Direct oxidation of cyclohexane to adipic acid in air over Co₃O₄@ZrO₂ nanostructured catalyst

Despite the vast industrial importance of adipic acid (AA), developing sustainable and selective catalysts for the oxidation of cyclohexane using air as an oxidant is a longstanding challenge for researchers. We have synthesized a Co₃O₄@ZrO₂ nanostructured catalyst and found that it converts cyclohexane to AA directly in air without adding any additional reagent. Under mild, environmentally friendly, solvent-free conditions and without any initiators, the catalyst showed a conversion of ∼40% with a selectivity of ∼43% towards adipic acid. NH₃-TPD analysis confirmed the presence of acidic sites on Co₃O₄@ZrO₂. According to the findings, loading Co₃O₄ in the ZrO₂ matrix generated a considerable increase in acidity (1.08 mmol/g), and the trend of different loading of cobalt followed the order: 5.4Co₃O₄@ZrO₂ > 3.2Co₃O₄@ZrO₂ > 2.5Co₃O₄@ZrO₂ > 1.6Co₃O₄@ZrO₂. The catalyst was thoroughly characterized by XRD, BET, HR-TEM, SEM, H₂-TPR, XPS, NH₃-TPD, Pyridine-IR, TGA, Raman, ICP-AES, UV–Visible and EPR spectroscopy. The recyclability and leaching test confirmed the true heterogeneous nature of the catalyst as there was no significant loss of catalytic activity even after 5 cycles.