Direct synthesis of dimethyl ether from syngas over mechanical mixtures of CuO/ZnO/Al2O3 and γ-Al2O3: Process optimization and kinetic modelling

This article explores the use of mechanical mixtures of a methanol synthesis catalyst (CuO/ZnO/Al2O3) and acid catalysts (gamma-Al2O3) for enabling the one-step formation of dimethyl-ether from syngas. The study involved the determination of the optimal catalytic system composition and operating conditions in terms of reactant conversion and dimethyl-ether yield. The catalysts mixture that exhibited the best performance consisted of 92.5% wt. of CuO/ZnO/Al2O3(7.5% wt. of gamma-Al2O3), when operated with an excess of H-2 (H-2/CO > 1.5) and a small amount of CO2 (4-6%) in the feed. The influence of temperature (250-270 degrees C) was less marked, due to the influence of the equilibrium. The final purpose of the study of these properties is to develop one of the first kinetic models for the use of mechanical mixtures of commercial catalysts for this reaction. The experimental data were used to fit and validate a kinetic model based on four reactions: synthesis of methanol from CO, CO2, water gas shift reaction and methanol dehydration. At the studied reaction conditions, synthesis of methanol is kinetically relevant whereas water gas shift reaction and methanol dehydration are close to equilibrium. The inhibition caused by water was also accounted for in the kinetic model.