Recent advances in the selective catalytic hydrodeoxygenation of lignin-derived oxygenates to arenes

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.