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  • Chapter 40

    Competitive Role of Organic and Inorganic Cations in Directing One-Dimensional

    Zeolitic Structures ZSM-48 and EU-1

    Girolamo Giordano1, Janos B. Nagy, Eric G. Derouane, Nicole Dewaele, and Zelimir Gabelica

    Laboratory of Catalysis, Center for Advanced Materials Research, Facults Universitaires Notre Dame de la Paix, Namur, 61 Rue

    de Bruxelles, B-5000 Namur, Belgium

    The Bis-quaternary ammonium ions (e.g. hexamethonium, HM++) favour the formation of two different one-dimensional zeolites, namely ZSM-48 and EU-1. The resulting structure essentially depends on the initial aluminium content in the starting hydrogel. The stability fields for both zeolites synthesized by using various reactant compositions have been established. Zeolite ZSM-48 is prepared from a silica hydrogel containing HM++ ions and alkali cations (Li, Na, and ), in presence or in absence of Al . For a higher in i t ia l Al content and increased crystallization time, zeolite EU-1 is obtained. The critical role of hexamethonium ions and inorganic cations on the crystallization rate of ZSM-48 was systematically studied. HM++ ions favor the ZSM-48 formation by interacting electrostatically with Al negative charges and stabilizing its structure by acting as pore fillers.

    The s y n t h e s i s o f pure and h i g h l y c r y s t a l l i n e z e o l i t e s from a l l u m i n o s i l i c a t e hydrogels r e q u i r e s the st u d y o f the s i m u l t a n e o u s e f f e c t s o f d i f f e r e n t parameters i n v o l v e d i n the s y n t h e s i s . Most of these z e o l i t e s are prepared i n presence of organic d i r e c t i n g

    Current address: Dipartimento di Chimica, Universit della Calabria, Arcavacata di Rende, 1-87030 Rende (CS), Italy

    0097-6156/89A)398-0587$06.00A) 1989 American Chemical Society

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    In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

  • 588 ZEOLITE SYNTHESIS

    agents whose r o l e can be d r a s t i c a l l y i n f l u e n c e d by the a c t u a l composition of the hydrogel.

    Recent work has p o i n t e d out t h e predominant t e m p l a t i n g r o l e of b i s - q u a t e r n a r y ammonium ions (e.g. hexamethonium i o n s ) i n d i r e c t i n g o n e - d i m e n s i o n a l z e o l i t i c s t r u c t u r e s namely z e o l i t e s ZSM-48 and EU-1, the f i r s t r e p r e s e n t a t i v e member of the EU0 fami l y ( 1 - 4 . ) .

    Z e o l i t e s having ZSM-48 t o p o l o g y a l s o c r y s t a l l i z e from hydrogels c o n t a i n i n g other l i n e a r organic s t r u c t u r e d i r e c t i n g agents, such as diamines (UL) or v a r i o u s (poly)alkylamines ().

    Z e o l i t e ZSM-50, another member of the EU0 f a m i l y p o s s e s s i n g the topology of EU-1, a l s o c r y s t a l l i z e s i n presence of dibenzyldimethylammonium ions (2).

    The framework of ZSM-48 i s composed of f e r r i e r i t e -type sheets, connected i n such a s p e c i f i c way as t o generate l i n e a r 10-membered r i n g channels. The s t r u c t u r e i s a random intergrowth of the Imma-Cmcm frameworks and contains 48-T atoms per u n i t c e l l (&).

    Recently B r i s c o e et a l . () d e s c r i b e d the framework t o p o l o g y o f z e o l i t e EU-1. I t c o n s i s t s o f a unidimensional 10-membered r i n g channel system with s i d e pockets formed at r e g u l a r i n t e r v a l s o f f the channels. The u n i t c e l l c o n t a i n s 112-T atoms and the framework symmetry i s Cmma.

    The aim o f t h i s work i s t o e v a l u a t e t h e (competitive) r o l e of other i n g r e d i e n t s i n s t a b i l i z i n g p r e f e r e n t i a l l y one or the other s t r u c t u r e . The most i n t e r e s t i n g are the c a t i o n i c s p e c i e s , namely the a l k a l i and HM + + ions, p o t e n t i a l n e u t r a l i z i n g agents t o A10 2~ n e g a t i v e l y charged framework c e n t e r s . Indeed, a l k a l i c a t i o n s were shown t o p l a y an important r o l e i n the f o r m a t i o n o f many z e o l i t e s , e i t h e r as s t r u c t u r e d i r e c t o r s i n the n u c l e a t i o n p r o c e s s ( 1 . - 1 2 . ) or as s t a b i l i z i n g m i n e r a l i z e r s during growth ( 1 1 , 1 2 . ) , thereby a f f e c t i n g the f i n a l s i z e , morphology and composition of the c r y s t a l l i t e s .

    Besides t h e i r obvious r o l e as templates or s t r u c t u r e d i r e c t i n g agents (13.) or as s t a b i l i z i n g pore f i l l e r s Qrl!) the organic c a t i o n s w i l l a l s o compete with a l k a l i i o n s f o r the s t a b i l i z a t i o n o f the n e g a t i v e l y charged framework (1, 11,1!) .

    The p r e s e n t approach c o n s i s t s i n examining the s t r u c t u r e and c o m p o s i t i o n o f s e l e c t e d c r y s t a l l i n e z e o l i t e s o b t a i n e d by h e a t i n g under a p p r o p r i a t e c o n d i t i o n s hydrogels having the general composition:

    x M 20 y HMBr2 A l 2 0 3 60 S i 0 2 3000 H 20

    by s y s t e m a t i c a l l y v a r y i n g the i n i t i a l c a t i o n i c , organic and A l content t o d e f i n e the fundamental r o l e of HM + + i o n s i n e a c h s y s t e m , a n d t o u l t i m a t e l y o p t i m i z e t h e o p e c i f i c c r y s t a l l i z a t i o n c o n d i t i o n s f o r each z e o l i t e .

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    In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

  • 40. GIORDANO ET AL. Competitive Role ofOrganic and Inorganic Cations 589

    Experimental

    A s e r i e s of hydrogels having the f o l l o w i n g molar composition :

    M 20 y HMBr2 A 1 2 0 3 60 S i 0 2 3000 H 20

    (where HMBr 2 stands f o r hexamethonium bromide; M= L i , Na or K; 0

  • 590 ZEOLITE SYNTHESIS

    3 I f 1 1 1 2

    x mole A\2O3/60 S1O2

    Figure 1. V a r i a t i o n of the percentage of c r y s t a l l i n i t y of z e o l i t e s ZSM-48 and EU-1 as a fu n c t i o n of the AI2O3 content i n the f o l l o w i n g precursor g e l phases: = ZSM-48 5Na 20 5HMBr2 x A l 2 0 3 60SiO 2 3000H 2O, (200C, 66 h) . = EU-1 10Na2O 10HMBr2 x A l 2 0 3 60SiO 2 3000H 2O, (200C, 120 h) . 0 = EU-1 + ZSM-48 (30%) Q = EU-1 + ZSM-48 (40%) + -quartz and c r i s t o b a l i t e .

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    In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

  • 40. GIORDANO ET AL. Competitive Role of Organic and Inorganic Cations 591

    A l content (expressed i n mole AI2O3 per 60 mole S1O2), i n each g e l system. Nearly 100% c r y s t a l l i n e ZSM-48 was o b t a i n e d i n A l - f r e e g e l systems. The c r y s t a l l i n i t y f u r t h e r markedly decreases u n t i l the upper l i m i t of 1 AI2O3, above which no ZSM-48 c r y s t a l s c o u l d be detected, even f o r longer r e a c t i o n times.In the A l - f r e e 10 Na 20 10 HMBr2 hydrogel, ZSM-48, c r i s t o b a l i t e and -quartz are the o n l y c r y s t a l l i n e phases detected, c o n f i r m i n g t h a t EU-1 can not be s t a b i l i z e d i n such a system (2.) . For 0.5 AI2O3 about 40% c r y s t a l l i n e EU-1 was detected along with ZSM-48 and smaller amount of c r i s t o b a l i t e and -quartz, and f o r 1 AI2O3 EU-1 and ZSM-48 are the only c r y s t a l l i n e phases. EU-1 i s the only phase present f o r 1.5 AI2O3, but over t h i s value i t s c r y s t a l l i n i t y s t a r t s t o decrease, i n agreement with the w e l l e s t a b l i s h e d i n h i b i t i n g r o l e o f l a r g e amounts o f A l i n t h e c r y s t a l l i z a t i o n r a t e of many z e o l i t e s (11).

    Influence of HMBr2 The amount of HM"1""1" ions i n the i n i t i a l h y d r o g e l

    d r a s t i c a l l y i n f l u e n c e s the nature of the z e o l i t e formed and i t s c r y s t a l l i z a t i o n k i n e t i c s . Pure and h i g h l y c r y s t a l l i n e ZSM-48 i s formed f o r a HMBr2 molar c o n c e n t r a t i o n c l o s e t o 2.5 ("Figure 2"). For h i g h e r HMB r 2 c o n t e n t s t he c r y s t a l l i n i t y shows a s l i g h t decrease, probably due to a marked m o d i f i c a t i o n of the hydrogel composition and/or of the s o l u b i l i t y of the va r i o u s r e a c t i v e s pecies, induced by the excess of organic molecules. For HM + + concentrations l y i n g between 1 and 2.5 mole, dense S1O2 pol y m o r p h i c phases coc r y s t a l l i z e w i t h ZSM-48, w h i l e f o r lower H M + + c o n c e n t r a t i o n s , e s s e n t i a l l y be