Effects of Solvent Acidity on the Free-Radical-Initiated Synthesis of Methanesulfonic Acid from CH₄ and SO₃

The effects of solvent acidity on K2S2O8-initiated synthesis of methanesulfonic acid (MSA) from CH4 and SO3 have been investigated. Dimethyl sulfoxide, sulfuric acid, and trifluromethanesulfonic acid were used as the solvents. The initial rate of MSA synthesis increased with solvent acidity, being lowest in DMSO and highest in triflic acid. This effect is attributed to the influence of solvent acidity on the extent of decompostion of the S2O82- anion, which dissociates to form SO4-•. The latter species is responsible for activation of CH4 to form CH3•. The maximum conversion of SO3 to MSA also depends on the solvent, increasing in the order DMSO < TFMSA < H2SO4. The highest conversion of SO3 to MSA observed for the conditions of these studies was 87% when H2SO4 containing 0.2 M TFMSA was used as the solvent. The limit on the maximum conversion of SO3 to MSA is set by the reaction CH3SO3H + SO3 ⇌ CH3(SO3)2H, which removes unconverted SO3 from solution as MSA is formed. Although the extent to which this reaction proceeds decreases with increasing solvent acidity, the use of TFMSA as the solvent is detrimental because it appears to stabilize the formation of CH3(SO3)2H, possibly in the form of CH3(SO3)2H·(CF3SO3H)2. The influences of the initial concentrations of SO3 and K2S2O8 and of the reaction temperature on the maximum conversion of SO3 to MSA are discussed in terms of a proposed mechanism for MSA synthesis.