Lanthanum oxides for the selective synthesis of phytosterol esters: Correlation between catalytic and acid–base properties

The selective synthesis of phytosterol esters from natural sterols and methyl esters was investigated on new grounds by a chemical process using basic solids that are reusable, active, and more selective than the currently used homogeneous catalysts (e.g., alkali hydroxides, carbonates). Various lanthanum oxides with different synthesis procedures and varying specific surface areas were tested. The catalytic performance of the new solids and, more specifically, the ability of La2O3 oxides in increasing phytosterol ester selectivity, was correlated with their acido-basic properties, which were fully characterized by infrared spectroscopy. All La2O3 oxides involved surface carbonate species with different structural states and strengths, as revealed by propyne adsorption. The selectivity to phytosterol ester varied from 90 to 96%, confirming that the side reaction of dehydration of β-sitosterol was strongly inhibited. The phytosterol ester yield could reach 89% and was related to the basic strength of the carbonate species. The lower the carbonate basicity, the higher the phytosterol ester yield. Moreover, IR spectra of catalysts diluted in KBr powder showed that the higher the intensity of unidentate carbonate bands, the higher the yield. It was concluded that the synthesis of phytosterol esters from fatty methyl esters and sitosterols in the presence of La2O3 catalysts requires ionic carbonate species of medium basic strength.