A Novel Method for the Direct Sulfonation of CH₄ with SO₃ in the Presence of KO₂ and a Promoter

Direct sulfonation of methane with SO3 to methanesulfonic acid (MSA) is accomplished in sulfuric acid in the presence of a small amount of KO2 as the free radical initiator and a metal chloride. Of the several metal chlorides examined, RhCl3 was found to be the most effective promoter. While KO2 alone can activate methane, the conversion of SO3 to MSA increases 2.3-fold when KO2 and RhCl3 are both present in the reaction mixture. The effects of different process parameters such as temperature, SO3 concentration, methane pressure, KO2 concentration, and RhCl3 concentration have been examined on the rate of reaction. The reaction is optimized at a KO2-to-RhCl3 molar ratio of 3.16. Strongly acidic solvents such as H2SO4 or CF3SO3H are necessary for the reaction. No MSA was formed when the reaction was carried out in DMSO. A mechanism is proposed to explain the activation of CH4 to form MSA. A critical part of the sequence is in situ formation of a metal−peroxo species via the reaction of KO2, acid solvent, and RhCl3.