Selective Aerobic Oxidation of Vanillyl Alcohol to Vanillin Catalysed by Nanostructured Ce-Zr-O Solid Solutions

Abstract

A series of Ce1 − x–ZrxO2 − δ (x = 0.2, 0.5, and 0.8) mixed oxides were prepared by coprecipitation method and explored for selective oxidation of vanillyl alcohol employing O2 and acetonitrile as the oxidant and solvent, respectively under base-free conditions. To ascertain the key factors responsible for vanillyl alcohol oxidation, the physicochemical properties of the synthesized catalysts were investigated by various characterization techniques namely, XRD, BET surface area, Raman, XPS, and H2-TPR. It was observed from this exercise that the catalytic activity dependents on the Ce:Zr mole ratio, which is related to the degree of reducibility of the catalyst. Interestingly, the catalytic activity is enhanced with the increase of Ce content in the Ce–Zr mixed oxide. Among the investigated catalysts, the Ce0.8Zr0.2O2 combination exhibited a high catalytic activity with ~ 98% conversion and ~ 99% selectivity to vanillin. Smaller crystallite size, large BET surface area, more number of oxygen vacancies, improved redox properties, and strong synergetic interaction are found to be the key factors to promote the oxidation ability of Ce0.8Zr0.2O2 catalysts towards vanillyl alcohol oxidation. Further, the influence of reaction parameters such as time, solvent, temperature, and oxygen pressure were also studied to optimize the catalytic process for vanillyl alcohol oxidation. As revealed by these studies, the high activity of Ce0.8Zr0.2O2 catalyst could be retained up to five cycles without appreciable loss in the activity and selectivity. Nanosized Ce0.8Zr0.2O2 catalyst exhibited an excellent catalytic activity and superior selectivity to vanillin in the liquid phase oxidation of vanillyl alcohol under ecofriendly conditions