Structure–Activity Relationships in Hydrodeoxygenation of Lignin Derivatives: Catalyst Design, Hydrogen Supply Strategies, and Reaction Mechanisms

As a renewable aromatic carbon reservoir, lignin exhibits great potential for the synthesis of sustainable fuels and high-value chemicals. Nevertheless, its intricate skeletons, irregular bond distribution, and abundant oxygenated functional groups complicate hydrodeoxygenation (HDO), resulting in inferior product selectivity and limited catalytic durability. Centered on structure–activity relationships, this review systematically correlates lignin structural features, catalytic performance, hydrogen supply modes, and HDO mechanisms. Impacts of linkage types, functional group distribution, and pretreatment-induced structural evolution on substrate adsorption and C–O bond cleavage are analyzed. The structure-performance rules of monometallic, bifunctional, atomic-scale, and carbon-based catalysts are clarified. Regulatory effects of various hydrogen supply strategies on active hydrogen generation and competitive direct deoxygenation/hydrogenation deoxygenation (DDO/HYD) pathways are elaborated. Critical bottlenecks restricting practical lignin conversion are summarized, and prospects for rational catalyst design and green biorefinery development are proposed.