Study On The Reaction Of Photoelectrochemical Synthesis Of Imines Coupled With Hydrogen Evolution Through Bismuth Vanadate

Imines are one of the most important multifunctional intermediates in the field of organic synthesis.Important nitrogen-containing organic compounds widely used in medicine,pesticide,biology and fine chemical industry can be produced by the reduction,addition,cyclization,nitrogen hybridization and other reactions of imines.Thus,the transformation from amines to imines is considered to be one of the most important functional group transformation reactions in organic chemistry,which has attracted extensive research by scientific researchers.In industry,imines are prepared by the condensation reaction between carbonyl compounds and amines.Some toxic oxidants and expensive dehydrators are essential for this process,which causes huge energy loss and pollutant emission,and the overall reaction efficiency is very low.The direct oxidation of amines to corresponding imines is the most ideal imines synthesis method at present,which includes two types of reactions-primary amines self-coupling and secondary amines oxidative dehydrogenation reactions.Various systems based on this synthesis method,such as metal-based catalylic system,photocatalylic system and electrocatalylic system have been developed and utilized,however,each of them has some shortcomings.Metal-based systems are mainly concentrated in rare and precious metals such as ruthenium and gold.The high cost limits their wide application.In the photocatalysis system,the activity of amines oxidation is not high enough,and long-time illumination is essential to obtain ideal conversion rate and yield.In the electrocatalysis system,the insufficient mass transfer at the interface between electrode and electrolyte will often lead to excessive oxidation of imine intermediates by further dehydrogenation to nitriles,resulting in poor selectivity of imine products.As a powerful emerging organic synthesis technology,photoelectrochemical can realize the constructively merging of photochemistry and electrochemistry,which uses the light responsive semiconductor photoanode to produce photogenerated holes and electrons driven reaction to reduce the external bias,save electric energy and avoid the peroxidation of organic substrates.High activity and high selectivity photoelectrochemical oxidation of many organic substrates have achieved at present,which indicates photoelectrochemical has great application potential in the organic synthesis of imines.In addition,many literatures reported that the oxidation of amines is usually a two-electron process,which is easier to be carried out than the four-electron water oxidation process in terms of kinetics or thermodynamics.Thus,the merging of amines oxidation and water reduction reaction in a photoelectrochemical reaction process can promote the hydrogen evolution reaction,realize solar production of hydrogen and high value-added fine chemicals,improve the overall energy conversion efficiency and reduce the cost of solar hydrogen production.Thus,this paper focuses on the photoelectrochemical direct oxidation of amines to imines coupled with hydrogen evolution.The main contents are as follows:In chapter 1,the structure,properties,classifications and applications of imines are briefly introduced.The research status of three synthesis methods of imines:condensation reaction between carbonyl compounds and amines,cross coupling reaction between alcohols and amines,direct oxidation reaction of amines are summarized.The reaction mechanism and existing problems of metal-based catalylic system,photocatalylic system and electrocatalylic system for the preparation of imines based on direct oxidation of amines are introduced.This paper focuses on the principle and research status of photoelectrochemical organic synthesis,expounds its application potential in the direct oxidation amines to imines with high activity and selectivity,and finally analyzes the feasibility of photoelectrochemical amines oxidation coupled with hydrogen evolution reaction.In chapter 2,the photoelectrochemical self-coupling of primary amines coupled with hydrogen evolution reaction through Mo:BiVO4 was researched by taking benzylamine to N-benzylidenebenzylamine as the model reaction.Pure BiVO4 and Mo:BiVO4 photoanodes were prepared by spin coating and annealing.It is proved that the doping of trace molybdenum can improve the conductivity of the original BiVO4 photoanode,promote the separation of photogenerated holes and electrons,and improve the carrier concentration,and make the photoanode have optimized photoelectrochemical performances,in which 3%is the best doping concentration.Acetonitrile was selected as the anodic solvent due to its inertia,excellent solubility and the photoanode had enhanced photocorrosion resistance in acetonitrile.The feasibility of the oxidation of benzylamine in organic phase coupled with water reduction reaction was analyzed.The experiment of the oxidation of benzylamine coupled with the hydrogen evolution was carried out in a closed two-chamber,three-electrode photoelectrochemical cell at constant voltage.The high activity and high selectivity transformation from benzylamine to N-benzylidenebenzylamine and high-speed production of hydrogen were realized,and the reaction mechanism was analyzed.We extended the range of organic substrates from benzylamine to different types of benzylamine derivatives,and preliminarily proved that the reaction of photoelectrochemical self-coupling of primary amines has certain universality.In chapter 3,the photoelectrochemical oxidative dehydrogenation of secondary amines coupled with hydrogen evolution reaction through Mo:BiVO4 was further studied by using N-(tert-Butyl)benzylamine to N-Benzylidene-2-methyl-2-propanamine and dibenzylamine to N-benzylidenebenzylamine as model reactions respectively.We chosed the 3%Mo:BiVO4 photoanode with the best photoelectrochemical performances selected in the experiment in Chapter 2 as the working electrode for a series of measurments.The 3%Mo:BiVO4 photoanode shows good stability in acetonitrile solution containing organic substrate.The feasibility of oxidative N-(tert-Butyl)benzylamine coupled with hydrogen evolution reaction were analyzed.The photoelectrochemical oxidative dehydrogenation of secondary amines in organic phase coupled with hydrogen evolution in aqueous phase were carried out in a closed two-chamber,three-electrode photoelectrochemical cell,and good yield and Faradic efficiency were obtained.Finally,the reaction mechanism was analyzed.In chapter 4,the research contents in the paper are briefly summarized,the main innovations and problems of this work are pointed out,and the future work is prospected.