6. Esterification in solvent-free systems coupled with
pervaporation
Table 5 presents an overview on lipase catalyzed esterifications
in solvent-free system coupled with pervaporation. In three papers
synthesis of fatty acid esters was studied [33,37,38] and in one
esterification of aroma ester, 3-methyl-n-butyl acetate [39].
Dormo˝ et al. have studied the enzymatic esterification of fusel oil
and oleic acid catalyzed by immobilized lipase Novozyme 435 in a
solvent-free system [33]. The same system was investigated by Koszorz
et al. [37]. Dormo˝ et al. have studied the influence of different
parameters on the reaction outcome (water content, temperature,
molar ratio of acid and alcohol, chain length of alcohol). Pervaporation
was used to remove water produced in the reaction in an
integrated system that consisted of bioreactor and a pervaporation
unit. The reaction was carried out at the previously determined
optimal conditions (shake flasks experiments – 0.5% water content,
1:2 oleic acid to isoamyl alcohol molar ratio, temperature of 60 ◦C).
Thus initial substrate conversion of 92% was increased up to 99.8%
in the integrated system [33]. Among the three membranes used,
CMC-VP-43 was found to be the most suitable. Koszorz et al. [37]
have proposed a mathematical model of the integrated process for
this system which could adequately describe their experimental
data.
Hazarika et al. investigated the pervaporation aided esterification
of different carboxylic acid (saturated fatty acids: lauric,
myristic, stearic and palmitic) and ethanol by lipase from porcine
pancreas. The effect of lipase and substrate concentrations in the
pervaporation aided process on the reaction outcome was studied
in their work. Higher lipase concentration enhanced the reaction
rate, thus water content in the reactor increased faster, as
well. Consequently this accelerated the pervaporation process. The
integrated process was simulated by a suitable kinetic model for
reaction and pervaporation incorporated in the batch system.
Another solvent-free system in which pervaporation aided
esterification was carried out was reported by Ehrenstein et al. [39].
The reaction studied was the synthesis of 3-methyl-n-butyl acetate
known as banana flavor. Novozyme 435 catalyzed the reaction. In
pervaporation aided system authors were able to achieve increased
ester yield in comparison to pervaporation-free system.
In all these papers authors discussed the importance of high
alcohol to acid molar ratio while it benefits the product yield.
However, it was found that high alcohol concentration can have
an inactivating effect on the enzyme, as well [37]. Most of the
papers deal with the mathematical model of the integrated process
[37–39]. In pervaporation aided systems substrate conversion was
significantly higher than that obtained without pervaporation due
to a favorable equilibrium shift via water removal. Efficiency of the
process depends on the ratio of the membrane area to the volume
(mass) of the mixture [38]. This parameter was also emphasized
out as important one in the work of Koszorz et al. If high alcohol to
acid molar ratio is used, quite small membrane areas can be used to
achieve great process efficiency [37]. This is an important parameter
for industrial plant design and pervaporation application. Of
course, only excess of water should be removed from the systems
by pervaporation, otherwise lipase activity decreases.