Original Research Article
Experimental investigation of CO2
absorption enthalpy in conventional
imidazolium ionic liquids
Yi Hu, State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology,
Wuhan, China and China-EU Institute for Clean and Renewable Energy, Huazhong University of
Science and Technology, Wuhan, China
Xiaoshan Li , Ji Liu, Liwei Li and Liqi Zhang
, State Key Laboratory of Coal Combustion,
Huazhong University of Science and Technology, Wuhan, China
Abstract: Ionic liquids (ILs) have elicited considerable attention in the field of gas absorption and
separation due to their unique properties. In addition to absorption capacity and absorption rate,
absorption enthalpy is an important indicator for sorbent evaluation. In this work, the absorption
enthalpy of CO2 in several kinds of ILs was measured directly using a Calvet calorimeter. The effects of
anions, cationic alkyl chain length, and temperature on absorption properties were discussed. Results
showed that anions influenced CO2 absorption enthalpy considerably. 1-Butyl-3-methylimidazolium
acetate presented the most appropriate enthalpy for CO2 absorption compared with the other ILs
investigated with different chain lengths. Absorption enthalpy was dependent on temperature, and the
isosteric absorption enthalpy decreased with an increase in temperature. This work provides a useful
basis for the appropriate selection of alternative forms of IL sorbents for post-combustion CO2 capture
C 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.
technology. Keywords: CO2 capture; ionic liquid; absorption capacity; absorption enthalpy
Introduction
s an important component of carbon
capture and storage (CCS) technology, current
post-combustion technology focuses on finding
low-cost and efficient absorbents.1 In recent years,
ionic liquids (ILs) have been recognized as promising
absorption solvents for CCS technology and have
elicited widespread international attention.2–4
Ionic liquids refer to organic liquid substances that
consist of anions and cations at room temperature.
Compared with traditional organic solvents, ILs exhibit
superior characteristics, as follows. (1) Ionic liquids
A
have low vapor pressure and do not release volatile
substances at high temperatures. They prevent
equipment corrosion and high energy consumption
due to the volatilization of traditional amine
absorbents. (2) They are molecularly designable. Thus,
their structure can be designed according to
requirements. For example, the theoretical CO2
absorption capacity of the currently available design
functional IL is considerably higher than that of
monoethanolamine solution. (3) Ionic liquids
demonstrate good thermal stability and chemical
stability, and thus, they have broad applications. (4)
They are reusable. Saturated ILs can be recycled and
Correspondence to: Xiaoshan Li, State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road,
Wuhan 430074, China. E-mail: lxs0721@hust.edu.cn; Liqi Zhang, State Key Laboratory of Coal Combustion, Huazhong University of Science
and Technology, Wuhan, China E-mail: lqzhang@mail.hust.edu.cn
Received February 10, 2018; revised March 16, 2018; accepted March 16, 2018
Published online at Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/ghg.1777
C 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.
Greenhouse Gas Sci Technol. 0:1–8 (2018); DOI: 10.1002/ghg
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Y Hu et al.
Original Research Article: Experimental investigation of CO2 absorption enthalpy in conventional imidazolium ionic liquids
reused under high-temperature and low-pressure
conditions. The energy consumption of ILs is also low.
(5) Ionic liquids provide industrial environmental
protection because they do not generate waste and
wastewater; hence, secondary pollution problems
caused by conventional absorbents can be avoided.5–7
Blanchard et al.8 found that CO2 can be dissolved in
large quantities in ILs, whereas ILs are nearly insoluble
in CO2 . This finding provides a new means to capture
CO2 . Since then, a number of studies on the absorption
of CO2 by ILs have been conducted. Blanchard et al.9
compared the solubility of CO2 in different ILs. Carbon
dioxide solubility in ILs is considerably influenced by
anions but the effect of cations is minimal. Bates et al.10
designed and synthesized the first functionalized IL for
absorbing CO2 , namely [NH2 p-bim][BF4 ], which
introduced a basic group, −NH2 , into the carbon chain
of the cationic imidazole ring. Compared with previous
studies, this functionalized IL significantly increased
the absorption capacity of CO2 . The work of Bates et al.
achieved a milestone in CO2 absorption by ILs. It
provides the pioneering idea of using functionalized
ILs for gas separation. Subsequently, scholars have
designed and synthesized various functionalized ILs
from anions and cations,11–14 grafting,15,16 and other
aspects.17,18 Functionalized ILs significantly increase
the amount of CO2 absorption;19–23 however, the high
prices of these ILs increase investment and operating
costs, which hinders their industrial applications.
Absorption enthalpy is an essential value for the
energy required in CO2 capture. Solubility and
absorption enthalpy data are essential for the flue gas
treatment operation of power plants.24 The enthalpy of
absorption during a reaction is closely related to energy
consumption.25–27 Although available data on CO2
solubility in new functionalized ILs are abundant, only
a few studies on the absorption enthalpy of ILs during
CO2 capture have been reported. Xie et al.28 calculated
CO2 absorption enthalpy using the non-random
two-liquid with Redlich–Kwong model. The value
consists of excess enthalpy and dissolution enthalpy,
according to Eqns (1) and (2), respectively. Li et al.29
studied the enthalpy of [Emim][Ac] with CO2 via
quantum chemical calculation.
These absorption enthalpy values are mainly obtained
from theoretical calculations30 and quantum chemical
simulations, but direct experimental measurements are
rare.1 In the current work, the absorption enthalpy of
CO2 in ILs was directly measured using a
micro-calorimeter. The effects of anions, cationic chain
2
C 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.
length, and temperature on absorption enthalpy were
investigated.
dis
2 ∂ ln Hco2 (T, P)
(1)
H = −RT
∂T
∂ ln γi
H ex = −RT 2
(2)
χi
∂T
Experimental
Experimental materials
Seven conventional imidazolium ILs were used in this
experiment: 1-hexyl-3-methyimidazolium acetate
([HMIm][AC]), 1-octyl-3-methylimidazolium
acetate ([OMIm][AC]), 1-butyl-3-methylimidazolium
tetra fluoroborate ([BMIm][BF4 ]), 1-butyl-3methylimidazole lactate ([BMIm][L-lactate]), and
1-butyl-3-methylimidazolium hexafluorophosphate
([BMIm][PF6 ]) were purchased from Lanzhou Institute
of Physical Chemistry, Lanzhou, China. 1-Ethyl-3methylimidazolium acetate ([EMIm][AC]), and
1-Butyl-3-methylimidazolium acetate ([BMIm][AC])
were procured from Shanghai Chengjie Chemical Co.,
Ltd, Shanghai, China. The experimental CO2 gas
(purity of 99.99%) was provided by Wuhan Yiming.
Prior to the experiment, the ILs were placed in a
vacuum oven at 80°C for 72 h.
Experimental apparatus
Carbon dioxide absorption capacity in ILs was
determined using a previously reported absorption
apparatus.31 Absorption enthalpy experiments were
conducted on a C80 microcalorimeter from Setaram
Instrumentation (Lyons, France), as shown in Fig. 1.
The instrument has two types of reaction cell, namely,
a reference cell and a sample cell, to eliminate the
impact of ambient temperature fluctuations. The
temperature of the calorimetric system is proportional
to thermal conductivity and corresponds to thermal
change in the reaction system. The reaction cell is
surrounded by a thermocouple ring to detect the
thermal efficiency of a reaction in all dimensions. In
accordance with the Calvet calorimetric principle, the
3D sensor passes the thermal signal to the
amplification system for amplification and to the
data-processing system for thermal efficiency
calculation. The temperature control accuracy of the
instrument is ±0.1°C. A set of gas circulation cells was
used to measure absorption enthalpy. The accuracy of
the absorption enthalpy was ±1%.
Greenhouse Gas Sci Technol. 0:1–8 (2018); DOI: 10.1002/ghg
Original Research Article: Experimental investigation of CO2 absorption enthalpy in conventional imidazolium ionic liquids
Y Hu et al.
Figure 1. Flowchart of C80 Calvet calorimeter.
At the beginning of the experiment, approximately
6 g of IL was preloaded into the sample cell. When the
sample temperature, instrument temperature, and heat
flow curve became stable, 50 mL/min CO2 was
delivered into the sample cell. The IL absorbs CO2 to
produce a thermal effect, and the heat flow curve
changes over time. When the heat flow curve becomes
stable, the IL is considered to be saturated by CO2 .
Then, the heat flow curve is integrated to obtain the
absolute absorption enthalpy of CO2 in the IL. The
absolute absorption enthalpy Habs (J/g) of the IL can
be obtained by integrating the positive peak of the heat
flow curve. The formula is presented in Eqn (3). The
isosteric absorption enthalpy Hiso (kJ/mol) is the
ratio of absolute absorption enthalpy Habs (J/g) and
absorption capacity AC(mol CO2 /mol IL). Hiso is
defined as the enthalpy change of 1 mol gas absorbed at
the same temperature and pressure. The formula is
provided in Eqn (4).
t2
q(t )dt
(3)
Habs = t1
mIL
Hiso =
Habs · M
AC · 1000
(4)
where t1 is the moment when CO2 passes into the IL, t2
is the moment when heat flow becomes stable, q(t) is
the instantaneous heat flow (mW), mIL is the IL mass
(g) loaded into the reaction cell, M is molar mass of IL
(g/mol) and AC is the CO2 absorption capacity of IL
(mol CO2 /mol IL).
C 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.
Table 1. Enthalpy and absorption capacity of ILs
with the same cation but different anions.
Absorption
capacity
(mol CO2 /
mol IL)
Absolute
absorption
enthalpy
(J/g)
Isosteric
absorption
enthalpy
(kJ/mol)
[BMIm][AC]
0.349
−80.573
−45.752
[BMIm][BF4 ]
0.051
−0.839
−3.801
ILs
[BMIm][PF6 ]
0.050
−0.854
−4.729
[BMIm][L-actate]
0.098
−12.118
−28.151
Results and discussion
Effect of anions
Carbon dioxide is non-dipolar; hence, numerous
studies have proven that ILs exhibit a specific
anion–CO2 interaction, in which anions play an
important role in CO2 absorption.32,33 Thus, CO2
absorption experiments were conducted using four
kinds of ILs with the same cation but different anions.
Many studies have shown that imidazolium-based ILs
demonstrate noticeable CO2 absorption capacity.34–37
The cation [BMIm]+ combined with different anions
([AC]− , [BF4 ]− , [L-lactate]− , and [PF6 ]− ) were
therefore investigated. The absorption capacity and
absolute absorption enthalpy of the four ILs were
measured experimentally. The isosteric absorption
enthalpy can be calculated using Eqn (4). The results
are presented in Table 1.
Greenhouse Gas Sci Technol. 0:1–8 (2018); DOI: 10.1002/ghg
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Y Hu et al.
Original Research Article: Experimental investigation of CO2 absorption enthalpy in conventional imidazolium ionic liquids
Figure 2. Optimum structure of the CO2 complexes of with (a) [AC]− , (b) [BF4 ]− , (c) [PF6 ]− , and
(d) [L-lactate]− .
[BMIm][AC] achieved the largest isosteric
absorption enthalpy of −45.752 kJ/mol, followed by
[BMIm][L-actate]. The isosteric absorption enthalpy of
[BMIm][BF4 ] was slightly smaller than that of
[BMIm][PF6 ]. Thermal effects were observed when the
ILs reacted with CO2 , and the isosteric absorption
enthalpy reflected the interaction between the ILs and
CO2 . The interactions of anions with CO2 were also
simulated via quantum chemistry calculations. The
optimum structures of the CO2 complexes with
different anions are shown in Fig. 2. All quantum
chemical calculations reported in this paper were
conducted in accordance with density functional
theory using Gaussian 09 software.
The anion [AC]− had the greatest influence in
disturbing the linear geometry of CO2 . The O–C–O
angle was bent when anions reacted with CO2 , thereby
explaining the strongest interaction between [AC]−
and CO2 . As shown in Table 2, OCO denoted the
change of O–C–O angle after optimization. The order
of OCO was [AC]− > [L-lactate]− > [PF6 ]− >
[BF4 ]− , which is in accordance with the order of the
isosteric absorption enthalpy.
4
C 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.
Table 2. Structural parameters of the anions and
CO2 .
ࢬOCO (deg)
OCO (%)
E(kJ/mol)
Isolated CO2
180.000
0.000
—
[AC]− -
Anion
139.093
22.700
−40.687
[BF4 ]− - CO2
174.281
3.200
−20.246
]− -
175.418
2.500
−15.681
170.022
5.500
−34.814
[PF6
CO2
CO2
[L-lactate]− - CO2
Effect of cationic alkyl chain length
The cationic alkyl chain, which is found on the
imidazolium ring, can also affect isosteric absorption
enthalpy. To study the effect of cationic alkyl chain
length on the absorption enthalpy of ILs, ILs with the
same anion but different cationic alkyl chain lengths
were analyzed in this study. The IL anion was [AC]− ,
and the four cations were [BMIm]+ , [EMIm]+ ,
[HMIm]+ , and [OMIm]+ .
As shown in Table 3, the CO2 isosteric absorption
enthalpy of [BMIm][AC] was the largest, followed by
[EMIm][AC], whereas that of [OMIm][AC] was the
Greenhouse Gas Sci Technol. 0:1–8 (2018); DOI: 10.1002/ghg
Original Research Article: Experimental investigation of CO2 absorption enthalpy in conventional imidazolium ionic liquids
Y Hu et al.
Table 3. Enthalpy and absorption capacity of ILs
with different cationic alkyl chain lengths.
Absorption
capacity
(mol CO2 /
mol IL)
Absolute
absorption
enthalpy
(J/g)
Isosteric
absorption
enthalpy
(kJ/mol)
[EMIm][AC]
0.276
−72.360
−44.625
[BMIm][AC]
0.349
−80.573
−45.752
[HMIm][AC]
0.062
−9.991
−36.650
[OMIm][AC]
0.042
−0.820
−4.973
ILs
Table 4. Viscosity of ILs with different cationic
alkyl chain lengths.
ILs
Viscosity
(mPa·s)
[EMIm]
[AC]
[BMIm]
[AC]
[HMIm]
[AC]
[OMIm]
[AC]
58.5
150
419
534
smallest. [BMIm]+ , [EMIm]+ , [HMIm]+ , and
[OMIm]+ can capture CO2 via physical interaction. It
was reported that free volume was increased with an
increase in the length of cationic alkyl chains.38 The ILs
with an increase in cationic alkyl chains length had a
large free volume to absorb CO2 . Thus, the isosteric
absorption enthalpy of [BMIm][AC] was larger than
that of [EMIm][AC]. However, the isosteric absorption
enthalpy was considerably reduced when the alkyl
chain length was increased from butyl to hexyl. Ionic
liquids with long cationic alkyl chains displayed high
viscosity, which reduced the absorption rate due to an
increase in mass transfer resistance.39–41 The viscosity
of ILs with different cationic alkyl chain lengths is
shown in Table 4.
When the alkyl chain belonged to the hexyl group,
the viscosity of the ILs was high, so free volume was
inhibited to capture abundant CO2 . The isosteric
absorption enthalpy was therefore a decreasing
function of cationic alkyl chain length. The order of the
isosteric absorption enthalpy was [BMIm][AC] >
[EMIm][AC] > [HMIm][AC] > [OMIm][AC]. From
ethyl to butyl, the difference in the isosteric absorption
enthalpy of the ILs was 1.127 kJ/mol. It indicated that,
when the anion was [AC]− , the isosteric absorption
enthalpy should increase with an increase in the length
of cationic alkyl chains. However, from butyl to hexyl,
the difference in the isosteric absorption enthalpy of
the ILs was −9.102 kJ/mol. As the length of cationic
C 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.
Figure 3. The heat flow of [BMIm][AC] capturing CO2 at
different temperatures.
alkyl chains increased, the isosteric absorption
enthalpy decreased drastically. These results indicated
that increasing the cationic alkyl chains length caused a
dramatic change in the viscosity of the ionic liquid and
viscosity had a significant influence on the isosteric
absorption enthalpy. Thus, alkyl chains length should
be considered when selecting the appropriate
absorption enthalpy of ILs.
Effect of temperature
It was reported that the appropriate absorption
enthalpy for the ILs-CO2 system was within −40 kJ/
mol to −60 kJ/mol, which was between physical
absorption and chemical absorption.42 Based on these
results, [BMIm][AC] with the isosteric absorption
enthalpy of −45.752 kJ/mol was the most appropriate
ILs for CO2 absorption among all the ILs used in the
experiment.
When selecting ILs for practical applications, the
effect of temperature on CO2 absorption should be
considered. The isosteric absorption enthalpy of
[BMIm][AC] was studied under a pressure of
0.1 MPa, a CO2 inflow of 50 mL/min, and different
temperatures to determine the most appropriate
operating temperature.
Figure 3 presents the heat flow of [BMIm][AC]
capturing CO2 at different temperatures. The
interaction between [BMIm][AC] and CO2 was
exothermic, so the the curve was convex upward.
Maximum heat flow became greater and the peak time
became shorter with an increase in temperature. These
can explain that the intensity of the interaction
Greenhouse Gas Sci Technol. 0:1–8 (2018); DOI: 10.1002/ghg
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Y Hu et al.
Original Research Article: Experimental investigation of CO2 absorption enthalpy in conventional imidazolium ionic liquids
Table 5. Enthalpy and absorption capacity of ILs
at different temperatures.
Absorption
capacity
(mol CO2 /
mol IL)
Absolute
absorption
enthalpy
(J/g)
Isosteric
absorption
enthalpy
(kJ/mol)
30
0.349
−80.573
−45.752
40
0.321
69.890
43.228
50
0.281
60.890
42.967
60
0.223
47.980
42.682
Temperature
(°C)
between the ILs and CO2 increased and the reaction
time was shortened with increasing temperature.
As shown in Table 5, absolute absorption enthalpy
decreased with an increase in temperature. High
temperature inhibited the IL to capture CO2 . As the
temperature increased, less CO2 reacted with the IL
and the absolute absorption enthalpy generated by the
reaction also decreased. The order of the isosteric
absorption enthalpy was 30°C > 40°C > 50°C > 60°C.
Although the maximum heat flow was the largest when
the temperature was 60°C, considering the reaction
time, the isosteric absorption enthalpy reached the
maximum when the temperature was 30°C. This result
indicated that the isosteric absorption enthalpy was
positively correlated with absolute absorption enthalpy
when the temperature changed.
Conclusions
The isosteric absorption enthalpy was measured and
discussed to explain the effect of different anions. Our
finding suggested that anions determined the isosteric
absorption enthalpy of CO2 . [AC]− achieved better
isosteric absorption enthalpy (−40.687 kJ/mol) than
the other anions. The comparison of the isosteric
absorption enthalpy of [BMIm]-based ILs with that of
the other anions results in the following order: [AC]−
> [L-lactate]− > [BF4]− ࣈ [PF6]− . The isosteric
absorption enthalpy of the ILs initially increased and
then decreased with an increase in alkyl chain length.
According to the experimental data, [BMIm][AC] was
the most appropriate IL, with an isosteric absorption
enthalpy between −40 kJ/mol and −60 kJ/mol.
Furthermore, a high temperature decreased the
isosteric absorption enthalpy and shortened the time
required to reach equilibrium. The effect of
temperature on the adsorption enthalpy was much less
6
C 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.
than that of the anions and the length of cationic alkyl
chains.
Acknowledgements
This research was financially supported by the National
Key Research and Development Program of China
(Grant no. 2016YFE0102500).
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Yi Hu
Xiaoshan Li
Yi Hu is a postgraduate at Huazhong
University of Science and Technology. Her
research interests focus on absorption
enthalpy of CO2 in ionic liquids.
Xiaoshan Li, PhD, a research assistant
at the state key laboratory for coal
combustion in Huazhong University
of Science and Technology, China.
Her research is focused on the flue-gas
treatment with ionic liquid sorbents,
including CO2 capture, SO2 and Hg0
removal.
C 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.
Greenhouse Gas Sci Technol. 0:1–8 (2018); DOI: 10.1002/ghg
7
Y Hu et al.
Original Research Article: Experimental investigation of CO2 absorption enthalpy in conventional imidazolium ionic liquids
Ji Liu
Liqi Zhang
Ji Liu is a postgraduate at HuaZhong
University of Science and Technology.
His main research interests focus on
amine blends for CO2 Capture.
Liqi Zhang is a professor at the state key
laboratory of coal combustion at
Huazhong University of Science and
Technology. His research interests are
carbon capture and storage (oxyfuel
combustion, post combustion), and
emission and control during coal
combustion and gasification.
Liwei Li
Liwei Li is a postgraduate at Huazhong
University of Science and Technology. His
reaserch interests focus on ionic liquids
for NO and SO2 capture, and absorption
mechanisms.
8
C 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.
Greenhouse Gas Sci Technol. 0:1–8 (2018); DOI: 10.1002/ghg