Recycling spent black tea as a novel heterogeneous acidic catalyst for efficient biodiesel production from waste cooking oil: Optimization, kinetics, and thermodynamic insights

Abstract

This study explores the potential of spent black tea (S.BT) as a heterogeneous acidic catalyst for biodiesel production from waste cooking oil (WCO) via transesterification. S.BT, a waste material, was sulfonated without carbonization, resulting in a catalyst with enhanced catalytic properties. The catalyst was characterized for key physicochemical properties, including an acid density of 34.6 mmol/g, surface area of 58.3 m²/g, and sulfur content of 3.14 wt%. Under optimal reaction conditions—50 °C, 70 min, 3 wt% S.BT catalyst loading, and a methanol/oil ratio of 10:1—transesterification of WCO with methanol yielded biodiesel with a maximum conversion of 97.3 %. A statistical optimization approach using response surface methodology (RSM) demonstrated that biodiesel yield can be increased to over 99 % by adjusting factors such as temperature, catalyst loading, and methanol/oil ratio. Kinetic analysis revealed that the transesterification followed pseudo-first-order kinetics with an activation energy (E_a) of 34.3 kJ/mol, indicating the reaction proceeds efficiently at moderate temperatures. The thermodynamic analysis, using the Eyring-Polanyi equation, revealed a positive activation enthalpy (ΔH* = 31.76 kJ/mol), indicating the endothermic nature of the process, and a positive entropy (ΔS* = 18.36 J/K·mol), reflecting increased disorder at the transition state. Gibbs free energy (ΔG*) values ranged from –5.35 kJ/mol at 30 °C to –5.71 kJ/mol at 50 °C, confirming the thermodynamic feasibility of the reaction at elevated temperatures. The biodiesel produced met international fuel standards (ASTM D6751 and EN 14214), with a cetane number of 65.3, flash point of 152.3 °C, viscosity of 3.62 mm²/s, and density of 0.837 g/cm³, confirming its suitability for diesel engines. These results underscore the efficiency, sustainability, and cost-effectiveness of S.BT as a catalyst for biodiesel production.