Co₃O₄/CeO₂ and Co₃O₄/ZrO₂ Composites as Heterogeneous Catalysts for Selective Aerobic Oxidation of Vanillyl Alcohol

There are significant benefits in achieving efficient, selective conversion of biomass-derived substrates into value-added chemicals. This study focuses on binary heterogeneous catalysts composed of Co₃O₄ supported on CeO₂ and ZrO₂, which were highly active for the selective oxidation of vanillyl alcohol to vanillin under base-free conditions. Co₃O₄/CeO₂ and Co₃O₄/ZrO₂ composites were synthesised using a facile coprecipitation method followed by calcination to activate the catalyst materials. Chemical and structural properties of these systems were characterized with X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and N₂ sorption measurements to establish the structure-activity relationship. Cyclic thermal gravimetric analysis experiments quantified the oxygen storage capacity of the catalysts, establishing oxygen buffering capacity and faster lattice oxygen exchange as key factors that govern catalytic turnover. The activity of Co₃O₄/CeO₂ for vanillyl alcohol oxidation was found to be superior compared to Co₃O₄/ZrO₂ catalyst under different temperatures and reaction times at atmospheric pressure. Enhanced activity of Co₃O₄/CeO₂ was explored using quenching studies, in situ DRIFTS and electron paramagnetic resonance experiments. These studies established the key role of superoxide (O₂ ^•-) in the oxidation reaction pathway, while hydroxyl radicals (HO^•) made a negligible contribution. A plausible catalytic pathway for the selective oxidation of vanillyl alcohol with Co₃O₄/CeO₂ catalyst was based on these results.