Alkali catalyzes methanethiol synthesis from CO and H₂S

Alkali metals are commonly used to promote heterogeneous catalysts. For instance, K can enhance the MoS₂-catalyzed synthesis of methanethiol, an important industrial chemical, from CO/H₂/H₂S. Herein, we not only demonstrate that the synergy increases with alkali cation size (Cs > Rb > K > Na) but also that alkali sulfides themselves are the active sites. The alkali-normalized methanethiol formation rate of Cs sulfide is nearly the same as that of Cs-promoted MoS₂ with a much lower yield of unwanted methane. Kinetic measurements show that methanethiol on alkali sulfides is formed via the reaction 3 CO + 2 H₂S → COS + CO₂ + CH₃SH. This represents a first example of H₂S as a hydrogen donor for the formation of the thiol counterpart of methanol. The role of MoS₂ is that of a promoter that disperses alkali and provides spillover H from H₂ dissociation.