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

Abstract

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.