| Cyclohexanone oxime（CHO）is a key precursor for the preparation of nylon 6 plastic monomer（caprolactam）.It is expected that by 2024,the global annual production of nylon 6 will reach 8.9 million tons,thus the demand for CHO will also increase significantly.At present,more than 90%of CHO in the world is prepared by ketone-oxime reaction between cyclohexanone and hydroxylamine salt.Among them,the raw material cyclohexanone is usually prepared from cyclohexane by catalytic oxidation.Hydroxylamine salt is prepared by hydroxylamine sulfate method,hydroxylamine phosphate method or NO reduction process.Because hydroxylamine（NH2OH）is quite unstable,meaning that using it as a free base is easy to cause safety accidents.In order to stabilize NH2OH,a lot of inorganic acids are usually employed to form solid hydroxylamine salts,which lead to some new problems,such as the easy corrosion of production equipment and the formation of a large number of inorganic salts as by-products in the process of CHO preparation.Therefore,it is of great significance to develop simple,efficient and environmentally friendly CHO synthesis process.Studies have shown that NH2OH can be prepared from nitric oxide（e.g.NO3–）by electrocatalytic reduction reaction（one of the active nitrogen intermediates in the process of ammonia synthesis）.Therefore,it is a green and feasible new path to prepare CHO by ketone-oxime reaction between cyclohexanone with the in situ NH2OH intermediates generated from the electrocatalytic reduction process of NO3–.It is worth noting that NO3–is not only a common pollutant in industrial and agricultural wastewater but also an important nitrogen resource.The preparation of CHO directly using NO3–as N source by electrocatalysis is of great significance for green chemical synthesis and efficient utilization of resources.In this paper,the preparation of CHO by electrocatalytic NO3–reduction coupled ketone-oxime reaction was studied.By designing and screening high-performance catalysts and optimizing reaction conditions,highly selective synthesis of CHO was successfully achieved.We revealed the mechanism of CHO electrocatalytic synthesis through a series of characterization methods,such as spontaneous reaction test,isotope labeling experiment,UV-visible spectrophotometry test and in situ infrared,and proved that the electrocatalysis strategy is universal for a series of ketone-oxime reactions.In addition,a novel（photo-）electrocatalytic paired reaction system was designed to achieve the oxidation of cyclohexane to cyclohexanone at the anode,while cathode coupled NO3-electrocatalytic reduction to prepare CHO,proving the feasibility of directly using cyclohexane and nitrate to prepare CHO.The main research contents and conclusions of this paper are as follows:（1）In order to screen out the best catalyst for the electrocatalytic NO3–reduction coupling ketone-oxime reaction to prepare CHO,we first prepared Cu,Cu O,In（OH）3,anatase phase Ti O2（A-Ti O2）and rutile phase Ti O2（R-Ti O2）,and evaluated their properties.Under the same reaction conditions,R-Ti O2 had the highest CHO productivity(127.3μmol cm-2 h-1)and Faradaic efficiency（68.3%）.By adjusting reaction temperature,current density and other experimental conditions,the highest FE of CHO over R-Ti O2 could reach 92.8%,and the productivity further increased to 207.5μmol cm-2 h-1.In addition,the yield of CHO was as high as 98.2%at high cyclohexanone conversion rate,which proved that this reaction strategy had application potential in the actual production of CHO.We explored the reaction mechanism by means of spontaneous reaction test and isotope experiment,and proved that NO3–was first electrocatalytic reduction to NH2OH over R-Ti O2 catalyst,and the NH2OH produced in situ was captured by cyclohexanone in the electrolyte and formed CHO through spontaneous dehydration and condensation process.Through UV-vis spectra,1H-MAS NMR spectra,and infrared testing,we found that R-Ti O2can promote the formation of NH2OH intermediate and has moderate adsorption of cyclohexanone,so that it has excellent reaction performance.Finally,we prove that this strategy is universal to the ketone-oxime reaction of a series of cycloketones,and successfully achieve the selective synthesis of several organic ketoxime products.（2）For CHO synthesis,cyclohexanone is also one of the important raw materials,which is usually obtained from cyclohexane by catalytic oxidation method.We further proposed the innovative idea of constructing（photo-）electrocatalytic paired reaction system in order to prepare CHO utilizing cyclohexane and nitrate.Specifically,the photo-electrocatalytic（PEC）method was used to achieve the oxidation of cyclohexane to prepare cyclohexanone at the anode,while the NO3–elctroreduction coupling ketone-oxime reaction was used to prepare CHO at the cathode.Based on this idea,we first prepared four common photoanode materials and used them to prepare cyclohexanone by PEC cyclohexane oxidation.The results showed that Ti O2had the best PEC activity,and the productivity of cyclohexanone over Ti O2photoanode was 0.91μmol cm-2 h-1 and the selectivity was as high as 99.6%under the condition of 1.5 V vs.RHE applied voltage.Through electron spin resonance test,radical capture experiment and isotope labeling experiment,we proved that cyclohexanone production by PEC cyclohexane oxidation follows the reaction mechanism mediated by hydroxyl radical and carbon central radical.Finally,we set up a paired system coupling PEC cyclohexane oxidation with electrosynthesis of cyclohexanone oxime to achieve the co-production of cyclohexanone and CHO(when the reaction current was 9 m A,the productivity of cyclohexanone and CHO in the paired system was 0.56μmol cm-2 h-1 and28.47μmol cm-2 h-1,respectively),which proves the feasibility of this（photo-）electrocatalytic paired system. |
