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