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.