Tailoring the activity of sulfonated coal catalyst during the sonication and infrared induced transesterification of castor (Ricinus communis) oil: Synergetic, optimization, kinetics, and CI engine assessment

The extraction of biodiesel using low-cost catalysts has been a critical issue in recent decades to match the world demand for energy supplies and environmental considerations. Sulfonated subbituminous coal was synthesized as acidic catalyst during the transesterification of castor oil into biofuel using three mixing and activation techniques (normal stirring, ultrasound irradiation, and infrared/ultrasound irradiation). The studies were accomplished based on Rotatable Central Composite Design (RCCD) to optimize the reactions. The determined biodiesel yield demonstrated strong enhancement by using ultrasonic and infrared irradiation, achieving the best results with the cooperation of both at the same time. Experimentally, the infrared/sonication system resulted in a 97.6% biodiesel yield after 15 min in the presence of 1 wt% of the catalyst and a 6:1 ethanol/oil molar ratio at 30 °C. Theoretically, this yield can be enhanced up to 99.8% by increasing the duration up to 20 min and reducing the temperature, catalyst load, and ethanol/oil ratio to 25 °C, 0.96 wt%, and 5.85:1, respectively. The reactions display pseudo-first-order kinetics with reaction rate constant, activation energy, and pre-exponential values of 0.123–0.155 min−1, 35.9 kJ.mol−1, and 104.9 × 104 min−1, respectively. The endothermic and nonspontaneous properties were detected from the thermodynamic functions (enthalpy (14.39 kJ/mol) and Gibb’s free energy (4.5–4.67 kJ. mol−1)). The CI engine investigation of biodiesel in blends with pterodiesel validated remarkable enhancements in the performance of the engine and reductions in toxic emissions. Therefore, the transesterification of castor oil using sulfonate coal based on infrared and sonication-induced systems is strongly recommended.