Atomically Dispersed WN3O Sites on Defective Graphitic Carbon Nitride Enabling Efficient and Selective Photocatalytic Depolymerization of Lignin Models and Larch Lignin.

Lignin valorization still is challenging of efficient and selective production of highly-functionalized aromatic compounds. Here atomically dispersed WN3O sites on defective g-C3N4 (W1/CV-OCN) are fabricated by an in situ route, in which WN3O sites are stabilized on CV-OCN through reinforcing W─N (O) coordination interactions. The W1/CV-OCN exhibit excellent photocatalytic oxidation activity and selectivity in the depolymerization of lignin models and larch lignin. Under the conditions of 25°C, O2 atmosphere and simulated sunlight irradiation, the optimal 0.96%W1/CV-OCN catalyst shows nearly complete conversion of a wide range of β-O-4 and β-1 lignin models, alongside more than 90% of Cα─Cβ bond cleavage selectivity; particularly, the depolymerization of larch lignin is achieved over 0.96%W1/CV-OCN, affording 8.4% of benzaldehyde and 24.7% of guaiacol as the main products after 4 h light irradiation. Reaction mechanism studies reveal that single WN3O sites not only facilitate lignin activation through photoinduced O-to-W charge transfer to produce alkoxy radical intermediates but also accelerate charge separation and transfer dynamics through directional electron transfer along W─N (O) bonds and dynamic transformation of W6+/W5+ states to ensure the formation of plentiful reactive oxygen species (ROSs). Precise β-scission (Cα─Cβ and/or Cβ─O bonds breakage) is achieved by synergy of alkoxy radical intermediates and ROSs.