(473g) Reactions of Aliphatic and Aromatic Oxygenates On Ruthenium

Biomass derived oxygenates can be used as feedstock for the production of alkanes or H₂ in aqueous phase processing over various metal catalysts [1,2]. While late transition metals, e.g., Pt or Pd, typically favor reforming yielding hydrogen, ruthenium catalysts were reported to show high selectivity for alkane formation [2], which demonstrates their potential as hydrodeoxygenation catalyst. We examined computationally transformations of selected model oxygenates over Ru(0001), applying a DFT-GGA periodic slab model approach.

We explored the decomposition of ethanol [3], the smallest alcohol containing a C-C bond, focusing on the competition between C-O and C-C bond cleavage steps. Preference of C-C bond cleavage has been associated with reforming, preference of C-O bond cleavage has been suggested to favor the formation of hydrocarbons via hydrodeoxygenation. We found that dehydrogenating ethanol yielding unsaturated centers is required for kinetically accessible C-C and C-O cleavage. In part, this is related to a stronger substrate-surface interaction, stabilizing transition structures. For most intermediates studied, C-O cleavage is both kinetically and thermodynamically favored over C-C scission. The latter becomes kinetically favored only for highly dehydrogenated species CH_kCO (k = 1, 2). Based on the calculated energetics we suggest a decomposition pathway of ethanol on Ru via formation of ketene CH₂CO and subsequent C-C cleavage, to yield methylene and CO. The crucial barrier, 77 kJ mol^–1, is associated with the initial dehydrogenation at the saturated ethyl group [3].

We went on to explore reactions of the aromatic oxygenate guaiacol (2-methoxy­phenol) on Ru. Due to its hydroxyl, alkoxyl and aromatic functionalities, guaiacol may serve as a model system for understanding the chemistry of lignin, one of the major components of woody biomass. Again we started with the evaluation of preferred adsorption complexes, addressing especially the interaction of  aromatic functionalities with Ru. We also studied the hydrodeoxygenation of guaiacol by evaluating pertinent reaction energies and activation barriers.

References:

[1] D.M. Alonso, J.Q. Bond, J.A. Dumesic, Green Chem. 2010, 12, 1493-1513.

[2] R.R. Davda, J.W. Shabaker, G.W. Huber, R.D. Cortright, J.A. Dumesic, Appl. Catal. B-Environ. 2005, 6,171-186.

[3] C.-c. Chiu, A. Genest, N. Rösch, Top. Catal. 2013, DOI: 10.1007/s11244-013-0051-0.