Solvent-Free Production of ε-Caprolactone from Oxidation of Cyclohexanone Catalyzed by Nitrogen-Doped Carbon Nanotubes
Zhihui Cai, Duo Liu, Jiangnan Huang, Jianning Feng, Hongjuan Wang, Guangxing Yang, Feng Peng, Yonghai Cao, Hao Yu
Abstract
As an important raw material for the synthesis of polycaprolactone, a significant alternative for the nondegradable plastic, ε-caprolactone (ε-CL) monomer is typically produced through the Baeyer–Villiger oxidation of cyclohexanone (Cy═O). In this work, a solvent-free, highly efficient, and nonmetallic catalytic system was developed to produce the ε-CL monomer with dioxygen as the oxidant and benzaldehyde (BEA) as the co-oxidant. Nitrogen-doped carbon nanotubes (NCNTs) as the catalyst boost the catalytic activity (10.09% conversion, 10.06% yield of ε-CL), much higher than other carbon catalysts. The normalized specific activity of NCNTs reached 2964.80 mmol/g/h and 20.02 mmol/m2/h, which are 2–4 orders of magnitude higher than the state-of-the-art catalysts. The structure-dependent activity result shows that the ratio between pyridine nitrogen (NP) and graphite nitrogen (NG) on the surface of NCNTs positively affects the catalytic performance. Oxygen functionalities on the NCNT surface inhibit the activity by suppressing the auto-oxidation of BEA, which unexpectedly increases aldehyde efficiency. Finally, this process could be scaled up to a molar-scale reaction with a similar activity, which lays down the foundation of the industrial application of the solvent-free Baeyer–Villiger oxidation catalyzed by nanocarbons.