Alkylation of Benzene with Methane over ZnZSM-5 Zeolites Studied with Solid-State NMR Spectroscopy

Abstract

Alkylation of benzene with methane was studied under oxidization condition over ZnZSM-5 zeolites by using in situ solid-state NMR spectroscopy and GC-MS analysis. The experimental results indicate that the alkylation reaction occurs with selective formation of toluene at ternperatures of 523-623 K using O-2 or N2O as the oxidant. Using C-13 isotope labeled reactants, the conversions of methane and benzene were independently monitored, and their respective role in the reaction was determined. It was found by NMR spectroscopy that methane was first activated into methoxy species and zinc methyl intermediates. As an electrophilic agent, the methyl group of methoxy species could directly attack phenyl ring to produce toluene via electrophilic substitution reaction, while the zinc methyl species was not directly involved in the alkylation reaction. However, the similar nature of zinc methyl species (Zn-CH3) to organozinc compounds allowed the facile oxidization of zinc methyl species (Zn-CH3) into methoxy species. As confirmed by GC-MS experiments, methane exclusively provided the methyl group of toluene product while benzene afforded the phenyl ring. Experimental results also indicated that neither methane nor benzene alone could generate toluene.