On the role of the alkali cations on methanol thiolation

The electronegativity effect of the alkali cations on the formation of methanethiol by reaction of methanol and H₂S was studied with K⁺, Rb⁺, and Cs⁺ supported on γ-Al₂O₃. Catalyst activation in H₂S led to the formation of sulfur oxyanions, including SO₃^-2, S₂O₃^-2, SO₄^-2 and S₂O₆^-2. The surface modified aluminas have strong basic properties. Accessible alkali cations on γ-Al₂O₃ are the active sites catalyzing the SN₂ nucleophilic substitution. These sites stabilize two Lewis acid-base pairs, formed upon H₂S and methanol dissociation, in proximity. Decreasing electronegativity of the alkali enhances the concentration of basic sites and their adsorption capacity. In consequence, the rates of methanethiol formation increased in the sequence K/γ-Al₂O₃ < Rb/γ-Al₂O₃ < Cs/γ-Al₂O₃. The activation energies also decreased in parallel with the decreasing electronegativity of the alkali, which is attributed to the increasing electron density at the SH^- groups that facilitates the nucleophilic attack.