One possible method of producing vanillin from biomass is through controlled oxidation of lignin. Direct oxidation of kenaf stalks was chosen without having to separate the cellulose and hemicellulose components from the lignocellulosic biomass. This makes the process greener, as well as saving time. In this paper, Ce/MgO catalysts were developed for oxidation of kenaf stalks and kenaf lignin under microwave irradiation. The catalysts were characterized for their physicochemical properties using XRD and N₂ adsorption–desorption isotherms. The synthesized MgO showed the presence of diffraction peaks assigned to cubic MgO while the 30Ce/MgO catalysts showed the presence of cubic fluorite of CeO₂. N₂ adsorption–desorption isotherms showed that all catalysts possess Type III isotherm according to IUPAC classification, indicating a nonporous structure. All catalysts were tested for direct oxidation of kenaf stalks under 300 W of microwave irradiation using H₂O₂ as the oxidizing agent at pH 11.5 and temperatures between 160 and 180 °C for 10–30 min with 5–15% catalyst loading. The highest vanillin yields of 3.70% and 2.90% for extracted lignin and direct biomass oxidation were achieved using 30Ce/MgO-48. In comparison, 7.80% and 4.45% were obtained using 2N of NaOH homogeneous catalyst for extracted lignin and direct biomass, respectively, at 170 °C for 20 min. The reusability test shows that 30Ce/MgO can be used up to three cycles without significant loss in catalytic activity. Other compounds detected were 4-vinylguaiacol, syringol and syringaldehyde.

Biomass lignin can be used to produce vanillin through an oxidation process. Although its purity is high, the processing time and separation efficiency are not ideal. This research aims to produce vanillin directly from Kenaf stalks without separating the lignin first from the lignocellulosic biomass. This method is greener because it does not require the separation of cellulose and hemicellulose from the biomass, thus minimizing the use of acid and alkaline solutions and saving time. A high oxygen storage capacity and release capacity of ceria as an oxidation catalyst contribute to the reversable redox properties between Ce⁴⁺ and Ce³⁺ in ceria lattice. Cerium oxide nanostructures were synthesized using a hydrothermal method treated under alkaline NaOH, followed by drying at 120 °C for 16 h and calcining at different temperatures between 400 and 600 °C for the direct oxidation of Kenaf stalks to vanillin under microwave irradiation. The catalysts were characterized for their physicochemical properties using XRD, N₂ adsorption–desorption isotherms and TEM. All synthesized CeO₂ nanostructures showed the presence of diffraction peaks assigned to the presence of cubic fluorite. The N₂ adsorption–desorption isotherms showed that all catalysts possess a Type IV isotherm, indicating a mesoporous structure. The TEM image shows the uniform shape of the CeO₂ nanostructures, while HRTEM images show that the CeO₂ nanostructures are single-crystalline in nature. All catalysts were tested for the direct oxidation of Kenaf stalks using H₂O₂ as the oxidizing agent in temperatures ranging from 160 to 180 °C for 10–30 min with 0.1–0.3 g catalyst loading under 100–500 W of microwave irradiation. The CeO₂-Nps-400 catalyst produced the highest vanillin yields of 3.84% and 4.32% for the direct oxidation of Kenaf stalks and extraction of lignin from Kenaf stalks, respectively. Compared to our earlier study, the highest vanillin yields of 2.90% and 3.70% for direct biomass and extracted lignin were achieved using a Ce/MgO catalyst.

Ceria’s exceptional reversible redox properties between Ce4+ and Ce3+ contributes to its high oxygen storage capacity and release capacity has received attention as an oxidation catalyst for a variety of process. However, ceria’s ability in catalytic performance can differ, corresponding to the selective exposition of different Ce facets in different nanostructures by controlling the shape of ceria crystal. The different CeO2 structures with the reactive Ce(100), Ce(110) and Ce(111) facets were synthesized using hydrothermal method and were characterized using XRD, BET, RAMAN, TPR, TPO and XPS. All synthesized CeO2 nanostructure shows diffraction peaks corresponding to the formation of cubic fluorite. N2 adsorption-desorption isotherms showed that all catalysts possess Type IV isotherm, indicating a mesoporous structure. The Raman spectra indicate a peak that dominated the characteristic of the F2g mode of cubic fluorite structure. The TPR and TPO analysis display formation peak corresponding to the surface-to-bulk ratio of reducibility and oxidized oxygen, which is responsible for the redox properties of ceria nanostructures. The XPS analysis of CeO2 nanoparticles proved that Ce exists in Ce3+ and Ce4+ oxidation states. All catalysts were tested for direct oxidation of kenaf stalks under 300 W of microwave irradiation using H2O2 as the oxidizing agent at pH 11.5 and temperatures of 170 °C for 20 min with 15% catalyst loading. The highest vanillin yield obtained by CeO2-Nps-400 heterogeneous catalyst was 3.93%, whereas 4.82% vanillin was produced using 2N NaOH as a homogeneous catalyst. The results clearly revealed the crucial role of catalyst morphology on the physicochemical properties and the catalytic behavior of the CeO2 nanostructures, with the particle-shaped catalyst exhibiting the highest catalytic performance.