(528c) Selective Oxidation of Methane to Methanol or Acetic Acid on Rhodium Single-Site Catalysts at Mild Conditions

The selective activation of the first C-H bond of methane on a metal site at low temperatures and its functionalization to produce oxygenates in a single step using molecular oxygen has been a long-standing goal in catalysis. Recently, the renewed interest in methane utilization has led to extensive studies on several catalytic systems, including low temperature gas phase reactions [1], electrophilic C-H bond activation in acidic media [2] and aqueous- phase conversion using peroxide [3]. However, these systems either require an expensive oxidant, e.g. peroxide or sulfuric acid that poses economic and environmental concerns, or the process is non-catalytic in nature. A complete catalytic cycle of this reaction involves three elementary steps: activation of CH₄ to a M-CH₃ complex, functionalization of the methyl species with OH, halides or CO groups and desorption of the product from the catalyst surface. Fundamental understanding of each of the three steps is necessary to design and further optimize the catalyst system that can activate C-H bond of methane.

In this presentation, we will show that over single-site, isolated Rh/ZSM-5 catalysts, methane can be oxidized directly to acetic acid in the presence of molecular oxygen and carbon monoxide in aqueous solutions under mild pressures and temperatures. The reaction is found to be catalytic and heterogeneous. The Brønsted acidity of the support is crucial for the selective production of acetic acid. On the other hand, methanol formation does not require acid sites and can be realized on Na ion-exchanged ZSM-5, and even on an open oxide support. Thus, anchoring rhodium single ions on the appropriate solid support offers a catalytic system with product tunability between methanol and acetic acid. Single batch production of acetic acid can reach 13,500 µmol/g-cat at more than 4.4% conversion of methane, and a fed-batch process is designed by replenishing limiting reactants to further push the conversion.

Acknowledgment

Financial support by the U.S. Department of Energy (ARPA-e grant# SU0433) is gratefully acknowledged.

References

[1] Alayon, E. M., Nachtegaal, M., Ranocchiari, M., and van Bokhoven, J. A. Chem. Comm. 48, 404 (2012).

[2] Periana, R. A., Taube, D. J., Gamble, S., Taube, H., Satoh, T., and Fuji, H. Science 280, 560 (1998).

[3] Hammond, C., Forde, M. M., and Ab Rahim, M. H., et al. Angew. Chem. Int. Ed. 51, 5129 (2012).