The conversion of methanol to olefins: a transient kinetic study

The conversion of methanol to light olefins over a H-ZSM-5 zeolite was studied by means of temporal analysis of products. Three experiment types were performed at 623-773 K and up to circa 100 Pa: stationary experiments by means of high-frequency pulse injection, single-pulse experiments and step experiments. Water, dimethyl ether, ethylene, propylene and butene are the most important products. Propylene is the major component in the olefin mixture. Methane and formaldehyde are produced in equal amounts at lower temperatures and low conversions. Less formaldehyde was detected at higher conversions and higher temperatures. Water, formaldehyde and methane are produced on a much smaller time scale than dimethyl ether and the olefins. Both propylene and ethylene are the primary gas-phase olefins. Their formation occurs via a common intermediate in a kinetically significant step. A second mechanism for the formation of water, simultaneously or subsequent to the kinetically significant step of the olefin formation, was observed. Adsorption of water and methanol on basic sites is proposed to reduce the olefin formation rate. A detailed reaction scheme is presented. (C) 1999 Elsevier Science Ltd. All rights reserved.