Synthesis and characterization of spherically-shaped CaO/?-Al^sub 2^O^sub 3^ catalyst and its application in biodiesel production

Loading CaO onto spherically-shaped ?-Al2O3 support was applied for the preparation of a cost-effective and environmentally-friendly heterogeneous catalyst for biodiesel production. Physico-chemical, textural and morphological characterization was performed using optical emission spectroscopy (ICP OES), Hg-porosimetry, N2 physisorption, scanning electron microscopy (FESEM), X-ray diffraction (XRPD), Fourier transform infrared spectroscopy (ATR FTIR) and Hammett indicators. The effects of catalyst preparation conditions (different precursor salts and calcination temperatures) on the catalyst activity, and the reaction conditions (methanol-to-oil molar ratio and catalyst loading) on the reaction rate and esters yield were studied. In addition, catalyst stability was investigated. The most active catalyst was derived from the nitrate precursor after calcination at 475 °C (inert atmosphere). Under mild reaction conditions (60 °C, 5 h, 900 rpm, methanol-to-oil molar ratio of 12:1 and catalyst loading (CaO wt% to the oil) of 0.5%), the equilibrium ester content of 94.3% was obtained in the methanolysis of sunflower oil in a batch reactor. The catalyst remained sufficiently active in two consecutive cycles without any pretreatment. Mechanical erosion caused by stirring and massive deposition of organic molecules from the reaction mixture caused the decrease of catalytic activity while CaO leaching was negligible.