Tuning the Active\nOxygen Species of Two-Dimensional\nBorophene Oxide toward Advanced Metal-Free Catalysis

Two-dimensional\n(2D) borophene materials are predicted\nto be ideal\ncatalytic materials due to their structural analogy to graphene. However,\nthe lack of chemical functionalization of borophene hinders its practical\napplication in catalysis. Herein, we reported a massive production\nof freestanding few-layer 2D borophene oxide (BO) sheets with tunable\nactive oxygen species by a moderate oxidation-assisted exfoliation\nmethod. State-of-the-art characterizations demonstrated the evolution\nof active oxygen species from surface B–O species at the initial\nstage to the intermediate B<sub><i>x</i></sub>O<sub><i>y</i></sub> (1.5 < <i>x</i>/<i>y</i> <\n3) species and eventually to bulk B<sub>2</sub>O<sub>3</sub> with\nan increasing oxidation duration. As a result, the 2D BO sheet with\nenhanced B–O species exhibited a strikingly high catalytic\nactivity for the aerobic oxidation of benzylamine into <i>N</i>-benzylidenebenzylamine. The formation rate of imine reaches as high\nas 29.7 mmol g<sub>catal</sub><sup>–1</sup> h<sup>–1</sup> under mild reaction conditions, higher than that of pristine borophene,\nboron oxides, graphene oxide, and other metal/metal-free catalysts\nin the reported literature. Density functional theory calculations\nfurther revealed the critical role of surface B–O species,\nwhich favor the adsorption and N–H activation of benzylamine\nfor high activity and suppress the deep dehydrogenation, yielding\nan outstanding imine selectivity (>90%). This work paves the route\nfor a moderate and scalable synthesis of few-layer BO sheets with\nhighly active B–O species toward advanced metal-free catalysis\nbeyond graphene.