(626a) Aqueous Phase Catalytic Conversion of Methane to Methanol on Bimetallic Catalysts Under Mild Conditions

The direct conversion of methane to methanol has long been considered the holy grail of catalysis due to the inertness of the C-H bond in methane and the ease of over-oxidation of the product. Methanol has the highest energy density among all the C1 oxygenates and it is also an important chemical feedstock in large demand. The current industrial processes to produce methanol from methane involves multiple reaction steps by first converting methane to syngas followed by the methanol synthesis reaction, and such process is only economical at large scale. A direct single step conversion circumvents the energy demanding steps, which makes it highly attractive. Despite extensive research, a fully catalytic system using molecular oxygen as oxidant with heterogeneous catalysts at low temperatures has not yet been reported. Liquid- phase conversion of methane to methanol is either conducted using H₂O₂ as oxidant¹ or using strong acid as the solvent² which limits their further applications. In the gas phase, although several zeolite- based catalysts have been shown to be active for this reaction³, the process requires a separate hydrolysis step to react and extract methanol from the active center, thus it is a non-catalytic process. The separate extraction step is not needed in the case of a liquid- phase system. Obviously, an aqueous phase catalytic system using O₂ is highly desirable.

Herein we report that iridium- based bimetallic catalysts in zeolites can directly convert methane to methanol with high selectivity in aqueous phase using O₂ as the oxidant and CO as a promoter. The monometallic Ir/ZSM-5 catalyst shows a relatively low activity, whereas the addition of a second metal significantly promotes both the activity and selectivity of the catalyst, leading to a high methanol yield. The promoter itself has a negligible activity and there is clearly a synergistic effect between iridium and the second metal. This study opens a new route for the development of methane- to- methanol catalysts using bimetallic systems.