Cooperative catalysis of metal and acid functions in Re-HZSM-5 catalysts for ethane dehydroaromatization

Abstract

A vapor-phase exchange method was used for the encapsulation of Re metal clusters in dehydrated HZSM-5 zeolites with Brønsted acid OH groups. The reaction pathway of ethane non-oxidative dehydroaromatization on Re-HZSM-5 catalysts was revealed by an experimental observation of the contact time effects on hydrocarbon product selectivity. The reaction pathway typically includes the dehydrogenation of ethane to form olefin reactant pool via ethylene formation and immediate dimerization to butene, further dehydrogenation of ethylene to acetylene, and eventual formation of benzene products. The possible elementary step of ethane non-oxidative dehydroaromatization was revealed by kinetic measurements of the dependence of ethane and hydrogen pressures on the turnover rates of benzene formation. The kinetic observation indicates that the dehydrogenation of ethylene to acetylene is the rate-determining step that mediates the whole reaction event. Cooperative catalysis of metal and acid functions is observed on Re-HZSM-5 catalysts. Re metal clusters contribute to ethane activation and formation of initial alkene intermediates. The acid sites would further promote the oligomerization and cyclization reaction and generate benzene and toluene. The bifunctional pathways involving metal and acid roles in ethane non-oxidative dehydroaromatization were further studied on Re-HZSM-5, HZSM-5, and Re-SIL-1/HZSM-5 catalysts. The site proximity between Re metal clusters and acid sites plays a critical role in benzene formation, exemplified by initial rates of benzene formation of 0.03 and 0.43 μmol(C) gcat−1 s−1 on Re-SIL-1/HZSM-5 and Re-HZSM-5 catalysts, respectively. These two catalysts contain long and short distances of approximately 75 and 1.3 nm between Re metals and acid domains.