Synthesis and Comparative Catalytic Study of Zirconia–MnCO₃ or –Mn₂O₃ for the Oxidation of Benzylic Alcohols

We report on the synthesis of the zirconia–manganese carbonate ZrO_x(x %)–MnCO₃ catalyst (where x=1–7) that, upon calcination at 500 °C, is converted to zirconia–manganese oxide ZrO_x(x %)–Mn₂O₃. We also present a comparative study of the catalytic performance of the both catalysts for the oxidation of benzylic alcohol to corresponding aldehydes by using molecular oxygen as the oxidizing agent. ZrO_x(x %)–MnCO₃ was prepared through co-precipitation by varying the amounts of Zr(NO₃)₄ (w/w %) in Mn(NO₃)₂. The morphology, composition, and crystallinity of the as-synthesized product and the catalysts prepared upon calcination were studied by using scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and powder X-ray diffraction. The surface areas of the catalysts [133.58 m² g⁻¹ for ZrO_x(1 %)–MnCO₃ and 17.48 m² g⁻¹ for ZrO_x(1 %)–Mn₂O₃] were determined by using the Brunauer–Emmett–Teller method, and the thermal stability was assessed by using thermal gravimetric analysis. The catalyst with composition ZrO_x(1 %)–MnCO₃ pre-calcined at 300 °C exhibited excellent specific activity (48.00 mmolg⁻¹ h⁻¹) with complete conversion within approximately 5 min and catalyst cyclability up to six times without any significant loss in activity. The specific activity, turnover number and turnover frequency achieved is the highest so far (to the best of our knowledge) compared to the previously reported catalysts used for the oxidation of benzyl alcohol. The catalyst showed selectivity for aromatic alcohols over aliphatic alcohols.