The utilization of lignin-derived compounds is an important part of biomass valorization. One promising way to achieve this goal is to reduce/eliminate the oxygen content via hydrodeoxygenation (HDO), producing valuable fuels and chemicals. However, most HDO catalysts are not selective for arene production; they have a tendency to saturate the aromatic rings and thus consume excess hydrogen. In this review, we summarize the recent advances in catalysts for phenol HDO and the understanding of the reaction mechanisms involved, aiming to provide a perspective for developing highly efficient HDO catalysts. In particular, we first present an overview of the HDO of phenolics, including the challenges in selectively removing oxygen without saturating the aromatic ring. Then, we focus on the characteristics of four types of HDO catalysts, i.e., sulfides, base metals (oxides), noble-metal-based catalysts, and carbides/nitrides/phosphides and the potential approaches to improve their arene selectivity. Finally, we attempt to generate a correlation between the oxophilicity of the catalysts and their performances in the HDO of phenolics, which can potentially serve as a basis for the design of selective HDO catalysts.

Ru species have a high redox capacity on a CeO₂ support, and the metallic Ru species could be easily oxidized back to RuO_x species.

Noble metals have been extensively employed in a variety of hydrotreating catalyst systems for their featured functionality of hydrogen activation but may also bring side reactions such as undesired deep hydrogenation. It is crucial to develop a viable approach to selectively inhibit side reactions while preserving beneficial functionalities. Herein, we present modifying Pd with alkenyl-type ligands that forms homogeneous-like Pd-alkene metallacycle structure on the heterogeneous Pd catalyst to achieve the selective hydrogenolysis and hydrogenation. Particularly, a doped alkenyl-type carbon ligand on Pd-Fe catalyst is demonstrated to donate electrons to Pd, creating an electron-rich environment that elongates the distance and weakens the electronic interaction between Pd and unsaturated C of the reactants/products to control the hydrogenation chemistry. Moreover, high H₂ activation capability is maintained over Pd and the activated H is transferred to Fe to facilitate C-O bond cleavage or directly participate in the reaction on Pd. The modified Pd-Fe catalyst displays comparable C-O bond cleavage rate but much higher selectivity (>90%) than the bare Pd-Fe (90%) in acetylene hydrogenation. This work sheds light on the controlled synthesis of selective hydrotreating catalysts via mimicking homogeneous analogues.