Ultrasound-assisted lipase catalyzed synthesis of propyl caprate: Process optimization, kinetic, and thermodynamic evaluation

The present work evaluates the influence of ultrasound on lipase-catalyzed esterification for the synthesis of propyl caprate in a solvent-free system. The optimum conditions were found as molar ratio of capric acid: 1-propyl alcohol 1:3, catalyst loading 1% (w/w), molecular sieves loading 5% (w/w), temperature 60 °C, agitation speed 100 rpm, sonication frequency 22 kHz, duty cycle 80% (10 min) and ultrasound power 100 W yielding 86.86% conversion of capric acid. Compared to conventional agitation, ultrasound technology significantly improved process productivity to around 3% more acid conversion for each batch with 50% less requirement of catalyst amount and 5 fold less reaction time with enhanced enzyme recycling up to 9 cycles and stability. The activation energy requirement was lower (6.22 kcal/mol) with ultrasound than the conventional approach (9.028 kcal/mol). Furthermore, the esterification process was observed to be endothermic and spontaneous, with enthalpy (ΔH), entropy (ΔS), and the Gibbs free energy change (ΔG) values of 83.66 kJ/mol, 250.23 J/mol/K, and -2.74 kJ/mol, respectively. An ordered bi−bi mechanism involving the dead-end complex formation with lipase and inhibition of the reaction by capric acid was presented. Ultrasound application was found to be an influential tool to enhance the performance of biocatalyst in this esterification reaction.