Hexane Isomerization and Cracking Activity and Intrinsic Acidity of H-Zeolites and Sulfated Zirconia−Titania

Adsorption of N2 was studied on zeolite H-Y, ultrastabilized H-Y (H-USY), H-mordenite, H-ZSM-5, H-beta, and on sulfated zirconia-titania (SZT) mixed oxide by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at 298 K and at N2 pressures up to 9 bar. The adsorption-induced DeltanuOH red-shift of the nuOH bands was used as a measure of the intrinsic acid strength of the Brnønsted acid sites. The intrinsic acid strength of the solids follows the order of H-ZSM-5 approximately H-mordenite approximately H-beta > H-USY > SZT approximately H-Y. The solids were characterized by their hexane conversion activities at 553 K and 6.1 kPa hexane partial pressure. The reaction was shown to proceed predominantly by a bimolecular mechanism, while the reaction was first order in hexane and zero order in alkenes. The site-specific apparent rate constant of the bimolecular hexane conversion was shown to parallel the intrinsic acid strength of the samples, suggesting that the ratio of the apparent and the intrinsic activity, that is, the KA' equilibrium constant of alkane adsorption on the hydrocarbon-covered sorption sites, is hardly dependent on the catalyst structure.