Ultrasound-assisted solvent-free synthesis of Citronellyl acetate using Fermase: Kinetics and intensification

Abstract

Citronellyl acetate is a flavour ester with a fresh-rosy, fruity smell. It can be synthesized by transesterification reaction of citronellol and vinyl acetate with Fermase as a biocatalyst where vinyl acetate acts as a reactant and a solvent. With advanced process intensification techniques like ultrasound, the maximum conversion can be achieved in reduced reaction time, as concluded by the current study. Under optimized process conditions with a 1:2 mole ratio of citronellol to vinyl acetate, 2 % enzyme loading at 60 °C and 300 RPM agitation speed, using ultrasound power 60 W with 30 % duty cycle at 25 kHz frequency, a maximum citronellol conversion of 99.8 % was achieved within 60 min of reaction time. The enzyme retained 90 % of its activity after six cycles of reuse. Several kinetic models governing the enzymatic reaction were explored, and a modified Ping pong bi-bi model that considered both reactant inhibitions was found to explain the reaction mechanism best. Advanced optimization techniques like the Response Surface Methodology (RSM) were used to compare the results obtained from the one factor at a time method. The ultrasound-assisted method is also compared in detail with the conventional batch stirring method to showcase the process intensification achieved using ultrasound.