Sulfation mechanism and catalytic behavior of manganese oxide in the oxidation of methanethiol

This work studies the behavior of a manganese dioxide catalyst during the total oxidation of methanethiol. Modifications of the working gamma-MnO2 catalyst were characterized by XPS, XRD, DRIFTS, and potentiometric measurements after reactions at different durations and at different temperatures. At temperatures below 200 degreesC, sulfur adsorbs on gamma-MnO2 in the form of sulfate species and poisons CH3SH adsorption sites. Accordingly, the methanethiol conversion is strongly inhibited. At higher temperatures (260-280 degreesC), gamma-MnO2 is strongly reduced and is converted to a moderately active manganese sulfate phase while the reaction proceeds. This contribution also shows that the activity and selectivity of the manganese sulfate phase formed in situ at high temperature are comparable to those of a commercial manganese sulfate. The variation of the reaction temperature between 280 and 150 degreesC controls the reversible hydration/dehydration of the manganese sulfate phase and its associated activation/deactivation. This important change in activity is accompanied by a change in the main byproduct formation. At 280 degreesC, the dehydrated manganese sulfate phase is fairly active and favors the production of dimethyl disulfide. On the contrary, the hydrated phase that exists only in the lower-temperature region is much more reactive and promotes the production of methanol.