A high-yield approach to the sulfonation of methane to methanesulfonic acid initiated by H2O2 and a metal chloride.

Methane is abundant and relatively inexpensive, making it an attractive feedstock for producing bulk chemicals. However, the low reactivity of methane makes it difficult to develop commercially viable processes for methane conversion. Because of favorable thermodynamics, many authors have investigated the oxidation and oxidative carbonylation of methane. By contrast, the sulfonation of methane to give methanesulfonic acid has not received as much attention despite its commercial importance. The current commercial process for the synthesis of methanesulfonic acid (MSA) relies on the chlorine-oxidation of thiomethane. While this process is highly productive, it produces six moles of HCl per mole of MSA, resulting in a coupling of the demand for the primary product and the by-product. It has been shown that K2S2O8 can be used as a free-radical initiator to sulfonate methane with SO3 in fuming sulfuric acid under very high methane pressure. The conversion of methane to MSA is reported to be 3–6% under 1000 psig methane pressure (1 psig= 6.8955 10 Pa). We recently have shown that metal peroxides can be used as an effective radical initiator for methane activation in sulfuric acid solvent. However, once the metal peroxides have activated methane they are converted into the metal sulfate salts, which cannot be recycled back to the metal peroxide. The economics of producing and handling safely the necessary amounts of metal peroxide must also be considered if a practical process is to be developed. Thus, there is considerable incentive to look for a way to generate metal peroxo species in situ from a cheap source such as H2O2. In this communication we show that methane will undergo liquid-phase sulfonation to MSA with SO3 in sulfuric acid. A small amount of urea/H2O2 is used as a free-radical initiator and a small amount of a metal chloride salt, particularly RhCl3, is used as a promoter [Eq. (1)].