Differentiating the {100} surfaces of Cu2O nanocrystals from {111} and {110} for benzylic Csp3-H bond oxidation: Oxidations of diphenyl methane to benzophenone and cumene to cumene hydroperoxide under mild conditions

Through this study, we have explored the basic questions like why do some facets show more photocatalytic activity towards some organic substrates resulting in photodegradation and some remain inactive at all? Do the organic reaction mechanism pathways (ionic or free radical) decide the catalyst's facets selectivity? To answer these questions, Cu2O nanocrystals (NCs) with different morphologies were synthesized by wet chemical methods. By powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM), it was confirmed that the synthesized material was Cu2O NCs with octahedral (o-Cu2O), dodecahedral (d-Cu2O) and cubic (c-Cu2O) morphologies with {111}, {110}, and {100} facets exposed, respectively. To confirm the phase purity and eliminate any probability of the presence of Cu or CuO as impurities in the material, fitting of the experimental XRD data was carried out by employing the Rietveld refinement. The synthesized Cu2O NCs with the above morphologies were employed as the catalysts for benzylic Csp3-H bond oxidation. For this purpose, a model reaction of oxidation of diphenylmethane (DPM) by using t-butyl hydroperoxide (TBHP) as an oxidant at RT was carried out in an acetonitrile medium. Out of all the morphologies evaluated, c-Cu2O NCs show the highest activity. They could complete the model oxidation reaction within 3 days with ∼99% yield and 100% selectivity for benzophenone as a sole product without any catalyst obliteration. Based on chemical kinetics experiments, electrochemical analyses, and DFT calculations, a reaction mechanism (similar to the Mars-van Krevelen reaction cycle for semiconductor metal oxide surfaces) involving (100) surfaces of c-Cu2O NCs was proposed. Furthermore, BET surface area analyses and ζ potential measurements were carried out to confirm the catalytic effect due to {100}, {111}, and {110} facets exposed. To demonstrate the efficiency of the developed oxidation protocol, industrially important oxidation of cumene to cumene hydroperoxide (CHP) was carried out under solvent-less conditions with 95% yield and 70% selectivity for CHP.