Influence of potassium on the synthesis of methanethiol from carbonyl sulfide on sulfided Mo/Al₂O₃ catalyst

Potassium-doped MoS₂ catalysts supported on Al₂O₃ were synthesized, characterized by using atomic absorption spectroscopy, N₂ physisorption, NO adsorption, X-ray diffraction, temperature-programmed sulfidation, and Raman spectroscopy, and tested in the synthesis of methanethiol from carbonyl sulfide (COS) and H₂. The results revealed that two phases, pure MoS₂ and potassium-decorated MoS₂ (formed at high potassium loadings), were present in the active catalysts. The main effect of potassium during sulfidation and during the catalytic reaction was to increase the mobility of surface oxygen or sulfur atoms. Thus, potassium promoted the disproportionation of COS to CO₂ and CS₂ and the production of CO from CO₂. Additionally, potassium cations hindered the reductive decomposition of COS to CO and H₂S and the hydrogenolysis of methanethiol to methane. Mars-van Krevelen-type mechanisms were proposed to explain the disproportionation of COS on alumina and on the MoS₂ phases. The catalytic site in the potassium-decorated MoS₂ phase was proposed to include a potassium cation as adsorption site. Potassium is the answer: Adding potassium to the MoS₂ catalyst is necessary to achieve a high activity and selectivity from CH₃SH from H₂S-containing syngas. Acting as adsorption centers, the surface potassium cations promote the disproportionation of carbonyl sulfide to CO₂ and CS₂, but hinder the O-C and C-S bond cleavage, which is the preferred reaction on the sulfur vacancies of MoS₂.