Factors Affecting the Synthesis of Pentasil Zeolites - ACS Symposium

12 Factors Affecting the Synthesis of Pentasil Zeolites ZELIMIR GABELICA and ERIC G.

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DEROUANE

Facultés Universitaires de Namur, Laboratoire de Catalyse, Rue de Bruxelles 61, B-5000 Namur, Belgium NIELS BLOM Haldor Topsøe Research Laboratories, Nymøllevej 55, DK-2800 Lyngby, Denmark Downloaded by UNIV LAVAL on November 5, 2015 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch012

Several techniques were used in combination to inves tigate the role of different synthesis variables go verning the crystallization of high silica zeolitic materials of the pentasil family. The use of Al-rich ingredients and polymeric silica generates a small number of nuclei which grow involving a liquid pha se ion transportation process and yield large ZSM-5 single crystals (synthesis of type A). When high Si/Al ratios and monomeric Na silicate are used (synthesis of type Β), numerous nuclei rapidly yield very samll ZSM-5 microcrystallites, which appear directly within the hydrogel and which are not de tectable by X-ray diffraction. Their formation and growth are strongly dependent on competitive inte ractions between alkali and Pr N cations with aluminosilicate polymeric anions. Some intrinsic pro perties of the alkali cations such as their size, their structure-forming (breaking) role towards wa ter or their salting-out power, modify the specifi city of such interactions, which greatly affect the morphology, size, chemical composition and homoge neity of the resulting crystallites. Et N , Pr N and Bu N cations specifically direct the structure of ZSM-8, ZSM-5 and ZSM-11 zeolites respectively. When tripropylamine or tributylamine is used, mixed ZSM-5/ZSM-11 phases are formed. Their nature seems to be determined more by the zeolitic channel fil ling than by the location of the organic molecules at the channel intersections. +

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Current address: Mobil Research and Development Corporation, Central Research Division, Princeton, NJ 08540. 0097-6156/84/0248-0219$09.25/0 © 1984 American Chemical Society

In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

Downloaded by UNIV LAVAL on November 5, 2015 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch012

220

CATALYTIC MATERIALS

Since the f i r s t attempts of B a r r e r ( J ) and K e r r (2) to synthesize counterparts of some n a t u r a l z e o l i t e m a t e r i a l s i n presence of t e tramethy1ammonium hydroxyde, a wide number of new z e o l i t e s have been prepared from various organic c a t i o n - c o n t a i n i n g mixtures ( 3 ) . Soon, z e o l i t e s w i t h high S i / A l r a t i o appeared a t t r a c t i v e because of t h e i r p o t e n t i a l thermal, hydrothermal and a c i d s t a b i l i t i e s . In that respect, a v a r i e t y of organics have proved s u c c e s s f u l i n pro ducing S i - r i c h z e o l i t e s ( 3 ) . Z e o l i t e s ZSM-5 and ZSM-11 are the most commercially important end-members of a continuous s e r i e s of intermediate s t r u c t u r e s be longing to the s o - c a l l e d p e n t a s i l f a m i l y (4,5). The f i r s t prepa r a t i o n of ZSM-5 was described i n 1972 (6) and s i n c e then, a number of elaborate synthesis r e c i p e s have been reported i n the patent l i t e r a t u r e . Because of the unique and f a s c i n a t i n g a c t i v i t y and (shape) s e l e c t i v i t y of t h i s m a t e r i a l f o r a v a r i e t y of c a t a l y t i c r e a c t i o n s c u r r e n t l y processed i n chemical i n d u s t r i e s , i n c r e a s i n g a t t e n t i o n has been devoted to a b e t t e r understanding of the va r i o u s mechanisms that govern the synthesis of ZSM-5 (7-33). As numerous f a c t o r s a f f e c t simultaneously, i n various ways, the nature and the r a t e of the n u c l e a t i o n and c r y s t a l l i z a t i o n pro cesses of h i g h l y s i l i c e o u s z e o l i t e s (3,34), our p r e l i m i n a r y e f f o r t s were d i r e c t e d towards the i n v e s t i g a t i o n of two d i f f e r e n t patent procedures d e s c r i b i n g the p r e p a r a t i o n of ZSM-5 from the c l a s s i c a l Na20-(Pr4N)20-Al 03-Si02-H 0 r e a c t i o n mixtures (6,35). In p a r t i c u l a r , c l a t h r a t e and template e f f e c t s played by the Pr4N c a t i o n s during s y n t h e s i s , were questioned (11). For the f i r s t time, e v i dence was obtained that two extreme synthesis mechanisms can go vern the formation and growth of ZSM-5 c r y s t a l l i t e s . E i t h e r a l i quid phase i o n t r a n s p o r t a t i o n (mechanism A) or a s o l i d hydrogel r e c o n s t r u c t i o n (mechanism B) can occur, depending e s s e n t i a l l y on the source of s i l i c a and on the r e l a t i v e concentrations of the r e a c t a n t s . Very r e c e n t l y , the existence of these two types of me chanisms was a l s o a s c e r t a i n e d to occur i n the case of the n u c l e a t i o n and growth of the (NH4"*", Pr^N" ")ZSM-5 z e o l i t e , i n absence of a l k a l i cations (30). High c r y s t a l l i z a t i o n r a t e s and the p o s s i b i l i t y to s t a b i l i z e X-ray amorphous phases, which e x h i b i t ZSM-5 l i k e p r o p e r t i e s , were among the reasons why we decided to i n v e s t i g a t e the procedure Β i n more d e t a i l . In order to optimize the p a r t i c l e s i z e , homoge n e i t y , morphology and composition, we have questioned more s y s t e m a t i c a l l y the i n f l u e n c e of secondary synthesis v a r i a b l e s such as the pH, solvent v i s c o s i t y or the nature of the a l k a l i c a t i o n , ad ded as c h l o r i d e . As f a r as we know,the i n f l u e n c e of the v i s c o s i t y of the me dium on the s i z e and d i s t r i b u t i o n of the ZSM-5 c r y s t a l l i t e s has never been reported i n the l i t e r a t u r e . The strong dependence of the i n d u c t i o n p e r i o d r e q u i r e d f o r a z e o l i t e nucleus to form, on a l k a l i n i t y (pH), has been emphasized by Barrer (36). Yet, ZSM-5 z e o l i t e s appear to behave r a t h e r d i f f e r e n t l y than other A l - r i c h z e o l i t e s . The OITVSiC^ r a t i o was recognized by Rollmann as the 2

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GABELICA ET AL.

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Synthesis of Pentasil Zeolites

dominant v a r i a b l e i n i n f l u e n c i n g n u c l e a t i o n and c r y s t a l l i z a t i o n rates of ZSM-5, as w e l l as the f i n a l c r y s t a l morphologies, s i z e s and agglomeration (37,38). Other s t u d i e s (14,24) have demonstra ted that an optimum 0H~ c o n c e n t r a t i o n i s needed to maintain enough d i s s o l v e d hydroxy S i and A l species but not i n such excess as to i n h i b i t the subsequent n u c l e a t i o n and growth of ZSM-5 c r y s t a l l i t e s . A l k a l i ions ( s a l t s ) i n f l u e n c e the formation of the precursor g e l f o r most of the s y n t h e t i c z e o l i t e s (3,34,39,40). Na ions we re shown to enhance i n v a r i o u s ways the n u c l e a t i o n process ( s t r u c t u r e - d i r e c t i n g r o l e ) (40-42), the subsequent p r e c i p i t a t i o n and c r y s t a l l i z a t i o n of the z e o l i t e ( s a l t i n g - o u t e f f e c t ) ( J ) and the f i n a l s i z e and morphology of the c r y s t a l l i t e s (34,43). Informa t i o n s on the v a r i o u s r o l e s played by the i n o r g a n i c ( a l k a l i ) c a t i o n s i n s y n t h e s i s of ZSM-5, such as reported i n some recent p u b l i c a t i o n s (7,8,10,14,17,29,30,44,45) remain fragmentary, sometines c o n t r a d i c t o r y and e s s e n t i a l l y q u a l i t a t i v e . Except f o r the work of Erdem and Sand, who i n v e s t i g a t e d the (Na , K , Pr^N" ") system (],&) , and the recent study of Nastro and Sand ( 2 9 1 ) of the ternary c a t i o n i c ( N H , M (M = L i , Na , K ) , Pr^N" ") systems, no other competitive i n t e r a c t i o n between Na , P r ^ N and an a l k a l i c a t i o n other than Na, has been reported. The l a s t p a r t of our work c o n s i s t s i n extending our i n v e s t i gations of v a r i a b l e s i n f l u e n c i n g the synthesis of type B, by exa mining the p o t e n t i a l i t y of v a r i o u s organic compounds to d i r e c t the formation of p e n t a s i l - t y p e s t r u c t u r e s . Pr^N* c a t i o n s were recognized to form complexes w i t h s i l i c a te (46) or a l u m i n o s i l i c a t e (23) species and subsequently to cause r e p l i c a t i o n of the so formed framework s t r u c t u r e v i a s t e r e o - s p e c i f i c i n t e r a c t i o n s (template e f f e c t ) . During t h i s process, they are p r o g r e s s i v e l y i n c o r p o r a t e d and s t a b i l i z e d w i t h i n the z e o l i t e f r a mework (21>ΑΖ»Μ). Rollmann (37) has shown that i n i t i a l P^N" "/ S1O2 r a t i o s above 0.05 are r e q u i r e d to f i l l the whole z e o l i t i c po re volume. These optimum P r ^ N concentrations have been r e c e n t l y r e d e f i n e d (18). The presence of n e a r l y 4 P^N* e n t i t i e s per u n i t c e l l of c r y s t a l l i n e ZSM-5 has been confirmed experimentally(11,23). While ZSM-11 and ZSM-8 ( i t s supposed s t r u c t u r a l analogue (49, 50)) need s p e c i f i c a l l y the presence of Bu^N (or BU4P ) (51,52) and Et^N* (53) to be formed, ZSM-5 can be prepared by u s i n g Pr^N* c a t i o n s , or i t s p r e c u r s o r s , or a wide v a r i e t y of other organic compounds, or even from r e a c t i o n mixtures f r e e of organic c a t i o n s , i n the presence of (Pr^N" ") ZSM-5 seeds (54). On the other hand, some organic compounds may y i e l d l e s s s p e c i f i c z e o l i t i c s t r u c t u r e s . For example, a number of ZSM-5/11 intergrowths could be pre pared u s i n g triethylpropylammonium c a t i o n s (55) or v a r i o u s other ALk^N c a t i o n s , e i t h e r as admixture or w i t h the a d d i t i o n of other organics (56). Since the a c t u a l r o l e of the v a r i o u s organics i n d i r e c t i n g s p e c i f i c z e o l i t i c s t r u c t u r e s remains confusing or at l e a s t obscure, we have t r i e d to e x p l a i n the nature and the s t r u c t u r e of the d i f f e rent p e n t a s i l z e o l i t e s synthesized under i d e n t i c a l experimental +

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CATALYTIC MATERIALS

conditions, using various Alk^N trialkylamines.

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c a t i o n s and the corresponding

Experimental M a t e r i a l s and s y n t h e s i s procedures I n s y n t h e s i s A ( e s s e n t i a l l y based on example 1 of r e f . 6 ) , a s o l u t i o n o f sodium aluminate, prepared by d i s s o l v i n g adequate amounts of pure aluminium powder (Al 99 % pure, Fluka) i n concentrated NaOH (U.C.B. a n a l , grade), i s added to a s o l u t i o n of s i l i c a (Davison grade 950), p r e l i m i n a r i l y d i s s o l v e d i n a s o l u t i o n of Pr^NOH (20 % i n H 0, F l u k a , purum). The so obtained g e l (pH - 11.5) i s s t i r r e d f o r 2 h before h e a t i n g . I n s y n t h e s i s B ( e s s e n t i a l l y based on example 1 of r e f . 3 5 ) , an a c i d i c s o l u t i o n , prepared by mixing A l s u l f a t e ( A 1 ( S 0 ) 3 . 18 H 0, Merck, purum), s u l f u r i c a c i d (98 % H S 0 , Merck, a n a l , grade) and P r N B r ( F l u k a , purum) p r e d i s s o l v e d i n H 0, i s added to a b a s i c s o l u t i o n o f aqueous sodium s i l i c a t e (Natronwasserglass Merck, 28.5 wt % SiO^, 8.8 wt % Na 0 and 62.7 wt % H 0 ) . The so formed g e l (pH - 2) i s s t i r r e d f o r 1 h before i t s pH i s adjusted to about 11 by adding dropwise a concentrated NaOH s o l u t i o n . The new g e l i s s t i r r e d f o r 3 h, l e f t f o r 65 h a t room temperature and then s t i r r e d again f o r 12 h, p r i o r t o h e a t i n g . Due to the very time consuming o f the above procedure, the l a t t e r was m o d i f i e d as f o l l o w s ( s y n t h e s i s B ) : equal volumes of s i l i c a t e and A l - P r N - H S O , s o l u t i o n s were added a t the same r a t e to an aqueous s o l u t i o n o f NaCl (Merck, a n a l , grade). The g e l which i s formed (pH - 3.5), i s s t i r r e d f o r 2 h before i t s pH i s adjusted to about 9-9.5 by f u r t h e r adding o f the remaining excess of Na s i l i c a t e s o l u t i o n . The so obtained g e l i s s t i r r e d f o r 3 h before heating. The molar compositions of thef i n a l mixtures f o r the syntheses A, Β and Β are thef o l l o w i n g • 2


2

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410 4020 2440

47.1

The t o t a l amounts of hydroxide i o n s , c a l c u l a t e d by summing moles of OH" added (as NaOH, Pr^NOH, sodium aluminate or sodium s i l i c a t e ) and by s u b s t r a c t i n g moles of a c i d added (as H S 0 o r a l u minium s u l f a t e ) , are 16.45, 19.3 and 8.8 f o r mixtures A, Β and Β r e s p e c t i v e l y . T h e i r f i n a l i n g r e d i e n t molar r a t i o s are compared i n Table I . 2

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Table

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A 0.68 0.085 0.60 17.0 12.2 0.97 0.14

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OH"/Si0 Na /Si0 Pr N /Si0 H 0/Si0 Si/Al Al/Na Ν320/(ΡΓ Ν) 0 4

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Comparison of the i n g r e d i e n t molar r a t i o s f o r v a r i o u s ZSM-5 s y n t h e s i s procedures

Ingredient molar r a t i o s



GABELICA ET AL.

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Β Β 0. 10 0.20 0.72 1.08 0.10 0.09 45.10 25.3 44.6 48.3 0.031 0.019 6.98 11.63

preferred 0.01-0.2 0.1-0.6 0.02-0.2 15-50 10-1000 1-0.003 5-30

broad 10-8-1.0 0.1-1.5 0.01-0.6 5-200 6.2-™ 1.6-0 0.17-150

N u c l e a t i o n and c r y s t a l l i z a t i o n of samples prepared according to procedures A and B were s t u d i e d as a f u n c t i o n of time, by d i v i ding the g e l a t i n o u s mixtures i n t o s e v e r a l p o r t i o n s and s e a l i n g them i n i d e n t i c a l 15 ml PYREX c o n t a i n e r s . The l a t t e r were heated at 120°C (procedures A and B) and at 130°C (procedure B ) i n an au t o c l a v e , under autogenous pressure, w i t h o c c a s i o n a l shaking, f o r given periods of time, and p r o g r e s s i v e l y removed i n order to ob t a i n m a t e r i a l s w i t h i n c r e a s i n g degrees of c r y s t a l l i n i t y . A f t e r c o o l i n g , each sample ( g e l + z e o l i t e ) was f i l t e r e d , washed w i t h c o l d d i s t i l l e d water and d r i e d at 120°C f o r 14h, before c h a r a c t e r i z a tion. 100 % c r y s t a l l i n e ZSM-5 z e o l i t e s were obtained a f t e r 312 h (type A)., a f t e r 48 h (type B) and a f t e r 110 h (type B ) . I n each case i t was checked that longer c r y s t a l l i z a t i o n times d i d not lead to more c r y s t a l l i n e ZSM-5 or to transformations i n t o other phases. Table I I gives parameters and o p e r a t i n g c o n d i t i o n s f o r the three procedures. 1

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M o d i f i e d syntheses To compare the i n f l Ç ^ 2 É P 5 crystalliz a t i o n , a g e l p o r t i o n , as prepared according to the procedure B , was d i v i d e d i n t o three p o r t i o n s , where the pH i s e i t h e r l e f t at 9, r a i s e d to 11 or lowered to 7, by adding small amounts of concentrated NaOH or H S 0 r e s p e c t i v e l y . To check the e f f e c t of v i s c o s i t y of the medium, the g e l , as obtained i n procedure B , was f i l t e r e d from i t s mother l i q u o r and an e q u i v a l e n t volume of g l y c e r o l was added to the s t i l l wet g e l . The a d d i t i o n of other a l k a J L i c a ^ i o n s than Na to the i n i t i a l mixture was r e a l i z e d by mixing equal v o l u mes of s i l i c a t e and a c i d s o l u t i o n s (procedure B ) w i t h a s o l u t i o n of the corresponding a l k a l i metal c h l o r i d e ( L i C l , N H 4 C I , K C 1 , RbCl, C s C l , a l l Merck, a n a l , grade). The e f f e c t of the ηatureοftheοrJ?i?Bl£—° E~~ k e d by s u b s t i t u t i n g P r N B r i n the procedure Β , by e q u i v a l e n t molar amounts of Bu NBr ( A l d r i c h ) , E t N B r ( A l d r i c h ) , t r i p r o p y l a m i n e (ΡΓβΝ) ( A l d r i c h ) , t r i b u t y l a m i n e (BU3N) (Merck, purum) and a Pr^N : η-propyl bromide ( A l d r i c h ) mixture. A l l the syntheses were stopped a f t e r the g e l a t i n o u s phase had disappeared, y i e l d i n g heavy c r y s t a l l i n e products and transparent mother l i q u o r s . The time r e q u i r e d f o r the c r y s t a l l i n e z e o l i t e to separate was u e n

o n

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ingredients

120 320

Time(h) r e q u i r e d f o r 100 % c r y s t a l l i n i t y

conditions

2(basic)

Heating temp. (°C)

Synthesis

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Other

4

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Si0

Source of S i 2

Na aluminate (Al+NaOH)(basic)

type

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Synthesis 4

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120

Hydrothermal, PIREX tubes

110

130

2(ac.)+3(basic)

1(ac.)+80(basic)

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Na s i l i c a t e (basic)

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sulfate+H S0 +Pr NBr (acid)

NaOH

Al

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Table I I . Comparison o f the parameters and operating c o n d i t i o n s f o r v a r i o u s ZSM-5 s y n t h e s i s procedures


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considered as a q u a l i t a t i v e e v a l u a t i o n o f the k i n e t i c s o f c r y s t a l l i z a t i o n . Hydrothermal heading was stopped u s u a l l y two days a f t e r the c r y s t a l l i n e phases had appeared. A l l the o r g a n i c - c o n t a i n i n g z e o l i t e precursors were f i l t e r e d , washed and d r i e d a t 120°C f o r 14 h. C h a r a c t e r i z a t i o n of the s o l i d phases S i , A l and Na contents were determined by proton-induced γ-ray emission (PIGE) ( 11,57) o r by high r e s o l u t i o n s o l i d s t a t e 29SÎ-NMR spectroscopy (49,50,58) (bulk a n a l y s i s ) , by e n e r g y - d i s p e r s i v e X-ray a n a l y s i s (EDX) (26) (outer r i m a n a l y s i s ) and by XPS (59) (surface a n a l y s i s ) , S i and A l being detected r e s p e c t i v e l y t o a depth of 8-10 μπι, 1-2 urn, and 5-10 nm. S i , A l , Na, K, Rb and Cs contents were a l s o estimated by probing by EDX both z e o l i t e p e l l e t s of 1 cm diameter and 0.1 cm thickness ( f o r average analyses) and i n d i v i d u a l z e o l i t e c r y s t a l l i tes d i s p e r s e d and d i r e c t l y glued onto the support ( f o r i n d i v i d u a l a n a l y s e s ) . The L i and N H contents were determined r e s p e c t i v e l y by PIGE (57) and TG (28). Morphologies and p a r t i c l e s i z e s were determined by Scanning E l e c t r o n Microscopy (SEM) (11). The natu re of the z e o l i t e s , t h e i r degree of c r y s t a l l i n i t y and the percen tage of ZSM-5 i n the a s - s y n t h e s i z e d intermediate ( g e l + z e o l i t e ) phases were evaluated from X-ray d i f f r a c t i o n (XRD) (11) and thermal a n a l y s i s data (60). I n the l a t t e r case, i t was assumed that both the area o f the DTA exotherm corresponding to the oxydative decom p o s i t i o n between 300 and 600°C of the P r N species occluded i n the c r y s t a l l i n e p r e c u r s o r , and the corresponding weight l o s s , as measured by TG, were p r o p o r t i o n a l to the a c t u a l amount o f P r ^ N and hence to the % of ZSM-5 present. The water and the organic content of the new p e n t a s i l phases were measured by TG/DTA, as p r e v i o u s l y reported (11,28,60). The nature of the o r g a n i c guest molecule occluded w i t h i n the precursors was i d e n t i f i e d by CP-MAS C NMR spectrometry (28,61). +

4

+

4

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Results and D i s c u s s i o n Composition of s y n t h e s i s procedures A, B , and B ' Table I gives the i n i t i a l r e a c t a n t molar r a t i o s used i n the A, Β and B proce dures. Although a l l of them do f a l l w i t h i n the range which f a vours the formation o f ZSM-5 from Si02-Al203-Pr N -Na20-H 0 mix tures (37), those used i n procedure A d i f f e r s u b s t a n t i a l l y from the r a t i o s chosen f o r Β or B . Rollmann has emphasized the i n fluence of most of these v a r i a b l e s on the mechanism that govern the n u c l e a t i o n and growth r a t e s of ZSM-5 (37) which, i n t u r n , i n fluence the s i z e and/or the morphology of the c r y s t a l l i t e s . For example, as the P r N / S i 0 2 r a t i o i n c r e a s e s , the p o l y c r y s t a l l i ne agglomerate i n t e r g r o w t h s , whose formation i s favoured by a h i g h 0H~/Si02 v a l u e , are p r o g r e s s i v e l y r e p l a c e d by an i n c r e a s i n g amount of w e l l d i s p e r s e d s i n g l e c r y s t a l s . The mechanisms d e s c r i b i n g the formation of ZSM-5 from procedures A and Β have been p r e v i o u s l y compared and discussed e x t e n s i v e l y (11). They can be summarized as f o l l o w s : T

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In the case of S Y n t h e s i s A , the d e p o l y m e r i z a t i o n of the a c t i ve s i l i c a to y i e l d u l t i m a t e l y monomeric s i l i c a t e anions, was shown to be the e s s e n t i a l r a t e l i m i t i n g s t e p . As a r e s u l t , a small num ber of n e g a t i v e l y charged monomers are formed. They can e i t h e r condense aluminate species to form a l u m i n o s i l i c a t e complexes or i n t e r a c t d i r e c t l y w i t h P r ^ N i o n s . The l a t t e r are known to order around them p r e f e r a b l y S i - r i c h e r (alumino) s i l i c a t e t e t r a h e d r a l u n i t s to form s t a b l e n u c l e i . Y e t , growing ZSM-5 c r y s t a l l i t e s are known to accomodate s i l i c a i n preference (9,21,30,62) and any i n c o r p o r a t i o n of aluminium i n t o t h e i r framework i s a d i s r u p t i v e , d i f f i c u l t process (12,37,38). Consequently, the c r y s t a l l i z a t i o n of the so formed S i - r i c h n u c l e i w i l l occur at l e a s t more r a p i d l y than a f u r t h e r n u c l e a t i o n , so that the number of c r y s t a l l i t e s i n i t i a l l y formed stays n e a r l y constant d u r i n g the s y n t h e s i s course. The p a r t i c l e s w i l l grow w i t h a r e l a t i v e l y f a s t r a t e at the expense of the s t i l l present a l u m i n o s i l i c a t e g e l which, at the l i m i t , can be occluded w i t h i n the growing c r y s t a l l i t e . As the s i l i c a t e spe c i e s a v a i l a b l e i n s o l u t i o n are p r o g r e s s i v e l y exhausted, the g e l w i l l continue to d i s s o l v e and b r i n g p r o g r e s s i v e l y A l - r i c h s o l u b l e species to the outer l a y e r of the growing p a r t i c l e . The r e s u l t i n g l a r g e and w e l l d e f i n e d ZSM-5 c r y s t a l l i t e s are t h e r e f o r e expected to present an inhomogeneous r a d i a l A l d i s t r i b u t i o n . An attempt to c o n f i r m t h i s hypothesis was proposed on the b a s i s of a simple mathematical treatment of the S i / A l r a t i o s , as obtained by PIGE analyses (11). In t y j 3 e B s y n t h e s e s , a hydrogel i s r a p i d l y formed from s o l u t i o n s a l r e a d y composed of monomeric (or low o l i g o m e r i c ) s i l i c a and alumina s p e c i e s . Y e t , i t s composition i s not too d i f f e r e n t from that expected from the reagents r a t i o s (Table I ) , as the supply of s i l i c a t e ions i s not l i m i t e d by any d e p o l y m e r i z a t i o n process. Smaller amounts of Pr^N* i o n s , as w e l l as s u b s t a n t i a l l y h i g h e r S i / A l and Na/Si r a t i o s than f o r the A s y n t h e s i s (Table I ) , favour a r a p i d n u c l e a t i o n . S t r u c t u r e - d i r e c t i n g Pr^N" " c a t i o n s , s t i l l p r e sent a l l throughout the g e l , can i n t e r a c t i n t i m a t e l y w i t h the nu merous r e a c t i v e a l u m i n o s i l i c a t e anions and a d i r e c t r e c r y s t a l l i z a t i o n process i n v o l v i n g the s o l i d hydrogel phase t r a n s f o r m a t i o n (or surface n u c l e a t i o n ) i s expected. Indeed, a r a p i d growth, y i e l d i n g a l a r g e number of s m a l l c r y s t a l l i t e s , which present a homogeneous A l r a d i a l d i s t r i b u t i o n , was confirmed e x p e r i m e n t a l l y (11). Recent r e s u l t s obtained by Sand and coworkers (21,30) have confirmed the e x i s t e n c e of these two d i f f e r e n t mechanisms : i n a g e l obtained from Si02~Pr NOH s o l u t i o n s , ZSM-5 was shown to grow w i t h i n a l i q u i d s l u r r y , w h i l e i t appears d i r e c t l y w i t h i n a wax l i k e g e l formed when Na s i l i c a t e and A l s u l f a t e are used as r e a gents. Figures 1 and 2 summarize the main f e a t u r e s of type A and Β syn theses, w h i l e F i g u r e 3 r e f l e c t s the d i f f e r e n t morphologies of 100% c r y s t a l l i n e ZSM-5 p a r t i c l e s grown from both procedures. The s y n t h e s i s of type Β o f f e r s s e v e r a l advantages among which the most promising f o r c a t a l y t i c a p p l i c a t i o n s i s the p o s s i b i l i t y


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Figure 1. Schematic r e p r e s e n t a t i o n of type A s y n t h e s i s (Adapted from r e f (11) and reproduced w i t h p e r m i s s i o n , E l s e v i e r S c i . P u b l . Co.).

F i g u r e 2. Schematic r e p r e s e n t a t i o n of type Β s y n t h e s i s . (Adapted from r e f (11) and reproduced w i t h permission, E l s e v i e r S c i . P u b l . Co.).



CATALYTIC MATERIALS

Figure 3. T y p i c a l morphologies of 100 % c r y s t a l l i n e A and B-type ZSM-5 (SEM micrographs) (Reproduced with permission, from r e f (11), E l s e v i e r S c i . Publ. Co.).


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to prepare r a p i d l y small 100 % c r y s t a l l i n e ZSM-5 p a r t i c l e s w i t h homogeneous A l d i s t r i b u t i o n and a l s o the p o s s i b i l i t y t o s t a b i l i z e very small "X-ray amorphous" ZSM-5 c r y s t a l l i t e s , embedded w i t h i n amorphous a l u m i n o s i l i c a t e g e l precursors (11). I n order t o con f i r m and improve these p r o p e r t i e s , the o p e r a t i n g c o n d i t i o n s o f synthesis Β were s l i g h t l y m o d i f i e d . P r e p a r a t i o n of X~ray amorphous ZSM-5 c r y s t a l l i t e s according to procedure B I t i s important that the g e l formation takes p l a c e as homogeneously as p o s s i b l e . Because of the p a r t i c u l a r s e n s i t i v i ty of v a r i o u s s i l i c a and alumina species to the pH (63,64) , the pH range between 4.5 and 8.5 was avoided. N u c l e a t i o n was performed at pH 3.5-4, where a low v i s c o u s g e l c o n t a i n i n g e s s e n t i a l l y mono meric s i l i c a species i s r a p i d l y formed (65).The. pH i s then r a i s e d to about 9, i n order to form t e t r a h e d r a l A1(0H) ~" e n t i t i e s and to favour the f u r t h e r A l i n c o r p o r a t i o n w i t h i n the z e o l i t i c framework. NaCl was used to increase the ( s u p e r ) s a t u r a t i o n of the g e l , which w i l l f l o c c u l a t e i n t o a macromolecular c o l l o i d ( J ) and i n i t i a t e the n u c l e a t i o n . This procedure y i e l d s 100 % c r y s t a l l i n e z e o l i t e a f t e r 110 h hydrothermal h e a t i n g at 130°C (Figure 4 ) . Longer h e a t i n g times (6 months) n e i t h e r a l t e r the p u r i t y nor the s t a b i l i t y of the c r y s t a l l i t e s (Figure,4). The c r y s t a l l i n i t y of the so formed intermediate phases was checked by v a r i o u s p h y s i c a l methods. No XRD c r y s t a l l i n i t y was de tected a f t e r the f i r s t 45 hours of h e a t i n g , w h i l e other techniques such as I n f r a r e d (20,6^,67), TG-DTA 0^,32,67) o r C NMR (32,33), which are s e n s i t i v e to the presence of very small amounts of P r N species occluded i n ZSM-5 c r y s t a l l i t e s , confirm that very small s i z e ZSM-5 p a r t i c l e s are present i n the e a r l y beginning of the syn t h e s i s process (Table I I I ) . F u r t h e r IR s t u d i e s of the ZSM-5 ske l e t o n v i b r a t i o n bands (20,66) o r c a t a l y t i c t e s t s (60) have c o n f i r med t h e i r presence i n B-type procedures. By c o n t r a s t , and as ex pected, XRD and DTA techniques give i d e n t i c a l c r y s t a l l i n i t y values i n the case of s y n t h e s i s A (Table I I I ) .


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Aluminium d i s t r i b u t i o n i n A and B intermediate and f i n a l phases Various compositional zonings have been reported i n s i l i c a - r i c h ZSM-5 z e o l i t e s prepared under p a r t i c u l a r c o n d i t i o n s (11-13,19,26, 27,31,32,59,68) w h i l e other s t u d i e s provided evidence f o r homoge neous A l d i s t r i b u t i o n throughout the c r y s t a l l i t e s (69-71). Ob v i o u s l y , the d i s t r i b u t i o n of aluminium i n ZSM-5 must depend on i t s mechanism of c r y s t a l l i z a t i o n . In order to b r i n g experimental c o n f i r m a t i o n of the A l r a d i a l d i s t r i b u t i o n w i t h i n the A-type c r y s t a l l i t e s and on homogeneous compo s i t i o n s of B-type phases (11) , we have reexamined t h e i r composition by u s i n g the PIGE, EDX and XPS techniques, which are s e l e c t i v e l y able to probe the presence of S i and A l a t v a r i o u s depths. Table 111 compares the S i / A l r a t i o s f o r s e v e r a l intermediate phases (gel + z e o l i t e ) , i s o l a t e d a f t e r d i f f e r e n t c r y s t a l l i z a t i o n times, from syntheses A and B . T



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S y n t h e s i s time (h) 1

F i g u r e 4. T y p i c a l c r y s t a l l i z a t i o n curves f o r A, Β and B type syntheses.


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Table I I I . V a r i a t i o n s of composition and c r y s t a l l i n i t y i n A B - t y p e intermediate and f i n a l phases.

and

T

Synthesis Type


A

Cryst.time(h) 0* 48 136 192 264 312 0* 14 20 26 32 38 42 49 64 110 Average values

S i / A l at. ratio TÏGE 12.2* 1.8 5.0 10.0 12.1 13.2 45.0* 33.6 32.0 34.2 34.1 34.3 34.3 34.3 32.0 36.1 : 35

EDX 3.5 11.7 16.7

XPS(73) 5.0 11.0 15.5

-

-

15.3

10.0

26.8 26.9 27.0 27.3 27.5 28.6 28.0 26.7 27.0

28.0

(34)

20 24 24 23

% crystallinity XRD

DTÂ

0 45 86 96 100

0 50 92 100

0 0 0 0 0 0 12 38 100

34 " 34 XRD 34 Amorphous 38 42 Zeolites 43 49 63 100

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27.5

S i / A l i n aqueous g e l , from i n g r e d i e n t s Figure 5 shows that f o r the type A s y n t h e s i s , the o r i g i n a l l y very low S i / A l r a t i o (PIGE) p r o g r e s s i v e l y increases and reaches, at the end of the process, values close to that of the s t a r t i n g i n g r e d i e n t s . This i s c o n s i s t e n t w i t h the assumption that S i - r i c h e r ZSM-5 c r y s t a l l i t e s grow from progressive d i s s o l u t i o n of the A l r i c h g e l phase. The l a t t e r , i f i s o l a t e d and d r i e d i n the beginning of the c r y s t a l l i z a t i o n ( a f t e r 48 h ) , must be e s s e n t i a l l y A l r i c h , as most of the s o l u b l e p o l y s i l i c a t e f r a c t i o n present i n a Pr^NOH-silica s o l i s not r e t a i n e d by f i l t r a t i o n ( 1 1 ,65,72). EDX measurements show the same t r e n d , except that the Si7Àl r a t i o s are always h i g h e r . Indeed, the EDX technique only probes the upper l a y e r of the s o l i d sample which i s supposed to c o n t a i n l e s s A l . As the g e l p r o g r e s s i v e l y d i s s o l v e s to y i e l d S i - r i c h e r z e o l i t i c phases, the EDX and PIGE S i / A l r a t i o s become c l o s e r . However, even f o r a 100 % c r y s t a l l i n e phase, l e s s A l i s s t i l l probed by EDX. This suggests that the Α-synthesis ZSM-5 c r y s t a l s must s t i l l c o n t a i n some A l - r i c h amorphous phase, deeply embedded w i t h i n the l a r g e p a r t i c l e s , as to p a r t i a l l y escape the EDX probing. For the same reason, the XPS a l s o continuously probes the outer S i - r i c h l a y e r of the growing p a r t i c l e s . However, at the end of the c r y s t a l l i z a t i o n , more A l i s detected on the outer rim than i n the



CATALYTIC MATERIALS

Synthesis time

(h)

F i g u r e 5. V a r i a t i o n of the bulk (PIGE), outer l a y e r (EDX) and surface (XPS) S i / A l r a t i o s and XRD c r y s t a l l i n i t i e s , of intermediate phases, as a f u n c t i o n of s y n t h e s i s time, f o r procedures A and B . 1


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bulk (73). This important f i n d i n g i s s t r o n g l y i n favour of the formation of c r y s t a l l i t e s having more A l i n t h e i r outer l a y e r . For s y n t h e s i s B , both PIGE and EDX S i / A l r a t i o s remain remarka b l y constant throughout the whole c r y s t a l l i z a t i o n process. This i n d i c a t e s that both the growing c r y s t a l l i t e s and t h e i r g e l precur sor must continuously keep the same composition. This i s c o n s i s tent w i t h the d i r e c t hydrogel transformation mechanism. In that case, however, EDX and XPS s y s t e m a t i c a l l y detect s l i g h t l y more A l near the s u r f a c e , suggesting that i n the beginning of the B* pro cess, more bulky S i - r i c h inhomogeneous domains are present w i t h i n the g e l . Through i n t e r n a l rearrangements, they w i l l transform p r o g r e s s i v e l y i n ZSM-5 n u c l e i . The l a t t e r w i l l grow w i t h a r a t e p r o p o r t i o n a l to t h e i r S i - c o n t e n t and y i e l d z e o l i t e c r y s t a l l i t e s w i t h d i f f e r e n t s i z e s but w i t h an homogeneous A l d i s t r i b u t i o n w i t h i n each phase. The A and B c r y s t a l l i z a t i o n procedures are s c h e m a t i c a l l y i l l u s t r a t e d i n Figure 6.


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Influence of pH on the c r y s t a l l i z a t i o n of ZSM-5 Three i d e n t i c a l Β'-type p o r t i o n s from the same batch have been heated at 130°C at three d i f f e r e n t pH v a l u e s . T h e i r s y n t h e s i s c h a r a c t e r i s t i c s and v a r i o u s p h y s i c a l parameters are compared i n Table IV. F i g u r e 7 i l l u s t r a t e s t h e i r morphological d i f f e r e n c e s . Table IV.

V a r i a t i o n of d i f f e r e n t parameters as a f u n c t i o n of pH (synthesis B ) T

EÎL Sam before/after Cryst. ple heating time(h) B >64 6.9 8.4 1 9.0 10.0 44 f

11.05

10.9

15

Si/Al (PIGE) 13.2 30.0 27.9

size (ym) 5x10 5 5-20 2

Crystals morphol. twins polycryst, aggreg.

Amorph. phase amount (mass%) (mass%) 70 30 100

Low pH values lead to long c r y s t a l l i z a t i o n times and incomplete g e l t r a n s f o r m a t i o n s . At h i g h pH v a l u e s , pure ZSM-5 c r y s t a l s are obtained a f t e r a very short time. They have the same morphology as those grown at pH 9 but appear s m a l l e r and homogeneously d i s t r i b u t e d . At low pH (which, i n t h i s case, i s p r o p o r t i o n a l to the 0H~/Si02 r a t i o ) , the d i s s o l u t i o n process would be l a r g e l y absent and w e l l defined i s o l a t e d c r y s t a l s are expected to form (37) , probably from more homogeneous S i - r i c h g e l domains, i n t e r m i x e d w i t h a l u m i n o s i l i c a t e - A l i O H ) ^ inhomogeneous phases, which remain amorphous. The low S i / A l r a t i o confirms these proposals and suggests that the hydrogel transformation i s not the only mechanism through which ZSM-5 i s formed. At h i g h e r 0 H / S i 0 2 r a t i o s , p o l y c r i s t a l l i n e



CATALYTIC MATERIALS

Figure 6. Schematic r e p r e s e n t a t i o n of the S i / A l v a r i a t i o n s i n the s y n t h e s i s A and B mixtures during t h e i r c r y s t a l l i zation. f


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Figure 7. SEM micrographs of ZSM-5 c r y s t a l l i t e s s y n t h e s i zed at d i f f e r e n t pH values ( s y n t h e s i s Β , 130°C.,.4 days). f


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aggregates are formed i n high y i e l d . T h e i r wide s i z e d i s t r i b u t i o n confirms the presence of S i - A l inhomogeneous d i s t r i b u t i o n w i t h i n the g e l . At very high 0H~/Si02, c r y s t a l growth and d i s s o l u t i o n would be competing and an e q u i l i b r i u m , producing smaller c r y s t a l s , would r e s u l t (37). D i s s o l u t i o n w i l l cause a d i f f u s e surface o r i e n t a t i o n of the s t r u c t u r e d i r e c t i n g P r N u n i t s and impede any c r y s t a l formation. +

4


Influence of the nature o f the l i q u i d phase on the c r y s t a l l i z a t i o n of ZSM-5 The aqueous part of a c l a s s i c a l Β'-type g e l was r e p l a ced by the e q u i v a l e n t volume of pure g l y c e r o l . C r y s t a l s obtained a f t e r hydrothermal h e a t i n g of the aqueous and g l y c e r o l gels are compared i n Figure 8 and Table V. Table V.

V a r i a t i o n of d i f f e r e n t parameters as a f u n c t i o n of the nature of the l i q u i d phase of the Β'-type s y n t h e s i s .

Sample L i q u i d phase Material Aver, s i z e (ym) S i z e d i s t r i b u t i o n (ym)

B'4 aqueous 100 % ZSM-5 „ 3.4 (1-7.5)

B'5 glycerol 100 % ZSM-5 9 (8.5-9.5)

Larger s i z e d and more homogeneous p a r t i c l e s are formed i n g l y c e r o l medium. The increase i s v i s c o s i t y probably impedes the d i s s o l u tion-growth competitive process and the growth predominates. On the other hand, the h i g h a f f i n i t y of g l y c e r o l towards water r e s u l t s i n important s u p e r s a t u r a t i o n of the s o l which favours a r a p i d n u c l e a t i o n and growth. As a resuit,numerous and l a r g e c r y s t a l s w i l l be formed. +

Role of a l k a l i and N H c a t i o n s i n the c r y s t a l l i z a t i o n of ZSM-5 Introduced i n an aqueous (alumino) s i l i c a t e g e l ( s o l ) , the bare a l k a l i c a t i o n s w i l l behave i n various ways : f i r s t l y , they w i l l i n t e r a c t w i t h water d i p o l e s and increase the (super) s a t u r a t i o n of the s o l . Secondly, once hydrated, they w i l l i n t e r a c t w i t h the a l u m i n o s i l i c a t e anions w i t h , as a r e s u l t , the p r e c i p i t a t i o n o f the so formed g e l ( s a l t i n g - o u t e f f e c t ) . T h i r d l y , i f s u f f i c i e n t l y s m a l l , they a l s o can order the s t r u c t u r a l subunits precursors to n u c l e a t i o n species of various z e o l i t e s (template f u n c t i o n - f u l f i l l e d by hydrated N a i n the case of ZSM-5 ( J J , 4 8 ) ) . Small bare cations ( L i , N a ) w i l l r e a d i l y order water molecu les i n forming r e g u l a r s t r u c t u r a l e n t i t i e s w i t h a l u m i n o s i l i c a t e species ( s t r u c t u r e - f o r m i n g e f f e c t ) . Larger c a t i o n s ( K , ΝΗ^ , Rb , C s ) i n t e r a c t l e s s w i t h H 0 and even d i s r u p t i t s r e g u l a r s t r u c t u r e s t a b i l i z e d through Η-bonding ( s t r u c t u r e - b r e a k i n g e f f e c t ) . The ve ry l a r g e P r N w i t h s h i e l d e d charge w i l l tend to b u i l d s t a b l e water c l a t h r a t e s (74). They are a l s o s t r u c t u r e - f o r m i n g towards water. 4

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Figure 8. SEM micrographs of ZSM-5 obtained i n aqueous l u t i o n and i n g l y c e r o l (synthesis B , 130°C., 3 days). T


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S i x ZSM-5 samples were s y n t h e s i z e d from gels c o n t a i n i n g the whole s e r i e s of a l k a l i c a t i o n s , added i n form of c h l o r i d e , as des c r i b e d i n procedure B . Synthesis data, p r i n c i p a l p r o p e r t i e s and analyses are summarized i n Tables VI and V I I . 1

Table V I . Synthesis data and p r o p e r t i e s of v a r i o u s (M)ZSM-5 samples (M = a l k a l i c a t i o n ) 'adapted from r e f ( 2 6 ) , by permission)

Zeolite

C r y s t . Aver. H2O content time size (mol./M ) (days) (ym) (mol./u.c.) 13.0 3 1.7 10.5 14.8 3.5 11.8 4.5 8.2 7.5 3.5 18 22 7.8 4 8.6 2.8 7 25 5.6 1.7 93 ( see t e x t ) 2.9


1

(Li)ZSM-5 (Na)ZSM-5 (K) ZSM-5 (Rb) ZSM-5 (Cs) ZSM-5 (NH )ZSM-5 4

Table V I I .

Si/Al

4

4

" S u r f a c e " (EDX) and bulk (PIGE) analyses ot v a r i o u s (M)ZSM-5 samples (adapted from r e f (26) , by permission) Al/Na

Zeolite (Li)ZSM-5 (Na)ZSM-5 (K)ZSM-5 (Rb)ZSM-5 (Cs)ZSM-5 (NH )ZSM-5

+

Pr N content (mol./u.c.) 3.4 3.6 3.7 3.4 3.6 3.9

(PIGE) 32.4 36.1 38.5 47.5 45.2 41.2

(EDX) 31.0 33.5 28.2 35.0 35.5 39.0

(PIGE) 14.2 3.2 13.9 9.1 10.0 70.0

Al/M A l / u . c.Na/u. c. M/u. c. (Na+M) / u.c. (EDX) (PIGE) (PIGE) (EDX) 3.5 2.87 0.07 0.81 0.88 2.4 2.56 0.80 0.80 2.2 2.43 0.17 1.10 1.27 1.8 1.98 0.22 1.10 1.32 1.1 2.08 0.21 2.00 2.21 1.4 2.27 0.03 1.68 1.71

These data show that most of the p h y s i c a l parameters and p r o p e r t i e s which c h a r a c t e r i z e each (M)ZSM-5 sample, show r e g u l a r v a r i a t i o n s as a f u n c t i o n of the nature of the c a t i o n , i . e . e s s e n t i a l l y of i t s s i z e . Except the p a r t i c u l a r behaviour of NH which i s envisaged s e p a r a t e l y , s t a r t i n g from L i t o Cs, the f o l l o w i n g trends are obser ved : c r y s t a l l i z a t i o n times and average p a r t i c l e s s i z e s i n c r e a s e w h i l e the s i z e d i s t r i b u t i o n becomes l e s s homogeneous (Figure 9 ) . The water and A l contents decrease w h i l e the amount of M c a t i o n s per u n i t c e l l of z e o l i t e i n c r e a s e s . For L i and Na,the morphology c o n s i s t s of c l u s t e r s of p o l y c r y s t a l l i n e aggregates. B e t t e r out l i n e d s i n g l e c r y s t a l s are observed f o r Κ and Rb and a d d i t i o n a l pro nounced twinning appears f o r (Cs)ZSM-5 (Figure 10). 4


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80 r

(Na) ZSM-5 0

10

30

20

40

20

(K) ZSM-5 0 20

jjEsL. 10

20

1

1

30 ...

40

j

(Rb) ZSM-5

30

40

c r y s t a l s i z e (μιη) — • Figure 9. P a r t i c l e s i z e d i s t r i b u t i o n f o r the v a r i o u s (M)ZSM-5 z e o l i t e s , as computed from SEM (Reproduction w i t h permission, from r e f (26), E l s e v i e r , S c i . P u b l . Co.).


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240

F i g u r e 10. SEM micrographs comparing s i z e s and morphologies of the v a r i o u s (M)ZSM-5 z e o l i t e s (Reproduced w i t h p e r m i s s i o n , from r e f (26), E l s e v i e r , S c i . P u b l . Co.).


12.

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241


AL.

The various chemical analyses y i e l d the f o l l o w i n g i n t e r e s t i n g conclusions : (a) Small s i z e d c r y s t a l l i t e s are e q u a l l y probed by EDX and PIGE ( S i / A l r a t i o s , Table V I I ) . The bigger the average s i z e , the greater the d i f f e r e n c e between " s u r f a c e and bulk r a t i o s . (b) EDX shows that the average " s u r f a c e " of a p e l l e t composed of v a r i o u s s i z e d c r y s t a l l i t e s , i s always enriched i n A l w i t h respect to the b u l k . On the other hand,(Si/Al) r a t i o s determined by EDX on i n d i v i d u a l c r y s t a l l i t e s i n d i c a t e that i n a l l cases except NH^"", small c r y s t a l l i t e s contain more A l than the l a r g e ones (Figure 11). Combining these two i n f o r m a t i o n s , i t appears that A l must be d i s t r i b u t e d homo geneously w i t h i n the c r y s t a l l i t e s , although each c r y s t a l l i te must have an A l concentration i n v e r s e l y p r o p o r t i o n a l to i t s s i z e . Such a c o n c l u s i o n s u b s t a n t i a l l y confirms that (M)ZSM-5 are produced w i t h i n the g e l by s t r u c t u r a l r e a r r a n gements, as expected f o r a Β'-type procedure. (c) When M (from MCI) i s present i n the g e l , very few Na (from Na s i l i c a t e ) i s incorporated i n the z e o l i t e l a t t i c e (Table V I I ) . A p o s s i b l e e x p l a n a t i o n i s to assume that the " f r e e " M ions (among which Na from NaCl) i n t e r a c t w i t h the a l u m i n o s i l i c a t e g e l (and e v e n t u a l l y d i r e c t i t s s t r u c t u r e t o wards a z e o l i t e framework by template e f f e c t ) , more r e a d i l y than the Na which were o r i g i n a l l y a s s o c i a t e d w i t h the s i l i c a t e anions. (d) (Al/M) r a t i o , greater than u n i t y f o r L i and Na z e o l i t e s , almost reaches u n i t y f o r (Cs) ZSM-5. I t i s concluded that i n the f i r s t case, Pr^N e n t i t i e s act as both s t r u c t u r e d i r e c t i n g u n i t s and exchange c a t i o n s , w h i l e i n the second ca se, they are e s s e n t i a l l y present w i t h i n the z e o l i t e l a t t i c e as Pr4N0H. Further s t u d i e s are i n progress to e l u c i d a t e the a c t u a l s t r u c t u r e of P r ^ N ions i n the (M) ZSM-5 z e o l i tes (75). (e) (Al/M) r a t i o i s constant f o r v a r i o u s c r y s t a l s i z e s (Figure 11), i n d i c a t i n g that the i n c o r p o r a t i o n of the a l k a l i i s go verned by the A l c o n c e n t r a t i o n . The general behaviour of the a l k a l i c a t i o n s i n presence of a l u m i n o s i l i c a t e gels and t h e i r i n f l u e n c e on z e o l i t e n u c l e a t i o n and c r y s t a l l i z a t i o n rates has been discussed i n d e t a i l elsewhere (26). T h e i r s p e c i f i c r o l e i n the formation of the v a r i o u s (M)ZSM-5 zeo l i t e s can be depicted on the b a s i s of the above described observa tions. 11


1

+

1

+

+

+

E f f e c t of C s , a s t r u c t u r e - b r e a k i n g type c a t i o n In a c i d i c medium, the s i l i c a anions w i l l i n t e r a c t p r e f e r e n t i a l l y w i t h the s m a l l e s t p o s i t i v e l y charged e n t i t i e s which appear to be the hydrated C s i o n s . Consequently, a small number of n u c l e i (because s i l i c a t e P r ^ N i n t e r a c t i o n s are l e s s p r e f e r r e d ) , e s s e n t i a l l y S i - r i c h (becau se s i l i c a t e - A l O ^ O ) " " i n t e r a c t i o n s are l e s s favoured) and c o n t a i ning very l i t t l e Na (because s i l i c a t e - N a ( H 2 0 ) i n t e r a c t i o n s are +

+

3

1

+


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242

0

20

40

60

0

Particle size (pm)

20

40 ~~

Figure 11. V a r i a t i o n of the ( S i / A l ) and (Al/M) r a t i o s , as measured by EDX, as a f u n c t i o n of c r y s t a l s i z e , f o r various (M)ZSM-5 z e o l i t e s .


βΟ

12.

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impeded), w i l l be formed. Moreover, because of t h e i r s t r u c t u r e breaking behaviour towards water, the C s c a t i o n s w i l l desorganize the c l a t h r a t e d water s t r u c t u r e of the s i l i c a t e anions, p r e v e n t i n g t h e i r " p r e c i p i t a t i o n a t the P r ^ N centers where the templating e f f e c t occurs. In b a s i c medium, negative A l species appear and condense w i t h the remaining s i l i c a t e e n t i t i e s , as to y i e l d an A l - r i c h a l u m i n o s i l i cate g e l . A few A l - r i c h e r n u c l e i can then form and y i e l d a small amount of A l - r i c h e r ZSM-5 p a r t i c l e s at the end of the process. +

1 1

+

+

+

E f f e c t of N a , a s t r u c t u r e - f o r m i n g c a t i o n Hydrated N a c a t i o n s have a l a r g e s i z e and a more d e l o c a l i z e d charge than the C s i o n s , so that the i n t e r a c t i o n between the s i l i c a t e complex anions and the P r ^ N c a t i o n s i s more favoured i n presence of Na(H2Û) . Even the hydrated A l c a t i o n i c species may i n t e r a c t a t t h i s stage of the process. I n a d d i t i o n , the N a ions w i l l favour the water o r d e r i n g i n the a l u m i n o s i l i c a t e species that appear at higher pH v a l u e s . Consequently, the hydrous g e l w i l l undergo n u c l e a t i o n r a p i d l y and a l a r g e number of centers are formed. Smaller c r y s t a l l i t e s are obtained due to the r e l a t i v e l y important A l c o n c e n t r a t i o n , e i t h e r present i n the o r i g i n a l n u l c e i or i n c o r p o r a t e d during growth. T h e i r p a r t i c u l a r morphology c o n s i s t s of medium-size (3-15 ym) u n i t s , s t i l l showing c r y s t a l f a c e s , which however are l e s s developed and p o o r l y o u t l i n e d (Figure 3, s y n t h e s i s B ) . They always appear to be covered w i t h a m u l t i t u d e of very small (0.1 ym) c r y s t a l l i t e s (Figure 12). Such a morphology suggests that the growth of the n u c l e i i n i t i a l l y s t a r t s from the s i l i c a t e species a v a i l a b l e . Once the g e l i s enriched i n A l , i t w i l l y i e l d , through a secondary n u c l e a t i o n , these small p l a t e l e t - l i k e c r y s t a l l i t e s , which e i t h e r agglomerate s e p a r a t e l y o r cover the primary S i - r i c h c r y s t a l s , sometimes so h e a v i l y that the p o l y c r y s t a l l i n e aggregates so formed develop a s p h e r u l i t i c shape (Figure 10). +

+

+


x

+

+

The p a r t i c u l a r behaviour of NH^ ions Because of t h e i r small s i z e , NH4"" ions develop an important charge and behave e s s e n t i a l l y as s t r u c t u r e - b r e a k i n g towards water. As i n the case of C s i o n s , few n u c l e i are formed and y i e l d very l a r g e w e l l defined c r y s t a l s . A number of u n u s u a l l y l a r g e (up to 25 χ 45 ym) c r y s t a l s appear indeed, i n agreement w i t h previous observations 0£,12>J3>30). Combined EDX and PIGE analyses r e v e a l that they must have a S i - r i c h core and an A l - e n r i c h e d outer r i m , as i t was observed f o r the l a r g e s i n g l e c r y s t a l s grown from syntheses of type A. Indeed, because of the very poor s a l t i n g - o u t e f f e c t of the NH^ i o n s , g e l a t i o n w i l l be impeded and a l i q u i d phase i o n t r a n s p o r t a t i o n mechanism can occur. On the other hand, we a l s o observed the formation o f numerous small hexagonal ZSM-5 p l a t e l e t s which appear, a f t e r a long h e a t i n g pe r i o d , e i t h e r w e l l separated o r s p r i n k l e d onto the primary l a r g e c r y s t a l s (Figure 13). They c o n t a i n more S i than the outer r i m of the b i g c r y s t a l s (EDX). T h e i r formation through a secondary 1

+

+



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Figure 12. SEM micrographs showing the p o l y c r y s t a l l i n e aggregate-type morphology obtained f o r (Na)ZSM-5.



GABELICA ET A L .


Figure 13. Secondary c r y s t a l l i z a t i o n of small hexagonal c r y s t a l l i t e s of (NH^)ZSM-5 on the o r i g i n a l s i n g l e c r y s t a l of the same (Reproduced w i t h p e r m i s s i o n , from r e f ( 2 6 ) , E l s e v i e r S c i . P u b l . Co.).


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n u c l e a t i o n i s explained as f o l l o w s . Because of the p r e f e r e n t i a l aluminate-NH i n t e r a c t i o n s , which predominate over s i l i c a t e - N H ^ i n t e r a c t i o n s (26), the A l - r i c h surface of the b i g c r y s t a l l i t e s w i l l have tendency to adsorb the excess of the NH ions and the growth process i s stopped. The r e s u l t i n g s o l u t i o n s t i l l contains Pr^N c a t i o n s and A l - d e p l e t e d a l u m i n o s i l i c a t e anions, w h i l e the concent r a t i o n of N H ions i s now very low. This favours the secondary formation of a l a r g e amount o f new S i - r i c h c r y s t a l l i t e s . T h e i r growth i s stopped as soon as the s o l u t i o n i s exhausted from i n g r e d i e n t s , which e x p l a i n s t h e i r average small s i z e . +

+

4

4

+

4

S p e c i f i c s t r u c t u r e - d i r e c t i n g e f f e c t s of some organic bases or c a t i o n s When i n the procedure B P r N i s replaced by other o r g a n i c s , v a r i o u s p e n t a s i l - t y p e z e o l i t i c precursors are formed. I t appears that s p e c i f i c z e o l i t e s are formed only when quaternary ammonium s a l t s are used, t h e i r nature ( s t r u c t u r e ) being e s s e n t i a l l y dependent on the length of the a l k y l chains : pure ZSM-8, ZSM-5 and ZSM-11 are obtained r e s p e c t i v e l y w i t h E t ^ N , P r ^ N and B u N c a t i o n s . TG data i n d i c a t e that the l a t t e r f i l l n e a r l y completely the z e o l i t i c channel system (Table V I I I ) . f

+


4

+

+

+

4

Table V I I I .

C h a r a c t e r i s t i c s o f some p e n t a s i l z e o l i t e s obtained from synthesis B u s i n g v a r i o u s organic bases or c a t i o n s (adapted from r e f (25) , by p e r m i s s i o n ) . f

Zeolite

+

(Et N )ZSM-8 (Pr N )ZSM-5 (Bu N )ZSM-11 (Pr N)ZSM-l1/5 (Bu N)ZSM-5/11 (Pr N+PrBr)ZSM-5 4

+

4

+

4

3

3 3

Chemical Mol.of Theor. % f i l l e d i S i / A l nature of %ZSM-5 orga- channel pore (PIGE) the organic present nics/u.c length volume occluded (XRD) (TG) (nm) (TG) (NMR) 31.3 Et,N (2.23)* * (^90)* 36.1 Pr N 100 3.64 8.8 98.8 43.0 Bu N 0 2.62 8.0 94.7 34.0 Pr N 10 1.72 8.08 38.1 29.4 Bu^N 60 1.61 8.48 41.2 32.9 Pr N 92 2.57 8.72 52.8 +

+

4

+

4

3

3

T h e o r e t i c a l channel length estimated - 4.65 nm, which should accomodate 2.45 mol of E t N , i f 100 % f i l l i n g i s reached (25). +

4

When t r i p r o p y l a m i n e or t r i b u t y l a m i n e i s used i n s t e a d of the c o r r e s ponding A l k N s a l t , ZSM-5/11 mixed phases (intergrowths ?) are formed, suggesting that the A l k N species are l e s s e f f i c i e n t i n d i r e c t i n g a s p e c i f i c s t r u c t u r e . Unexpectedly, Bu N y i e l d s essent i a l l y a ZSM-5-rich phase w h i l e Z S M - l l - r i c h phases are p r e f e r e n t i a l l y obtained w i t h P^ N (XRD d a t a ) . When an organic molecule acts as template towards ( a l u m i n o ) s i l i c a t e species t o form an o r dered z e o l i t i c framework, the l a t t e r i s supposed to organize i t s e l f around the host organic species i n such a way that a complete +

4

3

3

3


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f i l l i n g of the i n o r g a n i c framework i s achieved. The nature of the z e o l i t e formed w i l l then e s s e n t i a l l y depend on the pore f i l l i n g and not on the necessary l o c a t i o n of the organic molecules at the (four) channel i n t e r s e c t i o n s of the so formed u n i t c e l l . Experi mental data have suggested that f o r ZSM-5 and ZSM-11 the most ener g e t i c a l l y favourable f i l l i n g i s obtained when Pr^N* and Bu^N ions r e s p e c t i v e l y achieve an end-to-end c o n f i g u r a t i o n w i t h no other s t e r i c c o n s t r a i n t s (28,76). This a l s o corresponds to about 4 Pr^N ions per u n i t c e l l of ZSM-5, w h i l e , because of the l a r g e s i z e of the BU4N"*" u n i t s , only about 2.6 Bu4N u n i t s occupy (completely) one ZSM-11 u n i t c e l l on the average (28). Bearing such a "comple te pore f i l l i n g " model i n mind, one can a l s o c a l c u l a t e the maximum space f i l l e d by four P r N and f o u r BU3N when they d i r e c t the f o r mation of a h y p o t h e t i c a l z e o l i t e framework, (one molecule per i n t e r s e c t i o n of a f i c t i t i o u s p e n t a s i l - t y p e z e o l i t e ) (25). The f i c t i t i o u s u n i t c e l l space l e n g t h obtained f o r 4 P r N i s 7.16 nm and for 4 Bu N, 8.68 nm, i n d i c a t i n g that the best f i l l i n g w i t h P r N i s achieved f o r ZSM-11 ( t o t a l pore volume = 8.0 nm) and w i t h Bu N, for ZSM-5 ( t o t a l pore volume = 8.8 nm). The a c t u a l composition of the mixed phases i s c l o s e to t h a t r e s p e c t i v e l y p r e d i c t e d by our c a l c u l a t i o n s , assuming a complete pore f i l l i n g (Table V I I I ) . Ho wever, the a c t u a l percentage of the space f i l l e d , as experimental l y evaluated by TG (Table V I I I ) , does not correspond to the theo r e t i c a l p r e d i c t i o n s . This suggests that the complete f i l l i n g i s achieved only at the e a r l y stages of the n u c l e a t i o n - c r y s t a l l i z a t i o n processes, which are c r i t i c a l to determine the nature of the z e o l i t i c m a t e r i a l . Once a p a r t i c u l a r s t r u c t u r e i s achieved, i t can grow through s t e r e o s p e c i f i c s p a t i a l r e p l i c a t i o n s without ne c e s s a r i l y i n c o r p o r a t i n g the organic molecules. When P r N and η-propyl bromide are used, the z e o l i t e which i s formed appears to be a pure ZSM-5 phase. However, ^C-NMR expe riments i n d i c a t e that the occluded o r g a n i c species are P r N and not Pr^N*. However, P r ^ N must form because they s p e c i f i c a l l y d i r e c t ZSM-5 s t r u c t u r e s . ΡΓβΝ alone would have y i e l d e d a ZSM-11r i c h i n t e r g r o w t h . I t i s then assumed that Pr^N , formed i n small amounts ( t r a c e s ) by r e a c t i o n of P r N w i t h propylbromide, must ope r a t e as s t r u c t u r e - d i r e c t i n g agent o n l y during the n u c l e a t i o n s t a ge. Once ZSM-5 n u c l e i are formed, they can grow by p r o g r e s s i v e l y i n c o r p o r a t i n g the most abundant P r N s p e c i e s , which remain i n solution. +

+


3

3

3

3

3

3

3

+

3

3

General Conclusions The e x i s t e n c e of two d i s t i n c t mechanisms d e s c r i b i n g the s y n t h e s i s of ZSM-5 type m a t e r i a l s has been proposed and e x p e r i m e n t a l l y con firmed. The c o n t r o l of primary s y n t h e s i s v a r i a b l e s such as the source of s i l i c a and the composition of the r e a c t i o n mixtures ena b l e s us to prepare ZSM-5 p a r t i c l e s w i t h s p e c i f i c p r o p e r t i e s d e s i red f o r v a r i o u s c a t a l y t i c a p p l i c a t i o n s : l a r g e s i n g l e c r y s t a l s having a S i - r i c h core and an A l - e n r i c h e d outer r i m , smaller and homogeneous


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p a r t i c l e s or even very small "X-ray amorphous" ZSM-5 m i c r o c r y s t a l l i t e s embedded i n A l - r i c h a l u m i n o s i l i c a t e g e l phases. The f u r t h e r i n v e s t i g a t i o n of the i n f l u e n c e of some other secondary s y n t h e s i s v a r i a b l e s (pH, v i s c o s i t y , or a l k a l i c a t i o n s ) on the n u c l e a t i o n and c r y s t a l l i z a t i o n processes, enabled us to confirm, to extend and to g e n e r a l i z e the proposed mechanisms and to d e l i n e a t e more s p e c i f i c o p e r a t i n g c o n d i t i o n s which would y i e l d ZSM-5 c r y s t a l l i t e s w i t h con t r o l l e d s i z e , morphology, homogeneity and chemical composition. The c l a t h r a t i n g and templating r o l e of the P r ^ N c a t i o n s was reco gnized and emphasized. The chemical nature of the s t r u c t u r e d i r e c t i n g organic base or c a t i o n appears to be an e s s e n t i a l f a c t o r a f f e c t i n g the s t r u c t u r e of the z e o l i t i c m a t e r i a l which i s formed. E i t h e r the formation of pure p e n t a s i l z e o l i t e s o r t h e i r i n t e r m e d i a te intergrowths could be p r e d i c t e d and obtained from s p e c i f i c o r ganic templates. The s t r u c t u r e - d i r e c t i n g e f f e c t of the l a t t e r operates only at the n u c l e a t i o n stages of the s y n t h e s i s . The na ture of novel p o t e n t i a l z e o l i t i c phases can be b e t t e r e x p l a i n e d by s i m p l i f i e d models i n v o l v i n g t h e i r channel f i l l i n g by v a r i o u s templates, than by assuming the s p e c i f i c l o c a t i o n of the organic molecules at the channel i n t e r s e c t i o n s .


+

Acknowle dgment s The authors wish to thank Dr. J . Rostrup-Nielsen (Haldor Tops^e A.S.) f o r h i s continuous i n t e r e s t i n t h i s work. They are a l s o indebted to E l s e v i e r S c i e n t i f i c P u b l i s h i n g Co. f o r having granted the permission to p u b l i s h o r i g i n a l data. Literature 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

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Factors Affecting the Synthesis of Pentasil Zeolites - ACS Symposium

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