<p>The agricultural residue of corn cob was used as an inexpensive and abundant material for the manufacture of a green heterogeneous solid catalyst intended for the production of biodiesel from sunflower oil. The catalyst was synthesized through a straightforward calcination process at 500&#xa0;°C for 3&#xa0;h under ambient atmosphere. The resulting material was directly applied in the direct transesterification without further modification. The structural and physicochemical characteristics of the heterogeneous catalyst were investigated using XRD, SEM, EDS, BET, and FTIR analysis, while the alkalinity was evaluated by titration. The results revealed that the green heterogeneous catalyst consists of several active potassium phases, including KCl, K₂SO₄, K₆P₆O₁₈, K₂CO₃, and K₃AlO₃, which boost the basicity required to promote biodiesel synthesis. The catalyst reached its highest biodiesel yield (96.76%) under optimized transesterification conditions, namely 3 wt% catalyst dosage, a reaction temperature of 65&#xa0;°C, a methanol-to-oil ratio of 9:1, and a transesterification time of 120&#xa0;min. The catalyst was successfully reused for seven consecutive cycles while maintaining a biodiesel yield of approximately 78%. Biodiesel quality is closely compared to ASTM D6751 and EN 14,214 standards. Overall, work highlights the development of stable heterogeneous catalysts derived from inexpensive, environmentally friendly, and widely available materials, such as agricultural residue from corn cobs.</p> Graphical abstract <p></p>