(691b) Experimental and Computational Investigation of the Deoxygenation of Acetic Acid over Molybdenum Carbide

The catalytic upgrading of biomass pyrolysis vapors, prior to condensation, requires catalysts that can activate and incorporate H₂ at relatively low pressures and low hydrogen-to-carbon environments, are stable under acidic conditions, and favor cleaving C-O bonds over C-C bonds [1]. Transition metal carbides are promising materials for this process as they exhibit reactivities towards carbon and hydrogen similar to other transition metals (i.e., Ru and Ir), but have much higher reactivities towards oxygen [2]. Recently, transition metal carbides have been shown to selectively cleave C-O bonds and favor hydrodeoxygenation over decarbonylation/decarboxylation [3, 4]. However, there is limited understanding of (1) the metal carbide surface chemistry under reaction conditions and (2) the deoxygenation mechanism. This work combines density functional theory (DFT) modeling, X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier Transform spectroscopy (DRIFTS), and catalytic testing to investigate the deoxygenation mechanism of acetic acid, a biomass pyrolysis model compound, over molybdenum carbide (Mo₂C) catalysts. At moderate temperatures (250-400°C) in the presence of H₂, the Mo₂C catalyst favored the direct deoxygenation of acetic acid, forming primarily acetaldehyde and ethylene. XPS results revealed that the catalyst surface was partially covered by oxygen under reaction conditions, thus a Mo-terminated Mo₂C surface with a sub-monolayer of oxygen was used for DFT modeling. Good agreement was observed between experimental product selectivities, calculated reaction pathway energetics, and surface intermediates identified by DRIFTS. The results from this work provide insight into the deoxygenation mechanism of carboxylic acids over transition metal carbide materials and reveal opportunities for further catalyst enhancement.

References:

[1] Ruddy, D. A., Schaidle, J. A., Ferrell, J. R., Wang, J., Moens, L., and Hensley, J. E. Green Chem. 16, 454 (2014).

[2] Medford, A. J., Vojvodic, A., Studt, F., Abild-Pedersen, F., and Norskov, J. K. J. Catal. 290, 108 (2012).

[3] Ren, H., Yu, W., Salciccioli, M., Chen, Y., Huang, Y., Xiong, K., Vlachos, D. G., and Chen, J. G. ChemSusChem 6, 798 (2013).

[4] Ren, H., Chen, Y., Huang, Y., Den, W., Vlachos, D. G., and Chen, J. G. Green Chem. 16, 761 (2014).