Lipase-catalyzed long chain fatty ester synthesis in dense carbon dioxide: Kinetics and thermodynamics

Catalytic synthesis of n-octyl oleate by esterification of free fatty acid (FFA) with 1-octanol over immobilized lipase from Rhizomucor miehei (Lipozyme RM IM) was successfully performed using dense carbon dioxide as reaction medium. Process conditions (pressure, temperature, stirrer speed, water concentration, substrates molar ratio and incubation time) were optimised performing experiments in a high-pressure batch stirred tank reactor (BSTR). Experiments were carried out in the operative pressure and temperature ranges 8–30 MPa and 308.15–383.15 K, respectively. Pressure of 10 MPa, temperature of 323.15 K, stirring rate of 300 rpm, fatty acid/alcohol concentration molar ratio 40/60, water concentration of 166.7 μL/greaction bulk, enzyme concentration of 25 mg/cm3 and incubation time of 5 h were found to be the optimum reaction conditions. Operating at these conditions a maximum FFA conversion of 88% was attained. A comparison with reactions performed at atmospheric pressure in almost non-aqueous solvent-free medium and in n-hexane was also proposed. It resulted that CO2 expanded reaction mixture required lower enzyme concentrations, desired at commercial industrial scales, and enhanced the reaction kinetics with respect to the solvent-free system due to decrease of the interphase transport limitations. Higher fatty acid ester (FAE) concentration was also obtained in SC-CO2 when compared with those attained over n-hexane under identical reaction conditions.