Role of support in phenol hydrodeoxygenation over supported Pd catalysts: Insights from first-principles based microkinetic modelling

Catalytic Hydrodeoxygenation (HDO) of biomass pyrolysis-derived bio-oil is a critical step for reducing oxygen content in the bio-oil, before its utilisation as a fuel. In this regard, HDO of bio-oil model compounds have been investigated over numerous supported metal catalysts, but the role of support on the catalytic activity has been largely ignored. In this work, the reaction mechanism for HDO of phenol – a bio-oil model compound was investigated on Pd/Al₂O₃, Pd/ZrO₂ and Pd/TiO₂ catalysts. Comprehensive reaction map and energetics of the multiple reaction pathways for phenol activation to benzene and cyclohexanone were elucidated using first-principles Density Functional Theory (DFT) calculations. The metal-support interface provides a unique site for the adsorption of the reactants and the intermediates, which alter the reaction mechanism and consequently the product distribution. Over TiO₂ and ZrO₂ supports, phenol adsorbs on the Pd-support interface with the phenolic O binding to oxophilic cation of the oxide support and ring carbon to Pd, thereby weakening the C_aryl-O bond and facilitating its cleavage to form benzene. In the case of Al₂O₃, phenol adsorbs only through the ring carbon on the Pd, which results in ring-hydrogenated products like cyclohexanone. A microkinetic analysis revealed the direct deoxygenation pathway to benzene to be favoured over Pd/ZrO₂ and Pd/TiO_2, while cyclohexanone production via a phenoxy intermediate was favoured on Pd/Al₂O₃, in agreement with experimental results.