Deactivation Kinetics for Direct Dimethyl Ether Synthesis on a CuO-ZnO-Al2O3/γ-Al2O3 Catalyst

A kinetic model has been established for the deactivation of a CuO-ZnO-Al2O3/gamma-Al2O3 catalyst in the direct synthesis of dimethyl ether. The model allows calculating the effect of the operating conditions and the evolution of component concentration in the reaction medium (dimethyl ether, methanol, paraffins, H-2, CO, CO2, and H2O) with time on stream. The results of the model fit the experimental results in a wide range of conditions: 225-350 degrees C; 10-40 bar; space time, 0.1-68.0 (g of catalyst) h (mol of reactants)(-1); molar ratio (H-2/CO) in the feed, 2-4; time on stream, 30 h. The model considers the cause of deactivation to be coke deposition on the metallic function and that this coke is formed by degradation of oxygenates (dimethyl ether and methanol). The effect of water in the reaction medium has been quantitatively considered in the kinetic model, whose drawback is the attenuation of the methanol synthesis rate and whose benefit is the attenuation of deactivation by coke deposition. The kinetic model is useful for predicting the influence of water in the feed with syngas as a strategy to attenuate deactivation, which is insignificant for a H2O/(H-2 + CO) molar ratio of around 0.20, although it causes a slight decrease in initial catalyst activity.