Ultrasound-assisted conversion of waste frying oil into biodiesel using Al-doped ZnO nanocatalyst: Box-Behnken design-based optimization

In this research, ZnO and ZnO@Al nanocatalysts were synthesized and utilized to fabricate biodiesel from waste frying oil (WFO). The structural features of these nanocatalysts were evaluated by FTIR, XRD, Raman, SEM, EDX-mapping, BET, and CO2-TPD analyses. According to the Box-Behnken design, the optimal conditions for biodiesel generation were attained at ultrasonic time of 29.35 min and 28.04 min, catalyst concentration of 2.57% and 2.35%, and methanol/oil ratio of 10.46 and 10.29 for ZnO and Al-doped ZnO, respectively. In optimal condi-tions, the utmost yield of biodiesel by ZnO and ZnO@Al nanocatalysts was 91.79% and 96.13%, respectively. Also, the stability and recyclability of nanocatalysts displayed that the Al-doped ZnO nanocatalyst has remarkable recyclability compared to ZnO, and after 7 cycles, the yield of biodiesel by Al-doped ZnO decreased by only 12%. According to the kinetic study, the quasi-first order kinetic was in good agreement with the lab-oratory data, and the kinetic rate constant enhanced with enhancing temperature. The enthalpy changes for biodiesel generation by ZnO and ZnO@Al were 46.98 and 52.01 kJ/mol, respectively, illustrating that the biodiesel generation reaction is endothermic. Furthermore, the influence of adding WFO-derived biodiesel to petrodiesel on the performance of a CI diesel engine was carefully checked.

Automated summary

ZnO and ZnO@Al nanocatalysts were synthesized to convert waste frying oil into biodiesel. The optimal conditions for biodiesel generation were determined using various analytical techniques, and the highest yields were achieved with ZnO@Al nanocatalyst. The recyclability of the nanocatalysts was also evaluated, and Al-doped ZnO showed remarkable recyclability. The kinetics of the biodiesel generation reaction were studied, and it was found to be an endothermic reaction. Additionally, the performance of a CI diesel engine with WFO-derived biodiesel blended with petrodiesel was investigated.