Waste cooking oil(WCO) is a readily available and cheapfeedstockfor biodiesel production. However, WCO contains high levels of freefatty acids (FFAs), which negatively impact the biodiesel yield ifhomogeneous catalysts are used. Heterogeneous solid acid catalystsare preferred for low-cost feedstocks because the catalysts are highlyinsensitive to high levels of FFA in the feedstock. Therefore, inthe present study, we synthesized and evaluated different solid catalysts,pure & beta;-zeolite, ZnO-& beta;-zeolite, and SO4 (2-)/ZnO-& beta;-zeolite for the production of biodieselusing WCO as feedstock. The synthesized catalysts were characterizedby Fourier transform infrared spectroscopy (FTIR), pyridine-FTIR,N-2 adsorption-desorption, X-ray diffraction, thermogravimetricanalysis, and scanning electron microscopy, while the biodiesel productwas analyzed using nuclear magnetic resonance (H-1 and C-13 NMR) and gas chromatography-mass spectroscopy. Theresults revealed that the SO4 (2-)/ZnO-& beta;-zeolitecatalyst showed excellent catalytic performance for simultaneous transesterificationand esterification of WCO, with a higher percentage conversion thanthe ZnO-& beta;-zeolite and pure & beta;-zeolite catalyst, due tothe large pore size and high acidity. The SO4 (2-)/ZnO-& beta;-zeolite catalyst exhibits 6.5 nm pore size, a totalpore volume of 0.17 cm(3)/g, and high surface area of 250.26m(2)/g. Experimental parameters such as catalyst loading,methanol:oil molar ratio, temperature, and reaction time were variedin order to establish the optimal parameters. The highest WCO conversionof 96.9% was obtained using the SO4 (2-)/ZnO-& beta;-zeolite catalyst under an optimum reaction conditionof 3.0 wt % catalyst loading, 200 & DEG;C reaction temperature, and15:1 molar ratio of methanol to oil in 8 h reaction time. The WCO-derivedbiodiesel properties conform to the ASTM6751 standard specification.Our investigation of its kinetics revealed that the reaction followsa pseudo first-order kinetic model, with an activation energy (E (a)) of 38.58 kJ/mol. Moreover, the stabilityand reusability of the catalysts were evaluated, and it was foundthat the SO4 (2-)/ZnO-& beta;-zeolite catalystexhibited good stability, giving a biodiesel conversion of over 80%after three synthesis cycles.
