Electrochemical Hydrodehalogenation Of Chlorinated And Fluorinated Aromatic Compounds Catalyzed By Silver, Palladium And Rhodium

One of the most important chemical reactions in organic electrochemistry is the electrochemical reductive dehalogenation reaction, which has important applications in organic synthesis and treatment of environmental pollutants. After the mass production and widespread use for nearly a century, chlorinated organic compounds(COCs) have caused serious environmental problems all over the world. In recent years, the use of fluoroorganic compounds(FOCs) has become a great concern in the modern society for their wide and increasing applications. Similar to other halogenated organic compounds, transforms the C–F bond of FOCs to C–H can then greatly increase the biodegradability of the compounds. In order to perfect the dechlorination method of persistent COCs and develop a technology for the electrochemical treatment of fluoroaromatic pollutants under mild conditions, research contents carried out in this paper are as follows:(1) The electro catalytic reaction of benzyl chloride in acetonitrile solution was studied by cyclic voltammetry(CV), and the electron transfer coefficient(α) in reductive process was calculated to determine the electron transfer mechanism;Using rough silver electrodes(Ag(r)), preparaed by an electrochemical oxidation-reduction cycle(ORC) process, research focuses on the catalytic effect of silver cathodes on 3,4,5,6-tetrachloropicolinic acid(TeCP) dechlorination in aqueous solutions were carried out. Transmission electron microscopy(TEM) image and X-ray photoelectron spectroscopy(XPS) spectra were presented firstly to observe the surface structure and chemical state of Ag(r); in addition, density functional theory(DFT) calculations of the first electron transfer(ET) process integrated with an in situ electrochemical surface-enhanced Raman spectroscopy(SERS) study and a cyclic voltammetry(CV) experiment with the aid of H+ were carried out to characterize various surface species in different electrode potential regions.(2) An electrocatalytic hydrogenolysis(ECH) technique was tried to dechlorinate Clopyralid Manufacturing Waste(CMW), which mainly comprised of 3,5,6-trichloropicolinic acid(3,5,6-T), 3,6-dichloropicolinic acid(3,6-D), 3-chloropicolinic acid(3-ClPA) and 6-chloropicolinic acid(6-ClPA). Firstly, the effect of cathode substrates and Pd loading on the catalytic performance of the cathodes was performed using 3,6-D as the representative of CMW; In addition the effect of electrolyte composition on the product selectivity and current efficiency(CE) of the ECH dechlorination were evaluated; what’s more, the ECH dechlorination mechanism of 3,6-D was analyzed with regard to the effect of Pd loading and electrolyte composition. Finally, the practicability of dechlorinating of CMW to PA was assessed using the ECH technique.(3)A variety of rhodium modified catalytic cathodes were prepared by chemical substitution or electrochemical deposition method. Using 4-fluorophenol(4-FP) as a model molecule of fluoroaromatic(FA) pollutants, the effect of substrate materials, catalysts and various process parameters on the efficiency of fluoride removal were investigated under mild conditions(room temperature and pressure, water medium, air atmosphere, without the use of hazardous reagents). Newly developed electrochemical hydrodefluorination(HDF) system was used in the degradation of 18 representative FAs.(4) Scanning electron microscope(SEM),Energy dispersive spectrometer(EDS) and XPS were presented firstly to observe the surface structure and chemical state of rhodium modified cathodes. Effect of support materials on electrochemical HDF of 4-FP on rhodium modified cathode was investigated by CV and Chronopotentiometry(CP) experiments; Effects of pH and the type of anion in catholyte on rhodium modified cathode reductive behavior were tested by cyclic voltammetry; In addition, the catalytic mechanism and reaction pathway of 4-FP electrochemical HDF on Rh/foam Ni electrode were studied by CV, CP, and constant current electrolysis.Through the research contents above, following conclusions can be obtained:(1) Compared to GC electrode, reductive peak potential of benzenyltrichloride and benzyl chloride on Ag was significantly positive shifted. In a certain concentration, the dechlorination of benzyl chloride is controlled by diffusion, and that of benzenyltrichloride is controlled by both electrochemical and diffusion; Both TeCP and its intermediate(TeCP?-) can be adsorbed on Ag(r) electrode, and TeCP?- is the key intermediate in the dechlorination of TeCP. In acidic aqueous solution, C-Cl bond rupture of this radical anion is likely to be concerted with the first electron transfer, followed by the second electron transfer and further protonated, thereby generating 3,5,6-T. The dechlorination mechanism could drastically differ from the outer-sphere reduction at GC electrode.It is this great difference of dechlorination mechanism between Ag and GC causes large positive shift of the reduction potential of the Ag(r) electrode.(2) When the initial 3,6-D concentration region of 200–300 mM, moderate current density(208 A m-2) and Pd/Ni foam with a Pd loading of 2.25–3.6 mg cm-2 was used, the highest dechlorination efficiency can be obtained. Two Cl atoms in the 3,6-D molecule were eliminated stepwise at pH > 14, whereas the atoms were removed together in one step at pH < 0. The practicability research demonstrated that the 47 g L-1 ClPA mixtures(total concentration of ClPAs: approximately 250 m M) can be converted into PA with 99% yield, 76.3% CE, and 2.47 kW·h·kg-1 PA SEEC in the optimized ECH dechlorination system. The rate-limiting step of the dechlorination process changed along with the variation of reactant concentration in the catholyte and applied current density. The dechlorination process was kinetically limited by the generation of adsorbed hydrogen at low applied current density, whereas it may has been kinetically limited by the hydrogenation step and/or adsorption of reactants at low concentration of reactants in return.(3) When using water of pH = 2.7–3.2 as the reaction medium and Rh/Ni foam as the cathode, both the conversion of 4-FP and the yield of F–were 100%. Rapid and complete hydrogenation degradation of 18 representative FAs to form nonfluorinated organics and F- under mild conditions was realized on Rh/Ni foam electrode. The apparent HDF rate of the 18 FAs in the HDF system decreased in the following order: fluoropyridine > fluorophenol ≈ fluoroaniline fluoroanisole ≈ fluorotoluene ≈ fluorotoluene > fluorobenzoic acid > fluorobenzonitrile.(4) The degradation path of 4-fluorophenol in the optimizing HDF system was demonstrated as following: 4-fluorophenol â†' phenol â†' cyclohexanone â†' cyclohexanol. In addition, the easy of hydrogenation for the former three substances is decreased in the following order: phenol > 4-fluorophenol > cyclohexanone. Moreover, a reaction mechanism was proposed for the degradation process, in which the catalytic hydrogenation between adsorbed FAs and chemisorbed hydrogen on rhodium is the key step. Anion type and pH of the cathlyte can significantly influence the electrochemical behavior of the rhodium modified electrode. The cathode supports has an important influence on the electrochemical HDF of 4-FP over rhodium modified cathode. The reason lies in that the ease of oxygen reduction,hydrogen evolution,and hydrodefluorination varies with cathode support.