EXCHANGE THE

SURFACE

COMMUNICATION

OF

DEUTERIUM

WITH

OF

CATALYSTS

2.

ZEOLITE HNa-FORMS

OF TYPES

HYDROGEN

X AND

OF

Y ZEOLITES UDC

Kh. M. Minachev, G. Bremer,* R. V. Dmitriev, K. G. Shteinberg,* Ya. I. Isakov, and A. N. Detyuk

541.183:661.183.6

In o u r p r e v i o u s c o m m u n i c a t i o n [1] we c o n s i d e r e d the b a s e s of the u s e of the m e t h o d of i s o t o p i c h e t e r o e x c h a n g e of h y d r o g e n f o r the i n v e s t i g a t i o n of z e o l i t e c a t a l y s t s and c i t e d d a t a on the c o n c e n t r a t i o n and m o b i l i t y of h y d r o g e n in Ca f o r m s of t y p e s X a n d Y z e o l i t e s a s a f u n c t i o n of the m o l e r a t i o SiO2/A12Oa (x), the d e g r e e of e x c h a n g e of Na + (or), a n d the m e t h o d of p r o d u c t i o n ( h y d r o t h e r m a l s y n t h e s i s , d e a l u m i n a t i o n ) . S a m p l e s of NHaY with x = 4.2 a n d 5.0 and c~ = 75% w e r e a l s o i n v e s t i g a t e d by t h i s m e t h o d [2]. It w a s e s t a b l i s h e d t h a t with i n c r e a s i n g x, the m o b i l i t y of H in the z e o l i t e s i n c r e a s e s ; the p o s s i b i l i t y of d e t e r m i n i n g the d e g r e e of d e h y d r o x y l a t i o n of the s u r f a c e of the c a t a l y s t s and the r a t i o of the a m o u n t s of B r o n s t e d and L e w i s a c i d c e n t e r s w a s d e m o n s t r a t e d . T h e p u r p o s e of t h i s w o r k w a s to s t u d y the i n f l u e n c e of x and c~ on the c o n c e n t r a t i o n and m o b i l i t y of H in d e c a t i o n i z e d f a u j a s i t e s , p r o d u c e d f r o m the NH 4 f o r m s , a s w e l l a s the p r o p e r t i e s of h y d r o g e n of the u I t r a s t a b l e [3-5] z e o t i t e Y. EXPERIMENTAL

METHOD

T h e i n v e s t i g a t e d H N a - f o r m s of t y p e s X and Y z e o l i t e s w e r e p r e p a r e d b y e x c h a n g e of Na + i o n s in NaX (x = 2.5) and NaY (x = 5.2) f o r NH~, f o l l o w e d b y d e c o m p o s i t i o n of the N H 4 - f o r m s at 400 ~ in a s t r e a m of a i r . T h e d e g r e e of e x c h a n g e of Na + i s i n d i c a t e d b y n u m b e r s in f r o n t of the s y m b o l s f o r the z e o l i t e s a n d f r a c t i o n s . D e a l u m i n a t e d z ~ o l i t e Y w a s p r e p a r e d f r o m 0.974 NH4NaY a c c o r d i n g to the m e t h o d of [3]. B e f o r e the e x p e r i m e n t s , the s a m p l e s (0.5-1 g) w e r e t r e a t e d with v a p o r s of H20 (14 m m Hg) at 500 ~ and then e v a c u a t e d f o r 6 h a t 500 ~ and 10 -4 m m Hg. T h i s c o n d i t i o n i n g of the z e o l i t e s w a s c o n d u c t e d f o r c o m p l e t e r e p l a c e m e n t of the d e u t e r i u m i n t r o d u c e d into t h e m in r e a c t i o n s of i s o t o p i c e x c h a n g e b y p r o t i u m [2]. T h e a p p a r a t u s and m e t h o d of the e x p e r i m e n t w e r e d e s c r i b e d in [1]. T h e c o n c e n t r a t i o n s o f h y d r o g e n [H] w e r e found f r o m the e q u i l i b r i u m r a t i o of H and D in the g a s p h a s e a c c o r d i n g to the f o r m u l a [H] = 2a[H]~o/[D]oo, w h e r e a i s the a m o u n t of d e u t e r i u m i n t r o d u c e d into the s y s t e m , m o l e s p e r g r a m of c a t a l y s t ; [H]~o and [D]oo a r e the c o n c e n t r a t i o n s (in a t o m i c f r a c t i o n s ) of p r o t i u m a n d d e u t e r i u m in the g a s p h a s e at e q u i l i b r i u m . T h e r a t e s o f i s o t o p i c h e t e r o e x c h a n g e w e r e c a l c u l a t e d f r o m the k i n e t i c d a t a a c c o r d i n g to the e q u a t i o n W1 = 2.303al[H]~ {lg ([g]~ - - [K]~) - - lg ([HI~ - - [HI1)} t~ - - tl mote/rain,

g

a n d the m o b i l i t y of h y d r o g e n a c c o r d i n g to the e q u a t i o n W2 = 1.!5 (t - - [H/~) {lg ([H}~ - - [H]~) - - lg ([H]~ - - [Hh) } min-~ t~ - - t~ w h e r e [H] 1 a n d [H] 2 a r e t h e c o n c e n t r a t i o n s Of p r o t i u m in the g a s p h a s e a t the. m o m e n t s of t i m e ti and t2; t i s the r e a c t i o n t i m e [2]. T h e b a s i c k i n e t i c m e a s u r e m e n t s w e r e p e r f o r m e d a t a p r e s s u r e D 2 1 0 - 2 0 m m Hg and 400 ~ U n d e r t h e s e c o n d i t i o n s the t i m e of r e a c h i n g of e q u i l i b r i u m d i d not e x c e e d 2 - 5 h (for u l t r a s t a b l e * C o w o r k e r s of the K. S c h o r i e m m e r H i g h e r T e c h n i c a l School, M e r s e b u r g ,

German Democratic Republic.

N. D. Z e l i n s k i i I n s t i t u t e of O r g a n i c C h e m i s t r y , A c a d e m y of S c i e n c e s of the USSR, M o s c o w . T r a n s l a t e d f r o m I z v e s t i y a A k a d e m i i Nauk SSSR, S e r i y a K h i m i c h e s k a y a , No. 2, pp. 2 8 9 - 2 9 4 , F e b r u a r y , 1974. O r i g i n a l a r t i c l e s u b m i t t e d J u l y 18, 1973,

9 Consultants Bureau, a division of Plenum Publishing Corporation, 227 g'est 17th Street, New York, N. Y~ }00i1. No part of this publication may be reproduced, stored in a retrieval system, o/ transmitted, in an), form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

261

T A B L E 1. C o n c e n t r a t i o n s of S u r f a c e H y d r o g e n [H] i n HNaX and HNaY Z e o l i t e s , E v a c u a t e d at 10 -4 t o r r and 500 ~ and S t o i c h i o m e t r i c C o n t e n t of H, m m o l e s / g Zeolite Characteristics

HNaX

I

HNaY

Degree of exchange ofNa, 9o, 8tolehiomettie content*

1,8 20,030,837,75i,560,4 0 29,i38,7 50 75 97,4 0,ttl,3t2,052,533,544,t9 -- t,261,68 2,23,38 4,5

Concentration of [HI

10,8 li,O 10,851t,0 [i,25]1,661 0,20,260,520,651,6

3,0

*calculation. z e o l i t e s it was s e v e r a l m i n u t e s ) and p e r m i t t e d a s u f f i c i e n t l y r e l i a b l e m e a s u r e m e n t of the m o b i l i t y of h y d r o g e n of the c a t a l y s t s in i s o t o p i c e x c h a n g e with d e u t e r i u m .

DISCUSSION As was noted in [i, 2], the total concentration of hydrogen, which can be present in the form of H20, H3 O+, OH groups, and chemisorbed H2, is determined by the method of deuterium-hydrogen exchange. In the case of ammonium forms of zeolites, NH~ ions may be a supplementary source of H. Deammoniation of NH4X begins at temperatures somewhat higher than 20 ~ and is completed below 300 ~ In the case of NH4Y (x = 5.0), it ends at 375 ~ [6, 7]. The process of removal of NH 3 and dehydroxylation of zeolites are superimposed upon one another to a substantial degree. The maximum concentration of hydrogen in type X zeolites was observed after the decomposition of NH4X at 290 ~ [8], and in Y zeolites (x = 5.0) after heat treatment of NH4Y at 350 ~ [7]. The conditions of conditioning of the samples used in this work ensured the decomposition of all the NH~ ions, but also induced a substantial dehydroxylation of the zeolites. The values of [H] cited below correspond to the concentrations of the most thermostable OH groups and firmly chemisorbed H20. The hydrogen of these OH and H20 groups enters into exchange with D 2 at temperatures of 150-250 ~ Moreover, all the hydrogen of the zeolites participates in the reaction, which is evidenced by the coincidence of the equilibrium values of [H]~ in the interval 250-500 ~ for all the samples. Let us note that the values of [H] were well reproduced in the course of successive experiments. All the investigated zeolites, including NaX and NaY, after treatment under standard conditions, contain hydrogen chiefly in the form of OH groups (Table I). Their greatest concentration was detected in samples of HNaY and the least in NaX and NaY. For the latter, the presence of H is evidently the result of certain deficit of Na + cations and chemisorption of small quantities of H20 , as well as the presence of Si-OH groups on the faces of the zeolite crystals. It can be noted that [H] decreases with increasing X in Na forms of zeolites. Thus, for NaX (x = 2.5), [H] is equal to 0.8 mmole/g, -for NaY (x = 5.2) 0.2 mmole/g (see Table i), and for Na-mordenite with x = I0.0, after the same conditions of treatment, 0.05 mmole/g was found [2]. These data are in good agreement with the results of thermogravimetric investigations, a c c o r d i n g to which the a m o u n t of H20 r e m a i n i n g i n the z e o l i t e s a f t e r i d e n t i c a l t r e a t m e n t i n c r e a s e s with i n c r e a s i n g c o n c e n t r a t i o n of m e t a l c a t i o n s , i . e . , with d e c r e a s i n g m o l e r a t i o SiO2/AI20 3 i n t h e m [9, 10]. T h e v a l u e s of [H] f o r NHaX with o~ - 50% a r e c l o s e , 0 . 9 + 0.1 m m o l e / g (see T a b l e 1), i . e . , they do not T A B L E 2. I s o t o p i c C o m p o s i t i o n of H y d r o g e n above Z e o l i t e 0.515 HNaX (0.432 g, p 10.2 t o r r , 400 ~, H~o 34.5, [H] 1.25 m m o l e s / g , W 2 2.1 910 -3 m i n -t) Time, min 0 2 30 93 i5o 210 270 330 45O Equi~brium 262

......... H~ 0,t 0,2 1,0 3,1 3,8 6,6 ' 7,9 9,5 12,0 i2,3

I-ID

D:

0,9 2,7 13,2 25,8 32,9 38,4 40,4 42,2 46,3 46,2

98,3 97,1 85,8 71 ,t 63,3 55,0 5i ,7 48,3 4i,7 41,6

H, %

[a]oo [HJt

_[Hlt }

0,9 1,5 7,6' 16,0 20,2 25,8 28,1 30,6 35,2 35,4

34,5 . 32,9 27,8 19,4 i5,2 9,6 7,3 4,8 0,2 0

1,538 .1,517 1,443 1,288 t,182 0 983 0,863 0,681

~g([HL:[H]~

~

3

0,6 ! 80

i gO

Fig. 1

1 Igg

r, mln

J~

8D

fJQ r~

Fig. 2

Fig. 1. L o g a r i t h m i c dependences of the change in the p r o t i u m concentration in the gas p h a s e above type X zeolites: 1) 0.018 HNaX; 2) 0.20 HNaX; 3) 0.30 HNaX; 4) 0.377 HNaX; 5) 0.51.5 HNaX; 6) 0.604 HNaX. Fig. 2. L o g a r i t h m i c dependences of the change in the p r o t i u m concentration in the gas p h a s e above type Y zeolites: 1) NaY; 2) 0.291 HNaY; 3) 0.387 HNaY; 4) 0.50 HNaY; 5) 0.974 HNaY. differ f r o m [HI found for NaX. However, it should be noted that equality of [H] does not m e a n the s a m e nature of the hydrogen in the s a m p l e s . Thus, f o r HNaX with c~ < 20%, the concentrations of the s t r u c t u r a l groups of O H , calculated f r o m the s t o i c h i o m e t r i c composition of the zeolites, a r e l e s s than the exp e r i m e n t a l l y d e t e r m i n e d [H]. It can be a s s u m e d that a l a r g e contribution to [H] for HNaX at low d e g r e e s of exchange of Na + is made by w a t e r c h e m i s o r b e d by the c r y s t a l l i t e s . The f r a c t i o n of hydrogen of H20 d e c r e a s e s with d e c r e a s i n g content of Na + cations in the s a m p l e s . When a > 20%, [HI is l e s s than t:he stoichiometric content of hydrogen in HNaX, which is an indication of substantial dehydroxylation oll the zeolites (see Table i). In the ease of HNaY, the experimental values of [H] for all the samples are lower than the hydrogen concentrations calculated from the chemical composition. The degree of dehydroxylation of these zeolites varies from 30 to 80% and decreases with increasing oz. The highest [H] is observed for ultrastable Y zeolite, obtained from 0.974 NHtNaY (see Table I); the possibility remains that the hydrogen of AI(OH) 2+ and AI(OH) 2+ cations present in the ultrastable fau]asites also participates in exchange with D 2 [4, 5, ii]. Now let us consider data on the rate of exchange and mobility of hydrogen in various zeolites. Table 2 presents the isotopic composition of hydrogen above zeolite 0.515 HNaX during heteroexchange and the calculated concentrations of protium in the gas phase. A linear dependence of log ([H]~o-[H]t) on the time [the coordinates were selected in accord with Eq. (i)] is observed, i.e., a constant rate of the isotopic exchange (Fig. i, straight line 5). This is an indication of the kinetic equivalence of hydrogen in the sample 0.515 HNaX. An analogous dependence exists for 0.60 HNaX (see Fig. i, straight line 6). In the case of zeolites 0.018 HNaX, 0.20 HNaX, 0.308 HNaX, and 0.377 HNaX, certain deviations from linear functions are observed (see Fig. I), which is evidence of differences in the mobility of H in them, evidently associated with the different nature of hydrogen (OH and H20 groups). These results confirm the earlier hypothesis that in zeolites HNaX with ~ < 20%, hydrogen is inhomogeneous and is probably present in the form of H20 and Si-OH groups on the faces of the zeolite crystals. The fraction of more labile hydrogen is from 20 to 60% of the total H concentration. In the case of type Y zeolites, a linear dependence of log ([H]~o-[H]t) on t is observed for all the samples (Fig. 2). This is an indication of kinetic homogeneity of the surface hydrogen in Y zeolites. The rates of isotopic heteroexchange W I and W 2, determined from Figs. 1 and 2, are cited in Table 3. The values of W I, as a rule, are proportional to [H]. The average mobilities of hydrogen (W 2) in type X zeolites are approximately half as high as in type Y zeolites (x = 5.2). The change in a in the series of faujasites X and Y has little influence on W 2. These results do not confirm the hypothesis of certain authors [12], on the fact that not only the total acidity, but also the acid strength of the centers increases with increasing degree of exchange of Na + for NH~ in synthetic zeolites of the faujasite type. For Ca- and Lazeolites X and Y, this actually occurs [2, 13]. In the case of decationized forms, the question remains open. 263

TABLE 3. Rates of Isotopic Heteroexchange of Hydrogen and Mobilities of Hydrogen on HNaX and HNaY Zeolites (400 ~ Characteristics

I

Zeolite

HNaX ~' %of exchangeWI" l0 s, Rate moles/rain Mobilityof hydrogenW2"103. rain.1 ~ .

]

PINaY

t,4 2,5 2,4 2,7 2,8 6,0 0,8 1,t I 2,01 2,7l 8,0 2430, 1,8 2,5] 2,6 2,7 2,4[ 3,61 3,6] 4,2 3,8[ 4,2I 5,01 810

Of the samples that we investigated, HY, produced f r o m 0.974 NH4NaY , is sharply distinguished. According to the data of x - r a y diffraction study, this zeolite had a high degree of erystallinity, which was only 10-15% below the c r y s t a l l i n i t y of other samples of Y zeolite. As has already been noted, an equflibrium state of the s y s t e m D2-0.974 HNaY was reached within s e v e r a l minutes at 400 ~ which is evidence of the exceptionally high mobility of hydrogen in ultrastable zeolite. The values of W 1 and W2 for it are two o r d e r s of magnitude higher than for other catalysts (see Table 3). Such high r a t e s of heteroexehange were o b s e r v e d in the case of m e t a l - z e o l i t e catalysts [14]; m o r e o v e r , they were explained by activation of deut e r i u m by the metal (Ni, Pt) and by an i n c r e a s e in the rate of its migration along the surface. The causes of such high mobility of H in 0.974 HNaY are not yet c l e a r . L e t us note that according to the data of [15], for the dealumination of Y zeolites, the active strength of the c e n t e r s and their t h e r m a l stability are inc r e a s e d . The r e s u l t s cited above confirm the opinion of K e r r [5] that ultrastable faujasite is a v a r i e t y of dealuminated type Y zeolite. A c o m p a r i s o n of the data for H- and C a - f o r m s of faujasite s, studied e a r l i e r [1], shows that the concentrations of hydrogen and its mobility in HNaX o r HNaY a r e higher than in CaNaX or, correspondingly, in CaNaY with the same d e g r e e of exchange of Na +. In the decationization of synthetic type X and Y zeolites, the "threshold" of the degree of exchange of Na +, a f t e r which an i n c r e a s e in the acidity ([H]) and catalytic activity in carbonium ion reactions o c c u r s , and which is o b s e r v e d in the case of r e p l a c e m e n t of Na + by Ca 2+ [1], is p r a c t i c a l l y absent o r significantly s m a l l e r . Analogous r e s u l t s were obtained in [13], where the acidity of zeolites was d e t e r m i n e d by titration with n-butylamine in the p r e s e n c e of Hammett indicators. Thus, the method of d e u t e r o - h y d r o g e n exchange yields valuable information on ~he p r o p e r t i e s of hydrogen in zeolite catalysts. CONCLUSIONS 1. The concentrations and mobilities of hydrogen in decationized zeolites of type X and Y with various residual contents of Na + were d e t e r m i n e d by the method of isotopic exchange with deuterium. 2. The content and mobility of hydrogen are a maximum in the ultrastable type Y zeolite. LITERATURE

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 264

CITED

Kh. M. Minachev, R. V. Dmitriev, Ya. I. Isakov, and O. D. Bronnikov, Izv. Akad. Nauk SSSR, Ser. Khim., 2689 (1973). Kh. M. Minachev, R. V. Dmitriev, Ya. I. Isakov, and O. D. Bronnikov, Kinetika i Katatiz, 1__22, 712 (1971). C . V . MeDaniel and P. K. Makher, Molecular Sieves, P a p e r s Conference, London (Publ. 1968) (1967), p. 186. G . T . K e r r , J. Phys. Chem., 71, 4155 (1967). G . T . K e r r , J. Catalysis, 1_5_5,200 (1969). A . P . Bolton and R. L. Mays, Chimie et Industrie, 5___2,121 (1970). A . P . Bolton and M. A. Lanewala, J. Catalysis, 1._~8, 154 (1970). J . B . Uytterhoeven, L. G. C h r i s t n e r , and W. K. Hall, J. Phys. Chem., 6_.99, 2117 (1965). P . E . P i e k e r t , J. A. Rabo, E. Dempsey, and V. Schomaker, P r o c . 3rd Internal. Congr. Catalys., Vol. 1, A m s t e r d a m (1965), p. 714. J . W . Ward, J. Catalysis, 1__~4,365 (1969). P. Jaeobs and J. B. Uytterhoeven, ibid., 22, 193 (1971). J. T u r k e v i c h , F. Nozaki, and D. Stamires, P r o c . 3rd Internal. Congr. Catalys., Vol. 1, A m s t e r dam (1965), p. 586.

13. 14. 15.

H. Otouma, Y. Arai, and H. Ukihashi, Bulh Chem. Soc. Japan, 42, 2249 (1969). Kh. M. Minachev, R. V. Dmitriev, O. D. Bronnikov, V. I. Garanin, and T. I. Novruzov, Kinetika i Kataliz, 1__33,1095 (1972). R. Beaumont, P. Pichat, D. Barthomeuf, and Y. T r a m b o u z e , P r e p r i n t s of the Fifth Irternat. Congr. Catalys., No. 19, P a l m Beach (USA) (1972).

265