Esterification of Glycerol with Acetic Acid Using Nitrogen-Based Brønsted-Acidic Ionic Liquids

Glycerol esterification with acetic acid produces a mixture of mono, di, and triacetins, which are commercially important value-added products with a wide range of industrial uses including their application as fuel-additives, thus contributing to environmental sustainability and economic viability of the biorefinery concept. Glycerol esterification with acetic acid was studied using a range of nitrogen-based Brønsted-acidic ionic liquids. Cost-effective and easily synthesized Brønsted-acidic ionic liquids based on alkyl-pyrrolidone and alkyl-amine cations were synthesized and characterized using ¹H NMR spectroscopy. The catalytic activity of the Brønsted-acidic ionic liquids produced were investigated for the production of di and triacetin from glycerol and acetic acid. Amongst all ionic liquids evaluated in this study, N-methyl-2-pyrrolidinium hydrogen sulfate [H-NMP][HSO₄] was found to be the most active and cost-effective catalyst. The effect of significant reaction parameters on selectivity to the trisubstituted product, triacetin, was modeled using a design of experiment (DoE) approach with a response surface methodology involving a central composite design. The esterification process was optimized to maximize the production of triacetin. Optimizing the process this way naturally leads to lower levels of mono and diacetin. Amongst the reaction parameters evaluated, temperature had the greatest influence on product selectivity, followed by the glycerol to acetic acid molar ratio, and the model also showed dependence on the synergistic interaction between the temperature and mole ratio. It is worth noting that agitation speed had minimal influence on product selectivity. Under optimized reaction conditions, >99% glycerol conversion was achieved with 42.3% selectivity to triacetin, and a combined di and triacetin selectivity of >95% within 1 h.