Synthesis And Catalytic Cyclohexane Oxidation To KA Oil Based On Transition Metal Ion Coordinated Covalent Organic Polymer

Cyclohexane is a kind of saturated hydrocarbon with high C-H bond energy.The selective oxidation of sp³C-H bond at low temperature is a challenging research problem in the petroleum industry.The current method of activating C-H bond usually requires high temperature and excessive energy input,but this method often leads to uncontrolled by-product generation.Among all C-H activated processes,liquid phase oxidation of cyclohexane to KA oil（mixture of cyclohexanol and cyclohexanone）is widely used in the production of polyamide-6 and polyamide-66.Industrial processes are carried out with homogeneous cobalt/manganese carboxylate at 140-160℃,using 0.9-1.0 MPa air as oxidant.Nevertheless,with respect to homogeneous catalysts,they are to the disadvantage of recovering,and will lead to higher metal leaching,which reduce the cyclohexane conversion rate and the selectivity of cyclohexanol/cyclohexanone.As a consequence,the development of novel heterogeneous catalysts to oxidize cyclohexane is considered to effectively enhance the selectivity of products and the catalyst recovery ratio.Covalent organic polymers（COPs）can be regarded as a kind of ideal heterogeneous catalysts due to their abundant catalytic sites and rapid transport characteristics.At the same time,their high surface area generates well-dispersed active sites,which enable substrates to react efficiently and selectively.Secondly,the insolubility and high thermal stability of COPs result in high recovery after the reaction,making the catalyst repeatable and continuous.Furthermore,it was found through literature research that the metal sites formed by the coordination of chlorhexidine and Schiff base compounds with metal plays a certain role in the oxidation of cyclohexane.In view of the above research background,a sequence of covalent organic polymers based on chlorhexidine and Schiff bases were designed and synthesized in this paper,and their performance as heterogeneous catalysts for the oxidation of cyclohexane was explored.The structure of the ligand and catalyst was characterized by ¹H NMR,FT-IR,PXRD,N₂adsorption and SEM-EDS.In the meantime,the stability and catalytic effect of the catalyst on cyclohexane oxidation were tested to determine their structural characteristics and applications.In this paper,a succession of heterogeneous catalysts have been developed based on the fact that the metal sites formed by the coordination of chlorhexidine and Schiff bases with metal have certain catalytic activity for cyclohexane oxidation.The specific research contents are as follows:1.A variety of chlorhexidine covalent organic polymers（CHX-M-COPs,M=Mn,Co,Fe）were designed and synthesized using chlorhexidine with metal coordination and 1,3,5-triimidazolobenzene as ligands.Experimental results demonstrate that metal Mn as the catalytic sites have relatively high catalytic effect on cyclohexane oxidation.Based on CHX-Mn-COP,the catalytic conditions were optimized.It was found that when 20 mg catalyst was added at 75℃after 8 h reaction without solvent,the cyclohexane conversion rate could reach 13.72%and the selectivity of KA oil could reach 95.65%.At the same time,after 3 catalytic cycle tests,the catalyst still maintains favourable catalytic performance for the substrate.2.Schiff base Cl Salophen was synthesized from o-phenylenediamine and5-chloro-salicylaldehyde,and then a sequence of Schiff base covalent organic polymers（Cl Salophen-M-COPs,M=Mn,Co,Fe）were designed and synthesized by using Cl Salophen with metal coordination and 1,3,5-triimidazolobenzene as ligands.The experimental results show that Mn as catalytic sites have relatively high catalytic effect on cyclohexane oxidation.Based on Cl Salophen-Mn-COP,the catalytic conditions were optimized.It was found that when 20 mg catalyst was added at 70℃for 16 h without solvent,the cyclohexane conversion rate could reach 9.96%and the selectivity of KA oil could reach 76.09%.Meanwhile,after the catalytic cycle test for3 times,the catalyst still maintains favourable catalytic performance for the substrate.