The Role of Mg in CaO-MgO Composite Catalyst for Biodiesel Production via Transesterification of Soybean Oil

Abstract

Abstract To improve the activity and stability of catalysts in transesterification for biodiesel production, Mg-doped CaO-MgO catalysts were prepared by co-precipitation in this study. The physicochemical properties of the catalysts were characterized by BET, X-ray diffraction (XRD), scanning electronic microscopy (SEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR), thermal gravimetric analysis (TGA), CO2 temperature-programmed desorption (CO2-TPD), oxygen temperature-programmed oxidation (O2-TPO), and in-situ diffuse reflectance infrared spectroscopy (DRIFTS). The performance of the prepared catalysts in the transesterification reaction of soybean oil with methanol was evaluated. The results showed that the added Mg was present in both doped and supported forms. The doped form of Mg reduced the lattice spacing of CaO, improved the intensity and number of basic sites, and enhanced the activity of the catalyst in the transesterification reaction. The poor pore structure of the supported form of MgO weakened the adsorption and diffusion of reactant molecules to the basic sites, which was unfavorable to the activity, but it could protect the basic sites on the catalyst surface from water and CO2 contamination in the air, which was favorable to the stability of the catalyst. The 1Mg3Ca catalyst showed optimal performance in the transesterification reaction due to its higher Mg doping and suitable specific surface area. The main cause of catalyst deactivation is the formation of hydroxides and carbonates on the surface, which are removed in the order of H2O > Mg(OH)2 > Ca(OH)2 > CaCO3 during high-temperature activation.