Electrochemical Selective Hydrodechlorination Of Multi-chlorinated Pyridine

It has high application value that selective hydrodechlorination of2-chloro-5-trichloromethylpyridine（TCMP）to 2-chloro-5-choromethylpyridine（CCMP）or 2-chloro-5-methylpyridine（CMP）and pentachloropyridine（PCP）to2,3,5,6-tetrachloropyridine（2,3,5,6-TCP）,in the preparation of the green efficient pesticide such as imidacliprid and chlorpyrifos.The first part of this paper is to investigate the feasibility of electrochemical selective reductive hydrodechlorination of TCMP to CCMP or CMP.Firstly,we investigated the influence of the cathode material and the cathode electrolyte on the selectivity of the reaction by constant current electrolysis method with an H-cell.Secondly,we investigated the influence of current density and the concentration of TCMP on the hydrodechlorination reaction with a plate and frame cell.Moreover,we also compared the voltage of different composition of electrolyte and the different types of electrolytic cell.At last,we developed a method to treatment the by-product salt by bipolar membrance electrodialysis.The experiment results showed that 0.1mol·L^-1 TCMP could be selectively dechlorinated to CMP with 93%yield and 47%current efficiency under the optimized conditions(Catholyte:an methanol solution containing 10 v/v%CH₃COOH+5 v/v%water+0.4 mol·L^-1 CH₃COOLi;Anolyte:1 mol·L^-1 aqueous solution;Current density:333.3 A·m^-2).We could get LiOH and HCl by bipolar membrance electrodialysis,the yield of them are 77.6%and 96.8%,respectively.The second part of this paper is to study the electrochemical dechlorination mechanism of PCP to 2,3,5,6-TCP on stainless steel cathodes in an acetonitrile/water mixed solution containing ZnCl₂ by cyclic voltammetry and constant potential electrolysis methods.Moreover,an optimization of different operating conditions for the dechlorination process was performed with an H-cell.The experiment results showed that indirect electrochemical dechlorination using Zn/Zn²⁺as the redox mediator is the main pathway of PCP to 2,3,5,6-TCP,and 0.08 mol·L^-1 PCP could be selectively dechlorinated to 2,3,5,6-TCP with 88.7%yield and 59.1%current efficiency under the optimized conditions(Catholyte:an acetonitrile solution containing 0.025 mol·L^-1 HCl+15 v/v%water+0.2 mol·L^-1 sodium benzenesulfonate+0.16 mol·L^-1 ZnCl₂;Current density:1.25 A·dm^-2).