Potassium compounds-Al2O3 catalyst synthesized by using the sol-gel urea combustion method for transesterification of sunflower and waste cooking oils

Abstract

The nanostructured composite catalyst of potassium compounds-Al2O3, or briefly K/Al2O3, was prepared by using the sol-gel urea combustion method using potassium and aluminum nitrates as precursors, which was applied in the transesterification reaction of sunflower and waste cooking oils to produce biodiesel. The molar ratio of potassium nitrate to aluminum nitrate of 0.5 in the synthesizing solution of the catalyst and calcination temperature of 650 °C were obtained from the optimization experiments of the catalyst. The optimized K/Al2O3 catalyst was characterized by ICP, XRD, SEM, and TGA. Gas chromatography-mass spectrometry analysis of the produced biodiesel confirmed the formation of methyl esters. The effects of the operating conditions like methanol to oil molar ratio, catalyst dose, reaction temperature, and time on the biodiesel production were investigated, modeled, and optimized using the response surface methodology. The results of statistical analysis of the experimental model showed that linear term of temperature and interaction term of temperature and time had the strongest effect on the biodiesel yield. The optimized operating conditions were methanol to oil molar ratio of 17:1, catalyst dose of 8.25 wt.%, temperature of 70 °C, and time of 7 h. Under these conditions, the experimental yield of biodiesel production was 88.74% from sunflower oil and 82.01% from waste cooking oil. The properties of the produced biodiesels like density, pour point, cloud point, cetane number, and acid value were measured, which were comparable with the standards.