Mild oxidation of methane to methanol or acetic acid on supported isolated rhodium catalysts

Abstract

An efficient and direct method of catalytic conversion of methane to liquid methanol and other oxygenates would be of considerable practical value. However, it remains an unsolved problem in catalysis, as typically it involves expensive or corrosive oxidants or reaction media that are not amenable to commercialization. Although methane can be directly converted to methanol using molecular oxygen under mild conditions in the gas phase, the process is either stoichiometric (and therefore requires a water extraction step) or is too slow and low-yielding to be practical. Methane could, in principle, also be transformed through direct oxidative carbonylation to acetic acid, which is commercially obtained through methane steam reforming, methanol synthesis, and subsequent methanol carbonylation on homogeneous catalysts. However, an effective catalyst for the direct carbonylation of methane to acetic acid, which might enable the economical small-scale utilization of natural gas that is currently flared or stranded, has not yet been reported. Here we show that mononuclear rhodium species, anchored on a zeolite or titanium dioxide support suspended in aqueous solution, catalyse the direct conversion of methane to methanol and acetic acid, using oxygen and carbon monoxide under mild conditions. We find that the two products form through independent pathways, which allows us to tune the conversion: three-hour-long batch-reactor tests conducted at 150 degrees Celsius, using either the zeolite-supported or the titanium-dioxide-supported catalyst, yield around 22,000 micromoles of acetic acid per gram of catalyst, or around 230 micromoles of methanol per gram of catalyst, respectively, with selectivities of 60-100 per cent. We anticipate that these unusually high activities, despite still being too low for commercial application, may guide the development of optimized catalysts and practical processes for the direct conversion of methane to methanol, acetic acid and other useful chemicals.

Densely Populated Single Atom Catalysts

“…In addition to the electrocatalytic reactions mentioned above, SACs have also been reported to be used in other catalytic reactions, including NRR, WGSR, methane reforming, methanol steam reforming, CO oxidation, benzene oxidation, acetylene hydrogenation, and so on. Shan et al reported an atomically dispersed Rh on the zeolite (Rh/ZSM‐5) catalyst, which can convert methane into methanol or acetic acid in liquid phase mild oxidation at 150 °C. Qiao et al reported the SAC of Pt 1 /FeO x with excellent activity and high stability for CO oxidation, and preferential oxidation of CO in H 2 .…”

Nitrogen‐Doped Porous Carbon Supported Nonprecious Metal Single‐Atom Electrocatalysts: from Synthesis to Application

“…Due to lack of mature theoretical research and limitation of experimental conditions in early stage, the synthesis of metal single atoms involves multiple steps, and the preparation conditions are harsh, including coprecipitation method, atomic layer deposition method, impregnation method, anti‐Ostwald maturation method, stepwise reduction method, etc. With an in‐depth understanding of the coordination environment of metal single atoms, simple and efficient preparation solutions began to be explored.…”

Rh‐Based Nanocatalysts for Heterogeneous Reactions

“…Therefore, a simple heterogeneous catalytic system would be more desirable. Recently, Flytzani‐Stephanopoulos and co‐workers demonstrated that mononuclear Rh species anchored on a zeolite or TiO 2 support effectively catalyzed the direct conversion of methane to methanol and acetic acid in aqueous solution, using O 2 and CO under mild conditions (Figure ) . The conversion of methane can be tuned, yielding around 22000 micromoles of acetic acid per gram of zeolite supported catalyst or around 230 micromoles of methanol per gram of TiO 2 supported catalyst, with selectivities of 60–100%.…”