Tuning the Active Oxygen Species of Two-Dimensional Borophene Oxide toward Advanced Metal-Free Catalysis

Two-dimensional (2D) borophene materials are predicted to be ideal catalytic materials due to their structural analogy to graphene. However, the lack of chemical functionalization of borophene hinders its practical application in catalysis. Herein, we reported a massive production of freestanding few-layer 2D borophene oxide (BO) sheets with tunable active oxygen species by a moderate oxidation-assisted exfoliation method. State-of-the-art characterizations demonstrated the evolution of active oxygen species from surface B-O species at the initial stage to the intermediate B_<i>x</i>O_<i>y</i> (1.5 < <i>x</i>/<i>y</i> < 3) species and eventually to bulk B₂O₃ with an increasing oxidation duration. As a result, the 2D BO sheet with enhanced B-O species exhibited a strikingly high catalytic activity for the aerobic oxidation of benzylamine into <i>N</i>-benzylidenebenzylamine. The formation rate of imine reaches as high as 29.7 mmol g_catal^-1 h^-1 under mild reaction conditions, higher than that of pristine borophene, boron oxides, graphene oxide, and other metal/metal-free catalysts in the reported literature. Density functional theory calculations further revealed the critical role of surface B-O species, which favor the adsorption and N-H activation of benzylamine for high activity and suppress the deep dehydrogenation, yielding an outstanding imine selectivity (>90%). This work paves the route for a moderate and scalable synthesis of few-layer BO sheets with highly active B-O species toward advanced metal-free catalysis beyond graphene.