Thermal activation of ammonium forms of. Y zeolites. II. The dehydroxylation process of HNa-Y zeolites

j . H o f f m a n n and B. H u n g e r Sektion Chemie der Karl-Marx-Universitiit Leipzig Wissenschaftsbereich Physikalische Chemic DDR-7010 Leipzig, GDR

and R. Schliebs Paul-Flechsig-Institut J~r Hirnforschung der Karl-Marx-Universitiit, Abteilung Neurochemie DDR-7039 Leipzig, GDR (Received 10 May 1983; revised 19 December 1983)

Deammoniated NH4Na-Y zeolites have been studied thermodesorptometrically by various methods with respect to the dehydroxylation of the structural OH groups. In each case only one single dehydroxylation peak has been observed, obviously resulting from the intercombination of different kinds of OH groups. Different activation energies obtained for certain ranges of the kinetically evaluated t.p.d, peaks refer to a change of the rate-determining step of dehydroxylation with increasing temperatures. Complete dehyd- roxylation has not been observed (about one OH group per supercage obviously dehydroxylates only in connection with the framework collapse regardless of the total H + content) as is also shown in the results of catalytic investigations of the but-l-ene isomerization.

Keywords: Y zeolites; HNa-Y; dehydroxylation

I N T R O D U C T I O N

The dehydroxylation of H-Y zeolites has been described repeatedly in connection with investigations of the thermal stability of NH4-Y zeolites, i.e. weakly endothermic dehydroxylation step has already been identified in the range between 850 and 1100 K with d.t.a, methods 1-5. While deammoniation seems to be completely reversible there was for a long time much controversy regarding dehydroxylation. Because it is possible to re-load dehydroxylated(partially at the very least) zeolites with water and of thus reactivate parts of the catalytic activity, some authors have concluded that dehydroxylation is reversible 1'2"5-7.

Irreversible changes occurring however, in connec- tion with the water treatment of dehydroxylated zeolites (above all dealumination processes s-] i), make it certain that the rehydroxylated zeolite does not represent the original H + form any more. Later it was shown that such rehydroxylated forms provide entirely different d.t.a, and t.g. curves of water from the original H + zeolite, the irreversible changes obviously beginning even during the phase of dehydroxylation 12.

Meanwhile it seems to have become certain that the dehydroxylation process does not lead to coordina- tively unsaturated aluminium and silicon forms but to A10* species representing the Lewis centres 13'14, and obviously the formation of the latter species is responsible for the irreversibility of the entire process of dehydroxylation.

Quantitative studies of the dehydroxylation of a zeolite (NH4 chabazite) were first described on the basis of t.g./m.s, investigations 15. The authors suc- ceeded in separating the dehydroxylation processes from deammoniation by stopping the temperature programme at the point of complete deammoniation. The maximum temperature of the d.t.a, peak of dehydroxylation was found to be ~920 K 14.

Whilst the general course of the dehydroxylation process seems to be known 13 (a proton arising from deprotonization of a structural O H group is added to another O H group and the surface water molecule thus formed is removed by the splitting of a Si-O bond) so far no exact concepts exist about the kinetics and mechanism of the process. Here t.p.d, measure- ments could give new insights because of their kinetic relevance.

A question of particular interest is whether dehydroxylation occurs between uniform O H groups (as, for example, assumed in case of chabazite by reason of the existence of two overlapped dehydroxy- lation peaks) or between two O H groups of different acidity.

E X P E R I M E N T A L

The aEparatus used was as described in a preceding paper fT. Heating rates were used between 1 and 20 K

I min- . The heating process usually took place up to 1200 K. In order to determine the amounts of water

0144-2449/84/040235-05503.00 © Butterworth & Co. (Publishers) Ltd. ZEOLITES, 1984, Vol 4, July 235

Thermal activation of ammonium forms of Y zeolites Ih J. Hoffmann et al.

desorbed the areas of the water peaks were evaluated using a special calibration method. In order to assess the reversibility of the dehydroxylation process, samples of a 0.88 N H ~ I a - Y zeolite were completely or partially dehydroxyl-ated, then cooled and re- loaded with water by treatment with steam at different temperatures.

Zeolite samples used and the procedure ofbut-l-ene isomerization were as described earlier 17. The dehyd- roxylation process was studied after heating the NH4Na-Y samples up to 700 K in order to remove the ammonia completely.

RESULTS Typical desorption plots were shown in Figure 1 of the preceding paper 17. It can be seen that water is desorbed - - in a large peak at ~410 K (physisorbed w a t e r ) ; - at ~520 K (coordinatively bound probably at NH~- or Na + ions); - - in a peak above 850 K (water resulting from the dehydroxylation process).

In Figure 1 the dehydroxylation peaks for several degrees of exchange are recorded (heating rate 10 K min-I), indicating that water is lost in a process, expressed by a single, well-defined peak, the water amount and peak maximum strongly depending on the exchange degree.

In Figure 2 the normalized peaks are drawn, as is usual in case of t.p.d, experiments Is:9 (height of the peak maximum deliberately set at 1, Tmax also at 1), demonstrating the increasing peak broadening with decreasing degree of exchange.

Figure 3 shows peaks for 46, 74 and 88% ofexchange depending on ~, the heating rate, and the ordinates being normalized to height 1. The strong dependence of the peak-maximum temperature on the heating rate is obvious.

A >

r ~

Rgure 1 %)

800 900 ~000 . 0 0 1200 T(K)

Dehydroxylation peaks of HNa-Y zeolites (exchange degrees in

1.0

0 . 8 -

0 . 6 -

0 . 4 -

0 . 2 -

I I I I I I I 0.88 0.92 0.96 1.00 L04 1.08 1.12

T.

Rgure 2 Normalized dehydroxylation peaks of HNa-Y zeolites (Tn = T/Trnnx; Cn = relative gas-phase concentration) ( )91%; (--)88 %; (--(3--)74 %; (---0--)65 %; (--11--)46 %; (--12~)21%

i.o

0.5

o.a

0.~

0.0

a 3 4 5

b 873 973 1073 1 2 3 4 5

I.C

I C 873

I 2

I I 973

3 4 5

I 1073

1 . 0 -

0 .5

0 .0

873 973 r.

Rgure 3 Dehydroxylation peaks in dependence on the heating rate I~ (Cn = relative gas phase concentration) A: 0.46 HNa-Y; B: 0.74 HNa-Y; C: 0.88 HNa-Y 1:1 K min-1; 2 :2 K min-1; 3 :6 K min-1; 4:10 K min-1; 5: 20 K min -~

Table 1 shows results ofquantitative peak evaluation to give a comparison between the theoretically possible and the OH groups calculated from the peak area being listed in the last column (one water molecule desorbed corresponds to two O H groups).

The kinetic evaluation was carried out with both the peak maximum and the single peak method (see the preceding paper17).

2 3 6 ZEOLITES, 1984, Vol 4, July

Thermal activation of ammoinium forms of Y zeolites Ih J. Hoffmann et al.

Table 1 OH groups on deammonieted NHaNa-Y zeolites

Degree of Temp. of OH groups per supercage exchange peak max. calculated from (%) (K) (a) exchange (b) water

degree peak

Difference OH groups (a)-(b)

15 1111 1.1 0.2 0.9 21 1066 1.4 0.4 1.0 30 1036 2.0 0.9 1.1 46 990 3.0 1.9 1.1 64 975 3.6 2.3 1.3 65 956 4.3 2.8 1.5 74 941 4.9 3.6 1.3 80 936 5.3 4.0 1.3 88* 930 5.9 4.4 1.5 88* 925 5.9 4.6 1.3 91 919 6.1 5,2 0.9

*Different samples synthesized in various laboratories

Table 2 Arrhenius parameters for the dehydroxylation reaction of deammonieted NH4Na-Y zeolites (peak-maximum method)

Degree of Energy of Preexpon. Correlation exchange activation factors coefficient (%) (kJ mo1-1) (min -1)

15 264 _+ 13 6.88"1011 0.9988 21 300 + 33 1.93"1014 0.9882 30 290 -+ 33 1.35"10 TM 0.9873 46 258 + 8 1.38"10 TM 0.9992 64 258 +- 10 2.33-10 TM 0.9943 65 246 _+ 14 9.24.10 TM 0.9982 74 258 + 5 7.43"10 TM 0.9993 80 260 -+ 3 9.37'10 TM 0.9997 88* 245 -+ 12 2.53.10 TM 0.9965 88* 242 -+ 10 1.50.10 la 0.9977 91 245 - 12 2.93.1013 0.9966

heating rate of l 0 K min-I to simulate the first ord.er peak (peak 1 - - 613 kJ mol'-I), then after Galculation of the preexponential factors according to equation (3) of the preceding paper 17 in such a manner that the temperature of the peak maximum was not changed .for lower activation energies the same simulations were carried out (peak 2 - - 418 kJ mol-]; peak 3 - - 251 kJ mol-t).

Figure 5 represents a comparison of experimental results (dehydroxylation of 0.88 and 0.46 H - Y samples) with calculations according to Cvetanovit5 and Amenomiya la based on various kinetic models, the experimental activation energies being used.

Table 3 Arrhenius parameters for the dehydroxyletion reaction of deammonieted NH4Na-Y zeolites (single-peak evaluation)

Degree of Heating 1 =t order without reads. 2 nd order without reads. exchange rate ~ E~ In A Ed In A (%) (K rain -~) (kJ mo1-1) (A in min -1) (kJ mo1-1) (A in min -1)

88

1 655 88.25 2 611 81.16 6 627 81.81

10 613 78.58 20 549 70.06

74

46

1 622 81.29 2 496 62.11 6 430 53.52

10 385 47.34 20 412 50.65

6 263 30.23 280 32.95' 10 278 32.20 428 51.07 20 279 32.05 490 57.86

*Different samples synthesized in various laboratories *Dimension of A min -1 for 2 n¢ order because of the use of the coverage degree as a concentration measure

I n Table 2 the results of the peak-maximum method are listed, demonstrating a slight decrease of the activation energies with increasing degree of ex- change.

In Table 3 ArrhenilLs paramete: of the single-peak evaluation are shown lbr a few examples. From these it is evident that only in cases of intermediate degrees of exchange (46%) are the results in agreement with those of the peak-maximum method, whilst the highly exchanged zeolites yield much higher activation energies. In the case of the single peak method the activation energies obviously depend on the heating rate, resulting probably from weak diffusional influ- ences. One can expect that 'true' values of Eo are obtainable when extrapolating to zero heating rate. From such an extrapolation for the dehydroxylation process of the 0.88 H-Y sample (first order process without readsorption ) an activation energy o f~660 kJ tool -I can be calculated and for the dehydroxylation process of the 0.46 H -Y sample (second order process without readsorption) an activation energy of about 250 kJ mo1-1 results.

In order to find out the valid kinetic model, modelling calculations were carried out, the results of which are shown in Figures 4 and 5. Thus Figure 4 demonstrates results of a step procedure 2°. Here the Arrhenius parameters were first calculated using a

LO

qal~

I

o~-I " " - I I 850 900 950

T(K)

Rgure 4 T.p.d. peaks calculated for the 0.88 HNa-Y sample on the basis of a step procedure (1 =t order without readsorption) using (1) Ed ---- 613 kJ mol-1; (2) Ed = 418 kJ mol-1; (3) Ed = 251 kJ mo1-1. Heating rate 10 K min -1 (0); experimental points

ZEOLITES, 1984, Vol 4, July 2 3 7

0.8

0.6 L~

O.4

)

o

[04

a 1.0-

0.2

I 0.92 (196 IO

D I S C U S S I O N

b

/, I "°~1 " ' / I I

0.92 0.96 1.0 r.

0

~o

[04 LOS

Rgum 6 Calculated t.p.d, peaks for (a) 0.88 HNa-Y sample; Ed : 613 kJ mol-1; 1 ~ order without readsorption; (b) 0.46 HNa-Y sample; solid line: En = 278 kJ mol-1; 1 ¢ order without readsorption. Dashed line: Ed = 428 kJ mol-~; 2 nd order without readsorption. Heating rate 10 K min-1; (O) experimental points

The results of the kinetic experiments concerning the but-l-ene isomerization show the catalytic activity of the H-Y zeolites to be increasing strongly with increasing exchange degree (Figure 6) but deactivation increases also in the same direction.

Z O H ~ Z O - + H +

ZOH + H + ~ , Z O H ~ -

ZOH,_, + --," Z + + H20

Z O - + Z + ~ Z - O - Z

The release of water ti'om the zeolite surface starts by the attack of a proton on an intact OH group. Since all OH groups in H zeolites are acidic in character, the addition of a second proton to such an acidic OH group is particularly difficult and is a reason for the high thermal stability of the OH groups, as is the strong bond between the OH-group oxygen and the zeolitic framework (compared with OH groups in zeolites which are bound at polyvalent cations).

The mechanism of dehydroxylation can simply be outlined by:

(1)

(2)

(3) (4)

E

(Z representing the zeolite framework). In practice step (4) will be much more complicated (e.g. containing dealumination steps).

The experiments show that during the first phase of dehydroxylation, water is split according to a first order process, and from Figure 4 it can be seen that the experimental peak of the 0.88 H-Y sample is able to be described only when assuming a high activation energy in connection with an irreversible first order process.

Supposing a first order step to be rate-determining in the above mechanism, experimental results can be best interpreted assuming that, at the beginning of the non-isothermal process the splitting of the water molecule (step 3) is the slowest step, and steps (1) and (2) are to be considered as in equilibrium. The high activation energy for the low-temperature range of desorption then would have to be considered as an apparent activation energy, because from the reaction

equations (1)-(3) the rate equation is as follows:

d[ZOH] = K,K2k3" [ZOH]2 (5) ,~t [ z o - ]

[ZO-] represents the concentration of oxygen atoms available as proton acceptors and therefore is almost constant; K,, K2 are the equilibrium constants of the first and second steps, respectively; k3 is the rate constant of the third step. In order to explain the first order behaviour observed experimentally it is neces- sary to additionally assume the rate equation to be near-zero order with respect to proton concentration (this assumption is related to the low-temperature range of dehydroxylation where the H + or the OH group concentration, respectively, should not change very much). From this follows:

d[ZOH] _ K2k;" [ZOH] (6) dt

Assuming that at higher temperatures a change of rate-determiving step occurs (then step (2) with the lower activauon energy should be rate-determining), from the above mechanism the following equation results for steady-state conditions:

d[ZOH] _ Klk2k3 [ZOH] 2 - (7)

dt k-2 + k3 [ZO-]

When [ Z O ] is constant, the rate equation is of second order. This is in agreement with experimental results (Figure 5).

In the low-temperature range, where equation (6) is valid, the experimental activation energy is:

Ed = AH.., + E~.,

0.3 1

2

0.2

0.1

Thermal activation of ammonium forms of Y zeolites I1: J. Hoffmann et al.

I I I I I I0 20 30 40 50

Time on streom

Rgum 6 Rate constants of but-l-ene isomerization. (1) 0.88 HNa-Y; (2) 0.74 HNa--Y; (3) 0.15 HNa-Y

238 Z E O L I T E S , 1984, Vol 4, Ju ly

Thermal act ivat ion o f a m m o n i u m fo rms o f Y zeolites Ih d. Hof fmann et al.

With respect to equation (7), the Arrhenius plot is linear only if either k-2 ~ k3 or the reverse. Regarding the activation energies,

Ea, >> Ea_..,

should hold and therefore at sufficiently high tempera- tures in each case: k~ >> k-2. If one additionally assumes t ha t /~H 2 > AHI, E~ ~ it finally follows from equation (7) that: E,i ~ /kH~.-

Thus the assumption of a change of the rate- determining step with-ut any change of the mechan- ism permits an explanation of the experimental results.

Comparing the equations for the low and high temperature range of the dehydroxylation peak, it follows from the kinetic results that: - - the reaction heat orstep (2) /kH2 ~- 250 kJ mol-I; - - the activation energy of step (3) E~, ~- 410 kJ

mol - j , the main part of which is due to the break of the Si-O bond in the zeolite framework.

From Table 1 it follows that, for all.samples, a difference exists of about one O H group between the number of O H groups per supercage calculated on the one hand from the exchange degree and on the other from the area of the dehydroxylation peak.

This result can best be explained by the dehydroxy- lation of the last O H group per supercage only at temperatures higher than 1200 K, in connection with the collapse of the framework.

N.m.r. resuhs concerning the OH.~roup concentra- tion, which will be reported later '~, show that at activation temperatures of ~700 K all OH groups are still present, thus supporting this explanation.

Findings resulting from the but-l-ene isomerization studies also make this explanation likely: as can be seen from Figure 6 the 15% exchanged and deammoni- ated zeolite already shows a surprisingly high catalytic activity indicating that the majority of OH groups are present. In addition, the isomerization results clearly show that the zeolites with the highest degrees of exchange have a disprop(,rtionally higher catalytic activity compared to those with lower ones, which can be explained by the higher acid strength of the highly exchanged samples. In addition the most acidic O H groups seem to be blocked most quickly by poisoning, with polymerization products, as can be seen by the very rapid deactivation of the high- exchanged zeolite samples.

The very strong dependence of the maximum temperature of the dehydroxylation peak on the degree of exchange (see Table 1) can best be explained by the change of the ratio of O H groups inside and outside of the supercage (LF/HF OH groups, see our

previous paperl7). At high exchange this ratio is about 2, whilst it decreases at low exchange towards zero. Further, assuming the LF O H groups to be the less acidic 17 (i.e. more basic), the dehydroxylation process should occur more easily (at lower temperatures) in .the case of high exchange degrees because of the high concentration of LF O H groups (see the O H group spectra in Figure 4 of the preceding paperJT). Obviously these I,F OH groups are better acceptors for a second proton than the highly acidic HF O H groups.

It is not, therefore, the acidity of the O H groups but the relative facility with which oxygen linkages in the framework can be broken which is responsible for the tendency of O H groups to dehydroxylate. This is in agreement with results obtained for other systems. The much less acidic H - X zeolites 25 dehydroxylate at lower temperatures and the zeolites exchanged with polyvalent'cations 24'26'27 at even lower ones compared with the more acidic H-Y zeolites.

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