Catalysts with metal-acid dual sites for selective hydrodeoxygenation of lignin derivatives: Progress in regulation strategies and applications

Metal sites and acid sites in catalysts play an important role in promoting the hydrodeoxygenation (HDO) of lignin derivatives. Herein, the review first summarizes the intrinsic role and synergy of metal and acid sites in the HDO of model compounds. Metal sites tend to be responsible for activating hydrogen donors and hydrogenation reactions, and acid sites are the main active sites for promoting C-O bond cleavage. Then, this review discusses the modulation methods of dual active sites, including the introduction of the second metal, the modification of support, and the modulation of structure. In addition, an overview of the HDO reaction mechanisms of the single model compounds (guaiacol, vanillin, phenol, etc.), the mixture of multiple model compounds and bio-oil are presented. Ring saturation reactions are easy to occur in the HDO of model compounds or bio-oil, but dehydroxylation is relatively difficult. Finally, proposals for sustainable upgrades using renewable hydrogen (alcohols, acids, water, etc.) are presented, as well as ways for modifying the catalyst structure to improve the water resistance of catalysts in aqueous phase. The possibility of in situ characterization to explore the reaction mechanisms in depth is illustrated, and the importance of using the mixture of model compounds and bio-oil as primary research objects are emphasized.