Ultrasound-assisted synthesis of oleic estolide: Optimization, process intensification and kinetic study

Abstract

Bio-lubricants are crucial for overcoming environmental issues as they act as renewable substitutes for petroleum-based lubricants. Bio-lubricant derived from vegetable oils have excellent lubricating properties but are hampered by their low oxidation stability, low hydrolytic stability, and lack of cold-flow characteristics which can be mitigated by modification by estolide formation. The synthesis of estolides is limited by the higher reaction time and hence ultrasound-assisted technique was employed for intensification of the production of estolide from oleic acid. The reaction parameters (temperature, catalyst amount, and time) were optimized using the response surface methodology (RSM) to obtain the best characteristics in terms of acid value (AV) and pour point (PP). The optimized conditions were established using RSM and validated using an artificial neural network (ANN) based model. Successfully intensification using sonication in terms of reducing the reaction time of 6–12 h for the conventional approach to 2.623 h for the ultrasonic bath and to 1.5 h for the ultrasonic horn bath was observed. A kinetic study of the oligomerization reaction revealed the fitting of the second-order kinetics. Overall, the work elucidates the utility of ultrasound in synthesizing oleic estolide in terms of successful intensification and elucidating the reaction kinetics.