Intrinsic Catalytic Cracking Activity of Hexane over H−ZSM-5, H−β and H−Y Zeolites

Mono- and bimolecular intrinsic rate constants for the conversion of n -hexane over H−Y, H−ZSM-5, and H−β zeolites were measured at 623 K by combining conventional catalytic techniques with gravimetric adsorption measurements. Since H-zeolites are catalytically active, their adsorption properties under reaction conditions could not be directly obtained. Rather, adsorption experiments were carried out on the H-zeolites at temperatures below the onset of catalytic activity (450−530 K). Thermodynamic adsorption constants for the H forms at 623 K were then obtained by extrapolation. In addition, the catalytically inactive Na forms were chosen to characterize the overall temperature dependence of n -hexane adsorption on ZSM-5, Y, and β zeolites in the range 450−623 K. For the cracking experiments, monomolecular conditions were ensured by choosing a low hexane pressure (<0.57 kPa) and short contact times. The intrinsic activity for mono- and bimolecular n -hexane cracking over the zeolites, after taking into account the amounts of adsorbed hydrocarbon, was in the order H−ZSM-5 > H−β > dealuminated H−Y ≫ H−Y.