Low-Temperature Selective Oxidation of Methane into Formic Acid with H2–O2 Gas Mixture Catalyzed by Bifunctional Catalyst of Palladium–Heteropoly Compound

The selective oxidation of methane catalyzed by heteropoly compounds having the formulas MxCs2.5H0.5−2x+yPVyMo12−yO40 (M=Pd2+, Rh2+, Ru2+, Pt2+, Mn2+, Hg2+, Fe3+, Co2+, Cu2+; x=0–3, y=0–3) was investigated. It was demonstrated that addition of Pd and incorporation of V had a strong influence on oxidation with a H2−O2 gas mixture and that Pd0.08Cs2.5H0.34PVMo11O40 showed the highest yield of formic acid. The reaction proceeded at temperatures as low as 423–593 K. Moreover, the reaction rate reached 1.2×10−4 mol h−1 g−1 at 573 K; this value is about 300 times higher than that with FePO4 catalyst. Addition of steam promoted the production of formic acid, and the yield reached a maximum at a partial pressure of steam of 9.1 kPa. The coexistence of H2 and O2 was indispensable for the selective oxidation of methane. It is suggested that an active oxygen species is formed by the reaction of H2 with O2 catalyzed by Pd and acidic sites of supports. Pressure dependencies were expressed by −dPCH4/dt=kPH21.0PO21.0PCH41.0, and is consistent with the idea that the reaction of an active species formed from H2 and O2 with CH4 is rate determining.