Coupling of Dielectric Barrier Discharge and Cu–S-1 Catalyst for Direct Oxidation of Methane to Methanol

The industrial utilization of methane to methanol is based on the indirect pathway with syngas as an intermediate product, but the reaction conditions are very harsh and the costs in equipment and operations are extremely high. Therefore, the direct oxidation of methane to methanol (DOMTM) under mild conditions is more desirable. In this paper, coupling of non-thermal plasma and Cu–zeolite (Cu–ZSM-5 and Cu–S-1) catalysts have been studied for DOMTM. The effect of Si/Al ratios (Cu–ZSM-5 and Cu–S-1 catalysts) on the reaction performance has been investigated. Cu–S-1 catalyst exhibited the best catalytic performance, i.e., 5.8% methane conversion and 50.6% methanol selectivity. The employed catalysts have been characterized by XRD, UV–Vis, XPS and FT-IR. The results show that Cu–ZSM-5 catalysts with high Si/Al ratio are conducive to the formation of isolated Cu2+ species, and Cu–S-1 catalyst exhibits the most abundant isolated Cu2+ species. Furthermore, Cu–S-1 catalyst shows the most abundant Oads species. The plentiful surface silicon defects on S-1 is the reason why Cu–S-1 catalyst shows the most abundant isolated Cu2+ and Oads species, and these two species are interdependent to form a Si–O–Cu species, which may be the active site for DOMTM. Based on these findings, the possible catalytic mechanism was proposed.