Effect of fatty acid chain length on the efficiency of the lipase-catalyzed synthesis of alkyl esters in deep eutectic solvents

Abstract: Four DES solvents based on choline chloride (ChCl) and different hydrogen bond donors (HBDs: 1,4-butanediol, glycerol, isosorbide and urea) were tested as reaction medium for the lipase-catalyzed esterification of fatty acids with n-butanol.

A blend of seven even carbon number saturated fatty acids (from C6:0 to C18:0) containing 0.05 mMol of each one was incubated with 100 uL of butyl alcohol and 30 mg of immobilized lipase B from Candida antarctica in 3 g of DES solvent. Alternatively, distilled water was added to vials to achieve a molar ratio water/ChCl of 1, 2 or 3. Vials were incubated inside an orbital shaker at 50 °C and 250 rpm for different periods, and products were analyzed by GLC.

Although the strong hydrogen bonds between DES components were expected to lower the reactivity of the HBDs, all of them, except urea, competed in the esterification reaction. Thus the DES formed by ChCl/urea (2:1) was used to study the effect of the chain length of the fatty acid.

When the incubation was performed without adding water to the reaction medium, the shorter the fatty acid chain length, the higher the conversion achieved. While conversion of 32.8 % was achieved by caproic acid after 48 h of reaction, stearic acid was converted to less than 4 % after the same period.

Adding water to the reaction media drastically increased the esterification rate of all the fatty acids and, conversely to what occurred in the anhydrous media, the longest fatty acids achieved the highest conversions, and this effect intensified as the concentration of water increased. With a molar ratio water/ChCh of 3, fatty acids from C10 to C18 overpassed 95 % conversion after solely 3 hours of incubation, while caprylic and caproic acids achieved 73.8 and 49.5 % conversion in the same period, respectively.

Results suggest that several effects of water concentration in the DES medium (on enzyme activity, solvent viscosity and DES structure) are involved with the efficiency of the esterification reactions and the differences in the yield achieved with each fatty acid.