<p>Low-carbon fuels are crucial for mitigating climate change, with biodiesel from low-grade fats offering a sustainable alternative to fossil diesel. Free fatty acids (FFAs) in these fats are typically esterified using strong acids like sulfuric acid, but biomass-based heterogeneous catalysts, such as sulfonated hydrochars, provide a greener option. Recent research synthesized hydrochars via hydrothermal carbonization of corn starch under mild conditions, incorporating iron to study its effects on carbonization and magnetism. Raman spectroscopy revealed enhanced graphitization in Fe³⁺-modified hydrochars, while XRD confirmed Fe³⁺ reduction to Fe²⁺. As catalysts, hydrochars prepared with sulfuric acid and Fe salt achieved a 60% methyl oleate yield, compared to 51% for non-Fe³⁺ counterparts, and 70% using homogeneous sulfuric acid. Higher H₂SO₄ levels during synthesis improved catalytic activity due to increased sulfonation. These catalysts exhibited minimal water adsorption, maintaining stable bands at 3400&#xa0;cm⁻¹ post-reaction. Moreover, oleic acid conversion was rapid, suggesting fast saturation of active sites by intermediates. This study highlights sulfonated hydrochars as promising catalysts for biodiesel production, offering a sustainable alternative to conventional acid catalysts while maintaining competitive efficiency.</p> Graphical Abstract <p></p>