Catalytic selective oxidation of C2-C4 alkanes over reduced heteropolymolybdates

Catalytic oxidation of ethane, propane, and isobutane to corresponding unsaturated acids and acetic acid with molecular oxygen were carried out over reduced H3PMo12O40 catalysts above 300 degrees C. A highly reduced state of H3PMo12O40 was formed by the heat-treatment of the pyridinium salt under N2 flow up to 420 degrees C. This catalyst, denoted by H3PMo12O40(Py), gave the highest conversion of alkanes and selectivities to the objective products and its reduced state was highly stable under the conditions of catalytic oxidation. The highly stable reduced state was composed of defect Keggin unit with oxygen vacancies and a secondary structure formed by the defect Keggin unit and remaining pyridine in the structure. Another reduced catalyst generated by the heat-treatment of (NH4)3PMo12O40 was active for the propane and isobutane conversion but poorly selective to acrylic acid and methacrylic acid, because this catalyst was readily oxidized during the oxidation. Non-reduced H3PMo12O40, catalysts showed poor activity for all of the reactions. A reaction mechanism is proposed over the reduced H3PMo12O40(Py) catalyst.