Solvent-free selective oxidation of cyclohexane to KA oil in air over CoWO4@W18O49 catalyst

The challenge for researchers in heterogeneous catalysis is to develop robust catalysts while using benign re-action conditions, without producing unwanted by-products and preventing leaching of metal during the pro-cesses. We have synthesized nanosized CoWO4@W18O49 catalyst with oxygen vacancy sites for molecular oxygen activation from air, and this catalyst selectively oxidized cyclohexane to KA oil (KA oil referred to ketone-alcohol oil) in solvent-free conditions, where under optimum conditions,-24 % cyclohexane conversion with-93 % KA oil selectivity was achieved. The catalyst has been synthesized by the co-precipitation method and studied with the help of various analytical tools. DFT studies were also performed to calculate the energy needed for the oxygen vacancy generation over the CoWO4 (111), W18O49 (010) and CoWO4@W18O49 (010) surfaces. The energy required for the removal of oxygen atom at the CoWO4@W18O49 (010) surface was found to be -15.8 kJ/ mol, which is 96.2 kJ/mol and 46.9 kJ/mol lower than the CoWO4 (111) and W18O49 (010) surfaces respectively. The exceptional catalytic activity of CoWO4@W18O49 is an outcome of the combined contribution of the active CoWO4 and W18O49 interfaces, which have redox sites and a lot of vacancies of oxygen that benefit from the effective interaction.

Automated summary

The challenge for heterogeneous catalysis is to develop robust catalysts that can activate molecular oxygen from air and selectively oxidize cyclohexane without producing unwanted by-products or leaching metal. In this study, we synthesized a nanosized CoWO4@W18O49 catalyst with oxygen vacancy sites, which achieved-24% cyclohexane conversion with-93% KA oil selectivity under solvent-free conditions. DFT studies showed that the CoWO4@W18O49 surface required -15.8 kJ/mol of energy for oxygen vacancy generation, significantly lower than the CoWO4 (111) and W18O49 (010) surfaces. The exceptional catalytic activity of CoWO4@W18O49 is attributed to the active CoWO4 and W18O49 interfaces, which have redox sites and abundant vacancies of oxygen.