Upcycling black tea waste into a bifunctional catalyst for efficient biodiesel production from waste cooking oil with kinetic and thermodynamic insights

This study introduces a novel, sustainable method for biodiesel production using bifunctional heterogeneous catalysts derived from black tea waste. Catalysts were synthesized via sulfonation with H 2 SO 4 followed by alkaline treatment with NaOH and KOH to produce NaS.T and KS.T, respectively. Characterization showed enhanced surface areas of 59.7 m 2 /g (NaS.T) and 47.6 m 2 /g (KS.T), supporting improved catalytic activity. Transesterification of waste cooking oil using these catalysts achieved maximum biodiesel yields of 98.7% (NaS.T) and 97.8% (KS.T) under optimal conditions (3 wt% catalyst, 10:1 methanol/oil ratio, 40 °C, 45 min). Kinetic modeling confirmed pseudo-first-order behavior (R 2 > 0.90), with activation energies of 11.71 kJ/mol (NaS.T) and 24.56 kJ/mol (KS.T)—significantly lower than conventional ranges—indicating a diffusion-controlled reaction. The higher pre-exponential factor for KS.T (741.5 min −1 vs. 106.5 min −1 for NaS.T) suggests a greater frequency of effective molecular collisions. Thermodynamic analysis confirmed the endothermic nature of the process (ΔH* = 15.01–22.04 kJ/mol) and negative entropy values (− 25.81 to − 41.9 J/mol·K), reflecting the formation of an ordered transition state. Gibbs free energy values (ΔG* = 27.29–30.12 kJ/mol) affirmed the feasibility and spontaneity of the reactions under studied conditions. Optimization using Response Surface Methodology yielded models with high predictive accuracy (R 2 = 0.979 for KS.T; 0.944 for NaS.T), enabling predicted yields of up to 99.6%. These results demonstrate the potential of black tea waste-derived catalysts for low-cost, energy-efficient biodiesel production, aligning with sustainable fuel and waste valorization goals.