Synthesis Of Ionic Liquids And The Electroreductive Property Of Nitrobenzene In These Ionic Liquids

RTILs exhibit several desirable properties,such as negligible volatility,thermal and chemical stabilities,ability to dissolve a wide range of organic or inorganic compounds,wide electrochemical window and good intrinsic conductivity.They can be used to solve the problems in the process of nitrobenzene electroreduction,including poor solubility,many byproducts and serious pollution.In this paper,three typical sorts of ILs:RMimBF₄,RMimPF₆,HR₁imR₂CO₂,have been selected,designed and synthesized.The interaction between ILs and nitrobenzene has been investigated employing ultraviolet spectroscopy.The characteristics of nitrobenzene electroreduction in these ILs were studied systematically by cyclic voltammetry,stable polarization curve,constant potential electrolysis and altemate current（A.C.）impedance spectroscopy.The main results are as following.1.Three sorts of ILs:RMimBF₄、RMimPF₆、HR₁imR₂CO₂,were synthesized. Among them,HMimHCO₂ was synthesed firstly.These ILs have good conductivity, wide electrochemical window.2.The ultraviolet spectroscopies of nitrobenzene in ILs and cyclohexane were investigated.Compared with the ultraviolet spectroscopy in cyclohexane,red shift effects and hypochromic effects are observed in ILs.And no end absorption was found in ILs when the wavelength is smaller than 220 nm.These phenomena can be ascribed to the strong interaction between ILs and nitrobenzene.The interaction decreases in the order HMimHCO₂＞BMimPF₆＞EMimBF₄.3.The electroreductive characteristics of nitrobenzene in these three ILs were investigated in detail on platinum electrode.1)nitrobenzene can be electroreduced in RMimBF₄,RMimPF₆ and HMimHCO₂. In RMimBF₄ and RMimPF₆,the electroreduction process of nitrobenzene contains two molecules and three electrochemical reactions with 8 electrons transferred.In the first step nitrobenzene is reduced to the corresponding anion radical.Subsquently,the latter is further reduced to nitrosobenzene anion radical with the transfer of 2 electrons in the second step followed by the formation of azoxybenzene through coupling reaction of nitrosobenzene anion radical.When the electrode potential shifts to enough negative value,the azobenzene can be obtained from the 2-electron reduction of azoxybcnzene. While in HMimHCO2,nitrobcnzene is reduced to phenylhydroxylamine firstly accompanied with 4 electrons transferred;and then phenylhydroxylamine can be converted to aniline.Simultaneously,some other products（i.e.azobenzene and azoxybenzene）are detected in the electrolyte,which can be attributed to the coupling of reaction intermediates.2)The effects of nitrobenzene concentration in ILs on its electrochemical behavior were investigated by cyclic voltammetry.In EMimBF₄、BMimPF₆,with the increasing of nitrobenzene concentration,unrcmarkable changes can be found on the first electroreduction peak potential,while the second peak potential is shifted negatively. And currents of these two peaks are increasing.In HMimHCO₂,the reduction peak potential becomes more negatively and peak current is increasing with the increase of nitrobenzene concentration.3)Under the experimental conditions,the electroreduction of nitrobenzene in these three sorts of ILs is controlled by diffusion process.When the tcmperature of electrolyte increased or vigoroso agitatation applied,the controlled step can be transterred to electrochemical reaction step.4)With the increasing of nitrobcnzene concentration in ILs,the kinetic parameters ofα,k_s is decreased,while different trends of Do in these three ILs have been observed. Do is decreased in EMimBF₄ and no significant changes in HMimHCO₂ is found,while it is reduced at low concentration and increased at high concentration of nitrobenzene in BMimPF₆.At the same concentration of nitrobenzene,the value ofα,k_s,Do increases in the order HMimHCO₂＞EMimBF₄＞BMimPF₆.5)For the same kind of ILs,the reaction current and Do of nitrobenzene are decreased with more carbon numbers in side chain of imidazole cycle.6)With the increasing of water amount,the changes of kinetics parameters of nitrobenzene electroreduction arc different in these ILs.In RMimBF₄-H₂O solution,the peak potential is shifted positively,and the peak current is increased.So as the changes of k_s and Do.In HMimHCO₂-H₂O solution,the peak potential is shifted positively,but the peak current andαis increased at low water concentration and decreased at high water concentration,Do is reduced.7)The addition of sulfuric acid results in complicated influence on the behavior of nitrobenzene eleetroreduction in EMimBF₄.When total concentration of water and sulfuric acid is low,the electroreductive behavior is similar to that in EMimBF₄-H₂O solution.When total concentration of water and sulfuric acid is high,electroreductive behavior is similar to that in the acidic aqueous solution.If the total concentration of water and sulfuric acid exceeded a certain value,the electroreduction process is disturbed by the reaction of electrolyte itself.8)The voltammogram of nitrobenzene in HEimCF₃CO₂ is similar to that in HMimHCO₂.When the concentration of nitrobenzene is the same,the peak potential and the peak current are approximately equal in these two ILs.But the peak potential is more negative and the peak current is smaller in HMimCH₃CO₂ than those in HMimHCO₂.