Synthesis of vanillin via oxidation of kenaf stalks in the presence of CeO2: tuning the catalytic behaviour of CeO2 via nanostructure morphology.

Abstract

Different CeO2 nanostructures were synthesized using a hydrothermal method and treated with alkaline NaOH, followed by drying at 120 °C for 16 h and calcined at 400 °C for the direct oxidation of kenaf stalks to vanillin under microwave irradiation. The catalysts were characterized for their physicochemical properties using XRD, BET, Raman spectroscopy, TPR, TPO, and XPS. All synthesized CeO2 nanostructures show diffraction peaks corresponding to the formation of cubic fluorite, which agrees with Raman spectra of the F2g mode. The N2 adsorption-desorption isotherms showed that all catalysts possess a type IV isotherm, indicating a mesoporous structure. TPR and TPO analyses display formation peaks corresponding to surface-to-bulk reducibility and the oxidized oxygen ratio, which is responsible for the redox properties of ceria nanostructures. The XPS analysis of CeO2 nanostructures proved that Ce exists in the Ce3+ and Ce4+ oxidation states. All catalysts were tested for direct oxidation of kenaf stalks under microwave irradiation with the highest vanillin yield obtained by the CeO2-Nps-400 heterogeneous catalyst at 3.84%, whereas 4.66% vanillin was produced using 2 N NaOH as a homogeneous catalyst.