Electrochemical Study Of Gases In Hydroxyl Functionalized Ionic Liquids

The immediate need to reduce greenhouse gas emissions has brought great attention to CO2 capture and storage (CCS). While CO2 removal from natural gas increases the fuel value, H2S removal is also crucial, because it is extremely toxic. Combustion of fossil fuels (petroleum, coal, and natural gas) results in emission of SO2 has also been paid increasing attention all over the world. The most viable near-term approach to post-combustion acid gases (CO2, H2S and SO2) capture is chemical or physical absorption.Ionic liquids (ILs) have been extensively used as green solvents in organic synthesis, extraction and electrochemistry due to their unusual properties of high thermal stability, large electrochemical window, high conductivity and tunable solubility. Meanwhile, ILs attracted significant attention in acid gas capture because of their nearly zero partial pressure, about one-third of the specific heat capacity of water in a wide range of temperatures, and high solubility of acid gases. Thus, their use for acid gases capture may present various advantages over aqueous systems. The combination of ILs with alkanolamines for thermally reversible capture of CO2 has been become a popular research topic, although the solubility of the amine and hence the capturing capacity are amongst the concerns. Mono-ethanolamine (MEA) is fairly soluble in common ILs and the imidazolium groups in hydroxyl functionalized ionic liquids (HFIL). Both systems achieve enhanced or similar CO2 absorption compared with aqueous systems. Particularly, the acid gases capture requires further processing. Utilization of the captured acid gases can be achieved by prolonged potentiostatic electrolysis in MEA-HFIL.ILs are also being used in electrochemical applications, since they have intrinsic conductivity and wide electrochemical windows. Electrochemical gas sensing is an area in which ILs have been explored as solvents. ILs, especially after functional design, were found to have better solubility in wide range of gases.The high solubility of CO2, H2S, SO2 in ILs, combined with the ILs’favorable properties of often negligible vapor pressure and high thermal stability, suggests that these liquids may be successfully employed as gas sensors in conditions where sensors incorporating conventional solvents are prone to drying out or degrading over time. Our findings in this study suggest that the combination of ILs with alkanolamines can absorb large amounts of acid gases, which favor the sensitivity of gas sensors. Furthermore, the selectivity of ILs can also been improved due to the appearance of characteristic peak in cyclic voltammetry while CO2 or H2S react with the MEA.In this work, the electrochemical behavior and functions of MEA-HFIL in acid gas capture has been studied. Especially, we hope to establish a method to convert CO2 charged solution to other useful chemicals or materials. Moreover, the electrochemical oxidation of hydrogen gas (H2) has also been analyzed, which is of major importance in fuel cells and sensor application. The main contents and results are as follows:1. Several non-chloroaluminate ILs and task-specific ILs have been synthesized according to pervious works of our lab. The physicochemical properties and electrochemical characteristics of these synthesized ILs have been investigated. Our experimental data demonstrated that the HFILs showed better solubility in wide range of organic small molecules and melt salts.2. The oxidation of H2 at a platinum microelectrode was investigated by cyclic voltammetry in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6), 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Bmim]Tf2N), and 1-(3-hydropropyl)-3-methyl-imidazolium tetrafluoroborate ([C3OHmim]BF4). The oxidation waves were nearly electrochemically reversible at room temperature. A wide range temperature study (30～180℃) has also been carried out in [Bmim]Tf2N. Due to the high thermal stability and wide electrochemical window of [Tf2N]--based ILs, H2 oxidation could be performed and high current densities were observed at 180℃. 3. Enhanced and stabilized CO2 capture by MEA at 60℃was achieved in 1-(3-hydroxypropyl)-3-methylimidazolium ([C3OHmim]+) based ILs through replacing the common tetrafluoroboride (BF4-) ion with the chloride ion (Cl-). The absorbed CO2 and the MEA in the IL, in consistence with thermodynamic and kinetic analyses of the reaction:CO2+2HOC2H4NH2 (MEA)(?)HOC2H4NH3+(MEAH+)+ HOC2H4NHCOO- (MEACO2-).’H NMR spectra of the IL solutions of MEA before and after CO2 absorption revealed the Cl- ion to be more interactive than the BF4- ion with proton containing cations, i.e. [C30Hmim]+and MEAH+. These interactions retarded the backward reaction between MEAH+ and MEACO2- to release CO2 at higher temperatures, and also prevented the combination of MEACO2 with these cations into insoluble carbamate salts. Further enhanced and stabilised CO2 capture was achieved by electrolysis of the CO2 charged chloride solution. The anodic and cathodic reactions produced Cl2 and H2 gases, respectively, leading to continuous in situ regeneration of MEA and conversion of CO2 into the carbamate. This work may contribute to further development in carbon capture and reclamation (CCR) as an active approach to, at least, complement the passive strategy of carbon capture and storage (CCS).4. Electrochemical oxidation and reduction of H2S have been studied in [Bmim]PF6, [C3OHmim]BF4 and MEA-[C3OHmim]BF4 at a platinum microelectrode. The potential at which the H2S is detected is quite high in ILs. The use of MEA is an attractive option to decrease the potential at which H2S is detected, and may also offer some selectivity towards this specific gas. Upon imposing MEA, high adsorption ability of H2S will result in the formation of bulk MEAH+ and HS-. In addition, the oxidation of HS- will be expected to improving the selectivity of gas sensors. The H2S in MEA-[C3OHmim]BF4 solutions reflected high current densities and the subsequent treatment of H2S were also investigated by using electrolysis technique.5. [Bmim]PF6 and [C3OHmim]BF4 can absorb large amounts of SO2 gas (0.37-0.42 moles SO2 per mole IL) at room temperature and ambient SO2 pressure, indicating that ILs may be useful for SO2 removal regarding pollution control. Furthermore, MEA-[C3OHmim]BF4 solutions studied exhibited rapid SO2 uptake, and could be capable of capturing 1 mol of SO2 per 1 moles of dissolved ME A. The reduction of SO2 at a platinum microelectrode was investigated by cyclic voltammetry in [Bmim]PF6, [C3OHmim]BF4 and MEA-[C3OHmim]BF4 showed well-defined voltammetry and high current densities. However, the effects of these ILs with adding MEA were less than that of the H2S system.One possible reason may be the high solubility of SO2 in neat [C3OHmim]BF4. The high sensitivity of the system to SO2 also suggested that ILs and MEA-HFIL may be a viable solvent in gas sensing applications.