Recently, research groups led by Prof. DENG Dehui and Prof. XIAO Jianping from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) reported the direct electrochemical hydrogenative coupling of NO and cyclohexanone (e-HCNC) for constructing a C=N double bond to synthesize cyclohexanone oxime (CHO) over a carbon catalyst at room temperature and atmospheric pressure.
CHO is an important C=N organic compound in industry, which is used as the intermediate to produce nylon 6, one of the most versatile engineering thermoplastics. Traditional production of CHO is an energy-demanding multistep reaction process by using expensive NH3 as the nitrogen source and H2 as the reductant under high temperatures and pressures. To establish an environmentally friendly route for synthesizing CHO with low energy input, it is critically important to develop a low-cost substitutional nitrogen source together with an energy-saving process to directly produce CHO. NO is one of the major air pollutants leading to acid rain, photochemical smog, and ozone layer depletion. The one-step synthesis of valuable nitrogen-containing organic chemical CHO directly from NO is revolutionary and appealing.
Comparison between the routes of thermocatalytic and electrocatalytic synthesis of CHO.
Based on their previous work (“Direct Electrochemical Ammonia Synthesis from Nitric Oxide”, Angew. Chem. Int. Ed. 2020, 59, 9711-9718), the researchers newly developed an electrochemical route to synthesize CHO directly from NO and cyclohexanone. In contrast to the traditional thermocatalytic route for synthesizing CHO, this process uses water as the hydrogen source and proceeds over a carbon catalyst at room temperature and atmospheric pressure and thus is environmentally friendly with a lower energy cost. A high Faradaic efficiency of 44.8% for CHO with a formation rate of 10.7 mg cm−2 h−1 was achieved at −0.4 V versus the reversible hydrogen electrode. Controlled experiments with different nitrogen sources and isotope-labeled experiments with 15NO verify that the nitrogen in the CHO product originates from NO. Experiments and theoretical calculations illustrate that cyclohexanone can directly react with the NH2OH intermediate to produce CHO through nucleophilic addition on the graphene armchair edge, thus suppressing further hydrogenation of NH2OH to NH3 and self-disproportionation of NH2OH in the electrochemical NO reduction. The strategy presents a green route for synthesizing valuable CHO as well as an alternative method of electrocatalytic NOx transformation.
The research was published in Chem Catalysis on June 22 with the title “Direct electro-synthesis of valuable C=N compound from NO”.
This work was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and the DNL Cooperation Fund of the Chinese Academy of Sciences. (Image and text by ZHANG Xianhao)