Oxidation of KA oil to caprolactone with molecular oxygen using N-hydroxyphthalimide-mediated Ce(NH4)2(NO3)6 catalyst

Abstract

In traditional Baeyer-Villiger oxidation, peracids or hydrogen peroxide are usually adopted as the oxidants. When molecular oxygen is used as oxidant, the sacrificial agents are always indispensable, such as aldehydes that are transformed into cheap acids after reaction. In this work, KA oil (the industrial raw material, a mixture of cyclohexanol and cyclohexanone) has been oxidized to caprolactone by molecular oxygen using N-hydroxyphthalimide (NHPI) and cerium ammonium nitrate (CAN) as catalyst, in which the sacrificial agent is cyclohexanol, and it is converted into cyclohexanone, then into caprolactone rather than into byproducts. The selectivity of caprolactone was 98% with cyclohexanol conversion of 34% and it was still kept at 90% when the conversion reached to 46%. The mechanism investigation showed a bifunctional role of CAN, which performed both as a radical initiator for cyclohexanol oxidation and a Lewis acid for Baeyer-Villiger reaction. In the Baeyer-Villiger oxidation, a weak interaction between cerium and cyclohexanone was suggested by Fourier Transform Infrared Spectroscopy (FTIR), meanwhile, the active species generated from cerium and hydrogen peroxide was separated and characterized by FTIR. The detailed research also revealed an unusual effect between cerium and the Brønsted acid generated as a byproduct, which was critical for caprolactone synthesis.