Biodiesel Production Using a Biomass-Based Solid Acid Catalyst Synthesized from Agricultural Residue Garlic Peel

Garlic peel (GP), as a readily available agricultural waste, has been successfully employed to synthesize a carbon-based solid acid catalyst, which, in turn, has been used to catalyze the esterification of oleic acid with methanol for biodiesel production. Several GP-based catalysts have been synthesized by partial carbonization at various temperatures (300, 350, 400, 450, and 500 °C) followed by sulfonation at 105 °C for 120 min, and characterized by means of various techniques. The catalyst carbonized at 350 °C showed the highest density of sulfonic acid moieties of 1.24 mmol/g, corresponding to the maximum oleic acid conversion of 96.3%. This was achieved with optimum operating parameters of catalyst dosage 8 wt% (with respect to the mass of oleic acid), a molar ratio of methanol to oleic acid of 10, and reaction for 210 min at 60 °C. Importantly, the GP-based catalyst showed satisfactory recyclability, retaining 73.2% of its initial catalytic activity after a fifth cycle. Graphical Abstract