Catalytic properties of postsynthesis phosphorus-modified H-ZSM5 zeolites

A series of postsynthesis modified ZSM5 zeolites was prepared using orthophosphoric acid, trimethylphosphine, and trimethylphosphite. The density of strong and weak Brønsted acid sites was determined by the temperature-programmed desorption of pyridine. The ultimate effect of the phosphorus treatment was the removal of the strong Brønsted acid sites. The density of the weak Brønsted acid sites was found to increase after treatment with phosphoric acid and trimethylphosphite, whereas it was found to be unchanged after trimethylphosphine treatment. The turnover frequency in n-hexane cracking was constant for a given type of acid site with all samples investigated. The energy of activation increased with the increasing density of weak Brønsted acid sites indicating that the H+ participated in the rate-determining step. For alkylation of toluene with methanol, nearly constant energies of activation were observed over all samples investigated, indicating no influence of the Brønsted acid site upon the rate-determining step. The decrease in activity is attributed to a decreased surface concentration of the reacting species in the rate-determining step or to a substantial decrease in the transition entropy from the reactants to the activated complex.