In this study we explore the feasibility of TiO 2 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 photonabsorption. Kinetics 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, what explain the reduced conversion at lower temperatures. After 5 cycles of using, the catalytic activity remained a conversion up to 59% owing mostly to the organic deposition and catalyst agglomeration. In conclusion, it can also be seen that TiO 2 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. • Biodiesel was produced from WCO by TiO 2 photocatalysis. • TiO 2 /UVA-photocatalysis achieved a conversion to biodiesel of 83% ± 1.24% • Simultaneous esterification/transesterification reactions were carried out. • The E α as a function of temperature was determined. • ΔH, ΔS and ΔG indicate a irreversible and endothermic reaction.
Welter et al. (Sun,) studied this question.