Enhanced catalytic performance of zeolite ZSM-5 for conversion of methanol to dimethyl ether by combining alkaline treatment and partial activation

Abstract

Zeolite ZSM-5 (MFI) due to its excellent hydrothermal stability and high catalytic activity for methanol dehydration to dimethyl ether (MTD) has been considered for use in combination with a methanol synthesis catalyst, such as Cu/ZnO/Al2O3, in the conversion of syngas to dimethyl ether. However, the decline of dimethyl ether selectivity and catalytic activity over ZSM-5 by the formation of hydrocarbons and coke at optimum operation temperature of Cu/ZnO/Al2O3 catalyst impedes industrial application. In this work, for the first time the effects of alkaline treatment combined with partial activation on the catalytic performance of ZSM-5 catalysts with different Si/Al ratio have been studied for MTD reaction. The relationship between the physicochemical properties and catalytic performance has been assessed from the combined results of XRD, SEM, N2 physisorption, NH3-TPD, elemental analysis, online-GC and other characterization techniques. The results show that at a reaction temperature of 300°C and WHSV of 13gg−1h−1, all the parent and alkaline-treated ZSM-5 after full activation at 500°C exhibited a decline of dimethyl ether selectivity and methanol conversion over time. Alkaline treatment improved DME selectivity over ZSM-5 with an Si/Al ratio of 25, which could be ascribed to the formation of extra mesoporosity enhancing the diffusion capability and decreasing the probability of secondary reactions of DME to hydrocarbons. A decrease of the activation temperature led to a significantly improved DME selectivity for all parent and alkaline-treated ZSM-5 because ammonium cations were selectively retained in the structure and blocked the strong acid sites that brought about side-reactions. ZSM-5 with Si/Al ratio of 25 modified by combining alkaline treatment and partial activation, due to the synergy effect of moderated acidity and enhanced diffusion capability, exhibited improved catalytic performance with almost 100% DME selectivity, near 84% methanol conversion and excellent stability during 4 days of reaction.