Visible-light driven efficient cleavage of Cα-Cβ bond in lignin models and lignin over porous tubular carbon nitride

The photocatalytic selective breaking of recalcitrant C alpha-C beta bond in real lignin to product value-added chemicals under mild conditions is intriguing but remains challenging. Herein, a sustainable strategy for the efficient and selective breakage of C alpha-C beta bond in lignin beta-O-4 model compounds and lignin over porous tubular g-C3N4 (PTCN) photocatalyst was designed under ambient conditions. Compared with bulk g-C3N4, the construction of PTCN not only facilitates photogenerated charge separation and transfer, but also provides more exposed interior active sites, which is conductive to reactant adsorption on the surface of PTCN, thereby resulting in the higher photocatalytic efficiency. With this strategy, 2-phenoxy-1-phenylethanol as the typical model substrate was completely converted into aromatic monomers with C alpha-C beta bond cleavage selectivity of 97.2 %. Both mechanistic studies and theoretical calculations revealed that the photogenerated holes, electrons and center dot O2- respectively played an essential role in the C beta-H dehydrogenation and C alpha-C beta bond breakage. Native lignin samples (alkaline, pine and wheat straw) were applied to these photocatalytic systems and successfully depolymerized with C alpha-C beta bond cleavage under optimal reaction conditions. This work could provide an in-depth insight into photocatalyst design for the economical and efficient valorization of lignin.