Letter

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

Received:
Accepted:
Published online:

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 expensive1,2,3,4 or corrosive oxidants or reaction media5,6,7,8 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)9,10,11,12,13,14,15 or is too slow and low-yielding16 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 catalysts17,18. 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.

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Acknowledgements

The financial support of this work by the Department of Energy, DOE/ARPA-e grant DE-AR0000433, under subcontract from MIT, is gratefully acknowledged. The XAS work used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science, User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract DE-AC02-06CH11357. Aberration-corrected electron microscopy research at Oak Ridge National Laboratory was sponsored by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office, Propulsion Materials Program.

Author information

Author notes

    • Junjun Shan

    Present address: NICE America Research, Inc., Mountain View, California 94043, USA.

    • Junjun Shan
    •  & Mengwei Li

    These authors contributed equally to this work.

Affiliations

  1. Department of Chemical and Biological Engineering, Tufts University, Medford, Massachusetts 02155, USA

    • Junjun Shan
    • , Mengwei Li
    •  & Maria Flytzani-Stephanopoulos
  2. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

    • Lawrence F. Allard
  3. X-ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, USA

    • Sungsik Lee

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Contributions

J.S. conceived the research, designed the experiments, characterized the samples and drafted the manuscript. M.L. conceived the research and performed catalytic evaluation. M.F.-S. conceived the research and designed the experiments. L.F.A was responsible for the STEM characterization. S.L. helped with the XANES and EXAFS measurements and the interpretation of the results. All the authors discussed the results and participated in writing the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Maria Flytzani-Stephanopoulos.

Reviewer Information Nature thanks E. Pidko and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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