Study On The Method And Mechanism Of Electrochemical And Photochemical Synthesis Of Piridinecarboxylic Acid

Pyridinecarboxylic acid is an important chemical intermediate and has been widely used in the fields of medicine,pesticides and dyes.At present,chemical oxidation,gas-phase catalytic oxidation and liquid-phase catalytic oxidation are common methods for synthesizing picolinic acid.However,unfortunately,these methods have problems such as cumbersome operation,harsh reaction conditions,and large energy consumption and pollution during the synthesis of picolinic acid.Relevant previous studies have confirmed that electrochemical and photoelectrochemical oxidation methods can be used to achieve organic synthesis reactions,and have the advantages of simple operation,mild reaction conditions,and environmental protection.On the other hand,electrochemical and photoelectrochemical water-splitting strategies are commonly used to produce hydrogen,but their activities are often limited by the high overpotential of the oxidation and oxygen evolution reaction of anode water in the electrolytic cell.If kinetically preferential organic reactions occur on the anodes of electrochemical and photoelectrochemical cells,the hydrogen evolution activity of their cathodes will be greatly enhanced.Based on the above background and analysis,this paper systematically studies the feasibility and relatedness of the oxidation of 3-methylpyridine green to 3-Pyridinecarboxylic acid on the anode of the electrolytic cell through photoelectrochemical and electrochemical strategies,and the efficient production of hydrogen on the cathode Reaction mechanism.The main research contents and conclusions are as follows:（1）The theoretical calculation shows that the oxidation of 3-methylpyridine to 3-Pyridinecarboxylic acid（E^Θ=0.93 V vs.RHE）takes precedence over the water oxidation（E^Θ=1.23 V vs.RHE）in thermodynamics.The experimental results show that in a two chamber photoelectrochemical cell with WO₃ film as photoanode,Pt wire electrode as cathode and proton exchange membrane,the photogenerated hole of WO₃ drives the oxidation of 3-methylpyridine to 3-Pyridinecarboxylic acid,this reaction is kinetically preferred over the water oxidation reaction process.Therefore,3-Pyridinecarboxylic acid can be synthesized green by the photoelectric oxidation strategy of WO₃.The main active species of this reaction is the photogenerated hole of WO_3.At the same time,H₂O₂ generated in the process of photoelectric oxidation of water by WO₃ also participates in the oxidation of 3-methylpyridine to 3-Pyridinecarboxylic acid,the selectivity of the anode product 3-Pyridinecarboxylic acid is close to 100%.As the reaction potential of 3-Pyridinecarboxylic acid is lower than that of water oxidation,the hydrogen evolution activity of Pt wire cathode in WO₃ cell with 3-Pyridinecarboxylic acid electrolyte is significantly higher than that of Pt wire cathode without3-Pyridinecarboxylic acid electrolyte.The Faraday efficiency of hydrogen evolution reaction between the two cells is almost 99%.In addition,adding a small amount of Cr³⁺in the 3-methylpyridine electrolyte of WO₃ photoanode based cell can further improve the reaction efficiency of 3-Pyridinecarboxylic acid synthesis by means of Cr₂O₇^2-/Cr³⁺generated in the reaction process.（2）Based on the discovery in the previous chapter that H₂O₂ can drive the oxidation of 3-methylpyridine to 3-Pyridinecarboxylic acid,we first studied the formation of H₂O₂ in an electrolytic cell with In₂O₃ film as anode,graphite rod as cathode,KHCO₃ aqueous solution as electrolyte and proton exchange membrane.The experimental results show that H₂O₂（2H₂O→2 H⁺+2 e^-+H₂O₂,E^Θ=1.68 V vs.RHE）can be produced by the oxidation of water with two electron channels.The Faraday efficiency of the reaction is about 70%when E=2.2V vs.RHE.Using the same cell structure and KHCO₃/3-methylpyridine aqueous solution as electrolyte,the oxidation of 3-methylpyridine to 3-Pyridinecarboxylic acid can be realized at the constant current electrolytic reaction of 20 mA/cm².The experimental results show that H₂O₂ produced in the process of electrochemical oxidation of water on the anode of In₂O₃ film is the main active material to promote the oxidation of 3-methylpyridine to 3-Pyridinecarboxylic acid.In addition,two other isomers of 3-Pyridinecarboxylic acid,2-Pyridinecarboxylic acid and 4-Pyridinecarboxylic acid,can also be synthesized by the anodic oxidation of In₂O₃.The stability test shows that the physical and chemical properties of In₂O₃film anode have little change before and after 10 hours reaction at high current density,so it is an ideal electrode material.In addition,the selectivity of the anode target product Pyridinecarboxylic acid and the cathode hydrogen evolution efficiency are close to 100%.The electrochemical and photoelectrochemical synthesis of Pyridinecarboxylic acid proposed in this paper has the advantages of simple operation,mild reaction conditions,environmental protection,simultaneous production of hydrogen and so on,which conforms to the concept of sustainable development of organic synthesis.The research work of this paper can provide a reference for the green synthesis of picolinic acid,and also for the research of electrochemistry and photoelectrochemical synthesis of other organic compounds.