This study proposes an environmentally friendly and low-cost catalyst for producing biodiesel. This catalyst comprises 5% potassium hydroxide (KOH), 35% modified Iranian clinoptilolite (MIC), and 60% synthesized biochar from rapeseed meal. The synthesis was confirmed by SEM, XRD, and FTIR analyses. Different operating parameters such as acetone as co-solvent (0–20 wt%), catalyst concentration (4–10 wt%), reaction time (60–120 min), and oil-to-methanol volume ratio (1–3 v/v) on biodiesel purity were tested. There was 99% fatty acid methyl ester (FAME) content in the biodiesel produced at the co-solvent concentration of 12.4 wt%, the catalyst concentration of 8.5 wt%, the reaction time of 113 min, and the oil-to-methanol volume ratio of 2.6 v/v.
