K2S2O8-Initiated Sulfonation of Methane to Methanesulfonic Acid

The direct synthesis of methanesulfonic acid (MSA) from CH 4 and SO 3 has been successfully achieved in an H 2 SO 4 solvent using K 2 S 2 O 8 as an initiator. The maximum SO 3 conversion to MSA was achieved at 1000 psi of CH 4, 30 wt % of SO 3, and 350 μmol of K 2 S 2 O 8 at 333 K for a 2-h reaction time. SO 3 conversion to MSA is approximately independent of reaction time above 2 h, at which time the SO 3 conversion is observed to plateau at conversions much less than 100%. This plateau has been attributed to a loss of solution SO 3 as a result of the formation of CH 3 (SO 3 ) n H and (CH 3 SO 2 ) 2 O. The rate of MSA formation exhibits a positive-order dependence on the CH 4 pressure, and this pressure dependence is nonlinear, rising more rapidly for pressures equal to or greater than 800 psi. Addition of SO 3 to the solvent medium at concentrations greater than 10% results in a rapid decrease in SO 3 conversion as a result of the formation of significant byproducts and CH 3 (SO 3 ) n H. At both 333 and 363 K, the SO 3 conversion to MSA passes through a maximum with increasing K 2 S 2 O 8 concentration, and then declines to a plateau for higher K 2 S 2 O 8 concentrations. The position of the maximum is temperature-dependent and occurs at 100 μmol of K 2 S 2 O 8 at 363 K and 350 μmol at 333 K. A maximum in SO 3 conversion to MSA is also observed at 333 K when the temperature is increased from 313 to 363 K. A free radical mechanism has been proposed to account for the observed phenomena.