Etherification of glycerol to polyglycerols over MgAl mixed oxides

This work investigates the use of MgAl mixed oxides as base catalysts for the etherification of glycerol. Two different experimental strategies have been employed for the synthesis of the hydrotalcite precursors: coprecipitation and urea hydrolysis. The hydrotalcites and catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, CO2-temperature programmed desorption and N2 adsorption. The MgAl mixed oxides exhibit excellent textural properties, with specific surface areas higher than 200 m2 g−1 and average pore diameters in the mesoporous range. This series of catalysts have shown to be active in the formation of polyglycerols from glycerol without solvent, at 220 °C, in a batch reactor. The highest conversion (50.7%) is found for the catalyst prepared by coprecipitation using NaOH/Na2CO3 as precipitating agent, whereas full selectivity to diglycerols is only reached for low conversion values (14.6% for sample prepared by urea hydrolysis). Only diglycerols (DGs) and triglycerols (TGs) have been detected after 24 h of reaction, with a maximum DG yield of 43% with the catalysts having the highest specific surface area. The formation of DG is favored at low conversion and in the presence of small pores.