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-beta 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 beta 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-beta > dealuminated H-Y much greater than H-Y.