Fixation Of CO₂ By Electrocarboxylation Of Organic Compounds

Today,with growing awareness in industry,academia and the general public ofthe need for sustainable development,the international chemistry community is underincreasing pressure to change current working practices and to find greener alternatives.Green chemistry,also known as sustainable chemistry,refers to environmentally friendly chemicals and processes that result in:reduced waste,eliminating costly end-of-the-pipe treatments;safer products;and reduced use ofenergy and resources—all improving the competitiveness of chemical manufacturersand their customers.One principle of Green Chemistry is to maximize atom economy,which is todesign syntheses so that the final product contains the maximum proportion of thestarting materials.Electron has been used as reagent in electroorganic synthesis,avoiding the use of other reducing agent and oxidant.This feature is often cited asbeing environmentally favorable.So compared with traditional organic synthesis,electrochemical synthesis is greener.Another principle of Green Chemistry is to use safer solvents and reactionconditions.Because of claimed advantageous chemical,physical and green propertiesrelated to negligible vapor pressure,low toxicity,high chemical stability,high thermalstability and the ability to dissolve a wide range of organic and inorganic compounds,ionic liquids have been suggested to provided a useful alternative to the use of volatileorganic solvents in organic synthesis,solar cell applications,solvent extractionprocesses.On the basis of high conductivity,significant electrochemical stability andwide potential windows,ionic liquids can be directly used as solvent and electrolytein electrochemistry,avoiding the use of other supporting electrolyte.Hence,ionicliquids have been recognized as green solvent.Carbon dioxide is the largest contributor to the green house effect,which mayincrease the earth average temperature to such a value that may cause catastrophicevents.Therefore great effort has been placed toward the reduction of CO₂ atatmospheric loading.On the other hand,CO₂ can be proposed as a Cl building blockin organic synthesis.Electrochemical carboxylation is one of the most useful methodsfor the fixation of CO₂ to organic molecules because it is a clean and environmentallybenign process.It takes place efficiently even in an atmospheric pressure of CO₂ under neutral and mild conditions to give carboxylic acids in high yield.2-Phenylsuccinic acids and cyano-phenyl-propionic acids are of interest due totheir biochemical properties and also are key molecules in synthesis of medicaments.In these preparations,however,there are several drawbacks:the use of hazardousreagents,expensive and complicated catalysts,high pressure and high temperature.Therefore,alternative methods of synthesis are highly desirable.Under mild condition,fixation of greenhouse gas-CO₂,to synthesis ofcarboxylic acids and corresponding esters by electrochemical synthesis,is veryimportant.Especially,use of ionic liquids as solvent for electrochemical fixation ofCO₂ is significant.The details are given as follows:(1) Electrosynthesis of 2-phenylsuccinic acidElectrochemical dicarboxylation of styrene in the presence of atmosphericpressure of CO₂ with a Ti cathode and a Mg rod anode readily took place efficiently ina MeCN solution containing 0.1 mol L^-1 Et₄NBr to give 2-phenylsuccinic acid.Influences of the nature of the electrodes,the current density,the passed charge andthe temperature on electrolyses were studied to optimize the electrolytic conditions,with the maximal isolated yield to be 86.1%.The mechanism of theelelctrocarboxylation process has been studied by cyclic voltammetry.(2) Electrochemical reduction and carboxylation of ethyl cinnamateThe electrochemical reduction and carboxylation of ethyl cinnamate has beencarried out in undivided cell equipped with Mg sacrificial anode and using MeCN assolvent.Directly electroreduction leads to the formation of the hydrodimers andsaturated ester.And electrocarboxylation was carried out in the presence of CO₂.Theglobal yield and the ratio of mono-and dicarboxylic acids are strongly affected byvarious factors:electrode material,electrolysis potential,the substrate concentrationand temperature.The highest yield(78%)was obtained at the optimized reactioncondition(cathode:Ni;electrolysis potential:-1.7 V;substrate concentration:0.1 molL^-1;temperature:-10℃).(3) Electrocarboxylation of cinnamate estersElectrochemical carboxylation of cinnamate esters has been carried out bycathodic reduction of C=C bond in an undivided cell equipped with Mg sacrificialanode and using MeCN saturated with CO₂ as solvent.The yield and the ratio ofmono-and dicarboxylatic acids are strongly affected by various factors:cathodic material,current,charge and temperature.The highest yield(78.9%)was obtainedstarting from ethyl cinnamate.Cyclic voltammograms have been measured andreaction pathways have been proposed.(4) Electrochemical reduction and carboxylation of cinnamonitrileCyano-phenyl-propionic acids were synthesized simply and efficiently byelectrocarboxylation of cinnamonitrile in undivided cell using non-noble metal nickelas cathode and magnesium as anode.The radical anion generated by theelectroreduction of cinnamonitrile in the absence of CO₂,is involved in severalcompetitive reactions which lead to the formation of linear hydrodimer,cyclichydrodimer,saturated dihydro product and glutaronitrile derivative.While under 1atm of CO₂,the electrocarboxylation could easily be carried out in the absence ofadditional catalysts,and with good yield(84.8%).The influence of various synthesisparameters,such as the nature of the electrode,the working potential,theconcentration and the temperature,to the electrocarboxylation reaction wasinvestigated.(5) Electrocarboxylation of activated olefins in ionic liquidThe feasibility of electrocarboxylation of activated olefins has been investigatedin CO₂-saturated room-temperature ionic liquid BMEMBF₄ solution for the first time.The electrochemical behavior has been studied on GC electrode by cyclicvoltammetry,showing a diffusion controlled irreversible reduction process.Thesynthesis has been carried out under mild(P_CO2=1 atm,t=50℃)and safeconditions in undivided cell,and the use of volatile and toxic solvents and catalysts,as well as of any supporting electrolyte,was avoided.Monocarboxylic acids wereobtained in moderate yield(35%-55%).Furthermore,the ionic liquid has beenrecycled for five times,which did not affect the product yield greatly.(6) Electroreduction of coumarin in ionic liquid BMIMBF₄The electroredution of coumarin was studied in ionic liquid(BMIMBF4).Theinfluence of sweep rate,concentration and temperature by cyclic voltammetry,showsthat the reaction was irreversible and diffusion controlled.The diffusion coefficientD of coumarin in BMIMBF4 was 5.066×10^-7cm² s^-1.The product of constantpotential electrolysis is dimeric coumarin.And the same time the influence of waterand substituted group to the electroreduction was discussed.These preliminaryelectrochemical experiments appear as encouraging results for the use of ionic liquidsin a "green" electrochemistry. (7) Electrocatalysis of PhCH₂Cl and CO₂ by Co(L)in ionic liquidsThe electrochemical behavior of CoL(cobalt Schiff base complex)has beenstudied in ionic liquids.Its catalytic reactivity towards the reduction of PhCH₂Cl andthe reaction between PhCH₂Cl and CO₂ has also been investigated.CoL shows adiffusion controlled one-electron reversible peak couple in ionic liquids.Theelectrochemical behavior shows that the peak potential will not be influenced by theSchiff base ligand and the anion of the ionic liquids.CoL could catalyze the reductionof PhCH₂Cl by the voltammograms.The mechanism involves a one-electronreduction and a chemical reaction followed by another one-electron reduction.Andthis system could also catalyze the reaction between PhCH₂Cl and CO₂.Controlled-potential electrolysis led to (PhCH₂)₂CO,indicating that CO₂ can be fixedin ionic liquids by the catalysis of CoL.(8) Electrochemical properties of new ionic liquids dialcarbsThe viscosity,conductivity and electrochemical stability of several distillableroom temperature ionic liquids dialcarbs are reported.The temperature dependence ofviscosity and conductivity are analyzed for MEETCARB,and the influence of the ionsize on conductivity is discussed.The voltammetry of IUPAC recommended potentialscale reference system,cobalticenium/cobaltocene has been evaluated found to beideal in dialcarbs.Potential windows of dialcarbs have been compared on GC,Au,and Hg electrode,using Cc^+/0as reference.Diffusion coefficients and double layercapacitances have also been calculated.Steady-state behavior has been investigatedby using RDE.