| Although benefiting from fast development of economy, we unfortunately also had to face up the serious environmental pollution problems. Among all kinds of industries, chemical and energy-related ones make the most important contributions to the pollution. In order to control and fix the pollution from the chemical and energy-related industries, the concept of green chemistry, which focuses on avoiding pollutants before they are produced and using resource reasonably and reversibly, has been proposed and attracted significant interest in the decades.The removal of acidic gases, such as carbon dioxide (CO2), sulfur dioxide (SO2) is of significant importance for environmental protection, effective fuel utilization and chemical processes. Nowadays, technologies of trapping CO2using aqueous organic amines, inorganic base solutions and limestone as absorbents have extensively carried out in the chemical plants. However, these technologies often accompanied with resource consumption due to the irreversible reaction of absorbents with CO2(in case of inorganic base solutions and limestone) or organic pollutants releasing to the atmosphere and aqua-sphere (in case of aqueous organic amines). Additionally, high operation costs and large consumption of energy are also drawbacks of these processes. As an alternative to traditional organic solvents, ionic liquids (ILs) provoke a wide interest in scientific community and industrial fields, involving its applications in organic synthesis, catalysis, supercritical fluid reaction and gas separation due to their outstanding characteristics such as negligible vapor pressure, high thermal stability and so on. Comparing to the volatile amines as sequestering agents, ILs possess the non-volatile and thermal stable characteristics that make them environmentally benign gas absorbents without causing concurrent loss of the liquid into the gas and then polluting the environment. Some pioneering works have shown that ILs could reversibly trap acidic gases with high capacities. Inspired by these works, this paper designed and synthesized novel task-specific ionic liquids in order to capture acidic gases. For the first time, novel TSILs (such as [N2222][L-Ala]) with the low viscosity were prepared, which have shown good peformance of trapping CO2. Then, TSILs composed of asymmetric tetraalkylammnoium and carboxylic acid were synthesized. It is interesting to find that the full-deprotonted carboxylate ILs have the largest absorption capacity of CO2, while the half-deprotonted ones can effetively trap SO2even though they are weak acidic. These results have proved that the idea of designing and preparing TSILs for the separation of the acidic gases is worth to study in the future and the TSILs with amino acid or carboxylate anions are very attractive for industrial applications. After characterization of the physical chemical parameters and the determination of the absorption properties, ILs with advantages such as easy to prepare, low viscosity and high absorption rate were chosen to be additive species blended with aqueous organic amines (MDEA). Then, the physical chemical parameters such as density, viscosity and surface tension of the blended absorbents were determined as well as the absorption properties. Experimental results have shown that adding ILs into MDEA is a promising way to utilize ILs in the industry without large operation cost. Several important conclusions have been obtained and summarized as follows:(1) The relation between the viscosity and the structure of the ILs:normally, the addition of carbon atoms and brunches in the cation, as well as methyl group on the2-C in imidazolium cation and fluorination of the carbon atoms can enlarge the viscosity. The structure of anions has most important effects on the viscosity. The symmetric anions will arise the viscosity, while the two dimensional ones can decrease the viscosity. Assembling to the cation, large volume and molar mass of the anions as well as the enlargement of the carbon length will cause high viscosity. The viscosity will be diminished when water and other organic solvents are present in ILs, whereas reverse effects occur due to the remaining halide.(2) Amino acid ILs with low viscosity have large absorption rate. After consideration of the relation between the structure of the ions and the viscosity, we chose tetraalkylammonium and amino acid to construct ILs. Because of the large space between the cation and anion, and small size of the anions, the amino acid ILs have smaller viscosities than other functionalized ILs. The lowest viscosity of81mPa s at25℃belongs to [N2222][L-Ala] which has been chosen to determine the absorption prosperities. The absorption results have shown that the reaction mechanism between [N2222][L-Ala] and CO2was similar to that of organic amines (MEA or DEA) and amino-functionalized ILs ([p-NH2mim][BF4]), but with about2to5times larger absorption rate than the amines. Large absorption rate is the result of the low viscosity, which can improve the mass transfer of CO2in the liquid phase. In addition, the synthesis of amino acid ILs was simple and the final products were of high purity. In this case, amino acid ILs are considered as green, effective absorbents of potential use for CO2separation.(3) Full-deprotonated dicarboxylic acid ILs have large CO2absorption capacity. In order to have ILs with lower viscosity and lower cost, asymmetric tetraalkylammonium cation ([N2224]) and carboxylic acids were used to synthesis carboxylate ILs. It is found that these type anions have the hydrophilic nature and the water interacts with the anions. Because of the water, the ILs have low viscosity but suffer poor thermal stability. Experimental absorption results have shown that the carboxylate ILs have large CO2affinity and have almost the highest capacity than any other ILs based on carboxylic acids. It is further proved that the large molar mass of the anion, high water content, high absorption temperature, and other functional groups are negative effects to the absorption capacity.(4) Half-deprotonated dicarboxylic acid ILs have excellent SO2absorption properties. If one proton H atom in the dicarboxylic acid is merely neutralized, half-deprotonated carboxylic acid (H-carboxylate) ILs can be prepared. Experimental results have shown that these ILs have higher viscosities and more stable at high temperature than the full-deprotonated ones. It is interestingly found that H-carboxylate ILs have more attractive SO2absorption properties than any other TSILs. The reaction mechanism between ILs and SO2indicated that the SO2interruptted the original inter-molecular hydrogen bond and helped to form intra-molecular hydrogen bond through an octatomic ring, which also made the viscosity diminished after absorption. Additionally, this kind of ILs has low CO2affinity because of the weak acidic nature. These results strongly confirm that H-carboxylate ILs will be of significant importance for the separation of acidic gases from each other. (5) The mixture of MDEA and ILs show unique physical chemical properties and good performance of absorbing CO2. After thoughtful consideration, three ILs [emim][BF4],[N1111][Gly] and [N2222][L-Ala] have been chosen to blend with MDEA. The physical chemical properties such as density, viscosity and surface tension of the mixtures are quite different from neither pure MDEA nor pure ILs. The densities of the mixtures decreased not linearly with the rising temperature. Small coefficient of expansion indicates that the temperature has minor effect on the volume of mixture. At the same time, the excess molar volumes of all mixtures are negative values, implying that MDEA and the ILs have strong interaction and the molecular cumulated effectively. Unlike pure organic solvents, the relation between the viscosity of the mixture and the temperature can not be correlated by Arrhenius Equation but can be fitted to Vogel-Tammann-Fulchers Equation. Although the values of the surface tension is very close between the mixture and pure ILs, the temperature shows less importance to affect the surface tension of the mixture, making the mixture more attractive to the industrial applications. The ILs play a role as accelerant in the mixture from two aspects. Firstly, ILs take part in the chemical absorption process by providing active hydrogen which can accelerate the absorption rate. Secondly, ILs have no vapor pressure which can restrain the volatility of the MDEA and water, making the process more environmental benign. The experimental results have shown that the mixture composed by [emim][BF4] and MDEA act almost like aqueous MDEA mixtures, which proves the possibility of [emim][BF4] application as physical solvent but with outstanding ILs advantages. Task-specific ionic liquids, such as [N1111][Gly] and [N2222][L-Ala] can enlarge the absorption capacity at low partial pressure of CO2, and accelerate the absorption process significantly. Although the mixture trap less CO2at higher temperature, but the absorption rate would be very large due to the rising temperature. |
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