Kinetic and thermodynamic studies on the enzymatic synthesis of wax ester catalyzed by lipase immobilized on glutaraldehyde-activated rice husk particles.

Commercial lipase from Thermomyces lanuginosus has been immobilized on glutaraldehyde-activated rice husk particles via covalent attachment. It was reached maximum immobilized protein concentration of 27.5 ± 1.8 mg g-1 of dry support using the initial protein loading of 40 mg g-1 of support. The immobilized biocatalyst was used to synthesize cetyl oleate (wax ester) via direct esterification of oleic acid and cetyl alcohol. The influence of relevant factors on ester synthesis, such as reaction temperature, biocatalyst concentration, presence or lack of hydrophobic organic solvents, acid:alcohol molar ratio, and reaction time has been evaluated. The experimental data were well fitted to a second-order reversible kinetic model to determine apparent kinetic constants. Thermodynamic studies have revealed that the reaction was a spontaneous and endothermic process. Under optimal experimental conditions, it was observed maximum ester conversion of 90.2 ± 0.6% in 9 h of reaction time in hexane medium using 1 M of each reactant (cetyl alcohol and oleic acid), at 50 °C and biocatalyst concentration of 15% m/v of reaction mixture. Similar conversion (91.5 ± 0.8%) in a solvent-free system was also obtained within 24 h of reaction. The biocatalyst retained 85% of its initial activity after 12 cycles within 9 h of reaction in hexane medium. The physicochemical properties of purified ester have been determined in accordance with ASTM standards. The results indicate that the prepared biocatalyst has great potential for wax ester synthesis due to its satisfactory catalytic activity and operational stability.