Effects of Cu–ZnO Content on Reaction Rate for Direct Synthesis of DME from Syngas with Bifunctional Cu–ZnO/γ-Al₂O₃ Catalyst

Abstract

The effects of Cu–ZnO content on the performance of bifunctional Cu–ZnO/γ-Al₂O₃ catalysts for dimethyl ether (DME) synthesis from syngas were investigated by varying the weight ratios of Cu–ZnO/γ-Al₂O₃ prepared by the coprecipitation of Cu–ZnO in a slurry of γ-Al₂O₃. A higher rate of DME production with CO conversion of 47.6 % and DME selectivity of 61.1 % was observed with the bifunctional catalyst at an optimal weight ratio of CuO/γ-Al₂O₃ of two, providing a higher surface area of metallic copper and an abundance of weak acid sites. The number of acidic sites on solid-acid γ-Al₂O₃ is a more crucial factor to enhance DME yield, due to the faster dehydration rate of methanol to DME than that of CO hydrogenation to methanol. Although the first step of methanol synthesis on active copper sites is a rate-limiting step with a low equilibrium value, the second step of the dehydration of methanol to DME on acid sites adjusts the overall rate by enhancing the forward reaction rate of CO hydrogenation to methanol with a simultaneous formation of surplus hydrogen by a water–gas shift reaction. Therefore, the proper design of a high surface area of metallic copper with larger acid sites on the bifunctional CuO–ZnO/γ-Al₂O₃ catalysts at an optimal ratio, produced by adjusting the weight ratio of CuO/γ-Al₂O₃, is an important factor for improved catalytic performance.

Graphical Abstract