Biodiesel Production from Waste Cooking Oil via β-Zeolite-Supported Sulfated Metal Oxide Catalyst Systems

Waste cooking oil (WCO) is a readily available and cheap feedstock for biodiesel production. However, WCO contains high levels of free fatty acids (FFAs), which negatively impact the biodiesel yield if homogeneous catalysts are used. Heterogeneous solid acid catalysts are preferred for low-cost feedstocks because the catalysts are highly insensitive to high levels of FFA in the feedstock. Therefore, in the present study, we synthesized and evaluated different solid catalysts, pure β-zeolite, ZnO-β-zeolite, and SO₄^2-/ZnO-β-zeolite for the production of biodiesel using WCO as feedstock. The synthesized catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), pyridine-FTIR, N₂ adsorption-desorption, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy, while the biodiesel product was analyzed using nuclear magnetic resonance (¹H and ¹³C NMR) and gas chromatography-mass spectroscopy. The results revealed that the SO₄^2-/ZnO-β-zeolite catalyst showed excellent catalytic performance for simultaneous transesterification and esterification of WCO, with a higher percentage conversion than the ZnO-β-zeolite and pure β-zeolite catalyst, due to the large pore size and high acidity. The SO₄^2-/ZnO-β-zeolite catalyst exhibits 6.5 nm pore size, a total pore volume of 0.17 cm³/g, and high surface area of 250.26 m²/g. Experimental parameters such as catalyst loading, methanol:oil molar ratio, temperature, and reaction time were varied in order to establish the optimal parameters. The highest WCO conversion of 96.9% was obtained using the SO₄^2-/ZnO-β-zeolite catalyst under an optimum reaction condition of 3.0 wt % catalyst loading, 200 °C reaction temperature, and 15:1 molar ratio of methanol to oil in 8 h reaction time. The WCO-derived biodiesel properties conform to the ASTM6751 standard specification. Our investigation of its kinetics revealed that the reaction follows a pseudo first-order kinetic model, with an activation energy (E_a) of 38.58 kJ/mol. Moreover, the stability and reusability of the catalysts were evaluated, and it was found that the SO₄^2-/ZnO-β-zeolite catalyst exhibited good stability, giving a biodiesel conversion of over 80% after three synthesis cycles.