Mechanistic Aspects of the Hydrodeoxygenation of Phenolics and Furanics over Supported Metal Catalysts
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Elucidating the detailed reaction mechanisms of the hydrodeoxygenation (HDO) of phenolics has been the goal of a number of recent studies. Yet, there are aspects of the mechanism that remain unsettled and require further analysis. Microkinetic and theoretical studies have been conducted on a series of different metal supported on various oxide supports with varying degrees of reducibility and acidity. Depending on the metal and support used, different mechanisms are operational. Density functional theory (DFT) calculations show that the energy barrier for the direct dehydroxylation of m-cresol over Pt and Pd surfaces is too high, indicating that this path is unfavorable. Instead, a path via a ketone tautomer that undergoes hydrogenation of the carbonyl group followed by dehydration to form toluene and water is favorable on these noble metals. By contrast, over the more oxophilic Ru or Fe surfaces the direct dehydroxylation of m-cresol becomes more favorable than the tautomerization route. In addition, the selective deactivation of the different types of sites present on the metal catalyst as well as the effect of this deactivation on HDO selectivity have been investigated on a micro-pulse reactor over a series of metal catalysts of varying particle size and simulated with DFT calculations over FCC metal surfaces of varying defect densities.