Preparation, characterization of Lewis acidic solid acid catalyst Ce-Ag-PW and its catalytic application in biodiesel production via esterification

A solid acid catalyst Ce-Ag-PW with Lewis acidic active site as the main active sitewas prepared from the synergistic modification of phosphotungstic heteropoly acid (HPW) by Ce³⁺ and Ag⁺ through ultrasonic impregnation. Physicochemical properties of the obtained catalyst were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetry (TG), NH₃ temperature programmed desorption (NH₃-TPD), pyridine adsorption Fourier-transform infrared (Py-IR)and X-ray photoelectron spectroscopy (XPS).The catalytic activity and stability of Ce-Ag-PW were studied from the biodiesel production via the esterification of oleic acid and methanol. Results showed that Ce-Ag-PW had high catalytic activity and stability. The conversion of oleic acid reached 91% after 6 h with Ce-Ag-PW as catalyst when the molar ratio of methanol to oleic acid was 14:1, the amount of catalyst was 2% of the total mass of the reactant and the reaction temperature was 65 ℃. The conversion of oleic acid could still reach 80.5% after the Ce-Ag-PW catalyst was used for four cycles. The high catalytic activity and stability of Ce-Ag-PW can be attributed to the replacement of the protons in HPW by Ag⁺ and the strong electron-withdrawing effect exerted by Ce³⁺, which transforms it from Brönsted acid catalyst to Lewis acid catalyst. Since the Brönsted acid site is easily deactivated due to the hydration reaction between it and the water generated from the esterification reaction, the formation of Lewis acid sites helps reduce catalyst deactivation. Therefore, Ce-Ag-PW solid acid catalyst based on Lewis acidic active sites has high catalytic activity and stability in the catalytic synthesis of biodiesel by the esterification reaction of oleic acid and methanol.