The conversion of methanol to olefins: a transient kinetic study

The conversion of methanol to light olefins over a H-ZSM-5 zeolite was stud ied 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 me ans of high-frequency pulse injection, single-pulse experiments and step ex periments. Water, dimethyl ether, ethylene, propylene and butene are the mo st important products. Propylene is the major component in the olefin mixtu re. Methane and formaldehyde are produced in equal amounts at lower tempera tures and low conversions. Less formaldehyde was detected at higher convers ions and higher temperatures. Water, formaldehyde and methane are produced on a much smaller time scale than dimethyl ether and the olefins. Both prop ylene and ethylene are the primary gas-phase olefins. Their formation occur s via a common intermediate in a kinetically significant step. A second mec hanism for the formation of water, simultaneously or subsequent to the kine tically significant step of the olefin formation, was observed. Adsorption of water and methanol on basic sites is proposed to reduce the olefin forma tion rate. A detailed reaction scheme is presented. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.