The role of methoxy species on the transesterification reaction of castor oil on Ni-Mg-Al calcined hydrotalcites

In this work hydrotalcites with Ni/(Ni + Mg) ratios = 0.0, 0.33, 0.66 and ratio (Ni+Mg)/Al= 3, were synthesized. The hydrotalcite precursors were calcined and used as catalysts to study the effect of Ni content on the activity for transesterification of castor oil. The catalysts were characterized by physisorption of N2, X-Ray Diffraction (XRD), temperature programmed desorption (TPD) of CO2 and Fourier transform-infrared (FT-IR) spectroscopy of methanol adsorption. The reaction activity was measured by the production of each fatty acid methyl esters (FAMEs) using gas chromatography (GC). The highest activity and quantity of adsorbed methanol corresponds to the Ni2Mg1-C sample. It was found that the addition of nickel increases the number of sites of medium basic force. Methanol adsorption on the catalysts was studied by FT-IR and a relationship was established between the methoxy ions on the surface and the activity towards transesterification. Attenuated total reflectance infrared spectroscopy (ATR-IR) showed the existence of methoxy ions in solution when methanol was added to a calcined hydrotalcite and heated; with this data an alternative mechanism for basic heterogeneous catalysis was proposed. The reaction main steps are formation of a methoxy ion on the catalyst surface, followed by the reaction of the methoxy ion with triglycerides or fatty acid in the liquid phase.

Automated summary

Hydrotalcites with different Ni contents were synthesized and used as catalysts for the transesterification of castor oil. It was found that the addition of nickel increased the number of medium basic force sites, and the catalyst with Ni2Mg1-C ratio showed the highest activity and methanol adsorption. The relationship between methoxy ions and transesterification activity was established by analyzing the methanol adsorption behavior on the catalyst. A new mechanism for basic heterogeneous catalysis was proposed based on the existence of methoxy ions in solution when methanol was added to a calcined hydrotalcite and heated.