Concentrated aqueous solutions of protic ionic liquids as effective CO2 absorbents with high absorption capacities

Article abstract of DOI:10.1016/j.molliq.2017.08.035

Solubilities of CO₂ in a series of aqueous 3-(dimethylamino)-1-propylamine (DMAPA) cation-based protic ionic liquids (PILs) with different anions were determined at 308.2 K and pressure up to 3.5 bar. Impressively, the solubility of CO₂ in the [DMAPAH][F] aqueous solution reached up to 3.62 mol/kg at 1 bar and 308.2 K, which was much superior to the traditional amine solutions and other ILs reported in the literature. A novel absorption mechanism is proposed and verified by ¹³C NMR and ¹⁹F NMR spectra. Based on this, CO₂ absorption behaviors on pressure, temperature and concentrations of [DMAPAH][F] in aqueous solutions were studied. Meanwhile, the relevant parameters, such as the apparent absorption rate constant k, the absorption activation energy E_a, the dissolution enthalpy change ΔH_SOL and the entropy change ΔS_SOL were also calculated from the dynamic and thermodynamic data. Regeneration under the condition of 353.2 K and 15 kPa for 1 h showed that aqueous [DMAPAH][F] solution had good regeneration efficiency (over 94%). The absorption capacity (3.62 mol/kg), the ΔH_SOL (? 35 to ? 43 kJ·mol^{? 1}), as well as the good recyclability indicate that the aqueous [DMAPAH][F] is a kind of promising absorbent for the separation of CO₂ and believed to have potential use in gas decarburization.

Full text of DOI:10.1016/j.molliq.2017.08.035

Accepted Manuscript
Concentrated aqueous solutions of protic ionic liquids as effective
CO2 absorbents with high absorption capacities
Wen-Tao Zheng, Feng Zhang, You-Ting Wu, Xing-Bang Hu
PII:
DOI:
Reference:
S0167-7322(17)32774-5
doi: 10.1016/j.molliq.2017.08.035
MOLLIQ 7743
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
23 June 2017
6 August 2017
7 August 2017
Please cite this article as: Wen-Tao Zheng, Feng Zhang, You-Ting Wu, Xing-Bang Hu
, Concentrated aqueous solutions of protic ionic liquids as effective CO2 absorbents
with high absorption capacities, Journal of Molecular Liquids (2017), doi: 10.1016/
j.molliq.2017.08.035
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ACCEPTED MANUSCRIPT
Concentrated aqueous solutions of protic ionic liquids as effective CO2
absorbents with high absorption capacities
Wen-Tao Zheng, Feng Zhang, You-Ting Wu*, Xing-Bang Hu*
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu
210093, P. R. China
*Corresponding author: E-mail: ytwu@nju.edu.cn; huxb@nju.edu.cn.
Abstract: Solubilities of CO2 in a series of aqueous 3-(dimethylamino)-1-propylamine
(DMAPA) cation-based protic ionic liquids (PILs) with different anions were determined at
308.2 K and pressure up to 3.5 bar. Impressively, the solubility of CO2 in the [DMAPAH][F]
aqueous solution reached up to 3.62 mol/kg at 1 bar and 308.2 K, which was much superior to
the traditional amine solutions and other ILs reported in the literature. A novel absorption
13
19
mechanism is proposed and verified by C NMR and F NMR spectra. Based on this, CO2
absorption behaviors on pressure, temperature and concentrations of [DMAPAH][F] in
aqueous solutions were studied. Meanwhile, the relevant parameters, such as the apparent
absorption rate constant k, the absorption activation energy Ea, the dissolution enthalpy
change ΔHSOL and the entropy change ΔSSOL were also calculated from the dynamic and
thermodynamic data. Regeneration under the condition of 353.2 K and 15 kPa for 1 h showed
that aqueous [DMAPAH][F] solution had good regeneration efficiency (over 94%). The
absorption capacity (3.62 mol/kg), the ΔHSOL (-35 ~ -43 kJ∙mol-1), as well as the good
recyclability indicate that the aqueous [DMAPAH][F] is a kind of promising absorbent for the
separation of CO2 and believed to have potential use in gas decarburization.

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Keywords: Protic ionic liquid; CO2; High absorption capacity, Thermodynamics,
1. Introduction
The presence of CO2 in natural gas leads to the decrease of caloric value, which should
be eliminated to ensure the efficiency of fuel utilization [1]. Meanwhile, CO2 emissions from
fossil-fuel-fired power plants are considered the main contributor to global climate change [2].
Therefore, the removal of CO2 has been a hot spot [3]. One common method adopted in the
industry is chemical absorption by the amine solutions such as monoethanolamine (MEA),
diethanolamine (DEA) and N-methyldiethanolamine (MDEA) [4-7]. As we know, these
amine-based technologies still have their major drawbacks, including the corrosive nature and
volatility of amines, decomposition of the generated salts, and high regeneration energy cost
[8]. In consideration of the above-mentioned blemish, scientists have paid attention to ionic
liquid (IL) because of their excellent properties such as negligible vapor pressure, high
thermal stability, and tunable chemistry and so on [9-12]. The ionic liquids based on
imidazolium
(3-butyl-1-methylimidazolium
3-ethyl-1-methylimidazolium
bis(trifluoromethanesulfonyl)amide
bis(trimfluoromethanesulfonyl)amide
[bmim][Tf2N],
[emim][Tf2N], 1,3-di (2’-aminoethylene)-2-methy- limidazolium bromide), quaternary
ammonium salt (tetraethylammonium glycinate [N1111][Gly], tetraethylammonium lysinate
[N2222][Lys]), and aminopolycarboxylate (3-butyl-1-methylimidazolium iminodiacetic acid
[bmim][IDA]) and so on have been used for capturing CO2 [13-18]. These ionic liquids
generally have a high viscosity, exorbitant price and intensive energy input for regeneration to
limit their large-scale use in industry [19-21]. Therefore, to develop efficient absorbent and
trapping techniques that could reversibly and economically remove CO2 is highly desirable.

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In this connection, the protic ionic liquids (PILs) for CO2 removal has been investigated based
on their favorable absorption and regeneration properties and popular price [22,23].
Protic ionic liquid, an important branch of the ILs, is prepared from simple materials
through the direct neutralization between corresponding acid and base [24,25]. Compared
with those aprotic ionic liquids, the low cost and simple one-step synthesis process of PILs
are attractive to scientists and industry [23,26]. In addition, CO2 was found to be released
from PILs at relatively low temperature [22], which can significantly minimize the energy
needed, and avoid the loss of absorbent and the production of toxic compounds during the
regeneration of the absorbent. However, as the materials used to synthesize the PILs are
usually weak acid and base, the pure PILs are easily to decompose, not to mention the fact
that their viscosities are very high. Therefore, it is better to use PIL together with H2O so that
those disadvantages can be overcome. In this work, solubilities of CO2 in a series of 70 wt%
aqueous solutions of PILs based on 3-(dimethylamino)-1-propylamine cation (DMAPAH)
with different anions were measured to search the efficient absorbents for the absorption of
CO2. The absorption behaviors of CO2 in the efficient solvents on temperatures, pressures and
concentrations were investigated and desorption experiment was also conducted to evaluate
the recyclability of the absorbent. The dynamic parameters, thermodynamic parameters, as
well as pKa of the efficient PIL were also calculated.
2. Experimental section
2.1 Materials.
The analytical reagents of 3-(dimethylamino)-1-propylamine (DMAPA, 99 wt%), formic
acid (For, 99 wt%), acetic acid (Ac, 99 wt%), propionic acid (Prop, 99 wt%), butyric acid

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(Buty, 99 wt%), methoxyacetic acid (MOAc, 99 wt%), and hydrofluoric acid (HF, 41 wt% in
aqueous solution) are purchased from Sinopharm Chemical Reagent Co. Ltd and used as
received without further purification unless otherwise specified. CO2 (99.99 vt.%) is
purchased from Nanjing Tianze Gas Center, Nanjing. The PILs are prepared straightforwardly
by acid-base neutralization (Fig. 1). Typically, a round-bottom flask coupled with a magnetic
stirrer is charged with DMAPA (0.1 mol) and alcohol (50 mL), the acetic acid (0.1 mol) is
added slowly at 0oC over a period of 30 minutes. The resulting mixture is stirred for 30
minutes at this temperature and then slowly warmed to ambient temperatures over a period of
24 hours. After the reaction, the solvent is removed in vacuo resulting in a viscous liquid of
[DMAPAH][Ac], which is dried in vacuo at 50oC for 24 h. Other PILs, including
[DMAPAH][For], [DMAPAH][Prop], [DMAPAH][Buty], [DMAPAH][MOAc], are
performed using the same method. As for the preparation of [DMAPAH][F], the aqueous
solution of equimolar HF is added dropwise to an aqueous solution of DMAPA in a plastic
container with vigorous stirring. The rest of the synthesis process is exactly the same with that
o
of the [DMAPAH][Ac], except that the dried temperature in vacuo is about 65 C. The
molecular structures of the DMAPAH-based PILs prepared in this study were verified using
1H NMR spectra, 13C NMR spectra, which were determined on a Bruker 400 MHz
spectrometer, using CDCl3 or DMSO as a solvent with TMS as the internal standard. The
aqueous solutions of PILs are prepared by mass using a Sartorius BS 224S electronic analytic
balance.
2.2 Experimental apparatus.
The densities were determined using an Anton Paar DMA 5000 type automatic

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densiometer with a precision of 0.00001 g·cm-3, which was calibrated using distilled water.
Viscosities were measured on a HAAKE Rheostress 600 viscometer with an uncertainty of
±1% in relation to the full scale. The densities and viscosities are available in the
Supplementary Materials. A pH meter (model: PXSJ-216, Shanghai Rex Co-perfect
Instrument Co. Ltd) was used to determine the pH values of the aqueous PIL solution. The
high accuracy pH meter electrode was calibrated using buffer solutions. The buffer solutions
were provided with an accuracy of ± 0.01 for pH of 9.18 and 4.00.
2.3 Measurements of CO2 solubility.
Solubility data of CO2 in the aqueous solutions of PILs were measured by isothermal
gas-liquid equilibrium experiments at the temperature ranging from (298.2 to 328.2) K. The
experimental instrument and process, including data process, were identical to our previous
work [17]. Measuring temperature was kept constant by a circulation water bath with ±
0.01 K uncertainty. The system pressure was determined by a pressure gauge with an
accuracy of ±0.1 kPa. Duplicate experiments are run for each absorbent to obtain averaged
values of CO2 absorption. The averaged relative deviation of the absorption data in this work
was well within ± 1%. The absorptive capacities of CO2 both in the unit of molality (mol
CO2/kg solvent) and molar ratio (mol CO2/mol PIL) were introduced in this work to measure
the absolute and relative CO2 uptake of the absorbents.
2.4. Measurement of the dissociation constant (pKa).
The pH meter was calibrated using buffer solutions after setting the required temperature
(298.2 K) for titration. For titration of the PIL solution (0.9705 M, 20 mL), a 0.9674 M
aqueous hydrochloric acid solution (HCl) was used and added slowly to the aqueous PIL

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solution. pH values were recorded as soon as equilibrium was reached after the addition of the
titrant [27]. A thermostat water bath was used to keep the temperature constant during the
titration.
3. Results and discussions
3.1 Characterization of PILs
The 1H NMR and 13C NMR spectra data of all the prepared PILs are given as follows:
1
+
[DMAPAH][F]: H NMR (400 MHz, DMSO, 25 ℃, TMS), δ (ppm)： 7.67 (s, 3H, -NH3 ),
2.77 (s, 2H, -CH2-), 2.62 – 2.46 (m, 2H, -CH2-), 2.31 (s, 6H, -N(CH3)2), 1.83 – 1.66 (m, 2H,
-CH2-); 13C NMR (400 MHz, DMSO, 25 ℃, TMS), δ (ppm)： 55.49 (s, -CH2-), 44.21 (s,
-N(CH3)2), 37.45 (s, -CH2-), 24.30 (s, -CH2-).
[DMAPAH][For]: 1H NMR (400 MHz, CDCl3, 25 ℃, TMS), δ (ppm)：8.44 (s, 1H, HCOO-),
+
7.56 (s, 3H, -NH3 ), 2.87 (dd, J = 13.6, 6.9 Hz, 2H, -CH2-), 2.42 – 2.22 (m, 2H, -CH2-), 2.22 –
2.01 (m, 6H, -N(CH3)2), 1.70 (dd, J = 12.7, 6.2 Hz, 2H, -CH2-); 13C NMR (400 MHz, CDCl3,
25 ℃, TMS), δ (ppm)： 168.89 (s, HCOO-), 57.63 (s, -CH2-), 45.00 (s, -N(CH3)2), 39.15 (s,
-CH2-), 24.88 (s, -CH2-).
+
[DMAPAH][Ac]: 1H NMR (400 MHz, CDCl3, 25 ℃, TMS), δ (ppm)： 7.96 (s, 3H, -NH3 ),
2.95 (t, J = 6.7 Hz, 2H, -CH2-), 2.45 (t, J = 6.5 Hz, 2H, -CH2-), 2.24 (s, 6H, -N(CH3)2), 1.93 (s,
3H, -CH3), 1.80 (t, J = 6.6 Hz, 2H, -CH2-); 13C NMR (400 MHz, CDCl3, 25 ℃, TMS), δ
(ppm)： 178.31 (s, -COO-), 57.82 (s, -CH2-), 45.07 (s, -N(CH3)2), 39.33 (s, -CH2-), 25.22 (s,
-CH2-), 24.51 (s, -CH3).
+
[DMAPAH][Prop]: 1H NMR (400 MHz, CDCl3, 25 ℃, TMS), δ (ppm)：8.06 (s, 3H, -NH3 ),
2.94 (t, J = 6.8 Hz, 2H, -CH2-), 2.49 – 2.37 (m, 2H, -CH2-), 2.24 (s, 6H, -N(CH3)2), 2.17 (q, J

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13
= 7.6 Hz, 2H, -CH2-), 1.80 (p, J = 6.7 Hz, 2H, -CH2-), 1.06 (t, J = 7.6 Hz, 3H, -CH3); C
NMR (400 MHz, CDCl3, 25 ℃, TMS), δ (ppm)： 181.45 (s, -COO-), 57.70 (s, -CH2-), 45.07
(s, -N(CH3)2), 39.18 (s, -CH2-), 30.81 (s, -CH2-), 25.25 (s, -CH2-), 10.75 (s, -CH3).
+
[DMAPAH][Buty]: 1H NMR (400 MHz, CDCl3, 25 ℃, TMS), δ (ppm)：8.09 (s, 3H, -NH3 ),
2.94 (s, 2H, -CH2-), 2.44 (t, J = 6.4 Hz, 2H, -CH2-), 2.24 (s, 6H, -N(CH3)2), 2.20 – 2.08 (m,
2H, -CH2-), 1.80 (p, J = 6.6 Hz, 2H, -CH2-), 1.65 – 1.49 (m, 2H, -CH2-), 0.92 (t, J = 7.4 Hz,
3H, -CH3); 13C NMR (400 MHz, CDCl3, 25 ℃, TMS), δ (ppm)： 180.60 (s, -COO-), 57.82 (s,
-CH2-), 45.04 (s, -N(CH3)2), 39.93 (s, -CH2-), 39.33 (s, -CH2-), 25.17 (s, -CH2-), 19.75 (s,
-CH2), 14.19 (s, -CH3).
1
[DMAPAH][MOAc]: H NMR (400 MHz, CDCl3, 25 ℃, TMS), δ (ppm)： 7.82 (s,
+
3H,-NH3 ), 3.81 (s, 2H, -OCH2-), 3.36 (s, 3H, CH3O-), 2.98 (t, J = 6.7 Hz, 2H, -CH2-), 2.47 (t,
13
J = 6.5 Hz, 2H, -CH2-), 2.26 (s, 6H, -N(CH3)2), 1.83 (p, J = 6.6 Hz, 2H, -CH2-); C NMR
(400 MHz, CDCl3, 25 ℃, TMS), δ (ppm)： 176.13 (s, -COO-), 72.55 (s, -OCH2-), 58.39 (s,
-OCH3), 57.84 (s, -CH2-), 45.01 (s, -N(CH3)2), 39.28 (s, -CH2-), 24.56 (s, -CH2-).
1
The H NMR, 13C NMR spectra are found to be in good agreement with their
corresponding chemical structures. No impurities are found according to characterization
results.
3.2 Absorption of CO2
The solubilities of CO2 in the aqueous solutions of the DMAPAH-based PILs with
different anions are shown in Fig. 2(a). All curves show that the solubilities of CO2 in the
prepared solutions rise steeply with pressure in the low-pressure region (0 ~ 0.5 bar) and then
level off in the high-pressure region (0.5 ~ 3.5 bar). The chemical reaction dominates the

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absorption at low pressure, leading to the significant increase of absorption with the increase
of CO2 partial pressure. However, in high-pressure region, the physical absorption plays the
main role as the majority of absorbents have been reacted with CO2. Regarding the effect of
anions on the absorption of CO2, it is shown that the solubility of the CO2 in the ILs
containing [DMAPAH] as a common cation follows the subsequent trend: [F] > [For] >
[Ac] > [Prop] > [Buty] ≈ [MOAc]. The result reveals that lengthening the alkyl chain of the
anion leads to the decrease of the absolute absorption capacity of the PIL solutions, as the
moles of the PILs with larger anion are less than those with smaller anion in the same weight
of solutions. However, the relative uptake (CO2 loading per mole of PIL) of the PILs are very
close to each other (0.50 ~ 0.53 mol/mol), except for the aqueous [DMAPAH][F] solution
(0.61 mol/mol) (Fig. 2(b)). The relative uptake (0.50 ~ 0.53 mol/mol) implies that these PILs
(except [DMAPAH][F]) undergo the similar absorption mechanism with those
amine-functionalized ILs (0.50 mol/mol), which is two PIL molecules reacting with one CO2
molecule [17,28]. The main difference between these PILs and those amine-functionalized
ILs is that the primary amine groups on the PIL molecules are protonated while the
amine-functionalized ILs are not. However, it has been proposed and verified in literature that
the proton on the primary amine group of the PIL is transferred to the free tertiary amine to
form its equilibrium configuration, resulting in the exposure of the primary amine group so
that the reaction of CO2 with the primary amine is enabled [22]. Hence, the absorption
process of CO2 in these PILs is akin to in those amine-functionalized ILs, leading to the close
relative absorption uptake.
Impressively, the solubility of CO2 in the aqueous [DMAPAH][F] solution (3.62 mol/kg),

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as well as the relative uptake (0.61 mol/mol) is well over that of the other prepared PILs
solutions. This may be attributed to the large density of F electron cloud and its relatively
concentrated electron distribution, which is favorable for capturing CO2 as the partial
positively charged carbon atom orients close to the negative center [29]. It is proved by
quantum mechanical (QM) calculations that the ability of F- anion in reacting with CO2 is
strong, with the binding energy ca. -271.64 kJ/mol and the high bending of CO2, 138.2° [30,
31]. Therefore, it is believed that the capture of CO2 is mainly by F- anion not the primary
amine group on the [DMAPAH][F]. In order to investigate the absorption mechanism further,
the 13C NMR spectra and 19F NMR spectra were determined on a Bruker 400 MHz
spectrometer, using DMSO as a solvent with TMS as the internal standard (Fig. 3). It is found
13
that a new signal at 160 ppm appears in C NMR after the absorption of CO2 in comparison
with the fresh (unreacted) aqueous [DMAPAH][F] (Fig. 3(a)), which is assigned to the carbon
-
in HCO3 [32]. However, no change is observed at 165 ppm, which belongs to the carbon in
carbamate. The result indicates that the primary amine on the PIL does not react with CO2,
+
otherwise, the signal of the carbamate of RNH2 COO- will exist [32]. In addition, the
chemical shift of F and its peak shape have changed significantly after CO2 absorption (Fig.
3(b)), implying that it may be F- anion that reacts with CO2. According to the 13C NMR
spectra and 19F NMR spectra, a new reaction mechanism for the absorption of CO2 is
proposed. When the aqueous [DMAPAH][F] is applied for CO2 uptaking, the F- anion can
react with CO2 to form an intermediate [F-COO]-. It is unstable and easily to undergo
-
hydrolysis to form HF and HCO3 , so that the absorption system can be stabilized. The
bending structure of the carboxyl group on the intermediate is consistent with the result

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obtained by the QM calculation that CO2 is highly bent when reacting with F-. The formation
-
of HCO3 accounts for the new signal at 160 ppm appearing in 13C NMR spectra.
To compare the absorption capacity of the [DMAPAH][F] aqueous solution with other
absorbents, the solubility of CO2 in traditional amine solutions, ILs based on imidazolium,
quaternary ammonium salt, quaternary phosphonium salt, as well as some PILs and so on at 1
bar is summarized in Table 1. It is demonstrated that the aqueous [DMAPAH][F] in this work
(Entry 1) shows superior performance on absorption capacity compared with the traditional
amine solutions (Entries 2 ~ 6) and other ILs (Entries 7 ~ 24). In particular, aqueous MEA
and DEA are commonly used chemical solvents for the absorption of CO2 in industry. It is
noted that the uptake of the aqueous [DMAPAH][F] is 3.62 mol/kg, increasing by 33 % and
67 % in comparison with that of the MEA (2.73 mol/kg) and DEA (2.17 mol/kg) aqueous
solutions, respectively [33,34]. It is also remarkable that the CO2 solubility in the aqueous
[DMAPAH][F] (Entry 1) is well above in some fluorine-substituted ILs (Entries 16 ~ 19) and
other PILs (Entries 20 ~ 24), justifying the advantage of [DMAPAH][F] over other CO2
absorbents. In addition, the solubility of CO2 in the 70 wt% aqueous [DMAPAH][For] (Entry
24) is larger than in the anhydrous [DMAPAH][For], implying the positive effect of water on
CO2 absorption. It is known that the cost and viscosity of ILs are two major disadvantages
that hinder their applications in industry. For instance, normal imidazolium-based ILs are
often prepared by nucleophilic substitution of imidazole and anion exchange, where
expensive materials and tedious reaction/separation steps are usually required [1]. As a
contrast, the [DMAPAH][F] in this work can be easily produced by one step through the
direct neutralization of HF with DMAPA, not to mention the fact that all the reagents used are

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very cheap and easily available in industry. Considering the low cost, low viscosity and high
absorption capacity, the aqueous [DMAPAH][F] is believed to be an attractive alternative to
other absorbents for separation of CO2.
The absolute solubilities of CO2 in the aqueous solutions of [DMAPAH][F] with
different concentrations (60 wt%, 70 wt% and 80 wt%) are shown in Fig. 4(a). It is found that
the absorption capacity increases with the increasing concentration at CO2 partial pressure
Pe > 0.2 bar, as there are more moles of [DMAPAH][F] reacting with CO2 in concentrated
solutions. However, all these solutions show a tight absorption capacity at Pe < 0.2 bar. This
can be more easily discovered from Fig. 4(b), where the CO2 loading per mole of PIL
(relative uptake) decreases with the increasing concentration of the solution at low-pressure
region. It is the loss in the relative uptake of CO2 that leads to the tight absolute absorption
capacity. This is in consistence with the phenomenon of other solvents such as MEA [42],
DEA [43], ILs (e.g. [C2min][Gly] [44] and [N1111][Gly] [17]). This phenomenon may be
attributed to the intermolecular interactions of the PIL. The [DMAPAH][F] molecules in
concentrated solutions are more inclined to aggregate together, while CO2 at low partial
pressure possesses too low free energy to break up all the aggregation of the PIL. Therefore,
the availability of PIL in concentrated solution is lower than that in the dilute solutions.
However, with the increase of the CO2 partial pressure, the activity of the CO2 molecules is
high enough to break up the aggregation of PIL. Meanwhile, the concentrated solutions are
more easily to accommodate more moles of CO2 in physical absorption manner due to the
larger electron density of F. Therefore, the molar uptake of CO2 in the solutions tends to
increase with the increasing PIL concentration under high pressure (Fig. 4(b)).

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The uptake of CO2 in aqueous [DMAPAH][F] solutions with different concentrations at
initial CO2 partial pressure P0 = 1 bar and temperature T = 308.2 K, is illustrated in Fig. 5(a).
It is shown that both the 70 wt% and the 80 wt% solutions reach the equilibrium within 25
mins, while the 60 wt% needs more than 60 mins (Fig. 5(a)). To characterize the absorption
rate more precisely, the apparent absorption rate, k, is also introduced. The reaction rate
equation can be expressed as follows [45]:
r = k∙CIL∙CCO2
(1)
At the beginning of the reaction, it’s reasonable to assume that only a small amount of PILs
react with CO2. The concentration of the absorbent is nearly constant compared with the
pressure change. So the equation (1) can be rewritten as [45]:
r = - dCCO2/dt=k∙CCO2
(1(a))
where k is the apparent absorption rate constant. By integrating the equation (1(a)), we can get
the equation (2),
n
0
 n
e
ln
 kt
(2)
n  n
e
where n0 is the moles of CO2 at the beginning of the reaction; ne is the moles of CO2 at the
equilibrium; n is the moles of CO2 at time t. From the equation (2), the apparent reaction rate
constant k can be obtained by the slope of the plot of ln[(n0-ne)/(n-ne)] vs. t (Fig. 5(b)). The
values of k are summarized in Table 2. It shows that the apparent absorption rates of the
solutions are arranged in the following order: 70 wt% > 80 wt% > 60 wt%. The low
concentration of the 60 wt% solution may account for its slow absorption rate. As the
concentrated solution has high viscosity, the absorption rate of the 80 wt% solution is lower
than that of the 70 wt% solution as a result of high viscosity.

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It is noted that both the 70 wt% and the 80 wt% solutions show good absorption
performance. However, as the viscosity of the 80 wt% is larger, the energy input in mass
transfer and in practical transportation is higher correspondingly. Therefore, the 70 wt% is
more favored in this work. The exact concentration should be optimized from the combination
with engineering application in the future.
3.3. Thermodynamic parameters
The temperature dependency of the apparent absorption rate constant k is correlated with
the Arrhenius equation as follows [45]:
E
a
Ln k  
 A
(3)
RT
where k is the apparent absorption rate constant; Ea is the apparent reaction activation energy;
T represents the reaction temperature; A is a constant. It’s clear that the activation energy Ea
can be obtained by the slope of the plot of Lnk vs. 1/T. The values of k for the 70 wt%
[DMAPAH][F] aqueous solution at 298.2 K, 308.2 K, 318.2 K and 328.2 K were calculated
by the equation (2) (Fig. 6) and the results were listed in Table 2. As expected, the apparent
reaction constant k rises with the increasing temperature. According to the slope (-659.9) of
the plot in Fig. 7, the value of Ea is calculated to be 54.86 kJ∙mol-1 with a correlation
coefficient (σ) of 0.987. It is comparable to standard absorbents such as MEA, DEA, MDEA
(30 ~ 56 kJ∙mol-1) [46,47], implying the high reactivity of CO2 with [DMAPAH][F].
The enthalpy change ΔHSOL denotes the strength of the interaction between the
absorbents and CO2. A small absolute value of ΔHSOL represents a weak interaction so that the
energy input during the regeneration will decrease. The ΔHSOL and the entropy change ΔSSOL
can be calculated by the following equations [11]:

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




HSOL
 Ln P

1


R
(
)
T x
(4)
(5)
SSOL
R
 Ln P
 LnT


x
 



where x is the amount of CO2 absorbed in the unit of molality. The solubility data of CO2 at
different temperatures and pressures are shown in Fig. 8. It is found that the solubility of CO2
decreases with the increasing temperature, which is a typical phenomenon in the absorption of
CO2 [48]. Fig. 9 shows the dependence of LnP on 1/T and LnT at x = 2.27 mol/kg. The ∆HSOL
and ∆SSOL can thus be obtained by calculating the slopes of the two lines, with the values of
-42.8 kJ∙mol-1 and -137 J∙mol-1∙K-1, respectively. Following the same procedure, the values of
∆HSOL and ∆SSOL at different concentrations (x) were obtained and shown in Table 3. It is
found that the ΔHSOL of the 70 wt% [DMAPHA][F] aqueous solution (-35 ~ -43 kJ∙mol-1) is
about only half as much as that of the standard absorbents such as the MEA and the DEA
aqueous solutions (-70 ~ -101 kJ∙mol-1) [9]. It implies a significant reduction in the energy
required to break the chemical bonds between the aqueous [DMAPAH][F] and the absorbed
CO2 during the regeneration step in comparison with those traditional amine solutions, so that
the economic cost in carbon capture is cut down [49].
In contrast to the small absolute values of ΔHSOL, the aqueous [DMAPAH][F] solution is
accompanied with large negative entropy ΔSSOL (-114 ~ -137 J∙mol-1∙K-1). It is known that
ΔSSOL of the absorption process is determined by the entropy loss of the CO2 molecule during
the transformation from the gas phase to liquid phase, and most liquid CO2 absorbents have
very similar values of ΔSSOL under same conditions [49]. For the 1:2 chemical absorption of
CO2 (one CO2 molecule reacts with two IL molecules or amine groups, e.g. CO2 absorption in

ACCEPTED MANUSCRIPT
the aqueous MEA solution), the value of ΔSSOL is found to be about -100 J∙mol-1∙K-1 [49].
However, for the 1:1 chemical absorption of CO2 (one CO2 molecule reacts with one IL
molecule), the ΔSSOL is proved to be about -140 J∙mol-1∙K-1 [49]. The large negative entropy
ΔSSOL in our work is close to -140 J∙mol-1∙K-1, consistent with the 1:1 absorption mechanism
of F- anion bending with CO2. In addition, the Gibbs free energy is defined as:
∆GSOL = ∆HSOL – T∙∆SSOL
In comparison with the traditional amine solutions, the capture of CO2 by aqueous
[DMAPAH][F] solution is accompanied with less negative ∆HSOL and larger negative ∆SSOL
(6)
,
both of which are superimposed to result in the less negative ∆GSOL (lower absolute value).
Therefore, the aqueous CO2-saturated [DMAPAH][F] can be easily to undergo desorption at a
relatively low temperature, leading to less energy input in the regeneration process and
making the aqueous [DMAPAH][F] an energy-efficient absorbent.
An insight into the dissolution constant pKa is useful in measuring the proton acceptor
characterizing the PIL [50]. A potentiometric titration experiment was conducted at 298.2 K
for the determination of pKa of the PIL, following the same method as in literature [51]. The
standard titrant is HCl aqueous solution with a concentration of 0.9674 M. In order to
eliminate the dilution, the PIL aqueous solution is also prepared with an adjacent
concentration of 0.9705 M. From the titration curve as shown in Fig. 10, the pKa of the
DMAPA and PIL is calculated to be 10.50 and 8.68, respectively (The calculation can be seen
in Appendix A). In comparison with the pKa of DMAPA, the much lower pKa of the PIL
(8.68) verifies that the basicity of the amine site (proton acceptor) is reduced, which is
resultant from the proton transfer from HF to DMAPA during the synthesis process, leading

ACCEPTED MANUSCRIPT
to the low absolute enthalpy change ΔHSOL
3.4 Recycling of PILs.
.
To evaluate the recyclability of PILs during CO2 absorption, the CO2-saturated aqueous
[DMAPAH][F] solution in an airtight vessel was heated to 353.2 K for 1 h. The released CO2
is removed from the vessel by multiple evacuation. The time of each evacuation lasts for 5 s
so that the evaporation of water can be prevented. The regenerated aqueous [DMAPAH][F] is
reused directly for the next round to determine the CO2 solubility. The absorption–desorption
experiments were conducted for five cycles and Fig. 11 shows the solubility of CO2 in
aqueous [DMAPAH][F] at 308.2 K and 1 bar during the five cycles. It is seen that the
absorption capacity of the absorbent in each cycle is very close to each other, except for the
first to the second cycle where most of the capacity loss (about 6%) appears. This is because
high vacuum is not realized (the pressure of the system is higher than 15 kPa at the end of
regeneration). The following absorption-desorption cycles indicate the excellent reversibility
of the absorbent, meeting the requirement of green chemistry.
4. Conclusion
In summary, to find highly effective ILs for the capture of CO2, the absorption of CO2 in
a series of aqueous DMAPAH-based PILs with different anions were measured. It was found
that the solubility of CO2 in the [DMAPAH][F] aqueous solution reached up to 3.62 mol/kg,
which is superior to both the traditional amine solutions and other ILs. A novel absorption
mechanism of F- anion bending with CO2 is proposed and verified. It is further illustrated that
the aqueous [DMAPAH][F] of 70 wt% showed better performance on both the absorption rate
and the absorption capacity than that of 60 wt% and 80 wt%. Meanwhile, the relevant

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parameters, such as Ea, ΔHSOL and ΔSSOL were obtained from the dynamic and solubility data.
In particular, both the less negative ΔHSOL and the larger negative ΔSSOL were attributed to a
relatively low desorption temperature. In addition, the absorption–desorption experiments
were carried out for five cycles with no obvious loss in the absorption capacity of CO2. It is
believed that the aqueous [DMAPAH][F] investigated in the article is a class of potential
absorbent for the capture of CO2.
Appendix A
The reactions during the titration process are summarized as follows, where DA
2+
represents DMAPA, DAH+ represents DMAPAH+ (PIL), DAH22+ represents DMAPAH2 .
DA + H+ ⇌ DAH+
DAH+ + H+ ⇌ DAH2
F- + H+ ⇌ HF
(A.1)
(A.2)
(A.3)
2+
Thus we can get the following equations:
[DAH+ ]
c°
K 
1
[DA][H+ ]
Reaction equilibrium of equation (A.1):
(A.4)
(A.5)
(A.6)
2
c°
 
[DAH22+ ]
c°
K
2

[DAH+ ][H+ ]
Reaction equilibrium of equation (A.2):
2
c°
 
[HF]
c
K
3

[F- ][H+ ]
Reaction equilibrium of equation (A.3):
2
c
 
where the standard concentration c°= 1 M.
The mass balance of DA is

ACCEPTED MANUSCRIPT
2+
C0,DAV0 = ([DA] + [DAH] + [DAH2 ])(V0 + VHCl
)
(A.7)
(A.8)
The mass balance of F is
C0,FV0 = ([HF] + [F])(V0 + VHCl
The mass balance of the proton H is
C0,HFV0+CHClVHCl= ([HF]+[H+]+[DAH+]+2[DAH2 ])(V0+VHCl
The definition of pH is
pH = -Log10 [H]
Combining the equations (A.4) to (A.10), we can get the following equation:
)
2+
)
(A.9)
(A.10)
K
3
V
0
C0,Ac 10 pH
C
0,DA V
0
 K
1
(1 2K
2
10 pH )10 pH
 V
0
10 pH

 C0,HAc V
0
2
1 K
10 pH
3
1 K
1
10 pH  K
1
 K
2
 10 pH


VHCl

C
HCl 10 pH
(A.11)
where C0,i is the initial concentration of species i, C0,DA = C0,HF = 0.9705 M, CHCl = 0.9674 M,
V0 = 20.00 mL, K3 = 2832 (the reciprocal of HF dissociation constant at 298.2 K).
Equation (A.11) is adopted to fit the titration curve (Fig. 10) and the fitting results are
obtained: K1 = 3.15 × 1010, K2 = 4.81 × 108. Thus, the pKa value of the DMAPA and the
DMAPAH (PIL) are calculated to be: pKa (DMAPA) = Log K1 = 10.50, pKa (PIL) = Log K2
= 8.68, respectively.
Acknowledgment
This work was supported by the the National Natural Science Foundation of China (Nos.
21376115, 21576129 and 21676134).
References

ACCEPTED MANUSCRIPT
[1] K. Huang, X.M. Zhang, Y. Xu, Y.T. Wu, X.B. Hu, Protic ionic liquids for the selective
absorption of H2S from CO2: thermodynamic analysis, AIChE J. 60 (2014) 4232-4240.
[2] G.M. Maria, B. Jorge, A. Concepcion, P. Jose, R. Francisco, Anion effects on kinetics and
thermodynamics of CO2 absorption in ionic liquids, J. Phys. Chem. B 117 (2013) 3398−3406.
[3] M. Ishida, H. Jin, CO2 recovery in a power plantwith chemical looping combustion,
Energy Convers. Mgmt. 38 (1997) S187-S192.
[4] A. Rao, D. Rubin, A technical, economic, and environmental assessment of amine-based
CO2 capture technology for power plantgreen house gas control, Environ. Sci. Technol. 36
(2002) 4467-4475.
[5] S. Freguia, G.T. Rochelle, Modeling of CO2 capture by aqueous monoethanolamine,
AIChE J. 49 (2003) 1676-1686.
[6] J. Gabrielsen, M.L. Michelsen, E.H. Stenby,G.M. Kontogeorgis, A model for estimating
CO2 solubility in aqueous alkanolamines, Ind. Eng. Chem. Res. 44 (2005) 3348-3354.
[7] S. Paul, A.K. Ghoshal, B. Mandal, Removal of CO2 by single and blended aqueous
alkanolamine solvents inhollow-fiber membrane contactor: modeling and simulation, Ind.
Eng. Chem. Res. 46 (2007) 2576-2588.
[8] I. Niedermaier, M. Bahlmann, C. Papp, C. Kolbeck, W. Wei, S. . alder n, M. rabau,
P.S. Schul , P. asserscheid, .P. Steinr ck, F. Maier, Carbon dioxide capture by an amine
functionalized ionic liquid: fundamental differences of surface and bulk behavior, J. Am.
Chem. Soc. 136 (2014) 436-441.
[9] J.H. Huang, T. Ruther, Why are ionic liquids attractive for CO2 Absorption? An Overview,
Aust. J. Chem. 62 (2009) 298–308.

ACCEPTED MANUSCRIPT
[10] M.S.R. Shahrom, C.D. Wilfred, A.K.Z. Taha, CO2 capture by task specific ionic liquids
(TSILs) and polymerized ionic liquids (PILs and AAPILs), J. Mol. Liquids 219 (2016)
306–312.
[11] X.Y. Li, M.Q. Hou, Z.F. Zhang, B.X. Han, G.Y. Yang, X.L. Wang, L.Z. Zou, Absorption
of CO2 by ionic liquid/polyethylene glycol mixture and the thermodynamic parameters, Green
Chem. 10 (2008) 879.
[12] D.J. Tao, F.F. Chen, Z.Q. Tian, K. Huang, S.M. Mahurin, D.e. Jiang, S. Dai, Highly
efficient carbon monoxide capture by carbanion-functionalized ionic liquids through C-site
interactions, Angew. Chem. 129 (2017) 6947–6951.
[13] S. Raeissi, C.J. Peters, Carbon dioxide solubility in the homologous
1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide family, J. Chem. Eng. Data.
54 (2009) 382–386.
[14] C.M. Wang, H.M. Luo, X.Y. Luo, H.R. Li, S. Dai, Equimolar CO2 capture by
imidazolium-based ionic liquids and superbase systems, Green Chem. 12 (2010) 2019–2023.
[15] J.Z. Zhang, C. Jia, H.F. Dong, J.Q. Wang, X.P. Zhang, S.J. Zhang, A novel dual
amino-functionalized cation-tethered ionic liquid for CO2 capture, Ind. Eng. Chem. Res. 52
(2013) 5835–5841.
[16] Y. Zhang, Z.K. Wu, S.L. Chen, P. Yu, Y.B. Luo, CO2 Capture by imidazolate-based ionic
liquids: effect of functionalized cation and dication, Ind. Eng. Chem. Res. 52 (2013)
6069-6075.
[17] F. Zhang, C.G. Fang, Y.T. Wu, Y.T. Wang, A.M. Li, Z.B. Zhang, Absorption of CO2 in the
aqueous solutions of functionalized ionic liquids and MDEA, Chem. Eng. J. 160 (2010)

ACCEPTED MANUSCRIPT
691-697.
[18] F.F. Chen, K. Huang, Y. Zhou, Z.Q. Tian, X. Zhu, D.J. Tao, D.E. Jiang, S. Dai,
Multi-molar absorption of CO2 by the activation of carboxylate groups in amino acid ionic
liquids, Angew. Chem. Int. Ed. 55 (2016) 7166–7170.
[19] Y.Y. Jiang, G.N. Wang, Z. Zhou, Y.T. Wu, J. Geng, Z.B. Zhang, Tetraalkylammonium
amino acids as functionalized ionic liquids of low viscosity, Chem. Commun. (2008) 505-507.
[20] J.M. Zhang, S.J. Zhang, K. Dong, Y.Q. Zhang, Y.Q. Shen, X.M. Lv, Supported
absorption of CO2 by tetrabutylphosphonium amino acid ionic liquids, Chem. Eur. J. 12 (2006)
4021-4026.
[21] F. Zhang, Y. Gao, X.K. Wu, J.W. Ma, Y.T. Wu, Z.B. Zhang, Regeneration performance of
amino acid ionic liquid (AAIL) activated MDEA solutions for CO2 capture, Chem. Eng. J.
223 (2013) 371-378.
[22] R. Vijayraghavan, S.J. Pas, E.I. Izgorodina, D.R. MacFarlane, Diamino protic ionic
liquids for CO2 capture, Phys. Chem. Chem. Phys. 15 (2013) 19994-19999.
[23] C.M. Wang, H.M. Luo, D.E. Jiang, H.R. Li, S. Dai, Carbon dioxide capture by
superbase-derived protic ionic liquids, Angew. Chem. Int. Ed. 49 (2010) 5978–5981.
[24] P.K. Chhotaray, R.L. Gardas, Thermophysical properties of ammonium and
hydroxylammonium protic ionic liquids, J. Chem. Thermodyn. 72 (2014) 117-124.
[25] T. L. Greaves, C.J. Drummond, Protic ionic liquids: properties and applications, Chem.
Rev. 108 (2008) 206-237.
[26] D.S. Firaha, B. Kirchner, CO2 absorption in the protic ionic liquid ethylammonium
nitrate, J. Chem. Eng. Data 59 (2014) 3098-3104.

ACCEPTED MANUSCRIPT
[27] A. Albert, E.P. Serjeant, The Determination of Ionization Constants; A Laboratory
Manual, 3rd ed., Chapman and Hall, New York, 1984.
[28] E.D. Bates, R.D. Mayton, I. Ntai, J.H. Davis, CO2 capture by a task-specific ionic liquid,
J. Am. Chem. Soc.124 (2002) 926-927.
[29] E.I. Izgorodina, J.L. Hodgson, D.C. Weis, S.J. Pas, D.R. MacFarlane, Physical absorption
of CO2 in protic and aprotic ionic liquids: an interaction perspective, J. Phys. Chem. B 119
(2015) 11748-11759.
[30] B.L. Bhargava, S. Balasubramanian, Probing anion–carbon dioxide interactions in room
temperature ionic liquids: Gas phase cluster calculations, Chemical Physics Letters 444 (2007)
242–246.
[31] G.B. Damas, A.B.A. Dias, L.T. Costa, A quantum chemistry study for ionic liquids
applied to gas capture and separation, J. Phys. Chem. B 118 (2014) 9046−9064.
[32] B.H. Lv, B.S. Guo, Z.M. Zhou, G.H. Jing, Mechanisms of CO2 capture into
monoethanolamine solution with different CO2 loading during the absorption/desorption
processes, Environ. Sci. Technol. 49 (2015) 10728−10735.
[33] F.Y. Jou, A.E. Mather, F.D. Otto, The solubility of CO2 in a 30 mass percent
monoethanolamine solution, Can. J. Chem. Eng. 73 (1995) 140-147.
[34] N.E. Hadri , D.V. Quang, E.L.V. Goetheer, M.R.M.A. Zahra, Aqueous amine solution
characterization for post-combustion CO2 capture process, Applied Energy 185 (2017)
1433–1449.
[35] G. Hong, J. Jacquemin, P. Husson, M.F. Costa Gomes, M. Deetlefs, M. Nieuwenhuyzen,
O. Sheppard, C. Hardacre, Effect of acetonitrile on the solubility of carbon dioxide in

ACCEPTED MANUSCRIPT
1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, Ind. Eng. Chem. Res. 45
(2006) 8180–8188.
[36] M.J. Muldoon, S.N.V.K. Aki, J.L. Anderson, J.K. Dixon, J.F. Brennecke, Improving
carbon dioxide solubility in ionic liquids, J. Phys. Chem. B 111 (2007) 9001–9009.
[37] J.W. Ma, Z. Zhou, F. Zhang, C.G. Fang, Y.T. Wu, Z.B. Zhang, A.M. Li,
Ditetraalkylammonium amino acid ionic liquids as CO2 absorbents of high capacity, Environ.
Sci. Technol. 45 (2011) 10627–10633.
[38] M. Ramdin, T.J.H. Vlugt, T.W. de Loos, Solubility of CO2 in the ionic liquids
[TBMN][MeSO4] and [TBMP][MeSO4], J. Chem. Eng. Data 57 (2012) 2275–2280.
[39] C.M. Wang, X.Y. Luo, H.M. Luo, D.E. Jiang, H.R. Li, S. Dai, Tuning the basicity of
ionic liquids for equimolar CO2 capture, Angew. Chem. Int. Ed. 123 (2011) 5020–5024.
[40] X.M. Zhang, K. Huang, S. Xia, Y.L. Chen, Y.T. Wu, X.B. Hu, Low-viscous
fluorine-substituted phenolic ionic liquids with high performance for capture of CO2, Chem.
Eng. J. 274 (2015) 30–38.
[41] E.D. Bates, R.D. Mayton, I. Ntai, J.H. Davis, CO2 capture by a task-specific ionic liquid,
J. Am. Chem. Soc. 124 (2002) 926–927.
[42] J.I. Lee, F.D. Otto, AE Mather, Equilibrium between carbon dioxide and aqueous
monoethanolamine solutions, J. appl. Chem. Biotechnol. 26 (1976) 541-549.
[43]. A. Benamor, M.K. Aroua, Modeling of CO2 solubility and carbamate concentration in
DEA, MDEA and their mixtures using the Deshmukh–Mather model, Fluid Phase Equilibria.
231 (2005) 150–162.
[44]. Z.K. Wu, Y. Zhang, W.K. Lei, P. Yu, Y.B. Luo, Kinetics of CO2 absorption into

ACCEPTED MANUSCRIPT
aqueous 1-ethyl-3-methylimidazolium glycinate solution, Chem. Eng. J. 264 (2015) 744–752.
[45] G.N. Wang, W.L. Hou, F. Xiao, J. Geng, Y.T. Wu, Z.B. Zhang, Low-viscosity
triethylbutylammonium acetate as a task-specific ionic liquid for reversible CO2 absorption, J.
Chem. Eng. Data 56 (2011) 1125–1133.
[46] B. Arstad, R. Blom, O. Swang, CO2 Absorption in aqueous solutions of alkanolamines:
mechanistic insight from quantum chemical calculations, J. Phys. Chem. A 111 (2007)
1222-1228.
[47] F. Pani, A. Gaunand, R. Cadours, C. Bouallou, D. Richon, Kinetics of absorption of CO2
in concentrated aqueous methyldiethanolamine solutions in the range 296 K to 343 K, J.
Chem. Eng. Data 42 (1997) 353-359
[48] L.L. Zhang, J.X. Wang, Z.P. Liu, Y. Lu, G.W. Chu, W.C. Wang, J.F. Chen, Efficient
capture of carbon dioxide with novel mass-transfer intensification device using ionic liquids,
AIChE J. 59 (2013) 2957-2965.
[49] K. Huang, Y.T. Wu, S. Dai, Sigmoid correlations for gas solubility and enthalpy change
of chemical absorption of CO2, Ind. Eng. Chem. Res. 54 (2015) 10126−10133.
[50] A. Tagiuri, M. Mohamedali, A. Henni, Dissociation constant (pKa) and thermodynamic
properties of some tertiary and cyclic amines from (298 to 333) K, J. Chem. Eng. Data 61
(2016) 247−254.
[51]. N. Yang, D.Y. Xu, C.C. Wei, G. Puxty, H. Yu, M. Maeder, S. Norman, P. Feron,
Protonation constants and thermodynamic properties of amino acid salts for CO2 capture at
high temperatures, Ind. Eng. Chem. Res. 53 (2014) 12848−12855.

ACCEPTED MANUSCRIPT
Table 1. A summary of the solubility of CO2 in the amine solutions and some ILs at 1 bar.
Entry.
1
T (K)
308.2
313.2
313.2
313.2
313.2
313.2
313.2
313.2
313.2
298.2
333.5
298.2
313.2
297.2
313.2
313.2
313.2
313.2
293.2
293.2
Amine solutions
[DMAPAH][F] (70 wt%)
MEA (30 wt%)
DEA (30 wt%)
CO2 solubility (mol/kg)
Ref
this work
[33]
[34]
[34]
[34]
[34]
[35]
[16]
[16]
[17]
[36]
[37]
[38]
[39]
[40]
[40]
[40]
[40]
[41]
[22]
3.62
2.73
2.17
2.16
2.82
2.69
0.06
2.79
2.42
1.37
0.28
0.79
0.58
1.86
2.18
1.81
2.00
2.28
1.70
0.07
2
3
4
1A2P (30 wt%)
PZ (30 wt%)
5
6
AMP (30 wt%)
[emim][Tf2N]
7
8
[emim][Im]
9
[bmim][Im]
10
11
12
13
14
15
16
17
18
19
20
[N1111][Gly] (50 wt%)
[N4444][DOC]
[C2(N114)2][Gly] (40 wt%)
[P1444][MeSO4]
[P66614][Pyr]
[P4444][PhO]
[P4444][2-F-PhO]
[P4444][3-F-PhO]
[P4444][3-F-PhO]
[NH2p-bim][BF4]
[MEAH][Cl] (70 wt%)

ACCEPTED MANUSCRIPT
21
22
23
24
293.2
293.2
293.2
308.2
[N221H][Tf2N]
[DEEDAH][For]
0.15
2.86
1.89
2.50
[22]
[22]
[DMAPAH][For]
[22]
[DMAPAH][For] (70 wt%)
this work

ACCEPTED MANUSCRIPT
Table 2. Apparent absorption rate constants k of the [DMAPAH][F] aqueous solutions with
different concentrations at different temperatures.
Entries
T (K)
308.2
308.2
308.2
298.2
318.2
328.2
Concentration (wt%)
k (min-1)
1
2
3
4
5
6
60
70
80
70
70
70
0.173 ± 0.008
0.242 ± 0.003
0.220 ± 0.004
0.138 ± 0.001
0.324 ± 0.005
0.331 ± 0.006

ACCEPTED MANUSCRIPT
Table 3. ΔHSOL and ΔSSOL in the absorption of CO2 into the 70 wt% [DMAPAH][F] aqueous
solution at different uptake x of CO2.
x (mol/kg)
1.50
ΔHSOL (kJ∙mol-1)
-35.8 ± 5.6
ΔSSOL (J∙mol-1∙K-1)
-114 ± 18
1.85
-39.2± 4.2
-125 ± 14
2.27
-42.8 ± 0.8
-137 ± 3

ACCEPTED MANUSCRIPT
List of Figure Captions
Fig. 1. Synthesis route of the DMAPAH-based PILs.
Fig. 2. Absolute CO2 solubility (a) and relative CO2 uptake (b) in the aqueous
solutions of the DMAPAH-based PILs with different anions (☆: F, □: For, ○: Ac, △:
Prop, ▽: Buty, *: MOAc).
13
19
Fig. 3. C NMR spectra (a) and F NMR spectra (b) of the aqueous [DMAPAH][F]
before and after reacted with CO2.
Fig. 4. Absolute CO2 solubility (a) and relative CO2 uptake (b) in the aqueous
solutions of the [DMAPA ][F] with different concentrations (■:60 wt%, ●:70 wt%,
▲:80 wt%) at T = 308.2 K.
Fig. 5. Kinetic absorption (a) and the apparent absorption rate constants k (b) of the
[DMAPA ][F] aqueous solutions with different concentrations (■:60%, ●:70%,
▲:80%) at P1 = 1 bar, T = 308.2 K.
Fig. 6. Apparent absorption rate constants k of the 70 wt% [DMAPAH][F] aqueous
solution at different temperatures (■:298.2 K, ●:308.2 K, ▲:318.2 K, ▼:328.2 K).
Fig. 7. Apparent activation energy Ea of the reaction between CO2 and the 70 wt%
[DMAPAH][F] aqueous solution.
Fig. 8. Absorption capacities of the 70 wt% [DMAPAH][F] aqueous solution at
different temperatures (■:298.2 K, ●:308.2 K, ▲:318.2 K, ▼:328.2 K).
Fig. 9. Enthalpy change ΔHSOL (a) and entropy change ΔSSOL (b) of the 70 wt%
[DMAPA ][F] aqueous solution (■: experimental; straight line: fitted).
Fig. 10. Potentiometric titration curve of [DMAPAH][F].