ZnO/CuO composite catalyst to pre-esterify waste coconut oil for producing biodiesel in high yield

The study reported herein describes a two-stepped catalytic approach to produce biodiesel from waste-coconut oil in high (> 90%) yield. In this regard, pre-esterification of the Free Fatty Acid (FFA) content (9.58 mg KOH g−1) of waste coconut oil in the presence of a simple ZnO/CuO composite, as a heterogeneous acid-catalyst to prevent competitive saponification and hydrolysis side reactions caused by FFA, followed by the base-catalyzed transesterification of the triglyceride of oil was performed. The ZnO/CuO catalyst was synthesized using a simultaneous precipitation method, and characterized by spectroscopic (FTIR, UV–Vis), SEM, XRD and XRF techniques. The surface acidity of the catalyst and the FFA value (AV) of the oil before and after the pre-esterification was determined using the Hammett indicator method. The pre-esterification was performed at different temperatures (5–125 °C), time intervals (15–235 min), and using different weight percentages (wt%) of catalyst loading (0.005–2.665) and methanol-to-oil ratios. The optimum reaction conditions were identified using a central composite rotatable design (CCRD). The results of the study revealed that a small amount of the catalyst (1.66 wt%) is enough, and the catalyst could be easily recovered and reused 3–4 catalytic runs for reducing the AV of waste coconut oil by 94.53% under milder conditions (within 113 min, at 55 °C in the presence of 10.5:1 methanol-to-oil ratio) than those conditions reported so far. The biodiesel obtained this way was free from soap, and consistent with ASTM-D6751 and EN-14214 standards.