Mukhopadhyay and Bell have reported the first liquid phase oxidation of methane with SO₂ in triflic acid to methanesulphonic acid using K₂S₂O₈ as oxidant and a catalytic amount of a calcium salt as a promoter.  A maximum conversion of SO₂ to methanesulphonic acid of 22% could be achieved.  This method is considerably more environmentally than the commercial process for manufacturing methanesulphonic acid in which methylmercaptan is oxidised with chlorine and water generating 6 moles of HCl per mole of methanesulphonic acid.  (J. Am. Chem. Soc., 2003, 125, 4406).