Synthesis of Ce/MgO Catalysts for Direct Oxidation of Hibiscus cannabinus Stalks to Vanillin

Abstract

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.

Ce–Zr-based mixed oxide catalyst for oxidative depolymerization of kenaf stalk (biomass) into vanillin

“…The lignin was extracted from kenaf fiber using the Soxhlet extraction method (Anuar et al 2021 ; Orsi et al 2022 ). The Soxhlet extraction process was used to grind and get the extract out of the kenaf stalks.…”

“…The higher phase was put into a vial. A rotary evaporator extracted extra solvent at 50 °C and 400 mbar (Anuar et al 2021 ; Qu et al 2017 ) . Figure 1 illustrates an experimental design schematic diagram.…”

“…Using microwave radiation and a ferric sulfate catalyst, Zhu et al ( 2017 ) selectively depolymerized lignin into phenolic monomers, with phenolic dimers cleaving at a faster rate than non-phenolic dimers. Similarly, Anuar et al ( 2021 ) used microwave heating to produce aromatic monomer (vanillin) and concluded that microwave heating is quick, extremely effective, homogeneous, selective, and environmentally friendly.…”

“…Due to the high oxygen storage/release capacities, CeO 2 has been widely used as a catalyst or catalytic support for various redox processes (Ahniyaz et al 2004 ). According to Anuar et al ( 2021 ), the high base characteristics of Ce/MgO catalysts improved the cleavage of β-o -4 in the lignin structure, enhancing the reaction's selectivity.…”

Effect of CeO2 Nanostructures Morphology on Facet(s) Reactivity for Direct Oxidation of Kenaf Stalks to Vanillin

“…Our initial work on employing Ce and Zr catalysts supported by MgO to directly oxidize Kenaf stalks to produce vanillin indicates that CeO2 is active for vanillin production. The oxidation of lignin from the kenaf stalk using Ce/MgO catalyst under microwave gives a 3.70 yield of vanillin, while bare MgO and Zr/MgO do not give any vanillin product [11]. These facts provoked our interest to take up the present investigation on ceria.…”