Skeletal isomerization of hydrocarbons over zirconium oxide promoted by Platinum and Sulfate Ion

The genesis of the high activity of zirconium oxide promoted by platinum and sulfate ion (Pt/SO42−--ZrO2) for skeletal isomerization of butane and pentane in the presence of hydrogen is studied in terms of the interaction of the catalyst with molecular hydrogen. For skeletal isomerization of pentane at 523 K, Pt/SO42−-ZrO2 showed activity only in the presence of molecular hydrogen, and its activity persisted for a long period. For skeletal isomerization of butane at 523 K, the catalyst showed activity in the absence of hydrogen, but the activity was markedly enhanced in the presence of hydrogen. For pentane and butane skeletal isomerization, the products consisted exclusively of 2-methylbutane and 2-methylpropane, respectively. For a typical acid-catalyzed reaction of cyclopropane ring opening at 373 K, the presence of hydrogen enhanced the activity, but the hydrogen enhancement effect was small. The products consisted exclusively of propene even in the presence of hydrogen: hydrogenation of propene scarcely occurred. Infrared spectroscopic study of adsorbed pyridine showed that by heating the catalyst in the presence of hydrogen in the temperature range 423–623 K, protonic acid sites were formed with concomitant decrease in the number and strength of Lewis acid sites, demonstrating that the protonic acid sites originate from molecular hydrogen. The mechanisms of protonic acid site generation are discussed. It is suggested that molecular hydrogen dissociates on the platinum to hydrogen atoms which undergo spillover on the SO42−-ZrO2 and convert to an H+ and an e− or H−. The H+ acts as catalytic site for acid-catalyzed reactions.