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New liquid absorbents for the removal of CO2 from gas mixtures†
Dulce M. Mu~
noz,*a Ana Filipa Portugal,b Angel E. Lozano,a Jose G. de la Campaa and Javier de Abajoa
Downloaded on 21 March 2011
Published on 14 May 2009 on http://pubs.rsc.org | doi:10.1039/B901307E
Received 21st January 2009, Accepted 1st May 2009
First published as an Advance Article on the web 14th May 2009
DOI: 10.1039/b901307e
New liquid absorbents (LA) consisting of water solutions of CO2-complexing agents have been
developed and tested in an experimental lab-scale to be used in CO2 separation from gas mixtures.
The results here presented concern the preparation of these amino acid based new liquid absorbents and
the studies carried out to determine their performance as CO2 scrubbers. The novel systems show good
performance in terms of sorption and stability compared to the standard alkanolamines. Moreover, the
experimental results have shown that among the studied carriers, arginine and ornithine are the natural
amino acids with greater affinity towards CO2 and that some of the here synthesized amino acids show
outstanding absorption capacities, superior to MEA or any other amino acid tested. The developed
materials have a direct use as mobile carrier membranes for facilitated transport and present a great
potential for application in gas separation processes. Computational (DFT) and spectroscopic (1H and
13
C-NMR) methods have been applied to make clear the mechanism of carbamate formation from
the amine group of amino acids and CO2.
Introduction
Capture of carbon dioxide (CO2) by various techniques has
received great interest in recent years.1,2 The main CO2 separation technologies from gas mixtures are currently: adsorption,3,4
absorption,5,6 cryogenic methods7,8 and separation with
membranes.9,10 The first three methods are traditional and
present some limitations such as elevated energy consumption
and therefore high cost.11
Nevertheless, there have recently appeared some novel technologies based on membrane absorbents contactors12,13 which
a
Instituto de Ciencia y Tecnologı́a de Polı́meros, C.S.I.C, Juan de la
Cierva 3, E28006 Madrid, Spain. E-mail: [email protected]; Fax:
+34-915644853; Tel: +34-915622900
b
LEPAE, Departamento de Engenharia Quı́mica, Faculdade de
Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Porto,
Portugal
† Electronic supplementary information (ESI) available: Supplementary
Scheme S1, Fig. S1, Fig. S2 and Table S1. See DOI: 10.1039/b901307e
allow to selectively remove CO2 from gas streams. Membrane
contactors consist of special hollow fiber modules where the gas
flows on one side of the membrane and a liquid absorbent (LA)
flows on the other without phase dispersion, thus, offering
numerous advantages such as operational flexibility, high volumetric mass transfer rates and easy linear scale up. The
membrane itself does not perform the separation, but the
components in the solution.
The commonly used absorbents for CO2 removal from gas
mixtures in industry are aqueous solutions of alkanolamines such
as monoethanolamine (MEA) and diethanolamine (DEA). The
gas is contacted with the amine solution, which preferentially
absorbs the CO2. The amine solution is then heated and almost
pure CO2 is released from the stripper. However, the use of
amines has some disadvantages,14 for example MEA and DEA
are not very stable and under the process conditions some of
these amines suffer decomposition, what results in a lowering of
the scrubbing efficiency, increase of viscosity and excessive
foaming. Also, during its handling and the scrubbing process,
Broader context
Environmental concerns, such as global climate change, are now one of the most important and challenging environmental issues
facing the world community and have motivated intensive research on CO2 capture and sequestration. Carbon dioxide is currently
responsible for over 60% of the enhanced greenhouse effect. A wide range of technologies currently exist for separation and capture
of CO2 from gas streams. Absorption with amine-based absorbents (monoethanolamine, MEA, or diethanolamine, DEA) is the
most common technology for CO2 removal today. It is a process with considerable inherent problems. The processes require large
investment costs and high energy consumption, and the absorbents in use today are not stable and form degradation products that
need to be handled. Novel technologies based on membrane absorbents contactors which allow to selectively remove CO2 from gas
streams have recently gained a major interest. Membrane contactors consist of special hollow fiber modules where the gas flows on
one side of the membrane and a liquid absorbent flows on the other without phase dispersion, thus offering numerous advantages
such as operational flexibility, high volumetric mass transfer rates and easy linear scale up. The new developed liquid absorbents
based on amino acids here presented represent an interesting alternative to conventional methods and fulfil the requirements of cyclic
capacity and high reaction/absorption rate for CO2, as well as high chemical stability, low vapour pressure and bio-compatibility.
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they can be released to the atmosphere as contaminants. Besides,
the processes are very energy-intensive and too expensive for
most applications today. Consequently, there is a need to find
a process which fits the market demands, what involves finding
an absorbent that not only presents low vapour pressure, high
thermal stability and high absorption rates, but which is not
susceptible to degrade under industrial processes’ conditions.
Also, CO2 is continuously released in hermetically closed
travel crafts (submarines, spacecraft, etc.) by its occupants. The
technology nowadays used in these systems15–17 is very similar to
the one used in anaesthesia for removing carbon dioxide. This
gas is continuously generated by the patient and is traditionally
removed from the anaesthetic systems using soda lime packed in
a canister.18 This procedure presents some problems, namely: (i)
soda lime reacts with some anaesthetic additives, the halogenated
anaesthetics, producing by-products that are potentially harmful
for the patients; (ii) the canisters are hospital solid waste,
dangerous and expensive to treat and (iii) the soda lime–CO2
reaction produces water (which condenses in the tubing) and heat
(contributing to the rising of the system’s temperature above the
desirable patient-comfort values). Presently, there is no on-line
system available for continuous removal of carbon dioxide and
nitrogen from anaesthetic circuits, therefore, such technology
would be a significant step for the development of anaesthesia.
Membrane contactors present also an alternative to this problem
and can serve to remove carbon dioxide from such anaesthetics
closed systems, where energy-efficiency, bio-compatibility and
environmental safety are essential. The absorbents have to be
highly selective, posses a high absorption capacity, present as low
cost as possible and not be harmful to the environment.
Standard absorption technologies for trapping of CO2 and
acid gases in general using primary amines and amino alcohols as
efficient sorption media are limited for this purpose because they
do not fit the requirements of regeneration, bio-compatibility
and sterilization temperature imposed by the intended applications. Thus, a more complex system has to be developed. Ionic
liquids (low-temperature molten salts) have been shown to be
good CO2 scrubbing agents; they can facilitate the sequestration
of gases without loss of the capture agent or solvent into the gas
stream. The lack of vapour pressure due to the Coulombic
attraction between the ions of these liquids makes them useful for
gas processing.19,20 Moreover, some amino acid salts solutions
are considered a promising alternative to the conventional
absorbents mentioned above, where a positive balance between
water solubility, intrinsic basicity, thermal stability, gas recovery,
selectivity, and availability (processing and price) is sought. Over
recent years, only a few examples using amino acids21–23 as CO2
scrubbers have appeared. In this regard, absorption liquids based
on mixtures of amino acids and amino acid salts, CORAL,24
have been developed by TNO for CO2 recovery. Thus, due to the
growing concern on this area, there is an interest to extend this
application and to find alternative absorbents, particularly based
on amino acids.25
The aim of this work has been to find an amine replacement
with an improved CO2 absorption capacity. In our case, an
aqueous solution of an amino acid salt will act as the carrier
solution in the membrane contactor system. Herein, we report
a preliminary study on the performance of new liquid absorbents
based on natural and new synthetic amino acids of diverse
884 | Energy Environ. Sci., 2009, 2, 883–891
structures (Table 1) and the results of their performances, in an
experimental lab-scale, as CO2 absorbents.
Also, we describe here the full synthesis of these new nonnatural amino acids through various synthetic methods.
Experimental
Materials
Amino acids and other chemicals used in the preparation of
absorbents were of analytical grade, purchased form Aldrich
Chemical Co. and Lancaster, and were used without further
purification.
Synthesis of amino acid absorbents
The novel amino acids were prepared following various methods
of synthesis (a–f, Scheme S1).†
2-(Pyrimidin-2-ylamino)acetic acid (1a), 3-(pyrimidin-2-ylamino)propanoic acid (1b) and 4-(pyrimidin-2-ylamino)butanoic acid
(1c). 2-Chloropyrimidine (1.5 eq), K2CO3 (1 eq) and the corresponding a,b or g-amino acid (1 eq) were stirred in a 0.1 M
solution of EtOH at 90 C for 1 day. The solution was let to stand
and a solid appeared, which was filtered off and thoroughly
washed with acetone. The filtrate was concentrated to give a solid
that was washed with EtOAc, filtered and dried to obtain: 1a
(9.60 g, 0.06 mol, 67%); 1b (31.6 g, 0.20 mol, 98%); 1c (14 g,
0.08 mol, 77%).
Spectroscopic data for the three amino acids are as follows:
1a:1H-NMR (D2O, 300 MHz, ppm): 8.05 (d, 2H, J ¼ 4.9 Hz,
2H-6); 6.50 (t, 1H, J ¼ 4.9 Hz, H-7); 3.66 (s, 2H, 2H-2).
13
C-NMR (D2O, 75 MHz, ppm): 178.4 (C-1); 161.3 (C-4),
158.5 (C-7), 111.0 (C-6), 45.0 (C-2).
MS (ES): 154.1 (MH+, 100).
1b:1H-NMR (D2O, 300 MHz, ppm): 8.18 (d, 2H, J ¼ 5.0 Hz,
2H-7); 6.62 (t, 1H, J ¼ 5.0 Hz, H-8); 3.44 (t, 2H, J ¼ 6.9 Hz,
2H-3), 2.43 (t, 2H, J ¼ 6.9 Hz, 2H-2).
13
C-NMR (D2O, 75 MHz, ppm): 181.2 (C-1); 161.2 (C-5),
158.6 (C-7), 110.9 (C-6), 38.5 (C-2), 37.1 (C-3).
1c:1H-NMR (D2O, 300 MHz, ppm): 8.22 (d, 2H, J ¼ 5.0 Hz,
2H-8); 6.64 (t, 1H, J ¼ 5.0 Hz, H-9); 3.26 (t, 2H, J ¼ 7.0 Hz,
2H-4); 2.21 (t, 2H, J ¼ 7.6 Hz, 2H-2); 1.80 (m, 2H, 2H-3).
13
C-NMR (D2O, 75 MHz, ppm): 183.3 (C-1); 161.5 (C-6),
158.7 (C-8), 110.7 (C-9), 41.1 (C-4), 35.2 (C-2), 25.7 (C-3).
MS (ES): 182.1 (MH+, 100).
Piperazine-2-carboxylic acid (2). Pyrazine-2-carboxylic acid
(20 g, 0.16 mol) was dissolved in water (250 ml) and a 1 M KOH
solution (250 ml) was added. The resulting mixture was heated at
90 C and Al–Ni alloy (77.2 g) was added, with caution, in small
portions and heated at this temperature for 24 h. The reaction
mixture was then cooled to room temperature and filtered
through Celite. The obtained filtrate was evaporated to dryness
to give a brown solid which was crushed and washed several
times with methanol to give a beige solid (7.13 g, 0.05 mol, 34%).
1
H-NMR (D2O, 300 MHz, ppm): 3.84 (dd, 1H, J ¼ 3.8 and
11.1 Hz, 1H-2); 3.70 (dd, 1H, J ¼ 3.8 and 13.8 Hz, 1H-3), 3.57–
3.43 (m, 2H), 3.31–3.13 (m, 3H).
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Table 1 Chemical structures of the amino acids object of study
Arginine
Taurine
Glycine
BAPA
GABA
Proline
Ornithine hydrochloride
Histidine
Serine
Threonine
1a
1b
1c
2
3
4
5
6
13
C-NMR (D2O, 75 MHz, ppm): 171.0 (C-1); 55.7 (C-2), 43.9,
40.9 and 40.8 (C-3, C-5 and C-6).
(1-Piperazino)acetic acid (3). 2-Bromoacetic acid (10 g, 0.07
mol) was dissolved in water (50 ml) and piperazine (12.5 g, 0.14
mol) was added. The colourless solution was stirred at room
temperature for 1 day and then concentrated under vacuum to
give a slightly yellow solid that was washed with cold methanol to
give a white solid (9.07 g, 0.06 mol, 87%).
1
H-NMR (D2O, 300 MHz, ppm): 2.96 (s, 2H, 2H-2); 2.77
(t, 4H, J ¼ 5.1 Hz, 4H-4); 2.47 (m, 4H, 4H-5).
13
C-NMR (D2O, 75 MHz, ppm): 177.8 (C-1); 62.1 (C-2), 52.8
(C-4), 44.1 (C-5).
MS (ES): 145.0 (MH+, 100).
MS (CI): 157.1 (32), 144.1 (M+, 6), 99.1 (45), 56.1 (100).
(1-Piperazino)-2-propionic acid (4). 2-Bromopropionic acid
(35.2 g, 0.23 mol) was dissolved in propanol-2 (200 ml) and
piperazine (20 g, 0.23 mol) was added as 1 M piperazine in
propanol-2 with vigorous stirring. The precipitate was filtered
off, washed with propanol-2 three times, and dried in vacuum to
give a white solid (31.6 g, 0.20 mol, 87%).
1
H-NMR (DMSO, 300 MHz, ppm): 7.50 (brs, 1H, CO2H);
3.29 (q, 1H, J ¼ 7.1 Hz, 1H-2); 3.05 (m, 2H, 2H-5), 2.79 (m, 2H,
2H-4), 1.17 (d, 3H, J ¼ 7.1 Hz, CH3).
This journal is ª The Royal Society of Chemistry 2009
13
C-NMR (DMSO, 75 MHz, ppm): 173.3 (C-1); 61.2 (C-2),
45.3, 43.2 and 42.3 (C-4 and C-5), 14.1 (CH3).
MS (ES): 231.1 (25), 159.1 (MH+, 100).
MS (CI): 185.1 (36), 158.1 (M+, 6), 113.1 (100), 56.1 (48).
[4-(2-Aminoethyl)piperazin-1-yl]acetic acid (5). 2-Bromoacetic
acid (10 g, 0.07 mol, 1 eq) and 1-(2-aminoethyl)piperazine (14 ml,
0.11 mol, 1.5 eq) were stirred in MeOH (150 ml) in the presence
of Na2CO3 (15.8 g, 0.15 mol, 2.1 eq). The solution was stirred
under reflux for 24 h and the precipitate filtered and washed with
MeOH. The filtered solution was concentrated under vacuum to
give a brown gum which, after stirring in CHCl3, gave a slightly
yellow solid (7.29 g, 0.04 mol, 55%).
1
H-NMR (D2O, 300 MHz, ppm): 2.99 (t, 1H, J ¼ 6.9 Hz); 2.84
(s, 2H, 2H-2); 2.72 (t, 1H, J ¼ 7.2 Hz); 2.39–2.30 (m, 10H).
13
C-NMR (D2O, 75 MHz, ppm): 177.5 (C-1), 61.6 (C-2), 57.4
(C-11), 51.9 (C-5), 38.0 (C-3), 36.7 (C-2).
MS (ES): 188.1 (MH+, 100).
3-(2,5-Dimethylpiperazin-1-yl)propanoic acid (6). Methyl
acrylate (10 g, 0.12 mol, 10.5 ml, 1 eq), LiClO4 (12.8 g, 0.12 mol, 1
eq) and 2,5-dimethylpiperazine (20.5 g, 0.18 mol, 1.5 eq) were
placed in a round-bottomed flask and the mixture was stirred
under nitrogen atmosphere for 2 h. The orange solution was
stirred overnight and the solid formed was filtered and washed
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with CH2Cl2. The solid was purified by column chromatography
(acetone : MeOH 50%) to give the desired intermediate which
was hydrolyzed by stirring in a solution of NaOH 1 N (230 ml,
0.23 mol, 2.5 eq) in MeOH (180 ml) for 3 h. Recrystallization
from acetone gave 3-(2,5-dimethylpiperazin-1-yl)propanoic acid
as a pale yellow solid (7.29 g, 0.04 mol, 33% over 2 steps).
1
H-NMR (D2O, 300 MHz, ppm): 3.84–346 (m, 5H, 1H-8, 1H6, 1H-9 and 2H-3); 3.26 (dd, 1H, J ¼ 6.6 and 20 Hz, 1H-5); 3.24
(dd, 1H, J ¼ 12.9 and 20 Hz, 1H-5); 3.07 (dt, 1H, J ¼ 12.9 and
1.7 Hz); 1.36 (d, 3H, J ¼ 6.3 Hz, 3H-10 or 3H-11); 1.26 (d, 3H,
J ¼ 6.3 Hz, 3H-10 or 3H-11).
13
C-NMR (D2O, 75 MHz, ppm): 173.9 (C-1), 56.7 (C-5), 53.3
(C-9), 48.9 (C-6), 48.7 (C-8), 45.6 (C-3), 28.9 (C-2), 15.1 (C-10 or
C-11), 13.7 (C-10 or C-11).
MS (ES): 259.1 (100), 187.1 (MH+, 82).
Preparation of liquid absorbents (LA)
The liquid absorbents were prepared in 1 M concentration dissolving the natural and synthetic amino acids in deionized water
and using a 1 : 1 alkali salt : amino acid ratio. Potassium
hydroxide (KOH) was used as an alkali. Aqueous MEA solutions used for comparison were prepared by dissolving pure
amine in deionized water.
Absorption experiments
Absorption experiments were performed at 20 C in a set-up
basically composed of two connected tanks with well defined
volumes (one tank available only for the gas and the other one
containing the absorbent solution). Both tanks were submerged
in a thermostatic bath (Fig. S1).† Initially, vacuum was applied
to the system in order to remove all the gases present in the
solution. The pressure recorded at this initial moment is the
vapour pressure of the solution (Pvapor). After that, the valve
between the two tanks was closed and the gas tank (tank A,
Fig. S1)† was filled with carbon dioxide up to a certain pressure
(PA). The experiment was started by opening the valve that
separates the two tanks and leaving the system to reach the
equilibrium. During the whole experiment, the absorbent solution was vigorously stirred with a magnetic bar. Finally, when
the pressure stabilized, the value was recorded (Pfinal). The
amount of gas absorbed (nabs) was calculated applying the ideal
gases law to the system, as follows:
nA ¼
nfinal ¼
PA VA
RT
Pfinal Pvapor ðVA þ VB Vsolution Þ
RT
nabs ¼ nA nfinal
Theoretical calculations
Semiempirical calculations were performed using the original
parameters of the program AM1 based on the restricted Hartree–
Fock (RHF) method, included in MOPAC version 6.0,26 using as
graphics interface and data analysis the Cerius2 program.27
These semiempirical methods are commonly accepted to allow
a good description of the lone-pair/lone-pair repulsion in several
compounds. The MOPAC program ran on a Silicon Graphics
Origin 300 workstation.
Density functional theory (DFT) calculations were carried
out with the Gaussian 03 package28 using as data and graphical
interface the Cerius2 program. Through the Z-matrix input data
from an AM1 calculation, the geometry and the total electronic
energy were calculated by choosing the RHF method and the
B3LYP/6-31G* basis set. Geometries were optimized in internal
coordinates. For AM1 calculations, the optimization was
stopped when Herbert or Peter tests were satisfied in the
Broyden–Fletcher–Goldfarb–Shanno (BFGS) method29 after
placing the CO2 molecule. The PRECISE option was applied
during the optimization process with the gradient norm set to
0.01. The calculations were carried out with full geometry
optimization (bond lengths, bond angles and dihedral angles)
without any assumption of symmetry. The binding energies
have been corrected for the basis set superposition error (BSSE)
by using the Counterpoise method.30 This method estimates the
BSSE as the difference between the energies of the isolated
monomers and the energies of the monomers with the total
basis of the aggregate.
Table 2 pKa values of the natural amino acids object of study31
(1.1)
Amino acid
(1.2)
(1.3)
where nA and nfinal are the number of moles of carbon dioxide in
the gas phase at the beginning and the end of the experiment,
respectively; VA, VB and Vsolution are the volumes of tanks A and
B, and the volume of the absorbent solution, respectively; R is the
ideal gas constant and T is the absolute temperature.
886 | Energy Environ. Sci., 2009, 2, 883–891
Absorption was also monitored by spectroscopic methods.
C-NMR spectra were recorded at 20 C on a Varian XL
spectrometer at 75 MHz. D2O was used as solvent. The assays
were performed in a glass tube (1 cm 10 cm) having
a porous plate for a better gas diffusion and containing a 1 M
solution of the absorbents in D2O in both the absence and
presence of equimolecular quantities of base (KOH). Tubes
containing the absorbent solution were flushed with CO2 gas at
2 L min1 flow and the spectra recorded at different times of
gas exposure, to compare with the corresponding CO2 free
samples.
13
pKa (–NH3+)
Non polar, aliphatic R groups
Glycine
9.60
Proline
10.96
BAPA
10.15
GABA
10.43
Taurine
8.74
Polar, uncharged R groups
Serine
9.15
Threonine
9.62
Positively charged R groups
Ornithine
8.95
Histidine
9.17
Arginine
9.04
pKaR (R group)
—
—
—
—
—
—
—
10.53
6.20
12.48
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Results and discussion
The selection of the amino acids object of study was based on
their chemical and physical properties (aqueous solubility,
basicity and stability) and, thus, natural amino acids bearing
a variety of functional groups in their structure (aliphatic, polar
and charged groups) were studied. An important factor to be also
considered was the basic character of the amino acid; so that the
pKa values of the amino groups present in the amino acids
(Table 2) were taken into account.
In addition, absorbents based on hindered amines or cycloalkylamines, as well as amine blends, have been used for the
recovery of acidic gases. In the present study, we decided to
prepare a series of synthetic amino acids by chemical modification of natural amino acids and piperazine derivatives. Some
advantages of this type of reactants are: an adequate basicity for
a fast reaction with CO2, high N/C ratio, a high concentration of
amino groups, and low volatility.
The novel compounds based on chemical modification of
glycine, b-aminopropionic acid (BAPA), and g-aminobutyric
acid (GABA) were prepared by direct coupling of these natural
amino acids with 2-chloropyrimidine (a, Scheme S1),† giving rise
to the structures 1a–c (Table 1).
Also, the synthetic derivatives of piperazine were prepared
either by direct reduction of the commercially available pyrazine2-carboxylic acid giving rise to 2 (b, Scheme S1),† or by coupling
of piperazine with 2-bromoacetic acid (c, Scheme S1),† and
2-bromopropionic acid (d, Scheme S1),† to give compounds 3
and 4, respectively. Other amino acids containing piperazine
(5 and 6) were prepared by direct reaction of 2-bromoacetic acid
and 1-(2-aminoethyl)piperazine (e, Scheme S1),† or by reaction
of 2,5-dimethylpiperazine and methyl acrylate, followed by
saponification to yield the free acid 6 (f, Scheme S1).†
The performance of each liquid absorbent containing the
above synthetic compounds was investigated and compared to
those containing natural amino acids.
Amino acids dissolved in water exist preferentially as a zwitterionic form, which means that the amino group is protonated
and hence not able to react with CO2. Therefore, an equimolecular amount of base was added to free the amino group in
the aqueous solution.
After the absorption equilibrium was reached, desorption of
CO2 was initially performed at 20 C just applying vacuum to the
system. However, total CO2 removal was not complete. Thus, the
solution was heated to 85 C, whilst a nitrogen stream passed
through the solution, and cooled again to 20 C and the
absorption equilibrium measured once more. After this treatment, it could be verified that the second absorption capacity was
highly improved, even if it did not achieve the initial values;
which indicated that applying the thermal treatment, the absorbent regeneration was more efficient (over 76% in average).
When the regeneration at 85 C was repeated and another
equilibrium absorption point at 20 C was determined, the same
absorption capacity value was achieved. The absorbent loses
capacity after the first absorption but it remains constant after
the second one. Table 3 shows the molar ratio of absorbed CO2
per mol of amino acid at the equilibrium.
The combination of CO2 with the absorbents gives rise to
a carbamate following the sequence described in Scheme 1.
This journal is ª The Royal Society of Chemistry 2009
Table 3 Absorption capacity of natural amino acid salts (mol CO2 per
mol amino acid), before and after first regeneration, for a CO2 partial
pressure of 1 bara
Absorbent
First cycle
Second cycle
GABA
BAPA
Proline
Glycine
Taurine
Threonine
Serine
Histidine
Arginine
Ornithine HClb
MEA
0.78
0.86
0.89
0.70
0.78
0.73
0.70
1.00
1.70
1.65
0.88
0.43
0.45
0.45
0.50
0.52
0.58
0.65
0.77
0.80
0.90
0.60
a
All amino acid solutions (1.0M) were treated with an equimolar amount
of KOH. b Ornithine HCl solution was treated with a 2.0 M KOH
solution.
Scheme 1
Addition of a base abstracts a proton from the zwitterionic form
of the amino acid (II-A), releasing the free amino group (III-A).
This amino group interacts with CO2 to form a complex (IV-A)
by an equilibrium reaction. Afterwards, a basic group is needed
to remove a proton to give the final carbamate (V-A), this later
step being the key one. Thus, if a base is not used in this step, an
amino group of the amino acid has to take this role to give
a carbamate of ammonium derivative. This is the case for typical
amino acids containing aliphatic groups when just one mol of
base (used to release the proton of the compound II-A) is added;
and hence the theoretical CO2 load is 0.5 mol CO2 per mol amine.
Accordingly, the absorption values obtained for the tested amino
acids containing only aliphatic groups: glycine (0.5 mol CO2 per
mol amino acid), proline (0.45 mol CO2 per mol amino acid),
BAPA (0.45 mol CO2 per mol amino acid), GABA (0.43 mol
CO2 per mol amino acid) and taurine (0.5 mol CO2 per mol
amino acid) are equal or lower than 0.5 mol CO2 per mol amino
acid. From the results obtained with the amino acids tested,
which contain different aliphatic chemical structure, it seems that
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there is no direct influence of the amino acids chain length on the
absorption capacity.
Furthermore, the absorption values listed in Table 3 fairly
agree with data previously reported.32–34
Screening studies of natural occurring amino acids as potential
CO2 absorbents showed the high affinity to CO2 of amino acids
containing basic groups in their side chain [amine (ornithine),
guanidinium (arginine) and imidazonium (histidine) groups].
So, ornithine and arginine showed the best absorption
capacities (0.9 and 0.8 mol CO2 per mol amino acid in the second
cycle, respectively). For arginine and ornithine (Scheme 2), their
lateral chains have strong basic character as revealed by their pKa
values (12.48 and 10.53, respectively, Table 2) and, therefore, the
abstraction of the hydrogen from the CO2–amino acid complex
(V-B) to give the carbamate moiety (VI-B) is easily accomplished
by the lateral guanidinium or amine group. Among the many
organic bases known to have some promoting CO2 fixation
ability, amidines and guanidines are the ones with superior
effectiveness.35
Depending on the pKa value of the lateral chain groups, the
equilibrium constant rate (k3/k4), and the pH of the system, there
will be a relationship between the II-B and III-B species determined by the Henderson–Hasselbalch equation:
XHþ
X
pKa pH ¼
(2.1)
½X
Thus, when the value of pKa is very high, as happens with the
guanidinium group in arginine (12.48), III will be the predominant species at the solution working pH. Moreover, the abovementioned guanidinium group is able to efficiently remove the
proton of the CO2–amino acid complex (IV-B) to give the
carbamate (V-B) in high yield. When the pKa value is not so high
(10.53 for ornithine), III-B will also be the predominant species.
However, k7 will be lower and the carbamate formation will be
accomplished in a lower yield.
Scheme 2
888 | Energy Environ. Sci., 2009, 2, 883–891
Other amino acids containing a lateral chain with basic
character (imidazonium) is histidine (pKaR ¼ 6.20), which
showed very good absorption value (0.77 mol CO2 per mol
amino acid). Because the imidazole group has a pKa lower than
that of the amino group, the initial system at pH > 8 is
predominantly composed of II-B species (Scheme 2). Therefore,
the proton abstraction, necessary to form the carbamate, is not
especially favored and, consequently, the k9 kinetic constant will
be lower than in arginine and ornithine. However, the experimental CO2 load values of arginine are close to that of histidine
(Table 3).
A second possible mechanism for the absorption process when
a basic unit is part of the absorbent is the proposed by Jessop and
co-workers.35–39 They consider that on exposure to CO2, amidines and guanidines mixed with water will form amidinium and
guanidinium bicarbonate salts. Therefore, compound IV-B
will interact with CO2, with the consequent formation of the
bicarbonate salt [AAH+][HCO3] (VI-B).
Since basicity alone does not explain the behavior of histidine,
it could be explained by the nucleophilic character of the
heterocyclic moiety, which is able to interact with a Lewis acid to
give complexes type VI-B (this result is similar to that obtained
for MEA, as it will be commented below). The combination of
these two processes does determine the high load obtained for
histidine. Obviously, the kinetic process has to be very different
from arginine and ornithine, and a deeper study of apparent
constant rates is being carried out.
It should be noted that not only the amino groups can interact
with CO2 but that other groups present in the system are also
capable. So, as the pH of the solution is higher than 7, hydroxide
ions react with CO2 to give hydrogen carbonate ions eqn (3.1)
which can react again with hydroxide ions as follows eqn (3.2):
CO2 + OH $ HCO3
(3.1)
HCO3 + OH $ CO32 + H2O
(3.2)
CO2 + CO32 + H2O $ 2HCO3
(3.3)
Thus, two hydroxide ions react very fast with one CO2 molecule to give a system where only carbonate ions are theoretically
present at the end of the process. On the other hand, carbonate
ions react with CO2 molecules (eqn (3.3)) to give hydrogen
carbonate species with a much lower reaction rate.
All this can be tested by measuring the pH of the solutions
before and after the absorption process (Table 4). In all cases, the
initial pH values were basic, and after the first absorption and
regeneration they diminished to similar values.
Consequently, in the first absorption, CO2 is transformed into
carbamate (by interaction with –NH2) and into carbonate and
hydrogen carbonate ions (because of the high pH of the system),
being the ratio CO32/HCO3 a function of the pH. After the first
desorption, the carbamate formation is reversed but some of the
carbonate and hydrogen carbonate ions remain in the system
and, consequently, the CO2 removal is not completely achieved.
Therefore, in the second absorption, only the CO2 that can
become a carbamate or that can react with HCO3 (eqn (3.3)) is
absorbed. This means that second and subsequent absorptions
are lower than the first one.
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Table 4 pH of the solutions during the equilibrium measurementsa
Amino acid solution
Before 1st
absorption
Before 2nd
absorption
Final
Glycine
Taurine
Proline
BAPA
Ornithine HClb
GABA
Arginine
Serine
Threonine
Histidine
10.96
12.77
12.43
11.85
12.20
12.03
13.77
—
10.41
—
10.54
10.57
10.80
10.32
10.77
10.73
10.43
10.52
10.19
10.44
8.25
7.81
8.48
7.85
7.82
7.94
7.84
7.57
7.41
7.65
a
All amino acid solutions (1.0 M) were treated with an equimolar
amount of KOH. b Ornithine HCl solution was treated with an extra
equivalent of KOH.
In an attempt to establish the new absorbents competence, the
absorption capacity of MEA was also tested under the same
conditions and included here for comparison. This widely used
scrubber agent performed slightly less efficiently (0.60 mol CO2
per mol MEA) than many of the LA here studied (ornithine,
arginine, histidine and serine). The value is higher than that of
taurine (0.5 mol CO2 per mol), indicating that the hydroxyl
group of MEA should be acting as a weak base or, more plausibly, interacting with the CO2 molecules and forming complexes
comparable to the V-A and VI-B species (Scheme 1 and 2). This
behaviour is similar to that obtained for histidine, which has been
commented above.
With the purpose of enhancing the absorption capacity
through the design and preparation of synthetic amino acids, we
have also explored the absorption capacity of synthetic amino
acids prepared by us as described previously. On one side, the
multifunctional compounds prepared were based on natural
amino acids chemically modified with a pyrimidine ring so that
secondary amines of different chain length (1a–c) were obtained.
As shown in Table 5, the chemical modification of BAPA and
GABA generated compounds with higher absorption capacities,
while modification of glycine yielded a less productive absorbent.
Besides, new absorbents based on piperazine (2, 3, 4, 5 and 6)
and containing different architectures have been prepared. The
choice was based on the fact that piperazine has been previously
used for this application and it is known that secondary amines,
and also hindered amines with substituents next to the amino
group, can easily form the carbamate species and also release the
CO2 upon heating. Their absorption measurements are detailed
in Table 5.
Although most of these new absorbents presented similar
absorption capacities to those showed by the natural amino acids
(0.23–0.75 mol CO2 per mol amino acid), two absorbents (5 and
6) showed exceptional performances, with absorption capacities
(1.11 and 1.13 mol CO2 per mol amino acid) much higher than
any of the other absorbents tested. These molecules have several
amino groups as well as a carboxylic acid in their structure.
In conclusion, the LA based on arginine and ornithine solutions can absorb CO2 from simulated flue gas effectively with
a high absorption capacity. The molar uptake of CO2 per mole of
these amino acids approaches the value of 1 mol of CO2 per mol
of amino acid. This CO2 uptake is superior to that of a standard
This journal is ª The Royal Society of Chemistry 2009
Table 5 Absorption capacity of non-natural amino acids (mol CO2 per
mol amino acid), before and after first regeneration, for a CO2 partial
pressure of 1 bara
Absorbent
First cycle
Second cycle
1a
1b
1c
2
3
4
5
6
MEA
1.08
0.91
0.80
1.20
0.82
0.41
1.86
2.23
0.88
0.26
0.53
0.59
0.75
0.60
0.23
1.11
1.13
0.60
a
All amino acid solutions (1.0 M) were treated with an equimolar
amount of KOH.
sequestering amine such as MEA. Interestingly, synthetic amino
acids based on piperazine containing an alkyl chain presented
molar uptakes superior to 1 mol of CO2 per mol of amino acid.
These findings make these amino acids ideal materials to be used
in LA for CO2 recovery.
The process of CO2 uptake is reversible, being the CO2
released from the LA solution upon heating (85 C). Thus, the
recovered liquid absorbent was repeatedly recycled for CO2
uptake with no significant loss of efficiency.
Spectroscopic analysis of the carbamate formation
The sequestration of CO2 by an amino group via its fixation as
a carbamate has been borne out by comparison of the 13C-NMR
spectra of the gas-free and gas treated liquid absorbent. Initially,
a 1 M solution of arginine in D2O was thoroughly bubbled with
a vigorous nitrogen flux and the spectra were recorded. Afterwards, the solution was flushed for 30 min with CO2 in the
absence of base, at room temperature, and new spectra were
recorded. For the 13C-NMR spectra of the untreated and
CO2-treated arginine see (a) and (b) in Fig. S2.†
Prior to CO2 exposure, carbon signals of arginine appeared at
24.5, 31.9, 41.2, 55.7, 157.0 and 183.3 ppm. As previously
reported for similar systems;19 after CO2 bubbling, signals are
‘‘doubled’’ and new signals (24.8, 30.1, 41.1, 55.7, 164.0 and 181.5
ppm) appeared in addition to the previous ones, corresponding
to one half of the amine becoming a carbamate. Also, two
additional signals corresponding to the fluxed CO2 and carbamate carbonyl carbon are observed at 160.6 and 164.0 ppm,
respectively. Arginine was also CO2-treated in the presence of
base, but no differences in the spectra were observed when
compared with those obtained without base. This indicated that
the chemical interaction of arginine and CO2 occurs in the
absence of base due to the intrinsic basicity of the guanidinium
group, which possesses a pKaR value of 12.48.
In conclusion, NMR spectroscopy is a suitable tool to make
a first evaluation of the ability of the selected LA to bind CO2.
Theoretical calculations
A theoretical study of reactivity was performed using methods of
computational chemistry to get information of the affinity
Energy Environ. Sci., 2009, 2, 883–891 | 889
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uptake per mol. All of them present energy values higher than
MEA (3.93 Kcal mol1). The other studied molecules present
lower values of energy and, therefore, lower affinity to the gas. It
is interesting to note out the increase of the energy values when
the amino acid is modelled as the free anion, with values that go
from 5.05 Kcal mol1 to 17.02 Kcal mol1 (Table S1).†
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Fig. 1 AM1 semiempirical modelling before (a) and after complex
formation (b).
towards CO2 of the different functional groups present in the
tested natural amino acids.
First, the molecules with amine and hydroxy groups were
modelled by the AM1 semiempirical method. A CO2 molecule
was placed at 1.9 Å, perpendicular to the nitrogen–carbon or
oxygen–carbon bond, as depicted in Fig. 1a.
Then, the system was minimized, obtaining geometries
(Fig. 1b) that were the starting geometries for DFT calculations.
The main geometrical parameters obtained for these systems
than can account for the interaction between carbon dioxide and
amines are the distance C(CO2)–N and the angle O–C–O of
carbon dioxide. This angle ranges from 178 to 172 , depending
on the amine, in DFT calculations and takes values around 150
for AM1 calculations, denoting an electronic transfer from the
sp3 lone pair nitrogen orbital to the electrophile carbon dioxide,
much larger in the low-level method. The interatomic distances
between the nitrogen of the amine and the carbon atom of CO2
obtained by AM1 were always shorter than those obtained from
DFT calculations. For sake of simplicity and because DFT
calculations are very CPU-time consuming, we have only studied
in the majority of cases the individual interaction of each amino
group with a single CO2 molecule.
The most important parameter, which gives a clear indication
of complex formation, is the interaction energy. This energy,
formation enthalpy (DH), in semiempirical methodology and
electronic energy, DEelc, in DFT calculations, was calculated as
the difference between total energy of complex CO2–amino acid
and the sum of the individual energies of CO2 and the amino acid
(Fig. 2). The electronic energy values obtained by DFT, were
calculated for each amino (1 to 4) and hydroxy group in the
amino acid, and are shown in Table S1.† These values ranged
from 0.26 to 4.72 Kcal mol1 (values similar to medium-strong
interactions, such as hydrogen bonds are) depending on the
considered amine or hydroxyl. These results showed that arginine (4.72 Kcal mol1) together with GABA (4.68 Kcal
mol1), proline (4.23 Kcal mol1) and ornithine (4.00 Kcal
mol1) form the most stable complexes with carbon dioxide and,
therefore, seem to be the ones with higher affinity for the molecules of gas.
However, ornithine presents the advantage of having two
binding sites, which eventually enhances the carbon dioxide
Fig. 2 Electronic energy by DFT calculations.
890 | Energy Environ. Sci., 2009, 2, 883–891
Conclusions
New LA containing CO2-complexing agents have been tested in
an experimental lab-scale against CO2. The LA here studied can
absorb CO2 gas effectively with a high absorption capacity,
superior in some cases to the frequently used MEA. The experimental data pointed out that from the natural amino acids,
arginine and ornithine showed the greatest affinities to CO2. We
synthesized via several efficient methods new amino acids containing different chemical structures. Among the synthetic amino
acids, 5 and 6 showed outstanding absorption capacities, superior to MEA or any other amino acid tested in this study. From
the trends found in this study, it can stated that LA based on new
synthetic amino acids prepared and based on a piperazine ring
are efficient systems for CO2 absorption.
The absorbed CO2 can be reversibly desorbed by heating and
the LA reused. This method has potential applications such as
removal of CO2 pollutants or the separation of gas mixtures
when used in combination with a polymeric membrane. This
matter is currently under study and will be published in a near
future.
The experimental data are supported by 13C-NMR studies in
addition to theoretical calculations. These NMR results show
that the chemical shifts of the positions next to the amine group
of the amino acid can be used as a powerful parameter for predicting CO2 loading capacity without any previous measurement. The theoretical calculations allow getting a previous idea
of the affinity of the molecules selected to the gas, prior to the
study at a laboratory scale, and in most cases are in concordance
with the experimental data.
Acknowledgements
Financial support from the Spanish Ministerio de Ciencia e
Innovacion (MAT2007-62392) is gratefully acknowledged.
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