Methanol thiolation over Al₂O₃and WS₂catalysts modified with cesium

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.