Amino-anchored sulfonic acid-functionalized heteropolyacid ionic liquid: A highly selective and recyclable catalyst for the Baeyer-Villiger oxidation of cyclohexanone

A novel amino-anchored sulfonic acid functional heteropolyacid ionic liquid ([NHSO]3PW12O40) was designed, synthesized and used as a catalyst in the Baeyer-Villiger (BV) oxidation of cyclohexanone to synthetic ε-caprolactone (ε-CL). The catalyst was well characterized by FT-IR, XRD, 1H NMR, XPS, TG-DTA and SEM analysis methods, which showed that the tungsten phosphate anion (PW12O403-) was successfully modified from the sulphonate-functionalised IL precursor NHSO and formed a new hydrogen bond between the introduced –NH2 group and PW12O403-. The combined effect of ionic bonds and hydrogen bonds contributed to the good stability of the catalyst. Catalyst activity evaluation experiments confirmed that [NHSO]3PW12O40 exhibited superior performances such as a cyclohexanone conversion of 86% and ε-CL yield and selectivity of 82% and 95%, respectively. Furthermore, [NHSO]3PW12O40 could be recovered by simple treatment and the activity of the catalyst did not decrease significantly after five replicae experiments. In addition, the reaction kinetics and mechanism of the catalyst were investigated and a simple validation was given in conjunction with the bond energy changes of the catalyst during the reaction. The innovative of the green, stable and high-performance functionalised heteropolyacid IL catalyst provides a new solution for the efficient production of ε-CL.