Dehydroisomerization of n-butane over H-Y zeolite supported Pt and Pt,Sn catalysts

The direct one-step conversion of n-butane to isobutene was investigated over H-Y zeolite supported platinum and platinum-tin catalysts. Catalytic results over monometallic Pt catalysts showed that dispersion of Pt mainly affects the selectivity towards reaction products, larger Pt ensembles resulting in a higher degree of cracking/hydrogenolysis reactions and consequently to a lower n-butenes and isobutene formation. Addition of tin has been found to significantly improve both the selectivity to isobutene and the resistance to deactivation of the catalytic system. On the basis of characterization results (H-2 chemisorption, NH3-TPD, FT-IR of adsorbed CO) the higher isobutene selectivity of Pt,Sn catalysts have been accounted for a dilution effect of Pt ensembles induced by the second metal which inhibits hydrogenolysis reactions, requiring several contiguous Pt sites, and enhances the selectivity for the dehydrogenation reaction that can instead proceed over smaller metal ensembles. The improved stability of Pt,Sn catalysts has been related to the lower surface acidity of the system in the presence of Sn, which results in a reduced formation of carbonaccous deposits responsible for catalysts deactivation. (C) 2004 Elsevier B.V. All rights reserved.