Etherification of glycerol into short-chain polyglycerols over MgAl LDH/CaCO3 nanocomposites as heterogeneous catalysts to promote circular bioeconomy

Glycerol is a byproduct from biodiesel production via conventional transesterification processes, representing approximately 10 wt% of the mass of biodiesel produced. Because of increasing biodiesel consumption, the volume of glycerol being produced has grown significantly, leading to a large surplus and, consequently, a dramatic drop in its market value. Thus, the valorization of glycerol into chemicals is a promising pathway toward sustainability in biodiesel industries. This study focused on upgrading biodiesel plant-derived glycerol into short-chain polyglycerols (PG), which are used as intermediates for producing emulsifiers in several consumer products, via catalytic etherification. To enhance environmental sustainability, solvent-free etherification of glycerol was performed over mixed oxides derived from magnesium-aluminum layered double hydroxides (MgAl LDH). For the first time, natural dolomite, a mixed calcium and magnesium carbonate (CaMg [CO3]2), was used as an Mg source in the preparation of MgAl LDH/CaCO3 nanocomposites via hydrothermal synthesis. The calcined MgAl LDH/CaCO3 nanocomposites were characterized by highly dispersed small crystallites of magnesium oxide. Their textural and acid-base properties were tuned by varying the Mg:Al molar ratio. The MgAl LDH/CaCO3 (an Mg:Al molar ratio of 1:1) calcined at 500 °C exhibited a superior catalytic performance to the MgAl LDH available commercially and the one synthesized by conventional co-precipitation. The nanocomposite catalyst displayed selectivity of >99% toward short-chain PG at 52.1 mol% glycerol conversion.