Methanol to dimethyl ether conversion over a ZSM-5 catalyst: intrinsic kinetic study on an external recycle reactor

Abstract

In this work, several kinetic models have been studied for the methanol dehydration to dimethyl ether (DME), applying a commercial ZSM-5 catalyst rather than state-of-the-art alumina. The kinetic tests were carried out in a fixed-bed external-recycle reactor, in the absence of temperature and concentration gradients. Kinetic parameters of the different models were calculated and evaluated against physicochemical constraints and guidelines on entropy values. After discrimination, it was concluded that a modification of a model previously developed by Klusáček & Schneider for methanol dehydration over different catalyst formulations than alumina or zeolites describes best the methanol dehydration reaction, with excellent agreement between experimental data and calculated values. The selection of this model supports a mechanism in which methanol experiences dissociative adsorption and a surface reaction is the rate determining step (RDS) of the catalytic cycle. The kinetic measurements were performed at a total pressure of 1 bar, partial pressure of methanol (pm) from 0.3 bar to 1 bar, temperatures from 140 °C to 190 °C, weight hourly space velocities (WHSV) from 15 h⁻¹ to 100 h⁻¹, and two different amounts of water content (0 and 30 wt%). Experiments at different methanol concentrations in the feed revealed a weak dependence of the rate on pm. It could be determined to our knowledge for the first time that the reaction order with respect to water is negative, in the range of −0.80 to 0, thus providing quantitative proof of the inhibition effect of water on the methanol dehydration reaction rate over ZSM-5 catalysts, while DME showed no effect.