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.
Highlights
Vanillin, known as 4-hydroxy-3-methoxybenzaldehyde (Figure 1), is vanilla’s primary flavor constituent used as a flavoring and fragrance ingredient in the food and cosmetic industries [1]
The commercial vanillin market is served by three major sources, namely, natural vanilla from vanilla pods, vanillin produced from petrochemical sources and vanillin produced from kraft lignin [2]
We report the vanillin production from direct oxidation of kenaf stalks and kenaf lignin using 30Ce/MgO under microwave irradiation
Summary
Known as 4-hydroxy-3-methoxybenzaldehyde (Figure 1), is vanilla’s primary flavor constituent used as a flavoring and fragrance ingredient in the food and cosmetic industries [1]. The commercial vanillin market is served by three major sources, namely, natural vanilla from vanilla pods, vanillin produced from petrochemical sources and vanillin produced from kraft lignin [2]. The production of natural vanillin from vanilla pods covers only 0.2% of the market requirement and its production cost is very high as it requires manual labor to pollinate the flower [1,4]. Vanillin produced from petroleum-based intermediates, especially guaiacol, accounts for 85% of the world supply, while the remaining 15% is produced from lignin [5]. Kenaf (Hibiscus cannabinus) is a commodity crop with a high potential to produce renewable vanillin since it contains 13–15% lignin [16]. We report the vanillin production from direct oxidation of kenaf stalks and kenaf lignin using 30Ce/MgO under microwave irradiation
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