Investigation of the reaction pathway for synthesizing methyl mercaptan (CH3SH) from H2S-containing syngas over K–Mo-type materials

Abstract

The reaction pathway for synthesizing methyl mercaptan (CH₃SH) using H₂S-containing syngas (CO/H₂S/H₂) as the reactant gas over SBA-15 supported K–Mo-based catalysts prepared by different impregnation sequences was investigated. The issue of the route to produce CH₃SH from CO/H₂S/H₂ has been debated for a long time. In light of designed kinetic experiments together with thermodynamics analyses, the corresponding reaction pathways in synthesizing CH₃SH over K–Mo/SBA-15 were proposed. In the reaction system of CO/H₂S/H₂, COS was demonstrated to be generated firstly via the reaction between CO and H₂S, and then CH₃SH was formed via two reaction pathways, which were both the hydrogenation of COS and CS₂. The resulting CH₃SH was in a state of equilibrium of generation and decomposition. Decomposition of CH₃SH was found to occur via two reaction pathways; one was that CH₃SH first transformed into two intermediates, CH₃SCH₃ and CH₃SSCH₃, which were then further decomposed into CH₄ and H₂S; another was the direct decomposition of CH₃SH into C, H₂S and H₂. Moreover, the catalyst (K–Mo/SBA-15) prepared with co-impregnation exhibits higher catalytic activities than the catalysts (K/Mo/SBA-15 and Mo/K/SBA-15) prepared by the sequence of impregnation. Based on characterization of the oxidized, sulfided and spent catalysts via N₂ adsorption–desorption isotherms, XRD, Raman, XPS and TPR, it was found that two K-containing species, K₂Mo₂O₇ and K₂MoO₄, were oxide precursors, which were then converted into main K-containing MoS₂ species. The CO conversion was closely related to the amount of edge reactive sulfur species that formed the sulfur vacancies over MoS₂ phases.