Single-step selective oxidation of methane to methanol in the aqueous phase on iridium-based catalysts

[Display omitted] •Iridium-based catalysts are highly active for the catalytic oxidation of methane to methanol using molecular oxygen as the oxidant in the presence of CO in aqueous solution, under mild conditions.•The performance of Ir-ZSM-5 catalyst can be significantly enhanced by adding copper and palladium promoters. Addition of palladium can increase the overall catalytic activity, forming primarily formic acid, while addition of copper selectively promotes the formation of methanol.•The combination of Pd, Cu, and Ir together leads to the most active and selective catalyst, which can produce ∼1200 μmol/gcat or ∼3.9 g/gIr methanol with 80 % selectivity, at 150 °C in 1 h.•The in situ formed isolated Ir(I)(CO)2 sites are likely the active center for the C–H activation of methane. In this work, one-step conversion of methane to methanol using molecular oxygen as the oxidant in the presence of CO in aqueous solutions on copper or palladium promoted Ir-ZSM-5 catalyst is first reported. The addition of a second metal to Ir-ZSM-5 promotes the catalyst activity, while product selectivity can be tuned either to methanol on IrCu-ZSM-5 or to formic acid exclusively on IrPd-ZSM-5. Most effective is the combination of the three metal species together. Approximately 1200 μmol/gcat methanol, or ∼23.4 mol of methanol per mol of Ir, are formed on the IrCuPd trimetallic system (methanol selectivity ∼80 %) at 150 °C in 1 h. Our results also demonstrate that atomically dispersed Ir(I)(CO)2 species formed in the presence of CO can activate the C–H bond of methane to methyl species at temperatures below 150 °C. The good stability in cyclic operation is an additional attribute, rendering this type of catalyst a “front-runner” in future catalyst development for direct methane-to-liquid oxygenates.