four [N2224]-based AAILs that utilize the four simplest and
most readily available amino acids as the counter anions are of
low cost and viscosity, and are thought to meet the criteria for
engineering purposes that concern mass and heat transfer.
The global warming due to the increased atmospheric CO2
concentration results primarily from the excessive consumption
of fossil fuels and is becoming an important environmental
for CO
2
absorption (such as [N2224][b-Ala], [N2221][L-Ala],
[N2222][L-Ala], [N2222][b-Ala]) have an absorption capacity
(around 0.325 mole fraction) similar to that of [N2224][L-Ala].
In fact, since all the AAILs in this work have an exposed
amino group, they absorb CO chemically via the reaction of
2
CO with the amino group to form ammonium carbamate so
2
that their absorption capacities are close to that of DEA
2
3–25
issue today.
Carbon sequestration, which captures CO
2
(0.33 mole fraction of CO
2
in DEA at 40 1C). The formation
1
3
from large point sources such as fuel gas, natural gas, water
gas and waste gas from electrical power plants and stores it in
geological formations, has been proposed as a solution to this
problem. However, aqueous amines that are currently used
of ammonium carbamate has been evidenced from the
2
NMR analysis both in our laboratory and by the research
C
0
2
7
group of Davis. In addition, it is observed that the viscosities
of AAILs are low and the liquids can be stirred easily at the
beginning of the absorption. However, the transparent liquid
of [N2224][L-Ala] becomes a cloudy paste at the end of absorption,
2
most frequently in industry for large scale CO capture suffer
from many technical difficulties, i.e., the uptake of water into
the gas stream, high energy consumption during the regeneration
of the absorbing solution, and the volatile loss of amine
sequestering agent. As a result, innovative task-specific amino-
and the CO -saturated [N2224][L-Ala] sample collected for
2
analysis is found to have a viscosity of 1543 mPa s at 25 1C,
additionally indicating the formation of ammonium carbamate
species.
2
6,27
16
terminated
or amino acid-based ionic liquids were
developed and proposed to be stable and non-volatile CO
2
Even though DEA and AAILs have similar capacity in
2
8
absorbents for the replacement of traditional aqueous
amines. However, all these new ILs reported in the literature
still have high viscosity, which disfavors the heat and mass
transfer during the applications. Therefore, efficient separation
absorbing CO
organic amines are quite different. The results in Fig. 4
demonstrate that the equilibrium of CO absorption in
2 2
, the absorption rates of CO in the AAILs and
2
[N2224][L-Ala] can be reached within 30 min, half the time
of CO
2
using low viscosity [TAA][amino acid]s is not only a
required in the cases of [N2222][L-Ala] and [N2222][b-Ala]
potential addition to economically viable sequestration efforts,
but also a good system for the verification of enhanced CO2
mass transfer benefiting from low viscosity.
(E60 min). As for the absorption of CO in DEA, MDEA
and aqueous MDEA solutions, the times required for reaching
2
phase equilibrium are usually longer than several hours. In
The absorption of CO2 (99.99%) into [N2224][L-Ala],
particular, [N2224][L-Ala] is observed to absorb CO at a rate of
2
[
N
2224][b-Ala], [N2221][L-Ala], [N2222][L-Ala], [N2222][b-Ala],
8 to 10 min to reach 90% of its saturated absorption capacity,
vastly faster than that observed for other asymmetric
[TAA][amino acid]s, symmetric [TAA][amino acid]s and
organic amines (DEA, MDEA, aqueous MDEA). Analysis
shows that this is primarily due to the effect of viscosity on the
diethanolamine (DEA), methyldiethanolamine (MDEA), and
aqueous MDEA (51.28% w/w) was carried out at 40 1C and
ambient pressure, according to the standard procedure
1
6
reported in the literature. The absorbed CO
2
is almost
completely extruded from the CO -absorbed [N2224][L-Ala]
2
mass transfer of CO
2
.
upon heating at 60 1C for 4 h under vacuum (about 0.1 KPa).
The CO physical absorption mechanism was put
2
If the CO -saturated AAIL is heated at 110 1C at atmospheric
forward by Brennecke and co-workers. They disclosed that
CO2 at high pressure exhibits intrinsic solubility in the
‘‘conventional’’ ionic liquid 1-butyl-3-methylimidazolium
2
pressure for an hour, more than 90% of the absorbed
CO2 can be released, which is similar to the case of the
DEA–CO2 system. The recovered [N2224][L-Ala] has been
2
9,30
hexafluorophosphate.
liquids is dependent on their cations, anions, and substituents,
and the anions play a major role. The interaction between CO
The solubility of CO2 in ionic
repeatedly recycled for CO
in Fig. 4, with no apparent losses of absorption rate and
absorption capacity. The mole uptake of CO per mole
2224][L-Ala] is found to approach 0.5, equivalent to
.326 mole fraction of CO in the IL. All other AAILs tested
2
uptake (four cycles) as presented
2
2
and imidazolium-type ionic liquid, attributed to the activity of
H-2 in the imidazolium ring, accounts also for its higher
[
N
3
1,32
0
2
solubility of CO
groups present in the AAILs can increase dramatically the
CO solubility up to 0.33 mole fraction of CO , consistent with
2
.
However, in our work, the amino
2
2
2
7
the chemical mechanism proposed by Bates et al. that one
mole of 1-butyl-2-propylamineimidazole tetrafluoroborate
(
[(NH )pmim][BF ]) can absorb half of one mole of CO .
2 4 2
Therefore, the reaction between [TAA][amino acid] and CO2
can be represented by Scheme 1, and the reaction rate (r) can
be generally written as,
r = ÀdC/dt = KC
(1)
where K is the apparent absorption reaction rate constant and
À3
C is the concentration of CO in mol m . Scheme 1 represents
2
a pseudo-first order reaction for CO , since the concentrations
2
Fig. 4 Cycles of CO
2
absorption into [N2224][L-Ala] at 40 1C.
of [TAA][amino acid]s can be regarded as a constant at the
2
388 | New J. Chem., 2009, 33, 2385–2390
This journal is ꢀc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009