Catalytic Synthesis of Lactones from Alkanes in the Presence of Aldehydes and Carbon Nanotubes

Abstract

The catalytic synthesis of lactones from alkanes is a novel strategy from the viewpoint of environmentally benign technology. In this work, we propose a new protocol of using a typical alkane (cyclohexane, CyH) as a substrate to produce lactone (ε-caprolactone, ε-CL) in the presence of carbon nanotubes (CNTs) and aldehydes. The results demonstrated that CNTs could be used as an efficient catalyst for the production of ε-CL. Nitrogen doping would greatly enhance the activity, with 26.2% conversion of CyH and 87% selectivity of ε-CL. The specific N dopants served as complex active sites for improving the catalytic efficiency of NCNTs with an optimum content of N (3.38 atom %). Further investigations revealed that the ratio of pyridine N/ quaternary N probably played a significant role in the facilitation of this reaction, which displayed a linear relationship with the specific activity of NCNTs. A mechanistic study demonstrated that the activation of benzaldehyde for the production of carbonyl radicals was considered as the initial reaction, which activated CyH molecules to produce alkyl radicals, thus promoting the oxidation of CyH. The intermediate product CyO was found to be the main precursor of ε-CL, and during the reaction, cyclohexanol (Cy−OH) would likely be transformed into CyO.