Biodiesel (FAME) synthesis over bifunctional heterogeneous acid catalysts (Zr-S/SiO2), synthesized by Aerosol Assisted Sol-gel method

Fossil fuels account for 61% of global energy but contribute significantly to environmental pollution and climate change. Renewable energy sources, especially biodiesel derived from plant oils and animal fats, offer a promising alternative with lower greenhouse gas emissions and compatibility with existing diesel engines. Biodiesel is produced globally through the transesterification of vegetable oils with methanol. Basic homogeneous catalysts are commonly used but are non-recoverable and react with free fatty acids (FFA), forming soap and consuming the catalyst. Acid catalysts address this issue by catalysing the esterification of FFAs to biodiesel and water. On another hand, heterogeneous catalysts offer reusability, and are thus more ecologically interesting. Furthermore, bifunctional catalysts with both Brønsted and Lewis acid sites are ideal for biodiesel production, with Lewis acids catalyzing the transesterification and Brønsted acids catalyzing the esterification reaction. This master thesis aims to synthesize a bifunctional acid heterogeneous catalyst with zirconia and sulfur species on silica using the Aerosol Assisted Sol-Gel (AASG) method. During the thesis, various Zr/SiO2 catalysts doped with sulfur were prepared successfully. Zirconium was incorporated using the AASG technique, resulting in mesoporous catalysts with high specific surface areas. Sulfur was introduced either by impregnating ammonium persulfate over calcined Zr-SiO2 catalysts or by oxidizing thiols to sulfonates via H2O2 treatment. XPS confirmed the successful oxidation of thiols, while NH3-TPD and TGA-MS analyses provided insights into the catalysts' acidity and composition. However, the catalysts showed limited activity in transesterification and some exhibited leaching, rendering them unsuitable for simultaneous esterification and transesterification reactions. Future research should optimize biodiesel synthesis by testing catalytic activity with and without zeolite 3A to assess the impact of water absorption on yield. The transesterification would be catalyzed by sodium aluminate as a basic heterogeneous catalyst, followed by esterification with sulfuric acid as a homogeneous acid catalyst. Additionally, DR-UV diffuse reflectance spectroscopy should be used to study zirconium's chemical environment in the catalyst, providing insights into its stability and activity.