Microwave-assisted biodiesel production using -SO3H functionalized heterogeneous catalyst derived from a lignin-rich biomass

The synthesis of biodiesel from renewable resources has immense potential as a sustainable and cost-ffective energy alternative. In this work, a reusable SO3H functionalized heterogeneous catalyst that has a total acid density of 2.06 mmol/ g was prepared from walnut (Juglans regia) shell powder by low-temperature hydrothermal carbonization (WNS-SO3H). Walnut shell (WNS) contains more lignin (50.3%), which shows great resistance toward moisture. The prepared catalyst was employed for the effective conversion of oleic acid to methyl oleate by a microwave-assisted esterification reaction. The EDS analysis revealed the significant presence of sulfur (4.76 wt%), oxygen (51.24 wt%), and carbon (44 wt%) content. The results of the XPS analysis confirm the bonding of C-S, C-C, C=C, C-O, and C=O. Meanwhile, the presence of SO3H (the responsible factor for the esterification of oleic acid) was confirmed by FTIR analysis. Under the optimized conditions (9 wt% catalyst loading, 1:16 oleic acid to methanol molar ratio, 60 min reaction time, and 85 degrees C temperature), the conversion of oleic acid to biodiesel was found to be 99.01 +/- 0.3%. The obtained methyl oleate was characterized by employing C-13 and H-1 nuclear magnetic spectroscopy. The conversion yield and chemical composition of methyl oleate were confirmed by gas chromatography analysis. In conclusion, it can be a sustainable catalyst because the catalyst preparation controls the agro-waste, a great conversion is achieved due to the high lignin content, and the catalyst was reusable for five effective reaction cycles.