Biodiesel Production From Low-Grade Fats Using Sulfonated Hydrochar Catalysts

A more sustainable route to biodiesel involves converting free fatty acid (FFA)-rich feedstocks through Fischer esterification using acid catalysts. Replacing traditional homogeneous catalysts, typically strong inorganic acids like H₂SO₄, with heterogeneous acid catalysts enhances both environmental and economic sustainability by enabling catalyst reuse, reducing corrosion, and minimizing wastewater generation. In this study, the methanolysis of oleic acid, used to model low-value fat, was catalyzed by hydrochar-based heterogeneous acid catalysts derived from banana peels, orange peels, and sucrose. These feedstocks were hydrothermally carbonized at 150 °C for 5 h in the presence of H₂SO₄. The resulting hydrochars, characterized via scanning electron microscopy and Raman spectroscopy, exhibited varying degrees of graphitization. The sucrose-derived hydrochar delivered the best catalytic performance, achieving a 90.1% yield of methyl oleate (MO) after 180 min of reaction. Orange peel- and banana peel-derived hydrochars achieved maximum MO yields of 87.8% and 66.5%, respectively. In contrast, a conventional homogeneous H₂SO₄ catalyst yielded only 70.0% MO under the same reaction conditions. Higher graphitization correlated with increased hydrochar hydrophobicity, which appears to enhance catalytic performance by minimizing water adsorption and subsequent catalyst deactivation. Additionally, the sucrose-derived catalyst demonstrated the lowest sulfur leaching, indicating superior stability of sulfonated species.