Direct synthesis of calcium diglyceroxide from hydrated lime and glycerol and its evaluation in the transesterification reaction

• Ca-DG was easily prepared from low cost commercial hydrated lime.
• Ca-DG is the active phase for the transesterification reaction.
• Full conversion was achieved by Ca-DG at 55 °C and 120 min.
• Lixiviation of the active phase was corroborated by X-ray powder diffraction analysis of the recovered solids after reaction.
• Ca-DG crystalline phase is highly soluble in either water, biodiesel, methanol or castor oil.

In this work, calcium diglyceroxide (Ca-DG), was produced by reacting hydrated lime with glycerin varying reaction times and temperatures. The effect of removing the glycerin excess by a water washing and drying step was also considered. Samples’ purity and identification of the crystalline phases was determined by X-ray powder diffraction (XRD) and the purest sample was evaluated in the transesterification reaction of castor oil with methanol. Transesterification reaction products were characterized by 1H NMR and thin-layer chromatography analysis. The influence of the catalyst amount, methanol:oil ratio (vol.), reaction time, and reaction temperature was studied. From the XRD results it was evidenced that Ca-DG was obtained as the main crystalline phase although calcium hydroxide and calcite were identified in all samples. In this sense, the best reaction conditions for Ca-DG obtaining were achieved at 60 °C and 2 h. It was interesting that although in the first use full conversion of castor oil was achieved in the first recycling of spent catalyst, a conversion decrease was observed and the catalyst did not show any activity after the second reuse. In order to determine the reasons that caused catalyst’s deactivation the solid’s stability was tested immersing it in water, methanol, castor oil and biodiesel, respectively.

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