Study On Electrochemical Fixation Of CO₂ And Synthesis Of Organic Compounds

As the more paying to protect the environment of the world, the conception ofGreen Chemistry is appeared. Green Chemistry is fargoing which includs the processthat eliminate hazardous substances, producing environmentally friendly product orgreen process. Carbon dioxide (CO₂) is the primary component of greenhouse gases.Converting it to chemical products can both reduce the environmental pollution bycontrolling the emission of greenhouse gases and utilize the cheap and abundant C1resource. As it is thermodynamically stable how to active CO₂ becomes so difficult. Inaddition, electrochemistry is one of green synthesis route since it carries out throughgain or loss of electron. In principle, it can avoid other reagent which can decrease theconsumption of the substance and reduce the environmental pollution. Ionic liquidsare novel green solvents. They can be used as medium for organic electrochemistrywithout any additional supporting electrolytes because has their own conductivity.Organic carbonates are an important class of compounds whose versatility allowstheir application in several fields of the chemical and pharmaceutical industry.Dimethyl carbonate is one of the organic carbonates which can be used as amethylation and carboxylation agent in organic synthesis. It is not toxic andconsidered as "Green" fundamental chemical material. DMC is synthesized by thephosgene of methanol an oxidative carbonylation of methanol, etc. These routes havethe disadvantanges including pollution, the short life of the catalysts, difficulty toseparate and many operational steps. The direct synthesis of DMC from CO₂ andmethanol is one of the green routes for envioromental protection. If the reaction takesplace, it will need severe conditions including higher temperature, higher pressure andthe presence of the catalyst. Carboxylic acids andα-hydroxyacids are obtained greatinterest in the pharmaceutical industry and organic synthesis. Thoughelectrocarboxylation of aromatic ketone has been reported by several workers,α-hydroxyacids have been formed in low yields or highly toxic mercury electrode orexpensive Pt electrode.The study on electrochemical fixation of CO₂ to organic compound includingorganic carbonates,α-hydroxyacids and carboxyled acid is few especially in China.Electrochemical electrochemical fixation of CO₂ has the advantange including highefficiency, simply operation and mild conditions. Specially, application of ionic liquid in fixation of CO2 is a practical significance contents.The details are given as follows:1 Electrochemical behavior of CO₂ in organic solventsThe electrochemical behavior of CO₂ in organic solvent is first studied in detail.The influence of working electrode, temperature and solvents on CO₂ reduction isinvestigated by cyclic voltammetry. The results show that the reduction of CO₂ is anone-eletron irreversible process by diffusion control which generates an anion radical.The nature of working potential has some influence on the peak potential and currentsof CO₂ reduction. Though the solubility of CO₂ will decrease with increasing thetemperature, higher temperature is benefit for its reduction. The influence of solventssuch as acetonitrile (MeCN), dimethyl formamide (DMF) and dimethyl sulfoxide(DMSO) on CO₂ reduction is studied. Because solvent nature and the solubility ofCO₂ in three solvents are not the same with each other, the electrochemical behavioris different from one another. The transfer coefficient of CO₂ in MeCN, DMF andDMSO is 0.064, 0.042 and 0.058, respectively. The diffusion coefficient of CO₂ inMeCN, DMF and DMSO is 8.981×10^-6,1.019×10^-6 and 1.032×10^-6 cm²Â·s^-1,respectively. The results of this chapter can provide the academic basis forelectrochemical activation of CO₂ to organic compounds in organic solvents.2 Electrochemical behavior of CO₂ in ionic liquid (BMIMBF₄)The electrochemical behavior of CO₂ in ionic liquid, 1-butyl-3-methylimidazolium (BMIMBF₄) is first studied in detail. The influence of workingelectrodes, temperature on CO₂ reduction is investigated by cyclic voltammetry. Theresults show that the reduction of CO₂ is an one-eletron irreversible process togenerate an anion radical. Compared with other solvents, the peak potential inBMIMBF₄ is more positive shift (about 0.3～0.65 V). It shows this ionic liquid is moresuitable for CO₂ reduction. The nature of working potential has some influence on thepeak potential and currents of CO₂ reduction. Though the solubility of CO₂ willdecrease with increasing the temperature, higher temperature can decrease theviscosity of ionic liquid and is benefit for its reduction. The transfer coefficient ofCO₂ in BMIMBF₄ is about 0.30. Influence of the mixture of BMIMBF₄ and MeCN onCO₂ reduction was investigated. The peculiar property of the mixture as the properaddition of MeCN makes the reduction easier. The results of this chapter can providethe academic basis for electrochemical activation of CO₂ to organic compound inBMIMBF₄. 3 Electrocatalytic synthesis of propylene carbonate from CO₂ and propyleneoxideThe electrosynthesis of propylene carbonate from CO₂ and propylene oxide werecarried out under constant current. The effects of supporting electrolytes, electrodes,current densities and charge etc on the yield of propylene carbonate have beenexamined to optimize the synthetic conditions. After optimizing the syntheticparameters, the yield of PC reached 46.2ï¼…on Stainless steel-Mg couple electrodesunder a constant current until 1.5 F·mol^-1 of charge with a current density of 2.47mA·cm^-2 passed through the cell when 0.191 mol·L^-1propylene oxide was applied at25℃in MeCN solution containing 0.1mol·L^-1 TEABr as supporting electrolytes. Theresults showed that supporting electrolyte bearing halide ions act as both supportingelectrolyte to make the solution electric and catalyst. The electrosynthesis took placewith the catalytic effects of the Lewis alkali (halide ions) and Lewis acid (Mg²⁺).4 Electrocatalytic synthesis of dimethyl carbonate by esterifieation or ureamethodWe combined the electrochemical with esterification to realize electrocatalyticsynthesis of dimethyl carbonate and 1,2-propanediol from propylene carbonate andmethanol. The effects of supporting electrolytes, electrodes, current densities, charge,concentration of methanol etc on the yield of DMC have been examined to optimizethe synthetic conditions. After optimizing the synthetic parameters, the yield reached82.3ï¼…on Cu-C couple electrodes under a constant current until 2.0 F·mol^-1 of chargepassed through the cell with a current density of 10.69 mA·cm^-2 when 0.119mol·L^-1propylene carbonate was applied at 25℃in methanol solution containing0.1mol·L^-1 TEAI as supporting electrolytes. The results showed that supportingelectrolyte bearing halide ions act as both supporting electrolyte to make the solutionelectric and catalyst. The electrosynthesis took place under the cooporation of Lewisalkali (halide ions) and electrocatalyst. In addition, the electrocatalytic synthesis ofDMC from urea and methanol was primarily studied.5 Electrosynthesis of dimethyl carbonate from CO₂ and methanol in aeetonitrileElectroactivation of CO₂ to dimethyl carbonate with methanol in acetonitrile wasrealized. It provides a new route to synthesis DMC. The effects of supportingelectrolytez, solvents, working potentialz and concentration of methanol etc on theyield have been examined to optimize the synthetic conditions. After optimizing the synthetic parameters, the yield reached 14.9ï¼…on Cu-Mg couple electrodes at aconstant potential of -2.3 V vs. Ag/AgI until 1.0 F·mol^-1 of charge passed through thecell when 0.165 mol·L^-1methanol was applied at 25℃in MeCN solution containing0.1mol·L^-1 TEABF₄ as supporting electrolytes. When the alcohol is ethanol or benzylalcohol, the yield of corresponding carbonate is 15.7and 3.1ï¼…, respectively. The resultshowed that this method is suitable for electroactivation of CO₂ to synthesizecarbonates. The mechanism is presumable that the generation of CO_2- reacted withthe methanol to form the aim product after addition of CH3I.6 Eleetrosynthesis of organic carbonates from CO₂ and alcohols in BMIMBF₄Electroactivation of CO₂ to organic carbonates with alcohols in ionic liquid(BMIMBF₄) was realized. It provides a clean and new route to synthesis organiccaronates. The synthesis of DMC is as example. The effects of temperature,electrodes, working potentials and concentration of methanol etc on the yield of DMChave been examined to optimize the synthetic conditions. After optimizing thesynthetic parameters, the yield reached 73.4ï¼…on Cu-Mg couple electrodes at aconstant potential of-1.8 V vs.Ag/AgI until 1.0 F·mol^-1 of charge passed through thecell when 0.124 mol·L^-1 methanol was applied at 55℃in BMIMBF₄. Furthermore, theionic liquid can be recycled. When other alcohols is as the raw material, the yield ofaim product (diethyl carbonate, n-butylethyl carbonate, sec-butylethyl carbonate,benzylethylcarbonate and phenethyl ethyl carbonate) was 67.3, 52.3, 56.7, 37.6 and32.8ï¼…. The obtained results show that the primary and secondary alcohols areconverted in good yields, whereas tertiary alcohol and phenol are unreactive. Themechanism is proposed that CO_2-(which is formed by CO₂ reduction and stablethrough the formation of CO_2-—BMIM⁺ ion-pairing) reacted with the alcohol toform the aim product after addition of alkylating agent.7 Electrocarboxylation of acetophenone to 2-hydroxy-2-phenylpropionic acid inthe Presence of CO₂Electrocarboxylation of acetophenone to 2-hydroxy-2-phenylpropionic acid in thepresence of CO₂ was carried out in acetonitrile solution containing 0.1 mol·L^-1tetraethylammonium bromide. Influences of the nature of the electrodes, the workingpotentiasl, the passed charge and the concentration of acetophenone on theelectrocarboxylation have been studied. After optimizing the synthetic parameters, theisolated yield of 2-hydroxy-2-phenylpropionic acid reached 73.0ï¼…on Mg-stainless steel couple electrodes at a constant potential of-l.7 V vs. Ag/AgI until 2.2 F·mol^-1 ofcharge was passed when 0.1 mol·L^-1 acetophenone was applied at 25℃. Thereduction of acetophenone was studied by cyclic voltammetry and the mechanism hasbeen proposed on the basis of the results. It indicated clearly that the mechanism ofacetophenone was a first reduction followed by an EC type mechanism. And an ECEmechanism involving two CO₂ molecules has been proposed. A one-electronreduction of acetophenone took place to generate the radical anion. The radical anionreacted as a nucleophile with CO₂ forming a carboxylated radical anion. Furtherone-electron reduction followed by reaction with CO₂ led to formation ofintermedium. Through the protonation step, where one CO₂ molecule was lost, theaimed product formed. The Mg²⁺ from anodic oxidation dissolved in the electrolytereadily captured the aimed product to form stable magnesium carboxylate, which gave2-hydroxy-2-phenylpropionic acid after an acid treatment.8 Electrocarboxylation of Anthrone to Anthracene-9-carboxylic Acid in thePresence of CO₂Electrocarboxylation of anthrone with CO₂ to anthracene-9-carboxylic acid wasstudied. The reduction of anthrone in the absence and presence of CO₂ was studied bycyclic voltammetry. The reduction of anthrone is irreversibile process and the transfercoefficient is 0.24. The diffusion coefficient is 8.597×10^-7 cm²Â·s^-1. The mechanismhas been proposed on the results. Because the hydrogen on the C9 carbon (C9-2H) veryactive, it reacted with the hydroxyl and the rearrange of the ring to form theproduct(Anthracene-9-carboxylic Acid).Influences of supporting electrolyte, the natureof the electrodes, working potentials and the concentration of anthrone etc on theelectrocarboxylation were studied. After optimizing the synthetic parameters, themaximal yield of anthracene-9-earboxylic acid reached 96.1ï¼…on Mg-stainless steelcouple electrodes at -1.5 V until 2.0 F·mol^-1 of passed charge at 0℃. Under theoptimal conditions, the electrocarboxylation of other ketones was studied. There aresome primary conclusions that this method is suitable for aryl ketone but not for alkylketone.