Preparation, characterization and mechanistic study of alumina supported calcium oxide based catalysts in transesterification of refined cooking oil

Abstract

Biodiesel synthesized from the transesterification reaction using heterogeneous base catalyst has been extensively studied over the past decades. In this research, a series of alumina-supported catalysts were synthesized utilizing CaO and MgO via impregnation with transition metal oxides of Cu, Zn and Ni. Through the catalyst screening process, Cu/Ni/Ca/Al2O3 and Cu/Zn/Ca/Al2O3 were selected, and further studies were carried out for optimization of several reaction parameters. The optimum calcination temperature of Cu/Ni/Ca/Al2O3 was at 700ºC, while that of Cu/Zn/Ca/Al2O3 was at 800ºC. The ratio of co-catalyst to based for both the catalysts was 3:7:90 wt%, with two times of alumina coatings performed on the samples. Study of the surface morphology by FESEM and TEM revealed the existence of agglomerated platelet-shape particles on the catalysts surface. XRD analysis showed the crystallinity of synthesized catalysts were generally poor, with the particle sizes of less than 50 nm. The Cu/Ni/Ca(3:7:90)/Al2O3 catalyst with BET surface area of 140 m2/g, exhibited a higher amount of weak and moderate basic sites (4.02 mmol/g) when compared to the Cu/Zn/Ca(3:7:90)/Al2O3 catalyst, as obtained from the CO2-TPD data. The optimum conditions were found as reaction temperature of 65ºC, catalyst loading of 4 wt% and oil to methanol molar ratio of 1:16, to achieve 90.12% of biodiesel production for 90 minutes of reaction time, in the transesterification of refined cooking oil over the Cu/Ni/Ca(3:7:90)/Al2O3 catalyst. Meanwhile, the Cu/Zn/Ca(3:7:90)/Al2O3 with 10 wt% catalyst loading underwent 180 minutes of reaction time and subsequent achievement of 82.34% of biodiesel production. The validated data from RSM analysis indicated that the selected model was adequate with a percentage error less than 5%. Mechanistic study of the catalyst surface using FTIR and the GC-FID analysis of the transesterification product showed that both the catalysts obeyed the Langmuir mechanism rule and capable to produce the cis and trans isomers of oleic acid methyl ester. The biodiesel produced complied with the quality standard and specification recommended by the American Society for Testing Materials D6751.