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CO2 absorption characteristics of amino group functionalized imidazolium-based amino acid ionic liquids
Journal Pre-proof CO2 absorption characteristics of amino group functionalized imidazolium-based amino acid ionic liquids Sehee Kang, Yongchul G. Chung, Jo Hong Kang, Hojun Song PII:
S0167-7322(19)33531-7
DOI:
https://doi.org/10.1016/j.molliq.2019.111825
Reference:
MOLLIQ 111825
To appear in:
Journal of Molecular Liquids
Received Date: 24 June 2019 Revised Date:
21 September 2019
Accepted Date: 25 September 2019
Please cite this article as: S. Kang, Y.G. Chung, J.H. Kang, H. Song, CO2 absorption characteristics of amino group functionalized imidazolium-based amino acid ionic liquids, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/j.molliq.2019.111825. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
CO2 Absorption Characteristics of Amino Group Functionalized Imidazolium-based Amino Acid Ionic Liquids Sehee Kanga,b, Yongchul G. Chungb, Jo Hong Kanga and Hojun Songa* a
Green Materials & Processes R&D group, Korea Institute of Industrial Technology, 55 Jongga-ro, Jung-gu, Ulsan 44413, South Korea
b
School of Chemical and Biomolecular Engineering, Pusan National University, 2 Busandaehak-ro, 63beon-gil, Geumjeong-gu, Pusan 46241, South Korea
Abstract In this study, dual amino group functionalized imidazolium amino acid ionic liquids (AAILs) were synthesized from imidazole, bromoalkylamine, and amino acids. 1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium hydroxide ([Apaeim][OH]), 1-propyl-3-(2-aminoethyl)imidazolium hydroxide ([Paeim][OH]), and 1-ethyl-3-(2aminoethyl)imidazolium hydroxide ([Eaeim][OH]) were synthesized in two-steps, while [Apaeim][amino acid], [Paeim][amino acid], and [Eaeim][amino acid] were synthesized in three-steps. The structures of the synthesized AAILs were confirmed by 1H-NMR analysis. Moreover, the CO2 absorption mechanism of [Apaeim][OH] was confirmed by FT-IR and
13
C-NMR analyses. Subsequently, CO2 absorption-desorption tests were conducted
under at 15 vol% CO2, 313K and atmospheric pressure. The cation and anion effects on the ionic liquid absorbents were also investigated. Among the tested ionic liquid solutions, 30 wt% [Apaeim][OH] and 30 wt% 1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium alaninate ([Apaeim][ala]) displayed the best performances. Thus, both presented CO2 cyclic capacities 2.2-fold higher than that of the benchmark CO2 absorbent 30 wt% monoethanolamine (MEA), while their viscosities were comparable to that of 30 wt% MEA. Notably, 30 wt% [Apaeim][ala] show great potential as a CO2 absorbent due to its high CO2 cyclic capacity and low viscosity. Keywords: CO2 absorption, Amino group functionalization, Ionic liquids, Cyclic capacity, Viscosity * Corresponding author. Tel.: +82-52-980-6670; fax:+82-52-980-6669 E-mail address: [email protected].
1. Introduction From 2030 to 2050, the energy strategy and policy of the European Union (EU) will focus on increasing renewable energy sources (RES) and reducing greenhouse gas (GHG) emissions. The rising global average temperature due to the greenhouse effect has been attributed to the ever increasing GHG emissions [1]. Thus, the EU target is to reduce GHG emission by 40% by 2030 [2]. The Paris Climate Change Convention (PCCC) was adopted to maintain the global average temperature rise below two degrees to prevent global warming [3-4]. However, CO2 is a renewable carbon resource, which inexpensive, non-toxic, and nonflammable [5-6]. Moreover, simple organic materials can be produced through a green process using CO2 [7]. CO2 emissions account for 76% of the total industrial emissions [8-10] and thus, it is necessary to develop technology to capture and reuse industrially emitted CO2. Carbon dioxide capture, utilization, and sequestration (CCUS) technology is expected to be a powerful industrial tool to help reduce 45 % of the total CO2 emissions by 2050 [11]. One of the widest applied industrial techniques for capturing CO2 is chemical absorption by aqueous solutions of alkanolamines. Alkanolamines, such as monoethanolamine (MEA), have been used industrially for > 100 years to remove CO2 from flue gas emissions from fossil fuel combustion [12]. However, alkanolamines require a vast amount of energy to strip CO2 from CO2-saturated absorbents, necessary for the regeneration and reuse of CO2 absorbents [13]. Thus, operating costs are high due to high absorbent regeneration energy in the CO2 regeneration process [14]. Additionally, alkanolamines present many disadvantage such as absorbent loss due to the high regeneration temperature, corrosion, and oxidative and thermal degradation [15-18]. Ionic liquids have been attracted significant attention as alternative CO2 absorbents due their specific characteristics including negligible vapor pressures, low heat capacity, liquid state in a wide temperature range, and high thermal and chemical stabilities [19]. Generally, ionic liquids that are in the liquid phase in a wide temperature range are classified as room temperature ionic liquids (RTILs). Many RTILs, for example, ones that comprise nitrate ([NO3]-) and dicyanamide ([DCA]-) anions, effectively absorb CO2 under high pressures (> 90 bar) [20]. However, the utilization of conventional RTILs in CO2 absorption is restricted, because the pressure of most discharged flue gases approaches atmospheric pressure. The physical and chemical properties of ionic liquids can be specifically designed by altering the combination of cations and anions [21]. Thus, optimal ionic liquids can be synthesized with characteristics such as non-toxicity, low viscosity, low melting point, thermal stability, and low cost. Ionic liquids synthesized for a specific purpose are called task-specific ionic liquids (TSILs) [22,23].
The physical solubility of CO2 is greatly high in ILs. Moreover, extremely high CO2 capacity in TSILs can be obtained, for instance, amine-functionalized ILs can add tunable and specific chemical reaction with CO2 because CO2 take a cue from chemistry of amine solution [24-27]. Amine-functionalized ILs have large CO2 capacity, which is due to chemical absorption rather than physical absorption. One molecule of CO2 reacts with two molecules of amine; the reaction proceed via carbamate and ammonium intermediates (equations 1 and 2) [28, 29].
+ ↔ + ↔ 1 +
(1) (2)
Specifically, AAILs, in which the deprotonated amino acid is used as an anion or cation, display many advantages over other TSILs [30]. Thus, AAILs are biodegradable, inexpensive, easy to dissolve in biocompatible materials such as DNA and cellulose, and easily blended into organic solvents [31,32]. Additionally, amino acids can chemically absorb CO2 due to the presence of the amino group [33]. Several AAILs have been recently synthesized as CO2 absorbents. Shahrom et al. [34] synthesized polymer benzyl ammonium-based AAILs, using eight amino acid anions and varying the amino acid and cation polymer groups of their products. A comparison of the CO2 absorption characteristics of the different products revealed that polyvinlybenzyltrimethylammonium arginate (poly[VBTMA][arg]) displayed the highest CO2 absorption capacity of 1.14 mol CO2/mol ionic liquid (IL) under 1 atm CO2, 298 K. However, this CO2 absorption capacity may not be high enough due to the high molecular weight of the polymeric ionic liquids. Sistla et al. [35] synthesized imidazolium-based AAILs. They used nine amino acid anions with different numbers of amino groups and compared the CO2 absorption characteristics when varying the number of amino groups in the amino acid. Among them, butylmethylimidazolium arginate ([Bmim][arg]) displayed the highest CO2 absorption capacity of 0.62 mol CO2/mol IL under 1 atm CO2. Ionic liquids, comprising anions with many amino groups present higher CO2 absorption capacities. Although the CO2 removal performances of the above-stated AAILs are satisfactory, the synthetic ionic liquids reported in other studies are expensive because commercially available ionic liquids are used as precursors for their syntheses and the synthetic yields are relatively low. Filippov et al. [36] synthesized choline-based AAILs and compared their CO2 absorption characteristics according to the amino acid-type and alkyl group length of the cation. Among them, [N1,1,5,2OH][tau] exhibited the highest CO2 absorption of 0.55 mol CO2/mol IL under 1 atm CO2. Yuan et al. [37] synthesized choline-based AAILs, whereby 30 wt% cholinium glycinate ([Cho][gly]) presented the highest CO2 absorption of 0.578 mol CO2/mol IL under 1 atm CO2. The synthetic cost of choline-based ionic liquids would be lower than other TSILs,
because choline hydroxide solution is commercially available. However, although choline-based ionic liquids are less expensive than other ionic liquids, they display relatively lower CO2 absorption capacities than other ionic liquids such as poly[VBTMA][arg] and [Bmim][arg]. Saravanamurugan
et
al.
[38]
synthesized
ammonium-based
AAILs
whereby
13.1
wt%
trihexyl(tetradecyl)ammonium lysinate ([N66614][Lys]) presented the highest CO2 absorption of 2.1 mol CO2/mol IL under 1 bar CO2. Zhou et al. [39] synthesized amino group functionalized imidazolium-based AAILs with 1aminopropyl-3-methylimidazolium lysinate ([Apmim][Lys]) presenting the highest CO2 absorption of 1.8 mol CO2/mol IL under 30 mL/min CO2, 303.15 K. These absorbents display high CO2 absorption capacities but use other ionic liquids as precursors for their synthesis. Therefore, these ionic liquid absorbents would be expensive. Hence, the CO2 absorption test was carried out using a very small amount of ionic liquid absorbent and pure CO2. In this study, amino group-functionalized imidazolium-based AAILs were synthesized, with glycine, Lalanine, and valine as amino acid anions. Each amino acid comprised the same number of amino groups, but a different alkyl group attached to the carbon atom near the amino group. Dual amino group functionalized imidazolium-based AAILs were also synthesized to improve the CO2 absorption capacity. Other amino group functionalized imidazolium AAILs were synthesized to compare the alkyl group effect on the CO2 absorption performance.
Thus,
1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium
([Apaeim]),
1-ethyl-3-(2-
aminoethyl)imidazolium ([Eaeim]), and 1-propyl-3-(2-aminoethyl)imidazolium ([Paeim]) were also synthesized. Fig. 1 illustrates the structures of the synthesized ionic liquids identified by 1H-NMR spectroscopy. The CO2 absorption mechanism of 1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium hydroxide ([Apaeim][OH]) was also identified through FT-IR and
13
C-NMR spectroscopy. Subsequently, the CO2 absorption-desorption test was
conducted on each multi-amino group functionalized ionic liquids using 15 vol% CO2 balanced with N2. The CO2 absorption capacity and viscosity characteristics of the 30 wt% aqueous solutions were explored with respect to the structure of each anion and cation. In addition, the CO2 absorption characteristics of the 30 wt% AAILs were compared with those of 30wt% MEA, a benchmark alkanolamine solution.
2. Experimental Section 2.1. Chemicals The analytic grade reagents 2-bromoethylamine hydrobromide (98 wt%), 1-ethylimidazole (98 wt%), 1-(3aminopropyl)imidazole (97 wt%), glycine (99 wt%), L-alanine (99 wt%), and L-valine (98 wt%) were purchased from TCI Co. Ltd. (Japan). 1-Propylimidazole (98 wt%) were purchased from Aladdin Chemical Co.. Potassium
hydroxide flake (93 wt%) were purchased Deajung Chemicals & Metals Co. Ltd. (Korea). The anion exchange resin (AW90 OH form) were purchased from Iontech Co. Ltd (Korea), respectively. All the solvents, including anhydrous ethanol (99.5 wt%), methanol (99.5 wt%), anhydrous dichloromethane (99.9 wt%), and acetonitrile (99.5 wt%) were purchased from Deajung Chemicals & Metals Co. Ltd. (Korea). All the chemicals were as received without further purification, but the purities of all chemicals were considered. CO2 (99.99 vol%) and N2 (99.99 vol%) were supplied by Korea Gas & Electric technology Co. Ltd.(Korea). 2.2. Synthetic procedures The synthetic procedures of the amino group-functionalized imidazolium-based AAILs adopted in this study are as follows (Fig. 1): 1- 1-Ethylimidazole (as well as 1-propylimidazole, and 1-aminopropylimidazole) and 2-bromoethylamine hydrobromide were used as starting materials to synthesize the ionic liquids. Typically, an Erlenmeyer flask with the cap equipped with a magnetic stirrer was charged with 1-ethylimidazole (0.1 mol) and ethanol (70 mL). 2-Bromoethylamine hydrobromide (0.1 mol) was slowly added dropwise to the Erlenmeyer flask over 10 min at 298.15 K. The resulting mixture was stirred for 30 min at 298.15 K and then slowly warmed to 343.15 K over 24 h. The solvents were removed by rotary evaporation and a highly viscous colloidal liquid was formed. Subsequently, KOH aqueous solution (pH 8) (100 mL) was mixed strongly with the highly viscous colloid reactant to remove any Br ions remaining in the solution. The remaining solvent was removed by rotary evaporation to form solid 1-ethyl-3-(2aminoethyl) imidazolium bromide ([Eaeim][Br]), which was dried in vacuo, at 353.15 K for 24 h [40]. 2- An Erlenmeyer flask with the cap equipped with a magnetic stirrer was charged with [Eaeim][Br] (0.1 mol) and anhydrous dichloromethane (70 mL). Potassium hydroxide (0.1 mol) was slowly added dropwise to the Erlenmeyer flask over a period of 30 min at 298.15 K. The resulting mixture was stirred for 12 h using a magnetic stirrer, at 298.15 K. The resultant potassium bromide (KBr) liquid reactant mixture was filtered to removes the precipitated KBr. Subsequently, the solvent was removed by rotary evaporation to form a viscous liquid of 1-ethyl-3-(2-aminoethyl) imidazolium hydroxide ([Eaeim][OH]), which was dried in vacuo at 353.15 K for 24 h [41]. Another method for displacing the Br anions with OH anions was via anion exchange resin. In this procedure, a column filled with the anion exchange resin AW90 (OH- form) and [Eaeim][Br] (0.1 mol) dissolved in ethanol (100 mL) was passed through the resin. The solvent was removed by rotary evaporation to afford a viscous liquid of [Eaeim][OH], which was dried in vacuo, at 353.15 K for 24 h.
3- An Erlenmeyer flask with the cap equipped with a magnetic stirrer was charged with an aqueous solution of glycine, [Eaeim][OH] was slowly added dropwise at 298.15 K over a period of 1 h using a syringe pump. The resulting mixture was stirred for 24 h using a magnetic stirrer at 298.15 K. Once the reaction was complete, the water was removed by rotary evaporation. Subsequently, an acetonitrile/ methanol mixture (4:1 (vol)) was strongly mixed with the reactant to remove any glycine remaining in the water. The precipitated glycine was then remover by filtration; this step was repeated two or three fold. The remaining solvent was removed by rotary evaporation to form a viscous liquid of 1-ethyl-3-(2-aminoethyl) imidazolium glycinate ([Eaeim][gly]), which was dried in vacuo, at 303.15 K for 30 h [42]. Other ionic liquids, including 1-ethyl-3-(2-aminoethyl)imidazolium L-alaninate ([Eaeim][ala]), 1-ethyl-3(2-aminoethyl)imidazolium
valinate
([Eaeim][val]),
1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium
glycinate ([Apaeim][gly]), 1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium alaninate) ([Apaeim][ala]), 1(3-aminopropyl)-3-(2-aminoethyl)imidazolium valinate ([Apaeim][val]), [Apaeim][OH], 1-propyl-3-(2aminoethyl)imidazolium glycinate ([Paeim][gly]), 1-propyl-3-(2-aminoethyl)imidazolium L-alaninate ([Paeim][ala]), 1-propyl-3-(2-aminoethyl)imidazolium hydroxide ([Paeim][OH]) were also prepared using the same method. 2.3. Characterization of the synthesized multi-amino group-functionalized ionic liquids The molecular structures of the multi-amino group functionalized ionic liquids prepared in this study were identified via 1H-NMR spectroscopy (Bruker 400 MHz spectrometer) using CD3OD as the solvent with tetramethylsilane (TMS) as the internal standard [43]. The viscosities of the 30 wt% synthetic ionic liquid absorbents were measured under lean CO2 loading at 40
and atmospheric pressure. The viscosities were
measured using a DV-II+ Pro viscometer (Brookfield Co. Ltd., USA) with an uncertainty of 1% in relation to the full scale; spindle 18 (SC-18) was employed. At least three runs were conducted for each measurement. FTIR and
13
C-NMR spectroscopy were used to observe the CO2 absorption mechanism of [Apaeim][OH]. The
NMR spectra were recorded on a Bruker 400 MHz spectrometers using CD3OD as a solvent. The FT-IR spectra were recorded using a JASCO (FT/IR-4600 type A) instrument in the region 4000-500 cm-1 in diamond attenuated total reflectance (D-ATR) measurement mode. 2.4. CO2 absorption and desorption tests The CO2 absorbent screening test apparatus (Fig. 2) comprised of N2 and CO2 gas tanks, N2 and CO2 flow rate controllers, a bubble column reactor, and a gas analyzer. First, when N2 and CO2 valves were opened, the gas
flow rate was adjusted by each gas flow rate controller. The concentration of the CO2 gas before entering the reactor was monitored and adjusted using an NDIR gas analyzer (Multi Master, Sensoronic Co. Ltd.). The gas analyzer displayed the real time CO2 concentration in the reactor or gas mixer. The CO2 concentration was set to 15 vol%, a value similar to the typical CO2 concentration of the flue gas emitted from coal-fired thermal power plants, by adjusting the flow rate controller. About 100 mL prepared CO2 absorbent was fed to the reactor (internal volume = 250 mL). The water bath temperatures were set at 313.15 and 353.15 K for the CO2 absorption and desorption tests, respectively : typically, in an amine CO2 absorption process, CO2 absorption occurs at 313.15 K, while we believe that a temperature of 80
is reasonable for the fast screening of the CO2
absorbents [44, 45]. Second, 15 vol% CO2 gas was introduced into the liquid absorbent by bubbling. The pipe of the condenser connected to the reactor was connected to a gas analyzer that measured the CO2 concentration in the bubble reactor. When the valve connected to the reactor was closed and the valve connected to the analyzer was open, the analyzer displayed the composition inside the reactor. Normally, the CO2 concentration of the CO2 analyzer during CO2 absorption was <15 vol% and the CO2 absorption reaction was complete when the CO2 concentration reached 14.8 vol%. N2 gas was purged at 353.15 K during CO2 desorption. The CO2 desorption reaction was terminated when the analyzer CO2 concentration reached 0 vol%. Based on the logged gas analyzer data, we could calculate the reaction rate, absorption amount, and CO2 cyclic capacity : By integrating the CO2 concentration data, the CO2 loading can be calculated as CO loading CO loading
mol CO Accumulated absorption capacity (mol CO ) = mol IL Mole of ionic liquid (mol IL)
mol CO Accumulated absorption capacity (mol CO ) = L Volume of absorbent (L)
(1)
(2)
Then the CO2 absorption rate is calculated as CO absorption rate
3.
mol Absorption capacity of CO (mol) = L×s Volume of absorbent (L) × Unit time (s)
(3)
Results and Discussion
3.1. H-NMR and FT-IR data of the multi-amino group-functionalized ionic liquids The 1H-NMR spectra were recorded at 297 K on a Bruker Avance 400 MHz spectrometer, with CD3OD and TMS as the solvent and internal standard, respectively.
The 1H NMR data and yields of all prepared synthetic ILs are listed in Table 1 [46]: The synthetic yield depended on the cation type and was the highest for the ionic liquids comprising [Apaeim] cations. The imidazole protons were observed at 7–8 ppm in all the synthetic ionic liquids [47], while the CH3 peak was absent for all the ionic liquids comprising [Apaeim] cations. The 1H NMR spectra of the different ionc liquids were in good agreement with their corresponding chemical structures. According to the characterization results, the synthesized products did not contain any impurities. The chemical structures of the synthesized ionic liquids are displayed in Table 2. CO2 chemical absorption mainly proceeds via the cation and anion amino groups. For [Apaeim][OH], the CO2 molecules react with the cation amino groups. FT-IR data (Fig. 3) confirmed this CO2 chemical absorption. The broad peak at 3500–3000 cm-1 was assigned as the OH peak, while the small sharp peaks at 3500–2750 cm-1 were assigned to the imidazole group. Additionally, an imidazole C–N stretching peak appeared at 1229–900 cm-1, while the imidazole C–H peak appeared at 662 and 741–740 cm-1. An increase in the height of the CO2 gas peak at 2500–2250 cm-1 was observed after CO2 absorption [48,49]. This was attributed to the CO2 that did not chemically reacted with [Apaeim][OH]. A C=O and two N–H peaks also appeared at 1561, 1467, and 1307 cm-1 following CO2 absorption, attributed to the CO2 that chemically reacted with [Apaeim][OH]. The
13
C-NMR spectra (Fig. 4) provided information on the CO2 chemical absorption mechanism of the
synthetic ionic liquid [Apaeim][OH]. Fig. 4 (d), which presents the before, during (1 h), and after the completion of CO2 absorption clearly indicates that CO2 is chemically and physically absorbed into the synthetic ionic liquids. The
13
C-NMR spectra of the ionic liquids were in good agreement with their corresponding
chemical structures. The 13C-NMR spectrum during CO2 absorption, measured after one hour after the reaction was initiated, presented new peaks at 163 and 166 ppm, assigned to the amide (CO–NH) groups, as well as CO2 and HCO3- peaks at 125 ppm and 165 ppm [50,51]. Since two peaks assigned to the carbon connecting the N-H group appeared in 13C-NMR spectra, it was assumed that all of the amino groups (NH2) of the [Apaeim] cation react with CO2 molecules. The spectrum in Fig. 4 (d) also reveals that the CO–NH, CO2, and HCO3- peaks appear simultaneous during CO2 absorption. The CO2 peak at 125 ppm is a gas phase component, While the CO–NH and HCO3- peaks at 163, 165, and 166 ppm are liquid phase components. These data indicate that the CO2 chemical and physical absorptions of the amino groups occur simultaneously. Notably, an increase in the heights of the peaks at 163 and 166 ppm was observe following absorption. Thus, in the CO2 absorption
mechanism of [Apaeim] (Fig. 5), the CO2 chemical reaction of [Apaeim] with CO2 proceeds via the two [Apaeim] sites [10]. 3.2. CO2 absorption of multi-amino group-functionalized ionic liquids: cation and anion effects CO2 absorption experiments were carried out using 15 vol% CO2 under atmospheric pressure at 40
. The
CO2 absorption capacity of each multi-amino group-functionalized ionic liquid absorbent is listed in Table 3. All the multi-amino group-functionalized ionic liquids containing [Apaeim] cations presented better CO2 absorption (mol/L) units than that observed for 30 wt% MEA. The CO2 absorption results for each ionic liquid are displayed in Fig. 6. Fig. 6 (a) reveals that the CO2 absorption capacity of 30 wt% [Apaeim][OH] is approximately twice that of 30 wt% [Eaeim][OH] and 30 wt% [Paeim][OH]. This is because there are two reaction sites (amino groups) for [Apaeim][OH] and one reaction site for [Paeim][OH] and [Eaeim][OH]. Fig. 6 (b) indicates that 30 wt% [Paeim][amino acid] displays a slightly better CO2 absorption performance than 30 wt% [Eaeim][amino acid], while Fig. 6 (a) reveals that 30 wt% [Paeim][OH] and 30 wt% [Eaeim][OH] have similar CO2 absorption capacities. The
13
C-NMR spectrum of [Apaeim][OH] CO2 absorption (Fig 4.)
reveals that HCO3- molecules were formed by CO2 absorption, whereby both the amino acid and OH anions involved in the CO2 absorption process. The CO2 diffusion of a gas-liquid is affected by the mass transfer coefficient, so that the liquid film mass transfer coefficient decreases with an increase in the absorbent viscosity [45]. Fig. 7 illustrates the viscosities of 30 wt% [Paeim][OH] and 30 wt% [Eaeim][OH]. At lean loading, the viscosity of 30 wt% [Paeim][OH] is higher than 30 wt% [Eaeim][OH]; this viscosity difference increases at rich CO2 loading. Thus, due to its lower viscosity, 30 wt% [Eaeim][OH] present a higher gas-liquid mass transfer coefficient than 30 wt% [Paeim][OH]. Therefore, 30 wt% [Paeim][OH] and 30 wt% [Eaeim][OH] have similar CO2 absorption capacities. On the other hand, a comparison of the CO2 absorption capacities of 30 wt% [Eaeim][amino acid] and 30 wt% [Paeim][amino acid] reveals that the CO2 absorption performance of the latter is slightly better. The viscosities of 30 wt% [Paeim][amino acid] and 30 wt% [Eaeim][amino acid] are similar ; however, the [Paeim] cation has one more carbon number than the [Eaeim] cation. At high pressure, an ionic liquid absorbent having a long chain molecular structure presents a better absorption performance than one with a shorter or no chain [52]. Thus, under atmospheric pressure CO2 absorption conditions, ionic liquids comprising [Paeim] cations absorb more CO2 than those comprising [Eaeim] cations. Fig. 6 (b) reveals that for the ionic liquid absorbents comprising [Eaeim] and [Paeim] cations, the CO2 absorption capacity increases in the order [val] >[ala] >[gly]. However, Fig. 6 (c) illustrates that for the ionic liquid absorbents with [Apaeim], the CO2 absorption capacity increases in the order [ala] > [val] > [gly].
Because of the steric hindrance effect of the anion, synthetic ionic liquids comprising [ala] anions exhibit better CO2 absorption capacity than synthetic ionic liquids with [gly] anions [27]. However, no significant difference in the CO2 absorption capacity is observed. Alanine, glycine, and valine are non-polar aliphatic side chains consisting of only one primary amine functional group. Because the CO2 absorption experiment was conducted at atmospheric pressure, the cation effect on CO2 absorption is greater than that of the anion. For example, Zhang et al. [20] reported similar CO2 absorption capacities for the single amino-group functionalized phosphonium
ionic
liquids
(3-aminopropyl)tributylphosphonium
glycinate
([aP4443][gly]),
(3-
aminopropyl)tributylphosphonium alaninate ([aP4443][ala]), (3-aminopropyl)tributylphosphonium valinate ([aP4443][val]), and (3-aminopropyl)tributylphosphonium leucinate ([aP4443][leu]). Thus, the CO2 absorption of ionic liquids under atmospheric pressure seems to depend on the number of amino groups [53]. 3.3. CO2 desorption of multi-amino group-functionalized ionic liquids: cation and anion effects CO2 desorption experiments were conducted at atmospheric pressure and 80
under nitrogen gas bubbling.
The CO2 desorption results in Fig. 8 reveal the amounts of CO2 still present in the synthetic ionic liquid absorbents after CO2 desorption, i.e., the “lean CO2 loading”. Fig. 8 (a) illustrates that lean CO2 loading value of 30 wt% [Apaeim][OH] is higher than those of [Eaeim][OH] and [Paeim][OH], while the lean CO2 loading of 30 wt% [Eaeim][OH] is slightly higher than that of 30 wt% [Paeim][OH]. On the other hand, Fig. 8 (b) indicates that except for [Eaeim][gly], the lean CO2 loadings of 30 wt% [Eaeim][amino acid] are similar to those of 30 wt% [Paeim][amino acid]. Fig. 8 (c) indicates that the lean CO2 loadings of 30 wt% all the [Apaeim][amino acid] samples are similar at 1.5 mol/mol. Overall, the lean CO2 loadings of the synthetic ionic liquids comprising the [Apaeim] cation are higher than those of the synthetic ionic liquids comprising [Eaeim] and [Paeim]. These results therefore suggest, that the lean CO2 loading is more affected by the number of amino groups on the ionic liquid. Singh et al. [54] compared the lean CO2 loading of amines depending on the number of the amino group and effect of the side chain. They revealed the lean CO2 loading values increased with the increase in the number of amino groups in the tested amines, regardless of the CO2 absorption capacity. The comparison of the CO2 desorption results of 1-2-diamino propane and N-(2-hydroxyethyl)ethylenediamine), revealed that the effect of the side chain on the lean CO2 loading is less significant than the effect of the number of amino groups. The lean CO2 loading estimation model for amines proposed by Momeni et al. [55] is necessary to determine the number of NH2, and NH groups and the charge of the NH moiety. This model displayed a similar trend to that of the actual experimental results, when compared with the experimental data on 20 types of amines. This led to
the conclusion that lean CO2 loading is mainly affected by the number of amino groups. In the case of 30 wt% synthetic ionic liquids, the anion effect is less in lean CO2 loading because alanine, glycine, and valine contain the same number amino groups. 3.4. CO2 cyclic capacities,and CO2 absorption and desorption rates Table 3 lists the CO2 cyclic capacities, CO2 absorption and desorption rates of multi-amino group functionalized ionic liquids. The CO2 cyclic capacity is the difference between the rich and lean CO2 loadings. We defined the CO2 absorption rate at 90% absorption as the CO2 absorption rate at 90% rich CO2 loading. Similarly, the CO2 desorption rate at 90% desorption was defined as the CO2 desorption rate at 90% desorption. The CO2 absorption rates of the 30 wt% synthetic ionic liquids were slightly slower than that observed for 30 wt% MEA, while the CO2 cyclic capacities were more than twice that of 30 wt% MEA. The CO2 desorption rate of
the30 wt% synthetic ionic liquids were two- or threefold faster than that of 30 wt% MEA. Moreover, the
CO2 desorption rates of the synthetic ionic liquids comprising [Eaeim] and [Paeim] cations were slightly faster than those of the synthetic ionic liquids comprising the [Apaeim] cation. Table 3 and Fig. 3 and 4 illustrate that the CO2 absorption of the synthetic ionic liquids was accompanied by physical absorption. Theoretically, two moles of MEA [56] or one mole of [Apaeim] cations are required to chemically absorb one mole of CO2. The rich CO2 loadings of 30 wt% MEA and [Apaeim][OH] are 0.614 and 3.65 mol/mol, respectively (Table 3). These data clearly indicates that the synthetic ionic liquid [Apaeim][OH] absorbs more CO2 than MEA, and the theoretically predicted chemical CO2 absorption capacity. The amount of physical absorption of the synthetic ionic liquids is greater than that of MEA due to the small molecule number of the synthetic ionic liquids of the same mass [57]. The CO2–NH bonds are formed via CO2 chemical absorption while the CO2 molecules bind weakly with the N and F atoms via physical absorption. CO2 removal is easier for the physically absorbed CO2 than for the chemically absorbed CO2 and thus, the CO2 desorption rates of the 30 wt% multi-amino group functionalized-ionic liquids are faster than that of 30 wt% MEA. 3.5. Viscosity of the 30 wt% multi-amino group-functionalized ionic liquids The viscosity data of the 30 wt% synthetic ionic liquids are displayed in Fig. 7. The viscosities were measured at atmospheric pressure and 40
at the lean CO2 loading state. Overall, the synthetic ionic liquids with OH anions
presented high viscosities. A comparison of the three ionic liquids revealed that diethanolamine (DEA) is more viscous than aminoethylethanolamine (AEEA) and diethylenetriamine (DETA) [58, 59]. DEA comprises one amino group and two hydroxyl groups, AEEA comprises two amino groups and one hydroxyl group, and DETA
comprises three amino groups and no hydroxyl group. All three amines have similar alkyl groups. The comparison of the chemical structures of these amines, suggested that the viscosity increase with an increase in the strong intermolecular hydrogen bonding of the hydroxyl group. Among the synthetic ionic liquids with OH anions, 30 wt% [Apaeim][OH] was the most viscous because it has one more amino group than [Eaeim][OH] and [Paeim][OH]. In turn, [Paeim][amino acid] and [Paeim][OH] are more viscous than [Eaeim][amino acid] and [Eaeim][OH]. The experimental results indicate the viscosity increases with increasing cation alkyl chain length. The same trend was reported in other studies. Thus, for the imidazolium cation, the viscosity was higher for longer cation alkyl chains [60]. On the other hand, for the solutions, the valine anion, with a chain length longer than those of glycine and L-alanine, presented the lower viscosity. This occurred because water molecules reduce the intermolecular interactions between the cations and anions [61]. Moreover, valine has weak intermolecular interactions in the solvation state due to a more sterically hindered structure than those of alanine and glycine. Notably, 30 wt% [Apaeim][OH] and 30 wt% [Apaeim][ala] presented higher CO2 absorption capacities than the other synthetic ionic liquids. However, the high viscosity of 30 wt% [Apaeim][OH] is due to the strong intermolecular interactions caused by hydrogen bonding. Therefore, it was concluded that the best absorbent in this study is 30 wt% [Apaeim][ala]. Overall, the viscosities of the 30 wt% synthetic ionic liquids are not significantly higher than that of 30 wt% MEA.
4. Conclusion In this study, multi-amino group-functionalized ionic liquids were synthesized in reasonable yields. The structures of the synthetic ionic liquids were confirmed by 1H-NMR spectroscopy, while the CO2 absorption mechanism of the synthesized ionic liquids was confirmed by 13C-NMR and FT-IR analyses. All the synthesized 30 wt% AAILs presented similar viscosities compareble to that of 30 wt% MEA. For the CO2/absorbent L as the unit of the CO2 absorption capacity, 30 wt% [Apaeim][amino acid] and 30 wt% [Apaeim][OH] presented higher CO2 absorption capacities than that of 30 wt% MEA, a commercial amine. Among them, 30 wt% [Apaeim][OH] and 30 wt% [Apaeim][ala] displayed higher CO2 absorption capacities than those of the other amines. With respect the viscosity, the best absorbent in this study was 30 wt% [Apaeim][ala]. The rich CO2 loading of 30 wt% [Apaeim][ala] was 1.39-fold higher than that of 30 wt% MEA, while the CO2 cyclic capacity of 30 wt% [Apaeim][ala] was 2.2-fold better than that of 30 wt% MEA. The number of amino groups in the ionic liquid cations of ionic liquid has a dominant influence on the CO2 absorption. On the other hand, the steric hindrance of the anions in the ionic liquid has little effect on the rich CO2 loading of the absorbent. Therefore, it was concluded that manipulating the number of amino groups in the ionic liquid cations could be an effective way to control the CO2 absorption capacity.
Acknowledgements This study has been conducted with the support of the Korea Institute of Industrial Technology as “Development of Gas-phase pollutant removal technology” (Project No.: EO-19-0011). And this study has been conducted with the support of the Korea Institute of Industrial Technology as “Development of Carbon Dioxide Capture and Utilization Technology for Greenhouse Gas Reduction” (Project No.: IZ-18-0050). This study also has been conducted with the support of the Korea Institute of Industrial Technology as “Development of membrane contactor based movable gas scrubbing one-module” (Project No.: NS-19-0047).
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Table 1. The 1H NMR data and yields of all the prepared synthetic ionic liquids Table 2. Names and structures of the multi-amino group-functionalized ILs (ionic liquids) Table 3. CO2 cyclic capacities and CO2 absorption/desorption rates of the multi-amino group functionalized ionic liquids
Table 1. The 1H NMR data and yields of all the prepared synthetic ILs Ionic liquid [Apaeim][OH]
1
H-NMR, δ (ppm) 7.5 (s, 1H), 6.98 (s, 1H), 6.78 (d, 1H), 3.95 (t, 4H), 2.64 (q, 4H), 1.89 (q, 2H), 1.07 (t, 4H)
Yideld 100 %
[Eaeim][OH]
8.06 (s, 1H), 7.64 (m, 1H), 7.13 (d, 1H), 4.27 (tt, 2H), 3.08 (t, 2H), 2.78 (f, 2H), 1.55 (m, 2H), 1.14 (t, 3H), 0.9 (t, 3H)
100 %
[Paeim][OH]
7.66 (m, 1H), 7.12 (s, 1H), 6.96 (s, 1H), 4.23 (tt, 2H), 3.99 (t, 2H), 3.08 (br, 2H), 1.94 (m. 2H), 1.25 (t, 2H), 1.0 (t, 3H)
100 %
[Apaeim][ala]
7.7 (s, 1H), 7.2 (s, 1H), 7.0 (d, 1H), 4.29 (br, 2H), 4.16 (t, 4H), 2.97(br, 2H), 2.85 (q, 4H), 2.1 (q, 2H), 1.98 (m, 1H), 1.45 (d, 3H)
75.8 %
[Apaeim][gly]
7.7 (s, 1H), 7.15 (s, 1H), 7.0 (d, 1H), 4.25 (br, 2H), 4.12 (t, 4H), 3.05 (br, 2H), 2.67 (q, 4H), 1.97 (q, 2H), 1.43 (d, 1H)
78.7 %
[Apaeim][val]
7.67 (s, 1H), 7.15 (s, 1H), 6.97 (d, 1H), 4.27 (br, 2H), 4.10 (t, 4H), 3.05 (br, 2H), 2.66 (q, 4H), 1.97 (q, 2H), 1.44 (d, 1H), 1.02 (dd, 6H)
98 %
[Eaeim][ala]
7.66 (m, 1H), 7.13 (s, 1H), 6.95 (s, 1H), 4.28 (m, 2H), 4.06 (f, 2H), 3.08 (q, 2H), 2.91 (br, 2H), 1.56 (t, 3H), 1.44 (t, 2H)
53.17 %
[Eaeim][gly]
7.68 (m, 1H), 7.15 (s, 1H), 6.95 (s, 1H), 4.28 (m, 2H), 4.07 (f, 2H), 3.08 (m, 2H), 2.87 (t, 2H), 2.63 (br, 2H), 1.56 (t, 3H), 1.43 (t, 2H)
72.14 %
[Eaeim][val]
7.66 (m, 1H), 7.13 (s, 1H), 6.95 (s, 1H), 4.29 (m, 2H), 4.05 (f, 2H), 3.07 (m, 2H), 2.88 (br, 2H), 2.63 (br, 2H), 1.56 (t, 3H), 1.43 (t, 2H), 1.02 (dd, 6H)
68.28 %
[Paeim][ala]
7.64 (m, 1H), 7.11 (s, 1H), 6.93 (s, 1H), 4.21 (tt, 2H), 4.05 (t, 2H), 3.99 (t, 2H), 3.08 (t, 2H), 2.98 (br, 2H), 1.93 (m, 2H), 1.81 (m, 2H), 1.45 (d, 3H), 1.02 (t, 3H)
63.08 %
[Paeim][gly]
7.75 (m, 1H), 7.21 (s, 1H), 6.97 (s, 1H), 4.49 (t, 2H), 4.36 (t, 2H), 4.22 (t, 2H), 4.03 (t, 2H), 3.38 (f, 2H), 3.06 (br, 2H), 1.93 (m, 2H), 1.81 (m, 2H), 1.02 (t, 3H)
63.77 %
[Paeim][val]
7.68 (m, 1H), 7.15 (s, 1H), 6.97 (s, 1H), 4.34 (t, 2H), 4.21 (f, 2H), 4.00 (t, 2H), 3.2 (t, 2H), 3.08 (br, 2H), 1.94 (m, 2H), 1.81 (m, 2H), 1.02 (t, 3H)
68.18 %
Table 2. Names and structures of the multi-amino group-functionalized ILs (ionic liquids) IL name
IL abbreviation
1-(3-aminopropyl)-3-(2aminoethyl)imidazolium L-alaninate
[Apaeim][ala]
1-(3-aminopropyl)-3-(2aminoethyl)imidazolium glycinate
[Apaeim][gly]
1-(3-aminopropyl)-3-(2aminoethyl)imidazolium valinate
[Apaeim][val]
1-ethyl-3-(2-aminoethyl)imidazolium alaninate
L-
[Eaeim][ala]
1-ethyl-3-(2-aminoethyl)imidazolium glycinate
[Eaeim][gly]
1-ethyl-3-(2-aminoethyl)imidazolium valinate
[Eaeim][val]
IL structure
1-propyl-3-(2-aminoethyl)imidazolium Lalaninate
[Paeim][ala]
1-propyl-3-(2-aminoethyl)imidazolium glycinate
[Paeim][gly]
1-propyl-3-(2-aminoethyl)imidazolium valinate
[Paeim][val]
Table 3. CO2 cyclic capacities and CO2 absorption/desorption rates of the multi-amino group functionalized ionic liquids
Absorbents
30 wt% monoethanolamine (MEA) 30 wt% [Apaeim][OH] 30 wt% [Eaeim][OH] 30 wt% [Paeim][OH] 30 wt% [Apaeim][gly] 30 wt% [Apaeim][ala] 30 wt% [Apaeim][val] 30 wt% [Eaeim][gly] 30 wt% [Eaeim][ala] 30 wt% [Eaeim][val] 30 wt% [Paeim][gly] 30 wt% [Paeim][ala] 30 wt% [Paeim][val]
Rich CO2 loading (mol CO2/ mol IL)
Rich CO2 loading (mol CO2/ L absorbents)
CO2 cyclic capacity (mol CO2/ mol IL)
CO2 cyclic capacity (mol CO2/ L absorbents)
CO2 absorption rate ( 105mol CO2/L * s) at 90 % absorption
CO2 desorption rate ( 105mol CO2/L * s) at 90 % desorption
0.614
3.02
0.254
1.27
39.98
10.11
3.75
4.19
2.54
2.84
28.87
23.19
2.19
2.74
1.73
2.17
32.2
25.08
2.15
2.54
1.96
2.32
31.22
23
4.19
3.89
2.82
2.62
29.82
25.01
4.72
4.19
3.14
2.8
29
22.91
4.44
3.64
2.95
2.42
25.78
25.43
1.97
1.99
1.91
1.93
35.12
31.56
2.44
2.54
2.09
2.1
26.83
29.54
2.9
2.56
2.35
2.07
36.72
28.29
2.45
2.35
1.95
1.87
30.87
27.52
2.67
2.46
2.19
2.02
39.37
30.87
2.96
2.5
2.65
2.23
36.58
27.11
Fig. 1. Synthetic route of the synthesized amino group-functionalized imidazolium-based amino acid ionic liquid [Apaeim][gly] Fig. 2. Schematic diagram of the apparatus employed for CO2 absorbent screening Fig. 3. FT-IR data of [Apaeim][OH]: (a) before and (b) after CO2 absorption Fig. 4. 13C-NMR data of [Apaeim][OH]: (a) before,
(b) during – 1 h, and (c) after CO2 absorption
Fig. 5. CO2 chemical absorption mechanism of the synthesized amino-group functionalized imidazolium-based ionic liquids – imidazolium cation Fig. 6. Rich CO2 loading data of the synthetic ionic liquids. Comparison of the CO2 absorption performance of: (a) [Apaeim][OH], [Paeim][OH], [Eaeim][OH]; (b) [Paeim][amino acid] and [Eaeim][amino acid]; and (c) [Apaeim][amino acid] Fig. 7. Viscosity of the 30 wt% synthetic ionic liquids at atmospheric pressure and 40
(unit: cP)
Fig. 8. Lean CO2 loading data of the synthetic ionic liquids. Comparison of CO2 the desorption performance of: (a) [Apaeim][OH], [Paeim][OH], [Eaeim][OH]; (b) [Paeim][amino acid] and [Eaeim][amino acid]; and (c) [Apaeim][amino acid]
Fig. 1. Synthetic route of the synthesized amino group-functionalized imidazolium-based amino acid ionic liquid [Apaeim][gly]
Fig. 2. Schematic diagram of the apparatus employed for CO2 absorbent screening
Fig. 3. FT-IR data of [Apaeim][OH]: (a) before and (b) after CO2 absorption
(a)
(b)
Fig. 4. 13C-NMR data of [Apaeim][OH]: (a) before,
(a)
(b)
(c)
(d)
(b) during – 1 h, and (c) after CO2 absorption
Fig. 5. CO2 chemical absorption mechanism of the synthesized amino-group functionalized imidazolium-based ionic liquids
imidazolium cation
Fig. 6. Rich CO2 loading data of the synthetic ionic liquids. Comparison of the CO2 absorption performance of: (a) [Apaeim][OH], [Paeim][OH], [Eaeim][OH]; (b) [Paeim][amino acid] and [Eaeim][amino acid]; and (c) [Apaeim][amino acid]
(a)
(b)
(c)
Fig. 7. Viscosity of the 30 wt% synthetic ionic liquids at atmospheric pressure and 40
(unit: cP)
Fig. 8. Lean CO2 loading data of the synthetic ionic liquids. Comparison of CO2 the desorption performance of: (a) [Apaeim][OH], [Paeim][OH], [Eaeim][OH]; (b) [Paeim][amino acid] and [Eaeim][amino acid]; and (c) [Apaeim][amino acid]
(a)
(b)
(c)
CO2 Absorption Characteristics of Amino Group Functionalized Imidazolium-based Amino Acid Ionic Liquids Sehee Kanga,b, Yongchul G. Chungb, Jo Hong Kanga and Hojun Songa* a
Green Materials & Processes R&D group, Korea Institute of Industrial Technology, 55 Jongga-ro, Jung-gu, Ulsan 44413, South Korea b School of Chemical and Biomolecular Engineering, Pusan National University, 2 Busandaehak-ro, 63 beon-gil, Geumjeong-gu, Pusan 46241, South Korea
Highlights: 1) Amino acid ionic liquids are synthesized as CO2 absorbents for postcombustion CO2 capture; 2) CO2 absorption mechanisms are analyzed by 13C-NMR and FT-IR spectroscopy; 3) [Apaeim][amino acid] presented a superior CO2 absorption performance than 30 wt% monoethanolamine; 4) The number of amino groups in the ionic liquids affects their CO2 absorption capacity.
CO2 Absorption Characteristics of Amino Group Functionalized Imidazolium-based Amino Acid Ionic Liquids Sehee Kanga,b, Yongchul G. Chungb, Jo Hong Kanga and Hojun Songa* a
Green Materials & Processes R&D group, Korea Institute of Industrial Technology, 55 Jongga-ro, Jung-gu, Ulsan 44413, South Korea b School of Chemical and Biomolecular Engineering, Pusan National University, 2 Busandaehak-ro, 63 beon-gil, Geumjeong-gu, Pusan 46241, South Korea
None declared.
S0167-7322(19)33531-7
DOI:
https://doi.org/10.1016/j.molliq.2019.111825
Reference:
MOLLIQ 111825
To appear in:
Journal of Molecular Liquids
Received Date: 24 June 2019 Revised Date:
21 September 2019
Accepted Date: 25 September 2019
Please cite this article as: S. Kang, Y.G. Chung, J.H. Kang, H. Song, CO2 absorption characteristics of amino group functionalized imidazolium-based amino acid ionic liquids, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/j.molliq.2019.111825. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
CO2 Absorption Characteristics of Amino Group Functionalized Imidazolium-based Amino Acid Ionic Liquids Sehee Kanga,b, Yongchul G. Chungb, Jo Hong Kanga and Hojun Songa* a
Green Materials & Processes R&D group, Korea Institute of Industrial Technology, 55 Jongga-ro, Jung-gu, Ulsan 44413, South Korea
b
School of Chemical and Biomolecular Engineering, Pusan National University, 2 Busandaehak-ro, 63beon-gil, Geumjeong-gu, Pusan 46241, South Korea
Abstract In this study, dual amino group functionalized imidazolium amino acid ionic liquids (AAILs) were synthesized from imidazole, bromoalkylamine, and amino acids. 1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium hydroxide ([Apaeim][OH]), 1-propyl-3-(2-aminoethyl)imidazolium hydroxide ([Paeim][OH]), and 1-ethyl-3-(2aminoethyl)imidazolium hydroxide ([Eaeim][OH]) were synthesized in two-steps, while [Apaeim][amino acid], [Paeim][amino acid], and [Eaeim][amino acid] were synthesized in three-steps. The structures of the synthesized AAILs were confirmed by 1H-NMR analysis. Moreover, the CO2 absorption mechanism of [Apaeim][OH] was confirmed by FT-IR and
13
C-NMR analyses. Subsequently, CO2 absorption-desorption tests were conducted
under at 15 vol% CO2, 313K and atmospheric pressure. The cation and anion effects on the ionic liquid absorbents were also investigated. Among the tested ionic liquid solutions, 30 wt% [Apaeim][OH] and 30 wt% 1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium alaninate ([Apaeim][ala]) displayed the best performances. Thus, both presented CO2 cyclic capacities 2.2-fold higher than that of the benchmark CO2 absorbent 30 wt% monoethanolamine (MEA), while their viscosities were comparable to that of 30 wt% MEA. Notably, 30 wt% [Apaeim][ala] show great potential as a CO2 absorbent due to its high CO2 cyclic capacity and low viscosity. Keywords: CO2 absorption, Amino group functionalization, Ionic liquids, Cyclic capacity, Viscosity * Corresponding author. Tel.: +82-52-980-6670; fax:+82-52-980-6669 E-mail address: [email protected].
1. Introduction From 2030 to 2050, the energy strategy and policy of the European Union (EU) will focus on increasing renewable energy sources (RES) and reducing greenhouse gas (GHG) emissions. The rising global average temperature due to the greenhouse effect has been attributed to the ever increasing GHG emissions [1]. Thus, the EU target is to reduce GHG emission by 40% by 2030 [2]. The Paris Climate Change Convention (PCCC) was adopted to maintain the global average temperature rise below two degrees to prevent global warming [3-4]. However, CO2 is a renewable carbon resource, which inexpensive, non-toxic, and nonflammable [5-6]. Moreover, simple organic materials can be produced through a green process using CO2 [7]. CO2 emissions account for 76% of the total industrial emissions [8-10] and thus, it is necessary to develop technology to capture and reuse industrially emitted CO2. Carbon dioxide capture, utilization, and sequestration (CCUS) technology is expected to be a powerful industrial tool to help reduce 45 % of the total CO2 emissions by 2050 [11]. One of the widest applied industrial techniques for capturing CO2 is chemical absorption by aqueous solutions of alkanolamines. Alkanolamines, such as monoethanolamine (MEA), have been used industrially for > 100 years to remove CO2 from flue gas emissions from fossil fuel combustion [12]. However, alkanolamines require a vast amount of energy to strip CO2 from CO2-saturated absorbents, necessary for the regeneration and reuse of CO2 absorbents [13]. Thus, operating costs are high due to high absorbent regeneration energy in the CO2 regeneration process [14]. Additionally, alkanolamines present many disadvantage such as absorbent loss due to the high regeneration temperature, corrosion, and oxidative and thermal degradation [15-18]. Ionic liquids have been attracted significant attention as alternative CO2 absorbents due their specific characteristics including negligible vapor pressures, low heat capacity, liquid state in a wide temperature range, and high thermal and chemical stabilities [19]. Generally, ionic liquids that are in the liquid phase in a wide temperature range are classified as room temperature ionic liquids (RTILs). Many RTILs, for example, ones that comprise nitrate ([NO3]-) and dicyanamide ([DCA]-) anions, effectively absorb CO2 under high pressures (> 90 bar) [20]. However, the utilization of conventional RTILs in CO2 absorption is restricted, because the pressure of most discharged flue gases approaches atmospheric pressure. The physical and chemical properties of ionic liquids can be specifically designed by altering the combination of cations and anions [21]. Thus, optimal ionic liquids can be synthesized with characteristics such as non-toxicity, low viscosity, low melting point, thermal stability, and low cost. Ionic liquids synthesized for a specific purpose are called task-specific ionic liquids (TSILs) [22,23].
The physical solubility of CO2 is greatly high in ILs. Moreover, extremely high CO2 capacity in TSILs can be obtained, for instance, amine-functionalized ILs can add tunable and specific chemical reaction with CO2 because CO2 take a cue from chemistry of amine solution [24-27]. Amine-functionalized ILs have large CO2 capacity, which is due to chemical absorption rather than physical absorption. One molecule of CO2 reacts with two molecules of amine; the reaction proceed via carbamate and ammonium intermediates (equations 1 and 2) [28, 29].
+ ↔ + ↔ 1 +
(1) (2)
Specifically, AAILs, in which the deprotonated amino acid is used as an anion or cation, display many advantages over other TSILs [30]. Thus, AAILs are biodegradable, inexpensive, easy to dissolve in biocompatible materials such as DNA and cellulose, and easily blended into organic solvents [31,32]. Additionally, amino acids can chemically absorb CO2 due to the presence of the amino group [33]. Several AAILs have been recently synthesized as CO2 absorbents. Shahrom et al. [34] synthesized polymer benzyl ammonium-based AAILs, using eight amino acid anions and varying the amino acid and cation polymer groups of their products. A comparison of the CO2 absorption characteristics of the different products revealed that polyvinlybenzyltrimethylammonium arginate (poly[VBTMA][arg]) displayed the highest CO2 absorption capacity of 1.14 mol CO2/mol ionic liquid (IL) under 1 atm CO2, 298 K. However, this CO2 absorption capacity may not be high enough due to the high molecular weight of the polymeric ionic liquids. Sistla et al. [35] synthesized imidazolium-based AAILs. They used nine amino acid anions with different numbers of amino groups and compared the CO2 absorption characteristics when varying the number of amino groups in the amino acid. Among them, butylmethylimidazolium arginate ([Bmim][arg]) displayed the highest CO2 absorption capacity of 0.62 mol CO2/mol IL under 1 atm CO2. Ionic liquids, comprising anions with many amino groups present higher CO2 absorption capacities. Although the CO2 removal performances of the above-stated AAILs are satisfactory, the synthetic ionic liquids reported in other studies are expensive because commercially available ionic liquids are used as precursors for their syntheses and the synthetic yields are relatively low. Filippov et al. [36] synthesized choline-based AAILs and compared their CO2 absorption characteristics according to the amino acid-type and alkyl group length of the cation. Among them, [N1,1,5,2OH][tau] exhibited the highest CO2 absorption of 0.55 mol CO2/mol IL under 1 atm CO2. Yuan et al. [37] synthesized choline-based AAILs, whereby 30 wt% cholinium glycinate ([Cho][gly]) presented the highest CO2 absorption of 0.578 mol CO2/mol IL under 1 atm CO2. The synthetic cost of choline-based ionic liquids would be lower than other TSILs,
because choline hydroxide solution is commercially available. However, although choline-based ionic liquids are less expensive than other ionic liquids, they display relatively lower CO2 absorption capacities than other ionic liquids such as poly[VBTMA][arg] and [Bmim][arg]. Saravanamurugan
et
al.
[38]
synthesized
ammonium-based
AAILs
whereby
13.1
wt%
trihexyl(tetradecyl)ammonium lysinate ([N66614][Lys]) presented the highest CO2 absorption of 2.1 mol CO2/mol IL under 1 bar CO2. Zhou et al. [39] synthesized amino group functionalized imidazolium-based AAILs with 1aminopropyl-3-methylimidazolium lysinate ([Apmim][Lys]) presenting the highest CO2 absorption of 1.8 mol CO2/mol IL under 30 mL/min CO2, 303.15 K. These absorbents display high CO2 absorption capacities but use other ionic liquids as precursors for their synthesis. Therefore, these ionic liquid absorbents would be expensive. Hence, the CO2 absorption test was carried out using a very small amount of ionic liquid absorbent and pure CO2. In this study, amino group-functionalized imidazolium-based AAILs were synthesized, with glycine, Lalanine, and valine as amino acid anions. Each amino acid comprised the same number of amino groups, but a different alkyl group attached to the carbon atom near the amino group. Dual amino group functionalized imidazolium-based AAILs were also synthesized to improve the CO2 absorption capacity. Other amino group functionalized imidazolium AAILs were synthesized to compare the alkyl group effect on the CO2 absorption performance.
Thus,
1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium
([Apaeim]),
1-ethyl-3-(2-
aminoethyl)imidazolium ([Eaeim]), and 1-propyl-3-(2-aminoethyl)imidazolium ([Paeim]) were also synthesized. Fig. 1 illustrates the structures of the synthesized ionic liquids identified by 1H-NMR spectroscopy. The CO2 absorption mechanism of 1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium hydroxide ([Apaeim][OH]) was also identified through FT-IR and
13
C-NMR spectroscopy. Subsequently, the CO2 absorption-desorption test was
conducted on each multi-amino group functionalized ionic liquids using 15 vol% CO2 balanced with N2. The CO2 absorption capacity and viscosity characteristics of the 30 wt% aqueous solutions were explored with respect to the structure of each anion and cation. In addition, the CO2 absorption characteristics of the 30 wt% AAILs were compared with those of 30wt% MEA, a benchmark alkanolamine solution.
2. Experimental Section 2.1. Chemicals The analytic grade reagents 2-bromoethylamine hydrobromide (98 wt%), 1-ethylimidazole (98 wt%), 1-(3aminopropyl)imidazole (97 wt%), glycine (99 wt%), L-alanine (99 wt%), and L-valine (98 wt%) were purchased from TCI Co. Ltd. (Japan). 1-Propylimidazole (98 wt%) were purchased from Aladdin Chemical Co.. Potassium
hydroxide flake (93 wt%) were purchased Deajung Chemicals & Metals Co. Ltd. (Korea). The anion exchange resin (AW90 OH form) were purchased from Iontech Co. Ltd (Korea), respectively. All the solvents, including anhydrous ethanol (99.5 wt%), methanol (99.5 wt%), anhydrous dichloromethane (99.9 wt%), and acetonitrile (99.5 wt%) were purchased from Deajung Chemicals & Metals Co. Ltd. (Korea). All the chemicals were as received without further purification, but the purities of all chemicals were considered. CO2 (99.99 vol%) and N2 (99.99 vol%) were supplied by Korea Gas & Electric technology Co. Ltd.(Korea). 2.2. Synthetic procedures The synthetic procedures of the amino group-functionalized imidazolium-based AAILs adopted in this study are as follows (Fig. 1): 1- 1-Ethylimidazole (as well as 1-propylimidazole, and 1-aminopropylimidazole) and 2-bromoethylamine hydrobromide were used as starting materials to synthesize the ionic liquids. Typically, an Erlenmeyer flask with the cap equipped with a magnetic stirrer was charged with 1-ethylimidazole (0.1 mol) and ethanol (70 mL). 2-Bromoethylamine hydrobromide (0.1 mol) was slowly added dropwise to the Erlenmeyer flask over 10 min at 298.15 K. The resulting mixture was stirred for 30 min at 298.15 K and then slowly warmed to 343.15 K over 24 h. The solvents were removed by rotary evaporation and a highly viscous colloidal liquid was formed. Subsequently, KOH aqueous solution (pH 8) (100 mL) was mixed strongly with the highly viscous colloid reactant to remove any Br ions remaining in the solution. The remaining solvent was removed by rotary evaporation to form solid 1-ethyl-3-(2aminoethyl) imidazolium bromide ([Eaeim][Br]), which was dried in vacuo, at 353.15 K for 24 h [40]. 2- An Erlenmeyer flask with the cap equipped with a magnetic stirrer was charged with [Eaeim][Br] (0.1 mol) and anhydrous dichloromethane (70 mL). Potassium hydroxide (0.1 mol) was slowly added dropwise to the Erlenmeyer flask over a period of 30 min at 298.15 K. The resulting mixture was stirred for 12 h using a magnetic stirrer, at 298.15 K. The resultant potassium bromide (KBr) liquid reactant mixture was filtered to removes the precipitated KBr. Subsequently, the solvent was removed by rotary evaporation to form a viscous liquid of 1-ethyl-3-(2-aminoethyl) imidazolium hydroxide ([Eaeim][OH]), which was dried in vacuo at 353.15 K for 24 h [41]. Another method for displacing the Br anions with OH anions was via anion exchange resin. In this procedure, a column filled with the anion exchange resin AW90 (OH- form) and [Eaeim][Br] (0.1 mol) dissolved in ethanol (100 mL) was passed through the resin. The solvent was removed by rotary evaporation to afford a viscous liquid of [Eaeim][OH], which was dried in vacuo, at 353.15 K for 24 h.
3- An Erlenmeyer flask with the cap equipped with a magnetic stirrer was charged with an aqueous solution of glycine, [Eaeim][OH] was slowly added dropwise at 298.15 K over a period of 1 h using a syringe pump. The resulting mixture was stirred for 24 h using a magnetic stirrer at 298.15 K. Once the reaction was complete, the water was removed by rotary evaporation. Subsequently, an acetonitrile/ methanol mixture (4:1 (vol)) was strongly mixed with the reactant to remove any glycine remaining in the water. The precipitated glycine was then remover by filtration; this step was repeated two or three fold. The remaining solvent was removed by rotary evaporation to form a viscous liquid of 1-ethyl-3-(2-aminoethyl) imidazolium glycinate ([Eaeim][gly]), which was dried in vacuo, at 303.15 K for 30 h [42]. Other ionic liquids, including 1-ethyl-3-(2-aminoethyl)imidazolium L-alaninate ([Eaeim][ala]), 1-ethyl-3(2-aminoethyl)imidazolium
valinate
([Eaeim][val]),
1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium
glycinate ([Apaeim][gly]), 1-(3-aminopropyl)-3-(2-aminoethyl)imidazolium alaninate) ([Apaeim][ala]), 1(3-aminopropyl)-3-(2-aminoethyl)imidazolium valinate ([Apaeim][val]), [Apaeim][OH], 1-propyl-3-(2aminoethyl)imidazolium glycinate ([Paeim][gly]), 1-propyl-3-(2-aminoethyl)imidazolium L-alaninate ([Paeim][ala]), 1-propyl-3-(2-aminoethyl)imidazolium hydroxide ([Paeim][OH]) were also prepared using the same method. 2.3. Characterization of the synthesized multi-amino group-functionalized ionic liquids The molecular structures of the multi-amino group functionalized ionic liquids prepared in this study were identified via 1H-NMR spectroscopy (Bruker 400 MHz spectrometer) using CD3OD as the solvent with tetramethylsilane (TMS) as the internal standard [43]. The viscosities of the 30 wt% synthetic ionic liquid absorbents were measured under lean CO2 loading at 40
and atmospheric pressure. The viscosities were
measured using a DV-II+ Pro viscometer (Brookfield Co. Ltd., USA) with an uncertainty of 1% in relation to the full scale; spindle 18 (SC-18) was employed. At least three runs were conducted for each measurement. FTIR and
13
C-NMR spectroscopy were used to observe the CO2 absorption mechanism of [Apaeim][OH]. The
NMR spectra were recorded on a Bruker 400 MHz spectrometers using CD3OD as a solvent. The FT-IR spectra were recorded using a JASCO (FT/IR-4600 type A) instrument in the region 4000-500 cm-1 in diamond attenuated total reflectance (D-ATR) measurement mode. 2.4. CO2 absorption and desorption tests The CO2 absorbent screening test apparatus (Fig. 2) comprised of N2 and CO2 gas tanks, N2 and CO2 flow rate controllers, a bubble column reactor, and a gas analyzer. First, when N2 and CO2 valves were opened, the gas
flow rate was adjusted by each gas flow rate controller. The concentration of the CO2 gas before entering the reactor was monitored and adjusted using an NDIR gas analyzer (Multi Master, Sensoronic Co. Ltd.). The gas analyzer displayed the real time CO2 concentration in the reactor or gas mixer. The CO2 concentration was set to 15 vol%, a value similar to the typical CO2 concentration of the flue gas emitted from coal-fired thermal power plants, by adjusting the flow rate controller. About 100 mL prepared CO2 absorbent was fed to the reactor (internal volume = 250 mL). The water bath temperatures were set at 313.15 and 353.15 K for the CO2 absorption and desorption tests, respectively : typically, in an amine CO2 absorption process, CO2 absorption occurs at 313.15 K, while we believe that a temperature of 80
is reasonable for the fast screening of the CO2
absorbents [44, 45]. Second, 15 vol% CO2 gas was introduced into the liquid absorbent by bubbling. The pipe of the condenser connected to the reactor was connected to a gas analyzer that measured the CO2 concentration in the bubble reactor. When the valve connected to the reactor was closed and the valve connected to the analyzer was open, the analyzer displayed the composition inside the reactor. Normally, the CO2 concentration of the CO2 analyzer during CO2 absorption was <15 vol% and the CO2 absorption reaction was complete when the CO2 concentration reached 14.8 vol%. N2 gas was purged at 353.15 K during CO2 desorption. The CO2 desorption reaction was terminated when the analyzer CO2 concentration reached 0 vol%. Based on the logged gas analyzer data, we could calculate the reaction rate, absorption amount, and CO2 cyclic capacity : By integrating the CO2 concentration data, the CO2 loading can be calculated as CO loading CO loading
mol CO Accumulated absorption capacity (mol CO ) = mol IL Mole of ionic liquid (mol IL)
mol CO Accumulated absorption capacity (mol CO ) = L Volume of absorbent (L)
(1)
(2)
Then the CO2 absorption rate is calculated as CO absorption rate
3.
mol Absorption capacity of CO (mol) = L×s Volume of absorbent (L) × Unit time (s)
(3)
Results and Discussion
3.1. H-NMR and FT-IR data of the multi-amino group-functionalized ionic liquids The 1H-NMR spectra were recorded at 297 K on a Bruker Avance 400 MHz spectrometer, with CD3OD and TMS as the solvent and internal standard, respectively.
The 1H NMR data and yields of all prepared synthetic ILs are listed in Table 1 [46]: The synthetic yield depended on the cation type and was the highest for the ionic liquids comprising [Apaeim] cations. The imidazole protons were observed at 7–8 ppm in all the synthetic ionic liquids [47], while the CH3 peak was absent for all the ionic liquids comprising [Apaeim] cations. The 1H NMR spectra of the different ionc liquids were in good agreement with their corresponding chemical structures. According to the characterization results, the synthesized products did not contain any impurities. The chemical structures of the synthesized ionic liquids are displayed in Table 2. CO2 chemical absorption mainly proceeds via the cation and anion amino groups. For [Apaeim][OH], the CO2 molecules react with the cation amino groups. FT-IR data (Fig. 3) confirmed this CO2 chemical absorption. The broad peak at 3500–3000 cm-1 was assigned as the OH peak, while the small sharp peaks at 3500–2750 cm-1 were assigned to the imidazole group. Additionally, an imidazole C–N stretching peak appeared at 1229–900 cm-1, while the imidazole C–H peak appeared at 662 and 741–740 cm-1. An increase in the height of the CO2 gas peak at 2500–2250 cm-1 was observed after CO2 absorption [48,49]. This was attributed to the CO2 that did not chemically reacted with [Apaeim][OH]. A C=O and two N–H peaks also appeared at 1561, 1467, and 1307 cm-1 following CO2 absorption, attributed to the CO2 that chemically reacted with [Apaeim][OH]. The
13
C-NMR spectra (Fig. 4) provided information on the CO2 chemical absorption mechanism of the
synthetic ionic liquid [Apaeim][OH]. Fig. 4 (d), which presents the before, during (1 h), and after the completion of CO2 absorption clearly indicates that CO2 is chemically and physically absorbed into the synthetic ionic liquids. The
13
C-NMR spectra of the ionic liquids were in good agreement with their corresponding
chemical structures. The 13C-NMR spectrum during CO2 absorption, measured after one hour after the reaction was initiated, presented new peaks at 163 and 166 ppm, assigned to the amide (CO–NH) groups, as well as CO2 and HCO3- peaks at 125 ppm and 165 ppm [50,51]. Since two peaks assigned to the carbon connecting the N-H group appeared in 13C-NMR spectra, it was assumed that all of the amino groups (NH2) of the [Apaeim] cation react with CO2 molecules. The spectrum in Fig. 4 (d) also reveals that the CO–NH, CO2, and HCO3- peaks appear simultaneous during CO2 absorption. The CO2 peak at 125 ppm is a gas phase component, While the CO–NH and HCO3- peaks at 163, 165, and 166 ppm are liquid phase components. These data indicate that the CO2 chemical and physical absorptions of the amino groups occur simultaneously. Notably, an increase in the heights of the peaks at 163 and 166 ppm was observe following absorption. Thus, in the CO2 absorption
mechanism of [Apaeim] (Fig. 5), the CO2 chemical reaction of [Apaeim] with CO2 proceeds via the two [Apaeim] sites [10]. 3.2. CO2 absorption of multi-amino group-functionalized ionic liquids: cation and anion effects CO2 absorption experiments were carried out using 15 vol% CO2 under atmospheric pressure at 40
. The
CO2 absorption capacity of each multi-amino group-functionalized ionic liquid absorbent is listed in Table 3. All the multi-amino group-functionalized ionic liquids containing [Apaeim] cations presented better CO2 absorption (mol/L) units than that observed for 30 wt% MEA. The CO2 absorption results for each ionic liquid are displayed in Fig. 6. Fig. 6 (a) reveals that the CO2 absorption capacity of 30 wt% [Apaeim][OH] is approximately twice that of 30 wt% [Eaeim][OH] and 30 wt% [Paeim][OH]. This is because there are two reaction sites (amino groups) for [Apaeim][OH] and one reaction site for [Paeim][OH] and [Eaeim][OH]. Fig. 6 (b) indicates that 30 wt% [Paeim][amino acid] displays a slightly better CO2 absorption performance than 30 wt% [Eaeim][amino acid], while Fig. 6 (a) reveals that 30 wt% [Paeim][OH] and 30 wt% [Eaeim][OH] have similar CO2 absorption capacities. The
13
C-NMR spectrum of [Apaeim][OH] CO2 absorption (Fig 4.)
reveals that HCO3- molecules were formed by CO2 absorption, whereby both the amino acid and OH anions involved in the CO2 absorption process. The CO2 diffusion of a gas-liquid is affected by the mass transfer coefficient, so that the liquid film mass transfer coefficient decreases with an increase in the absorbent viscosity [45]. Fig. 7 illustrates the viscosities of 30 wt% [Paeim][OH] and 30 wt% [Eaeim][OH]. At lean loading, the viscosity of 30 wt% [Paeim][OH] is higher than 30 wt% [Eaeim][OH]; this viscosity difference increases at rich CO2 loading. Thus, due to its lower viscosity, 30 wt% [Eaeim][OH] present a higher gas-liquid mass transfer coefficient than 30 wt% [Paeim][OH]. Therefore, 30 wt% [Paeim][OH] and 30 wt% [Eaeim][OH] have similar CO2 absorption capacities. On the other hand, a comparison of the CO2 absorption capacities of 30 wt% [Eaeim][amino acid] and 30 wt% [Paeim][amino acid] reveals that the CO2 absorption performance of the latter is slightly better. The viscosities of 30 wt% [Paeim][amino acid] and 30 wt% [Eaeim][amino acid] are similar ; however, the [Paeim] cation has one more carbon number than the [Eaeim] cation. At high pressure, an ionic liquid absorbent having a long chain molecular structure presents a better absorption performance than one with a shorter or no chain [52]. Thus, under atmospheric pressure CO2 absorption conditions, ionic liquids comprising [Paeim] cations absorb more CO2 than those comprising [Eaeim] cations. Fig. 6 (b) reveals that for the ionic liquid absorbents comprising [Eaeim] and [Paeim] cations, the CO2 absorption capacity increases in the order [val] >[ala] >[gly]. However, Fig. 6 (c) illustrates that for the ionic liquid absorbents with [Apaeim], the CO2 absorption capacity increases in the order [ala] > [val] > [gly].
Because of the steric hindrance effect of the anion, synthetic ionic liquids comprising [ala] anions exhibit better CO2 absorption capacity than synthetic ionic liquids with [gly] anions [27]. However, no significant difference in the CO2 absorption capacity is observed. Alanine, glycine, and valine are non-polar aliphatic side chains consisting of only one primary amine functional group. Because the CO2 absorption experiment was conducted at atmospheric pressure, the cation effect on CO2 absorption is greater than that of the anion. For example, Zhang et al. [20] reported similar CO2 absorption capacities for the single amino-group functionalized phosphonium
ionic
liquids
(3-aminopropyl)tributylphosphonium
glycinate
([aP4443][gly]),
(3-
aminopropyl)tributylphosphonium alaninate ([aP4443][ala]), (3-aminopropyl)tributylphosphonium valinate ([aP4443][val]), and (3-aminopropyl)tributylphosphonium leucinate ([aP4443][leu]). Thus, the CO2 absorption of ionic liquids under atmospheric pressure seems to depend on the number of amino groups [53]. 3.3. CO2 desorption of multi-amino group-functionalized ionic liquids: cation and anion effects CO2 desorption experiments were conducted at atmospheric pressure and 80
under nitrogen gas bubbling.
The CO2 desorption results in Fig. 8 reveal the amounts of CO2 still present in the synthetic ionic liquid absorbents after CO2 desorption, i.e., the “lean CO2 loading”. Fig. 8 (a) illustrates that lean CO2 loading value of 30 wt% [Apaeim][OH] is higher than those of [Eaeim][OH] and [Paeim][OH], while the lean CO2 loading of 30 wt% [Eaeim][OH] is slightly higher than that of 30 wt% [Paeim][OH]. On the other hand, Fig. 8 (b) indicates that except for [Eaeim][gly], the lean CO2 loadings of 30 wt% [Eaeim][amino acid] are similar to those of 30 wt% [Paeim][amino acid]. Fig. 8 (c) indicates that the lean CO2 loadings of 30 wt% all the [Apaeim][amino acid] samples are similar at 1.5 mol/mol. Overall, the lean CO2 loadings of the synthetic ionic liquids comprising the [Apaeim] cation are higher than those of the synthetic ionic liquids comprising [Eaeim] and [Paeim]. These results therefore suggest, that the lean CO2 loading is more affected by the number of amino groups on the ionic liquid. Singh et al. [54] compared the lean CO2 loading of amines depending on the number of the amino group and effect of the side chain. They revealed the lean CO2 loading values increased with the increase in the number of amino groups in the tested amines, regardless of the CO2 absorption capacity. The comparison of the CO2 desorption results of 1-2-diamino propane and N-(2-hydroxyethyl)ethylenediamine), revealed that the effect of the side chain on the lean CO2 loading is less significant than the effect of the number of amino groups. The lean CO2 loading estimation model for amines proposed by Momeni et al. [55] is necessary to determine the number of NH2, and NH groups and the charge of the NH moiety. This model displayed a similar trend to that of the actual experimental results, when compared with the experimental data on 20 types of amines. This led to
the conclusion that lean CO2 loading is mainly affected by the number of amino groups. In the case of 30 wt% synthetic ionic liquids, the anion effect is less in lean CO2 loading because alanine, glycine, and valine contain the same number amino groups. 3.4. CO2 cyclic capacities,and CO2 absorption and desorption rates Table 3 lists the CO2 cyclic capacities, CO2 absorption and desorption rates of multi-amino group functionalized ionic liquids. The CO2 cyclic capacity is the difference between the rich and lean CO2 loadings. We defined the CO2 absorption rate at 90% absorption as the CO2 absorption rate at 90% rich CO2 loading. Similarly, the CO2 desorption rate at 90% desorption was defined as the CO2 desorption rate at 90% desorption. The CO2 absorption rates of the 30 wt% synthetic ionic liquids were slightly slower than that observed for 30 wt% MEA, while the CO2 cyclic capacities were more than twice that of 30 wt% MEA. The CO2 desorption rate of
the30 wt% synthetic ionic liquids were two- or threefold faster than that of 30 wt% MEA. Moreover, the
CO2 desorption rates of the synthetic ionic liquids comprising [Eaeim] and [Paeim] cations were slightly faster than those of the synthetic ionic liquids comprising the [Apaeim] cation. Table 3 and Fig. 3 and 4 illustrate that the CO2 absorption of the synthetic ionic liquids was accompanied by physical absorption. Theoretically, two moles of MEA [56] or one mole of [Apaeim] cations are required to chemically absorb one mole of CO2. The rich CO2 loadings of 30 wt% MEA and [Apaeim][OH] are 0.614 and 3.65 mol/mol, respectively (Table 3). These data clearly indicates that the synthetic ionic liquid [Apaeim][OH] absorbs more CO2 than MEA, and the theoretically predicted chemical CO2 absorption capacity. The amount of physical absorption of the synthetic ionic liquids is greater than that of MEA due to the small molecule number of the synthetic ionic liquids of the same mass [57]. The CO2–NH bonds are formed via CO2 chemical absorption while the CO2 molecules bind weakly with the N and F atoms via physical absorption. CO2 removal is easier for the physically absorbed CO2 than for the chemically absorbed CO2 and thus, the CO2 desorption rates of the 30 wt% multi-amino group functionalized-ionic liquids are faster than that of 30 wt% MEA. 3.5. Viscosity of the 30 wt% multi-amino group-functionalized ionic liquids The viscosity data of the 30 wt% synthetic ionic liquids are displayed in Fig. 7. The viscosities were measured at atmospheric pressure and 40
at the lean CO2 loading state. Overall, the synthetic ionic liquids with OH anions
presented high viscosities. A comparison of the three ionic liquids revealed that diethanolamine (DEA) is more viscous than aminoethylethanolamine (AEEA) and diethylenetriamine (DETA) [58, 59]. DEA comprises one amino group and two hydroxyl groups, AEEA comprises two amino groups and one hydroxyl group, and DETA
comprises three amino groups and no hydroxyl group. All three amines have similar alkyl groups. The comparison of the chemical structures of these amines, suggested that the viscosity increase with an increase in the strong intermolecular hydrogen bonding of the hydroxyl group. Among the synthetic ionic liquids with OH anions, 30 wt% [Apaeim][OH] was the most viscous because it has one more amino group than [Eaeim][OH] and [Paeim][OH]. In turn, [Paeim][amino acid] and [Paeim][OH] are more viscous than [Eaeim][amino acid] and [Eaeim][OH]. The experimental results indicate the viscosity increases with increasing cation alkyl chain length. The same trend was reported in other studies. Thus, for the imidazolium cation, the viscosity was higher for longer cation alkyl chains [60]. On the other hand, for the solutions, the valine anion, with a chain length longer than those of glycine and L-alanine, presented the lower viscosity. This occurred because water molecules reduce the intermolecular interactions between the cations and anions [61]. Moreover, valine has weak intermolecular interactions in the solvation state due to a more sterically hindered structure than those of alanine and glycine. Notably, 30 wt% [Apaeim][OH] and 30 wt% [Apaeim][ala] presented higher CO2 absorption capacities than the other synthetic ionic liquids. However, the high viscosity of 30 wt% [Apaeim][OH] is due to the strong intermolecular interactions caused by hydrogen bonding. Therefore, it was concluded that the best absorbent in this study is 30 wt% [Apaeim][ala]. Overall, the viscosities of the 30 wt% synthetic ionic liquids are not significantly higher than that of 30 wt% MEA.
4. Conclusion In this study, multi-amino group-functionalized ionic liquids were synthesized in reasonable yields. The structures of the synthetic ionic liquids were confirmed by 1H-NMR spectroscopy, while the CO2 absorption mechanism of the synthesized ionic liquids was confirmed by 13C-NMR and FT-IR analyses. All the synthesized 30 wt% AAILs presented similar viscosities compareble to that of 30 wt% MEA. For the CO2/absorbent L as the unit of the CO2 absorption capacity, 30 wt% [Apaeim][amino acid] and 30 wt% [Apaeim][OH] presented higher CO2 absorption capacities than that of 30 wt% MEA, a commercial amine. Among them, 30 wt% [Apaeim][OH] and 30 wt% [Apaeim][ala] displayed higher CO2 absorption capacities than those of the other amines. With respect the viscosity, the best absorbent in this study was 30 wt% [Apaeim][ala]. The rich CO2 loading of 30 wt% [Apaeim][ala] was 1.39-fold higher than that of 30 wt% MEA, while the CO2 cyclic capacity of 30 wt% [Apaeim][ala] was 2.2-fold better than that of 30 wt% MEA. The number of amino groups in the ionic liquid cations of ionic liquid has a dominant influence on the CO2 absorption. On the other hand, the steric hindrance of the anions in the ionic liquid has little effect on the rich CO2 loading of the absorbent. Therefore, it was concluded that manipulating the number of amino groups in the ionic liquid cations could be an effective way to control the CO2 absorption capacity.
Acknowledgements This study has been conducted with the support of the Korea Institute of Industrial Technology as “Development of Gas-phase pollutant removal technology” (Project No.: EO-19-0011). And this study has been conducted with the support of the Korea Institute of Industrial Technology as “Development of Carbon Dioxide Capture and Utilization Technology for Greenhouse Gas Reduction” (Project No.: IZ-18-0050). This study also has been conducted with the support of the Korea Institute of Industrial Technology as “Development of membrane contactor based movable gas scrubbing one-module” (Project No.: NS-19-0047).
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Table 1. The 1H NMR data and yields of all the prepared synthetic ionic liquids Table 2. Names and structures of the multi-amino group-functionalized ILs (ionic liquids) Table 3. CO2 cyclic capacities and CO2 absorption/desorption rates of the multi-amino group functionalized ionic liquids
Table 1. The 1H NMR data and yields of all the prepared synthetic ILs Ionic liquid [Apaeim][OH]
1
H-NMR, δ (ppm) 7.5 (s, 1H), 6.98 (s, 1H), 6.78 (d, 1H), 3.95 (t, 4H), 2.64 (q, 4H), 1.89 (q, 2H), 1.07 (t, 4H)
Yideld 100 %
[Eaeim][OH]
8.06 (s, 1H), 7.64 (m, 1H), 7.13 (d, 1H), 4.27 (tt, 2H), 3.08 (t, 2H), 2.78 (f, 2H), 1.55 (m, 2H), 1.14 (t, 3H), 0.9 (t, 3H)
100 %
[Paeim][OH]
7.66 (m, 1H), 7.12 (s, 1H), 6.96 (s, 1H), 4.23 (tt, 2H), 3.99 (t, 2H), 3.08 (br, 2H), 1.94 (m. 2H), 1.25 (t, 2H), 1.0 (t, 3H)
100 %
[Apaeim][ala]
7.7 (s, 1H), 7.2 (s, 1H), 7.0 (d, 1H), 4.29 (br, 2H), 4.16 (t, 4H), 2.97(br, 2H), 2.85 (q, 4H), 2.1 (q, 2H), 1.98 (m, 1H), 1.45 (d, 3H)
75.8 %
[Apaeim][gly]
7.7 (s, 1H), 7.15 (s, 1H), 7.0 (d, 1H), 4.25 (br, 2H), 4.12 (t, 4H), 3.05 (br, 2H), 2.67 (q, 4H), 1.97 (q, 2H), 1.43 (d, 1H)
78.7 %
[Apaeim][val]
7.67 (s, 1H), 7.15 (s, 1H), 6.97 (d, 1H), 4.27 (br, 2H), 4.10 (t, 4H), 3.05 (br, 2H), 2.66 (q, 4H), 1.97 (q, 2H), 1.44 (d, 1H), 1.02 (dd, 6H)
98 %
[Eaeim][ala]
7.66 (m, 1H), 7.13 (s, 1H), 6.95 (s, 1H), 4.28 (m, 2H), 4.06 (f, 2H), 3.08 (q, 2H), 2.91 (br, 2H), 1.56 (t, 3H), 1.44 (t, 2H)
53.17 %
[Eaeim][gly]
7.68 (m, 1H), 7.15 (s, 1H), 6.95 (s, 1H), 4.28 (m, 2H), 4.07 (f, 2H), 3.08 (m, 2H), 2.87 (t, 2H), 2.63 (br, 2H), 1.56 (t, 3H), 1.43 (t, 2H)
72.14 %
[Eaeim][val]
7.66 (m, 1H), 7.13 (s, 1H), 6.95 (s, 1H), 4.29 (m, 2H), 4.05 (f, 2H), 3.07 (m, 2H), 2.88 (br, 2H), 2.63 (br, 2H), 1.56 (t, 3H), 1.43 (t, 2H), 1.02 (dd, 6H)
68.28 %
[Paeim][ala]
7.64 (m, 1H), 7.11 (s, 1H), 6.93 (s, 1H), 4.21 (tt, 2H), 4.05 (t, 2H), 3.99 (t, 2H), 3.08 (t, 2H), 2.98 (br, 2H), 1.93 (m, 2H), 1.81 (m, 2H), 1.45 (d, 3H), 1.02 (t, 3H)
63.08 %
[Paeim][gly]
7.75 (m, 1H), 7.21 (s, 1H), 6.97 (s, 1H), 4.49 (t, 2H), 4.36 (t, 2H), 4.22 (t, 2H), 4.03 (t, 2H), 3.38 (f, 2H), 3.06 (br, 2H), 1.93 (m, 2H), 1.81 (m, 2H), 1.02 (t, 3H)
63.77 %
[Paeim][val]
7.68 (m, 1H), 7.15 (s, 1H), 6.97 (s, 1H), 4.34 (t, 2H), 4.21 (f, 2H), 4.00 (t, 2H), 3.2 (t, 2H), 3.08 (br, 2H), 1.94 (m, 2H), 1.81 (m, 2H), 1.02 (t, 3H)
68.18 %
Table 2. Names and structures of the multi-amino group-functionalized ILs (ionic liquids) IL name
IL abbreviation
1-(3-aminopropyl)-3-(2aminoethyl)imidazolium L-alaninate
[Apaeim][ala]
1-(3-aminopropyl)-3-(2aminoethyl)imidazolium glycinate
[Apaeim][gly]
1-(3-aminopropyl)-3-(2aminoethyl)imidazolium valinate
[Apaeim][val]
1-ethyl-3-(2-aminoethyl)imidazolium alaninate
L-
[Eaeim][ala]
1-ethyl-3-(2-aminoethyl)imidazolium glycinate
[Eaeim][gly]
1-ethyl-3-(2-aminoethyl)imidazolium valinate
[Eaeim][val]
IL structure
1-propyl-3-(2-aminoethyl)imidazolium Lalaninate
[Paeim][ala]
1-propyl-3-(2-aminoethyl)imidazolium glycinate
[Paeim][gly]
1-propyl-3-(2-aminoethyl)imidazolium valinate
[Paeim][val]
Table 3. CO2 cyclic capacities and CO2 absorption/desorption rates of the multi-amino group functionalized ionic liquids
Absorbents
30 wt% monoethanolamine (MEA) 30 wt% [Apaeim][OH] 30 wt% [Eaeim][OH] 30 wt% [Paeim][OH] 30 wt% [Apaeim][gly] 30 wt% [Apaeim][ala] 30 wt% [Apaeim][val] 30 wt% [Eaeim][gly] 30 wt% [Eaeim][ala] 30 wt% [Eaeim][val] 30 wt% [Paeim][gly] 30 wt% [Paeim][ala] 30 wt% [Paeim][val]
Rich CO2 loading (mol CO2/ mol IL)
Rich CO2 loading (mol CO2/ L absorbents)
CO2 cyclic capacity (mol CO2/ mol IL)
CO2 cyclic capacity (mol CO2/ L absorbents)
CO2 absorption rate ( 105mol CO2/L * s) at 90 % absorption
CO2 desorption rate ( 105mol CO2/L * s) at 90 % desorption
0.614
3.02
0.254
1.27
39.98
10.11
3.75
4.19
2.54
2.84
28.87
23.19
2.19
2.74
1.73
2.17
32.2
25.08
2.15
2.54
1.96
2.32
31.22
23
4.19
3.89
2.82
2.62
29.82
25.01
4.72
4.19
3.14
2.8
29
22.91
4.44
3.64
2.95
2.42
25.78
25.43
1.97
1.99
1.91
1.93
35.12
31.56
2.44
2.54
2.09
2.1
26.83
29.54
2.9
2.56
2.35
2.07
36.72
28.29
2.45
2.35
1.95
1.87
30.87
27.52
2.67
2.46
2.19
2.02
39.37
30.87
2.96
2.5
2.65
2.23
36.58
27.11
Fig. 1. Synthetic route of the synthesized amino group-functionalized imidazolium-based amino acid ionic liquid [Apaeim][gly] Fig. 2. Schematic diagram of the apparatus employed for CO2 absorbent screening Fig. 3. FT-IR data of [Apaeim][OH]: (a) before and (b) after CO2 absorption Fig. 4. 13C-NMR data of [Apaeim][OH]: (a) before,
(b) during – 1 h, and (c) after CO2 absorption
Fig. 5. CO2 chemical absorption mechanism of the synthesized amino-group functionalized imidazolium-based ionic liquids – imidazolium cation Fig. 6. Rich CO2 loading data of the synthetic ionic liquids. Comparison of the CO2 absorption performance of: (a) [Apaeim][OH], [Paeim][OH], [Eaeim][OH]; (b) [Paeim][amino acid] and [Eaeim][amino acid]; and (c) [Apaeim][amino acid] Fig. 7. Viscosity of the 30 wt% synthetic ionic liquids at atmospheric pressure and 40
(unit: cP)
Fig. 8. Lean CO2 loading data of the synthetic ionic liquids. Comparison of CO2 the desorption performance of: (a) [Apaeim][OH], [Paeim][OH], [Eaeim][OH]; (b) [Paeim][amino acid] and [Eaeim][amino acid]; and (c) [Apaeim][amino acid]
Fig. 1. Synthetic route of the synthesized amino group-functionalized imidazolium-based amino acid ionic liquid [Apaeim][gly]
Fig. 2. Schematic diagram of the apparatus employed for CO2 absorbent screening
Fig. 3. FT-IR data of [Apaeim][OH]: (a) before and (b) after CO2 absorption
(a)
(b)
Fig. 4. 13C-NMR data of [Apaeim][OH]: (a) before,
(a)
(b)
(c)
(d)
(b) during – 1 h, and (c) after CO2 absorption
Fig. 5. CO2 chemical absorption mechanism of the synthesized amino-group functionalized imidazolium-based ionic liquids
imidazolium cation
Fig. 6. Rich CO2 loading data of the synthetic ionic liquids. Comparison of the CO2 absorption performance of: (a) [Apaeim][OH], [Paeim][OH], [Eaeim][OH]; (b) [Paeim][amino acid] and [Eaeim][amino acid]; and (c) [Apaeim][amino acid]
(a)
(b)
(c)
Fig. 7. Viscosity of the 30 wt% synthetic ionic liquids at atmospheric pressure and 40
(unit: cP)
Fig. 8. Lean CO2 loading data of the synthetic ionic liquids. Comparison of CO2 the desorption performance of: (a) [Apaeim][OH], [Paeim][OH], [Eaeim][OH]; (b) [Paeim][amino acid] and [Eaeim][amino acid]; and (c) [Apaeim][amino acid]
(a)
(b)
(c)
CO2 Absorption Characteristics of Amino Group Functionalized Imidazolium-based Amino Acid Ionic Liquids Sehee Kanga,b, Yongchul G. Chungb, Jo Hong Kanga and Hojun Songa* a
Green Materials & Processes R&D group, Korea Institute of Industrial Technology, 55 Jongga-ro, Jung-gu, Ulsan 44413, South Korea b School of Chemical and Biomolecular Engineering, Pusan National University, 2 Busandaehak-ro, 63 beon-gil, Geumjeong-gu, Pusan 46241, South Korea
Highlights: 1) Amino acid ionic liquids are synthesized as CO2 absorbents for postcombustion CO2 capture; 2) CO2 absorption mechanisms are analyzed by 13C-NMR and FT-IR spectroscopy; 3) [Apaeim][amino acid] presented a superior CO2 absorption performance than 30 wt% monoethanolamine; 4) The number of amino groups in the ionic liquids affects their CO2 absorption capacity.
CO2 Absorption Characteristics of Amino Group Functionalized Imidazolium-based Amino Acid Ionic Liquids Sehee Kanga,b, Yongchul G. Chungb, Jo Hong Kanga and Hojun Songa* a
Green Materials & Processes R&D group, Korea Institute of Industrial Technology, 55 Jongga-ro, Jung-gu, Ulsan 44413, South Korea b School of Chemical and Biomolecular Engineering, Pusan National University, 2 Busandaehak-ro, 63 beon-gil, Geumjeong-gu, Pusan 46241, South Korea
None declared.