Ca–Mg–Al ternary mixed oxides derived from layered double hydroxide for selective etherification of glycerol to short-chain polyglycerols

Abstract

In this study, a magnesium (Mg) and aluminum (Al) layered double hydroxide (LDH) was used as a precursor in the preparation of calcium (Ca), Mg and Al ternary mixed oxides for selective conversion of glycerol into diglycerols via solvent-free glycerol etherification. The LDH-derived MgAl mixed oxide was saturated with an aqueous solution of Ca citrate complexes, followed by calcination at 700 °C, to obtain the Ca-MgAl mixed metal oxide (MMO) catalysts. The effects of the citric acid addition and Ca loading levels on the physicochemical and catalytic properties of the resulting Ca-MgAl MMO were investigated. The Ca-MgAl MMO series were characterized by a high Ca dispersion, high total basicity (236–482 μmol g−1), an enhanced amount of strong basic sites and a new mesoporosity. The highest diglycerols selectivity of 78.3 mol% was achieved at a glycerol conversion of 40.4 mol% when the reaction was performed over the Ca-MgAl MMO catalyst synthesized with a 7.5 wt% Ca loading level, at a 3 wt% catalyst loading, 220 °C for 24 h. The formation of ternary mixed oxides improved the structural stability of the pristine MgAl mixed oxide. The catalytic performance of Ca-MgAl MMO was compared with that of commercial MgAl mixed oxides and homogeneous base (Na2CO3). The presence of strong basicity in the MMO catalysts was a key part in the selective synthesis of diglycerols.