TiO2 photocatalysed biodiesel production through simultaneous esterification and transesterification of waste cooking oil

In this study, we explore the feasibility of TiO2 photocatalysis in converting high free fatty acid (FFA) waste cooking oil (WCO) to biodiesel using a one-step esterification-transesterification. This approach has potential to convert high-FFA feed stocks into biodiesel without any use of homogeneous catalysts under mild operational conditions, applying either UVA or natural sunlight. The highest overall yield (83%) was achieved in methanolic solution and under UVA irradiation. The greatest conversion of 89% was obtained from the FFA fraction in esterification, whereas that of transesterification from triglycerides was 81%. Thereby different reaction pathways were presented under photocatalytic conditions. With UVA replaced by natural sunlight, the conversion decreased to 32% as it should be considered on account of photon limitations. Using ethanol instead of methanol resulted in only 21% conversion, highlighting the structure-selectivity of alcohol reactivity to photo absorption. Kinetic data were established by five different models and apparent rate constants as well as activation energies were calculated. Thermodynamic parameters (ΔH = 92.58 kJ/mol, ΔS = 0.29 kJ/mol·K) indicated that the process is endothermic, entropy-driven process that becomes thermodynamically favourable above 45 °C, which explains the reduced conversion at lower temperatures. After 5 cycles of using, the catalytic activity remained at a conversion up to 59% owing mostly to the organic deposition and catalyst agglomeration. In conclusion, it can also be seen that TiO2 photocatalysis has a significant potential for sustainable biodiesel production from high-FFA waste feedstocks and, with scope for further optimisation and scale-up using renewable irradiation sources.