Reversible and robust CO2 capture by equimolar task-specific ionic liquid-superbase mixtures

Article abstract of DOI:10.1039/b927514b

Integrated sorption systems consisting of 1:1 mixtures of an alcohol-functionalized ionic liquid and a superbase were found to be effective for CO₂ capture under atmospheric pressure, eliminating the use of volatile n-alkanols or water. Conversely, by using the current approach, there is no longer a requirement for maintaining scrupulously dry conditions. The effect of ionic liquid structure, choice of superbase, their relative ratios, the sorption temperature, and the reaction time on the absorption and release of CO₂ were investigated. Our results demonstrate that (i) this integrated ionic liquid-superbase system is capable of rapid and reversible capture of nearly one mole of CO₂ per mole of superbase, (ii) the captured CO₂ can be readily released by either mild heating or bubbling with an insert gas (N₂, Ar), and (iii) this novel CO ₂ chemisorption platform can be recycled with minimal loss of activity. This efficient and fully reversible catch-and-release process using non-volatile, task-specific ionic liquids provides an excellent alternative to current CO₂ capture technologies, which are based largely around volatile alkanols or alkylamines. Furthermore, our integrated ionic liquid-superbase system can be used as a novel medium for supported liquid membranes, for which they demonstrate both good selectivity and permeability in model CO₂/N₂ gas separations.

Full text of DOI:10.1039/b927514b

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www.rsc.org/greenchem | Green Chemistry
Reversible and robust CO2 capture by equimolar task-specific ionic
liquid–superbase mixtures
Congmin Wang,a,b Shannon M. Mahurin,b Huimin Luo,c Gary A. Baker,b Haoran Lia and Sheng Dai*b
Received 4th January 2010, Accepted 12th February 2010
First published as an Advance Article on the web 16th March 2010
DOI: 10.1039/b927514b
Integrated sorption systems consisting of 1 : 1 mixtures of an alcohol-functionalized ionic liquid
and a superbase were found to be effective for CO2 capture under atmospheric pressure,
eliminating the use of volatile n-alkanols or water. Conversely, by using the current approach,
there is no longer a requirement for maintaining scrupulously dry conditions. The effect of ionic
liquid structure, choice of superbase, their relative ratios, the sorption temperature, and the
reaction time on the absorption and release of CO2 were investigated. Our results demonstrate
that (i) this integrated ionic liquid–superbase system is capable of rapid and reversible capture of
nearly one mole of CO2 per mole of superbase, (ii) the captured CO2 can be readily released by
either mild heating or bubbling with an insert gas (N2, Ar), and (iii) this novel CO2 chemisorption
platform can be recycled with minimal loss of activity. This efficient and fully reversible
catch-and-release process using non-volatile, task-specific ionic liquids provides an excellent
alternative to current CO2 capture technologies, which are based largely around volatile alkanols
or alkylamines. Furthermore, our integrated ionic liquid–superbase system can be used as a novel
medium for supported liquid membranes, for which they demonstrate both good selectivity and
permeability in model CO2/N2 gas separations.
and reduced hydrogen bonding in alkylcarbonate salts result in a
Introduction
less energy intensive CO2 release.15 Unfortunately, volatilization
Emissions of carbon dioxide (CO2), arguably the most significant
of alcohol, as well as the recombination of CO2 with volatilized
anthropogenic greenhouse gas, have received worldwide atten-
species (i.e., alcohols and/or base), can lead to the loss of
tion because of the possible implications for climate change.
organic solvents and increased operating costs associated with
Consequently, the development of efficient, reversible and
preventing CO2 recombination losses during desorption. Hence,
economic capture technologies for CO2 is becoming increasingly
there remains a strong need to develop alternative technologies
important.1,2 One of the most popular technologies for the
capture of CO2 is chemical absorption by a weakly basic
and approaches for the efficient and reversible capture of CO2
without incurring the loss of volatiles (e.g., alcohols or water).
aqueous solution of monoethanolamine. Despite the numerous
Within the last few years, room temperature ionic liquids
advantages, such as high reactivity, low cost and good absorp-
(RTILs) have attracted significant attention for their potential as
tion capacity, the use of monoethanolamine has some serious
alternative media for the capture of CO2,16–23 due to their unique
inherent drawbacks, including solvent loss, corrosion and high
properties, such as negligible vapor pressure, high thermal
energy demand for regeneration.3-7 An alternative method to
stability, non-flammability and excellent CO2 solubility.24–34
monoethanolamine is the use of chilled ammonia.8 However, the
A large number of experimental and theoretical studies on
very low temperatures necessary to minimize evaporative losses
the solubility of CO2 in RTILs have been carried out, with
of NH3 result in an increased energy usage and unfavorable
a focus on imidazolium-based RTILs, and the results have
operating costs.9 Recently, an innovative CO2 capture system
shown that imidazolium RTILs have remarkable absorption
based on the formation of amidinium or guanidinium alkylcar-
capacities, particularly at relatively high pressures.35–41 In reality,
bonate salts with good reactivity and high absorption capacity
however, the physical solubility of CO2 in RTILs still has
has shown interesting promise.10–14 This CO2 capture system
some way to go before industrial adaptation because of the
consists of an alcohol and an amidine (or guanidine) superbase.
need for high pressures to enhance gas sorption. The Davis
Compared with aqueous solution systems, the low specific heat
group has proposed a novel strategy for the chemisorption of
CO2 that employs an amino-functionalized, task-specific ionic
aDepartment of Chemistry, Zhejiang University, Hangzhou, 310027,
P. R. China
liquid (TSIL), and their results show that 0.5 moles of CO2
can be captured per mole of TSIL over 3 h under ambient
pressure.42 Han and co-workers studied the absorption and
desorption of CO2 by RTILs from renewable materials with
polyethylene glycol. They demonstrated that the molar ratio of
CO2 to neat RTIL could slightly exceed 0.5, and that addition
bChemical Sciences Divisions, Oak Ridge National Laboratory, Oak
Ridge, Tennessee, 37831, USA. E-mail: Dais@ornl.gov; Fax: +1
865-576-5235; Tel: +1 865-576-7307
cNuclear Science and Technology Division, Oak Ridge National
Laboratory, Oak Ridge, Tennessee, 37831, USA
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Table 1 The CO2 absorption of different RTIL–superbase systemsa
Superbase Time/min CO2 absorptionb State
of polyethylene glycol significantly enhanced the rates of CO2
absorption and desorption.43 Furthermore, Noble et al.44 have
reported the capture of CO2 by RTIL–alkanolamine solutions as
tunable solvents and found that an RTIL solution containing 50
mol% monoethanolamine was capable of efficient and reversible
capture of 0.5 moles of CO2 per mole of monoethanolamine
to give an insoluble carbamate precipitate, which helped to
drive the capture reaction. Although these approaches have
made important strides, the maximum absorption capacity is
generally limited to about 0.5 moles of CO2 per mole of RTIL,
and an additional inert solvent is needed to enhance the CO2
diffusion and capture rates due to the formation of highly viscous
gels or solids during CO2 capture by RTIL-based systems.15,45,46
Consequently, alternative TSIL systems able to achieve rapid,
reversible CO2 capture at higher sorption capacities are highly
sought, particularly those that eliminate the need for additional
inert co-solvents.
RTIL
[Im21OH][Tf2N] DBU
[Nip,211OH][Tf2N] DBU
[Im21OH][Tf2N] MTBD
[Nip,211OH][Tf2N] MTBD
[Im21OH][Tf2N] BEMP
30
30
30
30
60
1.04
1.02
1.02
0.98
0.81
0.49
Liquid
Liquid
Liquid
Liquid
Gel
[Im21OH][Tf2N] EtP2(dma) 60
Gel
a 4 mmol RTIL + 4 mmol superbase, 20 ◦C. b Moles of CO2 captured
per mole of superbase.
Herein, we report CO2 sorption behavior and CO2/N2
separation performance using integrated systems consisting of
1 : 1 mixtures of alcohol-containing TSILs and an appropriate
superbase. By incorporating alcohol groups into non-volatile
RTIL cations, the issues associated with the volatilization of
the alcohol can be mitigated. Superbases, neutral organic bases
with proton affinities so high that their protonated conjugate
acids (BH+) cannot be deprotonated by the hydroxide ion,47,48
play a key role as proton acceptors, thereby providing a
thermodynamic driving force for CO2 capture. The effects of
the RTIL and superbase choice, their molar ratios, the sorption
temperature and the reaction time on the absorption/desorption
of CO2 were investigated. It was found that these novel integrated
systems were very effective for the capture of CO2, which could
easily be released via gentle heating or inert gas bubbling to
regenerate the chemisorptive platform for future capture cycles.
The selective membrane separation of CO2 and N2 using this
TSIL–superbase system was also demonstrated in this work.
Fig. 1 The chemical structures of the RTILs and superbases used in
this work, and their designations.
20 times greater than that in neat [Im21OH][Tf2N].37 CO2
underwent a reaction with [Im21OH][Tf2N] and DBU to form
the liquid amidinium alkylcarbonate salt during the absorption
reaction of CO2, which was identified by NMR. Based on
previous reports related to use of neutral alcohols for the same
reaction11,12 and the observed reaction products, we propose the
reaction mechanism shown in Scheme 1, where CO2 is absorbed
by the RTIL–superbase system. For this RTIL–superbase
combination, the molar ratio of CO2 to DBU exceeds 1.0,
which is the theoretical maximum for the chemical absorption
of CO2, indicating that both chemical and physical absorption
mechanisms are active.
Results and discussion
Absorption of CO2
To investigate the gas separation performance and CO2
absorption of the RTIL–superbase system, two RTILs (1-(2-
hydroxyethyl)-3-methylimidazolium
fonyl)imide ([Im21OH][Tf2N]) and 2-hydroxyethyl(dimethyl)-
isopropylammonium bis(trifluoromethylsulfonyl)imide
bis(trifluoromethylsul-
([Nip,211OH][Tf2N])) and four superbases (1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU), 1,3,4,6,7,8-hexahydro-1-methyl-
2H-pyrimido[1,2-a]pyrimidine (MTBD), 2-tert-butylamino-
2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine
(BEMP)
and
1-ethyl-2,2,4,4,4-pentakis(dimethylamino)-
5
5
2l ,4l -catenadi(phosphazene) (EtP2(dma))) were selected and
explored (see Fig. 1 for the structures and their designations).
The DBU superbase was first combined with each of the
RTILs to investigate the effect of different RTILs on the
absorption of CO2 under atmospheric pressure at 20 ◦C
(see Table 1). It was seen that the absorption of CO2 by
both [Im21OH][Tf2N]–DBU and [Nip,211OH][Tf2N]–DBU was
excellent. A CO2 to DBU molar ratio of 1.04 was achieved
when the TSIL [Im21OH][Tf2N] was used, which is at least
Scheme 1
The entries in Table 1 also illustrate the effect of employ-
ing different superbases on the efficiency of CO2 absorption
in the presence of an equivalent of alcohol-bearing TSIL.
Essentially, unit CO2 absorption capacity was achieved when
either a bicyclic amidine (DBU) or a guanidine (MTBD)
superbase was used, whereas CO2 absorption in the presence
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Table 2 Effects of molar ratio and temperature on the capture of CO2
of a phosphazene-type superbase was reduced by roughly 20%
and 50% in the cases of BEMP and EtP2(dma), respectively.
This distinctly diminished performance may be related in part
to the excessive viscosity of the latter RTIL–superbase systems
and their corresponding alkylcarbonate salts. Specifically, while
the [Im21OH][Tf2N]–DBU and [Im21OH][Tf2N]–MTBD systems
remain fluid at all stages of CO2 capture, the corresponding
systems containing BEMP or EtP2(dma) produce gelatinous
phosphazenium alkylcarbonate systems, which clearly hampers
subsequent CO2 absorption.
by [Im21OH][Tf2N]–superbase systemsa
Superbase
Molar ratiob
T/◦C
CO2 absorptionc
State
DBU
DBU
EtP2(dma)
EtP2(dma)
DBU
DBU
DBU
1.2
1.5
1.0
1.5
1.0
1.0
1.0
20
20
20
20
35
50
75
1.11
1.13
0.49
0.75
0.96
0.85
0.68
Liquid
Liquid
Gel
Liquid
Liquid
Liquid
Liquid
a Superbase (4 mmol), 30 min. b Molar ratio of [Im21OH][Tf2N] to
superbase. c Moles of CO2 per mole of superbase.
An important feature of CO2 absorption by RTIL–superbase
systems is that the absorption is mildly exothermic, lending
toward a favorably rapid absorption rate. Fig. 2 shows a
typical CO2 absorption by the [Im21OH][Tf2N]–DBU system as
a function of time. From Fig. 2, it is clear that for this system,
absorption is nearly complete within the first 10 min of CO2
exposure. Explicitly, the CO2 absorption increases initially to
nearly 80% uptake within 5 min, with almost no additional
uptake evident beyond 10 min. This rapid CO2 sorption, in the
range of a handful of minutes as opposed to tens of minutes
to hours, is a distinct advantage of the alcoholic RTIL–superbase
platform.
Release of CO2
The stabilities of the CO2-captured RTIL–superbase systems
were further studied by thermogravimetric analysis (TGA),
which is an effective method for evaluating CO2 release
capability.49,50 Fig. 3 shows the scanning TGA results for various
RTIL–superbase–CO2 systems with a 10 ◦C min-1 temperature
ramping rate to 800 ◦C. The decomposition of the RTIL–
superbase–CO2 systems was slow at low temperature, with
no increase in the decomposition rate being observed until
the heating temperature reached approximately 50 ◦C. After
reaching a temperature of 120 ◦C, the [Im21OH][Tf2N]–DBU
system lost approximately 12 wt%, indicating that the release
of CO2 was almost complete. Furthermore, no obvious weight
loss was observed from 200 to 350 ◦C due to the high stability
of the RTILs. Indeed, the decomposition temperature (Tdcp) of
a number of◦alcohol-containing RTILs has been reported to be
around 400 C.51 In stark contrast, the n-hexanol–DBU–CO2
◦
system started to decompose at 50 C with a fast reaction rate
and ◦lost approximately 44 wt% when the temperature reached
120 C. The volatilized n-hexanol and DBU would react with
CO2 to form highly viscous n-hexanol–DBU–CO2, which would
result in an increase in the energy required for the release of the
CO2 and so increase operating costs. Clearly, these results show
that loss of the volatile alcohol component is avoided because
of the use of TSILs during the release of CO2.
Fig. 2 A plot of percentage CO2 uptake by [Im21OH][Tf2N]–DBU as a
function of time at 20 ◦C. The points are the actual data and the curve
shown is the fit: %CO2 = 103(1 - e-0.294t) (N = 21, r2 = 0.992).
Table 2 shows the effect of the RTIL–superbase molar ratio on
the absorption of CO2, which was found to be quite strong for
some systems. For the [Im21OH][Tf2N]–DBU system, the CO2
absorption increased only modestly from 1.04 to 1.11 moles
of CO2 per mole of DBU as the RTIL–superbase molar ratio
increased from 1.0 to 1.2, with little increase thereafter. However,
the absorption of CO2 relative to superbase increased dramati-
cally from 0.49 to 0.75 when the molar ratio of [Im21OH][Tf2N]
to EtP2(dma) went from 1.0 to 1.5. This increase most likely
stems from the increased diffusion/absorption rates for CO2
due to the reduced viscosity when excess RTIL is present. In this
case, these conditions help the system remain liquid as the CO2 is
captured. The data in Table 2 also show the pronounced effect of
temperature on the absorption of CO2 for the [Im21OH][Tf2N]–
DBU system. The CO2 absorption was found to decrease steadily
with increasing temperature from 20 to 75 ◦C. Therefore, the
results show that the captured CO2 can be stripped out simply
by heating.
Fig. 3 Scanning TGA profiles for selected RTIL–superbase–CO2
systems (N2 atmosphere, 10 ◦C min-1 ramp rate).
The captured CO2 was easy to strip out by heating or by
bubbling N2 through the solution. Fig. 4 shows the effect of
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Table 3 The selective separation of CO2 and N2 by [Im21OH][Tf2N]–
superbase membranesa
time on the release of CO2 under heating for the [Im21OH][Tf2N]–
DBU system. The results show that the release of CO2 proceeded
rapidly at a temperature of 120 ◦C. For example, the CO2
release was essentially complete within 15 min at 120 ◦C.
Fig. 4 also reveals that CO2 absorption into and release from
the [Im21OH][Tf2N]–DBU solution can be repeatedly recycled
with only a slight loss of absorption capability, indicating that
the process of CO2 absorption by RTIL–superbase systems is
reversible.
CO2 permeance/
mol m-2 s-1 Pa-1
Superbase
T/◦C
CO2/N2 selectivity
DBU
DBU
MTBD
20
90
85
1.20 ¥ 10-11
1.43 ¥ 10-9
1.89 ¥ 10-9
—
7.1
8.0
a The feed pressure was 35 kPa.
indicated in Table 3, both the [Im21OH][Tf2N]–DBU system and
the[Im21OH][Tf2N]–MTBD system exhibited good performance
for the separation of CO2 and N2 at elevated temperatures.
For example, the CO2 permeance was measured to be 1.43 ¥
10-9 mol m-2 s-1 Pa-1 at 90 ◦C, with a CO2/N2 selectivity of 7.1.
This permeance value is comparable to that of [Im21][Tf2N], a
widely used RTIL for SILM-based CO2 separation. An even
better performance, with a CO2 permeance of 1.89 ¥ 10-9 mol
m-2 s-1 Pa-1 and a CO2/N2 selectivity of 8.0, was achieved when
a [Im21OH][Tf2N]–MTBD SILM was used. In contrast, the CO2
permeance for the [Im21OH][Tf2N]–DBU SILM was only 1.20 ¥
10-11 mol m-2 s-1 Pa-1 at room temperature because the RTIL–
superbase strongly absorbs CO2 but does not easily release it, as
previously discussed.
Fig. 4 Three consecutive cycles of CO2 absorption (cyan, 30 ◦C) and
release (pink, 120 ◦C) by the [Im21OH][Tf2N]–DBU system. In this
representation, 100% CO2 uptake denotes 1.0 mole of CO2 per mole
of [Im21OH][Tf2N].
Experimental
Materials
Selective separation of CO2 and N2
MTBD, DBU, BEMP, EtP2(dma) and n-hexanol were purchased
from Aldrich. All chemicals were obtained in the highest purity
grade possible and used as received, unless otherwise stated. The
RTILs [Im21OH][Tf2N] and [Nip,211OH][Tf2N] were synthesized
according to methods detailed in the literature.51–54
The selective separation of CO2 and N2 in these RTIL–superbase
systems was investigated using a non-steady-state permeation
method. Each RTIL–superbase solution was loaded onto a
porous polymer support to form a supported ionic liquid
membrane (SILM) for the measurement of permeance and
selectivity. The feed pressure was typically 35 kPa, the permeate
pressure was initially about 40 mTorr. By measuring the pressure
of the permeate side as a function of time, we could calculate the
permeance, Qi, of the SILM for each gas by using the following
equation:
Absorption of CO2
In a typical absorption of CO2, CO2 at atmospheric pressure was
bubbled through about 2.0 g of RTIL–superbase solution in a
glass container with an inner diameter of 10 mm at a flow rate
of about 30 mL min-1. The glass container was partly immersed
in an oil bath at the desired temperature. The amount of CO2
absorbed was determined at regular intervals by an electronic
balance with an accuracy of 0.1 mg. The RTIL–superbase
system was regenerated by heating or bubbling N2 through the
solution.
V
dP"
Qi =
×
(1)
RTA(P'− P") dt
where V is the permeate volume, R is the Boltzmann gas
constant, T is the absolute temperature, A is the membrane
area, P¢ is the upstream pressure, P¢¢ is the downstream pressure
and dP¢¢/dt is the rate of gas pressure increase on the permeate
side. The selectivity was obtained by calculating the ratio of the
CO2 permeance over N2 permeance.
Gas permeation experiments
Gas permeation experiments were performed using a cus-
tomised test apparatus.55 The RTIL–superbase membrane was
prepared by immersing a 47 mm Supor-100 polyethersulfone
membrane (Pall Corp.) in 1 mL of RTIL–superbase solution.
The membrane was then placed in a vacuum desiccator for
12 h for de-gassing. After removal from the desiccator, the
membrane was carefully wiped with filter paper to eliminate
excess RTIL–superbase solution. The permeation properties of
the RTIL–superbase membrane was then analyzed in a stainless
steel permeation cell that consisted of feed and permeate
chambers separated by the membrane. Single gas permeance
Qi
ai =
(2)
Qj
Here, Qi and Qj are the permeances of CO2 and N2, respec-
tively.
In conventional industrial processes of CO2 capture from flue
gas streams, significant energy consumption is associated with
the cooling of the flue gas to ambient temperature prior to
the release of CO2.50 Therefore, an ideal CO2 capture system
should be capable of operating at elevated temperature. As
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measurements were separately acquired for CO2 and N2 at 90 ◦C.
Both the feed and permeate sides were initially evacuated to
approximately 40 mTorr, followed by the introduction of the gas
to the feed side at a pressure of 35 kPa. The pressure of the
permeate side was recorded as a function of time for 30 min.
19 J. E. Bara, S. Lessmann, C. J. Gabriel, E. S. Hatakeyama, R. D.
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Conclusions
In summary, TSIL–superbase systems are highly efficient for the
capture of CO2, eliminating the use of volatile alcohols or water.
The capture of CO2 by [Im21OH][Tf2N]–DBU occurs rapidly,
and the CO2 capture capacity is more than 1 mole per mole of
superbase, which is superior to those captured by traditional
RTILs. The captured CO2 is easily released and the system
recyclable with only a slight loss of activity. This efficient and
reversible process by the combination of TSILs and superbases
provides a potential method for the capture of CO2 in industry.
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Acknowledgements
This work was supported by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences,
U.S. Department of Energy. The authors thank Dr Je Seung
Lee for his kind assistance with CO2 sorption measurements.
C. M. W. also gratefully acknowledges the support of the
National Natural Science Foundation of China (nos. 20704035
and 20773109).
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