Selective C-C and C-O bond cleavage strategies for the thermochemical upgrading of (hemi)cellulosic biomass

Utilizing lignocellulose as an alternative green carbon resource is recognized as a vital approach to drive the ongoing transition towards a sustainable manufacture of chemicals, fuels, and materials with a low-carbon footprint. Unlike fossil resources, lignocellulose and its derivatives are featured by ubiquitous C-C and C-O bonds constituting carbon skeletons and various oxygenated moieties, respectively. Breaking these C-C/C-O bonds, in particular, the selective cleavage of specific C-C/C-O bonds present in biomass is of pivotal importance for the selective valorization of lignocellulose into targeted products. Hence, considerable attention has paid to searching for selective bond-breaking methods/strategies capable of tailoring different C-C/C-O bonds. Numerous conversion pathways based on selective C-C/C-O bond cleavage reactions have been developed to upgrade biomass into a broad spectrum of valuable products. In this review, we attempt to present a summary of existing selective C-C/C-O bond cleavage strategies frequently used for the upgrading of (hemi)cellulose, the dominant components of lignocellulose and also the major source of non-food carbohydrates. Each strategy is illustrated in terms of bond cleavage mechanism, utility in synthesizing various chemicals, representative catalyst systems, and coupling with other reactions to form cascade upgrading processes. Finally, pending challenges and potential opportunities related to selective bond cleavage strategies are discussed to propose future research directions. We hope that this review will help inspire further progress in designing new selective approaches for (hemi)cellulose valorization.