Catalytic activity of Brønsted acid sites in zeolites: Intrinsic activity, rate-limiting step, and influence of the local structure of the acid sites

The catalytic activity of Brønsted acid sites in zeolites was studied by the monomolecular conversion of propane over zeolites with varying framework topologies and Si/Al ratios. The rates and apparent activation energies of cracking and dehydrogenation were determined. The activity of the Brønsted acid sites depends on the rate-limiting step of the reaction. In the cracking reaction, the protonation of the alkane is the rate-limiting step, and the heat of reactant adsorption dominates the differences in the observed activity. The similar intrinsic activities over the different zeolites show that the ability of zeolitic Brønsted acid sites to transfer a proton to an alkane does not vary significantly, suggesting that the acid sites that participate in the reaction have very similar strengths. In the dehydrogenation reaction, the rate-limiting step is the desorption of the alkoxide species. The rate is determined by the stability of the alkoxide species, which is influenced by the local geometric and electronic structure of the Brønsted acid site and is affected by zeolite structure and Si/Al ratio. Implications of these conclusions are related to other reactions, such as catalytic cracking and alkylation.