5 Aluminophosphate Molecular Sieves: A New Class of Microporous Crystalline Inorganic Solids
Downloaded by MONASH UNIV on June 6, 2013 | http://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.ch005
STEPHEN T. WILSON, BRENT M. LOK, CELESTE A. MESSINA, THOMAS R. CANNAN, and EDITH M. FLANIGEN Union Carbide Corporation, Tarrytown Technical Center, Tarrytown, N Y 10591 A novel class of crystalline, microporous aluminophosphate phases has been discovered. It represents the f i r s t class of molecular sieves with framework oxide compositions free of silica. The new class of materials encompasses some fourteen reported three-dimensional microporous framework structures, and six two-dimensional layer-type structures. The three-dimensional structures include structural analogues of the zeolites sodalite and erionite-offret i t e . The novel phases can be synthesized hydrothermally i n the presence of organic amines and quaternary ammonium templates. The template is entrapped or clathrated within the crystallizing aluminophosphate network. After thermal decomposition of the template the three-dimensional molecular sieves have the general composition of Al2O3•1.0± 0.2 P2O5. The various structures exhibit intracrystalline adsorption pore volumes from 0.04 to 0.35 cm 3 /g, and pore sizes from 0.3 to 0.8nm. The aluminophosphate frameworks are hydrophilic. The aluminophosphate molecular sieves represent a new class of microporous inorganic solids with the potential to be as useful and as s c i e n t i f i c a l l y challenging as the aluminosilicate zeolites. These molecular sieves are the latest chapter in the history of framework oxide molecular sieves, a history which began with the discovery of the naturally occurring mineral zeolites. These mineral zeolites stimulated the very successful research that led to the preparation of synthetic zeolites. As this research progressed, ever more s i l i c a - r i c h frameworks were synthesized, culminating i n the essential Al-free microporous s i l i c a polymorphs, such as silic a l i t e . These latter materials preserve the molecular sieving property of the zeolite but lose the ion-exchange capabilities as Al is removed from the framework.
0097-6156/83/0218-0079$08.00/0 © 1983 American Chemical Society In Intrazeolite Chemistry; Stucky, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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INTRAZEOLITE CHEMISTRY
Background S i l i c o n and aluminum, of course, are not unique i n t h e i r a b i l i t y t o form t e t r a h e d r a l l y coordinated oxide networks. The element phosphorus, at the r i g h t of s i l i c o n i n the p e r i o d i c t a b l e , f r e q u e n t l y assumes t e t r a h e d r a l c o o r d i n a t i o n w i t h oxygen. With phosphorus i n the +5 o x i d a t i o n s t a t e as phosphate, aluminum phosphate possesses many s t r u c t u r a l s i m i l a r i t i e s t o s i l i c a : 1) AIFO4 i s i s o e l e c t r o n i c w i t h S i 0 . 2) The average of the i o n i c r a d i i of A1 (0.39A) and P ( 0 . 1 7 A ) i s 0.28A, v e r y c l o s e t o the i o n i c r a d i u s of S i * ( 0 . 2 6 A ) ( 1 ) . 3) AlPO* and S i 0 form isomorphic dense phases w i t h A l alternating with P i n a t e t r a h e d r a l oxide n e t work. There are dense phases of A1P0* corresponding t o seven s t r u c t u r a l forms of S i 0 : a - and B-quartz; a - , and y - t r i d y m i t e ; and o- and B - c r i s t o b a l i t e ( 2 ) . These s t r u c t u r a l analogies between A1P0* and S i 0 served as one stimulus f o r examining aluminophosphates as a p o t e n t i a l source of microporous frameworks. A second stimulus was the extensive aluminophosphate s y n t h e s i s l i t e r a t u r e , and, i n p a r t i c u l a r , the ease of hydrothermal s y n t h e s i s . S e v e r a l metastable hydrated networks have been synthesized. For example, two forms of A1P0**2 H 0 are known, m e t a v a r i s c i t e and v a r i s c i t e . These have s t r u c t u r e s composed of a l t e r n a t i n g A l and P, w i t h A l e x h i b i t i n g o c t a h e d r a l c o o r d i n a t i o n and P, t e t r a h e d r a l c o o r d i n a t i o n (3,_4). The two waters of h y d r a t i o n occupy c i s p o s i t i o n s i n the c o o r d i n a t i o n sphere of A l . M e t a v a r i s c i t e contains 4-, 6-, and 8- r i n g s of a l t e r n a t i n g alumina octahedra and phosphate t e t r a h e d r a . The p a r t i a l removal of the waters of h y d r a t i o n from m e t a v a r i s c i t e or v a r i s c i t e i s r e v e r s i b l e but complete removal r e s u l t s i n s t r u c t u r a l c o l l a p s e . I n a d d i t i o n t o these hydrated forms of A1P0*, the dense phase AlP0
