Modular calcination strategy to construct defect-rich nitrogen-doped Nb2O5 for boosting photocatalytic oxidation of cyclohexane to cyclohexanone in solvent-free conditions

Abstract

The partial oxidation of cyclohexane (CHA) with oxygen to cyclohexanone (CHA-one) is one of the most transformation in the chemical industry in view that CHA-one is an irreplaceable raw material in the production of nylon. Herein, hydrogen, vacuum and air modules combined with ammonia module, respectively, were used to construct defect-rich nitrogen-doped Nb2O5. And, the calcination order and temperature were also considered to establish a modular calcination strategy. A series of modified N-doped Nb2O5 were prepared by the proposed strategy, which were used to catalyze the photo-oxidation of CHA in solvent-free and room temperature conditions. Among these modified N-doped Nb2O5 samples, the HN-Nb2O5 possessed abundant chemical defects of oxygen vacancies and Nb4+, and achieved the best photocatalytic performance. Surprisingly, HN-Nb2O5 achieved an amount of 130.35 μmol for CHA-one, which was approximately 2.2 and 26 times that of the pure Nb2O5 and commercial Nb2O5, respectively, with up to 96.68% selectivity to CHA-one. The outstanding photocatalytic performance of the HN-Nb2O5 can be ascribed to the presence of chemical defect-rich and the NOx species, enhanced light absorption and effective electron-hole separation. DFT calculations explained the promotion of the chemical defects and N doping on Nb2O5 photocatalytic activity in terms of the band electron and band-gap level. The results of EPR analysis and scavenger tests found that e− is the main active species resulted in photocatalytic activity, while O2− is the dominant active species for producing CHA-one in the photo-oxidation of CHA. The present work developed an efficient strategy to construct the defect-rich N-doped metal oxide for the improved photocatalytic oxidation of hydrocarbon.