Innovative method for hydrocalumite-like compounds' preparation and their evaluation in the transesterification reaction

Abstract Hydrocalumite-like compounds (HLCs) were synthesized by a simple, economical and environmentally friendly method. The solids were characterized by X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), nitrogen adsorption–desorption at − 196 °C, scanning electron microscopy (SEM) and CO2 temperature-programmed desorption (CO2-TPD). Transesterification of castor oil with methanol was selected as a probe reaction to stress the effect of HLC purity and the presence of distinct anions in the interlayer region. By X-ray powder diffraction it was demonstrated that by only mixing hydrated lime and boehmite either in deionized or in deionized–decarbonated water, HLCs were obtained as the main crystalline phase, together with hydrogarnet, calcite and calcium hydroxide as secondary crystalline phases, respectively. The effect of the system's pH and of the purity of the calcium source was confirmed by adjusting the pH and using calcium hydroxide; in this way, pure nitrate-intercalated hydrocalumite was obtained after aging for 3 h at 80 °C. Although, pure HLCs were effectively synthesized, after their thermal activation all samples (including those where other crystalline phases were detected) presented the same crystalline phases. When the calcined solids were tested as heterogeneous catalysts in the transesterification reaction, the highest conversions were attained with solids with a combination of both weak and strong basic sites, revealing that initial crystalline phase purity is not a requirement to prepare an active catalyst for this reaction.