Biodiesel production from waste cooking oil through transesterification catalyzed by the strontium-zinc bifunctional oxides

The basic active sites are rendered inactive by the free fatty acids (FFAs) in the feedstock. The aim of this study was to maximize the use of cheap and readily available waste cooking oil (WCO) and Sr-Zn oxide catalyst (prepared by sol-gel method) for biodiesel production. The best performing catalyst with a molar ratio of ZnO to SrO of 0.6 gave a biodiesel yield of 91.99% even with the addition of 11 wt.% oleic acid. The morphology, crystal size and acid-base value of the Sr-Zn oxide catalysts were determined using various characterization methods. The results showed that the total basicity and acidity of the Sr-Zn oxide catalysts was 0.402 mmol/g and 0.367 mmol/g, respectively. After Zn doping, the specific surface area increased from 0.28 m2 g−1 to 0.41 m2 g−1. The adsorption energy of acetic acid on the surface of Sr-Zn oxide was calculated to be the maximum of -268.9 kJ/mol based on the density functional theory. Zinc doping stimulated the C=O bond of acetic acid with the charges of 0.028 e. As a result, Sr-Zn oxide catalysts demonstrated acceptable activity and reusability, indicating bright futures for the heterogeneous catalyst-mediated manufacturing of biodiesel.