13C MAS NMR mechanistic study of benzene alkylation with propane over Ga-modified H-ZSM-5 catalyst

Abstract

13C MAS NMR has been performed in situ to investigate the mechanism of benzene alkylation with propane over Ga H-ZSM-5 catalyst. Propane 2- 13C was used as a labelled reactant. In order to clarify the main reaction pathways, conversions of the individual starting materials and some of the reaction intermediates such as cumene and n-propylbenzene were also studied under similar conditions. Benzene alkylation with propane begins at 573 K, toluene and ethylbenzene being the major primary products. The reaction is preceded by the induction period caused by stronger adsorption of benzene that prevents propane activation. The main reaction pathway includes bifunctional propane activation on Brønsted and Ga sites which leads to protonated pseudocyclopropane (PPCP) intermediate, which in turn decomposes preferentially to CH 4 and C 2H + 5, or C 2H 6 and CH + 3. The reaction of C 2H + 5 or CH + 3 carbenium ions with benzene leads to toluene or ethylbenzene, respectively. The less favourable reaction routes give propenium and cycloproponium ions upon PPCP decomposition, and result in observation of the traces of cumene and n-propylbenzene. These routes are reversible, the equilibrium being shifted towards reactants at 573 K.