| Environmental problems caused by greenhouse gases have received more and more attention. The emission of carbon dioxide, which is a major greenhouse gas, has increased greatly with each passing day. Nowadays, technologies of trapping carbon dioxide using aqueous organic amines, limestone and inorganic base solutions have extensively carried out in the industry. However, these technologies are not environmental friendly because of high operation costs, severe corrosion of equipments and organic pollutants. Ionic Liquids (ILs) have been paid much attention by industrial and academic as novel liquid materials, which are totally composed of ions at room temperature or similar temperature. They have outstanding characteristics such as negligible vapor pressure, designable structure and high thermal stability. Therefore, task-specific ionic liquids for CO2absorption can be designed and synthesized.In this work, systematic task-specific ionic liquids were designed and synthesized based on symmetrical tetraalkylammonium cation and amino acid anion. After determining the structures and physico-chemical properties of the obtained ionic liquids, tetraethylammonium L-alanine(N2222L-Ala) has been chosen to examine the absorption property due to its lowest viscosity. The experiment result shows it has good performance of trapping CO2. In order to reduce the cost and viscosity, ionic liquids are prepared by coupling asymmetrical tetraethylbutylammonium cation and carboxylate anion. It is found that N2224acetate, N2224propionate and N2224butyrate have relatively low viscosity. The absorption experiment shows each of them has high absorption capacity and fast absorption rate. However, due to the relatively high cost and viscosity of ionic liquid, it is hard to be applied in industry. Methyldiethanolamine aqueous solutions widely used in industrial processes has the advantage of high absorption capacity, while it has slow absorption rate. Therefore, N1111Gly, N2222L-Ala and N2224CH3COO have been chosen as accelerants to mix with aqueous amine solution due to their readily preparation, relatively low cost and fast absorption rate. The researches on the absorption properties of different proportion are carried out. It is supposed that IL can accelerate the rate, enlarge the capacity and reduce the pollution. Several important results have been obtained and summarized as follows:(1) Three tetraalkylammonium cation([N1111],[N2222] and [N4444]) and four amino acid anion ([Gly],[L-Ala],[β-Ala] and [Val]) are coupled to form twelve symmetrical task-specific ionic liquids. Then, the structures and physic-chemical properties of these TSILs are characterized. The symmetry of cation is shown to have a significant influence on the increase in viscosity of the ionic liquid. Besides, the addition of carbon atoms and brunches of the anion will enlarge the viscosity. N2222L-Ala has the lowest viscosity among the amino acid ionic liquids.(2) Sixteen novel asymmetrical ionic liquids are prepared by coupling tetraethylbutylammonium cation and eleven carboxylate anion ([acetate],[propionate],[butyrate] and so on). The characterizations of the structures and physic-chemical properties show that full-deprotonted carboxylate ILs are highly hydrophilic. However, a lot of energy has to be cost to obtain the ILs without water. Take N2224acetate as an example, anhydrous samples can be got after dried at80°C under vacuum for30days. The absorption capacity decreases with the increase in water content; samples with one mole ratio of water shows the highest absorption capacity, the same as samples without water. However, the existence of water can greatly lower the viscosity, so it has more advantages in trapping CO2.(3) Three ionic liquids (N1111Gly, N2222L-Ala and N2224CH3COO) are chosen as additive to mix with aqueous amine solution. It is found the addition of ILs has great influence in80%MDEA aqueous solution. The absorption capacity increases with the content of ILs. ILs does not have obvious impact on60%MDEA aqueous solution. As for40%MDEA aqueous solution, the absorption capacity is affected by CO2loading and the mole of valid amine. With the relatively low content of ILs, the increase of CO2loading play an important role, the absorption capacity will increase as a result. With the relatively high content of ILs, the increase of the mole of valid amine is the dominant factor, the absorption capacity will decrease. The addition of2.5%N1111Gly in MDEA aqueous solution has the highest CO2loading, which is1.058mole ratio at0.94MPa. Moreover, the higher temperature will reduce the capacity at low pressure (<0.8MPa); the temperature does not have remarked effect in absorption property at high pressure (0.81MPa). Thus, this mixture has good heat tolerance.This work about the synthesis of amino acid ionic liquids, carboxylate ionic liquids and blended absorbents not only theoretically supported the optimization of existing CO2capture process, but also give the guidance for the applications of ionic liquids. |
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