Synthesis of CaO[sbnd]SiO2 compounds and their testing as heterogeneous catalysts for transesterification of sunflower oil

Abstract

The powder mixtures of calcium oxide (CaO) and silica gel (SiO2) in molar ratios of 1:1, 1.5:1, 2:1 and 3:1 were mechanochemically treated with the addition of water, and were subsequently calcined with a goal of synthesizing CaSiO3, Ca3Si2O7, Ca2SiO4 compounds and CaO/Ca2SiO4 two-phase mixture. The prepared materials were characterized by XRD, FTIR, SEM/EDS, particle size laser diffraction (PSLD), UV–vis diffuse reflectance spectroscopy (DRS), N2 adsorption/desorption isotherms, Hammett indicator for basic strength and volumetric analysis for free CaO content. The catalytic activity of calcium silicates with different Ca/Si ratios was tested in the transesterification of triacylglycerols (sunflower oil) with methanol. Samples obtained with initial composition 2CaO·SiO2 and 3CaO·SiO2 calcined at 700 °C, and 3CaO·SiO2 calcined at 900 °C had high catalytic activity, resulting with triacylglycerols conversion and fatty acids methyl ester formation (FAME or biodiesel) above 96%. The activity of these samples can be attributed to the existence of free CaO defined by CaO/Ca2SiO4 complex mixture. The effect of different amount of catalyst used for transesterification (0.2–2 wt%) was analyzed using the most active catalyst i.e. 3CaO·SiO2 calcined at 700 °C as well as possibility of its reuse for biodiesel synthesis. It was also found that CaSiO3, Ca3Si2O7 and Ca2SiO4, phases did not possess catalytic activity.