From waste to clean Energy: Green synthesis of hybrid Nano-Catalysts for biodiesel production from waste cooking oils and animal fats.

A successful green biodiesel process was achieved through the full utilization of catalysts derived from waste. The catalysts in this study were composed of nano CaO (31.0 nm, calcined eggshell undergoing hydration-dehydration cycles) and MgAlOx mixed oxides (14.0 nm, derived from Mg-Al layered double hydroxide). Transesterification of waste cooking oil using only CaO resulted in 95.9 ± 0.8% biodiesel. Using a combination of CaO and MgAlOx in a mass ratio of 70:30 achieved an improved biodiesel yield of 97.8 ± 0.5% under optimal conditions (molar ratio of methanol to oil at 12:1, catalyst mass fraction of 3 wt%, temperature of 60 °C, duration of 45 min, and ultrasonic support). The recycled CaO/MgAlOx hybrid catalyst exhibits greater stability compared to pure CaO, achieving 91.4 ± 1.2% of its initial activity after five reuse cycles for the mixed catalyst, whereas pure CaO attains only 87.9 ± 1.5%. Feedstocks with elevated Free Fatty Acid (FFA) content can be transformed into biodiesel using the hybrid catalyst, yielding 96.9 ± 0.7% from waste cow fat. All produced biodiesels meet American Society for Testing and Materials (ASTM) D6751 and European Committee for Standardization (EN) 14,214 standards. All biodiesel-diesel blends with varying biodiesel ratios- B5 (5%), B10 (10%), and B20 (20%) meet ASTM D7467 standards; therefore, the biodiesel can be used in existing diesel engines. This demonstrates a circular economy approach where various waste products can be recycled into high-value biodiesel using sustainable catalysts under green conditions.