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Generation of synergistic sites by thermal treatment of HY zeolite. Evidence from the reaction of hexane isomers

To, Anh T., Jentoft, Rolf E., Alvarez, Walter E., Crossley, Steven P., Resasco, Daniel E.
Journal of catalysis 2014 v.317 pp. 11-21
Bronsted acids, Lewis acids, adsorption, aluminum oxide, catalytic activity, cracking, dehydrogenation, heat treatment, hexane, isomers, thermogravimetry, zeolites
We have studied the conversion of hexane isomers on HY zeolites (Si/Al=40 and 2.6) after different thermal pretreatments (400–600°C) on a micropulse reactor, which is ideal for evaluating catalyst activity and evolution of product distribution over fresh zeolites at very short times on stream, when coke formation is negligible. We have observed that the catalytic activity of the zeolites is enhanced upon high-temperature pretreatment. This activity enhancement can be ascribed to dehydroxylation, as observed by thermogravimetric analysis upon heating the zeolite above 500°C. However, this dehydroxylation does not result in a decrease in the density of the original Brønsted acid sites (BAS). Therefore, the newly created Lewis acid sites (LAS) must be associated with either extra-framework alumina (EFAl) or silanol species. We propose that the activity enhancement is associated with the generation of synergistic sites resulting from the interaction between LAS created by the dehydroxylation and the original BAS. Both, protolytic cracking and dehydrogenation reactions are enhanced with paraffin feeds, but such enhancement is not apparent for olefin feeds. The synergistic sites were also deactivated very quickly by strong adsorption of hydrocarbon species, but could be regenerated by a subsequent high-temperature treatment.