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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 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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Figure 1: Catalytic performance of supported rhodium catalysts in the conversion of methane to oxygenates.
Figure 2: TEM images and CO-DRIFTS spectra of various Rh-ZSM-5 catalysts.

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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.

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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.

Corresponding author

Correspondence to Maria Flytzani-Stephanopoulos.

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The authors declare no competing financial interests.

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

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Figure 1 XRD patterns of bare ZSM-5, as-synthesized 0.5 wt% Rh-ZSM-5 and 0.5 wt% Rh-ZSM-5 after 1 h and after 3 h of reaction at 150 °C.

XRD patterns of as-synthesized Rh-ZSM-5 show no observable difference compared with pure ZSM-5, indicating that the impregnation of Rh does not change the lattice structure of ZSM-5. The XRD patterns of Rh-ZSM-5 catalysts after the 1-h reaction and the 3-h reaction are preserved, suggesting that the oxidative conversion of methane to liquid oxygenate products does not alter the lattice structure of ZSM-5.

Extended Data Figure 2 Aberration-corrected HAADF/STEM images of as-synthesized Rh-ZSM-5.

a, Raw HAADF/STEM image. b, HAADF/STEM image after five-point smoothing and small contrast enhancement (Fig. 2a in main text). The images are of a thin edge of the as-synthesized Rh-ZSM-5 flake, where it is possible to image the Rh atoms. Contrast points consistent with the imaging of single Rh atoms are circled.

Extended Data Figure 3 TEM images and XPS characterization of Rh-ZSM-5.

a, b, Ac-HAADF (a) and bright-field (b) image pairs, acquired simultaneously, of as-synthesized 0.5 wt% Rh-ZSM-5. c, d, Ac-HAADF (c) and bright-field (d) image pairs of 0.5 wt% Rh-ZSM-5 after use in the reaction. e, f, Ac-HAADF (e) and bright-field (f) image pairs of 0.5 wt% Rh-ZSM-5washed. The ac-HAADF images show that there are a few Rh nanoparticles on the external surface of zeolite for the samples without washing, whereas for the sample of Rh-ZSM-5washed there are no Rh nanoparticles present. g, The Rh 3d photoemission spectra of as-synthesized 0.5 wt% Rh-ZSM-5 before and after Ar+ ion sputtering. h, The Si 2p photoemission spectra of as-synthesized 0.5 wt% Rh-ZSM-5 before and after Ar+ ion sputtering. Before sputtering, no photoemission peak of Rh was observed on Rh-ZSM-5. After Ar+ ion sputtering for 5 min, a Rh 3d feature arising from the Rh species anchored on the internal walls of the zeolite was clearly identified. XPS characterization therefore confirms the presence of Rh species inside the micropores of the zeolite.

Extended Data Figure 4 Catalytic performance of various Rh-ZSM-5 catalysts in the direct conversion of methane to liquid oxygenates.

a, Yields of liquid oxygenates on Rh-ZSM-5 catalysts with different Rh loadings, as well as 0.5 wt% Rh-ZSM-5washed and 0.5 wt% Rh-ZSM-5 O2. Reaction conditions: 20 mg catalyst, 20 bar CH4, 5 bar CO, 4 bar O2, 150 °C, 1 h. b, Product yields and liquid oxygenate selectivity at various reaction times with a of 4 bar, using the as-synthesized 0.5 wt% Rh-ZSM-5 catalyst. Reaction conditions: 20 mg catalyst, 20 bar CH4, 5 bar CO, 4 bar O2, 150 °C, and various reaction times. All data points were replicated three times and average values are reported with uncertainty below 10%.

Extended Data Figure 5 UV–Vis absorption spectra and XANES spectra of various samples.

a, UV–Vis absorption spectra of bare ZSM-5 (bottom trace), as-synthesized 0.5 wt% Rh-ZSM-5 (top trace), and 0.5 wt% Rh-ZSM-5 O2 (middle trace). b, Normalized XANES spectra of 0.5 wt% Rh-ZSM-5washed, 0.5 wt% Rh-ZSM-5washed suspended in water, 0.5 wt% Rh-ZSM-5washed after reaction, as-synthesized 0.5 wt% Rh-ZSM-5, 0.5 wt% Rh-ZSM-5 O2, as well as various Rh standards (Rh+: [Rh(μ-OH)(COD)]2, Rh2+: [Rh(CH3COO)2]2, Rh3+: Rh2O3).

Extended Data Figure 6 EXAFS characterization of 0.5 wt% Rh-ZSM-5washed samples under different conditions and Rh foil standard.

a, Rh K-edge EXAFS spectra of 0.5 wt% Rh-ZSM-5washed, Rh foil, and the fitting of 0.5 wt% Rh-ZSM-5washed. b, Rh K-edge EXAFS spectra of 0.5 wt% Rh-ZSM-5washed, 0.5 wt% Rh-ZSM-5washed suspended in water, and Rh foil. EXAFS spectra of the Rh-ZSM-5 sample were collected at room temperature in fluorescence mode. EXAFS spectra of Rh foil were collected at room temperature in the transmission mode. Note that in these cases, radial distances are given without phase correction. Quantitative analyses of Rh–O and Rh–Rh contributions in 0.5 wt% Rh-ZSM-5washed are shown in Extended Data Table 2, Entry 1.

Extended Data Figure 7 Possible reaction pathways of the catalytic conversion of methane to methanol and acetic acid on Rh-ZSM-5.

The conversion to methanol and acetic acid follow independent reaction pathways. See the main text for more information.

Extended Data Figure 8 EXAFS characterization of various Rh-ZSM-5 catalysts.

a, Rh K-edge EXAFS spectra and fitting of 0.5 wt% Rh-ZSM-5 without washing. EXAFS spectra clearly show Rh–Rh bonding, indicating the presence of Rh nanoparticles in the 0.5 wt% Rh-ZSM-5 sample without washing. b, Rh K-edge EXAFS spectra and fitting of 1.0 wt% Rh-ZSM-5washed. EXAFS spectra clearly show Rh–Rh bonding, indicating the presence of Rh nanoparticles. c, Rh K-edge EXAFS spectra and fitting of used 0.5 wt% Rh-ZSM-5washed catalyst. The sample was obtained after methane conversion reaction using the 0.5 wt% Rh-ZSM-5washed catalyst at 20 bar CH4, 5 bar CO, 2 bar O2 and 150 °C for 3 h. EXAFS spectra clearly show Rh–Rh bonding, indicating the formation of Rh clusters during the methane conversion reaction. The quantitative analyses for Rh–O and Rh–Rh contributions in these samples are shown in Extended Data Table 2.

Extended Data Table 1 Calculated conversion of methane at different reaction times
Extended Data Table 2 Quantitative analyses of Rh–O and Rh–Rh contributions as measured by EXAFS

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Shan, J., Li, M., Allard, L. et al. Mild oxidation of methane to methanol or acetic acid on supported isolated rhodium catalysts. Nature 551, 605–608 (2017). https://doi.org/10.1038/nature24640

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