Comparative study of direct methylation of benzene with methane on cobalt-exchanged ZSM-5 and ZSM-11 zeolites

Abstract

Methane exists in massive quantity on the Earth, and considerable efforts have been made to develop effective methods to convert chemically inert methane into value-added products. Recently, cobalt-exchanged ZSM-5 zeolite has been found to be a competent catalyst for non-oxidative methylation of benzene using methane as a methylation agent. Herein, cobalt-exchanged ZSM-11 zeolite, with close topological structure to that of ZSM-5 zeolite, was discovered to be capable of catalyzing non-oxidative methylation of benzene with methane for the first time. ZSM-5 and ZSM-11 zeolites both with Si/Al molar ratio of ca. 36 and 18 were synthesized. However, higher cobalt loadings were obtained on ZSM-11 zeolites, and cobalt-exchanged ZSM-11 catalysts displayed superior toluene production rates and turnover frequencies. In addition, low silica zeolite catalysts possessed higher methylation selectivity. Diffuse reflectance visible–near infrared spectra revealed that divalent cobalt cations on the ion exchange sites existed mainly in two different coordination states, and the cobalt cation on the α site, coordinating weakly to the framework oxygen atoms, was supposed to be the active site because of the positive correlation of its concentration and the methane activation performance. Since more α-type Co2+ could be created in ZSM-11 zeolites than in ZSM-5 zeolites with the same Si/Al molar ratio, it was concluded that cobalt-exchanged ZSM-11 catalyst was a promising candidate for the non-oxidative methylation of benzene with methane.