Catalytic activity of SAPO5 for cracking of butane and hexane

The catalytic properties of three SAPO5 samples for cracking of n-hexane and n-butane have been investigated. For characterization, both adsorption and temperature-programmed desorption of ammonia and pyridine were used. Only bridging hydroxyl groups (strong Brønsted-acid sites) were found to be active for both reactions. The catalytic activites of these sites were the same for all samples investigated. Differences exist with respect to the accessibility of n-butane and n-hexane. The variations of the turnover frequencies suggest that n-butane was converted over all Brønsted-acid sites present, whereas n-hexane was converted only over those sites that were able to adsorb pyridine. The product distribution and the apparent energies of activation suggest the prevalence of a monomolecular mechanism for cracking. As both molecules, n- butane and n-hexane, have nearly identical kinetic diameters, the transition state for n-hexane must be more bulky than that for n-butane.