Methanol thiolation over Al2O3 and WS2 catalysts modified with cesium

Abstract

Thiolation of methanol with H2S was studied on Al2O3, WS2/Al2O3, and the Cs-modified counterparts Cs/Al2O3, and Cs-WS2/Al2O3. On Cs-free catalysts, methanol reacts with similar rates to dimethyl ether and to methanethiol. Secondary steps yield dimethyl sulfide and trace amounts of dimethyl disulfide and methane. On Cs-containing catalysts, the dominating reaction is methanol thiolation. This drastic change in selectivity is related to the low strength and concentration of Lewis acid sites in the presence of Cs+. The active sites for condensation and thiolation of methanol are concluded to be Lewis or Brønsted acid-base pairs formed by dissociation of H2S on the Lewis sites. Thiolation and condensation follow Langmuir-Hinshelwood mechanisms with nucleophilic attack of SH or methoxy groups to adsorbed methanol as rate determining step. The formation of dimethyl ether on Cs+ Lewis acid sites is suppressed due to low coverage of methanol, whereas the rate of the nucleophilic attack of SH to methanethiol is enhanced.