Research progress of CO 2 separation technology by solvent absorption

. The combustion of fossil fuels emits a large amount of CO 2 , which causes the greenhouse effect and leads to global warming and poses a serious threat to life on earth. CO 2 capture technology can effectively reduce the concentration of CO 2 in the atmosphere, alleviate the greenhouse effect, and improve the environment. CO 2 capture technologies include absorption, membrane separation and adsorption separation, among which chemical absorption and separation have the advantages of high efficiency, low cost and easy availability of materials. In this paper, the advantages and disadvantages of three main chemical absorption and separation methods (inorganic reagents, organic amines and ionic liquids) adsorb CO 2 are summarized. Among inorganic adsorbents, NH 3 ·H 2 O can achieve rapid and efficient absorption of CO 2 , and it is relatively stable and not easy to degrade. Common types of organic amine adsorbents are monoethanolamine, methanolamine, and sterically hindered amine. However, it is difficult for a single organic amine adsorbent to meet the requirements of high absorption rate, high absorption capacity and low reaction heat at the same time. Therefore, mixing organic amine absorbents with different characteristics can improve their performance in absorbing CO 2 . Ionic liquids have the advantages of good thermal stability, very low saturation vapor pressure, and designable structure, and are a new type of CO 2 adsorbent, but ionic liquids have high viscosity themselves. Combining ionic liquids with organic or inorganic porous materials to form loaded ionic liquid materials, which can be used as CO 2 adsorbent not only to improve the separation effect, but also to avoid the problem of high viscosity caused by direct absorption of ionic liquids, thus improving CO 2 adsorption efficiency.


Introduction
With the continuous development of China's economy, the consumption of energy has been increasing, and the total primary energy consumption in China reached 4.3 billion tce in 2015, of which fossil energy accounted for 88%. The burning of large amounts of fossil fuels makes China the country with the largest carbon emissions at present, which reached 9.1 billion tce in 2015, accounting for 27.3% of the world's total [1]. The emission of large amounts of CO2 causes the greenhouse effect, leading to global warming, glacier melting, sea level rise, and precipitation change, which will eventually pose a serious threat to life on Earth. How to effectively control and reduce CO 2 emissions while maintaining economic development and energy demand, so as to achieve a "win-win" situation of sustainable economic development and environmental protection, has become a serious issue for mankind [2]. At the 75th session of the United Nations General Assembly, President Xi Jinping made a major announcement to the world that China will strive to peak its CO2 emissions by 2030 and achieve carbon neutrality by 2060.
In order to achieve carbon peaking and carbon neutralization, our current goal is to reduce CO 2 . CO 2 reduction methods include improving energy efficiency, carbon capture, utilization and storage (CCUS), and Clean energy substitution, etc. Currently, CCUS technology is considered to be one of the most important technologies to control greenhouse gas emissions in the short term [3].CO 2 capture technologies include absorption, membrane separation, and adsorption separation. Adsorption separation is divided into two aspects: physical adsorption and chemical adsorption, and chemical absorption separation has the advantages of high efficiency, low cost, and easy availability of materials. The absorbent is the core of chemical absorption method to remove CO2. The ideal CO 2 absorber should have the characteristics of fast absorption rate, high absorption capacity, and low energy consumption for regeneration, as well as the characteristics of safety and stability, environmental friendliness, low corrosion of equipment, and good economy [1]. In this paper, the advantages and disadvantages of three main chemical absorption separation methods (inorganic reagents, organic amines and ionic liquids) for CO 2 adsorption are summarized in order to provide a reference for the subsequent chemical absorption separation of CO 2 .

Chemical absorbents
Commonly used chemical absorbents include inorganic reagents, organic amines and ionic liquids, etc. Their main types and advantages and disadvantages of CO 2 absorption are shown in Table 1. They are introduced separately below.

Inorganic adsorbents
Commonly used CO 2 inorganic adsorbents are Ca(OH) 2 , K 2 CO 3 , Na 2 CO 3 and NH 3 ·H 2 O, etc. Ca(OH) 2 reacts quickly with CO 2 at room temperature to 750°C, the concentration of Ca(OH)2 solution affects the absorption of CO 2 , and saturated Ca(OH) 2 solution can absorb more CO 2 than the theoretical amount. However, Ca(OH) 2 preparation energy consumption is high, and the preparation process will release CO 2 to limit the application of this method. K 2 CO 3 is low cost, non-volatile, less toxic and less degradable than organic amines. However, at low temperatures, the solubility of K 2 CO 3 is low, resulting in a slow absorption rate of CO 2 . In addition, the KHCO3 generated by the reaction of K 2 CO 3 and CO 2 is highly corrosive, and some corrosion inhibitors need to be added, such as V 2 O 5 , As 2 O 3 , etc. NaCO 3 is generally used to absorb carbon dioxide from the flue, and the purity of carbon dioxide absorbed by this method is very high, up to 99%. The disadvantage is that the cost is too high and the water consumption is serious. NH 3 ·H 2 O achieves fast and efficient absorption of CO 2 , and is relatively stable and not easily degraded. In addition, The products produced by NH 3 ·H 2 O and CO 2 can be used as scrap [4]. In addition to the commonly used inorganic adsorbents for CO 2 , there are also some new inorganic adsorbents, Lixia Yang et al. prepared a series of sodium-magnesium complex salt materials for CO 2 uptake using sodium carboxymethyl cellulose as a crystal guiding agent, and it was found that the CO 2 saturation uptake of homogeneous layered sodium-magnesium complex salt was 22.8%. And this reagent has good cycling stability and high potential for industrial application [2].

Organic amine adsorbents
The common types of organic amine adsorbents include monoethanolamine, methanolamine, and sterically hindered amine, etc. Monoethanolamine (MEA) is a primary amine that is highly alkaline and can react quickly with CO 2 to form a carbamate compound, and when heated, the carbamate will decompose to desorb CO 2 . Monoethanolamine has the advantages of fast absorption rate, high absorption capacity, high selectivity and removal efficiency, and low solvent consumption, but CO 2 desorption requires high temperature and therefore high energy consumption, in addition, monoethanolamine is prone to foaming and degradation, and the addition of organic solvents and active agents to the solution can solve these problems [5]. Methyl diethanolamine (MDEA) has good chemical stability, the solvent is not easily degraded, and the absorption volume is large, but the absorption time is long.MEDA plays a catalyst-like role in the absorption of CO2, but when reacting with CO 2 , MEDA must be hydrolyzed before slowly reacting with CO 2 and generating sub-stable bicarbonate, which can lead to a slower absorption rate of MDEA, which is its biggest defect. and some additives can be added to MEDA to speed up its reaction rate. In addition, sterically hindered amine (AMPs) have also attracted attention. These organic amine molecules are characterized by at least one amino group connected to a secondary or tertiary carbon atom in their structure, which makes them have a space-site hindering effect, resulting in unstable carbamates and easy to react with H2O to form amines and HCO 3 -. Therefore, it is relatively easy to desorb CO 2 after absorption of sterically hindered amine, and their maximum theoretical absorption capacity of CO 2 is 1 molCO 2 /mol absorber [6].
Because a single organic amine absorbent is difficult to meet the requirements of high absorption rate, high absorption capacity and low reaction heat. Therefore, mixing organic amine absorbents with different characteristics can improve their performance in absorbing carbon dioxide. The main idea is to absorption rate higher than primary amine or secondary amine as the main body, adding other amines to reduce regeneration energy consumption and improve the overall performance of the absorbent. For example, the addition of MDEA in MEA aqueous solution can significantly reduce the regeneration energy consumption of CO2 capture [1]. In addition, the mixture of organic amines and alcohols can also improve the CO 2 capture capacity.
For example, Sha et al.prepared a butanediol-ethylenediamine system and found that CO 2 can be rapidly activated and converted into a solid CO 2 storage material (CO 2 SM) under mild conditions. When the butanediol-ethylenediamine mixed solvent system absorbs CO 2 at 20 °C and desorbs CO 2 at 98.6 °C, the solution has no obvious loss and is considered to be green, efficient and low-cost method for carbon dioxide capture and utilization [7].
In recent years, some CO2 absorption demonstration project using chemical solvents method were built in China (Table 2), their CO2 adsorption capacity is in the range of 60000-1000000 t/a.

Ionic liquids
The ionic liquid is a room-temperature molten salt composed of anions and cations [9], the cations mainly include imidazole, pyridine, pyrrolidine, quaternary amine and quaternary phosphorus ions, and the anions are mainly halogen, boric acid, carboxylic acid and amino acid ions. Ionic liquids have good thermal stability, extremely low saturation vapor pressure, and a designable structure. The environmental pollution caused by volatilization is small, and it is easy to be separated from other substances for recycling. Ionic liquids capture CO2 mainly through its basic group directly capturing CO 2 [10]. The special properties of ionic liquids make them promising for a wide range of applications in CCUS, for

example, Lv et al. based on the preparation of ionic liquids [C2OHmim]
[Gly] containing multiple amine sites, which have good thermal stability, regenerability and oxygen tolerance, and 1 mol of this ionic liquid can trap 0.575 mol of CO2 [11]. The CO2 adsorption capacity of some other ionic liquids is illustrated in Figure 1 [12]. In addition, temperature and pressure affect the solubility of CO2 in ionic liquids, the solubility of CO2 increases with increasing pressure and decreases with increasing temperature. The type of anion in the ionic liquid also significantly affects the solubility of CO 2 . SHARMA et al. found that the order of CO 2 absorption capacity of different ionic liquids is: BF 4 − <DCA − <PF 6 − < TfO − <Tf 2 N − [13]. However, the high viscosity of ionic liquids themselves and the formation of hydrogen bonding network structure of ionic liquid-CO 2 system during the absorption process further increases the viscosity. For example, the viscosity of amino acid ionic liquids increases by about 15-30 times after absorbing CO 2 [14], which limits the application of ionic liquids in absorption and separation. In order to solve the above problems, ionic liquids have been combined with organic or inorganic porous materials to form loaded ionic liquid materials, which have the advantages of both ionic liquids and porous materials, As a CO 2 adsorbent, it can not only improve the separation effect, but also avoid the high viscosity problem caused by the direct absorption of ionic liquids, thereby improving the CO 2 adsorption efficiency [15]. Dong et al.loaded two ionic liquids on type A and type B silica gel by impregnation method to investigate the effects of different influencing factors (silica gel type and activation temperature, ionic liquid type and loading, adsorbent pore size distribution) on CO 2 adsorption efficiency. It was found that the obtained adsorbents all had well-developed microporous structures with pore diameters of 0.4-0.8 nm, and the loaded samples had better CO 2 adsorption performance of 3.68% (mass fraction) when the type A silica gel was activated at 500°C and the amount of [bmim]PF6 was 20% [16]. In addition, the method of combining ionic liquid with membrane separation can also improve its CO 2 adsorption performance. This method can greatly reduce the amount of ionic liquid and reduce the complexity and cost of ionic liquid preparation to a certain extent. It also overcomes the disadvantage of slow diffusion of CO 2 in ionic liquids. Vollas et al. developed pyridine-based polyionic liquid composite membranes, and when the mass fraction of ionic liquids was at 45% mass fraction of ionic liquid, the CO 2 flux of the polyionic liquid composite membrane was 11.8 barrer and the CO 2 /CH 4 selectivity was 35, and the separation performance was significantly better than that of the pure membrane [17].

Conclusion and outlook
The chemical absorption separation method for CO 2 capture has the advantages of high efficiency, low cost and easy availability of materials. This paper summarizes the advantages and disadvantages of three main chemical absorption separation methods (inorganic reagents, organic amines and ionic liquids) for CO 2 adsorption. Commonly used inorganic adsorbents for CO 2 are Ca(OH) 2 , K 2 CO 3 , Na 2 CO 3 and NH 3 ·H 2 O, among which NH 3 ·H 2 O can achieve rapid and efficient absorption of CO 2 and is relatively stable and not easily degraded. The common types of organic amine adsorbents are monoethanolamine, methanolamine, sterically hindered amine, etc. However, it is difficult for a single organic amine adsorbent to meet the requirements of high absorption rate, high absorption capacity and low reaction heat at the same time. Therefore, mixing organic amine absorbents with different characteristics can improve their performance in absorbing CO2. Ionic liquids have the advantages of good thermal stability, very low saturation vapor pressure, and designable structure, and are a new type of CO 2 adsorbent. However, ionic liquid has high viscosity, Combining ionic liquids with organic or inorganic porous materials to form loaded ionic liquid materials, which can be used as CO 2 adsorbent not only to improve the separation effect, but also to avoid the problem of high viscosity caused by direct absorption of ionic liquids, thus improving CO 2 adsorption efficiency.
In general, single organic amines have limited CO 2 absorption capacity, and mixed amine absorbers can combine the advantages of multiple single absorbers, thus having higher absorption capacity and absorption  rate as well as lower regeneration energy consumption. However, there are still more problems, such as absorber stability and process system development, which need further research. In addition, the current types of loaded ionic liquids are mainly conventional ionic liquids and amino and carboxylic acid functional ionic liquids, which are limited in variety and are mainly based on single-site interaction with CO2. In the future, these ionic liquids need to be combined with emerging materials to develop more types of loaded ionic liquids to meet the efficient adsorption and separation of CO 2 . Meanwhile, the mechanism of interaction between loaded ionic liquids and CO 2 is still unclear, and more characterization tools are needed to probe the microstructure and macroscopic properties of loaded ionic liquids and to understand the interaction between loaded ionic liquids and CO 2 , so as to provide theoretical guidance for the design and preparation of loaded ionic liquids for CO 2 adsorption.