Surface sulfonation of biosilica derived from Navicula salinicola and Cyclotella striata as highly active solid acid catalysts for ethyl levulinate production

The marine environment contains abundant diatoms that produce biosilica. Porous architecture and high surface area make biosilica a promising support material for heterogeneous catalysts. Converting biosilica into catalysts increases its value while contributing to sustainable chemical production. Ethyl levulinate (EL), a bio-based diesel additive, is typically produced by esterifying levulinic acid (LA) with ethanol in the presence of mineral acid catalysts, such as sulfuric acid. However, the use of mineral acids causes equipment corrosion and presents challenges, including difficulties in recovery and reuse, making a shift to solid acid catalysts highly desirable. This research investigated sulfonated biosilica derived from two marine diatoms, Navicula salinicola (SiO 2 (NS)) and Cyclotella striata (SiO 2 (CS)), as solid acid catalysts for EL production. The biosilica particles were functionalized with 3-mercaptopropyltriethoxysilane to introduce thiol groups, followed by oxidation with nitric acid to produce sulfonic acid groups on the biosilica. The resulting sulfonated biosilica (SO 3 H@SiO 2 ) exhibited excellent catalytic performance for the esterification of LA to EL. Notably, SO 3 H@SiO 2 (CS) achieved 100% LA conversion and 91% EL yield, while SO 3 H@SiO 2 (NS) reached 95% conversion and 82% yield. Both biosilica-derived solid acid catalysts demonstrated promising reusability, with SO 3 H@SiO 2 (CS) retaining 90% of its initial activity after three consecutive cycles. The catalytic performance of both SO 3 H@SiO 2 catalysts was superior to that of sulfonated commercial fumed silica, which gave 81% conversion and 75% yield. These findings highlight the potential of diatom-derived biosilica as a catalyst support, positioning it as a green and cost-effective alternative to conventional synthetic silica supports.