ABSTRACT
Waste cooking oil (WCO) as a feedstock used for biodiesel production is recognized as an attractive alternative to vegetable oil. However, the high concentration of free fatty acid (FFA) coupled with other impurities in WCO may require several pretreatments prior to transesterification. This study explored a novel method of estrifying WCO by employing a photocatalytic process using ZnO/SiO2 as the catalyst. The developed catalysts with various proportions of ZnO loading onto the silica support (10-ZnO/SiO2, 20-ZnO/SiO2, 30-ZnO/SiO2 and 40-ZnO/SiO2) were synthesized by mechanochemical method followed by solid state dispersion. It was characterized using X-Ray Diffraction (XRD), X-ray Fluorescence Spectroscopy (XRF), Scanning Electron Microscopy (SEM), pH at point of zero charge (pHPZC) and Specific Surface Area Analysis. The composition of the raw and esterified WCO was analyzed using Gas chromatography Mass spectrometry (GC-MS) technique. The photocatalytic activity of the synthesized catalysts was studied by monitoring the kinetics of degradation of methylene blue (MB) and benzoic acid (BA) under solar irradiation. The percentage FFA conversion in WCO for the developed catalysts in the order 10-ZnO/SiO2, 20-ZnO/SiO2, 30-ZnO/SiO2 and 40-ZnO/SiO2 was 76.4, 79.9, 81.5, and 90.3 in the dark (6 hr) and 78.7, 82.0, 84.10, 94.0 after irradiation time of 4 hours, with 40-ZnO/SiO2 having the highest percentage FFA conversion. The GC-MS results revealed that more esters were being formed in the presence of light than in the dark with no occurrence of soap formation during the photocatalytic esterification process.