The role of external acid sites of ZSM-5 in deactivation of hybrid CuZnAl/ZSM-5 catalyst for direct dimethyl ether synthesis from syngas

Abstract

Direct synthesis of dimethyl ether (DME) from syngas was investigated on a series of hybrid Cu–ZnO–Al2O3/ZSM-5 catalysts prepared by kneading. It was found that the initial performance of the catalysts was a function of both zeolite crystallite sizes and Si/Al ratio. The activity of the hybrid catalysts gradually decreased with time on stream because of simultaneous copper sintering, copper oxidation and ion exchange with zeolite hydroxyl groups. Copper sintering led to the decrease in the number of metal active sites, while ion exchange of Cu2+ ions with the hydroxyl groups of the zeolite resulted in lower concentration of acid sites for methanol dehydration. Copper sintering and ion exchange could be slowed down by selective neutralization of the acid sites on the zeolite outer surface by silylation. Both catalyst stability and dimethyl ether productivity were significantly improved.