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10-1000. 0.97. 0.031 0.019. 1.6-0. 1-0.003. 0.14. 6.98 11.63. 0.17-150. 5-30. Nucleation and ... To compare the influ e n Ç^2ÉP5 o n crystalli- zation...
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12 Factors Affecting the Synthesis of Pentasil Zeolites ZELIMIR GABELICA and ERIC G.

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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.

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

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

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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

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

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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

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Synthesis of Pentasil Zeolites

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

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

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

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)

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Synthesis

Gel aging time (h)

Other

4

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Si0

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Na aluminate (Al+NaOH)(basic)

type

Source o f A l

Synthesis 4

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120

Hydrothermal, PIREX tubes

110

130

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1(ac.)+80(basic)

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sulfate+H S0 +Pr NBr (acid)

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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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C/5

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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). +

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

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

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Synthesis of Pentasil Zeolites

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.).

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

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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.).

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

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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

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

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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.

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

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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.

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Synthesis Type

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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

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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

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

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Synthesis time

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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

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

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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

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

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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

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

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Synthesis of Pentasil Zeolites

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12.

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

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

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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. +

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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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Synthesis of Pentasil Zeolites

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

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

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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

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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

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

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80 r

(Na) ZSM-5 0

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jjEsL. 10

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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.).

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

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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.).

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

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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

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1

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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 +

+

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

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0

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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 .

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

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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). +

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x

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

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CATALYTIC MATERIALS

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.

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

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Synthesis of Pentasil Zeolites

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.).

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

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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

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4

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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

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(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

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4

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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 +

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4

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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 +

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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. +

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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

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

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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 .

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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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RECEIVED

November 8, 1983

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