Synthesis and microstructural characterization of metals@MOF-5 · Analytical Chemistry for teaching me how to work the MAS-NMR instrument and all his patience during the sometimes

Synthesis and Microstructural

Characterization of Metals@MOF-5

Dissertation

by

Felicitas Schröder

Synthesis and Microstructural


Dissertation

Zur Erlangung der Doktorwürde

der Fakultät für Chemie und Biochemie

an der Ruhr-Universität Bochum

Vorgelegt von

Dipl. Chem.

Felicitas Schröder

2008

III

This work has been performed in the time between January 2006 and August 2008 at the

Chair of Inorganic Chemistry II, Organometallics & Materials Chemistry of the Ruhr-

University Bochum.

Herein I declare that I have written this thesis independently and without unauthorised help.

Further, I assure that I have used no other sources, auxiliary means or quotes than those

stated.

I further declare that I have not submitted this thesis in this or in a similar form to any other

university or college.

Besides, I declare that I have not already undertaken an unsuccessful attempt to obtain a

doctorate from another college or university.

Felicitas Schröder, November 2008

Day of examination: 19.12.2008

1st referee: Prof. Dr. Roland A. Fischer

2nd referee: Prof. Dr. Christof Wöll

IV

I am grateful to my supervisor

Prof. Dr. Roland A. Fischer

for giving me great support and scientific freedom throughout the work on my thesis.

I always enjoyed being part of your group.

V

Acknowledgements

First I would like to thank all members of the MOF group for a great time and working

atmosphere: Maike Müller, Daniel Esken, Sebastian Henke, Denise Zacher, Xiaoning Zhang,

Saeed Amirjalayer, Mikhael Meilikhov and Dr. Kirill Yusenko.

I would also like to thank Prof. Dr. Bruno Chaudret from the Laboratoire de Chimie de

Coordination, CNRS, Toulouse for being my second supervisor in my thesis and very helpful

discussions.

I sincerely thank Prof. Dr. G. Buntkowsky from the Friedrich Schiller Universität, Jena, Dr.

Bernadeta Walaszek and Prof. Dr. H.-H. Limbach from the Freie Universität, Berlin for

readily accepting my request to measure my samples and useful discussions.

I would also like to thank Dr. Oleg I Lebedev and Stuart Turner from the EMAT institute in

Antwerp, Belgium for the detailed TEM measurements and evaluations of my metal@MOF-5

samples.

Of course I would also like to thank Hans-Jochen Hauswald from the Department of

Analytical Chemistry for teaching me how to work the MAS-NMR instrument and all his

patience during the sometimes time consuming measurements.

I sincerely thank Dr. Harish Parala for introducing me to the XRD and his help during

technical difficulties.

I would also like to thank Dr. Bernd Marler, Faculty of Geoscience, for help with the Rietveld

refinement and many helpful discussions.

Many thanks also go to Dr. Konstanze Schröck and Prof. Dr. Martina Havenith-Newen for an

interesting collaboration within the scope of the THz project.

I would like to give my warmest regards to Sabine Pankau for her help in organizing things of

everyday and scientific life.

VI

Also, I would like to thank all group members of AC II for a nice working atmosphere:

Saeed Amirjalayer, Daniela Bekermann, Timo Bollermann, Thomas Cadenbach, Jun. Prof.

Anjana Devi, Dr. Sandra Gonzalez Gallardo, Dr. Eliza Gemel, Dr. Christian Gemel, Vanessa

Gwildis, Markus Halbherr, Malte Hellwig, Ursula Herrmann, Todor Hikov, Dr. Ramasamy

Pothiraja, Dr. Ganesan Prabusankar, Heike Gronau-Schmid, Andrian Milanov, Dr. Rochus

Schmid, Dr. Maxim Tafipolski, Tobias Thiede, Manuela Winter, Ke Xu

Of course, warm regards also go to the former group members:

Dr. Arne Baunemann, Dr. Raghunandan Bhakta, Dr. Stephan Hermes, Dr. Andreas Kempter,

Dr. Jayaprakash Khanderi, Dr. Eva Maile, Dr. Daniel Rische, Dr. Marie-Kathrin Schröter, Dr.

Tobias Steinke, Dr. Urmila Patil

Further, I would like to thank the German National Academic Foundation (Studienstiftung des

Deutschen Volkes) for granting me a PhD fellowship over the last three years.

I would also like to thank the Research School of the Ruhr-University Bochum for granting

me a fellowship.

Financial support from the Deutsche Forschungsgemeinschaft (DFG) within the scope of the

Sonderforschungsbereich 558 – Metal-Support Interactions in Heterogeneous Catalysis is also

gratefully acknowledged.

Also I would like to thank, Dr. André van Veen, André Rittermeier and of course Angelika

Kruse-Fernkorn and Ruth Knödlseder-Mutschler for a nice but sadly too short time in the GK

of the SFB 558.

I sincerely thank Prof. Dr. Christof Wöll for accepting being co-referee within my PhD

defence and for the past years of collaborations.

Besides, I would also like to thank the following persons without whom it would not have

been possible to realise this work:

VII

- Prof. Dr. Wolfgang Grünert and Dr. Maurits van den Berg, Department of Technical

Chemistry at the Ruhr-University Bochum, for the XAS measurements at Hasylab,

Hamburg

- Dr. Andreas Trautwein (Südchemie AG, Heufeld) and Karin Bartholomäus

(Department of Analytical Chemistry, Ruhr-University Bochum) for elemental

analysis measurements.

- Susanne Buse for the N2 sorption measurements

- Jutta Schäfer and Sabine Bendix (Department of Analytical Chemistry, Ruhr-

University Bochum) for the GC-MS measurements

- The entire staff from the chemical lager, the glassblowing and the fine mechanics

factories of the Faculty of Chemistry and Biochemistry of the Ruhr-University

Bochum

- Dr. Alexander Birkner for help with TEM measurements at the Ruhr-University

Bochum

Finally I would like to thank Mirza Cokoja for being on my side always. Your love and

support always were and always will be everything I could have ever wished for in my life.

VIII

For my parents and Mirza

IX

Table of contents

1. Motivation and objectives ............................................................... 1

2. Introduction ................................................................................ 3

2.1. Metal-organic frameworks – an introduction .............................................................. 3

2.2. Loading of MOFs with functional molecules ............................................................. 8

2.2.1. Large organic molecules ....................................................................................... 9

2.3. Towards nanoparticles in metal-organic frameworks ............................................... 11

2.3.1. Loading with MOCVD precursors ...................................................................... 12

2.3.2. Reactions inside MOFs ....................................................................................... 16

2.4. Nanoparticle synthesis inside metal-organic frameworks ......................................... 18

2.4.1. General synthesis ................................................................................................. 18

2.4.2. Metal nanoparticles inside MOF-5 ...................................................................... 19

2.4.2.1. Pd@MOF-5 ................................................................................................ 20

2.4.2.2. Cu@MOF-5 and Au@MOF-5 ................................................................... 21

2.4.2.3. Metal nanoparticles in MOF-177 ............................................................... 22

2.4.3. Metaloxide@MOF-5 and metal/metaloxide@MOF-5 ........................................ 22

2.5. Other frameworks and other loading techniques ....................................................... 24

2.5.1. Noble metal particle formation at redox-active frameworks .............................. 24

2.5.2. Grafting of metal nanoparticles inside MOFs ..................................................... 26

2.6. Applications of nanoparticles loaded MOFs in catalysis .......................................... 28

3. Synthesis and characterisation of Ru nanoparticles in MOF-5 ................. 31

3.1. Loading of MOF-5 with [Ru(cod)(cot)] .................................................................... 33

3.1.1. Synthesis .............................................................................................................. 33

3.1.2. Characterization .................................................................................................. 33

3.1.2.1. Elemental/AAS analysis and packing density of

[Ru(cod)(cot)]3.5@MOF-5 .......................................................................... 33

3.1.2.2. 13C MAS-NMR spectroscopic measurements ............................................ 36

3.1.2.3. FT-IR spectroscopic measurements ........................................................... 39

3.1.2.4. X-ray diffraction studies of [Ru(cod)(cot)]3.5@MOF-5 ............................. 40

3.2. Attempts of the structural analysis of [Ru(cod)(cot)]3.5@MOF-5 by the

Rietveld method ........................................................................................................ 41

X

3.3. Hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5 at mild conditions ............................ 45

3.3.1. Synthesis .............................................................................................................. 45


3.3.2.1. 13C MAS NMR spectroscopic investigations of

{[Ru(cod)]/Ru}@MOF-5 ........................................................................... 45

3.3.2.2. PXRD structural investigation of {[Ru(cod)]/Ru}@MOF-5 ..................... 48

3.3.2.3. TEM analysis of {[Ru(cod)]/Ru}@MOF-5 ............................................... 49

3.4. Quantitative hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5 to give Ru@MOF-5 ..... 50

3.4.1. Synthesis .............................................................................................................. 50


3.4.2.1. 13C MAS-NMR investigations of Ru@MOF-5 .......................................... 50

3.4.2.2. PXRD structural investigations and calcination studies of Ru@MOF-5 ... 51

3.4.2.3. X-ray absorption spectroscopy (XAS) measurements of Ru@MOF-5 ...... 54

3.5. Microstructural investigation of Ru@MOF-5 by advanced TEM techniques .......... 56

3.6. Investigation of the host-guest interactions in Ru@MOF-5 ..................................... 62

3.6.1. CO-Adsorption on Ru@MOF-5 .......................................................................... 62

3.6.2. Hydride Mobility of Ru@MOF-5 in comparison to Ru Nanoparticles

stabilized by organic surfactants ......................................................................... 63

3.7. Catalytic Test Reactions of Ru@MOF-5 .................................................................. 66

3.7.1. Oxidation of benzyl alcohol ................................................................................ 66

3.7.2. Hydrogenation of benzene .................................................................................. 68

3.8. Conclusion ................................................................................................................. 69

4. Investigations of the loading of MOF-5 with two precursor components .... 71

4.1. Loading of MOF-5 with [Fe(η6-toluene)(η4-C4H6)] and [CpPtMe3] ........................ 73

4.1.1. Synthesis .............................................................................................................. 74


4.1.2.1. Elemental analysis ...................................................................................... 74

4.1.2.2. MAS-NMR investigations .......................................................................... 75

4.1.2.3. PXRD structural investigations .................................................................. 76

4.2. Loading of MOF-5 with [CpPd(η3-C3H5)] and [CpPtMe3] ....................................... 78

4.2.1. Synthesis .............................................................................................................. 79


4.2.2.1. Elemental analysis ...................................................................................... 79

XI

4.2.2.2. MAS-NMR investigations .......................................................................... 80


4.3. Loading of MOF-5 with [Ru(cod)(cot)]/[Pt(cod)Me2] .............................................. 84

4.3.1. Synthesis .............................................................................................................. 84


4.3.2.1. 13C MAS NMR spectroscopic investigations ............................................. 85


4.4. Co-Hydrogenolysis of [Ru(cod)(cot)] and [Pt(cod)Me2] in MOF-5

at mild conditions ...................................................................................................... 88

4.4.1. Synthesis .............................................................................................................. 88


4.4.2.1. MAS-NMR spectroscopic investigations ................................................... 88


4.5. Quantitative Co-Hydrogenolysis of [Ru(cod)(cot)] and [Pt(cod)Me2] in MOF-5 .... 91

4.5.1. Synthesis .............................................................................................................. 91


4.5.2.1. 13C MAS-NMR spectroscopic investigations ............................................ 91


4.5.2.3. TEM investigations .................................................................................... 95

4.6. Conclusion ................................................................................................................. 97

5. Investigations of MOF-5-water interactions ..................................... 100

5.1. Loading of MOF-5 with 4wt.% and 8 wt.% of water ............................................. 104

5.1.1. Synthesis ............................................................................................................ 104

5.1.2. Characterization ................................................................................................ 104

5.1.2.1. PXRD structural investigations ................................................................ 104

5.1.2.2. THz-TDS spectroscopic investigations .................................................... 106

5.1.3. FT-IR investigation of MOF-5 + 8 wt.% H2O .................................................. 109

5.1.4. 13C MAS-NMR of MOF-5 + 8 wt.% H2O ........................................................ 110

5.2. Conclusions ............................................................................................................. 112

6. Summary and Outlook ................................................................ 114

XII

7. Experimental ........................................................................... 118

7.1. Analytical methods and instrumental details .......................................................... 118

7.1.1. Specific surface area determination from N2 adsorption measurements ........... 118

7.1.2. X-ray powder diffraction ................................................................................... 120

7.1.3. Transmission electron microscopy .................................................................... 121

7.1.4. X-ray absorption spectroscopy .......................................................................... 123

7.1.5. Solid State Nuclear Magnetic Resonance-general ............................................ 125

7.1.6. THz spectroscopy .............................................................................................. 127

7.1.7. IR spectroscopy ................................................................................................. 129

7.1.8. Elemental/Atom Absorption analysis ................................................................ 129

7.1.9. Gas chromatography-mass spectroscopy .......................................................... 129

7.2. Syntheses of the materials ....................................................................................... 130

7.2.1. Synthesis of MOF-5 ([Zn4O(bdc)3]) powder .................................................... 131

7.2.2. Synthesis of MOF-5 ([Zn4O(bdc)3]) crystals .................................................... 131

7.2.3. Synthesis of [Ru(cod)(cot)]3.5@MOF-5 ............................................................ 133

7.2.4. Hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5 at mild conditions .................... 134

7.2.5. Synthesis of Ru@MOF-5 by quantitative hydrogenolysis of

[Ru(cod)(cot)]3.5@MOF-5 ................................................................................. 135

7.2.6. Deuterium adsorption at Ru@MOF-5, sample preparation for solid state 2H-NMR measurements .................................................................................... 135

7.2.7. CO adsorption at Ru@MOF-5 for FT-IR measurements .................................. 136

7.2.8. Oxidation of benzyl alcohol using oxidized Ru@MOF-5 as catalyst ............... 136

7.2.9. Hydrogenation of benzene using Ru@MOF-5 as catalyst ................................ 137

7.2.10. Synthesis of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5 .......................... 138

7.2.11. Synthesis of [CpPd(η3-C3H5)]/[CpPtMe3]@MOF-5 ......................................... 139

7.2.12. Synthesis of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 ......................................... 141

7.2.13. Removal of the precursor molecules from

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 .............................................................. 142

7.2.14. Co-Hydrogenolysis of [Ru(cod)(cot)]/[Pt(cod)(CH3)2] in MOF-5 at mild

conditions .......................................................................................................... 143

7.2.15. Quantitative Co-Hydrogenolysis of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 .... 144

7.2.16. Loading of MOF-5 with H2O ............................................................................ 145

XIII

8. References ............................................................................. 147

9. Appendix ................................................................................ 147

9.1. List of publications .................................................................................................. 147

9.2. Poster presentations ................................................................................................. 148

9.3. Curriculum Vitae ..................................................................................................... 150

XIV

Abbrevations

AAS Atom absorption spectroscopy

at.% Atomic percent

BTB 4,4′,4′′-Benzene-1,3,5-triyl-tribenzoate

BTC Benzene-1,3,5-carboxylate

bdc 1,4-Benzene-dicarboxylate

bpdc 4,4′,-Biphenyldicarboxylate

BPTC 1,1´-Biphenyl-2,2´,6,6´-tetracarboxylate

bpy 4,4′-Bipyridine

CNtBu Tertiary-butyl isonitrile

cod cis,cis-1,5-Cyclooctane

COF Covalent organic framework

cot cis,cis,cis-1,3,5-Cyclooctatriene

Cp Cyclopentadienyl-anion (C5H5)

cyclam 1,4,8,11-Tetraaza-cyclotetradecane

dmf Dimethylformamide

DEF Diethylformamide

DMA N,N-Dimethylacetamide

EDX Energy dispersive spectroscopy

EXAFS Extended X-ray absorption fine structure

FT-IR Fourier transform infrared spectroscopy

FWHM Full width at half maximum

fcc Face centered cubic

GC-MS Gas chromatography coupled with mass spectroscopy

HAADF High-angle annular dark field

hcp Hexagonal close packed

JCPDS Joint committee on powder diffraction standards

JUC Jilin University China

Me Methyl

MOCVD Metal-organic vapor deposition

MOF Metal-organic framework

MIL Matérial Institute Lavoisier

mtb methanetetra-benzoate

XV

OiPr Iso-propoxide

pzdc Pyrazine-2,3-dicarboxylate

pyz Pyrazine

SAED Selected area electron diffraction

STEM Scanning transmission electron microscopy

TATB triazine-1,3,5-tribenzoate

TEM Transmission electron microscopy

THz-TDS Terahertz time-domain spectroscopy

THF Tetrahydrofuran

wt.% Weight percent

XANES X-ray absorption near-edge structure

XAS X-ray absorption spectroscopy

XRD X-ray powder diffraction

ZIF Zeolite A imidazolate framework

1. Motivation and objectives

1


Metal/support-interactions play an important role in heterogenous catalysis. In this

context, the Cu/ZnO based methanol catalyst represents a prototype of the so-called strong

metal support interactions in heterogeneous catalysis. This work was performed within the

scope of the research center SFB 558 ‘Metal support-interactions in heterogeneous catalysis’

at the Ruhr-University Bochum, which focuses on these interactions. Beside common porous

support materials such as silica or alumina, metal-organic frameworks arise as new support

matrices. MOFs exhibit large surface areas (up to 5900 m2/g) and temperature stability of up

to 500 °C with yet a rather different support-interaction than other common porous supports.

MOFs appear to behave rather like solid solvent cages with comparably low interaction with

embedded metal nanospecies. On the other hand, MOF-5 exhibits an unusual promotional

effect on embedded Cu nanoparticles in methanol catalysis. Zn-free Cu colloids were found to

be completely inactive in methanol catalysis, yet Cu@MOF-5 showed low catalytic activity in

methanol catalysis. Obviously MOFs exhibit unique novel properties as stabilization matrices

for metal nanospecies in heterogeneous catalysis.

Metals@MOF-5: Microstructural and Metal/Support-interaction

Although investigations of the synthesis of metal and metal oxide nanoparticles in

MOFs have already been performed, there has never been a direct proof as to where the

obtained nanoparticles are actually located inside the MOF cavities or at the outer surface of

the framework. A detailed investigation of the framework-guest (either nanoparticles or guest

molecules) interaction has also never been performed. The general objective of this work was

therefore to provide a deeper understanding of the nanoparticle-guest interaction and the

location of nanoparticles@MOF-5. Due to its easy accessibility, relatively large surface area

and pore opening/diameter (see discussion above), MOF-5 was chosen as model metal-

organic framework host material in this work. In order to investigate the framework-

nanoparticle interaction in more detail, especially investigation of the synthesis of ruthenium

nanoparticles in MOF-5 was selected as study case. The synthesis of ruthenium nanoparticles

on solid supports or ruthenium colloids in solution has already been extensively studied (see

next chapter), and an extended database of results from TEM, PXRD, XAS, MAS NMR and

FT-IR measurements is available in the literature. Therefore, the obtained corresponding

analytical data from Ru@MOF-5 can easily be compared to these data and new insights of the

properties of nanoparticles@MOF-5 appear possible. In addition, Ru nanoparticles are also of


2

relevance in catalytic oxidation and hydrogenation reactions, studies of the catalytic

properties of the composite Ru@MOF-5 are hence interesting as well.

Bimetallics@MOF-5

With the perspective of synthesizing bimetallic nanoparticles in MOF, loading of

MOF-5 with two different metal precursor molecules was studied. Bimetallic nanoparticles

such as FePt, PdPt and PtRu have already been studies in the literature due to their superior

magnetic (FePt) or catalytic properties (PdPt, PtRu). For example, the catalytic properties of a

single metal catalyst can be greatly enhanced by adding an additional metal. Three binary

systems of Pd/Pt, Fe/Pt and Ru/Pt precursors were examined. Here the main objective was to

investigate whether the MOF material can be loaded with distinct precursor ratios, which is an

important prerequisite of the synthesis of distinct metallic alloys. Preliminary results on the

synthesis of PtRu nanoparticles were also obtained.

Water@MOF-5

Since many catalytic processes also demand the presence or production of water, the

interaction between MOF-5 and water guest molecules was another objective of this work.

For this, Terahertz spectroscopy, which is frequently used to investigate protein-water

interactions, was applied. The examined MOF-5 materials, loaded with 4 wt.% and 8 wt.%

water were investigated additionally by PXRD, FT-IR and MAS-NMR.

2. Nanoparticles synthesis in MOFs

3

2. Introduction

2.1. Metal-organic frameworks – an introduction

The phrase coordination polymer first appeared in the literature in the early 1960s

with the corresponding research area already being reviewed in 1964.[1] However, it was not

until the early 1990s that more detailed research on porous coordination polymers started to

increase considerably. Early papers on these new polymeric compounds already pointed out

the great possibilities for new material structures and properties offered by these materials.[2-6]

Ever since, this novel class of hybrid inorganic-organic soft solid state materials, largely

based on Werner-type coordination chemistry, has become a major field of research. Beyond

the scope of pure inorganic porous materials, i. e. zeolites, alumophosphates etc; metal-

organic frameworks (MOFs) are completely regular, have a high porosity and are highly

designable. Their synthesis usually occurs under mild conditions by using a choice of a

certain combinations of discrete molecular building units which, in the ideal case, leads to the

desired extended network (see Figure 2.1). The crucial chemical parameters of MOF

syntheses are pH (mostly acidic), concentrations and temperature (< 100 °C classical

coordination chemistry,

Figure 2.1. The building block principle behind formation of metal-organic frameworks.[3]


4

>100 °C solvothermal conditions). These are directly linked to the overall possibility of

designing MOF structures. During MOF synthesis, the organic linkers remain invariant

whereas the nuclearity and dimensionality of the inorganic brick can change if the synthesis

parameters are not controlled; leading to possible undesired framework structures. Due to the

infinite number of possible combinations of organic and inorganic building parts, basic

principles of classification of the resulting MOF structures are clearly needed. O’Keeffe and

Férey were the first to develop two different approaches concerning the topology of

structures.[7,8] O’Keeffe’s concept of ‘augmented nets’ describes every solid as a geometric

figure (net) resulting from the connection of the entities of the structure. For instance, in an

[N,M] connected net, some vertices are connected to N and some vertices to M neighbors. An

illustration of this is provided by the example of platinum oxide Pt3O4 as parent structure (see

Figure 2.2). In this solid, the Pt ions are surrounded by four oxygen atoms in a square planar

mode, whereas the oxygen ions are surrounded by three Pt ions in a triangular mode, creating

a [3,4] net and a structure based on the three-dimensional assembly of square planes. In the

augmented net, the Pt ions will be replaced by squares (with the same connectivity) and the O

ions will be replaced by triangles. Through linkage by a line, the vertices of the polygons

create the augmented net. These polygons can also be referred to as topological SBUs

(secondary building units), representing species whose connectivity is four for the squares and

three for the triangles, not regarding their chemical nature. The structure of Cu3(BTB)2 (BTB

= 4,4′,4′′-benzene-1,3,5-triyl-tribenzoate) was described as augmented Pt3O4 net by this

Fig.2.2. Principle of augmented nets, (a) description of Pt3O4 in terms of connection of squares, (b) balls and

sticks representation of Pt3O4 (Pt, blue; O, red) showing the fourfold coordination of Pt and the threefold coordination of O, (c) augmented version of Pt3O4; O is replaced by a triangle and Pt by a square. Both polygons are related by linkers; keeping the same topology in copper(II) 4,4′,4′′-benzene-1,3,5-triyl-tribenzoate, the triangles correspond to the connecting points of the central phenyl ring of 4,4′,4′′-benzene-1,3,5-triyl-tribenzoate, (d) and (e) the square to the Cu dimer linked to four carbons of the carboxylate functions.[9]


5

approach[10] but other MOF structures can be classified in the same way.[11] From this concept

combined with the defined selection of certain molecular building blocks for a desired MOF

structure, the concept of ‘recticular synthesis’ was developed[12] along with the synthesis of

the Zn carboxylate based so-called IR-MOFs (see Figure 2.3) with IR-MOF-1 also known as

MOF-5 being one of the most studied metal-organic frameworks to date.[4] Férey’s ‘scale

chemistry’ concept started from the analysis of solids in terms of secondary building units

(SBU). Instead of describing MOF structures by the connection of single polyhedra, it was

shown that it was possible to analyze structures by using larger units (SBUs). The SBUs will

then, by translation and/or rotation and further sharing of vertices, build up the final structure.

Thereby the topology of the structures remains unchanged when, e. g. a larger SBU is

used.[9,14] The so-called MILs (Matérial Institute Lavoisier),[7,9,15] with MIL-53 and MIL-88

demonstrating large and reversible structural changes upon guest exchange and MIL-101[16]

being the most porous material known to date with an equivalent Langmuir surface area of

5900 m2/g, were derived from this principle. In general, porous coordination compounds can

be classified in three categories, 1st, 2nd and 3rd generation (see Figure 2.4).[17a] The 1st

generation compounds are sustained only with guest molecules and show irreversible

framework collapse upon guest removal. Second generation compounds are stable and robust

frameworks, showing permanent porosity without any guest molecules.

Figure 2.3. Series of Zn based IR-MOFs with linkers differing in dimensionality and functionality. While expansion of the linkers increases the internal void space (represented by yellow spheres), it also allows the formation of catenated phases (IR-MOF-9,-11,-13,-15).[13]


6

Figure 2.4. Classification of porous coordination compounds as 1st, 2nd and 3rd generation.[17a]

Figure 2.5. Classification of dynamic porous coordination polymers upon guest removal/exchange,

“recoverable collapsing” (Type-I), “guest-induced transformation (Type-II) and “guest-induced reformation” (Type-III).[17b]

Third generation compounds are flexible, dynamic frameworks, responding to external stimuli

(i.e. light, electric field, guest molecules) and change their channels and pores reversible.

Flexibility of the MOF structures has long been considered a disadvantage addressing the

robustness of the frameworks upon guest molecules removal. These structures interacting

with exchangeable guest species in a switchable fashion (see Figure 2.5) can however be

applicable for molecular sensing. Here, especially the work of the group of S. Kitagawa

emphasized the importance of soft porous coordination polymers based on hydrogen bonds

with responsive and adaptive properties for applications in gas storage, sensing and

catalysis.[17] So far, the most investigated applications of MOF materials clearly are gas,

especially hydrogen, storage and gas separation as well as solvent separation.[18–26] In this

context, zinc imidazolate based MOFs (ZIFs) were recently shown to be excellent storage

materials for CO2 gas.[27] ZIFs are expanded analogues of zeolites in which transition metal

atoms (M, specifically Zn and Co) replace the tetrahedral linker (such as Si, Al, P) and


7

Figure 2.6. The sodalite based net (stick diagram (left) and tiling (center)) and largest cage (right) of ZIF-

8 with ZnN4 tetrahedra in blue.[29]

imidazolates (IM) replace the bridging oxygens with an M–IM–M angle close to 145°,

coincident with the Si–O–Si angle preferred in zeolites.[28] Among this class of new MOF

materials, especially ZIF-8 (Figure 2.6) showed excellent chemical and temperature stability,

sustaining its structure even after boiling in water and 8 M aqueous NaOH solution for up to

24 h.[29] Rather different from the class of porous coordination polymers but clearly an

extension to the family of designable porous materials in general, the boronic ester based

frameworks (COFs) were introduced.[30] They exhibit very low crystal densities and high H2

strorage capacities.[30b-c] In these materials, the organic building units (see Figure 2.7) are held

together by strong covalent bonds (C–C, C–O, B–O and Si–C) rather than metal ions to

produce materials with high porosity (BET surface areas of 3472 m2/g for COF-102 and 4210

m2/g for COF-103) and low crystal density (0.17 g/cm3 and for COF-108).[30b] The last decade

of research on MOFs and other designable porous materials has brought fascinating new

structures and applications. Beside the research on new MOF structures and the investigation

of gas storage and solvent separation, new applications are also emerging. The controlled

growth of MOF thin films at surfaces (SURMOFs) has been demonstrated by a few groups

recently.[31,32] This paves the road for integration of MOF materials into more complex

functional devices such as smart membranes and chemical sensors. Similar to zeolites,

mesoporous silica and other inorganic porous materials, the grafting of functional molecular

species at the internal surface of MOFs and the loading of the pores, cavities or channels of

MOFs with functional nanoparticles is relevant for quite a number of potential applications

including catalysis, hydrogen storage and sensing.[33–38] In addition, the use of MOFs as host

materials for the formation of nanosized metals or metal oxides is of considerable interest to

study the resulting specific properties and host-synergetic functions.[39–41] The following


8

Figure 2.7. Molecular structures of building units (a) and crystal structures of COFs (b-g). Hydrogen atoms

are omitted for clarity. Carbon, boron, oxygen, and silicon atoms are represented as gray, orange, red, and blue spheres, respectively.[30c]

introductory chapter of this work will cover the current literature related to the doping of

MOFs with nanoparticles with some emphasis on the involved precursor concepts.The

discussion will include relevant aspects of the loading of metal-organic frameworks and

loading-related properties in general. This appears to be very important since the loading of

metal-organic frameworks using molecular precursors to provide the components for the

nanoparticle growth inside the framework clearly relies on the supramolecular host-guest

chemistry of MOFs in general.

2.2. Loading of MOFs with functional molecules

Initially, the pores or channels of MOFs are usually occupied with solvent molecules

and sometimes also with unreacted, excess linkers. These (small) molecules are supposed to

act as templates during the growth in solution and also stabilize the framework structure due

to hydrogen bond formation.[17] They should be removable or exchangeable by other solvent

or gas molecules without any structural change of the remaining framework in order to load

MOFs with more complex functional molecules. This is apparently mandatory, since in some

cases structural changes of the host or even complete collapse of the frameworks structure can

occur upon solvent removal.[17a-b] Metal-organic frameworks are typically studied in the


9

context of their gas storage and/or separation properties mainly focusing on rather non-polar

hydrocarbons.[18-26] Most MOFs exhibit non polar, i.e. hydrophobic inner surfaces, and here

the discussion will be largely restricted to these types of MOFs. The general possibility to

adsorb non-trivial, large functional organic molecules inside the cavities of MOFs was

demonstrated, for example, by Yaghi et al. using dyes[42] and by Férey et al. applying

pharmaceutically relevant molecules.[43] Obviously, for larger guest molecules, the potential

host frameworks should exhibit pore diameters or window openings that allow the in-

diffusion of this compounds. This will be discussed later in this chapter.

2.2.1. Large organic molecules

Only a few key examples of metal-organic frameworks have been tested in loading

with larger, more complex molecules so far. In their first report on the zinc-1,3,5-

benzenetribenzoate based MOF-177 in 2004,[42] Yaghi et al. introduced the loading of this

highly porous framework with the dye molecules of Astrazon Orange R, Nile red and

Reichardt’s dye as well as with C60 molecules. The inclusion of these compounds in the

framework was followed by UV- as well as Raman-spectroscopy (Figure 2.8). Quantitative

uptake analysis of the materials revealed 16 molecules

Figure 2.8. Loading of MOF-177, a) Raman spectra of A) bulk C60, B) evacuated MOF-177 crystal, C) whole

MOF-177 crystals loaded with C60 and D) sliced MOF-177 crystals loaded with C60. b) MOF-177 crystal loaded with dye molecules.[42]


10

of Astrazon Orange R, two molecules of Nile red and one molecule of Reichardt’s dye per

MOF-177 unit cell, already demonstrating the size dependant sorption properties of MOF-

177. Quiu et al. presented the synthesis of the porous framework [Cd3(bpdc)3(dmf)]·5 dmf·18

H2O (bpdc = 4,4′,-biphenyldicarboxylate, dmf = dimethylformamide, JUC-48, JUC = Jilin

University China) and the assembly of Rh6G dye molecules in its pores.[44] Dye molecules

were infiltrated by either adding an ethanolic solution of the dye to the mother liquor of the

MOF or immersion of the framework in the dye solution. The dye@MOF composite showed

temperature dependent fluorescence properties. Férey et al. studied the loading of the

chromium terephthalate based MIL-101 with the Keggin anion [PW11O40]7- in order to show

the selective inclusion of very large guest molecules into the spacious cages of the MIL-101

(Figure 2.9).[16] The loading was followed by PXRD, N2 adsorption and 31P-NMR. From

elemental analysis 0.05 Keggin anions per chromium were found, corresponding to a loading

of five Keggin moieties per large cage of MIL-101. The volume of five Keggin anions

represents 10,100 Å3 in volume, the volume of the large cage of MIL-101, however, is 20,600

Å3; the authors therefore assume that the remaining space is filled with cations and water

molecules. Together with the C60 inclusion in MOF-177, the inclusion of Keggin anions in

MIL-101 already points at the potential of MIL-101 and other MOFs to host nanoobjects. The

Férey group also introduced the first drug (Ibuprofen) release study of a metal-organic

framework.[43] The Ibuprofen uptake of the frameworks as well as the subsequent release was

investigated. Leaving the well known toxicity of chromium aside, the drug release study of

MIL-101 and MIL-100 shows the potential of metal-organic frameworks not only for the

Figure 2.9. (A) Schematic view of the insertion of Keggin anions within the largest pore of MIL-101.

(B) XRD of MIL-101 (1) and MIL-101(Keggin) (2); θ, in degrees. (C) TGA of MIL-101 (1) and MIL-101(Keggin) (2); T, temperature (K). (D) Nitrogen sorption-desorption isotherms at 78 K of MIL-101 (1) and MIL-101(Keggin) (2); Vads, volume adsorbed in cm3g-1. (E) 31P solid-state NMR spectra of the Keggin salt and MIL-101(Keggin); δ, chemical shift in ppm.[16]


11

loading but also the controlled release of an imbedded compound.[43a] MIL-101 is able to take

up four times as much Ibuprofen as MCM-41 which has comparable cage sizes. It also shows

a slower delivery rate which supposes advantages for larger pharmacological molecules. Very

recently, the ‘breathing’ chromium and iron based MIL-53 frameworks have also been used

as matrices for the delivery of Ibuprofen.[43b] Very slow, controlled and complete delivery was

achieved under physiological conditions which is due to the frameworks ability to adapt to the

dimensions of the drug.

These and similar studies employing more complex organic molecules as guests prove distinct

features of the corresponding frameworks, i.e. extraordinary cavity size or adsorption

properties. Thus, these studies focus on the function of the MOF itself as matrix for

adsorption. The loading of MOFs with organometallic precursor molecules is motivated by

combination of the functionality of the precursors to serve as source for nanoparticles with the

function of the MOF to restrict the growth and the aggregation of the particles by caging

effects. Interestingly, the characteristic sizes of cavities, channels and pores of MOFs cover a

size regime between that of classical zeolites ≤ 1 nm and that of mesoporous materials ≥ 2–3

nm. In contrast to these latter materials however, MOFs are expected to show a much weaker

particle/host interaction.

2.3. Towards nanoparticles in metal-organic frameworks

Metal doping of metal-organic frameworks seems to be a promising field of research

not only for catalytic applications but also for enhanced capacity in gas storage as compared

to the pure metal-organic framework.[45] In general, one could think of two different

approaches for the synthesis of metal nanoparticles inside MOFs, the infiltration of preformed

nanoparticles stabilized by organic molecules (surfactants) in solution or the stepwise

infiltration of suitable precursors and their conversion into nanoparticles inside the

framework’s cavities. For other porous host materials, such as silica and alumina, several

approaches for imbedding nanoparticles into their cavities via immersion in colloidal

solutions are literature known.[46] However, the typical cavity sizes of MOFs are too small to

match surfactant stabilized nanoparticles with hydrodynamic radii typically larger than 3 nm.

In order to utilize the metal-organic framework as stabilizing agent and host material at the

same time, stepwise precursor infiltration and subsequent decomposition appears to be the

most suitable approach. In this way, the size and shape of the nanoparticles, synthesized


12

directly in the pores of the framework, should be controlled by the pore size, shape and

channel structure of the host material. Suitable precursor molecules for the synthesis of metal

nanoparticles in MOFs can in general be molecules that are also commonly used in the

synthesis of colloidal metal nanoparticles in solution or metal nanoparticles in the solid state.

These molecules are often also known from thin film formation processes such as MOCVD

(Metal-Organic Vapor Deposition) or ALD (Atomic Layer Deposition) and are basically

metal-organic coordination compounds or so-called organometallic, often all-hydrocarbon

ligand molecules featuring metal carbon bonds. Upon decomposition, the ligands of these

molecules are cleaved off the metal center, leaving ‘free’ metal atoms that will then fuse

together to form metal clusters.[47] Space confinement of the MOF cavities should ideally limit

the growth of the particles to the size of the corresponding pore diameter. Note that the host

framework should be inert towards the imbedded precursor itself as well as to decomposition

products or free ligands of the precursor molecules. Here, common approaches known from

colloid and nanoparticle chemistry in general, like the ‘polyol process’ for coinage and noble

metal colloids, seem to have a somewhat lower importance, due to the reactivity of many

metal-organic frameworks toward acidic conditions (protons) and halides, especially at

elevated temperatures. It should also not be neglected that typical metal-organic precursor

molecules often exhibit an intrinsic reactivity toward protic solvent residues, hydroxyl groups

or other reactive surface groups inside the host material. Hence, a careful choice of the metal-

organic framework and the precursor is mandatory for a controlled nanoparticle synthesis

inside MOFs.

2.3.1. Loading with MOCVD precursors

Loading of metal-organic frameworks with organometallic molecules can be seen as

an extension of the loading with larger non-trivial organic molecules mentioned above. Some

of these coordination compounds are highly volatile already at room temperature; others are

sublimable at elevated temperature and pressure. Loading of MOFs with these compounds

can, therefore, be compared to the loading with gases or volatile solvents and is usually

performed in vacuo. With respect to the subsequent controlled decomposition of the included

molecules to nanoparticles, MOCVD (Metal-Organic Vapor Deposition) precursors are a very

suitable class of compounds for the loading of MOFs. For a controlled loading, the

characterization of the primary inclusion compounds precursor@MOF is mandatory to

warrant the yield of a well defined metal@MOF composite in a second step. In addition,


13

studying the loading process of the MOF with precursor molecules also gives interesting

insights into the host-guest interactions of this composite which may help to elucidate the

interactions between imbedded metal nanoparticles and the MOF. This is highly relevant

since, e.g., catalytic processes often demand well defined host guest interactions to enable

stabilization of free adsorption sites for molecules in the catalytic reaction. Depending on the

guest molecules, loading of the metal-organic frameworks can be generally performed via gas

phase in vacuo or via solution. In all studies discussed above, loading of MOFs with dye

molecules,[42,44] C60,[42,16] Keggin anions[16] and Ibuprofen[43] was performed via solution

impregnation of the MOF powders. The MOF materials were immersed in saturated solutions

of the compounds, letting the guest molecules slowly diffuse into the MOF cavities. When

loading via solution, the competition in diffusion between the guest and the solvent molecules

has to be taken into account. A uniform distribution of the guest molecules throughout the

framework is not easy to achieve due to the inclusion of solvent molecules at the same

time.[42] Kaskel et al. have used the incipient wetness technique to load MOF-5 with the Pd

precursor [Pd(acac)2] (acac = acetylacetonate).[45] The advantage of this technique is the rather

precise control of the loading just by choosing a certain concentration of the precursor in the

solution. With this technique, however, the loading with precursors is limited to the solubility

of the precursor molecule in the solvent used. Notably, Kaskel and co-workers introduced

only 1 wt.% Pd into the MOF, which is fine for many catalytic applications.

Due to its facile synthesis even in larger scales, temperature stability up to 400 °C in argon,

high Langmuir surface area of up to 4400 m2/g[48] and the relatively large pore opening of 7.8

Å,[4] MOF-5 is quite a nice test system for various types of loading studies. Of course, the

reactivity of MOF-5 towards water and humid air[49,50] limits its application in technical

processes to some extent. The first studies on the loading of MOFs with organometallic

molecules and the subsequent synthesis of metal nanoparticles in MOF (which will be

discussed later) have been performed by infiltration of [CpPd(η3-C3H5)], [CpCu(PMe3)] and

[Au(CH3)(PMe3)] in MOF-5 (Zn4O(bdc)3, bdc = benzene-1,4-dicarboxylate) via gas phase.[51]

Prior to that, the freshly prepared [Zn4O(bdc)3] was activated in vacuo and then exposed to

the vapor of the different metal-organic molecules. The loading was followed by 1H and 13C

MAS-NMR, FT-IR, powder X-ray diffraction and elemental analysis. It was shown that,

similar to the loading with the large organic molecules, the structure of the host framework

remains intact after inclusion of the organometallic compounds.


14

Figure 2.10. Cut-out of the crystal structure[52] of the inclusion compound ferrocene7@MOF-5.

The presence of the unchanged guest molecules inside the MOF cavities was detected by the

corresponding solid state 1H and 13C MAS-NMR signals and elemental analysis. In a second,

more detailed study, loading of MOF-5 was extended to a variety of metal and metal oxide

precursors; here different aspects of the loading were investigated.[53] The metal loading was

determined to be in a range of 10–40 wt.%. The reversible loading of MOF-5 was presented,

without changing the chemical properties of the precursors or the host framework. Ferrocene,

as representative example for many organometallic compounds of interest here, can first be

infiltrated into the MOF-5 cavities and then be removed without any change of the host

material. Also, the dependence of the size of the precursor molecules on the loading was

shown. MOF-5 cavities exhibit an opening diameter of 7.8 Å. Ideally, only one of the three

principal axes (x, y, z; see Table 2.1) of the enveloping ellipsoid representing the van der

Waals volume of the respective precursor molecule should exceed this diameter in order to

allow diffusion into the MOF cavities. For that reason, [Cu(OCHMeCH2NMe2)2] is not

adsorbed by the MOF-5 matrix since its characteristic dimensions exceed the pore opening in

all three dimensions (see Table 1.1). Not surprisingly, the loading is also dependent on the

vapor pressure of the precursor molecule. Rapid desorption was observed for small,

comparably volatile compounds like [Fe(CO)5] or [Zn(C2H5)2]. Cyclopentadienyl complexes

like [FeCp2] form more stable inclusion compounds with the MOF-5 matrix (see Figure 2.10).

More or less stoichiometric inclusion compounds of the formula precursorn@MOF-5 (where n


15

is the average number of precursor molecules per MOF-5 cavity) are obtained in all cases

according to elemental analysis. In addition to this, also the loading of MOF-177, as another

member of the zinc carboxylate based MOFs, with organometallic precursor molecules has

already been studied.[54,55] The larger pore opening and pore volume of MOF-177 (10.8 Å and

1.59 m2/g[42]) in comparison to MOF-5 (7.8 Å and 1.04 m2/g[4]) allows the diffusion of larger

molecules like [Cu(OCHMeCH2NMe2)2] into the cavities and the overall absorption of more

molecules per cavity than in MOF-5.[54]

Table 2.1 Characteristic molecular dimensions of organometallic precursors absorbed by MOF-5.[53]

Precursor x y z Max.

[CpPd(η3-C3H5)] 4.5 4.5 4.5 5.5

[CpPtMe3] 4.3 4.7 4.7 6.5

[FeCp2] 3.5 4.5 4.5 5.2

[CpCu(P(CH3)3)] 5.0 5.0 7.5 7.5

[Au(CH3)(P(CH3)3)] 4.5 4.5 7.0 7.0

[Sn(C4H9)2(OOC2H3)2] 6.5 7.8 10.0 10.0

[Zn(C2H5)2] 1.8 3.0 8.0 8.0

[Fe(CO)5] 4.2 4.6 5.9 5.9

[Cu(OCHMeCH2NMe2)2] 6.5 7.9 8.7 8.7

x, y, z in Å

Besides following the loading of MOFs with precursor molecules by MAS-NMR

spectroscopy and elemental analysis, investigation of ordering of these compounds in the

MOF cavities is clearly an interesting aspect of loading MOFs in general. Only in a few cases

a crystal structure of the host-guest composite has been obtained so far. The first example for

such a substructure was investigated by Kim et al.[52] Similar to the first reports by Hermes et

al., MOF-5 was loaded with ferrocene via gas phase in vacuum. Interestingly, the authors

attempted loading of MOF-5 in DMF (dimethyl formamide) solution as well, however in this

case inclusion of the guest molecules was not successful as shown by UV/VIS spectroscopy

(see the above comments regarding solution impregnation). By applying synchrotron radiation

at 100 K, a single crystal structure of the compound [FeCp2]7@MOF-5 was obtained.

Shrinkage of the unit cell volume of 3.7 % is observed compared to the evacuated host. The

unit cell is defined by the space group Pa-3. The smaller pores of MOF-5 are filled with six,

the larger pores with eight ferrocene molecules (see Figure 2.10).[52] Depending on the tilting

of the bdc linkers, MOF-5 exhibits two kinds of pore diameters with 11.0 Å and 15.1 Å.[4] The

results of the crystal structure were confirmed by elemental analysis, giving seven ferrocene

molecules per formula unit of MOF-5, which corresponds to 56 ferrocene molecules per


16

Figure 2.11. a) The orientation of the ferrocene molecules in the larger pore of MOF-5. b) The π-stacked

ferrocene molecules are 3.53 Å apart.[52]

elementary cell of the MOF-5 structure. The six ferrocene molecules in the smaller pore adopt

an octahedral arrangement with the ferrocene molecules close to the faces of the cube shaped

cavity; whereas the eight molecules in the larger pore are positioned near the corners of the

MOF-5 pores (see Figure 2.11). Here, extensive π−π interactions exist between the guest

molecules as well as between the guests and the framework itself. The packing of the

ferrocene molecules in the pores leaves only 1.6 % of the crystal volume accessible to other

guest molecules. In another study, Kim et al. have also shown the loading of porous

[Tb16(TATB)16(DMA)24](DMA)91(H2O)108 (TATB = triazine-1,3,5-tribenzoate, DMA = N,N-

dimethylacetamide) with ferrocene.[56] In this case, a crystal structure of the composite was

not obtained. Loading was followed by emission spectroscopy and 1H NMR spectroscopy.

Elemental analysis suggests 65 ferrocene molecules per formula unit. However, the crystal

structure of ferrocene@MOF-5 nicely shows the influence of the space limitation of the MOF

cavities on the ferrocene arrangement and vice versa the subtle effects of the loading on the

MOF-5 framework itself. This leads to another aspect of loading metal-organic frameworks.

The space confinement of the MOF cavities may as well support stabilization of reactive

species in order to observe reactive intermediates of organometallic or organic reactions.

2.3.2. Reactions inside MOFs

The interior of porous hosts may be very different from that of the exterior

surroundings, leading to a novel kind of reactive of species included in these pores. For

example, reactive intermediates from organometallic or organic reactions could possibly be

stabilized in the cavities of the host material. At this point, it is notable that detailed studies on


17

Figure 2.12. Schematic presentation of the supramolecular tetrahedral assembly Ga4L6 and a guest

molecule included in the nanocage.[58]

the stabilization of reactive species inside metal-organic polyhedra have already been

performed. Here, the group of K. Raymond has given interesting insights into the rich host-

guest chemistry of the assemblies of the type M4L6 (see Figure 2.12).[57] For instance, it was

shown that the ionic [(Cp)Ru(cod)]+ and [(Cp*)Ru(cis-1,3,7-octatriene)]+ species, which

usually rapidly decompose in water, are stabilized inside the cluster [Ga4L6]12- in aqueous

solution.[58] Despite their stabilization within the host, the guest molecules are still able to

react stoichiometrically with CO. In addition, also the ability of the tetrahedral assemblies to

act as nanoenzymes, catalyzing the hydrolysis of acetals and ketals in basic solution, was

presented.[59] In a similar metal-organic polyhedron, even the observation of a reactive

intermediate of the photodissociation of [Cp'Mn(CO)3] by crystal structure was shown by

Fujita et al.[60] The in situ generated [Cp´Mn(CO)2] is directly observed by X-ray diffraction.

A discussion about the species’ geometry could be clarified by the resulting crystal structure,

the species adopts pyramidal geometry.

These results suggest that MOFs as extended metal-organic assemblies could show a similar

host-guest chemistry. In uncharged metal-organic frameworks however space confinement

should be the most effective stabilizing effect on these species. So far only few reports on

reactions inside MOF and the trapping of reactive intermediates have been published. The

first example was reported by Long et al. who showed functionalization of MOF-5 bdc linkers

with {Cr(CO)3} fragments and subsequent photoreactions.[61] The fragments were introduced

by heating the MOF-5 powder in a solution of Cr(CO)6 in THF/Bu2O. Photoreactions of the

{Cr(CO)3} fragment with N2 and H2 lead to stable (η6-arene)Cr(CO)2(N2) and (η6-

arene)Cr(CO)2(H2) species which are usually only accessible in frozen gas matrices or


18

Figure 2.13. Reaction of [Zn4O(bdc)3] with Cr(CO)6.

[61]

supercritical fluids (see Figure 2.13). Obviously, the MOF framework shows a remarkably

stabilizing effect on the reactive fragments. From these first examples it can be concluded that

metal-organic frameworks, especially MOF-5, show potential as stabilizing matrices for

reactive intermediates in metal-organic reactions. Therefore this field of research is definitely

worth being investigated in more detail.

2.4. Nanoparticle synthesis inside metal-organic frameworks

The synthesis of nanoparticles inside MOFs is another, yet special example for

reactions inside the framework. Nanoparticles “trapped” inside MOF cavities, with a high

number of reactive surface atoms, are indeed reactive species as well. Their synthesis inside

the porous hosts, starting from molecular precursors, anticipates the caging effect of the

framework. To prevent the clusters from growing to larger, bulk agglomerates, the space

confinement of the framework’s pores is utilized naturally.

2.4.1. General synthesis

As mentioned above, in most of the still rather few reports on the synthesis of

nanoparticles inside metal-organic frameworks, the formation of the nanoparticles is obtained

in two steps: first, loading of the porous host with precursor molecules and second, the

decomposition of the precursor inside the porous host. Depending on the properties of the

intercalated precursor, the decomposition conditions have to be chosen. In general,


19

decomposition of MOCVD precursors can be obtained by treatment with reactive gases such

as H2 at a suitable temperature, treatment at elevated temperature or by photolysis. The

formation of nanoparticles from metal salts, i.e. loading the MOFs with metal cations or

inorganic metal complexes, e.g. [PdCl4]2- as precursors, will be addressed as a special case

later. It is mandatory to choose precursors with decomposition conditions that will be

tolerated by the host framework. Therefore, the temperature stability of the framework should

match the corresponding decomposition temperature of the precursor in order to obtain

nanoparticles in an unchanged host matrix. Also, the stability of the MOF toward possible

additional reactive gases or UV radiation has to be confirmed. For different precursors,

different decomposition protocols have to be applied. The obtained nanoparticle@MOF

composites can subsequently be investigated by standard analytical techniques such as

powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), N2 sorption

measurements and transmission electron microscopy (TEM). Similar to the related research

on nanoparticles hosted by zeolites or mesoporous silica etc., the challenge here clearly is to

check whether the nanoparticles are located inside or outside the framework and to investigate

and control the distribution of the particles inside the matrix. Beside TEM, the routine

analytical methods give only indirect proof of the existence of embedded particles. The few

reports on the synthesis of either metallic or oxidic nanoparticles in MOFs so far have mostly

been performed with MOF-5, however some other metal-organic frameworks have been

studied as well. Therefore, this part of the introductory chapter will be divided into two sub

chapters. First, nanoparticles synthesis in MOF-5 will be discussed. Second, nanoparticles

synthesis in other MOFs will be addressed.

2.4.2. Metal nanoparticles inside MOF-5

In the first report on the synthesis of metal nanoparticles inside MOF-5, the formation

of Pd, Cu and Au nanoparticles inside this framework was presented.[51] After loading with the

corresponding precursors [CpPd(η3-C3H5)], [CpCu(PMe3)] and [Au(CH3)(PMe3)] (see

above), decomposition of the precursor to nanoparticles was achieved by either photolysis

(UV radiation) or hydrogenolysis. Both, UV radiation and H2 treatment, even at elevated

temperature, left MOF-5 unchanged, controlled decomposition of precursor molecules only is

therefore ensured. First, the synthesis and characterization of Pd nanoparticles inside MOFs

will be discussed in more detail, after that the synthesis of copper and gold nanoparticles will

be addressed.


20

2.4.2.1. Pd@MOF-5

Palladium nanoparticles with a dimension of 1.4 nm were obtained by photolysis of

[CpPd(η3-C3H5)] in MOF-5 at room temperature or below (with cooling) in the absence of

additional hydrogen, leaving a perfectly intact MOF-5 matrix as confirmed by powder X-ray

analysis and N2 sorption measurements.[51] The powder XRD of the corresponding sample

shows an additional broad reflection (FWHM = 5.4°) at 2θ = 40.99°, typical for

nanocrystalline Pd particles. The size of the nanoparticles derived from TEM and PXRD data

(Scherrer equation) is in good agreement with the diameter of the MOF-5 cavities (see above)

hinting at nanoparticles embedded in the porous host. Treatment of the same

precursor@MOF-5 composite with H2 gas at -35 °C led to Pd nanoparticles in the same size

regime, here, however, the MOF-5 matrix appeared to have lost its 2D long range order,

indicated by the absence of some Bragg reflections of the host material. Elemental analysis in

both cases gave a metal loading of 35.6 wt.% Pd.

Another route of introducing Pd nanoparticles into MOF-5, was presented by Kaskel et al., as

already mentioned above.[45] Here, MOF-5 powder was loaded with a solution of [Pd(acac)2]

in CHCl3 following standard recipes of the incipient wetness technique. Decomposition of the

precursor was obtained by thermal treatment at 150–200 °C or hydrogenolysis at 150–200 °C.

From elemental analysis, the metal loading was determined to be 1 wt.%. In the PXRD of the

composite, no additional Bragg reflections for Pd were observed. Due to the low metal

loading, detection of the embedded Pd species and their chemical nature, whether it is fully

reduced to Pd0 nanoparticles or whether there are still some remaining PdII species, was

presumably rather difficult. These aspects were not reported and discussed in detail by the

authors. The BET surface area of the composite material was, however, reduced in

comparison to the starting material, from 2885 g/m2 to 958 g/m2 which is most probably due

to the embedding of Pd nanoparticles. In another approach, Kaskel and coworkers also

applied coprecipitation for the preparation of Pd in MOF-5.[62] In this case, Pd(NO3)2 was

directly added during the synthesis of the MOF, leading to Pd contents of 0.43–0.64 wt.%.

Again, no Pd reflections were observed in the corresponding PXRD data, however a slightly

reduced surface area was observed. Obviously the detection of the location and the nature of

imbedded metal species in MOF-5 remains a particular challenge, especially if the

corresponding metal content is rather low. Here the detection limit of most analytic

techniques obviously anticipates a detailed examination.


21

2.4.2.2. Cu@MOF-5 and Au@MOF-5

Beside the already discussed metal@MOF-5 composite Pd@MOF-5, the synthesis of

copper and gold nanoparticles in MOF-5 has also been investigated. Similar to the synthesis

protocol discussed above, these materials were obtained by hydrogenolysis of

[CpCu(PMe3)][51,63] or [CpCu(CNtBu)][63] and [Au(CH3)(PMe3)][51] as precursors in MOF-5 at

elevated temperatures. Inspection of the PXRD patterns of the parent MOF-5 as well as the

ones of the Cu precursors in MOF and Cu@MOF-5 synthesized from these composites

showed that the characteristic reflections of the MOF-5 host is retained in all cases. The

structural quality of the Cu@MOF-5 composite derived from [CpCu(CNtBu)]@MOF-5

appears to be better than the one of the material derived from [CpCu(PMe3)]@MOF-5,

deduced from the signal-to-noise ratio of the corresponding PXRDs. This effect may be

caused by the interaction of the PMe3 ligand with the MOF-5 matrix.[63] From PXRD and

TEM measurements, the size of the Cu nanoparticles was determined to be in a range of 1–3

nm with a metal loading of 10–11 wt.%.

In contrast to this, TEM and PXRD data of Au@MOF-5 showed polydisperse Au particles in

a size range of 5–20 nm, the metal loading of Au@MOF-5 was determined to be 48 wt.%.

The gold particles appear to interact more weakly with the host matrix than the Pd and Cu

particles and thus larger agglomerates are formed possibly by diffusion of the particles to the

outer surface. Similar observations have been made for the loading of mesoporous silica in

which Au and Ag nanoparticles grew larger than the pore diameter of the host, here

presumably due to a destructive growth mechanism of the embedded particles.[64–66] The

results from the latest publication by Haruta et al.[67] also add to this finding to some extent.

Here, gold nanoparticles were synthesized on various 3D and 1D porous coordination

polymers beside MOF-5, such as [Cu3(btc)2] (BTC = benzene-1,3,5-carboxylate), Al-based

MIL-53, CPL-1 ([Cu2(pzdc)2(pyz)]n (pzdc = pyrazine-2,3-dicarboxylate, pyz = pyrazine) and

CPL-2 ([Cu2(pzdc)2(bpy)] (bpy = 4,4’-bipyridine) by solid grinding (SG) of the corresponding

MOF powders with the precursor [Me2Au(acac)] or CVD loading with the precursor as

described above and subsequent reduction of the precursor in H2 at 120 °C. The resulting Au

nanoparticles were found to be in size range of 2.2±0.3 nm for the SG loading and in a size

range of 3.1±1.9 nm for the CVD loading. The corresponding TEM pictures show that most

of the Au nanoparticles are indeed located at the outer surfaces of the MOF supports. Both

loading procedures clearly result in Au nanoparticles that are larger than the pore sizes of the

porous support materials, yet they are smaller than the Au nanoparticles prepared from

[Au(CH3)(PMe3)] in MOF-5 (5–20 nm). However here, the Au content was relatively high


22

(48 wt.%) in comparison to the Au content in the composites synthesized by Haruta et al.

(0.5–1 wt.%), which might be a reason for this finding.

2.4.2.3. Metal nanoparticles in MOF-177

Apart from detailed studies on nanoparticle synthesis in MOF-5, additional studies on

the synthesis of Pd, Cu and Pt nanoparticles inside the chemical related MOF-177 have been

published as well.[54,55] The synthesis procedure here was very similar to the procedure

published by Hermes et al.[51] with the precursors [CpPd(η3-C3H5)],[54] [CpCu(PMe3)][54] and

[Pt(η5-C5H4(CH3))(CH3)3][55] being first infiltrated into MOF-177 in vacuo and then

decomposed by UV radiation or H2 gas. As deduced from TEM and PXRD data of the

resulting composites, Pd and Cu nanoparticles of about 2.6 nm[54] and Pt of about 2.2 nm[55]

were obtained, matching the pore size of the MOF-177. Due to the larger pore size of MOF-

177 as compared to MOF-5 (see above), the overall obtained metal contents were relatively

high with 32.5 wt.% (Pd), 10.6 wt.% (Cu) and 41 wt.% (Pt). These first studies nicely prove

the utility of the general concept for nanoparticles synthesis in MOF-5 described above and its

transfer to other metal-organic frameworks stemming from the same class of compounds as

MOF-5. Furthermore, for nanoparticle synthesis in other frameworks, varying synthetic

techniques have been applied as well.

Although in the discussed examples of Cu and Pd nanoparticles in MOF-5 and MOF-177 the

particles sizes mostly correspond to the pore diameters of the host materials, from the given

analytical data (XRD, TEM) it is not easy to verify whether the particles are really located

inside or outside the support matrices. In the composite Au@MOF, this challenge appears

even bigger. Altogether, so far, a more detailed examination of the location of the different

nanoparticles within the networks has not been performed and would truly add to the deeper

understanding of the nanoparticle synthesis in MOFs.

2.4.3. Metaloxide@MOF-5 and metal/metaloxide@MOF-5

This section will refer to the formation of metal oxide and metal/metal oxide species in

MOF-5 similar to the formation of metal nanoparticles@MOF-5. After the infiltration of

precursor molecules, oxidation of the precursor molecules to oxide species by O2 gas can be

performed. However, classical sol-gel chemistry might be another synthetic strategy to yield

oxide species inside MOFs[68] and will surely be investigated in the future. Due to the


23

Figure 2.14. Left, Powder X-ray diffraction patterns of MOF-5 (a), [ZnEt2]@MOF-5 (b), as-ZnO@MOF-5 (as

synthesized by the wet method) (c), ZnO@MOF-5 (after annealing and derived by the wet method) (d), as-ZnO@MOF-5 (as synthesized by the dry method) (e), ZnO@MOF-5 (after annealing and derived by the dry method) (f) and ZnO-reference sample obtained from controlled hydrolysis of ZnEt2 and annealing in air (g). The positions of the characteristic reflections of hexagonal ZnO are marked. Right, TEM images of unloaded MOF-5 (top) and ZnO@MOF-5 (bottom after annealing and derived by the dry method).[63]

sensitivity of MOF-5 towards moisture, a sol-gel approach is not feasible. The sol-gel

chemistry inside MOFs needs water stable structures. Recently, the synthesis of Cu/ZnO

species in MOF-5, the preparation of nanometer sized ZnO species in MOF-5 and the

subsequent introduction of Cu nanoparticles were reported.[63] ZnEt2 was used as ZnO

precursor and was first adsorbed inside MOF-5 and then converted to ZnO species by either

exposure to O2 gas (dry method) or by very careful hydrolysis (wet method), followed by

annealing at 250°C. 17O labeling studies, using H217O revealed, that neither the bdc linkers

nor the central oxide ion of the Zn4O unit exchange oxygen atoms/ions with the imbedded

ZnO species.[63] Depending on the preparation conditions, Zn contents from 10–35 wt.% were

introduced in MOF-5. Results from PXRD, TEM, UV-VIS and 17O MAS-NMR spectroscopy

gave evidence for largely intact MOF-5 matrix with imbedded ZnO nanoparticles < 4 nm (see

Figure 2.14). Langmuir surface areas of the composite gave values of 900 m2/g (wet method)

and 1750 m2/g (dry method) which shows the advantage of the dry method, leaving a

remarkably high surface area at a Zn loading of 35.5 wt.%. The corresponding TEM

micrographs reveal the typical faceted MOF-5 nanocrystallites, due to the low contrast of the

ZnO in the Zn-based MOF matrix; however, the single particles could not be detected. In

addition to the synthesis of ZnO@MOF-5, also TiO2@MOF-5 was obtained by the oxidation


24

of Ti(OiPr)4 inside the MOF cavities.[69] The resulting metal oxide aggregates are presumably

quite small and neither showed reflections in the PXRD nor in the SAED.

The attempt to synthesize CuO or Cu2O species in MOF-5 by oxidizing the composite

Cu@MOF-5 with O2 led to a complete collapse of the framework as indicated by powder X-

ray diffraction.[63] Yet, soft oxidation of the embedded Cu nanoparticles with N2O yields core-

shell Cu2O/Cu nanoparticles inside the framework with the host matrix remaining completely

unchanged. The oxidation is completely reversible, upon treatment with H2 gas

Cu2O/Cu@MOF-5 is fully re-reduced to Cu@MOF-5.[63] Cu/ZnO@MOF-5 was obtained by

gas phase loading of ZnO@MOF-5 with [CpCuL] (L = PMe3, CNtBu) followed by

hydrogenolysis of the precursor. Here, a Cu loading of 1.4 wt.% together with ZnO loading

of 9.9 wt.% was obtained. The composite exhibited a surface area of 920 m2/g, indicating an

intact host matrix. In this case, the order in introducing the different nano species is a crucial

point. Although both precursors of ZnO and Cu nanoparticles were infiltrated in the MOF

simultaneously as unchanged molecules, the simultaneous conversion by pyrolysis, photolysis

or hydrogenolysis of both to Cu/ZnO@MOF-5 failed and led to collapse of the host matrix.

Therefore, the Cu precursor had to be infiltrated after the formation the ZnO species in MOF-

5. Below, the catalytic properties of the obtained composite Cu/ZnO@MOF-5 will be

discussed as well.

2.5. Other frameworks and other loading techniques

Due to its easy accessibility and photochemical as well as thermal stability MOF-5 has

been the typical study case for nanoparticle@MOF synthesis and characterisation. However,

few studies of other frameworks are known as well and are summarized in the following.

2.5.1. Noble metal particle formation at redox-active frameworks

Suh et al. studied the nanoparticle formation at MOFs, using metal salts as precursors

which are reduced to form metal clusters by a special redox-active framework.[70] Silver

nanoparticles of ~3 nm are formed when the metal-organic framework

[{Ni(C10H26N6)}3(bpdc)3]·2C5H5N·6H2O (bpdc = 4,4’-biphenyl-dicarboxylate; Figure 2.15) is

immersed in a methanolic solution of Ag(NO3). The reaction proceeds stoichiometrically with


25

Figure 2.15. X-ray structure of [{Ni(C10H26N6)}3(bpdc)3]·2C5H5N·6H2O. a) Structure of the linear

coordination polymer. b) Double network of threefold braids where macrocycle grooves are created by bpdc2- ligands. c) View showing the stacking of the linear chains to generate 1D channels.[70a]

a relation of Ni2+(in the host) , Ag+ = 1:1. The host framework remains unchanged upon

oxidation of its Ni-centers from Ni2+ to Ni3+. Due to the positive charging of the MOF during

the redox reaction, however, NO3- ions are adsorbed in the framework channels as well. The

size of the Ag nanoparticles (3 nm) exceeds the window size of the host framework of 7.3 Å

(see Figure 2.16) which might be due to diffusion of the particles to the framework’s

surface.[70a] Similar to this, in a second report Suh et al. presented the synthesis of Ag and Au

nanoparticles by the redox reaction of Ag(NO3) or HAuCl4 with the 2D framework

{[Ni(cyclam)]2[BPTC]}n·2nH2O (cyclam = 1,4,8,11-tetraaza-cyclotetradecane and BPTC =

1,1´-biphenyl-2,2´,6,6´-tetracarboxylate).[70b] Ag nanoparticles of 4 nm and Au nanoparticles

of 2 nm are obtained which exceed the void size between two layers in the framework. As in

the similar case above, the authors assume particle diffusion to the surface of the framework

as reason for this finding. In addition, the authors also reported Pd nanoparticle synthesis in

the metal-organic framework [{Ni(cyclam)]2(mtb)}n]⋅8n H2O⋅4n DMF (mtb =

methanetetrabenzoate) from Pd(NO3)2 solution in acetonitrile.[70c] The presumably

stoichiometric reaction between the NiII centers of the framework and the Pd salt, gives Pd0

and PdII coexisting in the framework as deduced from XPS measurements. TEM analysis

reveals that again the size of the obtained Pd nanoparticles exceeds the size of the MOF

channels. In this case, the authors describe that Pd nanoparticles were already spontaneously

formed from the solution of Pd(NO3)2 in MeCN.[70c] In all examples of metal nanoparticles

formed at the redox-active MOFs, the sizes of the obtained nanoparticles largely exceed those

of the host channels (i.e. 3 nm particles in a framework with 7.3 Å pore diameter). It is not

clear whether the redox reaction needs a penetration of the noble metal salt into the MOF or


26

Figure 2.16. HRTEM images of [{Ni(C10H26N6)}3(bpdc)3] after immersion in methanolic Ag(NO3) solution at

room temperature for a) 10 min, b) 18 h and c) after removal of the host by heating the solid of b) in dioctylether solution.[70a]

whether these reactions occur only at the surface. It is not inevidently reported whether the

particles are actually still located in the framework (outer surface) or indeed outside. From the

presented TEM pictures it cannot really be ruled out that at least some of the larger particles

are located outside the framework.

2.5.2. Grafting of metal nanoparticles inside MOFs

A method known from nanoparticle synthesis in silica materials has been introduced

very recently to direct the loading with metal precursors and in parallel to reduce the problems

resulting from the rather low interaction of the host materials with embedded particles. The

synthesis of Pd, Pt and Au nanoparticles inside chromium based MIL-101 was reported by

Férey et al.[71] First, the Lewis-acidic sites in MIL-101 were activated by drying and

desolvation at 150 °C in vacuo for 12 h. The resulting Lewis acidic sites were then grafted

with the chelating reagent ethylenediamine (ED). A subsequent treatment with HCl resulted in

the formation of ammonium groups inside the cavities in order to facilitate ionic interactions

with [PdCl4]2-, [PtCl6]4- or [AuCl4]- as carrier species for the noble metal component. Finally,

the adsorbed anionic noble metal complex was reduced with NaBH4. Again, the particle

formation has no influence on the host framework’s crystallinity and no additional Bragg

peaks are visible in the corresponding powder X-ray diffraction patterns. This is possibly due

to the surprisingly low metal loading of about 1 wt.%. TEM analysis reveals particle sizes of

2–4 nm, matching the pore dimensions of MIL-101 (2.4 nm and 3.9 nm). However, larger

particles outside the framework were also detected by TEM which might be due to a leaching

process during the reduction step of the metal salt (Figure 2.17).


27

Figure 2.17. TEM images of as-synthesized MIL-101 and precious metal immobilized EDMIL-101. (a) as-

synthesized MIL-101, (b) Pd/ED-MIL-101, (c) Au/ED-MIL-101, (d) Pt/EDMIL-101, (e) Pd- impregnated MIL-101 and (f) Pd/APS-SBA-15. Insets are EDX profiles of impregnated precious metal nanoparticles.[71]

The grafting of the MIL-101 frameworks obviously has a great effect on the strength of the

interaction between the framework and the imbedded particles. Through grafting of the

framework with functional amine groups, it was possible to obtain Au nanoparticles imbedded

inside the framework. As discussed above, Au nanoparticles might otherwise easily diffuse

out of the framework to form bigger agglomerates. The overall metal content of Pd, Pt and Au

in MIL-101 is rather low, so it will be interesting to see whether in the future an increased

loading can be obtained at all and what effect it will have on the particle sizes in the MIL

materials.


28

In addition to this, synthesis of Pd@MIL-101 was also performed using the “incipient

wetness technique” by Kaskel et al.[72] Similar to the procedure published previously on

loading MOF-5 by the same technique,[45] Pd nanospecies in MIL-101 at rather low Pd

content (1 wt.%) were obtained. As in the previous work of the same group,[45] no additional

Bragg reflections for palladium in the corresponding PXRD of Pd@MIL-101 were observed.

The results from N2 and H2 sorption measurements however indicate the loading of the MIL-

101 pores.

2.6. Applications of nanoparticles loaded MOFs in catalysis

Most of the already discussed examples of metal@MOFs and metaloxides@MOFs

have been synthesized in order to obtain novel kinds of supported nanoparticles with

potentially advantageous properties for catalytical applications. The catalytic properties of the

composite materials Pd@MOF-5 and Cu@MOF-5 were among the first to be tested. Here, the

Pd@MOF-5 composite that was obtained by the gas-phase loading/photolysis synthetic

protocol (35.6 wt.% Pd) showed moderate activity in catalysis of hydrogenation of

cyclooctene.[51] The Pd@MOF-5 (1 wt.%) synthesized by the incipient wetness technique

from [Pd(acac)2] was tested as catalyst in hydrogenation of styrene, 1-octene and cis-

cyclooctene, exhibiting a slightly higher catalytic activity than Pd supported on activated

carbon (Pd/Norit A; see Figure 2.18).[45] Possibly, the observed enhanced catalytic

performance of Pd@MOF-5 (1 wt.%) in comparison to Pd/Norit A (1 wt.%) and Pd@MOF-5

(35.6 wt.%)[45] is due to the higher dispersion and accessibility of the active Pd sites in this

sample. Also, Pd@MOF-5 was tested in hydrogen adsorption, revealing a capacity of up to

1.86 wt.% (at 77 K, 1 bar), clearly exceeding the value of Pd-free MOF-5 (1.32 wt.% at 77 K,

1 bar) by about 40 %. The material Pd@MOF-5 material obtained from coprecipitation[62] was

also tested in hydrogenation catalysis of ethyl cinnamate, revealing an activity twice as high

as the one of Pd supported on activated carbon. Again, the higher performance was attributed

to the higher dispersion and better accessibility of palladium; however, the actual

microstructure of the catalyst is still unknown. From N2 measurements of the composite

material after subsequent catalytic test reactions, it was found that Pd is most probably bound

to the outer surface of the MOF-5 crystals. Therefore, the as-prepared composite might be less

stable in prolonged catalytic runs than a sample with Pd particles located in the MOF-5 pores.


29

Figure 2.18. Ethylbenzene formation with different Pd supported catalysts. ■ represents Pd@MOF-5 (1

wt.%), ○ represents Pd/Norit A (1 wt.%) and ∆ represents Pd/C (purchased from Aldrich,1

wt.%).[45]

The Pd loaded MIL-101 composite was tested as a catalyst of the Heck coupling reaction of

acrylic acid with iodobenzene. Its activity is comparable to that of commercially available

Pd/C catalysts.[62] Kaskel et al. also performed catalytic test reactions of the composite

Pd@MIL-101.[72] The material exhibited a higher catalytic activity in the hydrogenation of

styrene and octane than Pd@MOF-5, furthermore, sustained activity in gas phase

hydrogenation of acetylene/ethylene mixtures was found. The works of Kaskel and Férey

show that for distinct catalytic applications, incipient wetness loading or solution loading in

general might be more suitable than gas phase loading since the metal content of the resulting

metal@MOF composite can be controlled more easily. However, other applications demand a

high metal content of a catalyst sample (e.g. Cu in methanol synthesis). Here, the gas phase

loading may be more advantageous, since comparably high metal loadings can be obtained in

a rather straightforward way. Accordingly, the composite Cu@MOF-5[51] was tested in

methanol catalysis from synthesis gas (H2/CO/CO2) and showed some catalytic activity,

matching the performance of Cu/ZnO@MCM-41.[73] This is surprising, since usually the

promotion of Cu by ZnOx species is essential for catalytic activity in methanol synthesis.[74–77]

Apparently, in Cu@MOF-5 this is not necessary or provided in a novel form by the zinc

carboxylate based host material MOF-5. A significant decomposition of Cu@MOF-5 during

the catalytic test reactions to yield promoting ZnO has been ruled out by characterization of

the catalyst used. In order to study the influence of additional ZnO species as a promoter for

Cu@MOF-5, the Cu/ZnO@MOF-5 system discussed above was also tested as catalyst for

methanol synthesis. The material expectedly showed enhanced catalytic activity peaking at


30

about 60 % of an industrial reference catalyst.[63] With a comparably very low Cu loading of

only 1.4 wt.%, obviously a superior interfacial contact between the Cu and ZnO nanospecies

must exist. However, the MOF-5 host material collapsed after several hours under catalytic

conditions, leading to poor final activities. These observations are attributed to the instability

of MOF-5 against water. Water is a by-product in many oxidation reactions. Due to its

sensitivity to water, MOF-5 appears to be less suitable as support material in catalytic

reactions that demand or release water. Still, the Au@MOF-5 composites synthesized by

Haruta et al.[67] showed noticeable high catalytic activity in benzyl alcohol oxidation in the

liquid phase as well as product selectivity depending on the porous support material.

Au/MOF-5 and Au/MIL-53 were selective to methyl benzoate, whereas the Au nanoparticles

on the copper-based frameworks were selective to benzaldehyde. Note that in this case the

composites structures were not investigated after the catalytic reactions. Therefore,

information about possible changes or deformations in the framework’s structures upon

exposure to a protic medium is clearly missing. Yet, all the presented results are encouraging

and show the general potential of MOFs as support matrices in catalytic reactions.

3. Synthesis and characterisation of Ru nanoparticles in MOF-5

31

3. Synthesis and characterisation of Ru nanoparticles in

MOF-5

In order to investigate the nanoparticle@MOF-5 interaction in more detail, the system

ruthenium@MOF-5 was chosen as case study for the following reasons. The catalytic

application of Cu, Pd and Au nanoparticles in MOFs has already been discussed in the

introductory chapter. Ruthenium is another element being relevant in many catalytic

applications. Organometallic ruthenium complexes are prominent in the homogeneous olefin

methathesis.[78,79] Ba-promoted Ruthenium supported on Al2O3 is interesting as catalyst for

ammonia synthesis.[80] Oxidation reactions of organic compounds can be catalyzed by Ru

supported on zeolites[81–84] and surfactant stabilized ruthenium nanoparticles are very well

known to be catalysts in hydrogenation reactions.[85,86] The synthesis of ruthenium

nanoparticles on solid supports or as surfactant stabilized colloids in solution is well

documented in the literature. Ruthenium colloids can be obtained via the polyol process from

RuCl3[87] as well as by the reduction with NaBH4, as typical examples.[88] However, in terms

of particle purity and perfect control over size and shape distribution by the surface chemistry,

the apparently most successful preparation method is the hydrogenolysis of highly reactive

all-hydrocarbon organometallic precursors, such as [Ru(cod)(cot)] (cod = 1,5-cyclooctadiene,

cot = 1,3,5-cyclooctatriene).[89] Under hydrogenolysis this purely olefinic complex gives rise

to cyclooctane which does not react with the host material MOF-5. Therefore, [Ru(cod)(cot)]

appears to be a suitable precursor for the synthesis of ruthenium nanoparticles in MOF-5.

Usually all liquid-phase routes to nanoparticles need suitable surfactants such as alkylamines,

-thiols, -alcohols or -silanes in the appropriate amounts to prevent particle agglomeration and

to control the growth by adsorption to the particle surface. Alternatively, the growth can be

limited by caging effects combined with particle support interaction as in the case of nano Ru

inside porous silica and alumina membranes.[90–91] As mentioned above, MOFs offer a novel

way of combining caging effects of porous solid state matrices with surfactant/particle

interaction in a very defined and molecularly controlled manner, extending the previously

mentioned concepts and going beyond the imbedding into pure organic polymers.[92]

Altogether, the interesting catalytic properties of ruthenium, together with the well

documented Ru colloid and nanoparticle synthesis from [Ru(cod)(cot)], make the composite


32

Ru@MOF-5 a feasible target system for a detailed study of the general properties of

nanoparticles embedded in MOF-5 and MOFs in general.


33

3.1. Loading of MOF-5 with [Ru(cod)(cot)]

3.1.1. Synthesis

For the synthesis of Ru nanoparticles in MOF-5, the purely olefinic [Ru(cod)(cot)]

was chosen as precursor. This compound has successfully been used before as precursor for

surfactant stabilized colloids.[89,92–94] It readily decomposes under 1–3 bar of H2 in solution

already at room temperature, to give Ru0 and cyclooctane as the only byproduct resulting

from hydrogenation of the olefinic ligands catalyzed by Ru0 (see Scheme 3.1). Furthermore,

it can be sublimed without decomposition at 30 °C and 10-5 mbar. To allow adsorption of

[Ru(cod)(cot)] in MOF-5, the molecular dimensions of the precursor should not exceed those

of the MOF-5 pore opening of 7.8 Å.[4] From the single crystal structural data of

[Ru(cod)(cot)][95] the dimensions of the precursor molecules where estimated to be 4.9 Å (x),

5.0 Å (y), 6.5 Å (z), therefore diffusion of the precursor molecules into the MOF-5 cavities is

possible.

Ru

1 - 3 bar H2, RT

solvent

Ru + 2

Scheme 3.1. Hydrogenation of [Ru(cod)(cot)].

3.1.2. Characterization

3.1.2.1. Elemental/AAS analysis and packing density of [Ru(cod)(cot)]3.5@MOF-5

Loading of the MOF-5 material was performed analogously to the procedure described

previously by Hermes et al.[51] In order to ensure adsorption equilibrium, the empty activated

MOF-5 was loaded with the Ru precursor under static vacuum at 10-5 mbar, 30 °C for 6 days

until visibly no further diminishment of the precursor material was observed. Thereby the off-


34

white MOF-5 powder turned yellow-orange. To demonstrate the inclusion of the precursor

inside the MOF-5 pores, loading experiments with larger, mm-sized single crystals were

Figure 3.1. Images of mm sized MOF-5 crystals after loading with [Ru(cod)(cot)]. In order to show the

macroscopically uniform distribution of the precursor, the crystals were cut (middle) and turned to examine the cross section (right).

performed as well. When the loaded crystals were cut open, the typical yellow-orange colour

stemming from the intercalated precursor molecules appeared to be homogeneously

distributed over the cross-section (Figure 3.1). This hints at a more or less uniform

distribution of the precursor molecules throughout the framework. From elemental analysis

results (see Table 3.1) the number of [Ru(cod)(cot)] molecules per MOF-5 formula unit was

determined as described elsewhere[96] to 3.5(±0.1). By comparing the pore volume per MOF-5

formula, i. e. the molar volume of the empty cavities, with the molar volume of the precursor

in the solid state, derived from DFT calculations, an estimation of the packing density of the

precursor molecules in MOF-5 can be achieved.[53] Given, that the unit cell of MOF-5 has a

total volume of 17344 Å3 and only 28 % of this volume is accessible for guest molecules,[4]

the average pore volume of MOF-5 per cavity is 1692 Å3. The molecular volume of

[Ru(cod)(cot)] was calculated from single crystal structural data applying Gaussian98[97] to be

347 Å3. An average number of 3.5 precursor molecules per MOF-5 cavity therefore

corresponds to a packing density of 71.5 %. In comparison to packing densities of other

precursors adsorbed by MOF-5 (see Table 3.2), this indeed corresponds to a rather high

loading with [Ru(cod)(cot)].

Table 3.1. Elemental Analysis of [Ru(cod)(cot)]3.5@MOF-5.

Elemental analysis Calculated

[found/calculated] number of molecules

Ru [%] C[%] H[%] per cavity

[Ru(cod)(cot)]@MOF-5 18.7/18.8 51.1/51.2 4.7/4.8 3.5


35

Only from the loading data of [FeCp2]7@MOF-5,[52] where “saturated” loading was verified

by single crystal structure analysis, a higher packing density of 81.5 % was calculated so far.

This confirms the intended rather “saturated” loading of MOF-5 with [Ru(cod)(cot)]. MOF-5

exhibits two types of cavities with different van der Waals pore diameters of 15.1 (cavity A)

and 11.0 Å (cavity B) as caused by the tilted bdc linkers and pore volumes of 0.61 and 0.54

cm3cm-3, respectively.[4] Therefore, the number of caged molecules in each type of cavity is

not necessarily the same and in the example of [Ru(cod)(cot)]3.5@MOF-5 was found to be

four precursor molecules in the larger and three precursor molecules in the smaller cavity of

MOF-5 (see Figure 3.2).

Table 3.2. Comparison of loading characteristics of various precursors adsorbed by MOF-5.

The data for [Ru(cod)(cot)] obtained during this work is marked orange.

Precursor Calculated number of molecules per

cavity Volume [Å3]

Packing density @MOF-5/@solid

state

[CpPd(η3-C3H5)] 4[53]

192[97]

47 %

[CpPtMe3] 3[53]

216[97]

40 %

[CpCu(P(CH3)3)] 3[53]

243[97]

28 %

[CpCu(CNtBu)] 1[53]

249[97]

15 %

[Au(CH3)(P(CH3)3)] 3[53]

160[97]

40 %

[Sn(C4H9)2(OOC2H3)2] 0.75[53]

445[97]

20.6 %

[Zn(C2H5)2] 2[53]

174[97]

21.2 %

[Fe(CO)5]

4[53] 216[97] 52 %

[FeCp2] 7[22]

198[97]

81.9 %

[Ru(cod)(cot)] 3.5 346 71.5 %

Loading experiments at higher temperature, intending to increase the packing density

precursor (60 °C) even above 71.5 %, led to uncontrolled isomerization of the precursor (see

NMR discussion below). Therefore loading at 30 °C, 10-5 mbar appeared to be the most

reasonable procedure in order to obtain a well defined [Ru(cod)(cot)]3.5@MOF-5 composite.

The obtained [Ru(cod)(cot)]3.5@MOF-5 was investigated further by means of 13C MAS-

NMR, FT-IR and PXRD.


36

Figure 3.2. Model of the inclusion compound [Ru(cod)(cod)]3.5@MOF-5. The exact distribution of the precursor molecules over the cavities is not known.

3.1.2.2. 13C MAS-NMR spectroscopic measurements

Figure 3.3 presents the 13C MAS-NMR spectra of [Ru(cod)(cot)]3.5@MOF-5 (a) and

of the mixture [Ru(cod)(cot)]/[Ru(η5-C8H11)2] in MOF-5 (b), the latter resulting from partial

isomerization of [Ru(cod)(cot)] to [Ru(η5-C8H11)2] due to loading at 60 °C. The carbon

resonance signals of the bdc moiety of MOF-5 were found in both spectra at 175.3 ppm

(COO), 136.5 ppm (C(COO)) and 131.3 ppm (C6H4), which nicely matches to the signals of

pure MOF-5.[4] In spectrum (a), the characteristic signals of [Ru(cod)(cot)] were clearly

observed at 101.4, 99.1, 76.7 and 31.6 ppm for the cot ligand, as well as 70.5 and 33.7 ppm

for the cod ligand, which stem from the intact Ru complex. This is in good accordance to the

literature known 13C-resonances of [Ru(cod)(cot)],[98] confirming the unchanged nature of the

intercalated Ru precursor. In spectrum b, the precursor signals are also observed, however

additional signals of the thermal isomerization product of [Ru(cod)(cot)], displayed in Figure

3.3.b (right), are also observed. Slight deviations in the chemical shifts in both spectra result

from the relative broad signals in solid state NMR spectra in general and the thus resulting

difficulty in marking the center of the signals. It is known that the precursor undergoes

thermal isomerization when exposed to temperatures above 60 °C in solution.[98] Obviously,


37

Figure 3.3. 13C MAS NMR spectra of (a) [Ru(cod)(cot)]3.5@MOF-5 and (b) mixture of [Ru(cod)(cot)]/[(Ru(η5-C8H11)2] in MOF-5. The carbon signals of MOF-5 are marked shady.

upon [Ru(cod)(cot)] infiltration in MOF-5 at 60 °C in vacuo, the same process can be

observed. Thereby, it was not possible to determine the ratio between the two Ru isomers.

With respect to further decomposition of the [Ru(cod)(cot)] molecules in MOF-5 to Ru

nanoparticles, it is mandatory to obtain a precursor@MOF-5 composite that is as defined as

possible. Therefore, all further investigations were accomplished with the defined

[Ru(cod)(cot)]3.5@MOF-5 composite obtained by loading at 30 °C. Interestingly, the


38

adsorbed precursor molecules exhibit the same number of signals and very similar chemical

shifts in the 13C MAS-NMR (Figure 3.3a) as in the conventional solution 13C-NMR as

reference (in C6D6).[98] In contrast to this, the 13C MAS NMR spectrum of pure, crystalline

[Ru(cod)(cot)] shows 16 different carbon atoms (see Figure 3.4.). Due to the absence of

symmetry of the molecule in the crystalline state, as result of crystal packing effects and the

very reduced molecular mobility and conformational flexibility, all carbon atoms of the cod

and cot ligands exhibit a different chemical environment, resulting in 16 carbon signals in the

corresponding 13C MAS-NMR. The broadness of the signals in both spectra is rather similar.

The signal of the cot ligand at 70.5 ppm (olefin.) appears to be rather broad in the 13C MAS-

NMR of [Ru(cod)(cot)]3.5@MOF-5 (Figure 3.3a) but the same is also observed in the 13C

NMR of the precursor in C6D6 solution.[98] Upon dissolving of the precursor molecules in

C6D6 or infiltration in MOF-5, the [Ru(cod)(cot)] molecules become fluxional. The

fluxionality of the molecules leads to

Figure 3.4. 13C MAS-NMR spectrum of pure crystalline [Ru(cod)(cot)]. Assignment of the signals was performed by simulation of the spectrum with Gaussian 03 (B3LYP/GIAO; Ru: Stuttgart RSC 1997; C,H: 6-31 G*).[98]


39

chemical equivalence of certain C atoms of the cod and cot ligands and only 6 different

signals can be observed in the corresponding 13C NMR spectra. However, the four olefinic C

atoms of the cot ligands appear to be less chemical equivalent (even in C6D6 solution),

probably due to limited fluxtionality of the Ru precursor molecules in certain dimensions,

which results in the detection of a comparably broad 13C NMR signal.

Obviously, when intercalated in MOF-5, the precursor molecules are more fluxional and

mobile and behave almost as in solution. The Ru precursor molecules behave as being

“dissolved” in the solid solvent MOF-5. Also, interaction with the host framework appears to

be rather weak as well, since no splitting of the carbon signals of the MOF-5 linkers, being a

hint for interaction with intercalated molecules is observed. The striking differences in the 13C

MAS-NMR spectra of [Ru(cod)(cot)]3.5@MOF-5 and the pure precursor, additionally show

that the loading of MOF-5 with the precursor does not lead to a simple physical mixture of the

two materials but indeed to a new composite material.

3.1.2.3. FT-IR spectroscopic measurements

The composite [Ru(cod)(cot)]3.5@MOF-5 was furthermore investigated by FT-IR

spectroscopy. Figure 3.5 shows the FT-IR spectra of pure empty MOF-5 (a) and

[Ru(cod)(cot)]3.5@MOF-5 (b). Both, the spectra of empty and loaded MOF-5 exhibit the

typical adsorption bands for C-O vibrations, with two very strong bands between 1400 and

1700 cm-1. The two bands at 1572 and 1507 cm-1 can be assigned to the asymmetric stretching

νas(COO), whereas the band at 1391 cm-1 can be assigned to the corresponding symmetric

stretching vibration νas(COO). These values are consistent with the presence of CO2- groups

coordinating to zinc. The vibrational bands in the range of 700–1200 cm-1 can be assigned to

the out-of-plane vibration of the MOF-5 terephthalates. In the FT-IR spectrum of the

composite [Ru(cod)(cot)]3.5@MOF-5, the vibrational bands of the host framework are still

dominating the spectrum without noticeable changes. However in the 1900–3100 cm-1 region,

where MOF-5 exhibits no adsorption bands, the vibrational bands of the precursor can be

observed. The observed bands at around 3000 cm-1 and 1970 cm-1 correspond well to the C-H

and C=C stretching vibrations of the olefinic cod and cot ligands of the precursor molecule.

Other bands of the precursor below 1500 cm-1 are more or less superimposed by the strong

MOF-5 vibrations.


40

Figure 3.5. FT-IR spectra of (a) MOF-5 and (b) [Ru(cod)(cot)]3.5@MOF-5 in dry KBr.

Together with the results from the 13C MAS-NMR measurements, these results show that

[Ru(cod)(cot)] can be infiltrated intact in MOF-5 without any change of the spectroscopic

properties of the precursor molecules.

3.1.2.4. X-ray diffraction studies of [Ru(cod)(cot)]3.5@MOF-5

The powder XRD diagram of [Ru(cod)(cot)]3.5@MOF-5 is given in Figure 3.6

(bottom). It clearly shows the two prominent reflections of MOF-5 at 2θ = 13.7° and 15.4° in

the same intensity ratio as in pure activated (Ar loaded) MOF-5 pointing at an intact host

material. The decrease of the overall reflection intensity with respect to the parent, empty

MOF-5 (Figure 3.6c) is a consequence of the inclusion of guest molecules in the framework

and has well been studied for zeolites and mesoporous silica materials.[99,100] In the powder

XRD diagram of [Ru(cod)(cot)]3.5@MOF-5, the intensities of the reflections at 6.9° and 9.7°

are inverted compared to the diagram of pure MOF-5 (Figure 3.6c). This corresponds to the

loading of the MOF-5 pores with guest molecules[101] meaning that each cavity of the host

framework is filled with guest molecules. Indeed, the intensity changes of powder X-ray

reflections of porous materials below 10° 2 θ generally indicate the occupation of the pores by

guest molecules. The XRD of the composite material also exhibits new reflections, mainly

between 10-20° 2θ with the most prominent new reflection at 13.8°. These reflections most


41

Figure 3.6. Powder X-ray diffraction patterns of (a) [Ru(cod)(cot)]3.5@MOF, (b) [Ru(cod)(cot)] and (c)

pure, activated MOF-5.

presumably result from some sort of ordering of the caged [Ru(cod)(cot)] in MOF-5. Clearly,

the XRD of the composite is not a simple superposition of the XRD pattern of [Ru(cod)(cot)]

(Figure 3.6b) and MOF-5 (Figure 3.6c). Obviously the space confinement induced by the

MOF-5 host and the preferred adsorption sites have an influence on the packing of

[Ru(cod)(cot)] in the cavities as found for [FeCp2]7@MOF-5.[52] Thus, the packing of the

[Ru(cod)(cot)] molecules inside the MOF-5 cannot resembles its solid state structure as a pure

compound.

3.2. Attempts of the structural analysis of [Ru(cod)(cot)]3.5@MOF-5 by

the Rietveld method

In order to elucidate the structure of [Ru(cod)(cot)]3.5@MOF-5, Rietveld

refinement[102] of the obtained PXRD of data was attempted. The Rietveld method is a

structure refinement and not a structure solution method. In the Rietveld method least square

refinements are carried out until the best fit is obtained between the entire observed powder

diffraction pattern taken as a whole and the entire calculated pattern, based on the


42

simultaneously refined models for the crystal structure, diffraction optics, instrumental factors

and other specimen characteristics (e.g. lattice parameters).[103] No effort is made in advance

in allocating observed XRD reflection intensities to particular Bragg reflections, nor to

resolve overlapped reflections. Consequently a reasonably good starting model for the crystal

structure is needed. In this case, the structure of pure, empty MOF-5 was chosen as starting

model. At the beginning of the Rietveld refinement, (PXRD) profile and lattice parameters are

refined separately. Here, the structure of [Ru(cod)(cot)]3.5@MOF-5 was refined using the

FULLPROOF 2K software.[104] As a first step, the obtained PXRD reflections of

[Ru(cod)(cot)]3.5@MOF-5 could be indexed on the basis of a tetragonal lattice showing that

all reflections belong to one single crystalline phase. The corresponding lattice parameters

revealed that the symmetry of the structure of [Ru(cod)(cot)]3.5@MOF-5 was lowered to a

tetragonal unit cell with a = b = 25.6716(3) Å and c = 25.4741(5) Å as compared to the

structure of pure, empty MOF-5 which has a cubic symmetry with a = b = c = 25.8849(7) Å.

Also, the deduced unit cell volume of [Ru(cod)(cot)]3.5@MOF-5 was found to be 3.2 %

smaller (16788 Å3) than the unit cell volume of the pure, empty MOF-5 (17343 Å3 at

169K).[4]

Figure 3.7. Le Bail fit with reasonable good quality factors (Rwp = 2.12 %, Rexp = 1.39%, chi2 = 2.3, for a

“good” fit, Rwp/Rexp should be close to 1.5 and chi2 should be close to 2) based on the PXRD of [Ru(cod)(cot)]3.5@MOF-5. Y(obs) is the observed PXRD intensity data, Y(calc) is the calculated PXRD intensity data, the Y(obs-calc) is the difference of these data.


43

A similar behaviour of framework shrinkage was also found for the inclusion compound

[FeCp2]7@MOF-5.[52] From systematic absences in the observed reflections of

[Ru(cod)(cot)]3.5@MOF-5, seven possible space groups were deduced: P4, P-4, P4/m, P422,

P4mm, P-42m, P4/mmm. Typically in Rietveld refinements, as a first step, a LeBail fit of the

obtained XRD data set is performed to derive the reflection intensities of the powder X-ray

diffraction pattern. Further refinement of, e.g., the lattice parameters is performed in

subsequent steps. An illustration of the LeBail fit of [Ru(cod)(cot)]3.5@MOF-5 is given in

Figure 3.7. In the LeBail fit, the measured PXRD data was approximated by a calculated

profile based on the given tetragonal lattice parameters. The refinement of the profile

parameters led to a good fit with the result being the same for any of the allowed space groups

(see Figure 3.7.). This once more confirms the assumption of a tetragonal symmetry with the

corresponding lattice parameters (see above) for the structure of [Ru(cod)(cot)]3.5@MOF-5.

To further refine the structure, the reflection intensities obtained by the LeBail fit were

converted into the corresponding electron density map to find additional atoms that were not

considered in the starting structure model. The density map in the space group P-42m, which

appeared to be the most reasonable space group due to the symmetry of the MOF-5

elementary cell, is given in Figure 3.8. The host framework structure of MOF-5 is clearly

revealed. The residual electron density found in the MOF-5 cavities was however to low to be

stemming from Ru precursor molecules. Difference

Figure 3.8. Preliminary electron density map of [Ru(cod)(cot)]3.5@MOF-5 in space group P-42m (ρ is the

relative electron density).


44

electron density maps showed the electron density within the MOF-5 cavities to be highly

smeared out indicating highly disordered Ru precursor molecules since the presence of these

guest molecules was explicitly shown by the presented elemental analysis, 13C MAS-NMR

and FT-IR results. Therefore, it was not possible to obtain a satisfying crystal structure of the

inclusion compounds [Ru(cod)(cot)]n@MOF-5 (n ≤ 3.5). Noteworthy, in the case of

[FeCp2]7@MOF-5, the single crystal structure was determined only by using synchrotron

radiation at low temperature to measure a sufficient number of the (weak) superstructure

reflexes of the guest molecules.[52] In our case, one additional problem may be possible a non-

saturated loading of MOF-5 with the [Ru(cod)(cot)] throughout the micro crystals, which

leads to an increased disorder of the rather mobile guest molecules. A similar observation was

made for [FeCp2]n@MOF-5 with n < 7. However, the attempts to rigorously ensure

thermodynamic equilibrium and a maximum loading at higher temperatures led to the

discussed isomerization of the precursor. An even severer problem of disorder of the

intercalated molecules resulted. Nevertheless, all reflections observed in the X-ray powder

diffractogram of [Ru(cod)(cot)]3.5@MOF-5 could be indexed applying the same tetragonal

space group with preliminary values for the cell constants. Since pure MOF-5 crystallizes in a

cubic space group, this difference provides evidence for a significant distortion and symmetry

change of the MOF-5 framework itself, induced by intercalation of the guest molecules. In

fact, [Ru(cod)(cot)]3.5@MOF-5 can be described as one single new phase. So far this has not

been observed for MOF-5 but similar changes of the framework symmetry upon guest

inclusion are well known for MIL materials.[7] Although the elucidation of the ordering of

caged organometallic molecules inside the MOF matrices certainly warrants attention, the

system [Ru(cod)(cot)]n@MOF-5 is possibly not the best study object for that purpose because

of its chemical lability. Furthermore, 13C MAS-NMR investigation (see above) of the

composite at 25 °C showed that the precursor molecules inside MOF-5 behave almost as in

solution. Obviously, interactions between the [Ru(cod)(cot)] molecules and the host

framework are rather weak. This is possibly due to the lack of extended π − systems in the

ligands of [Ru(cod)(cot)] which could undergo more pronounced π − π interactions with the

aromatic MOF-5 linkers and hence help to increase the ordering of the precursor molecules as

in the case of [FeCp2]7@MOF-5. The refined PXRD data (Figure 3.7) was furthermore

measured at 25 °C due to the lack of a PXRD instrument with a cooling set-up. In order to

minimize molecular motions of the precursor molecules in MOF and the MOF-5 network

itself, PXRD data collection at lower temperature might help to solve the structure of the

composite.


45

Nevertheless, 13C MAS-NMR and FT-IR spectroscopical measurements, as well as PXRD

data of the composite [Ru(cod)(cot)]3.5@MOF-5, prove the inclusion of intact precursor

molecules in an intact MOF-5 matrix. Therefore the mandatory prerequisite for further

investigations of a well defined hydrogenolysis of [Ru(cod)(cot)] in MOF-5 was achieved.

3.3. Hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5 at mild conditions

3.3.1. Synthesis

Studies of the hydrogenolysis of [Ru(cod)(cot)] in solution have shown that the

precursor decomposes already at 1 bar H2 at 25 °C.[89] The same gentle reactions conditions

were chosen for the hydrogenolysis of the precursor in MOF-5. When the composite

[Ru(cod)(cot)]3.5@MOF-5 was exposed to a stream of hydrogen (1 bar, 1 sccm) at 25 °C for

30 min, a colour change from yellow orange to brown was observed. The obtained brown

pyrophoric powder was identified as {[Ru(cod)]/Ru}@MOF-5 (see below) resulting from

incomplete hydrogenolysis of the Ru precursor and caging effects of the host framework. The

corresponding Langmuir surface area of the composite material was calculated to 1600 m2/g

from N2 sorption measurements (Type I Isotherms). This is a decrease of nearly 50% in

comparison to the “empty” MOF-5 powder which had a specific surface area of 3300 m2/g

prior to the loading with precursor and subsequent hydrogenation. The decrease of the surface

area can be attributed to the formation of Ru nanoparticles inside the framework and was also

observed upon loading of MOF-5 with Pd, Cu and Au.[51]


3.3.2.1. 13C MAS NMR spectroscopic investigations of {[Ru(cod)]/Ru}@MOF-5

The 13C MAS-NMR spectrum of {[Ru(cod)]/Ru}@MOF-5 (Figure 3.9) shows the

characteristic signals for the MOF-5 matrix (marked shady), but five additional signals that

are not expected upon quantitative hydrogenolysis of the precursor molecules, are observed

too. When comparing the 13C MAS-NMR of the composite [Ru(cod)(cot)]3.5@MOF-5 before

(Figure 3.3a) and after hydrogenolysis (Figure 3.9) at room temperature, it is evident that

these additional signals are not derived from remaining, unchanged [Ru(cod)(cot)]. The

resonances of the cot ligand have completely disappeared. The three broad signals at 177.5


46

Ru Ru

Rn

Rn

1 bar H2, 25 °C +

Scheme 3.2. Ligand exchange reaction of [Ru(cod)(cot)] under H2.

ppm (COO), 91.8 ppm (C(COO)) and 81.4 ppm (C6H4) are assigned to a more rigid and now

Ru - coordinated (η6 fashion) bdc linker and the two narrower signals at 66.1 and 33.4 ppm

are assigned to the more fluxional η4-coordinated cod ring at the Ru (Figure 3.9). A signal at

27.3 ppm stems from cyclooctane as byproduct of hydrogenolysis of [Ru(cod)(cot)], which

was not fully desorbed prior to the NMR experiment. Therefore, it is reasonable to suggest the

coordination of a 12-electron [(cod)Ru(0)] fragment at the arene moiety of the bdc linker to

form a typical 18-electron [(η6-arene)Ru(cod)] complex in order to explain these NMR data.

This assignment is in full accordance with known arene/cot exchange reactions taking place

when [Ru(cod)(cot)] is treated with hydrogen (1 bar) in the presence of arenes to yield [(η6-

arene)Ru(cod)] (see Scheme 3.2), which are quite stable against hydrogen in aromatic

solvents. The mild conditions (25 °C) of the treatment with H2 (1 bar) do obviously not lead

to a full splitting of the olefin ligands by hydrogenolysis. It is generally assumed that the

hydrogenolysis mechanism of [Ru(cod)(cot)] in solution starts with hydrogenolysis of the cot

ligand, releasing the [Ru(cod)] fragment which can be trapped by suitable ligands.[105,106] It is

evident that not all bdc linkers of the framework are coordinated by [Ru(cod)] since the

original MOF-5 signals are clearly detected and Ru nanoparticles are formed in parallel (see

below). In fact, the obtained material {[Ru(cod)]/Ru}@MOF-5 exhibits a total molar content

of Ru being unchanged as compared with [Ru(cod)(cot)]3.5@MOF-5. Ligand exchange

reaction of [Ru(cod)(cot)] with dimethylterephthalate (as model for the bdc linker) under 1

bar H2 in solution was attempted. But even in case of large excess of dimethylterephthalate

with respect to [Ru(cod)(cot)] (40:1) only quantitative formation of a black Ru metal

precipitate was observed. There is actually only one report in the literature on molecular

complexes of the [(cod)Ru] fragment with aromatic carboxylic acids and its derivatives. Here,

the synthesis of these complexes proceeds via lithiation and electrophilic substitution of

[(bromoarene)Ru(cod)] complexes.[107]


47

Figure 3.9. 13C MAS-NMR spectrum of {[Ru(cod)]/Ru}@MOF-5. The MOF-5 signals are marked shady.

FT-IR data of the composite (see experimental part) also show the presence of some

hydrocarbon residues from the remaining cod ligands besides the typical MOF-5 vibrational

bands. Taken together these comparisons, it can be concluded, that the observed trapping of a

significant fraction of the Ru in form of [Ru(cod)] species coordinated at the bdc linkers is an

indication of a caging effect on the kinetics of the hydrogenolysis of [Ru(cod)(cot)] inside

MOF-5. Obviously, the coordination of the reactive intermediate [Ru(cod)] at the bdc linkers

effectively competes with the hydrogenolysis and diffusion of [Ru(cod)(cot)] to yield Ru

particles. A similar trapping has not been observed for the related Cu@MOF-5 and

Pd@MOF-5 which were derived from the hydrogenolysis of [CpCu(PMe3)] and [CpPd(η3-

C3H5)], possibly because of the absence of stable η6-arene complexes of Cu(0) and Pd(0).[51]

The diffusion of caged molecules is affected by the particular environment of the host and it is

also dependent on the loading. These effects have been recently analyzed in detail for the

diffusion of benzene inside MOF-5.[108] The growth of Ru nanoparticles of more than 100 Ru

atoms (see below) via the hydrogenolysis of isolated [Ru(cod)(cot)] molecules is likely to


48

follow a diffusion limited kinetics which may be slowed down to some extent inside the solid

matrix MOF-5 as compared with conditions in solution. On the other hand, reactive

intermediates, such as [Ru(cod)], may be stabilized by caging effects in some way and

reaction paths may be altered as has been shown for a number of cases that have already been

discussed in the introductory chapter. Among these recent publications the formation of

{[Ru(cod)]/Ru}@MOF-5 can indeed be seen as another first examples.

3.3.2.2. PXRD structural investigation of {[Ru(cod)]/Ru}@MOF-5

The results of the powder X-ray diffraction measurements of {[Ru(cod)]/Ru}@MOF-5

compared to empty MOF-5 are presented in Figure 3.10. As already discussed above, the

overall decrease of the reflection intensities of the composite when compared to pure MOF-5

(Figure 3.10a) is due to the increased X-ray absorption caused by the embedded Ru

species.[63,64] Indeed, the PXRD of {[Ru(cod)]/Ru}@MOF-5 looks strikingly different from

the PXRD of the unchanged precursor in MOF-5 (see Figure 3.10a) and almost identical to

the one of the empty MOF-5 material (see discussion of the reflection intensities < 2 θ =10°

below) indicating an unchanged host matrix.

Figure 3.10. Powder X-ray diffraction patterns of (a) pure, activated MOF-5 and (b) {[Ru(cod)]/Ru}@MOF-5 (derived by treatment of [Ru(cod)(cot)]3.5@MOF-5 with 1 sccm H2, 25 °C, 30 min). (Lines: Ru JPDS reference No. 6-0663)


49

A very broad, low intensity reflection can be observed at 41-45 ° 2θ which can be attributed

to the two overlapping Bragg (002) and (101) reflections of hexagonal ruthenium at 42.1 °

and 44.0 ° 2θ. The finding of a very broad, low intensity reflection is in accordance with

earlier studies on the formation of Ru nanoparticles in porous silica materials[90] and

corresponds to nanocrystalline hexagonal Ru particles in a size regime certainly below 3

nm.[109] The single broad reflection at 44.0° 2θ does however not allow a reliable estimation of

the particle size by the Scherrer equation and line profile analysis especially since there is an

overlapping of two reflections at 42.1° and 44.0° 2θ and the superposition of the peaks from

the host matrix. Nevertheless, these results already hint at the formation of Ru nanoparticles

inside MOF-5 by hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5, even at mild conditions, and

an intact MOF-5 matrix.

3.3.2.3. TEM analysis of {[Ru(cod)]/Ru}@MOF-5

The TEM image of {[Ru(cod)]/Ru}@MOF-5 (Figure 3.11. left) presents the typical

rectangular shape of MOF-5 crystallites (larger image) indicating an unchanged host matrix.

Figure 3.11. TEM image of {[Ru(cod)]/Ru}@MOF-5 (left) and the corresponding EDX spectrum (right). The

sample was prepared on a Cu grid.


50

The Ru nanoparticles can be observed as regions of higher contrast, confirmed by EDX

analysis, due to the higher number of electrons in the metal compared to the metal oxide

based host material. The particles are in a size range of 1.6-1.8 nm which matches the pore

diameter of MOF-5[4] and also corresponds to the results from XRD analysis.

Taking together all analytical data from the analysis the composite {[Ru(cod)]/Ru}@MOF-5

it can be concluded, that the precursor cannot be fully hydrogenolyzed under 1 bar H2, 25 °C

when intercalated in MOF-5. An almost unchanged MOF-5 matrix was found as well as the

formation of small Ru nanoparticles matching the pore sizes of the host material. Side product

formation of (η6-arene)Ru(cod) complexes with the MOF-5 bdc linkers was observed, yet

most probably (see discussion of the XAS results below) most of the Ru precursor molecules

were decomposed to form Ru nanoparticles.

3.4. Quantitative hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5 to give

Ru@MOF-5

3.4.1. Synthesis

In order to obtain pure Ru nanoparticles in MOF-5, quantitative hydrogenolysis of

[Ru(cod)(cot)]3.5@MOF-5 was attempted. Finally, prolonged treatment (48 h) of the

composite under 3 bar H2, 150 °C gave Ru@MOF-5 with a total loading of 30.6 wt.% of Ru

metal. The Langmuir surface area, calculated from N2 sorption measurements was found to be

860 m2/g which again corresponds to the formation of Ru nanoparticles in the pore system of

MOF-5.[51] The composite {[Ru(cod)]/Ru}@MOF-5 could also be converted into Ru@MOF-

5 by the same procedure and appeared to be rather stable against H2 under more gentle

reaction conditions. The pure Ru@MOF-5 composite was subject to more thorough

investigations of the Ru nanoparticles, i.e. their chemical nature and especially location in

MOF-5 as well as their interactions with the host framework.


3.4.2.1. 13C MAS-NMR investigations of Ru@MOF-5

The product of the hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5 at 3 bar H2, 150 °C


51

Figure 3.12. 13C MAS NMR spectrum of Ru@MOF-5 (derived by treatment of [Ru(cod)(cot)]3.5@MOF-5 with 3 bar H2, 150 °C, 48 h).

for 48 h was studied by 13C MAS-NMR spectroscopy. The spectrum (see Figure 3.12.) shows

the typical signals of the host framework MOF-5 at 175.3, 136.9 and 130.4 ppm (see above)

only, no other carbon signals can be observed. No hydrogenated bdc linkers were detected in

the 13C MAS-NMR. The FT-IR data of the composite correspond to these results and can be

found in the experimental data part of this work. Quantitative hydrogenolysis of the Ru

precursor leaving an intact host matrix can be concluded.

3.4.2.2. PXRD structural investigations and calcination studies of Ru@MOF-5

The powder X-ray diffractogramm of Ru@MOF-5 (Figure 3.13b) shows perfectly

retained MOF-5 reflections with an overall intensity change of the reflections compared to

pure MOF-5 (Figure 3.13a). As discussed above, this is assigned to the higher X-ray

absorption of the embedded Ru nanoparticles which has an influence on the reflection

intensities of the framework as well.[99,100] As in the PXRD of {[Ru(cod)]/Ru}@MOF-5 (see


52

Figure 3.13 Powder X-ray diffractogramms of (a) pure, activated MOF-5 and (b) Ru@MOF-5 (Lines: Ru JPDS reference No. 6-0663).

Figure 3.10b), a broad additional reflection between 41-45° 2θ with a superposition of sharp

MOF reflections is observed. Here however, the reflection centred at 44.0° 2θ can be assigned

more clearly as the single (101) Bragg reflection of hcp Ruthenium with an estimated FWHM

of 1.5(±0.2)° (calculated by Bruker Diffrac Plus Eva 2004 software, version 10.0). The reason

for this small but significant difference may be that prolonged hydrogenolysis at 150 °C leads

to Ru particles with somewhat larger crystalline domains in case of Ru@MOF-5 as compared

with {[Ru(cod)]/Ru}@MOF-5. However, other reflections of metallic ruthenium were not

detected, which points to very small sizes of the nano crystallites again below 3 nm.[109] Due

to superposition of this reflection with other sharper MOF-5 reflections, this value is rather

inaccurate and clearly overestimating the real broadness of the Ru reflection. As discussed

above, loading of MOF-5 pores with guest molecules can be followed by the change of the

framework X-ray reflections intensities below 10° 2 θ. Low angle region PXRDs of

Ru@MOF-5 and pure, empty MOF-5 are given in Figure 3.14. In pure, activated MOF-5 (see

Figure 2.14b) the relative intensity ratio of the reflections at 6.9° and 9.7° was found to be

1:0.51. In the low angle XRD of [Ru(cod)(cot)]3.5@MOF-5 (see Figure 3.6c), the intensities

of these two peaks are inverted with an intensity ratio of 0.25 (6.9 °):1 (9.7°). For Ru@MOF-

5 (see Figure 3.14a), the peaks at 6.9° and 9.7° exhibit an intensity ratio of 1: 0.75. Here, the

intensity decrease of the reflection at 6.9° is not as pronounced as in [Ru(cod)(cot)]3.5@MOF-

5 but a difference in intensity compared to the low angle PXRD of pure MOF-5 (Figure


53

Figure 3.14. Low angle powder XRD diffractogramms of (a) Ru@MOF-5 (I(6.9°)/I(9.7°) = 1:0.75) and (b) pure, activated MOF-5 (I(6.9°)/I(9.7°) = 1:0.51)

3.14b) is clearly detected, indicating the presence of the Ru nanoparticles actually in the

MOF-5 pores. The less pronounced intensity change hints at a more or less randomly

distribution of the Ru nanoparticles (see TEM discussion below). In order to further

investigate the embedding of the Ru nanoparticles in MOF-5, calcination studies in vacuo (10-

3 mbar) were performed at 200 °C, 400 °C and 500 °C. The results are presented in Figure

3.15. After calcination at 200 °C, 14 h the PXRD of the obtained Ru@MOF-5 material

(Figure 3.15b) appears nearly unchanged compared to the PXRD of the starting composite

(Figure 3.15a), meaning that no broadening of the MOF-5 reflections or sharpening of the Ru

reflections is observed. The powder X-ray diffractogramm of the composite after calcinations

at 400 °C exhibits an increased background noise as well as a slight sharpening of the Ru

reflections, pointing at an already started decomposition of the host framework. Under inert

conditions, MOF-5 is however stable up to 450 °C.[3a] Calcination at 500 °C leads to complete

decomposition of MOF-5 to ZnO, assigned by the complete loss of the sharp MOF-5

reflections and the new reflections at 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, 67.9° and

69.10°. Yet, even after calcination at 500 °C, almost no sharpening of the Ru reflection at

44.0° 2θ is observed compared to the original broad reflection shown in Figure 3.15a.


54

Figure 3.15. Powder XRD diffractogramms of (a) Ru@MOF-5 as synthesized; (b) Ru@MOF-5 after calcination at 200 °C; 14 h, (c) Ru@MOF-5 after calcination at 400 °C, 14 h; (d) Ru@MOF-5 after calcination at 500 °C, 14 h (Lines: Ru JPDS reference No. 6-0663).

In contrast to that, Ru nanoparticles located outside the framework should exhibit sharper

reflections after calcination at elevated temperatures due to agglomeration processes.

However, no sharp superpositions of the reflection at 44.0° are observed. The calcination

experiments therefore already hint at the embedding of the Ru nanoparticles in MOF-5 – and

even the stabilisation after calcination and matrix break-down (possibly by the ZnO formed,

as known from typical supported catalysts).

3.4.2.3. X-ray absorption spectroscopy (XAS) measurements of Ru@MOF-5

In order to confirm the metallic nature of the Ru species inside the pores of MOF-5

after the quantitative decomposition of intercalated [Ru(cod)(cot)], a detailed investigation

and comparison of the composites [Ru(cod)(cot)]3.5@MOF-5, {[Ru(cod)]/Ru}@MOF-5 and

the material Ru@MOF-5 by X-ray absorption spectroscopy (XAS) was performed. The X-ray

absorption near edge structure (XANES) of Ru@MOF-5 (Figure 3.16 left, curve a) is

indicative for metallic ruthenium. The curves of the Ru@MOF-5 and the reference Ru foil

(curve d) overlap to a large degree, and moreover, the edge-shift suggests that no oxidation of

the ruthenium has taken place during sample manipulation for the ex-situ XAS measurements.


55

Figure 3.16. XANES (left) and EXAFS (right) of (a) Ru@MOF-5, (b) [Ru(cod)(cot)]3.5@MOF-5, (c) Ru-ox@MOF-5 and (f) {[Ru(cod)]/Ru}@MOF-5 compared to (d) Ru foil and (e) RuO2. All data were recorded at liquid nitrogen temperature.

The amplitude of the first Ru-Ru shell in the FT (curve a) of Ru@MOF-5 is very low when

compared to that of the Ru foil. This suggests very small ruthenium particles, matching the

space confinement in the MOF-5 pores. In the EXAFS (Figure 3.16., right) it is seen that the

Ru-Ru shell (at 2.44 Å, uncorrected FT) is flanked on the left by a large shoulder (1.95 Å,

uncorrected FT). In order to get some insight into the origin of this shoulder, XAS of the

material [Ru(cod)(cot)]3.5@MOF-5 was recorded (Figure 3.16. left, curve b). The

corresponding XANES looks strikingly different to both Ru@MOF-5 and the Ru foil (curve

d), with a single white line, and an edge position of ~22125 eV. In the respective EXAFS

(Figure 3.16. right, curve b), a peak at 1.78 Å (uncorrected FT) is likely to arise from a Ru-C

bond. Modelling this peak with a single Ru-C scattering path proved unsuccessful and indeed,

the Nyqist theorem prohibits fitting of many Ru-C contributions to a single peak. In the

precursor [Ru(cod)(cot)] many Ru-C bonds are found within the range of 2.1-2.7 Å. Right of

the Ru-C peak, a smaller contribution is visible, but farther away from the central atom no

higher shell structure appears in the EXAFS. On the other hand, in Ru@MOF-5 (Figure 3.16

right, curve a), the higher shell contributions matching those of the metallic state are seen.

The peak position of the left-side shoulder next to the main contribution (1.8 Å, uncorrected)

is comparable to that of the Ru-C contribution (1.78 Å, uncorrected) in the EXAFS of


56

[Ru(cod)(cot)]3.5@MOF-5, which results from the adsorbed, intact precursor molecule. In

order to investigate whether this left shoulder may arise from unintentional partial oxidation,

intentionally oxidized Ru@MOF-5 was also analyzed by XAS measurements. A sample of

Ru@MOF-5 was exposed to a stream of 4 vol. % O2 in argon (1 sccm) for 30 min at room

temperature (curve c). The XRD diagram recorded after the O2 treatment shows the presence

of the intact MOF-5. The XANES and EXAFS of this oxidized sample are akin to those of the

anhydrous RuO2 structure.[110] The peak height in the EXAFS is slightly smaller when

compared to the reference oxide (curve e), which is indicative for a smaller particle size. The

long-range order of these oxide particles is manifested in the higher shells, which follow the

pattern seen for RuO2 (EXAFS). Likewise, the XANES show the characteristic double feature

associated with RuO2, with a slightly smaller distance between the first two maxima. This is

also interpreted in the light of the small particle size of the ruthenium oxide species in the

MOF-5 material. From the EXAFS of the oxidized sample it can also be concluded that the

shoulder in the Ru@MOF-5 EXAFS does not arise from partial oxidation, i.e. a significant

contribution of Ru-O contacts are absent for Ru@MOF-5. Taken all the arguments together,

we suspect that the shoulder stems from the coordination/interaction of the very small

ruthenium particles to the arene carbon atoms of the MOF-5-linkers which is another

indication for the Ru nanoparticles being located within MOF-5. Finally, the XANES and

EXAFS of Ru@MOF-5 (spectra a) and {[Ru(cod)]/Ru}@MOF-5 (spectra f) correspond to a

great extend, indicating that most of the Ru precursor is converted to Ru nanoparticles in the

metallic state already after hydrogenolysis under mild conditions.

3.5. Microstructural investigation of Ru@MOF-5 by advanced TEM

techniques

The discussed results from 13C MAS-NMR, XRD and XAS measurements show that

pure Ru nanoparticles in MOF-5 can be synthesised by the subsequent loading/hydrogenolysis

of [Ru(cod)(cot)] in MOF-5. In nanoparticle synthesis in porous host materials, gaining

spatial information on the location of nanoparticles always remains a great challenge. A key

question is the distribution of the formed Ru particles over the MOF-5 crystallites. An

enrichment of Ru at the pores near the edges of the crystallites could be possible, as one

would expect the ruthenium complexes within the pores on the periphery to be reduced first

and thus these would be the initial seeds for nanoparticle nucleation. Hydrogenolysis


57

Figure 3.17. In order to show the macroscopically uniform distribution of the Ru nanoparticles after

hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5, after treatment with H2, MOF-5 crystals loaded with Ru precursor (Figure 3.1) were cut (middle) and turned to examine the cross section (bottom).

experiments with individual mm-sized macrocrystals homogeneously loaded with

[Ru(cod)(cot)] (Figure 3.17) were performed. After hydrogenolysis, the black crystals were

cut open and the cross-section was examined by using a light microscope. The series of

images in Figure 3.17 of one representative example gives evidence of a rather uniform

distribution of the black color arising from the Ru particles over the whole cross-section of the

crystal. Significant enrichment of Ru particles at the edges of the macro crystallites is not

observed in these images, although the inner section of the crystals appears to be lighter than

the edges. The actual position of the particles, e.g. whether they are situated within, at the

surface or outside the MOF-5 crystallites, can however not directly be deduced from standard

analytical methods like N2 sorption, XRD and XAS. Since one of the main objectives of this

work was to provide a thorough understanding of the location of nanoparticles synthesised in

MOF-5, advanced TEM techniques were applied to gain local structural information on the

MOF-5 crystals as well as on the Ru nanoparticles. A combination of techniques was applied

for the following reasons. High-resolution TEM (HR-TEM) can help to directly image the

particles within the framework. Chemical information can be given by high-angle annular

dark field scanning transmission electron microscopy (HAAD-STEM). Finally electron

tomography provides spatial information, i.e. the uniform or random distribution of the

particles throughout the MOF matrix. Figure 3.18 provides the results from TEM and HAAD-

STEM measurements of Ru@MOF-5. Although stemming from one single batch of

Ru@MOF-5, crystallites with varying degrees of particle filling can be observed (Figure

3.18a).


58

Figure 3.18. (a) Bright field TEM image of Ru@MOF-5 with sections of loaded (bottom) and empty crystals (right top) side by side with the ED pattern of the Ru-loaded MOF-5 crystallite as inset, (c) enlarged image of the loaded crystallite showing a densely loaded matrix and local ordering of the Ru nanoparticles marked by arrows and (c) HAADF image of Ru@MOF-5 showing the Ru nanoparticles evenly distributed white spots.

Whereas the crystal indicated as MOF-5 is almost empty, the crystal indicated as Ru@MOF-5

is densely packed with particles ranging from 1-3 nm and looks almost identical to the TEM

image of {[Ru(cod)]/Ru}@MOF-5 (Figure 3.11), despite prolonged treatment under H2 at

elevated temperature. The displayed sample exhibits a Ru content of 30.6 wt.%. Since Ru

nanoparticles in a size regime of the larger MOF cavity A (1.5 nm) are likely to consist of

approximately 140-150 atoms (e.g. an ideal three shell icosahedral model cluster is composed

of 147 atoms),[111] only about 2 % of all the cavities in the MOF-5 matrix would be filled with

Ru particles. Due to the fluxionality and mobility of the precursor molecules within the

framework (see MAS-NMR discussion above), the precursor molecules and possibly small

Ru clusters (nuclei) are as well able to move around different MOF-5 cavities. Upon


59

hydrogenolysis of [Ru(cod)(cot)] “liberated” Ru atoms from different molecules will fuse to

nanoparticles, leaving some MOF cavities empty. This was also verified by the low angle

PXRD of Ru@MOF-5 (see Figure 3.14). The inset electron diffraction pattern in Figure 3.18

was taken from the densely filled crystal and evidences crystalline Ru nanoparticles adopting

the hexagonal structure of bulk ruthenium (P63/mmc, space group 164, with lattice parameters

a = b = 2.7 Å and c = 4.3 Å). Figure 3.18b presents an enlargement of Figure 3.18a. The

closely packed particles appear in many cases to adopt some degree of cubic ordering

resulting from the cubically arranged pore structure of the host matrix MOF-5. This short-

range ordering, indicated by arrows (Figure 3.18b), stretches only 4-5 particles since not all

pores of the framework are filled. The local arrangement of the particles is however a strong

indication that the particles are located within the framework. The HAADF-STEM image of a

similar loaded MOF-5 crystallite is shown in Figure 3.18c. The contrast in this type of image

depends on both the thickness and the atomic number Z, and therefore contains chemical

information. The heavier Ru nanoparticles appear as bright white dots evenly distributed

within the darker MOF-5 framework. Although the TEM images were taken at low electron

beam dosage, the ordering of the metal organic framework is very difficult to record and

visualize. Reflections from electron diffraction of MOF-5 itself were not observed,

presumably due to degradation of the material in the electron beam.[112] So far, only for the

chromium terephthalate based porous framework MIL-101, the pore structure was directly

imaged and an electron diffraction pattern of the matrix could be measured.[112]

A set of HRTEM images of Ru@MOF-5 is given in Figure 3.19. In Figure 3.19a Ru

nanoparticles in a size range of 1-3 nm can be observed, whereas the MOF-5 matrix is mainly

by the electron beam of the high resolution measurement. The particles are monocrystalline

ruthenium, with lattice fringes in agreement with the hexagonal Ru structure. Figure 3.19b

displays a MOF-5 crystallite region with smaller particles (> 2 nm). Less lattice fringes are

visible as charging of the sample occurs due to the small size of the particles, distorting the

HRTEM image. For Ru nanoparticles embedded in MOF-5, a particle size of 1.1-1.5 nm

corresponding to the MOF-5 pore diameters[4] is expected. For metal nanoparticles embedded

in zeolites, it is known that the particle sizes can exceed the sizes of the zeolitic cages. It was

found, that the porous zeolitic structure is locally distorted by the imbedded particles but the

particles remain situated inside the porous structure.[113] It is reasonable to assume, that this is

also the case for larger Ru nanoparticles of about 3 nm inside MOF-5.


60

Figure 3.19. HRTEM images of Ru@MOF-5 exhibiting regions of MOF-5 crystallites loaded with (a) Ru nanoparticles in a size range of 1-3 nm and (b) Ru nanoparticles of 1-2 nm.

Because of beam damage and the two-dimensional aspects of the TEM images in Figure 3.18

and 3.19, it is not possible to resolve the spatial position of the particles within the MOF-5

framework. The exact positioning of the metal nanoparticles within the framework can be

determined by electron tomography. In Figure 3.20 a bright field TEM image of Ru@MOF-5

and the corresponding reconstructions from electron tomographical measurements are

presented. The bright field TEM image (Figure 3.20a) gives the impression that the

distribution of the Ru nanoparticles within the MOF-5 nanocrystallite is homogeneous and

densely packed with nanoparticles in a size range of 1-5 nm, although the structures larger

than 3 nm appear to be agglomerates of two or more single nanoparticles with smaller sizes.

The 3D tomographic reconstruction of the Ru nanoparticles within the crystallite (Figure

2.20b), however, shows a complete different picture. The slices through the tomographically

reconstructed volume (Figure 3.20c and d) clearly show that the particles are in fact located at

or close to the surface of the crystallite and have a maximum penetration of 20 nm. These

surface particles probably form during the prolonged hydrogenolysis of

[Ru(cod)(cot)]3.5@MOF-5 to Ru@MOF-5.


61

Figure 3.20. (a) Bright field image of Ru@MOF-5, (b) tomographically reconstructed Ru nanoparticles

(matrix not imaged), (c) and (d) slice through the tomographically reconconstructed volume of Ru@MOF-5.

Figure 3.1 shows that the Ru precursor molecules are evenly distributed throughout the mm-

sized MOF-crystals in the starting composite material. Hydrogenolysis of the composite will

presumably start at the surface of the MOF-5 matrix, due to diffusion limitation of the H2 gas.

The obviously highly mobile [Ru(cod)(cot)] precursor molecules from the inner core of the

MOF-5 crystallites will then diffuse as well to the outer surface of the crystallites as well to

form Ru clusters. This finding is in contrast to the light microscopic picture of Ru@MOF-5

(Figure 3.18), which presents a rather uniform metal distribution but also exhibits brighter,

probably less densely filled inner sections of the cut crystals. It is noteworthy that the

presented tomographic measurements only represent one more or less typical nanocrystallite

of Ru@MOF-5. Since only about 2 % of all MOF-5 cavities are filled with Ru nanoparticles,

also other completely empty crystals must exist in the sample. However, these results show

that electron tomography is an extremely important tool to determine the exact position of

nanoparticles in MOF-5 which cannot be derived by conventional TEM measurements.


62

3.6. Investigation of the host-guest interactions in Ru@MOF-5

After thorough structural investigation of Ru@MOF-5, also the accessibility of the

surface of the embedded Ru nanoparticles which offers insights into the nanoparticles-host-

interaction, was examined. This was performed with respect to potential further application of

the Ru@MOF-5 composite in catalytic reactions.

3.6.1. CO adsorption on Ru@MOF-5

CO is a particularly useful probe molecule for the study of the surface of metal

nanoparticles. The surface accessibility and chemical nature of surfactant stabilized metal

nanoparticles has been probed by CO adsorption/desorption followed by FT-IR.[114] Several

adsorption studies have been performed on Ru colloids stabilized by organic polymers in

solution.[89a,92] Accordingly, a sample of the Ru@MOF-5 material was exposed to 1 bar CO at

25 °C for 30 min. The corresponding FT-IR spectrum is shown in Figure 3.21. Prior to the

absorption study of CO on Ru@MOF-5, the empty MOF-5 material was also exposed to CO

gas to check whether CO interacts with pure MOF-5. The corresponding FT-IR spectrum did

not show additional vibrational bands despite the typical MOF-5 bands (as seen in Figure

2.21). For Ru@MOF-5, two CO vibrational bands at 2000 and 1890 cm-1 were observed in

addition to the bands of the host material MOF-5 (compare Figure 3.21 a and b) as well as

some minor hydrocarbon residues from precursor decomposition. Generally, CO bands

between 2100 and 2000 cm-1 are assigned to CO molecules bound in a linear mode to the

metal surface. Vibrational bands between 2000 and 1850 cm-1 are assigned to CO molecules

bound in a bridging mode to adjacent metal atoms.[115] Here, the CO bands at 2000 cm-1 and

1890 cm-1 were therefore assigned to CO in the linear and bridging position respectively,

being adsorbed at the surface of the caged Ru particles. This finding is in accordance to CO

adsorption on colloidal Ru nanoparticles. For Ru nanoparticles capped by cellulose acetate or

tetranitrocellulose, also two vibrational bands at 2030 cm-1 for linear and 1968 cm-1 for

bridging CO were found. The particle sizes were in a similar size range of about 2 nm as

compared to Ru@MOF-5.[92] Ru nanoparticles stabilized by polyvinylpyrolidone (PVP),

exhibit vibrational CO bands at 2040 cm-1 and 1967 cm-1. The average particle size was

determined to be closed to 1 nm, which possibly is more accurately described as molecular Ru

cluster.[116] In other studies on Ru single crystals[115] and earlier reports on Ru nanoparticles


63

supported on oxide materials,[117] CO was found to bind only in the linear and not in the

bridging mode. Recently, bridging CO was also indirectly observed on Ru nanoparticles of

Figure 3.21. FT-IR spectra of (a) Ru@MOF-5 as synthesized and (b) after 30 min of exposure to 1 bar CO in

dry KBr.

1.3-2.8 nm size, supported on Al2O3 and SiO2.[118,199] However, the corresponding FT-IR

spectrum was more complex, dominated by vibrational CO bands for linearly bound CO on

different Ru0 and Run+ and sites. Indeed, the band for bridging the CO was very weak and

only determined by fitting of the spectra.[119] Therefore, prominent bands for CO in the

bridging mode, clearly distinguishable from linear bound CO, as shown in Figure 3.21, appear

to be a unique feature of Ru colloids and the Ru nanoparticles imbedded in MOF-5. The wave

numbers of the vibrational bands of CO on Ru@MOF-5 and CO on Ru colloids deviate

slightly. This may be due to the different situation given by the surfactants and the particle

wall interaction respectively. Nevertheless, it is notable that Ru@MOF-5 shows similar CO

adsorption behavior as the Ru colloids capped by organic ligands such as PVP. Note that the

CO adsorption on Ru@MOF-5 is fully reversible, after evacuation (10-3 mbar) at 120 °C for

30 min, the FT-IR spectrum of the starting material was obtained.

3.6.2. Hydride mobility of Ru@MOF-5 in comparison to Ru nanoparticles stabilized by

organic surfactants

As already mentioned above Ru nanoparticles and colloids are known catalysts in

hydrogenation reactions.[85,86] Thereby, hydrogen or hydrides species bound to the surface of


64

the metal nanoparticles constitute active species of the catalytic behavior of metal

nanoparticles. Hence it is interesting to study the interaction of these species with the surface

of the metal nanoparticles. In this respect, investigation of the hydride mobility on the surface

of Ru nanoparticles or colloids gives insights into the behavior of these surface species.

Surfactant stabilized (e.g. HDA) Ru colloids derived from hydrogenolysis of [Ru(cod)(cot)]

have been shown to be surface covered by Ru-H species which arise from the chemisorption

of dihydrogen during particle formation.[120] In addition, H/D exchange at the hydrocarbon

backbone of the HDA ligands was observed as result of the catalytic activity of the Ru

particles in C-H activation reactions.[120] Inspired by these studies, similar experiments were

performed with the Ru@MOF-5 material. Figure 3.22 compares experimental 2H-solid-echo-

NMR spectra and the simulated 2H-FID-NMR spectra of Ru@MOF-5 after saturation with D2

gas in the temperature regime from 23 K to 200 K. Obviously, the Ru particles promote H/D

exchange with the bdc linker of MOF-5 (but without hydrogenation of the bdc). Thus, a pure

sample of D2 on Ru@MOF-5 cannot be stabilized without partial isotope scrambling, in

particular at higher temperatures. As a result of this isotope scrambling, a part of D2 gas inside

the sample is converted to HD or H2 and the organic ligands are partially deuterated in this

experiment, resulting in the presence of C-D groups. The stoichiometry of this isotope

exchange depends not only on the relative ratio of D/H, but also on the possibility of contact

between the bdc linker and the Ru particle surface and is not easy to interpret.[120] Note that

only a fraction of the cavities is likely to be occupied by Ru particles, as discussed above.

Since the measured samples contain several inequivalent deuterons, their spectra are complex

superpositions of different sub spectra. The shape of these spectra depends on the motional

state of the deuterons and changes with temperature. From line shape analysis of each

spectrum, the corresponding quadrupolar coupling constants Qzz can be obtained which offers

information on the nature of the observed surface species. At the lowest temperature of 23 K,

the principal component is a broad Pake spectrum with Qzz=130-140 kHz. The second major

component is a more narrow Pake spectrum with Qzz=60 kHz. In addition there is a weak

narrow Lorentzian component in the center of the spectrum which corresponds to less than

3% of the whole intensity. Upon increase of the temperature over 40 K to 70 K and 100 K the

line shape of the narrow Pake spectrum changes strongly, indicating the presence of motions

on the NMR time scale, while the broad component is practically not affected by the

temperature change. This however changes drastically above 100 K. In the spectra measured

at 150 K the broad Pake component has almost completely disappeared, except a broad socket


65

in the spectrum and a narrow Lorentzian line has appeared in the center of the spectrum. Upon

further increase of the temperature to 200 K, the narrow Pake spectrum also disappears and

Figure 3.22. Experimental and simulated static solid state 2H-NMR spectra of Ru@MOF-5 saturated with D2

gas.

only the central Lorentzian line is visible. The same result is also found at 300 K (not shown).

All spectra were measured at a repetition time of 1 s. This repetition time favors fast relaxing

species and suppresses slow relaxing species. For a quantitative determination of the

contributions of individual 2H-species, a series of experiments with variations of the recycle

time, pulse width and delay time in the solid echo experiment has to be performed. Yet, the

spectra shown above indicate the presence of surprisingly high mobility of the deuterons at

Ru@MOF-5 even at very low temperatures in contrast to the situation known for the HDA

stabilized Ru colloids which show a significantly reduced mobility at the same conditions. In

that latter case a reduced mobility of the deuterons was already observed at 200 K.[120] The

broad component with Qzz=130-140 kHz is typical for immobile C-D groups.[121] The narrow

Pake line with Qzz=60 kHz is typical for deuterons bound to ruthenium nanoparticles. For

both a rigid hydride type Ru-D and C-D bond a nearly axial symmetric quadrupolar tensor

with η close to zero is expected. Thus, we attribute the Qzz=130-140 kHz component mainly

to the bdc linkers deuterated by some interaction with the Ru surface and the Qzz=60 kHz

component to D2 chemisorbed on the metal nanoparticle. If the deuterium undergoes fast

reorientations the value of the quadrupolar tensor and thus also the quadrupolar coupling

constants are changed, depending on the type and speed of the motion. Thus, the line shape

changes observed in the narrow Pake spectrum of the Ru-D species are also a clear indication


66

of a high mobility of the deuterons on the Ru surface already at 40 K, while the metal-organic

framework is rigid at least up to 100 K. The disappearance of this component above 100 K is

not completely clear yet. It may be an indication of a starting mobility of the organic ligands,

which causes very short T2-relaxation and suppresses the signal in the echo sequence.

However, a full elucidation of the observed effects will be the subject of future investigations.

In conclusion, the high mobility of the deuterons on the Ru nanoparticles in MOF-5 even at

40 K hints at a rather weak interaction between the particles and the stabilizing framework,

i.e. the bdc linkers, as compared with a typical surfactant such as HDA which is needed to

stabilize Ru nanoparticles in colloidal solution.

3.7. Catalytic Test Reactions of Ru@MOF-5

From the discussed results of XAS, TEM, CO adsorption and solid state 2H MAS-

NMR it was found, that the interaction between the embedded Ru nanoparticles and the host

MOF-5 are rather weak and that the surface of the nanoparticles is easily accessible for

catalytic relevant gases. Therefore catalytic test reactions were carried out (see Scheme 3.3)

to study the impact of the low nanoparticle-host interaction on the catalytic performance of

Ru@MOF-5.

OH

H

H

O

HRuO2

+1/2 O2+ H2O

Ru@MOF-5

+ O2

+ 3 H2Ru

Scheme 3.3 Catalytic test reactions with Ru@MOF-5 and RuO2@MOF-5 as catalyst.

3.7.1. Oxidation of benzyl alcohol

According to the XAS experiments (see section 3.4.2.3), the Ru@MOF-5 material can

be easily converted to RuOx@MOF-5 by oxidation with diluted O2 gas. Recent studies have


67

shown that RuO2 supported on Al2O3[82] or zeolite[83] can be used for the oxidation of a large

variety of alcohols. We therefore decided to probe the catalytic activity of RuOx@MOF in the

oxidation of benzyl alcohol to benzaldehyde. The test reaction was performed in analogy to a

recent report.[81] For the catalytic test, a sample of Ru@MOF-5 was oxidized as discussed

above for the XAS measurements. Then it was suspended in toluene, the suspension was

saturated with O2 and benzyl alcohol was added. The reaction was then performed at 80 °C.

The supernatant was analyzed by GC-MS to investigate the progress of the reaction. Due to

superposition of the signals of benzyl alcohol, benzaldehyde and toluene in the corresponding 1H NMR spectrum, estimation of the product/educt ratio by integration of the corresponding

signals was not possible. The observed

Figure 3.23. GC spectrum of the supernatant from the catalytic oxidation of benzyl alcohol by RuOx@MOF-

5 and separated mass spectra of 1) benzaldehyde (left) and 2) benzyl alcohol (right).

conversion of the benzyl alcohol to benzaldehyde was only about 25 % (as estimated from

GC-MS results, see Figure 3.23) after 48 h. XRD examination of the material after the test

reaction revealed a breakdown of the MOF-5 structure which has lost its high porosity as


68

well. We attribute this observation to the stoichiometric release of water during this oxidation

reaction (see Scheme 3.3). MOF-5 is well known to be rather water sensitive.[49,50] Theoretical

studies by Allendorf et al. have shown that the MOF-5 structure becomes instable at levels

above 4-6 wt.% of adsorbed water.[50] Nevertheless, the expected catalytic activity of

RuOx@MOF-5 has been observed and other water-resistant MOF support materials for the

imbedding of Ru and RuOx can be selected for further studies.

3.7.2. Hydrogenation of benzene

In addition, Ru@MOF-5 was preliminary tested as catalyst in hydrogenation of

benzene (Scheme 3.3).

Figure 3.24 1H NMR spectrum (in C6D6) of the filtrate from hydrogenation of benzene to cyclohexane

catalyzed by Ru@MOF-5. The corresponding integrals are assigned in H atoms. The conditions were chosen according to literature[85] and the reaction was performed under 3

bar H2 at 75°C. Consumption of H2 was followed by pressure decrease. After 20 h reaction

time no further pressure decrease was observed and the reaction was stopped. From 1H NMR

of the filtrate the conversion of benzene to cyclohexane was detected to be approximately


69

25% (see Figure 3.24). The powder XRD diffractogramm of Ru@MOF-5 after catalysis

reveals perfectly intact MOF-5 structure as well as the typical broad Ru reflection at 44.0°.

All together, this shows the potential of Ru@MOF-5 as catalyst in hydrogenation of benzene.

For a complete conversion of benzene to cyclohexane however, the ideal reaction parameters

(p,T) have certainly to be adapted to the Ru@MOF-5 system in the future.

3.8. Conclusion

In summary, the synthesis of Ru nanoparticles in MOF-5 via hydrogenolysis of the

precursor [Ru(cod)(cot)] in MOF-5 was presented along with detailed studies on the

interaction between precursor molecules and finally Ru nanoparticles with the porous host

MOF-5. Loading of MOF-5 with the precursor molecules led to a completely changed PXRD

of the resulting composite [Ru(cod)(cot)]3.5@MOF-5. From the obtained PXRD data

measured at room temperature, the crystal structure of the composite could however not be

elucidated by Rietveld refinement. The Ru precursor molecules appear to be highly disordered

inside the MOF-5 cavities. 13C MAS-NMR spectroscopical investigations showed that the

precursor molecules behave almost as dissolved in MOF-5 which explains the finding of

highly disordered guest molecules. Further PXRD measurements at low temperature might

help to increase the degree of order of the [Ru(cod)(cot)] molecules and therefore to refine the

crystal structure by the Rietveld method.

Ru@MOF-5 was obtained from [Ru(cod)(cot)]3.5@MOF-5 by hydrogenolysis at 3 bar H2, 150

°C, 48 h, whereas hydrogenolysis at milder conditions led to a side reaction of {Ru(cod)}

fragments with MOF-5 bdc linkers alongside formation of Ru nanoparticles. XAS

measurements revealed the existence of small metallic Ru nanoparticles. Characterisation of

Ru@MOF-5 by TEM and electron tomography revealed the presence of Ru nanoparticles

below 3 nm, confirmed by PXRD measurements. The particles were however mostly located

at the surface or close to the surface of the investigated MOF-5 nanocrystallites which is most

probably due to the rather prolonged treatment under 3 bar H2 at 150 °C. Though a perfectly

even distribution of the precursor molecules was found prior to the hydrogenolysis, this rather

harsh treatment obviously caused diffusion of precursor molecules and smaller Ru clusters to

the outer regions of the MOF-5 crystallites. However the Ru nanoparticles still remain caged

by the MOF-5 host since temperature stability of up to 400 °C without pronounced particles

agglomeration was observed. CO adsorption studies and investigation of the hydride mobility

on the surface of Ru@MOF-5 showed that the Ru nanoparticles embedded in MOF-5 behave


70

similar to Ru colloids stabilised by organic surfactants which presents MOF-5 as novel kind

of solid support matrix.

However the catalytic performance of Ru@MOF-5 was rather moderate in the oxidation of

benzyl alcohol. This was attributed to the sensitivity of MOF-5 towards water. Ru@MOF-5

showed catalytic activity in benzene hydrogenation as well, here however the catalytic

conditions (p,T) need to be adapted to this system in the future.

4. Investigations of the loading of MOF-5 with two precursor components

71

4. Investigations of the loading of MOF-5 with two

precursor componentS

Beside investigations of loading MOF materials with one metal or metal oxide

precursor compound, followed by decomposition to metal or metal oxide nanoparticles

embedded in MOFs, also simultaneous loading with two metal precursor components and

their co-decomposition appears to be a fruitful field of research. Among the plethora of

applications for bimetallic nanoparticles only a few will be addressed. For instance, the fully

ordered fct (face-centered tetragonal) structured FePt alloy exhibits an anisotropy constant K

that is among the highest of all known hard magnetic materials.[122] Therefore bimetallic

superparamagnetic FePt nanoparticles are expected to become the density limit for magnetic

memory devices.[123,124] In catalytical applications, the addition of a second metal can improve

the activity and/or selectivity of metallic catalysts.[125,126] The addition of Pd to Au catalysts

significantly enhances the catalytic performance for H2O2 and, at an optimum Pd-Au

composition, exceeds the H2O2 production rate of pure Pd catalysts which are significantly

more active than pure gold catalysts.[127] In the hydrogenation reaction of cinnamaldehyde to

cinnamyl alcohol, PtRu catalysts showed a notably improved selectivity which was up to 2

times higher than in the corresponding monometallic catalysts.[128] Furthermore PtRu

nanoparticles are the so far most promising anode catalysts in direct methanol fuel cells since

PtRu nanoparticles exhibit an excellent tolerance towards the poisoning CO formed during

methanol oxidation.[129] MOF-5 and MOFs in general as new kind of stabilization matrices

for nanoparticles might support these enhancement effect by the extraordinary low interaction

between the host matrix and embedded guests.

As discussed above, the hydrogenolysis of all-hydrocarbon precursors embedded in MOF-5

appears to be advantageous over other classical routes for the synthesis of metal nanoparticles

due to the formation of unreactive side products. For the synthesis of bimetallic nanoparticles

in MOF-5 materials, especially the co-hydrogenolysis of two all hydrocarbon metal

precursors under hydrogen pressure, analogously to strategy the developed by the group of B.

Chaudret for the synthesis of colloidal alloy nanoparticles in solution,[130] appears to be

feasible. The crucial point in bimetallic nanoparticle synthesis in MOF-5 and MOF materials

as solid support matrices in general, is the adjustment of defined metal (1)/metal(2) ratios.


72

Since so far no research on the simultaneous loading of MOFs with two precursor

components has been performed, the following chapter has a rather explorative character.

In contrast to bimetallic colloids in solution the stoichiometry of the intercalated precursor

compounds cannot be directly controlled by simply adding defined amounts of the two

precursor components. Since the loading with precursor molecules is usually performed via

gas phase in vaccum, a certain control over the precursor volatilities and diffusion properties

needs to be achieved when the MOF material is simultaneously loaded with two precursor

components. As in the previous chapter on Ru nanoparticle synthesis in the model metal-

organic framework compound MOF-5, the loading experiments were also carried out with

MOF-5. Different precursor ratios were applied for the simultaneous loading experiments,

followed by elemental/AAS analyses, spectroscopical and structural investigations of the

obtained composites. Finally, the co-decomposition of two precursors embedded in MOF-5

was also investigated in order to examine the nature of the obtained metal nanoparticles.


73

4.1. Loading of MOF-5 with [Fe(η6-toluene)(η4-C4H6)] and [CpPtMe3]

For a first general investigation of the simultaneous loading of MOF-5 with two

different precursor molecules the compounds [Fe(η6-toluene)(η4-C4H6)] (A) and [CpPtMe3]

(B) were chosen (see Scheme 4.1). [Fe(η6-toluene)(η4-C4H6)] can only be obtained from co-

condensation of Fe vapour and the corresponding ligands in a high vaccum apparatus with

cooled walls[131] and was kindly provided by Prof. U. Zenneck, University of Erlangen-

Nürnberg. With respect to possible future decomposition experiments of the precursors in

MOF-5, as discussed above, all-hydrocarbon precursors were chosen to avoid unwanted side

reactions between the host framework and free ligands after decomposition. Both molecules A

and B are literature known MOCVD precursors[131,132] and the loading of MOF-5 with

[CpPtMe3] was successfully performed in a previous work.[53] Due to the lack of single

crystal structural data of the highly air sensitive [Fe(η6-toluene)(η4-C4H6)], the precursor

dimensions were estimated from other similar literature known aromatic and olefinic iron

complexes. The dimensions of the Fe precursor A were estimated to be below 6 Å and 5 Å in

the x and y direction which generally allows precursor diffusion through the MOF-5 opening

of 7.8 Å.[53] Furthermore both precursor can be sublimed in the same temperature range and

already at room temperature which enables simultaneous loading of MOF-5 with the

components A and B.

Fe PtH3C CH3

CH3

A B

Scheme 4.1. Precursors [Fe(η6-toluene)(η4-C4H6)] (A) and [CpPtMe3] (B) to be infiltrated in MOF-5

simultaneously.


74

4.1.1. Synthesis

Especially FePt nanoparticles with a near equal atomic percentage of Fe and Pt are an

important class of magnetic nanomaterials.[122] Investigations of the loading of MOF-5 with

two different precursor molecules was performed to obtain possible composite materials

suitable for further decomposition studies yielding functional bimetallic nanoparticles

embedded in MOF-5. Therefore the molar ratio of the precursor materials [Fe(η6-toluene)(η4-

C4H6)] and [CpPtMe3] used for the loading of MOF-5 was set to 1:1.

The loading of MOF-5 with [Fe(η6-toluene)(η4-C4H6)] and [CpPtMe3] was performed at 25

°C, 10-3 mbar (static vaccum) for 12 h by placing pure, activated MOF-5 powder in a Schlenk

tube together with the two precursor components A (red-brown) and B (colorless) in separated

glass vials. The loading was perpetuated until no further uptake of the two precursor materials

was observed. Thereby a colour change of the MOF-5 material from off-white to red-brown

resulted. The obtained material was analyzed by elemental/AAS analysis, FT-IR and MAS-

NMR spectroscopy.


4.1.2.1. Elemental analysis

Loading of MOF-5 with the precursors [Fe(η6-toluene)(η4-C4H6)] and [CpPtMe3] in a

molecular ratio of 1:1 was attempted. The results from the elemental/AAS analysis of the

obtained composite are combined in Table 4.1. Although the stochiometric ratio of the

precursors was set to Fe precursor:Pt precursor = 1:1 prior to the start of the loading

experiment, elemental and AAS analysis of composite gave an overall ratio of 1.37:1 of the

precursors embedded in MOF-5. Enrichment of the Fe precursor in composite is most

probably due to the higher vapour pressure of [Fe(η6-toluene)(η4-C4H6)] and lower molecular

mass (202.08 g/mol) in comparison to [CpPtMe3] (305.277 g/mol), enabling faster diffusion

of A over B into MOF. Although the vapour pressure of the Fe precursor is not literature

known, it should be higher than the vapour pressure of the Pt precursor (0.06 mbar at 23

°C)[132b] since it is a liquid at room temperature whereas [CpPtMe3] is a solid, crystalline

material under the same conditions.


75

Table 4.1. Comparison of the molar ratios of [Fe(η6-toluene)(η4-C4H6)] and [CpPtMe3] before and after absorption by MOF-5.

Precursor

used found

molar ratio molar ratio [Fe(η6-toluene)(η4-C4H6)]/ [CpPtMe3]

1:1 1.37:1

4.1.2.2. MAS-NMR investigations

Figure 4.1. presents the 13C (Figure 4.1b) and 195Pt MAS-NMR (Figure 4.1c) spectra

of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5. For comparison the 13C-MAS-NMR

spectrum of [Fe(η6-toluene)(η4-C4H6)]@MOF-5 (Figure 4.1a) is presented as well. The 13C-

MAS NMR spectra are comparably well resolved, once more pointing at MOF-5 behaving

almost as a solid solvent cage (see MAS-NMR discussion of [Ru(cod)(cot)]3.5@MOF-5).

Table 4.2. Assignment of the MAS-NMR data of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5

Precursor 13C MAS-NMR 195Pt MAS-NMR δ [ppm] δ [ppm] [Fe(η6-toluene)(η4-C4H6)](A) 93.8(η6-C6H5CH3), 84.5(η6-C6H5CH3), _______ 82.8(η6-C6H5CH3), 81.4(η6-C6H5CH3), 75.7(η4-C4H6), 33.8(η4-C4H6), 21.1(η6-C6H5CH3) [CpPtMe3](B) 96.3(η5-C5H5), -18.8(CH3, J(Pt,C) = 706 MHz) -5224.9

The observed 13C MAS-NMR signals of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3] can be clearly

assigned to the carbons in the two different intercalated molecules (see Table 4.2) matching

the literature data of the precursor molecules in C6D6.[133] The MOF-5 carbon signals are

observed at 174.9 (COO), 137.0 (C(COO)) and 131 (C6H4) ppm. Furthermore, the Pt

precursor signal at -18.8 shows the typical splitting due to Pt-C coupling with a coupling

constant of 706 MHz.[132a] The 195Pt MAS-NMR of the composite exhibits one signal at -

5224.9 ppm also matching the literature data of [CpPtMe3].[132a]

The small deviations in the 13C MAS-NMR signals of the Fe precursor in MOF-5, when

loaded separately and together with the Pt precursor, arise from the relative broadness of the


76

Figure 4.1. (a) 13C MAS-NMR spectrum of [Fe(η6-toluene)(η4-C4H6)]@MOF-5, (b) 13C MAS-NMR and (c) 195Pt

MAS-NMR spectra of [Fe(η6-toluene)(η4-C4H6)](A)/[CpPtMe3](B)@MOF-5. signals in solid state NMR in general (see above) and the resulting difficulty in marking the

exact centre of the NMR signals. The FT-IR spectrum of [Fe(η6-toluene)(η4-

C4H6)]/[CpPtMe3]@MOF-5 also confirms the absorption of the precursor molecules A and B

in unchanged MOF-5. However both precursors are all hydrocarbon precursors and the

vibrational bands of the ligands are not very characteristic. The spectrum resembles very

much the FT-IR spectrum of other all-hydrocarbon precursors (i.e. [Ru(cod)(cot)]3.5@MOF-5)

and is therefore only given in the experimental part of this work.

The MAS-NMR investigations of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5

unambiguously showed that both Fe and Pt precursor molecules can be infiltrated in MOF-5

unchanged.

4.1.2.3. PXRD structural investigations

The powder X-ray diffractogramm of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5

and pure, empty MOF-5 is given in Figure 4.2. When compared to the diffractogramm of


77

empty MOF-5 (Figure 4.2a), the PXRD of the composite (Figure 4.2b) shows all typical

MOF-5 reflections with however very different intensity ratios than in pure MOF-5. The

PXRD of MOF-5 (Figure 4.2a) shows the reflections at 13.7°(400) and 15.4°(420) 2θ to be

the most intense. In the PXRD of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5, the most

intense reflection is the (511) reflection at 17.8° 2θ with the reflections at 15.4°(420) and

19.4° (440) exhibiting also rather high intensities and one new reflection at 11.9° (marked by

an asterisk) with about the same intensity. These results are very different from the PXRD

results of the discussed composite [Ru(cod)(cot)]3.5@MOF-5, here all reflections of the host

MOF-5 were found in the same intensity ratios as in the empty material, in addition to various

new reflections. In the case of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5, the change of

the PXRD, when compared to pure MOF-5 must be due to a high degree of disorder of the

composite structure, induced by the loading of MOF-5 with two different precursor

molecules. Similar observations have been made upon absorption of highly disordered solvent

molecules in MOF-5.[49] It is rather likely that the different precursor molecules are not evenly

distributed throughout the MOF-5 frame, with maybe some regions exhibiting higher

Figure 4.2. Powder x-ray diffractogramms of (a) [Fe(η6-toluene)(η4-C4H6)](A)/[CpPtMe3](B)@MOF-5 and (b) empty MOF-5.


78

concentrations of the Fe precursor and other regions exhibiting higher concentrations of the Pt

precursor. The single new reflection observed in the PXRD of [Fe(η6-toluene)(η4-

C4H6)]/[CpPtMe3]@MOF-5 compared to the PXRD of MOF-5 must be attributed to the

absorption of the organometallic molecules but a superstructure of the intercalated precursor

molecules appears to be rather unlikely. Since the typical MOF-5 reflections are preserved in

the PXRD of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5, it can still be concluded that the

host structure does not change upon loading with two kinds of precursor molecules and

remains mostly intact. Taking together the results from the investigation the loading of MOF-

5 with [Fe(η6-toluene)(η4-C4H6)] and [CpPtMe3] it can be concluded that in general two

different precursor molecules can be infiltrated simultaneously. However a better control over

the stochiometric ratios of the two precursors still needs to be achieved.

4.2. Loading of MOF-5 with [CpPd(η3-C3H5)] and [CpPtMe3]

In order to gain a better control over the molar ratios of two different precursor

systems when intercalated in MOF-5, a combination of the precursors [CpPd(η3-C3H5)] (C)

and [CpPtMe3] (B) (see Scheme 4.2) was chosen as another binary model system for

investigations of the simultaneous loading of MOF-5 with two precursor compounds. Both

precursors are solid materials at room temperature and both precursors were already

successfully infiltrated in MOF-5 separately followed by detailed spectroscopic and structural

investigations of the composites.[53] Therefore these precursors appeared to be rather suitable

for a more detailed study on the simultaneous loading of MOF-5.

PdPtH3C CH3

CH3

B C Scheme 4.2. [CpPtMe3] (B) and [CpPd(η3-C3H5)] (C) to be infiltrated in MOF-5 simultaneously.


79

4.2.1. Synthesis

For the loading of MOF-5 with B and C, three different precursor ratios were applied

followed by elemental/AAS analysis, FT-IR, MAS-NMR and PXRD investigations of the

obtained composites. In each case, the loading was performed at 25 °C, 10-3 mbar for 4 h by

placing the empty activated MOF-5 powder in a Schlenk tube together with the two

compounds [CpPd(η3-C3H5)] and [CpPtMe3] in separated glass vials. From prior

investigations of the loading of MOF-5 with B and C separately,[53] it is known that saturation

of the MOF-5 material with the precursors is achieved already after 4 h, indicated by no

further consumption of the precursor materials after that time period. The same was observed

upon simultaneous loading of MOF-5 with B and C. Due to the sensitivity of C towards light,

the Schlenk tube was kept in the dark during the loading procedure. Upon loading with the

two different precursor compounds B (colorless) and C (dark red) the MOF-5 material turned

from off-white to dark red.


4.2.2.1. Elemental analysis

The results from the elemental/AAS analysis of the three obtained composites are

presented in Table 4.3. Compared to the molar ratios applied for the preparation, composites 2

and 3 exhibit clearly deviating molar ratios of the precursors in MOF-5, indicating preferred

absorption of the Pd precursor C over the Pt precursor B. Only in composite 4, the found

molar ratio of [CpPd(η3-C3H5)] (C) and [CpPtMe3] (B) almost matches the used molar ratio.

From the vapour pressure of B (0.06 mbar at 23 °C[132b]) and C (0.03 mbar at 23 °C[134]) it is

evident that the Pt precursor has a higher volatility than the Pd precursor. The precursor

volume (192 Å3, see Table 2.2) and molar mass (212.59 g/mol) of C is however clearly

smaller than the molar volume (216 Å3, see Table 2.2) and molar mass (305.28 g/mol) of B,

therefore favouring the diffusion of C over B into MOF-5. It should be noted that after

preparation of all composites excess precursor materials of B and C were left. Therefore, the

precursor amounts applied for the loading experiments exceeded the saturation level of MOF-

5, e.g. the maximum amounts of precursor molecules that can be absorbed. This allowed

precursor component C, obviously exhibiting a higher diffusion velocity into MOF-5, to be

absorbed preferentially by MOF-5. Hence, the resulted molar ratios of B and C in MOF-5


80

Table 4.3. Comparison of the precursor ratios of [CpPd(η3-C3H5)] and [CpPtMe3] before and after absorption by MOF-5.

Precursor

used found

molar ratio molar ratio

[CpPd(η3-C3H5)]/

[CpPtMe3]

1:1 2.7:1 (2)

3:1 4.1:1 (3)

6:1 5.9:1 (4)

deviate from the molar ratios of the precursor starting amounts applied for the loading

experiments. A saturated loading of MOF-5 with two different precursor compounds in such a

way that all cavities of the host are completely saturated with precursor molecules in a defined

molar ratio is obviously not easy to achieve. Therefore for future loading experiments it

appears more reasonable to perform the loading below the maximum uptake of each precursor

by MOF-5 assuring complete absorption of the starting amounts of both precursor

components by MOF-5. Since the spectroscopical and structural properties of the obtained

composites 2, 3 and 4 were found to be very similar; in the following discussion only the

results from the investigations of composite 2 will be presented.

4.2.2.2. MAS-NMR investigations

The results from the MAS-NMR spectroscopic investigations of [CpPd(η3-

C3H5)]/[CpPtMe3]@MOF-5 are presented in Figure 4.3. The 13C MAS-NMR signals of the

composite (see Table 4.4) can be assigned to the unchanged precursors [CpPd(η3-C3H5)] and

[CpPtMe3] intercalated in MOF-5 and correspond to the MAS-NMR data of both precursors

absorbed by MOF-5 separately.[53] The signals of the MOF-5 carbons at 175.0, 137.0 and

131.1 ppm also correspond well to the literature data.[4] Table 4.4. Assignment of the MAS-NMR data of [CpPd(η3-C3H5)]/[CpPtMe3]@MOF-5.

Precursor 1H MAS-NMR 13C MAS-NMR 195Pt MAS-NMR δ [ppm] δ [ppm] δ [ppm] [CpPd(η3-C3H5)](C) 5.6(η5-C5H5), 4.7 (η3-C3H5), 94.7(η5-C5H5), _______ 3.4(η3-C3H5), 2.1(η3-C3H5) 46.3(η3-C3H5) [CpPtMe3](B) 5.6(η5-C5H5), 1.1 (CH3) -19.1(CH3), -5216.9 99.5(η5-C5H5)


81

Figure 4.3. (a) 1H MAS-NMR spectrum, (b) 13C MAS-NMR spectrum and (c) 195Pt MAS-NMR spectrum of [CpPd(η3-C3H5)](C)/[CpPtMe3](B)@MOF-5. MOF-5 signals are marked by asterisks.

Interestingly also the 1H MAS-NMR spectrum of [CpPd(η3-C3H5)]/[CpPtMe3]@MOF-5 is

very well resolved allowing the assignment of the signals to the various protons in [CpPd(η3-

C3H5)] and [CpPtMe3], except the superimposing signal of the protons of the Cp rings at 5.6

ppm (Table 4.4, Figure 4.3). The chemical shift of the 195Pt MAS-NMR signal of the Pt

precursor slightly deviates from the chemical shift in composite [Fe(η6-toluene)(η4-

C4H6)]/[CpPtMe3]@MOF-5 but lies well within the error margin of signal marking caused by

the broadness of the MAS-NMR signal. The Pt-H and the Pt-C coupling could not be resolved

in the corresponding MAS-NMR spectra of the composite. Taking together these results from

the MAS-NMR analysis, it can be concluded that also the precursors [CpPd(η3-C3H5)] and

[CpPtMe3] can be infiltrated into MOF-5 unchanged, almost as into a solid solvent cage. The


82

corresponding FT-IR spectrum confirming this finding is given in the experimental part of

this work (see above).


The powder X-ray diffractogramm of [CpPd(η3-C3H5)]/[CpPtMe3]@MOF-5 is

presented in Figure 4.4b together with the PXRD of pure MOF-5 (Figure 4.4a). Very similar

to the PXRD of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5 (Figure 4.2), the PXRD of

[CpPd(η3-C3H5)]/[CpPtMe3]@MOF-5 exhibits all typical MOF-5 reflections with very

different intensity ratios from the reflections in the PXRD of the parent MOF-5. As in the case

of composite [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5, in the PXRD of [CpPd(η3-

C3H5)]/[CpPtMe3]@MOF-5 (Figure 4.4b) also, the most prominent reflection has become the

(511) reflection at 17.8 ° 2θ with the two reflections at 15.4° (420) and 19.4° 2θ (440) being

rather prominent and one new reflection at 11.9° (marked by an asterisk). The similarity in the

PXRD results of the two different composites confirms that the simultaneous loading of

MOF-5 with two different molecules leads to a high degree of disorder in the composites’

structures indicated by the change in the corresponding PXRDs when compared to the PXRD

of empty MOF-5 or to composites with only one embedded precursor compound (see

discussion of [Ru(cod)(cot)]3.5@MOF-5). This also indicates that the distribution of the two

different precursors throughout the MOF-5 is very likely to be asymmetric. Most probably

some parts of the MOF-5 frame are enriched with the first precursor compound and other

parts are enriched with the second compound. A uniform mixture of both components

throughout the host framework’s cavities, as it is the case in a solvent is not obtained. This is

most likely caused by the different diffusion velocities of the two different compounds and

probably also by preferred intermolecular interactions between precursor molecules of the

same kind.


83

Figure 4.4. Powder X-ray diffractogramm of (a) composite [CpPd(η3-C3H5)]/[CpPtMe3]@MOF-5 and (b)

pure MOF-5.

The loading experiments of MOF-5 with [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3] and

[CpPd(η3-C3H5)]/[CpPtMe3] did not yield composites with defined precursor ratios,

respectively the molar ratios of the precursor molecules absorbed by MOF-5 did not

correspond to the starting precursor ratios used for the loading. In all loading experiments

described above, saturated loading with the respective two precursors was attempted. Due to

different vapour pressures, molar masses and thus resulting diffusion velocities, enrichment of

the respective more volatile precursor component in MOF-5 was found. This leads to the

conclusion that confined precursor ratios in MOF-5 can only be obtained when a simultaneous

loading of MOF-5 with two precursors below or right at the saturation level of the MOF

material with the two different precursor components is performed. When a complete uptake

of the starting amounts of precursors by MOF-5 is assured, the molar ratio of the precursors

embedded in MOF-5 must be the same as the applied precursor ratio before the loading. This

limits the amount of introduced precursor and the resulting metals to some extend. However

for certain catalytical applications, doping of the MOF material with only few active centres

was shown to be advantageous.[29]


84

4.3. Loading of MOF-5 with [Ru(cod)(cot)]/[Pt(cod)Me2]

The catalytic properties of PtRu nanoalloys have already been discussed above. Since

the synthesis of Ru nanoparticles in MOF-5 via hydrogenolysis of [Ru(cod)(cot)] embedded

in the host matrix was successfully performed, synthesis of a Ru alloy appears to be another

promising research aspect. Therefore, a suitable Pt precursor needed to be chosen. In a

previous work it was shown that the hydrogenolysis of [CpPtMe3] in MOF-5, even at very

low temperature (0 °C) leads to the formation of large Pt nanoparticles and complete

decomposition of the host framework MOF-5.[135] For the simultaneous loading of MOF-5

with Ru and Pt precursor molecules, as an alternative Pt precursor [Pt(cod)Me2] (E) (see

Scheme 4.3) was chosen. The loading of MOF-5 with this precursor has also previously been

studied[136] and the hydrogenolysis of this precursor embedded in MOF-5 was expected to

proceed more gentle. This time loading of MOF-5 with the two precursor components below

the saturation level of MOF-5 with the precursor molecules was performed in order to obtain

a defined precursor ratio in MOF-5.

RuPt CH3

CH3

D E

Scheme 4.3. Precursors [Ru(cod)(cot)](D) and [Pt(cod)Me2](E) to be infiltrated in MOF-5.

4.3.1. Synthesis

To allow complete uptake of the used precursor amounts, loading below the saturation

level of MOF-5 with the two precursor components was attempted. Therefore different ratios

between [Ru(cod)(cot)]/[Pt(cod)Me2] (the ratio between the precursors was kept 1:1 always)

and pure MOF-5 were tested in loading experiments. For instance it was found that 100 (0.13


85

mmol) mg of MOF-5 completely absorb 100 mg (0.3 mmol) of [Pt(cod)Me2] and 94 mg (0.3

mmol)of [Ru(cod)(cot)] simultaneously.

The loading experiments of MOF-5 with [Ru(cod)(cot)](D)/[Pt(cod)Me2](E) were performed

in static vacuum (10-5 mbar) at 40 °C in a glass tube for 2 days. A color change of the MOF-5

powder from off-white to yellow was observed. As discussed above the starting precursor

ratio was chosen to be [Ru(cod)(cot)]:[ Pt(cod)(Me)3] = 1:1 and complete absorption of the

used precursor amounts by MOF-5 was observed.


Characterization of the composite [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 by

elemental/AAS analysis gave an overall ratio of the precursors [Ru(cod)(cot)]:[Pt(cod)Me2] =

1:1. This nicely proves that a confined ratio of two precursors embedded in MOF-5 can be

obtained as long as the MOF material is loaded with precursor amounts below the saturation

level of MOF-5.

4.3.2.1. 13C MAS NMR spectroscopic investigations

Figure 4.5 presents the 13C MAS-NMR spectrum of the composite

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5.

Figure 4.5. 13C-MAS-NMR of composite [Ru(cod)(cot)](D)/[Pt(cod)Me2](E)@MOF-5. The MOF-5 signals are marked by asterisks.


86

Beside the unchanged carbon signals of the MOF-5 host material at 175.6, 136.8 and 131.6

ppm (marked by asterisks) the carbon signals of the intercalated precursors [Ru(cod)(cot)] and

[Pt(cod)Me2] can be observed (see Table 4.5). The chemical shifts of the precursor signals

correspond to the 13C MAS-NMR literature data of the precursor molecules in C6D6

solution,[98,137] confirming that both precursors were infiltrated in MOF-5 chemically

unchanged. Due to technical difficulties, the 195Pt MAS-NMR signal of the embedded Pt

precursor in [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 could not be detected. However, in the 13C-MAS-NMR of the composite, the typical Pt-C coupling of the carbon signal

corresponding to the methyl groups of [Pt(cod)Me2] was observed with a coupling constant of

JPt-C = 785 MHz also matching the literature data.[138]

Table 4.5. Assignment of the 13C MAS-NMR signals of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5.

Precursor 13C MAS-NMR 195Pt MAS-NMR δ [ppm] δ [ppm] [Ru(cod)(cot)](D) 101.4 (cot, olefin.), 98.8 (cot, olefin.), 77.2 (cot, olefin.), 70.2 (cod, olefin.), _______ 34.1(cod, aliphat.), 32.0 (cot, aliphat.) [Pt(cod)Me2](E) 98.8 (cod, olefin), 30.8 (cod, aliphat.), signal could not be detected 5.5 (CH3, J(Pt-C) = 785 MHz)


The PXRD of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 is given in Figure 4.6 (top). In

comparison to the PXRD of pure, empty MOF-5, a drastic decrease of the overall reflection

intensities can be observed in the PXRD of the composite, which must be attributed to the

inclusion of the precursor molecules [Ru(cod)(cot)] and [Pt(cod)Me2] in MOF-5 and was also

observed in the PXRDs of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5 and [CpPd(η3-

C3H5)]/[CpPtMe3]@MOF-5. The PXRD of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 exhibits the

prominent MOF-5 reflections at 13.7° and 15.4° 2θ in about the same intensity ratio as in the

pure activated MOF-5. The intensity ratio between these reflections and another prominent

MOF-5 reflection below 10° 2θ, at 9.7° 2θ has drastically changed. The intensity of the

reflection at 9.7° in the PXRD of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 has become smaller

than the intensities at of the reflections at 13.7° and 15.4°. As discussed above, decrease of the

intensities of MOF-5 reflections below 2θ = 10° is a result of the loading of MOF-5 cavities.

Remarkably in the PXRD of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5, the characteristic MOF-5


87

Figure 4.6. Powder X-ray diffractogramms of (c) [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5, (b) the same

composite after removal of the intercalated precursors [Ru(cod)(cot)] and [Pt(cod)Me2] and (a) pure MOF-5.

reflection at 6.9° is not observed. However, the single new reflection at 11.9° that was also

observed in the PXRDs of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5 and [CpPd(η3-

C3H5)]/[CpPtMe3]@MOF-5, is also present in the PXRD of

[Ru(cod)(cot)]/[Pt(cod)(Me)2]@MOF-5. The drastic decrease of the MOF-5 reflections in the

PXRD of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 when compared to the PXRD of the parent

MOF-5 material, together with a remarkably lower signal-to-noise ratio must be a

consequence of the loading with two different precursor compounds, introducing a high

degree of disorder in the composite structure.

To verify the unchanged nature of the host material upon inclusion of the Ru and Pt precursor

molecules, the precursor molecules were removed from the composite

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 by immersion in dry n-pentane (both precursor

components dissolve well in n-pentane) and filtering the solution. The procedure was repeated

three times until the supernatant remained colourless. An off-white powder was obtained

which, after careful drying in vacuum (10-3 mbar) at 100°C overnight, exhibited the PXRD

presented in Figure 4.6b. The PXRD of the powder is identical to the PXRD of pure, empty

MOF-5 (Figure 4.6a). N2 sorption measurements gave the typical Langmuir surface area of


88

MOF-5 with a value of 3300 m2/g and the removal of the precursor molecules was verified by

an FT-IR measurement. This proves that the MOF-5 host framework of the composite

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 remains unchanged upon inclusion of the Pt and Ru

precursor molecules, although the PXRD of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 looks

rather different from pure MOF-5. The results from the elemental/AAS analysis,

spectroscopical and structural investigations confirm the simultaneous loading of MOF-5 with

[Ru(cod)(cot)] and [Pt(cod)Me2] in a defined precursor ratio.

4.4. Co-hydrogenolysis of [Ru(cod)(cot)] and [Pt(cod)Me2] in MOF-5 at

mild conditionS

4.4.1. Synthesis

Since PtRu alloys are known hydrogenation catalysts, rather gentle conditions for the

first attempt of co-hydrogenolysis of [Ru(cod)(cot)] and [Pt(cod)Me2] in MOF-5 were chosen,

to avoid decomposition of the host framework MOF-5. [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5

was exposed to a stream of hydrogen (1 bar, 1 sccm) at 25 °C for 10 minutes until a colour

change of the material from yellow to dark-brown was observed.


4.4.2.1. MAS-NMR spectroscopic investigations

The 13C MAS-NMR spectrum of the co-hydrogenolysis product of

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 at mild conditions is given in Figure 4.7. The spectrum

exhibits the carbon signals of unchanged MOF-5 at 175.2, 136.7 and 131.0 ppm. In addition

to that, the 13C MAS-NMR signals of the [(η6-arene)Ru(cod)] complex with the MOF-5 bdc

linkers at 178.5, 91.0, 83.2, 68.3 and 33.4 ppm are also detected. Beside that a signal at 27.3

ppm is assigned to not fully desorbed cyclooctane, stemming from the side product of

hydrogenolysis of [Ru(cod)(cot)]. This is in accordance to the results from the decomposition

of the composite [Ru(cod)(cot)]3.5@MOF-5 at mild conditions. The 13C MAS-NMR of the

composite furthermore shows the carbon signals of the Pt precursor [Pt(cod)Me2] unchanged.


89

Figure 4.7. 13C-MAS-NMR spectrum of the co-hydrogenolysis product of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-

5 at mild conditions. The MOF-5 signals are marked by asterisks.

These results show that co-hydrogenolysis of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 at mild

conditions proceeds incomplete yielding only partly hydrogenolysed Ru precursor molecules

and most probably completely unchanged Pt precursor molecules. The 13C MAS-NMR

investigations of the hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5 have shown that

hydrogenolysis at mild conditions leads to incomplete conversion of the Ru precursor.

However particularly XAS investigations of the obtained composite {[Ru(cod)]/Ru}@MOF-5

showed that beside the formation of a [(η6-arene)Ru(cod)] complex with the MOF-5 bdc

linkers also metallic Ru nanoparticles were formed. This is most probably also the case in the

co-hydrogenolysis product of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 at mild conditions


The powder X-ray diffractogramm of the composite (see Figure 4.8b) looks strikingly

different from the PXRD of the composite material before hydrogen treatment. It exhibits the

typical MOF-5 PXRD reflections in the same intensity ratios as in the parent MOF-5 material

(Figure 4.8a) hinting at an unchanged MOF-5 matrix.


90

Figure 4.8. Powder X-ray diffractogramms of (a) empty MOF-5 and (b) the co-hydrogenolysis product of

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 at mild conditions.

The overall decrease in the intensities of the reflections is due to the remaining embedded Pt

precursor molecules and possibly also Ru nanoparticles. Additional Bragg reflections from

Ru, Pt or RuPt are however not observed. Therefore, no conclusions, whether formation of

nanoparticles inside the MOF-5 cavities proceeded can be made from the PXRD of this

composite. Due to the remaining Pt precursor molecules in the composite, determination of

the Langmuir surface area could not be performed. From the agreement of the positions of the

PXRD reflections in the PXRD of the co-hydrogenolysis product of

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 at mild conditions and pure MOF-5 it can however be

concluded that the MOF-5 matrix remained unchanged upon hydrogen treatment.


91

4.5. Quantitative co-hydrogenolysis of [Ru(cod)(cot)] and [Pt(cod)Me2] in

MOF-5

4.5.1. Synthesis

In order to achieve quantitative co-hydrogenolysis of [Ru(cod)(cot)] and [Pt(cod)Me2]

in MOF-5 hydrogen treatment at elevated temperature and pressure was performed. Keeping

in mind the catalytic activity of PtRu alloys in hydrogenation reactions, the treatment was

performed under milder conditions than in the case of hydrogenolysis of

[Ru(cod)(cot)]3.5@MOF-5 to give Ru@MOF-5. [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 was

treated in 1 bar H2 at 150 °C for 3 h. Thereby a colour change of from yellow to black was

observed. From N2 sorption measurements the Langmuir surface area of the obtained

composite was determined to be 380 m2/g which is very poor, compared to the surface area of

the starting material MOF-5 of 3300 m2/g, already indicating the impact of the co-

hydrogenolysis of [Ru(cod)(cot)] and [Pt(cod)Me2] in MOF-5. From elemental analysis/AAS

results, the molar ratio in the composite was also determined to Ru : Pt = 1:1 (± 0.1).


4.5.2.1. 13C MAS-NMR spectroscopic investigations

Figure 4.9a presents the 13C MAS-NMR of the composite. The signals of unchanged

MOF-5 linkers are observed at 175.0, 136.9 and 130.7 ppm. In addition to that, five new

rather broad signals at 186.3, 45.1, 41.1 and 29.6 ppm are observed that can neither be

assigned to remaining Ru or Pt precursor molecules nor to the discussed [(η6-arene)Ru(cod)]

complex with MOF-5 bdc linkers. The 13C carbon signals of these compounds are completely

missing, indicating quantitative hydrogenolysis of [Ru(cod)(cot)] and [Pt(cod)Me2] in MOF-

5. The comparison with the 13C MAS-NMR spectrum of cis/trans-1,4-

cyclohexanedicarboxylic acid which is a possible hydrogenation product of the MOF-5 bdc

linkers (Figure 4.9b) shows that the signals in the 13C MAS-NMR spectrum of the

quantitative co-hydrogenolysis product of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 must stem

from hydrogenated MOF-5 bdc linkers. The signal at 27.2 ppm most probably stems from not

fully desorbed cyclooctane which is a side product in hydrogenolysis of the Pt and Ru


92

precursor molecules. Obviously complete conversion of the embedded Ru and Pt precursor

molecules was achieved however partly hydrogenation of the MOF-5 linkers proceeded as

well. Compared to the carbon signals in the 13C MAS-NMR spectrum of the free acid

molecules (Figure 4.9b), the corresponding signals in the 13C MAS-NMR of the composite

material are rather broad. This indicates that the hydrogenated linkers remain bound to the

Zn4O units of the MOF-5 and are therefore more rigid than the free acid molecules. Recently

the synthesis of a metal-organic framework based on Zn4O tetrahedrons linked by

cyclohexanedicarboxylates was

Figure 4.9. 13C MAS-NMR spectra of (a) the quantitative co-hydrogenolysis product of

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 and (b) cis/trans-1,4-cyclohexanedicarboxylic acid.


93

reported.[139] Therefore the remaining MOF-5 matrix might stay intact even though a fraction

of the bdc linkers was converted to cyclohexanedicarboxylates. Since unchanged MOF-5

signals can be also observed in the 13C MAS-NMR spectrum of the quantitative co-

hydrogenolysis product of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 (Figure 4.9a) at least part of

the host matrix is most probably intact. Hydrogenation of the linkers possibly occurred in the

close environment of the PtRu species. Upon decomposition [Ru(cod)(cot)]3.5@MOF-5 no

hydrogenation of the MOF-5 linkers was observed in the corresponding 13C MAS-NMR

spectrum. Obviously, the presence of additional Pt species together with Ru nanoparticles

leads to partial hydrogenation of the MOF-5 linkers. The great superior catalytic activity of

RuPt catalysts in hydrogenation of benzene, in contrast to pure Ru or Pt catalyst is literature

known.[140] This effect might be also the reason for the partly hydrogenation of the MOF-5

linkers in the discussed case here. Contributing to that, the hydrogenolysis of [CpPtMe3] in

MOF-5[135] led to a complete break-down of the MOF lattice and large Pt nanoparticles which

hints at a partial hydrogenation of the MOF-5 linkers in that case also. The formation of Pt

nanoparticles in MOF-177 via hydrogenolysis of [Pt(η5-C5H4(CH3))(CH3)3] was shown as

well[55] The authors did not detect hydrogenated MOF-177 linkers in the corresponding 13C

MAS-NMR. The corresponding spectrum was however rather poorly resolved, with spinning

side bands possibly overlapping signals of partially hydrogenated linker molecules.


The powder X-ray diffractogramm of the composite material (top) in comparison to

the powder X-ray diffarctogramm of pure MOF-5 (bottom) is given in Figure 4.10. The

PXRD of the composite shows only very weak remaining reflections of the host matrix MOF-

5, indicating a great loss of crystallinity of the host matrix most probably due to

hydrogenation of a fraction of the MOF-5 bdc linkers. In addition also four broad Bragg

reflections are observed with the most prominent centered at 39.91° 2θ exhibiting a FWHM

(full width at half maximum) of 2.496°. The Bragg reflections stem from the metal

nanospecies formed upon quantitative hydrogenolysis of [Ru(cod)(cot)] and [Pt(cod)Me2] in

MOF-5. Comparison of these Bragg reflections with the simulated PXRD patterns of bulk

(hcp) Ru and (fcc) Pt metal (Figure 4.10b) shows that the reflections cannot be assigned to Ru

nanoparticles. The additional reflections rather exhibit a face centered cubic structure more

similar to platinum. However the enlargement in Figure 4.10b shows that the maximum of the

most prominent


94

Figure 4.10. Powder X-ray diffractogramms of (a) pure, empty MOF-5 and (b) the quantitative co-hydrogenolysis product of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5(Lines: Ru (black) JPDS reference No. 6-0663, Pt (red) JPDS reference No. 4-0802).

reflection at 39.84 ° 2θ is slightly shifted in comparison to the center of the Pt (111) reflection

centered at 39.76° 2θ. This behaviour has been observed before in the synthesis of PtRu

nanoalloys.[141] In alloyed PtRu nanoparticles, the Ru atoms substitute the Pt atoms in the Pt

fcc lattice, shrinking the lattice size and therefore resulting in slightly shifting of the

corresponding Bragg reflections. Thereby, up to 62 atomic % of Ru, the (fcc) structure of Pt is

maintained (see Figure 4.11), then up to 80 at.% of Ru, a mixture of (hcp) Ru and (fcc) Pt

exists and above 80 at.% of Ru, the (hcp) Ru structure will be adopted. The shift is dependent

on the atomic ratios of Pt and Ru in the alloy and becomes more pronounced with increasing

Ru content. For instance, alloyed Pt50Ru50 nanoparticles with a size of 3.5 nm exhibited a

(111) reflection at 39.80° 2θ.[141] Therefore the Bragg reflection at 39.84°, in the PXRD of the

quantitative co-hydrogenolysis product of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5, might

indicate the existence of alloyed Pt50Ru50 nanoparticles, which is also in agreement with the

elemental analysis of the composite exhibiting an equimolar ratio of Pt and Ru. From the

FWHM of this reflection a crystallite size of 3.8 nm was calculated with the Scherrer

equation. This of course exceeds the diameter of the MOF-5 cavities of max. 1.5 nm but is


95

most likely due to the distortion of the host framework by partial hydrogenation of the MOF-5

bdc linkers.

Figure 4.11. Phase diagram of the binary PtRu alloy.[141b]

4.5.2.3. TEM investigations

A TEM image of the composite is given in Figure 4.12. The metal nanoparticles in

MOF-5 can be observed as darker spots within the host matrix due to the higher contrast in

Figure 4.12. TEM image of the quantitative co-hydrogenolysis product of

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 (left), the SAED pattern (upper left) and the EDX spectrum (right). The TEM sample was prepared on a carbon coated Cu grid.


96

the electron beam and were found to be in a size range of 4-6 nm. The observed particle sizes

deviate from the crystallite sizes calculated from the PXRD of the composite (3.8 nm) since in

the PXRD only crystalline domains give rise to the observed Bragg reflection. The SAED

pattern of the composite exhibits diffraction rings that can be assigned to (fcc) Pt. The

observed pattern exhibits rather broad diffraction rings typical for nanocrystalline materials.

This limits the accuracy in the calculation of the corresponding lattice constants d. As

discussed above, the shrinkage of the lattice size due to a possible alloyed Pt50Ru50 phase

causes only slight deviations in the PXRD reflections in comparison to pure Pt which is also

the case for the electron diffraction reflections. The EDX analysis, by focusing on the

displayed area, however clearly reveals the presence of both Ru and Pt in however too small

amounts to determine the atomic ration of the two elements. In order to check the distribution

of Ru and Pt throughout the composite material, several TEM images and EDX spectra of

different sample regions were taken. Figure 4.13 presents another TEM image of quantitative

co-hydrogenolysis product of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5, due to overlapping of

several layers of the composite material, the image appears less well resolved than the image

presented in Figure 4.12. However metal nanoparticles in MOF-5 can also be observed

(marked by arrows) and are in a size range of 3-4 nm. The corresponding EDX spectrum

shows the presents of Ru and Pt with 65.15 at.% Ru and 34.85 at.% Pt, which contrasts the

overall stochiometric equimolar ratio of Ru and Pt determined by AAS.

Figure 4.13. TEM image (left), SAED pattern (upper left) and EDX spectrum (right) of rather agglomerated powder of the quantitative co-hydrogenolysis product of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5.


97

However the EDX analysis provides information of the elemental composition of the sample

only over a very small area. Surprisingly, the SAED pattern of the corresponding area, can be

assigned to (fcc) Pt. From the phase diagram of PtRu alloys (see Figure 4.11.) it can be

deduced that an alloy with more than 62 at.% Ru should be a mixture of (fcc) Pt and (hcp) Ru.

Here, only reflections of (fcc) Pt can be observed, but the existence of another, possibly

amorphous (hcp) Ru phase cannot be ruled out.

To gain a better insight into the distribution of Ru and Pt within the MOF-5 host, a more

detailed TEM analysis needs to be performed with several EDX spectra and SAED patterns

taken from various parts of the sample. The results from the EDX analysis hint at non uniform

distribution of the precursor molecules throughout the framework, leading to sections of the

host enriched with Ru precursor molecules and others possibly enriched with Pt precursor

molecules. Yet, the overall ratio of Ru and Pt was found to be equimolar by AAS and

elemental analysis.

4.6. Conclusion

The loading of MOF-5 with the three different two component precursor systems

[Fe(η6-toluene)(η4-C4H6)] / [CpPtMe3], [CpPd(η3-C3H5)] / [CpPtMe3] and [Ru(cod)(cot)] /

[Pt(cod)Me2] was successfully performed. Spectroscopic and structural investigations of the

obtained composites confirmed the absorption of unchanged precursor species in an

unchanged host matrix. It was found that distinct ratios of the two precursor components can

only be achieved when a loading of the MOF-5 material below or at the saturation level with

the precursor components is performed. Otherwise favoured absorption of the respective

precursor component exhibiting a higher diffusion velocity was observed. The loading of

MOF-5 with a distinct 1:1 molar ratio of the precursors [Ru(cod)(cot)]/[Pt(cod)Me2] was

obtained. First attempts of the co-hydrogenolysis of these precursor compounds in MOF-5

showed that quantitative conversion of the precursors resulted in partial hydrogenation of the

MOF-5 bdc linkers. The powder diffraction analysis of the obtained composite gave first hints

at the existence of alloyed PtRu nanospecies in MOF-5. TEM analysis showed a non uniform

distribution of Ru and Pt species throughout the host material, contrasting the results from

elemental analysis, which gave a 1:1 ratio. Further TEM and also XAS experiments have to

be performed in order to verify the alloying of the Pt and Ru nanospecies. Also additional

experiments of the co-hydrogenolysis at lower temperature are necessary to test the


98

quantitative conversion of the precursor molecules while maintaining the unchanged nature of

the host framework MOF-5.

5. Investigations of MOF-5-water interactions

100


The metal-organic framework MOF-5 is probably the most intensively studied porous

coordination polymer to date, due to its comparably facile synthesis even in larger scales. It is

stable under inert, dry conditions, but easily decomposes upon exposure to water or moist air.

This is clearly a major disadvantage for large scale industrial applications. Therefore already

some research on the interaction of MOF-5 and water has been performed.

When adding 0.04 mol of water per bdc linker to the standard MOF-5 synthesis recipe, Yaghi

et al. observed formation of the the new monoclinic phase Zn3(OH)2(bdc)2·(DEF)2 (MOF-

69C, bdc = 1,4-benzenedicarboxylate), identified by single crystal X-ray structural

analysis.[141,142] The cubic MOF-5 structure only exhibits Zn centers tetrahedrally bound to

three carboxylate groups of the bdc linkers and one O2- ion. In contrast to this, MOF-69C

exhibits Zn atoms bridged by four carboxylate groups of the bdc linkers but also bound to two

OH- ions, resulting in tetrahedral and octahedral Zn centers (see Figure 5.1).[142,143] The 3D

network of MOF-69C is hence constructed from infinite Zn-O-C units running along the [001]

direction. Its structure is very similar to MOF-69C (Figure 5.1), which is however based on

bpdc (4,4′-biphenyldicarboxylate) instead of bdc linker molecules. The formation of a

complete different coordination polymer upon addition of even small amount of water to the

MOF-5 mother liquor emphasizes the sensitivity of the MOF-5 structure. Huang et al.

investigated the influence of addition of aqueous H2O2 solution to the MOF-5 mother

liquor.[49] A new phase with tetragonal structure was obtained, with the chemical formula

[Zn4O(bdc)3(H2O)] derived from TGA experiments. By stirring of MOF-5 powder in

Figure 5.1. The crystal structure of MOF-69A (a) ball and stick (b) space filling model (Zn : blue, O: red; C

gray).[142]


101

Figure 5.2. Powder X-ray diffractogramms of (a) MOF-5 [Zn4O(bdc)] powder, (b) [Zn4O(bdc)2(H2O)2] and

(c) MOF-5 after stirring in 0.1 m HCl obtained by Huang et al.[49]

water for 0.5 h, the authors obtained the same tetragonal phase. This phase was found to

exhibit two strongly absorbed water molecules per MOF-5 formula with possibly one of the

water molecules replacing one of the MOF-5 bdc linkers, resulting in the chemical formula

[Zn4O(bdc)2(H2O)2]. Stirring of MOF-5 powder in 0.1 m HCl solution resulted in complete

dissoziation of the framework derived from PXRD results (see Figure 5.2). Molecular

dynamics simulations by Allendorf et al. have shown that upon loading with water molecules,

the MOF-5 framework structure is maintained up to 2.3 wt.% of water.[50] At 3.9 wt.% and 9.5

wt.% of water the structure collapses completely. Thereby, Allendorf et al. identified three

different ways of interaction between the water molecules and the parent MOF-5 material:

Water molecules can attack the Zn4O tetrahedrons of the secondary building units (SBU) of

MOF-5 in which a water O atom replaces the central O atom of the SBU. The water

molecules can also undergo hydrogen-bonding with the O atoms of the linker molecules.

Additionally, a hydrogen-bonded network of water molecules can be tethered to one or more

Zn4O tetrahedral. Allendorf et al. detected all three scenarios upon a water loading of the

MOF-5 structure of 2.3 wt.% (see Figure 5.3).[50] In a detailed study on the transformation

processes of MOF-5 when exposed to water, Mertens et al. identified Zn(bdc)·H2O and

Zn5(OH)4(bdc)3 beside MOF-69C as the hydrolysis products of MOF-5.[144] For the

investigation of the transformation processes, MOF-5 was exposed to H2O/DEF mixtures of

different concentrations, at different temperatures ranging from 60 – 130 °C.


102

Figure 5.3. Simulated lattice parameters of the MOF-5 structure as a function of the water content (left)

and disruption of the MOF-5 structure (Zn : purple, O : red, C : gray, H : white) at 2.3 wt.% water (right) derived by Allendorf et al.[50]

In this work, the investigation of the interaction of MOF-5 with water molecules by means of

Terahertz time-domain spectroscopy (THz-TDS) along with PXRD and FT-IR investigations

was attempted, according to the investigations of Allendorf et al.[50] The THz spectra were

recorded and evaluated at the chair of Physical Chemistry II by Dr. Konstanze Schröck,[145]

while the sample preparations and all other sample characterisation were performed by the

author of this work. In studies of water dynamics around biomolecules, THz-TDS was shown

to excite collective motions of the water molecules.[146,147] For instance, water molecules form

hydrogen bonds with three to four other neighbour water molecules at any given time, these

bonds are subject to constant breaking and reformation on a sub-picosecond time scale.

Terahertz spectroscopy allows the observation of picosecond collective motions of so called

hydration water as well as motions of crystal lattices. These are exactly the processes that are

important in the observation of the interaction of MOF-5 with water molecules as predicted

by Allendorf et al.[50] Also the absorption of water plays a significant role in the THz region

between 1–5 THz. The dynamical reorientation of the water dipole moment, when water

molecules within the hydrogen bonding network tumble around and diffuse, has a resonance

around 4–5 THz. Upon investigations of water molecules in MOF-5 two limiting cases should

be distinguishable: The addition of water to the MOF-5 framework results in an increase of

absorption in the THz frequency range. In case that the water network motions and the

crystalline phonon modes of MOF-5, which both have absorption features rather in the THz

regime, are uncoupled, the absorption of the hydrated (intercalated) water can be described as

the sum of both contributions. Since the THz absorption of water can be described as a

featureless linear increasing absorption, a nearly linear increase of the MOF-5 material is


103

expected upon loading with water molecules. In case of coupling of the dynamics of the

intercalated water molecules and the MOF-5 crystal lattice, a rather different net absorption is

expected. As in the case of the hydrated tri-peptide Alanin,[148] a strong crystal-water

coupling, or a reformation of the crystal structure might be detectable in the THz region,

resulting in considerable frequency shift and appearance of new absorption features in the

corresponding THz spectrum. In addition to the THz measurements, also the structural

properties of the MOF-5 materials after exposure to water were investigated by PXRD

analysis. FT-IR measurements were performed as well to observe possible changes in the

vibrations of the MOF-5 bdc linkers.


104

5.1. Loading of MOF-5 with 4wt.% and 8 wt.% of water

5.1.1. Synthesis

According to the results of Allendorf et al.,[50] the MOF-5 framework structure

completely collapses upon loading of the framework with 3.9 wt.% of water. To verify this

result from molecular dynamics simulations, dry, activated MOF-5 powder was exposed to 4

wt.% of water in static vaccum (10–3 mbar) at 25 °C. For the loading experiment, the

respective amount of water and MOF-5 material were placed in two separated glass vials in a

Schlenk tube, the loading experiment was performed until complete absorption of the applied

amount of water was observed by the MOF-5 powder. The same experiment was also

performed with 8 wt.% of water to investigate the impact of an even larger amount of water

on the MOF-5 structure and THz spectrum.



The powder X-ray diffractogramms of MOF-5 after loading with 4 wt.% (Figure 5.4b)

and 8 wt.% (Figure 5.4c) of water are presented in Figure 5.4. By comparing the PXRD of

pure, dry MOF-5 powder (Figure 5.4a) with the PXRD of MOF-5 after loading with 8 wt.%

of water, it is obvious that the exposure of the MOF-5 material to 8 wt.% of water has led to a

complete change of the MOF-5, resulting in a new phase. The amount of 8 wt.% of water

molecules is equivalent to 3.4 mole of H2O per mole MOF-5 ([Zn4O(bdc)3]), which

corresponds to 1.1 molecules of water per MOF-5 bdc linker. Therefore it is reasonably to

assume that complete hydrolysis of the parent MOF-5 structure by the introduced water

molecules has occurred. The PXRD of the observed new phase is however not similar to the

new phase [Zn4O(bdc)2(H2O)2] Huang et al.[49] observed upon stirring MOF-5 in water (see

Figure 5.2). By stirring MOF-5 in water, the framework structure is exposed to an even

greater excess of water which obviously leads to the formation of a different decomposition

product of MOF-5 than in the case of exposure of MOF-5 to 8 wt.% of water. The new phase

resulting from exposure of MOF-5 to 8 wt.% water is also not similar to the decomposition

products of MOF-5 observed by Mertens et al.[144] However the authors exposed the MOF-5


105

Figure 5.4. Powder X-ray diffractogramms of (a) pure MOF-5 (argon filled) as synthesized, (b) MOF-5 + 4

wt.% H20 and (c) MOF-5 + 8 wt.% H2O.

powder to solutions of water in DEF which is most probably the reason for the finding of

different decomposition products. The treatment of MOF-5 with 8 wt.% of water was

performed in vacuum with, prior to the loading experiment, perfectly dry and empty MOF-5

material. The PXRD of MOF-5 after loading with 4 wt.% H2O (Figure 5.4b) looks very

similar to the PXRD of the parent dry, argon filled MOF-5. This is in contrast to the

molecular simulations by Allendorf et al.[49] However the PXRD of MOF-5 after loading with

4 wt.% H2O exhibits five additional reflections that can also be observed in the PXRD of

MOF-5 after loading with 8 wt.% of water and hints at an already started decomposition

process. The amount of 4 wt.% of water corresponds to 1.7 mole of water per mole MOF-5

([Zn4O(bdc)3] which is equivalent to 0.6 molecules of water per MOF-5 linker. Complete

decomposition of the MOF-5 framework through protonation of the bdc linkers has obviously

not occurred. Although, the additional small intensity reflections observed in the PXRD of

MOF-5 after loading with 4 wt.% of water must stem from partial decomposition, it can be

concluded that most of the framework structures remains unchanged.


106

5.1.2.2. THz-TDS spectroscopic investigations[145]

The THz-TD spectra of dry MOF-5, MOF-5 + 4 wt.% H2O and MOF-5 + 8 wt.% H2O,

in the frequency range between 5 and 46 cm–1, are presented in Figure 5.5. The THz spectra

of the three materials do not exhibit discrete absorption bands in this frequency range. Above

35 cm–1, the absorption of the materials increases with increasing water content as discussed

above. However, the absorption coefficient α of the spectra of MOF-5 + 4 wt.% H2O and

MOF-5 + 8 wt.% H2O is decreased compared to α in the spectrum of dry MOF-5 between 5–

30 cm–1 and increased between 30–45 cm–1. For a more detailed inspection of the features of

the absorption spectra, Figure 5.6 compares the THz spectra of dry MOF-5, MOF-5 + 4 wt.%

of H2O and MOF-5 + 4 wt.% of H2O after evacuation at room temperature. The spectrum of

the evacuated material agrees very well with the spectrum of the dry MOF-5, whereas the

spectrum of MOF-5 + 4 wt.% of water exhibits an increased absorption (see discussion

above). This supports the conclusion that the framework structure remains almost unchanged

when exposed to 4 wt.% of water. It can therefore be concluded that the loading of MOF-5

with 4 wt.% of water is reversible.

Figure 5.5. THz TDS spectra of MOF-5 (full squares), MOF-5 + 4 wt.% H2O (open squares) and MOF-5 + 8 wt.% H2O (full triangles).[145]


107

Figure 5.6. Comparison of the THz spectra of dry MOF-5 (full squares), MOF-5 + 4 wt.% H2O (open squares) and MOF-5 + 4 wt.% H2O after evacuation (full triangles). [145]

The same comparison for MOF-5 + 8 wt.% H2O is given in Figure 5.7. The THz spectra of

dry MOF-5, MOF-5 + 8 wt.% H2O and MOF-5 + 8 wt.% of H2O after evacuation in dynamic

vacuum at 200 °C for 24 h are presented. Since the PXRD of MOF-5 + 8 wt.% H2O looked

strikingly different from the starting material MOF-5, evacuation at elevated temperature was

performed in order to reobtain the parent material. However, the PXRD of the evacuated

material looked akin to the loaded material MOF-5 + 8 wt.% H2O. Also, the THz spectrum of

the evacuated material looks rather different from the THz spectrum of dry MOF-5. Different

from MOF-5 + 4 wt.% H2O, evacuation of the material loaded with the higher water content

did not lead to a THz spectrum similar to dry MOF-5. Obviously hydration above 4 wt.% of

water has led to irreversible decomposition of the MOF-5 material. The changed crystal

structure gives rise to measurable changes in the collective network motions of the framework

as can be probed by the corresponding THz TD spectrum. This confirms the thesis of

Allendorf et al.,[49] that exposure of MOF-5 to water contents of or above 4 wt.% H2O leads to

complete decomposition of the MOF-5 structure to some extent. From the close similarity of

the THz spectra of MOF-5 + 4 wt.% H2O after prolonged evacuation and dry MOF-5 it was

concluded that the loading of MOF-5 with 4 wt.% H2O is reversible. This indicates that there

is no coupling between the vibrations of the water molecules and the MOF-5 lattice. In this

case, the both contributions of additional water absorptions due to the loading of MOF-5 with

water and the low frequency spectrum of the MOF-5 excitations in the THz should be simply


108

additive. To further verify this conclusion, the difference of the two spectra was calculated. In

case of independent vibrations of the water molecules and the MOF-5 lattice, the difference

spectrum should correspond to a THz spectrum of pure water (scaled to 4 %). The THz

spectra of dry MOF-5 and MOF-5 + 4 wt.% H2O as well as the difference spectrum are

presented in Figure 5.8. The difference spectrum (Figure 5.8b) shows an increase in the

absorbance at higher frequencies (> 32 cm–1), which could be attributed to a linear bulk like

spectrum of pure water. At lower frequencies (< 32 cm–1) however, a decrease in the total

absorption is observed, which is a clear disagreement to the assumption of an additive

contribution of the MOF-5 and the hydration water absorbance to the total THz spectrum. The

observation of nonlinear frequency dependent changes is an indication of a coupling between

the lattice modes of MOF-5 and the network motions of the hydration water. If the water

binds to the Zn4O clusters as proposed by Allendorf et al.,[50] a rearrangement of the hydrogen

bond network, which is sensitive in the studied frequency range, is expected. This will result

in distinct absorption features for the hydrated MOF-5 in contrast to dry MOF-5. The

observed decrease of the THz absorbance at lower frequencies compared to absorbance of dry

MOF-5 is a clear indication of a strong coupling of the crystal modes with the adsorbed water

hydrogen bond network motions. These results confirm the finding of partial decomposition

already in MOF-5 + 4 wt.% H2O deduced from the corresponding PXRD.

Figure 5.7. THz spectra of dry MOF-5 (full squares), MOF-5 + 8 wt.% of H2O (open squares) and MOF-5 + 8 wt.% of H2O after evacuation (full triangles). [145]


109

Figure 5.8. Comparison of (a) the THz spectrum of dry MOF-5 (full squares) with MOF-5 with 4 wt.% H2O

(open squares) and (b) the difference of both spectra. [145]

5.1.3. FT-IR investigation of MOF-5 + 8 wt.% H2O

The PXRD of MOF-5 + 8 wt.% H2O showed a complete loss of the parent MOF-5

structure and the formation of a new phase. To further examine the nature of this phase, the

composite was examined by FT-IR spectroscopy. The corresponding FT-IR spectrum is

presented in Figure 5.9b in comparison to the FT-IR spectrum of dry MOF-5. The IR

spectrum of MOF-5 + 8 wt.% H2O shows the characteristic symmetric and asymmetric C–O

vibrations of the MOF-5 network at 1578 and 1384 cm–1 for COO– groups coordinating to

zinc.[49] In contrast to the IR spectrum of dry MOF-5 it also shows vibrations typical of

adsorbed water at 3424 cm–1 and of protonated bdc at 1678 cm–1.[49] In addition another sharp

vibration at 3606 cm–1 can be assigned to a ν(O–H) vibration which possibly stems from Zn–

OH moieties. This findings strongly support the assumption that exposure of MOF-5 to water

results in protonation of the bdc linkers and replacement of the linkers by OH– ions as

suggested by Huang et al.[49] and Allendorf et al.[50] Furthermore additional “free” water

molecules must be present as well.


110

Figure 5.9. FT-IR spectra of (a) dry MOF-5 and (b) MOF-5 + 8 wt.% H2O.

5.1.4. 13C MAS-NMR of MOF-5 + 8 wt.% H2O

The 13C MAS-NMR spectrum of MOF-5 + 8 wt.% H2O is given in Figure 5.10b. The

spectrum exhibits five different signals at 176.8, 173.0, 137.3, 135.2 and 131.0 ppm. The 13C

MAS-NMR of dry MOF-5 exhibits three signals at 175.1 (COO), 136.3 (C(COO) and 130.0

(C6H4) ppm for the bdc linker molecules. The additional signals in the 13C MAS-NMR

spectrum of the composite MOF-5 + 8 wt.% H2O are very likely to stem from reprotonated

MOF-5 bdc linkers, namely terephtalic acid. The 13C NMR spectrum of terephthalic acid

dissolved in d6-DMSO exhibits three signals at 168 (COO), 135 (C(COO) and 130 (C6H4)

ppm.[149] Especially the broadness of the signals in the 13C MAS-NMR of MOF-5 + 8wt.% of

H2O hints at various numbers of different terephtalate species with very little deviations in

their chemical shifts. These species might be either fully reprotonated and no longer bound to

the Zn ions in the MOF-5 framework or partially reprotonated and bound to one Zn ion with

one carboxylate function or interacting with intact water molecules via hydrogen bonds. The

signals at 176.8 and 173.0 can be assigned to carbon atoms of carboxylate groups of the

terephtalate species, with the signal at 176.8 ppm probably stemming from Zn-bound

terephthalate (COO– group) and the signal at 173.0 ppm stemming from reprotonated

terephthalate (COOH group).


111

Figure 5.10. 13C MAS-NMR spectra of (a) dry MOF-5 and (b) MOF-5 + 8 wt.% H2O.

The signals at 137.3 and 135.2 ppm can be assigned to the carbon atoms directly bound to the

carboxylate groups of the therephthalate species, with the signal at 137.3 ppm possibly

stemming from reprotonated terephthalate (C(COOH) group) and the signal at 135.2 ppm

stemming from Zn-bound terephthalate (C(COO–) group). Finally the very broad signal at

131.0 ppm can be assigned to the aromatic ring carbon atoms of different bound and

reprotonated terephthalate species.


112

5.2. Conclusions

The interaction of MOF-5 and water was investigated by means of PXRD, THz, FT-IR

and 13C MAS-NMR spectroscopy. The loading experiments were performed in vacuum with

dry, empty MOF-5 powders. Complete decomposition of MOF-5 upon exposure to 8 wt.% of

water was found, whereas upon exposure to 4 wt.% H2O, the framework remained almost

unchanged. THz experiments confirmed the reaction of the intercalated water molecules and

the MOF-5 framework. Coupling of the crystal modes of the framework and the absorbed

water hydrogen bond network motions was observed already in MOF-5 + 4 wt.% of H2O.

This proves the sensitivity of MOF-5 towards even small amount of water. The PXRD of

MOF-5 + 8 wt.% of H2O confirmed the formation of a new phase that has not yet been

observed in the literature. However, in the reports on the reactivity of MOF-5 towards water,

the water was either added during the synthesis of MOF-5[49,141,142] or the MOF-5 materials are

stirred in pure water or water/DEF solutions.[49,143] Treatment of MOF-5 with water vapour in

vacuum led to a different kind of decomposition product. The examination of the

decomposition product by FT-IR and 13C MAS-NMR spectroscopy showed the existence of

partially reprotonated bdc linkers and additional water molecules in this material which is in

accordance to earlier reports on the reactivity of MOF-5 towards water.

6. Summary and Outlook

114


Synthesis and characterisation of Ru@MOF-5

This work presents the synthesis of Ru@MOF-5, by hydrogenolysis of [Ru(cod)(cot)]

in MOF-5. Since Ru nanoparticle and colloid synthesis in other porous supports or in solution

is well documented in the literature, a classification of MOF-5 as support matrix by

comparing the properties of Ru@MOF-5 to the properties of other Ru nanospecies is possible.

The intercalated Ru precursor molecules were found to be highly fluxional and behaving

almost as in solution. This behavior clearly differentiates MOF-5 from other solid support

matrixes like zeolites, which undergo rather strong interactions with intercalated guest

molecules and nanoparticles, due to, e.g., reactive surface Si–OH groups. The hydrogenolysis

of [Ru(cod)(cot)] in MOF-5 at mild conditions resulted in a side reaction between precursor

{Ru(cod)} fragments and MOF-5 linkers, as it is known from hydrogenolysis of the precursor

in aromatic solvents. Quantitative hydrogenolysis could only be obtained by prolonged

treatment under elevated H2 pressure and temperature. The resulting Ru nanoparticles

exhibited a surprisingly high surface hydride mobility. Due to obviously rather weak

interactions between the MOF-5 host and the embedded particles, the surface hydrides adopt a

mobility that is even higher than the mobility on Ru colloids stabilized by organic surfactants,

which hints at a possibly superior activity of Ru@MOF-5 in hydrogenation catalysis.

However, a preliminary catalytic test revealed rather low activity of the composite, certainly

the reaction conditions need to be optimized for Ru@MOF-5. Tomographical TEM

measurements showed that most of the Ru nanoparticles are located at the surface of the

MOF-5 nanocrystallites. This might be due to the rather harsh conditions for the quantitative

hydrogenolysis of the Ru precursor molecules and possibly also due to the weak MOF-5-

nanoparticle interactions which might even allow small metal clusters to diffuse to the outer

regions of the nanocrystallites. Precursor molecules that can be decomposed to metal

nanoparticles at more gentle conditions may lead to a more uniform distribution.

Loading of MOF-5 with two precursor components

In this work, the simultaneous loading of MOF-5 with two metal precursor

components was investigated, using the precursor compounds: [Fe(η6-toluene)(η4-


115

C4H6)]/[CpPtMe3], [CpPd(η3-C3H5)]/[CpPtMe3] and [Ru(cod)(cot)]/[Pt(cod)Me2]. The

synthesis of bimetallic nanoparticles in MOF-5 and MOFs in general is clearly a reasonable

continuation of the synthesis of metal or metal oxide nanoparticles in MOFs performed so far.

However, especially the adjustment of a certain molar ratio between the intercalated precursor

molecules in MOF-5, which is an important prerequisite for the synthesis of functional alloys,

proved to be rather difficult to achieve. When saturated loading, i.e. loading of MOF-5 with

two different precursor components in such a way, that all MOF-5 cavities are completely

filled with precursor molecules, was attempted, enrichment of the precursor component,

which is absorbed faster, was observed. However loading below the maximum loading level

of MOF-5 with the two precursor components [Ru(cod)(cot)] and [Pt(cod)Me2] was obtained

in a equimolar precursor ratio. In a similar way, it should be possible to load MOF-5 with

other two precursor components in a defined molar ratio.

Co-decomposition of two precursor components in MOF-5

In this work the co-hydrogenolysis of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 was

attempted. Quantitative decomposition of the precursors was achieved, however partial

hydrogenation of the MOF-5 bdc linkers occurred at the same time. Analysis of the obtained

composite material by TEM, revealed a non uniform distribution of Ru and Pt throughout the

MOF-5 host. Although, elemental and AAS analysis of the composite revealed an Pt/Ru = 1/1

ratio, EDX analysis revealed enrichment of Ru in parts of the nanocrystallites of the

composite material. This hints at an also non uniform distribution of the Ru and Pt precursor

molecules in MOF-5, prior to the hydrogenolysis. In addition, the hydrogenolysis of the Ru

precursor obviously occurs faster than that of the Pt precursor. Hydrogenolysis of

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 at mild conditions led to partial decomposition of the

Ru precursor molecules, whereas the Pt precursor molecules remained unchanged. The

formation of Ru nanoparticles prior to the start of formation of Pt clusters can also lead to Ru

enriched parts of the host framework and furthermore also complicates the formation of a

nanoalloy. The PXRD (and SAED) of the obtained Pt/Ru@MOF-5 composite revealed

reflections assignable to (fcc) Pt, with the reflection maxima however slightly shifted to

higher 2θ values. This finding is a hint at possibly alloyed Pt/Ru species, since PtRu alloys

adopt the (fcc) Pt structure up to 62 at.% of Ru. However, further TEM and also XAS


116

measurements need to be performed to examine the nature of the obtained species in more

detail.

Investigations of the MOF-5-water interactions

The interactions of 4 and 8wt.% water with MOF-5 were investigated by THz

spectroscopy in combination with PXRD, FT-IR and MAS-NMR spectroscopy. The loading

of MOF-5 with 8 wt.% H2O led to a complete conversion of the parent MOF-5 structure and

to the formation of a new phase. Upon loading with 4 wt.% of H2O, the MOF-5 structure was

remained, however, in the corresponding PXRD, additional reflections from the

decomposition product were detected. The analysis of the composites by THz spectroscopy

showed that loading of MOF-5 with 4 wt.% H2O is reversible, the water can be removed in

vacuum and the resulting material exhibited a THz spectrum akin to dry MOF-5. However,

coupling of the crystal modes of the framework and the absorbed water hydrogen bond

network motions was observed already in MOF-5 + 4 wt.% of H2O and was found to be even

more distinct in the case of MOF-5 + 8wt.%. This proves the sensitivity of MOF-5 towards

even small amount of water.

Future perspectives for the synthesis of metals@MOF-5

The obtained results on the properties of Ru@MOF-5 classify MOF-5 as a novel

stabilization matrix with properties beyond conventional porous support matrices. Especially

the weak interactions with the embedded nanoparticles, which still allow stabilization of very

small nanoparticles (< 3 nm) at the same time, assign MOF-5 to be a very fascinating support

matrix for nanoparticles, especially in catalytic reactions. However, the sensitivity of MOF-5

towards even small amount of water clearly limits the application of metal@MOF-5 to a

certain extent. For future investigations of Ru nanoparticles embedded in metal-organic

frameworks other suitable MOFs need to be investigated. Especially the ZIF materials appear

to be very promising stabilization matrices due to their extremely high chemical stability.

Also, unwanted side reactions with the imidazolate linkers during precursor hydrogenolysis

are rather unlikely. The rather small pore diameters of these materials (comparable to those of

zeolites) might be the only limitation in loading with MOCVD precursor molecules. Also

MIL-53 (Al based) and MIL-101 (Cr based) immerge as suitable support matrices for

nanoparticles.


117

The first results from co-hydrogenolysis of two precursor components in MOF-5 show the

general possibility of synthesizing bimetallic nanoparticles in MOFs. Different from co-

hydrogenolysis in solution which usually occurs under rapid stirring of the solution, the

reaction in MOFs is clearly limited by the distribution of the precursor molecules within the

host matrix. However, additional experiments need to be performed to investigate the

obtained already synthesized Pt/Ru species. Certainly, co-hydrogenolysis experiments at

various temperatures and pressures, followed by detailed TEM and XAS analysis have to be

performed in order to find the optimum reaction conditions for the formation of PtRu species

in an intact host matrix. The use of a more labile Pt precursor that undergoes hydrogenolysis

more readily might also be examined. Also the investigation of the synthesis of other

functional bimetallic nanoparticles in MOF, whose synthesis in solution is very well studied,

e.g. CuZn, AuPd, AuPt, appears feasible.

7. Experimental

118

7. Experimental

7.1. Analytical methods and instrumental details

7.1.1. Specific surface area determination from N2 adsorption measurements

Adsorption is defined as the concentration of gas molecules near the surface of a solid

material. It takes place because of the presence of an intrinsic surface energy. When a material

is exposed to a gas, an attractive force acts between the exposed surface of the solid and the

gas molecules. The result of these forces is characterized as physical adsorption. Regarding

the classification of pores according to their size,[150] MOF-5 with a pore opening of 7.8 Å[4]

can be regarded as microporous material. The IUPAC classification of adsorption isotherms is

shown in Figure 7.1. These adsorption isotherms are characteristic of adsorbents that are

microporous (type I), nonporous and macroporous (types II, III and VI) and mesoporous

(types IV and V). The adsorption by a mesopore is dominated by capillary condensation,

which is responsible for a sharp adsorption rise around the mid relative pressure region. This

effect is not attributable to molecule-solid interactions but to a purely geometrical

requirement, which is illustrated by the Kelvin equation.

Figure 7.1. IUPAC classification of adsorption isotherms.[17a]

7. Experimental

119

The adsorption in the micropore should not be considered as that of molecules onto a solid

surface but as the filling of molecules into a nanospace where a deep potential field is

generated by the overlapping of all wall potentials.[17a] In this case, the adsorption isotherm

shows a steep rise at very low relative pressure and a plateau after saturation. The differences

between the adsorption isotherms types II and III and between types IV and V arise from the

relative strength of fluid-solid and fluid-fluid attractive interactions. When the fluid –solid

attractive interaction is stronger than that of fluid-fluid, the adsorption isotherm should be of

types II and IV, the opposite situation leads to types III and V. The type VI isotherm

represents adsorption on nonporous or macroporous solid surfaces where stepwise multilayer

adsorption occurs.

The adsorption isotherms of microporous materials such as MOF-5 are described by the

Irving Langmuir model. The basic idea behind the Langmuir model is the coverage of the

surface by a monolayer of adsorbed molecules. Between the remaining free and the adsorbed

gas molecules a dynamic equilibrium will exist. Per time unit there will be as much molecules

adsorbing as desorbing. The rate of adsorption will be proportional to the equilibrium pressure

of the gas and the free surface. The Langmuir isotherm (for a given sorption capacity Θ,

sorption equilibrium constant K and pressure p) can then be described as:

pKpK⋅+

⋅=Θ

1 Equation 7.1

Analysis of adsorption isotherms of microporous materials according to the BET method[151]

is not suitable. The BET method is based on multilayer adsorption of gas molecules above

one monolayer (see adsorption isotherms types II and IV). The corresponding adsorption

isotherms do not exhibit a saturation plateau but a futher increase of adsorption at higher

pressures. The specific surface area of the microporous materials can be calculated by

linearization of the Langmuir equation (Equation 7.2):

0)(

11

0pp

CC

CVp

VmonoVmonopp⋅

⋅−

+⋅

=−

Equation 7.2

From the slope and intercept of the resulting graph, the constant C and the volume of the gas

monolayer can be determined. Assuming that a N2 molecule adopts an adsorption place of

7. Experimental

120

16.2·10–20 m2, the specific surface area of the given material can be calculated from Vmono.

Thereby the quality of the assumption of monolayer absorption can be deduced from the

goodness of fit between the regression line and the measured data.

The data derived from the Langmuir model may however overestimate the surface area of the

MOF materials. With respect to the large cavities of these materials, monolayer absorption

may not be strictly correct. Snurr et al. indicated that the BET model might be more suitable

for the determination of the MOF surface areas.[152]

Instrumental details

N2 sorption measurements were performed using a Quantachrome Autosorp-1 MP

instrument and optimized protocols by Susanne Buse, Department of Technical Chemistry,

Ruhr-University Bochum. The specific surface areas of the composite materials or empty

MOF-5 were calculated applying the Langmuir model in a pressure range of p/p0 = 0.1–0.3 at

-196 °C. Prior to the measurements, all samples were evacuated (10–6 mbar) at 110 °C for 3

h.

7.1.2. X-ray powder diffraction

The principle of X-ray diffraction is based on the irradiation of a crystalline powder

sample by monochromatic X-ray photons and the subsequent elastic scattering of the photons

by atoms in a periodic lattice.[153,154] The scattered photons, which are in phase, exhibit a

constructive interference. The angles, under which the photons leave the crystal, are

characteristic for each lattice plane. This dependence is described by Bragg’s law (Equation

7.3):

)sin(2 θλ ⋅⋅= d Equation 7.3

where λ is the wavelength of the X-ray photons, d is the distance between two lattice planes

of the same orientation in the crystal, and θ is the angle between the reflected X-ray and the

lattice plane. The XRD pattern is measured with a stationary X-ray source and a moveable

detector, which scans the intensity of the diffracted radiation as a function of the angle

2θ between the incoming and the diffracted beams. The observed intensity maxima in the

XRD pattern represent the lattice planes of the crystal geometry of the sample. The reflection

width depends on the crystallinity, i.e. the long range order of the sample, as well as on the

7. Experimental

121

size of crystallites. The relation of the crystal size and the reflection width is expressed by the

Scherrer equation (Equation 7.4):

)cos(θβλ

⋅⋅

=Kd

Equation 7.4

where d is the average crystallite size, K is a constant, which varies for different crystallite

shapes (for spherical crystallites K ~ 1), λ is the X-ray wavelength, β is the full width at half

maximum (FWHM) of a reflection (in radian), and θ is the position of the reflection. With

decreasing crystallite size, e.g. in nanoparticles, the X-ray reflections become broader, due to

an incomplete destructive interference of scattered photons that are out of phase, and the

FWHM cannot be accurately measured. Thus, for small crystallites (< 10 nm), the size

determination via Scherrer’s equation is only a rough estimation. As in the metal@MOF-5

composites discussed in this work, if crystallites become too small, they have no longe-range

order, and must be regarded as either amorphous or nanocrystalline. The X-rays are not

diffracted in phase, and there is no constructive periodic interference of the reflected X-rays,

but rather a diffuse scattering in all directions of space.


All powder X-ray diffractogramms were recorded by the author of this work on a D8-

Advance Bruker-AXS-diffractometer (Cu-Kα radiation 1.51478 Å, accelerating voltage: 40

kV, heating current: 30 mA, scan step: 0.0141 2θ) in Bragg-Brentano θ-2θ geometry, using a

Göbel mirror as monochromator and a position sensitive detector. The detector was calibrated

to the reflections of crystalline α-Al2O3. The specimens (powder) were prepared in

Lindemann capillaries in the glovebox (diameter: 0.5 or 0.7 mm). The capillaries were flame

sealed prior to the measurements.

7.1.3. Transmission electron microscopy

Transmission electron microscopy is a straightforward technique to determine the size

and shape of nanoparticles.[153,154] Beyond the visual observation of the size, geometry and

dispersion of the particles, TEM can also reveal information on the composition and further

structural features. Highly crystalline nanoparticles, which exhibit a long-range order of

crystallinity, allow further structural characterization. State-of-the-art TEM instruments

exhibit a level of magnitude in the interatomic range, so that the lattice planes of the material

7. Experimental

122

become visible, from which the lattice constants can be calculated. Besides, the TEM electron

beam can be employed for diffraction on a selected particle area (SAED), which is, in

principle analogous to X-ray diffraction. The obtained pattern appears as diffraction rings or

distinct dots, which contain information on the measured sample. However, if the particles are

amorphous or too small, no pattern will be observed. By energy dispersive X-ray

spectroscopy (EDX), the atoms in the specimen can be excited by the electron beam to release

an X-ray photon upon relaxation, which is characteristic for all elements. In principle, the

detected photon intensities in the sample can be quantified, giving information on the

quantitative composition of the sample. However, to allow quantification, great care during

the measurement has to be taken, since i.e., the intensities of the detected signals are

dependent on the measurement time.


The TEM measurements presented in this work were mainly carried out at the

University of Antwerp, Belgium by Dr. Oleg I. Lebedev and Stuart Turner, M.Sc.. Bright-

field TEM and ED experiments were performed on a Philips CM20 microscope, operated at

200kV. High resolution TEM was performed on a JEOL 4000EX, operated at 400 MHz and

having a 1.7 Å resolution. Tomography and HAADF-STEM experiments were performed on

a JEOL 3000FTEM-STEM microscope, operated at 300 kV with a -70° to +70° tomography

tilt stage and holder. Images for tomographic reconstruction were taken using a 2° interval,

over the largest possible angle (preferably 140°). A reference image was taken at 0° tilt before

and after image acquisition, to ensure no changes in the sample structure due to beam damage

occurred during acquisition. Tomographic reconstruction was performed using the MATLAB

tomography toolbox. HAADF-STEM images were taken at a nominal spot size of 0.5 nm.

The HAADF inner collection semiangle was 35 mrad. For all techniques, low intensity beam

conditions (lowest possible magnification, low beam intensity and short exposure times) were

used as much as possible to minimize the electron dose and possible beam damage of the

MOF-5 host. The images for the tomographic acquisition were taken in bright field TEM

instead of HAADF-STEM as prolonged STEM illumination damaged the samples.

Also measurements at the Hahn-Meitner Institute in Berlin on a Philips CM30 instrument

with an accelerating voltage up to 300 kV were performed by T. Hikov, M.Sc. In addition,

also TEM measurements on a Hitachi-H-8100 instrument (accelerating voltage up to 200 kV)

were performed by X. Zhang, M.Sc. at the Ruhr-University, Bochum. All samples were

prepared on Cu grids (300 mesh, holey carbon films) under inert gas atmosphere in a glove

7. Experimental

123

box by suspending the composite powders in dry n-pentane, then placing a few drops on the

Cu grids and allowing drying in the glovebox for at least 12 h.

7.1.4. X-ray absorption spectroscopy

Whereas PXRD requires a long range order of the specimen crystallites, XAS allows a

determination of the local environment of an atom. The principle of XAS is based on the

irradiation of atoms with highly monochromatic X-rays, which leads to absorption, as long as

the binding energy EB of the core electron is higher than the photon energy. In the case that

the photon energy is sufficient to eject the electron from a core orbital, characteristic

absorption edges appear. XAS studies are typically performed at 5–30 KeV, which is for most

atoms in the range of photoelectron emission of the K orbital. The scattered photoelectron

itself can be scattered back from a neighbor atom. Since electrons have both particle and wave

character, the emitted, backscattered photoelectrons interfere, which is observed in the fine

structure oszillations, spanning a range of several hundred eV behind the edged. These

oscillations contain structural information, since the photoelectrons are scattered back from

either atoms of the same kind, but different lattice position, or different atoms. The resulting

X-ray absorption spectrum (Figure 7.2) exhibits two major areas: the X-ray absorption near

edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS). The

XANES gives information on the oxidation state of the measured atom and its

Figure 7.2. Illustration of a typical X-ray absorption spectrum with an absorption edge for Eb = hν.

7. Experimental

124

coordination geometry. The EXAFS exhibits data on the type of nearest neighbor and the

distance of nearest neighbor shells. Besides, for an identification of the measured material,

EXAFS spectra can be compared with calculated EXAFS spectra of reference compounds.

The EXAFS-function χ(k), which contains the structural information of the sample, is

expressed by Equation 7.5:

( )∑ +⋅=j

jjj )k(krsin)k(A)k( φχ 2 , with ( )be Ehmh

k −= νπ 22 Equation 7.5

where k is the wavenumber of the photoelectron, h is the Planck constant, me is the mass of an

electron, j is the label of the coordination shells around the electron-emitting atom, rj is the

distance of the electron-emitting atom and the atoms in the jth shell, and φ(k) is the phase shift

of the absorbing and the backscattering atoms. Aj(k) is the amplitude, i.e. the scattering

intensity, caused from j coordination shells (Equation 7.6), which contains the most desirable

information. Nj is the coordination number of atoms in the jth shell, S0 is the correction for

relaxation effects in the emitting atom, Fj is the backscattering factor of atoms in the jth shell,

( )2220 2

2

jjj

j

jj kexp)k(F)k(Srk

)k(r

expN)k(A σ

λ⋅−⋅⋅⋅

⋅

⎟⎟⎠

⎞⎜⎜⎝

⎛ −

⋅= Equation 7.6

λ is the mean free path of the photoelectron, and σ2 is the mean squared displacement of atoms

on the specimen. The radial distribution function of the EXAFS function χ(k) is obtained by a

Fourier transformation, giving the information of the distance of the neighbour atoms of the

different coordination shells.


All XAS measurements were performed on powder samples that were prepared inside

a glove box by Dr. M. W. E. van den Berg, Department of technical Chemistry, Ruhr-

University Bochum. The absorption edge of Ru at 22117 eV was measured at Hasylab X1

station (Hamburg, Germany). This beamline was equipped with a Si(311) double-crystal

monochromator that was used to detune to 50% of the maximum intensity in order to exclude

higher harmonics present in the X-ray beam. Samples were transferred to the synchrotron

station under argon, and pressed together with dry boron nitride into wafers inside an argon-

filled glove box. The wafers were encapsulated by Kapton sticky tape. Immediately preceding

7. Experimental

125

the recording of XAS spectra, samples were cooled rapidly to liquid nitrogen temperature.

The spectra μ(k) were measured in transmission mode using ionisation chambers. A thin self-

supporting wafer of RuO2 (between the second and the third ionisation chamber) was

measured at the same time for energy calibration purposes (E0 = 22130 eV). Data treatment

was carried out using the software package VIPER.[155] For background subtraction, a

Victoreen polynomial was fitted to the pre-edge region. A smooth atomic background μ0(k),

was evaluated using smoothed cubic splines. The radial distribution function FT[k2χ(k)] was

obtained by Fourier transformation of the k2-weighted experimental function χ(k) = {μ(k)-

μ0(k)} / μ0(k) multiplied by a Bessel window. The k-range was chosen to be from 3.60 to

13.95. Duplicate spectra were recorded to ensure data reproducibility.

7.1.5. Solid State Nuclear Magnetic Resonance-general

As described in this work, solid materials can be measured by solid-state NMR.

However, solid samples consist of many crystallites that have random anisotropic

orientations, and are thus not equally orientated upon application of an external magnetic

field. Dipole-dipole and quadrupole-field-gradient interactions cannot be neglected. This leads

to an extreme signal broadening, in contrast to high-resolution NMR in solution, where the

resonances adopt an average frequency, due to a rapid tumbling of the molecules. In the solid

state, the reorientation of nuclei can be stimulated by magic-angle-spinning (MAS) NMR.

The heteronuclear interaction Hamiltonian, which describes the dependence of the chemical

shift and the dipolar coupling from the angle of orientation to the magnetic field, is given in

Equation 7.7:

( )134

23

0 −⋅⋅⋅⋅⋅

⋅⎟⎠⎞

⎜⎝⎛−= )(cosSI

rH zz

SIheteroDD θγγ

πμ h Equation 7.7

where m0 is the magnetic momentum of the dipole, γI and γS are the magnetogyric ratios of the

spins I and S, ZI and ZS are the operators of the z component of each nuclear spin, r is the

distance between the two dipoles, and θ is the orientation angle of the dipole upon application

of a magnetic field. If the term [3cos2(θ)-1] = 0, then the contribution of the dipole-dipole

interaction is zero, which means that the line broadening effect of dipolar coupling is

removed. The angle θ, for which the above term is zero, is called the Magic Angle (θ =

7. Experimental

126

arcos(3-1/2) ≈ 74.74°). As a consequence, fast spinning of a solid sample (> 5 KHz) around an

axis in a magic angle of 54.74° from the external magnetic field, thus averaging the

anisotropy of the nuclei and the dipole-dipole coupling, and resulting in line sharpening,

which is comparable to high resolution NMR in solution.

2H Solid State NMR

The basic theory of solid state 2H-NMR is well-documented[156] and only briefly

summarized here. A detailed discussion of its application to the study of hydrogen interacting

with transition metal complexes is given in a recent review.[157] The leading interaction in 2H

solid state NMR is the quadrupolar interaction. In high field a pair of orientation dependent

resonance frequencies is observed:

( )2 21 3cos 1 sin cos 22Q zzQω ϑ η ϑ ϕ± = ± − − Equation 7.8

The asymmetry parameter η gives information about the shape of the electric field gradient

and Qzz is a measure for the strength of the quadrupolar interaction, ϑ and ϕ are the azimuth

and polar angles with respect to the main magnetic field B0. In a non-oriented powder sample

the average over all possible orientations have to be calculated by integration over the polar

angles ϑ and ϕ, which gives rise to the well-known Pake pattern. The quadrupolar coupling

constant Qcc is obtained from the experiment as

4

3cc zzQ Q= Equation 7.9

The parameters of the quadrupolar interaction are extracted from line-shape analysis,

employing laboratory written Matlab programs which are based on the theory of solid state 2H

NMR described elsewhere.[158] The Matlab program used in for the spectra presented in this

work, allows simulation of spectra consisting of up to 8 different sub spectra taking into

account effects of finite band width excitation by the RF-pulses.

7. Experimental

127


Solid state MAS-NMR spectra were recorded on a Bruker DSX 400 MHz instrument in

ZrO2-rotors (Ø = 2.5 mm) with rotational frequencies of 20 kHz by the author of this work.

All 13C MAS-NMR spectra were measured applying cross polarization (CP) pulse programs

written by H.-J. Hauswald at the Analytical Chemistry Department at the Ruhr-University

Bochum and based on standard parameters. All 2H solid state wide line NMR experiments

were performed at the institute of Physical Chemistry of the Freie Universität Berlin,

Germany at a field of 7.03 T at a 2H resonance frequency of 46.03 MHz. An Oxford wide

bore magnet (89 mm) equipped with a room-temperature shim unit was used. The home-build

three channel spectrometer has been described recently by Buntkowsky et al.[159,160] For the

experiments a home-build 5 mm 2H NMR probe was used.[161] Low temperature

measurements were performed in a dynamic Oxford CF1200 helium flow cryostat. An Oxford

ITC 503 temperature controller was used to control the temperature. The 90o pulse width was

5.5 μs. Owing to the weakness of the signal the solid echo technique with an echo spacing of

60 μs was used to suppress artifacts from the RF pulses. The repetition time of the

experiments was 1s. To acquire the deuterium powder patterns with reasonable signal-to-

noise ratio between 6000 and 32000 scans per spectrum were accumulated.

All NMR spectra in solution were recorded by the author of this work on a Bruker PPX 250

spectrometer.

7.1.6. THz spectroscopy

The THz spectra presented in this work were recorded and evaluated by Dr. Konstanze

Schröck at the Chair of Physical Chemistry II, Ruhr-University Bochum. As a radiation

source for the short pulses a titanium-sapphire (Ti:Sa) femtosecond (fs) laser from KMLabs

Inc., pumped by a 532 nm Verdi (CoherentTM) laser was applied. The mode locked Ti:Sa laser

emitted pulses at ≈ 800 nm with a 25 fs pulse width, a repetition rate of 80 MHz and an

average output power of 600 mW. The laser beam was split into two parts, which allowed a

coherent generation and detection of the THz pulse. One train of the fs beam was focussed on

the emitter which is a low temperature grown gallium arsenide (LTG-GaAs) photoconductive

antenna from GigaopticsTM.[162] A bias voltage of 50 V is applied to the antenna and the

impinging of the fs pulses results in the generation of free charge carriers in the conduction

band of the semiconductor that are accelerated and hence THz radiation is emitted. The THz

7. Experimental

128

beam was focussed by a parabolic mirror onto the sample and was finally focussed onto a

ZnTe crystal. The whole THz setup was enclosed in a box that was purged with dry air to

avoid the absorption of certain frequencies by water vapour.[163] Free-space electro-optic

sampling was used for detection.[164] When applying an electric field to a nonlinear crystal, the

optical properties of this medium are changed due to the Pockels effect. The nonlinear crystal

becomes birefringent and the change is proportional to the electric field. The transmission of

the THz pulse within the birefringent ZnTe crystals leads to a rotation of the linear polarized

fs beam. The propagation through a quarter waveplate results in the change from the linear to

an elliptic polarisation. The elliptic light is splitted by a Wollaston prism into a horizontal and

a vertical polarized beam. The change of polarization was detected using photobalanced

detection.[165,166] In Figure 7.3 a sketch of the THz setup is shown.

We have recorded eight scans for MOF-5, PE and air, respectively and averaged for data

analysis. An integration time of 20 ms at the Lock-In amplifier was used.

For sample preparation an amount of 100 mg polycrystalline MOF-5 powder was ground

together with an amount of dry 100 mg polyethylene (PE) powder (PE U.H.M.W. powder,

150 mikron, Goodfellow) inside a glove box (Ar). Pellets with a diameter of 13 mm and a

thickness of 1 to 1.5 mm were pressed at 10 tons for ≈ 3 min with a manual hydraulic press

from Perkin Elmer Inc.. The measurements were carried out using a PE pellet as a reference

that was prepared in the same way.

Figure 7.3. The scheme shows the THz TDS setup that is enclosed in a box and purged with dry air. The

fesce laser is focussed onto a a photoconductive antenna (Em) which emits THz radiation is

emitted. The THz pulse probes the sample and is the detected using electro optic sampling

(EOS).

7. Experimental

129

7.1.7. IR spectroscopy

FT-IR spectra were recorded as KBr pellets using a Perkin-Elmer 1720x and a Bruker

Alpha spectrometer. The samples were prepared in a glove box (MBraun, O2 and H2O

continuously monitored with levels below 1 ppm). KBr was dried at 300 °C in 10-3 mbar for

16 h prior to specimen preparation. All measurements were performed by the author of this

work.

7.1.8. Elemental/Atom Absorption analysis

Elemental analysis was performed by the Analytical Laboratory of the Catalysis

Research Center of Süd Chemie AG, Heufeld, Germany, using a Spectro Modula

spectrometer. The measurements were performed by standard protocols employing

Inductively Coupled Plasma Atom Emission Spectroscopy (ICP-AES). Also elemental

analyses were performed at the Ruhr-University Bochum, using a Vario 6 AAS instrument. In

all cases, the samples were dissolved in aqua regia prior to the measurements.

7.1.9. Gas chromatography-mass spectroscopy

GC-MS measurements were performed on a HP MSD GC/MS spectrometer by J. Schäfer

and S. Bendix from the Department of Analytical Chemistry, Ruhr-University Bochum.

7. Experimental

130

7.2. Syntheses of the materials

All manipulations and chemical reactions were conducted using Schlenk-line and

glove box techniques (Ar, H2O, O2 < 1 ppm) and sealed Fischer-Porter vessels (Andrews

Glass, volume: 90 mL). All solvents were catalytically dried, deoxygenated and saturated with

argon using an automatic solvent purification system by MBraun. The residual water content

was determined by Karl-Fischer titration exhibiting levels of 1 ppm. For the loading of the

MOF-5 materials, home build glass tubes and glass vial were used. (see Figure 7.4)

Figure 7.4. Glass tube set-up for MOF-5 loading in vaccum.

The following precursors were synthesized according to literature:

• [Ru(cod)(cot)][167]

• [CpPd(η3-C3H5)][168]

• [CpPtMe3][169]

• [Pt(cod)Me2][170]

The precursor [Fe(η6-toluene)(η4-C4H8)] was kindly provided by the group of Prof. U.

Zenneck, University of Heidelberg.

7. Experimental

131

7.2.1. Synthesis of MOF-5 ([Zn4O(bdc)3]) powder

0.665 g (4 mmol) terephthalic acid and 3.14 g (15 mmol) Zn(NO3)2 · 4 H2O were

dissolved in 100 ml of DEF in a 100 ml glass jar with a teflon tined lid. The reaction mixture

was heated in an oven at 105 °C for 12 h to yield large (2 mm) cube shaped colorless crystals.

After cooling to room temperature, the excess solvent was decanted and quickly exchanged

by 100 ml chloroform to avoid contact with most air. The procedure was repeated three times

to remove residual DEF absorbed by the crystals, followed by stirring of the crystals in 100

ml chloroform at room temperature overnight. The obtained off-white powder was filtered via

a glass frit under argon atmosphere and dried in dynamic vacuum (10-3 mbar) at 110 °C

overnight. Yield: 0.92 g (90.3 % based on terephthalic acid).

Elemental analysis calculated for analysis calculated for Zn4O13C24H12 (wt %): Zn 34.0, C

37.4, H 1.6, O 27. Found (wt %): Zn 33.9, C 37.2, H 1.6 (O 27.3 calcd. from difference to 100

%).

7.2.2. Synthesis of MOF-5 ([Zn4O(bdc)3]) crystals

0.33 g (2.0 mmol) terephtalic acid and 1.8 g (6.1 mmol) Zn(NO3)2 · 6 H2O were

dissolved in 50 ml DEF in a glass jar with a teflon tined lid. The mixture was first heated in an

oven at 80 °C for 12 h and then at 105 °C until formation of small crystals (below 1 mm) was

observed. After cooling to room temperature the supernatant solvent was decanted and

quickly exchanged by 50 ml of chloroform. The procedure was repeated three times and the

crystals were immersed in 100 ml chloroform for 48 h. For activation, the crystals, together

with the supernatant chloroform were quickly transferred to a Schlenk tube under argon

atmosphere. Excess chloroform was then removed by a syringe. The crystals were then

evacuated (10-3 mbar) very carefully at room temperature until absorbed chloroform was

removed and then dried in dynamic vacuum (10-3 mbar) at 110 °C overnight. Yield: 0.37 g

(73 % based on terephthalic acid).

Elemental analysis calculated for Zn4O13C24H12 (wt %): Zn 34.0, C 37.4, H 1.6, O 27. Found

(wt %): Zn 33.8, C 37.7, H 1.7, (O 26.8 calcd. from difference to 100 %)

Here only the analytical data of the MOF-5 crystals is presented, the data of the MOF-5

powder is in good agreement.

7. Experimental

132

Figure 7.5. FT-IR spectrum of MOF-5 [(Zn4O(bdc)3] in dry KBR.

IR: ν~ max KBr (cm-1): 1572 (s), νas(COO), 1506 (m), νas(COO), 1391 (vs), νsym(COO), 1316

(vw), 1176 (vw), 1154 (w), 1108 (w), 882 (w), 831 (w), 747 (w), 533 (m) ν(ZnO)

Figure 7.6. Powder X-ray diffractogramm of MOF-5 [(Zn4O(bdc)3] (argon filled).

7. Experimental

133

PXRD: Capillary: 2θ (°) (Intensity): 6.7 (100), 9.6 (29.0), 11.3 (4.5), 13.6 (17.2), 14.9 (4.4),

15.2 (10.4), 16.8 (2.7), 17.8 (3.9), 19.4 (3.4), 20.3 (5.6), 20.6 (3.9), 22.5 (5.2), 24.5 (4.5), 25.7

(2.6), 26.5 (6.9), 28.2 (3.0), 29.3 (2.2), 29.9 (4.7), 31.5 (4.1), 33.0 (2.0), 34.5 (3.6), 35.4 (2.0),

36.0 (2.3), 37.3 (2.0), 38.7 (1.2), 39.9 (1.5), 41.2 (1.7), 42.6 (2.7), 43.2 (2.1), 43.6 (1.4), 44.3

(1.3), 44.9 (1.7), 45.5 (1.2)

13C MAS-NMR: δ (ppm): 175.1 (COO), 136.3 (C(COO)), 130.0 (C6H4)

Figure 7.7. 13C MAS-NMR spectrum of dry, argon filled MOF-5.

The equivalent Langmuir surface area was determined to 3300 m2/g which is in accordance to

the literature report.12

7.2.3. Synthesis of [Ru(cod)(cot)]3.5@MOF-5

In a typical experiment, a sample of 50 mg (0.065 mmol) dry, activated MOF-5

powder and 100 mg (0.32 mmol) [Ru(cod)(cot)] were placed in two separate glass vials in a

Schlenk tube. The tube was then evacuated (10-5 mbar, turbo molecular pump) for 10 min,

sealed and kept at 30 °C for 6 days in static vacuo. A yellow composite material denoted as

[Ru(cod)(cot)]3.5@MOF-5 was obtained and stored in the glove box afterwards. Yield: 123

mg (98.5 % based on Ru).

7. Experimental

134

Elemental analysis calculated for [Ru(cod)(cot)]3.5@MOF-5 (wt.%): Ru 18.8, Zn 13.9, C

51.2, H 4.8 (O 11.3 calcld. from difference to 100 %). Found: Ru 18.7, Zn 14.2, C 51.1, H 4.7

(O 11.3, calcd. from difference to 100 %).

IR: ν~ max KBr (cm-1): 3008 (vw), 2964 (vw), 2923 (w), 2866 (w), 2817 (w), 1969 (w), 1603

(s), 1506 (m), 1432 (m), 1396 (vs), 1336 (vw), 1321 (w), 1162 (w), 1069 (w), 1027 (w), 887

(w), 831 (w), 746 (m). 13C MAS-NMR: δ (ppm): 175.3 (COO MOF-5), 138.5 (C(COO) MOF-5), 131.3 (C6H4

MOF-5), 101.4 (C olefin. cot), 99.1 (C olefin. cot), 76.7 (C olefin. cot), 70.1 (C olefin. cod),

33.7 (C aliphat. cod), 31.6 (C aliphat. cot).

PXRD: Capillary: 2 θ (°) (Intensity): 6.7 (30.1), 9.6 (57.0), 11.3 (25.8), 11.9 (97.3), 12.3

(27.8), 12.8 (32.0), 13.8 (100), 14.1 (49.6), 14.5 (47.5), 15.0 (34.0), 15.3 (73.9), 15.7 (32.0),

16.1 (23.1), 16.8 (43.9), 17.5 (36.5), 17.9 (25.4), 18.5 (24.8), 19.4 (26.8), 20.3 (33.0), 22.6

(21.4), 24.7 (21.9), 25.9 (22.4), 26.5 (39.7), 28.5 (22.3), 29.4 (18.9), 30.0 (20.0), 31.6 (24.4),

33.0 (16.3), 34.6 (19.5), 35.4 (14.6), 37.3 (14.2), 38.7 (13.6), 40.0 (13.0), 42.6 (12.7), 43.2

(14.7)

7.2.4. Hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5 at mild conditions

A sample of 50 mg of the inclusion compound [Ru(cod)(cot)]3.5@MOF-5 was placed

in a glass tube and exposed to a stream of pure H2 gas (1 sccm, 99.999 %) at 25 °C for 30

min. A fast color change from yellow to dark brown was observed. Yield: 48 mg.

Elemental analysis (wt.%): Ru 19.2, C 49.2, H 5.2 (O 26.4 calcd. from difference to 100 %).

IR: ν~ max KBr (cm-1): 2922 (w), 2854 (w), 1606 (s), 1508 (m), 1432 (m), 1396 (s), 1322 (vw),

1293 (vw), 1262 (vw), 1158 (vw), 1106 (vw), 1020 (w), 884 (w), 824 (w), 746 (m). 13C-MAS-NMR: δC (ppm): 177.5 (COO, η6-terephthalate), 175.0 (COO MOF-5), 136.8

(C(COO) MOF-5), 130.5 (C6H4 MOF-5), 91.8 (C(COO) η6-terephthalate), 81.4 (C6H4 η6-

terephthalate), 68.1 (cod, olefin), 33.4 (cod, aliphat.), 27.3 (cyclooctane).

PXRD: Capillary: 2 θ (°) (Intensity): 11.4 (5.6), 13.7 (100), 15.0 (10.4), 15.4 (53.0), 16.8

(2.25), 17.9 (11.2), 19.4 (5.6), 20.4 (18.8), 22.6 (14.7), 24.6 (11.3), 25.8 (3.3), 26.5 (21.7),

28.3 (4.8), 29.4 (2.2), 30.0 (12.8), 31.6 (11.5), 33.1 (2.7), 34.5 (9.4), 35.4 (3.9), 36.0 (5.1),

37.4 (7.3), 38.7 (1.0), 40.0 (4.7), 41.3 (3.4), 42.0 (5.2), 42.6 (11.0), 43.2 (7.4), 45.0 (5.3), 50.0

(2.6), 50.5 (2.4), 55.6 (2.5), 56.1 (1.7), 57.5 (1.3), 62.2 (2.1), 64.9 (1.4)

Langmuir Surface area (calculated from N2 sorption measurements): 1600 m2/g

7. Experimental

135

7.2.5. Synthesis of Ru@MOF-5 by quantitative hydrogenolysis of [Ru(cod)(cot)]3.5@MOF-5

A sample of 100 mg (0.054 mmol) of the inclusion compound

[Ru(cod)(cot)]3.5@MOF-5 was placed in a Fischer-Porter-Bottle and evacuated for 5 min

(dynamic vacuo, 10-3 mbar). The bottle was then filled with 3 bar of H2 gas (99.999 %) at 25

°C and heated to 150 °C for 48 h. A color change from yellow to brown was observed within

the first 15 minutes. Some slight further darkening was visible during the treatment. After

cooling to room temperature, H2 was removed in dynamic vacuo (10-3 mbar, 15 min) and

thereafter the bottle was refilled with Ar (1 mbar). Yield, 61 mg (0.054 mmol, 100 %),

Elemental analysis (wt.%) calculated for Ru3.5@MOF-5.: Ru 31.5, Zn 23.3, C 25.7, H 1.08

(O 18.4, calcd. from difference to 100 %). Found: Ru 30.6, Zn 23.1, C 25.5, H 1.06 (O, 19.7,

calcd. from difference to 100 %).

IR: ν~ max KBr (cm-1): 1576 (s), 1505 (m), 1390 (s), 1259 (w), 1176 (w), 1153 (w), 1095 (w),

1017 (w), 878 (w), 829 (w), 810 (w), 745 (m). 13C MAS-NMR: δ (ppm): 175.1 (COO MOF-5), 136.9 (C(COO) MOF-5), 130.6 (C6H4

MOF-5).

PXRD: Capillary: 2 θ (°) (Intensity): 11.3 (3.9), 11.8 (15.8), 13.7 (77.4), 15.3 (100), 16.8

(6.6), 17.8 (27.8), 19.4 (36.9), 20.3 (51.2), 20.6 (24.9), 22.6 (31.5), 22.8 (14.2), 24.6 (30.9),

25.8 (6.6), 26.5 (44.2), 27.0 (7.8), 28.3 (11.3), 29.3 (3.8), 30.0 (22.7), 31.6 (21.7), 33.1 (5.2),

34.6 (17.9), 40.0 (11.3), 41.3 (11.4), 42.0 (18.3), 42.6 (28.9), 43.2 (26.0), 43.7 (25.6), 44.0

Ru[101] (23.3), 44.4 (23.0), 44.9 (18.6), 50.0 (4.5), 50.5 (4.3), 55.6 (5.5), 56.1 (2.9), 57.6

(2.9), 62.3 (3.45), 65.0 (2.9)

Langmuir surface area (calculated from N2 sorption measurements): 860 m2/g

For the calcinations experiments 40 mg of Ru@MOF-5 were filled into a glass ampoule in the

glove box. The ampoule was evacuated(10-3 mbar) and then sealed. The calcinations were

performed at 200 °C, 400 °C and 500 °C for 14 h by placing the ampoules in an oven.

7.2.6. Deuterium adsorption at Ru@MOF-5, sample preparation for solid state 2H-NMR

measurements

A sample of 200 mg Ru@MOF-5 was placed in a NMR tube and evacuated for 15 min

(10-7 mbar, turbomolecular pump). Subsequently, 1 bar of D2 gas was added at 25 °C for 1 h.

7. Experimental

136

Then, the D2 gas was removed in vacuo (10-7 mbar, 25 °C) and the sample was evacuated for

5 more minutes before the NMR tube was flame sealed.

7.2.7. CO adsorption at Ru@MOF-5 for FT-IR measurements

A sample of 50 mg of Ru@MOF-5 was filled in a Fischer-Porter-Bottle and evacuated

for 5 min (at 10-3 mbar, oil pump). Then, the material was treated with 1 bar of CO for 30 min

at 25 °C in the sealed bottle. Afterwards, the non absorbed gas was removed in vacuo (10-3

mbar, 25 °C), the sample was introduced to a glove box immediately and a KBr pellet for FT-

IR measurements was prepared inside the box.

IR: ν~ max KBr (cm-1): 2922 (vw), 2854 (vw), 2000 (m), 1890 (m), 1590 (s), 1506 (m), 1390

(s), 1310 (m), 1250 (vw), 1153 (vw), 1018 (vw), 882 (vw), 824 (vw), 811 (vw), 781 (vw), 745

(m)

7.2.8. Oxidation of benzyl alcohol using oxidized Ru@MOF-5 as catalyst

A sample of 20 mg (0.011 mmol) of Ru@MOF-5 was exposed to a stream of 4 vol.%

O2 diluted in argon (1 sccm) at room temperature for 30 min.

Figure 7.8. Powder X-ray diffractogramm of Ru@MOF-5 after oxidation in 4 vol.% O2 diluted in argon at

25 °C, 30 min.

7. Experimental

137

PXRD: Capillary: 2 θ (°) (Intensity): 13.6 (100), 14.9 (11.9), 15.3 (69.2), 16.7 (3.58), 17.8

(13.7), 19.4 (14.4), 20.3 (31.8), 20.6 (14.7), 22.5 (22.0), 22.8 (7.2), 24.6 (18.5), 25.8 (6.1),

26.5 (35.7), 28.3 (8.9), 29.3 (3.6), 30.0 (22.8), 31.5 (18.0), 33.0 (3.7), 34.5 (14.4), 35.4 (5.9),

36.0 (8.0), 37.4 (7.6), 40.0 (6.4), 41.2 (3.5), 42.0 (5.32), 42.6 (12.7), 43.2 (8.0), 45.0 (7.1),

50.0 (4.0), 55.6 (5.64)

The obtained powder was then introduced into a Schlenk tube, suspended in 5 ml of toluene

and 0.1 mL of benzyl alcohol were added. The solution was saturated with O2 and stirred for

48 h at 80 °C. Then the suspension was filtered and the filtrate was analyzed by GC-MS.

PXRD analysis of the remaining catalyst powder after the catalytic reaction showed complete

decomposition of the MOF-5 matrix.

7.2.9. Hydrogenation of benzene using Ru@MOF-5 as catalyst

A sample of 50 mg Ru@MOF-5 (0.011 mmol) of Ru@MOF-5 were placed in a

Fisher-Porter bottle and 0.7 ml of benzene (7.8 mmol) were added. After short evacuation, 3

bar of H2 gas (99.999 %) were added and the mixture was heated to 75°C for 20 h. 1.43 bar of

H2 were consumed after 20 h, prolonged heating did not lead to additional gas consumption.

The reaction mixture was investigated by 1H NMR.

Figure 7.9. Powder X-ray diffractogramm of Ru@MOF-5 after hydrogenation catalysis (Lines: Ru JPDS

reference No. 6-0663).

7. Experimental

138

1H NMR (C6D6): δ (ppm): 7.27 (18 H, C6H6), 1.53 (12H, C6H12)

PXRD: Capillary: 2 θ (°) (Intensity): 11.8 (11.5), 13.6 (94.0), 15.3 (100), 17.8 (18.0), 19.4

(49.5), 20.3 (50.0), 20.6 (30.0), 22.5 (33.2), 22.8 (17.0), 24.6 (26.8), 25.8 (6.9), 26.5 (37.9),

28.3 (13.5), 30.0 (18.7), 31.5 (24.7), 34.5 (19.0), 35.4 (10.1), 36.0 (9.6), 37.3 (6.2), 38.6 (4.0),

40.0 (8.8), 41.2 (9.8), 42.0 (14.4), 42.6 (22.3), 43.2 (18.6), 45.0 (12.7), 50.0 (5.6), 50.5 (3.6),

55.5 (5.5), 62.2 (3.6)

7.2.10. Synthesis of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5

100 mg (0.13 mmol) of dry MOF-5 powder, 120 mg (0.39 mmol) of [CpPtMe3] and 79

(0.39 mmol) mg of [Fe(η6-toluene)(η4-C4H6)] were placed in three different glass vials in a

Schlenk tube. The tube was evacuated for 5 min (10-3 mbar), sealed and kept at 25 °C for 12

h. A red-brown composite denoted as [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5 was

obtained. Yield: 212 mg (95 % based on the amounts of absorbed Fe and Pt precursor

molecules).

Elemental Analysis (wt %): Fe 7.5, Pt 18.5, C 43.1, H 3.9; deduced number of precursor

molecules per MOF-5 cavity: n([Fe(η6-toluene)(η4-C4H6)]) : n([CpPtMe3]) = 2.2 : 1.6,

resulting overall stochiometric ratio of [Fe(η6-toluene)(η4-C4H6)] : [CpPtMe3] = 1.4 : 1

IR: ν~ max KBr (cm-1): 3026 (w), 2970 (w), 2920 (w), 1605 (s), 1505 (m), 1395 (vs), 1157

(vw), 1018 (w), 1105 (vw), 909 (vw), 885 (vw), 824 (w), 746 (m), 731 (m), 695 (w), 576 (w),

517 (m), 465 (vw) 13C MAS-NMR: δ (ppm): 174.9 ((COO) MOF-5), 137.0 (C(COO) MOF-5), 131.2 (C6H4

MOF-5), 96.3 ([Pt(η5-C5H5)(CH3)3]), 93.8 ([Fe(η6-C6H5CH3)(η4-C4H6)]), 84.5 ([Fe(η6-

C6H5CH3)(η4-C4H6)]), 82.8 ([Fe(η6-C6H5CH3)(η4-C4H6)]), 81.4 ([Fe(η6-C6H5CH3)(η4-

C4H6)]), 75.7 ([Fe(η6-C6H5CH3)(η4-C4H6)]), 33.8 ([Fe(η6-C6H5CH3)(η4-C4H6)]), 21.1

([Fe(η6-C6H5CH3)(η4-C4H6)]), -18.8 ([Pt(η5-C5H5)(CH3)3], J(Pt-C) = 706 MHz) 195Pt MAS-NMR: δ (ppm): -5224.9 ([Pt(η5-C5H5)(CH3)3])

PXRD: Capillary: 2 θ (°) (Intensity): 11.3 (11,8), 11.8 (37.2), 13.7 (10.2), 14.9 (20.8), 15.3

(40.2), 16.8 (18.1), 17.8 (100), 19.4 (32.2), 20.3 (19.0), 22.5 (5.3), 23.8 (5.6), 24.8 (5.1), 26.4

(6.3), 29.3 (7.0), 30.0 (13.5), 31.5 (6.1), 34.5 (4.8), 36.0 (9.7), 37.3 (6.7), 40.0 (3.2), 42.5

(5.6), 43.1 (6.6)

7. Experimental

139

Figure 7.10. FT-IR spectrum of [Fe(η6-toluene)(η4-C4H6)]/[CpPtMe3]@MOF-5 in dry KBr.

7.2.11. Synthesis of [CpPd(η3-C3H5)]/[CpPtMe3]@MOF-5

In a series of three loading experiments, 50 mg (0.06 mmol) of dry MOF-5 powder, X

mg of [CpPd(η3-C3H5)] and Y mg of [CpPtMe3] were placed in two different glass vials in a

Schlenk tube. The tube was evacuated for 5 min (10-3 mbar), then sealed and kept in the dark

at 25 °C for 4 h. Dark red composites, denoted as [CpPd(η3-C3H5)]/[CpPtMe3]@MOF-5,

exhibiting different precursor ratios, were obtained.

Elemental Analysis: Table 7.1. Amounts of [CpPd(η3-C3H5)]/[CpPtMe3]@MOF-5 applied in the described loading experiments

of MOF-5.

1. 2. 3.

Amount X of

[CpPd(η3-C3H5)] [mg] 100

(0.47 mmol) 100

(0.47 mmol) 223

(1.04 mmol)

Amount Y of

[CpPtMe3]

[mg]

144 (0.47 mmol)

50 (0.16 mmol)

53 (0.17 mmol)

Elemental analysis

results

Pd 16.4, Pt 11.1,

C 38.9, H 3.2

Pd 18.7, Pt 8.3,

C 39.5, 3.2

Pd 20.8, Pt 6.5 C 40.0, H 3.2

7. Experimental

140

Table 7.2.Calculated number of [CpPd(η3-C3H5)]/[CpPtMe3] molecules per MOF-5 cavity and the resulting

precursor ratios in MOF-5

1. 2. 3. Calculated number

of [CpPd(η3-C3H5)]/[CpPtMe3] molecules per MOF-5 cavity

2.4/0.9

2.7/0.7

3.1/0.5

Resulting overall stochiometric ratio of

[CpPd(η3-C3H5)]/[CpPtMe3]

2.7/1 4.1/1 5.9/1

Due to the similarity of the spectroscopic and structural properties of the obtained [CpPd(η3-

C3H5)]/[Pt(η5-C5H5)(CH3)3]@MOF-5 materials, as an example the data for the composite

obtained in the 1. loading experiment is listed. The data of the other composites are akin.

IR: ν~ max KBr (cm-1): 2961 (w), 2898 (w), 2814 (vw), 1603 (s), 1504 (m), 1395 (vs), 1258

(w), 1215 (w), 1156 (vw), 1103 (w), 1015 (m), 910 (vw), 882 (vw), 823 (w), 793 (m), 766

(m), 744 (s), 574 (m), 550 (w), 514 (m)

Figure 7.11. FT-IR spectrum of [CpPd(η3-C3H5)]/[CpPtMe3]@MOF-5 in dry KBr.

1H MAS-NMR: δ (ppm): 8.5 (C6H4, MOF-5), 5.6 ([Pd(η5-C5H5)(η3-C3H5)], [Pt(η5-

C5H5)(CH3)3]), 4.7 ([Pd(η5-C5H5)(η3-C3H5)]), 3.4 ([Pd(η5-C5H5)(η3-C3H5)]), 2.1 ([Pd(η5-

C5H5)(η3-C3H5)], 1.1 ([Pt(η5-C5H5)(CH3)3])

7. Experimental

141

13C MAS-NMR: δ (ppm): 175.0 (COO MOF-5), 137.0 (C(COO) MOF-5), 131.1 (C6H4

MOF-5), 99.5 ([Pt(η5-C5H5)(CH3)3]), 94.7 ([Pd(η5-C5H5)(η3-C3H5)]), 46.3 ([Pd(η5-C5H5)(η3-

C3H5)]), -19.1 ([Pt(η5-C5H5)(CH3)3]) 195Pt MAS-NMR: δ (ppm): -5216.9 ([Pt(η5-C5H5)(CH3)3])

PXRD: Capillary: 2 θ (°) (Intensity): 11.3 (12.5), 11.9 (40.1), 13.7 (5.6), 14.9 (32.8), 15.3

(27.8), 16.8 (20.3), 17.8 (100), 19.4 (35.1), 20.3 (19.8), 21.7 (5.0), 22.6 (4.0), 23.9 (4.9), 24.9

(5.9), 28.3 (7.6), 29.3 (8.8), 30.0 (10.4), 31.6 (4.7), 34.7 (5.8), 36.0 (9.6), 37.3 (6.4), 40.0

(2.8), 42.6 (3.7), 43.2 (4.6)

7.2.12. Synthesis of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5

In a typical experiment 50 mg (0.06 mmol) dry MOF-5 powder, 53 mg (0.16 mmol)

[Pt(cod)Me2] and 50 mg (0.16 mmol) [Ru(cod)(cot)] were placed in a Schlenk tube in two

different glass vials. The tube was evacuated for 10 min, sealed (10-5 mbar) and then kept at

30 °C until complete absorption of both precursors was observed (duration: 48 h). A yellow

powder, denoted as [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 was obtained. Yield: 151 mg (99 %

based on complete absorption of both precursor materials)

Elemental Analysis (wt %): Ru 9.8, Pt 17.0, Zn 13.5, C 41.3, H 4.39 (O 14.0 calcd. From

difference to 100 %); deduced number of precursor molecules per MOF-5 cavity:

n([Ru(cod)(cot)]):n([Pt(cod)Me2]) = 1.9:1.7, resulting overall stochiometric ratio of

[Ru(cod)(cot)] : [Pt(cod)Me2]): 1:1 (± 0.1).

IR: ν~ max KBr (cm-1): 3057 (vw), 2994 (vw), 2920 (w), 2869 (w), 2833 (vw), 2792 (vw), 1953

(w), 1603 (s), 1504 (m), 1393 (vs), 1265 (w), 1156 (w), 1092 (w), 1017 (w), 977 (w), 883

(vw), 858 (vw), 821 (w), 745 (m), 574 (w), 514 (w) 13C MAS-NMR: δ (ppm): 175.6 (COO MOF-5), 136.8 (C(COO) MOF-5), 131.6 (C6H4

MOF-5), 101.4 ([Ru(cod)(cot)], cot, olefin.), 98.8 ([Ru(cod)(cot)], cot, olefin. &

[Pt(cod)(CH3)2], cot olefin.), 77.2 ([Ru(cod)(cot)], cot, olefin.), 70.2 ([Ru(cod)(cot)], cod,

olefin), 34.1 ([Ru(cod)(cot)], cod, aliphat.), 32.0 ([Ru(cod)(cot)], cot, aliphat.), 30.8

([Pt(cod)(CH3)2], cod, aliphat.), 5.5 ([Pt(cod)(CH3)2], J(Pt-C) = 785 MHz)

PXRD: Capillary: 2 θ (°) (Intensity): 9.6 (100), 11.8 (99.5), 13.7 (83.5), 15.3 (68.7), 16.7

(48.2), 17.8 (52.4), 19.3 (43.3), 20.3 (44.5), 26.5 (40.0)

7. Experimental

142

Figure 7.12. FT-IR spectrum of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 in dry KBr.

7.2.13. Removal of the precursor molecules from [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5

For removal of the precursor molecules, a sample of 60 mg of

[Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 was washed four times with 20 ml of dry n-pentane.

The obtained off-white powder was dried at in vacuum (10-3 mbar) 100 °C overnight. Yield:

22 mg (98 %).

IR: ν~ max KBr (cm-1): 1571 (s), 1504 (m), 1389 (vs), 1310 (vw), 1175 (vw), 1150 (w), 1106

(w), 881 (w), 828 (w), 745 (w), 530 (m)

PXRD: Capillary: 2 θ (°) (Intensity): 6.8 (100), 9.6 (23.2), 13.6 (14.2), 14.9 (1.8), 15.3 (7.6),

17.8 (1.5), 19.4 (1.1), 20.3 (3.3), 20.6 (1.75), 22.5 (2.5), 24.6 (2.1), 25.8 (1.0), 26.4 (4.1), 28.2

(1.1), 30.0 (2.6), 31.5 (2.5), 33.0 (1.6), 34.5 (2.6), 35.4 (1.4), 35.9 (1.4), 37.3 (1.2), 40.0 (1.1),

42.5 (1.7), 43.1 (1.2), 44.9 (1.0)


7. Experimental

143

7.2.14. Co-Hydrogenolysis of [Ru(cod)(cot)]/[Pt(cod)(CH3)2] in MOF-5 at mild conditions

36 mg of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 (prepared as described above) were

placed in a glass tube, the glass tube was evacuated (dynamic vacuum, 10-2 mbar, 5 min) and

the sample was then exposed to a stream of hydrogen (1 bar, 1 sccm, 99.999 %) at 25 °C for

10 min. A fast color change from yellow to black was observed. Yield: 28 mg.

Elemental Analysis (wt %): Ru 9.6, Pt, 16.8, Zn 14.2, C 40.8, H 4.28 (O 14.3 calc.from

difference to 100 %)

IR: ν~ max KBr (cm-1): 2918 (s), 2829 (m), 1599 (s), 1506 (m), 1387 (vs), 1261 (m), 1152 (w),

1102 (w), 1083 (w), 1018 (m), 872 (w), 822 (w), 744 (m), 572 (w), 513 (m) 13C MAS-NMR: δ (ppm): 178.5 ([(η6-terephthalate)Ru(cod)], COO) 175.2 (COO MOF-5),

136.7 (C(COO) MOF-5), 131.0 (C6H4 MOF-5), 98.7 ([Pt(cod)(CH3)2], cot olefin.), 91.0 (

[(η6-terephthalate)Ru(cod)], C(COO)), 83.2 ([(η6-terephthalate)Ru(cod)], C6H4), 68.3 ([(η6-

terephthalate)Ru(cod)], cod, olefin.), 33.4 ([(η6-terephthalate)Ru(cod)], cod, aliphat.), 30.4

([Pt(cod)(CH3)2], cod, aliphat.), 27.3 (cyclooctane), 5.9 ([Pt(cod)(CH3)2])

Figure 7.13. FT-IR spectrum of the co-hydrogenolysis product of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 at

mild conditions in dry KBr.

7. Experimental

144

PXRD: Capillary: 2θ (°) (Intensity): 11.8 (29.6), 13.7 (74.1), 15.3 (100), 16.8 (10.8), 17.8

(35.4), 19.4 (14.2), 20.3 (25.7), 22.6 (17.7), 24.6 (8.5), 25.8 (3.5), 26.5 (18.1), 28.3 (6.6), 30.0

(13.8), 31.6 (12.1), 33.2 (4.3), 34.6 (8.1), 35.5 (4.4), 36.0 (4.4), 37.4 (3.8), 40.0 (3.1), 42.6

(6.4), 43.2 (4.5)

7.2.15. Quantitative Co-Hydrogenolysis of [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5

A sample of 60 mg [Ru(cod)(cot)]/[Pt(cod)Me2]@MOF-5 was placed in a Fischer-

Porter bottle and evacuated for 5 min (dynamic vacuum, 10-2 mbar). The bottle was then filled

with 1 bar of H2 gas (99.999 %) at 25 °C and heated to 150 °C for 3 h. After cooling to room

temperature, H2 was removed in dynamic vaccum (10-2 mbar, 15 min) and the bottle was

refilled with Ar. Yield: 48 mg.

Elemental analysis: (wt.%): Ru 12.6, Pt 21.8, Zn 17.3, C 31.6, H 2.9 (O 16.7 calcd. from

difference to 100 %); resulting overall stochiometric ratio of Ru : Pt = 1 :1 (± 0.1)

IR: ν~ max KBr (cm-1): 2921 (s), 2852 (m), 1581 (s), 1508 (m), 1390 (vs), 1262 (m), 1096 (m),

1020 (m), 878 (vw), 807 (m), 746 (m), 573 (vw), 516 (w)

Figure 7.14. FT-IR spectrum of the quantitative co-hydrogenolysis product

of [Ru(cod)(cot)] /[Pt(cod)(CH3)2]@MOF-5.

7. Experimental

145

13C MAS-NMR: δ (ppm): 186.3 (COO, cis-/trans-1,4-cyclohexanedicarboxylate), 175.0

(COO MOF-5), 136.9 (C(COO MOF-5), 130.7 (C6H4 MOF-5), 45.1 (C(COO) cis-1,4-

cyclohexanedicarboxylate), 41.1 (C(COO) trans-1,4-cyclohexanedicarboxylate), 29.6 (ring C

atoms cis-/trans-1,4-cyclohexanedicarboxylate), 27.2 (cyclooctane)

PXRD: Capillary: 2θ (°) (Intensity): 11.8 (34.4), 13.6 (48.8), 15.3 (65.5), 17.8 (56.5), 19.4

(23.4), 20.3 (27.7), 22.6 (15.8), 24.6 (17.0), 26.5 (36.7), 30.0 (20.4), 39.8 Pt [111] (100), 46.6

Pt [200] (31.5), 67.5 Pt [220] (27.7), 81.6 Pt [311] (38.6)


7.2.16. Loading of MOF-5 with H2O

8 wt.%:

200 mg (0.26 mmol) of dry MOF-5 powder and 16 μl H2O (8 wt.%, 0.89 mmol) were

placed in a Schlenk tube in two different glass vials. The tube was evacuated (10-3 mbar, 25

°C) until boiling of the water was observed (2 min) and then sealed. The loading was

perpetuated until complete absorption of the water by MOF-5 was observed (max. duration 3

h). Yield: 214 mg (99 %).

IR: ν~ max KBr (cm-1): 3606 (m), 3432 (m), 1678 (m), 1580 (s), 1500 (m), 1386 (vs), 1296 (w),

1254 (w), 1148 (vw), 1106 (vw), 1016 (w), 884 (vw), 818 (m), 748 (m), 666 (w), 554 (vw),

522 (vw) 13C MAS-NMR: δ (ppm): 176.8 (COO MOF-5), 173.0 (COOH re-protonated terephthalate),

137.3 (C(COOH re-protonated terephthalate), 135.2 (C(COO) MOF-5, 131.0 (C6H4 MOF-5)

PXRD: Capillary: 2θ (°) (Intensity): 12.1 (75.3), 14.1 (12.4), 14.7 (9.5), 15.0 (6.3), 15.6

(77.7), 16.9 (100), 17.7 (80.8), 18.5 (3.7), 19.2 (31.4), 20.6 (18.4), 21.5 (17.6), 23.0 (4.0),

23.9 (8.5), 24.9 (52.4), 25.4 (58.5), 26.1 (10.0), 26.7 (9.8), 27.2 (6.8), 27.8 (27.9), 29.9 (9.5),

30.6 (16.7), 31.1 (18.3), 31.9 (23.0), 32.6 (20.9), 34.5 (3.3), 35.5 (16.8), 36.7 (7.0), 40.0 (3.9),

40.5 (8.8), 41.4 (9.6), 41.7 (10.6), 42.3 (4.8), 42.9 (5.9), 43.5 (5.5), 45.3 (22.9), 45.9 (3.1)

4 wt.%: 200 mg of dry MOF-5 powder (0.26 mmol) and 8 μl H2O (4 wt.%, 0.44 mmol) were

placed placed in a Schlenk tube in two different glass vials. The tube was evacuated (10-3

mbar, 25 °C) until boiling of the water was observed (30 s) and then sealed. The loading was

perpetuated until complete absorption of the water by MOF-5 was observed (max. duration 1

h). Yield: 206 mg (99 %)

7. Experimental

146

PXRD: Capillary: 2θ (°) (Intensity): 11.3 (6.1), 13.6 (100), 14.9 (11.5), 15.3 (50.6), 15.6

(11.8), 16.7 (2.4), 17.8 (14.6), 19.4 (5.8), 20.3 (20.4), 20.6 (9.5), 22.5 (14.9), 22.8 (5.0), 24.6

(12.2), 25.8 (3.6), 26.5 (24.9), 28.2 (5.4), 28.7 (2.6), 29.3 (3.0), 30.0 (14.7), 31.5 (13.0), 32.1

(5.3), 33.0 (2.6), 34.5 (10.5), 35.4 (3.7), 35.9 (6.6), 37.3 (4.8), 39.9 (5.2), 40.7 82.3), 41.2

(2.3), 41.9 (3.8), 42.5 (9.2), 43.1 (5.8), 43.6 (2.4), 44.4 (2.0), 44.9 (4.8), 45.4 (2.4)

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9. Appendix

147

9. Appendix

9.1. List of publications

The results of this work are partly comprised in the following publications:

[1] Ruthenium nanoparticles inside porous [Zn4O(bdc)3] by hydrogenolysis of adsorbed

[Ru(cod)(cot)]: a solid state reference system for surfactant-stabilized ruthenium

colloids.

Schröder, F.; Esken, D.; Cokoja, M.; van den Berg, M.W. E.; Lebedev; O. I.; Van

Tendeloo, G.; Walaszek, B.; Buntkowsky, G.; Limbach, H.-H.; Chaudret, B.; Fischer,

R.A., J. Am. Chem. Soc. 2008, 130, 6119-6130.

[2] Direct imaging of loaded metal−organic framework materials (Metal@MOF-5).

Turner, S.; Lebedev, O. I.; Schröder, F.; Esken, D.; Fischer, R. A.; Van Tendeloo, G.,

Chem. Mater. 2008, 20, 5622-5627.

[3] Characterization of interfacial water in MOF-5 (Zn4(O)(BDC)3) - a combined

spectroscopic and theoretical study.

Schröck, K.; Schröder, F.; Heyden, M.; Fischer, R. A.; Havenith, M., Phys. Chem.

Chem. Phys. 2008, 10, 4732-4739.

Patents:

[1] Highly porous layers made of MOF materials and methode for producing such layers.

Fischer, R. W.; Fischer, R, A.; Wöll, Ch.; Schröder, F.; Hermes S.,

WO/2007/014678; DE 10 2005 035 762.8 2007.

Book contributions:

[1] Metal doping of metal-organic frameworks.

Schröder, F; Fischer, R. A., in Topics in Current Chemistry 2009(Ed. M. Schroeder),

Springer, Heidelberg.

9. Appendix

148

Contributions to other projects:

[1] Chemistry in Confined Geometries: Reactions at an Organic Surface.

K. Rajalingam, A. Bashir, M. Badin, F. Schröder, N. Hardman, T. Strunskus, R. A.

Fischer, C. Wöll, ChemPhysChem 2007, 8, 657-660.

[2] Synthesis of Periodic Mesoporous Organosilicas with Chemically Active Bridging

Groups and High Loadings of Thiol Groups.

W.-H. Zhang, X. Zhang, L. Zhang, F. Schröder, H. Parala, S. Hermes, J. Shi, R. A.

Fischer, J. Mater. Chem. 2007, 17, 4320-4326.

[3] Synthesis, Bifunctionalization and Application of Isocyanurate-Based Periodic

Mesoporous Organosilicas.

W.-H. Zhang, X. Zhang, Z. Hua, H. Parala, F. Schröder, S. Hermes, T. Cadenbach, J. Shi,

R. A. Fischer, Chem. Mater. 2007, 19, 2663-2670.

[4] Loading of Porous Metal-Organic Open Frameworks with Organometallic CVD-

Precursors: Inclusion Compounds of the Type [LnM]a@MOF-5.

S. Hermes, F. Schröder, S. Amirjalayer, R. Schmid, R. A. Fischer, J. Mater. Chem. 2006,

16, 2464-2470.

[5] Selective Nucleation and Growth of Metal-Organic Open Framework Thin Films on

Patterned COOH/CF3-Terminated Self-Assembled Monolayers on Au (111).

S. Hermes, F. Schröder, R. Chelmowski, C. Wöll, R. A. Fischer, J. Am. Chem. Soc. 2005,

127, 13744-13745.

[6] Insertion Reactions of GaCp*, InCp* and In[C(SiMe3)3] into the Ru-Cl bonds of [(p-

cymene)RuIICl2]2 and [Cp*RuIICl]4.

M. Cokoja, C. Gemel, T. Steinke, F. Schröder, R. A. Fischer, Dalton Trans. 2005, 44-55.

9.2. Poster presentations

[1] Thin Films of Metal Organic Framework Compounds: Design and Characterization of

New Functional Surfaces.

F. Schröder, S. Hermes, C. A. Bauer, A. J. Skulan, B. A. Simmons, M. D. Allendorf,

C. Wöll, R. A. Fischer, 231st ACS National Spring Meeting, March 26-30, 2006,

Atlanta, USA.

9. Appendix

149

[2] Ru@MOF-5: A Solid State Reference System for Surfactant Stabilized Ru Colloids.

F. Schröder, D. Esken, M. W. E. van den Berg, O. I. Lebedev, G. Buntkowsky, R. A.

Fischer, 3rd International Symposium on Chemistry of Coordination Space, December

9-12, 2007, Awaji Island, Japan.

9. Appendix

150

9.3. Curriculum Vitae

Name: Felicitas Schröder Nationality: German Birthday: 15.02.1981 Place of birth: Bochum Marital status: single

EDUCATION

06/2005-12/2008 PhD at the Chair of Inorganic Chemistry II, Ruhr-University Bochum Topic: Synthesis and Microstructural


09/2005-12/2005 Internship at the Sandia National Laboratories, Livermore, California, USA: MOF Thin Films

10/2000-05/2005 Study of Chemistry at the Ruhr-University

Bochum

10/2004-05/2005 Diploma Thesis at the Chair of Inorganic Chemistry II, Ruhr-University Bochum: Topic: Metal-Organic Preparation of

Cu/ZnO@MCM-48 Catalysts: Comparison of Liquid and Gas Phase Impregnation

08/1991-06/2000 High School Education at the Graf-Engelbert

Gymnasium, Bochum

FELLOWSHIPS AND AWARDS

01/2006-12/2008 Doctoral Fellowship of the German National Academic Foundation (Studienstiftung des Deutschen Volkes)

Since 02/2007 Fellowship of the Research School of the Ruhr-

University Bochum 09/2005-12/2005 Foreign Exchange Fellowship of the German

National Academic Foundation 06/2005 Award of the Ruth and Gerd Massenberg

Foundation for the Diploma 06/2005 Wilke-Award of the Faculty of Chemistry and

Biochemistry for the Diploma Thesis 03/2003-05/2005 Undergraduate Fellowship of the German

National Academic Foundation

Documents

Synthesis and microstructural characterization of metals@MOF-5 · Analytical Chemistry for teaching me how to work the MAS-NMR instrument and all his patience during the sometimes