Characterization and reactivity of transition metal modified sulfated zirconia catalysts for low temperature n-butane isomerization

It has been reported that promotion of sulfated zirconia with iron and manganese increases the activity of sulfated zirconia by several orders of magnitude for n-butane isomerization near room temperatures. Addition of platinum to sulfated zirconia stabilizes the catalysts against deactivation under a dihydrogen atmosphere.; In this work, samples of sulfated zirconia, sulfated zirconia promoted with iron and manganese and platinum doped sulfated zirconia were synthesized and their acidity and reactivity were studied under a variety of conditions to elucidate the mechanism for promotion.; From x-ray absorption spectroscopy we found that iron was present in a hematite-like nanocluster outside the tetragonal crystallites of zirconia. The acidities of Fe/Mn-promoted and unpromoted sulfated zirconia were studied by ammonia TPD and infrared spectroscopy of adsorbed pyridine and carbon monoxide. This characterization revealed that Bronsted and Lewis acid site densities were similar on the promoted and unpromoted samples. Reactions with n-butane-1-{dollar}sp{lcub}13{rcub}{dollar}C carried at low temperatures indicated that the isomerization of n-butane on these two materials proceeds via a bimolecular pathway. The iron promoter is proposed to be involved in the dehydrogenation of butane.; In agreement with classical acid chemistry, the admission of butene increased the activity for n-butane isomerization by a chain reaction mechanism and the increase of activity was nearly independent of the promoter.; In contrast with the iron and manganese promoted sulfated zirconia, which deactivated continuously, the platinum doped sulfated zirconia exhibited an extremely stable activity in a helium atmosphere at low temperatures.; Studies on neat sulfated zirconia were performed by carefully controlling the amount of gas phase butenes in the reactant mixture. Results indicated that below a critical concentration level of gas phase butenes, sulfated zirconia is incapable of catalytically sustaining the isomerization of n-butane. Even though sulfated zirconia is a strong solid acid, all evidence from this work suggests that it is not superacidic.