Continuous synthesis of ε-caprolactone in a microreactor and kinetics insights into its side reactions

Nowadays ε-caprolactone, the monomer of biodegradable polycaprolactone, is mainly produced via the strong exothermic Baeyer–Villiger oxidation of cyclohexanone in semi-batch reactors. In this work, the continuous synthesis of ε-caprolactone was conducted in a self-designed microreactor system to address its strong exothermic feature, resulting in a cyclohexanone conversion of 90.3% and an ε-caprolactone yield of 82.6%. Analysis using a liquid chromatography equipped with high resolution time-of-flight mass spectrometer suggested that the byproducts mainly consist of ε-caprolactone oligomers in the form of dimer, trimer, and tetramer. Such oligomers were produced via hydrolysis of ε-caprolactone, followed by esterification of the hydrolysis product, 6-hydroxyhexanoic acid. Kinetic studies suggest that the hydrolysis reaction orders for ε-caprolactone and water are 0.75 and 2.52, respectively, while dimerization of 6-hydroxyhexanoic acid is a zero-order reaction. The activation energies of the hydrolysis and dimerization were ~ 77.5 kJ·mol⁻¹ and ~ 55.4 kJ·mol⁻¹, respectively. Density functional theory calculations revealed the significant catalytic effect of acetic acid on both side reactions, where the dimerization of 6-hydroxyhexanoic acid proceeds through an alkoxy pathway.