Outstanding activity of a biodiesel coated K₂O/fumed silica catalyst in the transesterification reaction

In this study, several KOH loadings (10, 20, and 30 wt %) were impregnated on fumed silica (FS) using the wet impregnation method followed by calcination at 500 °C for 3 h. The catalysts were characterized by N₂ adsorption-desorption, X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR) techniques. Catalysts were tested in the transesterification of sunflower oil. The catalyst with a 30 wt % loading of KOH (30KOH/FS) showed the best catalytic performance by exhibiting the highest FAME yield. Optimum conditions for biodiesel production over this catalyst were, a reaction time of 4 h at 60 °C, a catalyst to oil ratio of 10 wt % and a methanol to oil molar ratio of 12:1 giving a final FAME yield of 99.9 %. The reusability study revealed a slight deactivation after 3 consecutive runs, attributed to the adsorption of hydrocarbons on the active sites. A breakthrough was achieved when the catalyst was coated with biodiesel (3 wt % with respect to oil in the initial methanol-oil mixture) which formed a protective layer over the active sites. This resulted in a better stability of the 30KOH/FS catalyst as it maintained the maximum FAME yield (99.9 %) over 3 consecutive runs. The properties of the produced biodiesel complied with the ASTM requirements. The coated catalyst also attained and maintained the maximum FAME yield (99.9 %) over 3 consecutive runs in the transesterification of waste cooking oil demonstrating its potential for industrial application.