Source of Activity in Zeolites

Source of Activity in Zeolites.—Most of the experimental work designed to elucidate the catalytically active sites in zeolites has used faujasitic zeolites. This has been reviewed recently in detail.Much is broadly applicable to non-faujasitic zeolites, but in this section three factors influencing catalytic activity are emphasized silica/alumina ratio crystal structure modification of the zeolite by thermal treatment, cation exchange, etc. 

The form of the site as a Bronsted acid is essentially identical with that described above for amorphous silica-alumina and the strong acidity, in contrast with crystalline or amorphous silica, can be attributed to the distribution of the residual negative charge over the A104 tetrahedron. As with silica-alumina, the nature of the Lewis acid site is much less clear. From studies of the dehydration of X and Y zeolites, Uytterhoeven, Christner, and Hail d suggested that Lewis sites were formed from Bronsted sites by dehydration reaction  

This model has had wide acceptance. Trigonal aluminium in the zeolite structure will plausibly show Lewis properties, but there is little evidence for the trigonal silicon. A recent examination of Y zeolite by Y-rays has shown that silicon remains tetra-co-ordinated, even after dehydroxylation. All the aluminium in a true hydrogen Y zeolite was found to be tetra-co-ordinated, in agreement with the normal model for a Bronsted site. However, about 19% of the aluminium in a stabilized hydrogen Y zeolite was hexa-co-ordinated. No 

Electron donor-acceptor centres have been identified in zeolites and their relevance to catalysis discussed. Shih has found the cation radical C5H8 +, on H-mordenite treated with cyclopentene, to react to give a carbonium ion and an allylic radical  

However, it is uncertain how far radical cations play a significant part in hydrocarbon catalysis over zeolites. 

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