Chapter
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Borosilicate Molecular Sieves
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Nancy A. Kutz Amoco Research Center, Amoco Chemical Company, P.O. Box 400, Naperville, IL 60566
Crystalline borosilicate molecular sieves have been the object of an intensive investigation effort since they were reported in the open literature at the Fifth International Conference on Zeolites by Taramasso, et al. (1) A wide range of structures containing framework boron have been synthesized. The physical properties of these borosilicate molecular sieves have been studied by such techniques as X-ray diffraction, infrared and nuclear magnetic resonance spectroscopies, and temperature programmed desorption of ammonia. In addition, the catalytic performance of borosilicate molecular sieves has been reported for such reactions as xylene isomerization, benzene alkylation, butane dehydroisomerization, and methanol conversion. This paper will review currently available information about the synthesis, characterization, and catalytic performance of borosilicate molecular sieves. The s y n t h e s i s o f z e o l i t e s and z e o l i t i c m a t e r i a l s has been p u r s u e d f o r n e a r l y 50 y e a r s ( 2 ) , and t h e l i t e r a t u r e i s f i l l e d w i t h r e p o r t s o f s t r u c t u r e s , methods o f p r e p a r a t i o n , and uses f o r these m a t e r i a l s . The s u b s t i t u t i o n o f aluminum o r s i l i c o n i n t h e framework s t r u c t u r e has been p e r f o r m e d u s i n g many main group elements (3.4) as w e l l as some t r a n s i t i o n m e t a l s ( 3 , 5 ) . New f a m i l i e s o f m o l e c u l a r s i e v e s w h i c h a r e b a s e d on an aluminophosphate framework have been r e p o r t e d r e c e n t l y , some o f which a r e a l s o m i c r o p o r o u s ( 6 . 7 ) . Of t h e v a r i o u s new m a t e r i a l s w h i c h have b e e n r e p o r t e d , t h i s " r e v i e w w i l l f o c u s on c r y s t a l l i n e b o r o s i l i c a t e molecular sieves. The growing i n t e r e s t i n t h e p r o p e r t i e s o f b o r o s i l i c a t e m o l e c u l a r s i e v e s h a s l e d t o a s i g n i f i c a n t i n c r e a s e i n t h e number o f r e p o r t s w h i c h s p e c i f y a b o r o s i l i c a t e m o l e c u l a r s i e v e as t h e i m p o r t a n t component i n c a t a l y s t c o m p o s i t i o n s (8-11). The range o f r e a c t i o n s which have been c l a i m e d i n c l u d e a v a r i e t y o f h y d r o c a r b o n c o n v e r s i o n s such as x y l e n e i s o m e r i z a t i o n (8,12),
0097-6156/88/0368-0532$06.00/0 © 1988 American Chemical Society
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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Borosilicate Molecular Sieves
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butane dehydroisomerization (13.) , c a t a l y t i c dewaxing (9.14) , and methanol conversion (10.15).
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Synthesis B o r o s i l i c a t e s have been prepared v i a hydrothermal synthesis i n a l k a l i n e solutions (1.16-24). A l t e r n a t i v e l y , synthesis has been successful from neutral or s l i g h t l y a c i d i c media i n the presence of f l u o r i d e anions (25.) · B o r o s i l i c a t e molecular sieves have been prepared through secondary synthesis techniques as reported by Derouane, et a l . (2j5) , i n which the aluminosilicate ZSM-5 was treated with boron t r i c h l o r i d e to replace aluminum with boron i n tetrahedral s i t e s . Typical hydrothermal synthesis consists of the preparation of a gel comprising a source of s i l i c a , a source of boron, an organic compound, and a source of a l k a l i . The gel i s digested at a temperature generally between 80°C and 200°C u n t i l such time as a c r y s t a l l i n e molecular sieve i s obtained. The asprepared molecular sieve contains the organic synthesis agent as weLJ. as any cations which may have been present (such as Na+ or Ca ). After c a l c i n a t i o n or f i r i n g , usually at temperatures i n excess of 400°C, a mixed hydrogen/cation form r e s u l t s which can be converted to the hydrogen form through ammonium ion exchange followed by c a l c i n a t i o n , or by treatment with hydrochloric acid to generate the hydrogen form d i r e c t l y . The preparation of a b o r o s i l i c a t e molecular sieve (termed by the authors "borozeosilite") at r e l a t i v e l y low pH, i n the neutral to a c i d i c range, has been reported (25.). A reaction mixture comprising a s i l i c a source, boric acid, tetrapropylammonium bromide and an ammonium f l u o r i d e s a l t was digested hydrothermally at 170°C. Subsequent c a l c i n a t i o n of the product i n a i r at 550°C was performed to remove organic template and to provide the hydrogen form of the molecular sieve. In a method which i s analogous to the dealumination of z e o l i t e s by s i l i c o n tetrachloride (27), Derouane and co-workers reported the replacement of aluminum i n the NH -ZSM-5 framework by boron from BC1 (26). By c o n t r o l l i n g the reaction conditions, p a r t i c u l a r l y the exposure time to boron t r i c h l o r i d e , the degree of substitution was varied. The z e o l i t e was dried i n i t i a l l y i n a stream of flowing dry nitrogen, then was exposed to the boron t r i c h l o r i d e at elevated temperature (500°C), followed by an additional treatment with dry nitrogen. After the reaction with BC1 , the molecular sieve was ammonium exchanged and a i r - d r i e d to preserve the ammonium form of the sieve f o r subsequent characterization. A number of b o r o s i l i c a t e molecular sieves have been discovered using these synthesis techniques. By changing the organic compound and other reaction variables, i t i s possible to prepare various b o r o s i l i c a t e structures (1.11.24.28). Modifications of z e o l i t e s and molecular sieves with boron compounds which do not lead to tetrahedral (framework) boron w i l l not be addressed i n this a r t i c l e . 2
4
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Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
PERSPECTIVES IN MOLECULAR SIEVE SCIENCE
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Characterization X-Ray D i f f r a c t i o n . B o r o s i l i c a t e m o l e c u l a r s i e v e s have been s t u d i e d by X - r a y d i f f r a c t i o n (1.16-20). X-ray d i f f r a c t i o n t e c h n i q u e s have been d e v e l o p e d t o d e t e r m i n e t h e degree o f s u b s t i t u t i o n o f t h e s i l i c a t e framework by b o r a t e t e t r a h e d r a (29). The boron-oxygen bond i s s h o r t e r t h a n t h e t h e s i l i c o n - o x y g e n bond, which l e a d s t o a c o n t r a c t i o n o f t h e u n i t c e l l f o r a b o r o s i l i c a t e m o l e c u l a r s i e v e as b o r o n s u b s t i t u t i o n increases. The u n i t c e l l volume d e t e r m i n e d from peak p o s i t i o n s i n t h e ranges 2O°
