The salt-based catalytic enhancement of CO2 absorption by a tertiary amine medium

Article abstract of DOI:10.1039/c6ra13978g

The rise in atmospheric CO₂ levels due to the effects of human activities poses a serious threat to the world's ecosystems because of global warming and climate change. Efficient methods are needed to limit the elevation of CO₂ level

Full text of DOI:10.1039/c6ra13978g

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The Salt-Based Catalytic Enhancement of CO2 Absorption by a
Tertiary Amine Medium
Dharmalingam Sivanesana, Young Eun Kima, Min Hye Youna, Ki Tae Parka, Hak-Joo Kima, Andrews
Received 00th January 20xx,
Accepted 00th January 20xx
Nirmala Graceb,*, Soon Kwan Jeonga,*
DOI: 10.1039/x0xx00000x
The rise in atmospheric CO2 levels due to the effects of human activities poses a serious threat to the world’s ecosystems
because of global warming and climate change. Efficient methods are needed to limit the elevation of CO2 level in the
atmosphere. Here, we propose an improved CO2 sequestration method that uses new catalysts, specifically a series of
tertiary amine nitrate salts, in an aqueous tertiary amine medium. We synthesized the new catalysts and characterized
them by using 1H and 13C NMR, single crystal X-ray spectroscopy, and FT-IR spectroscopy. The effects of the catalysts on
CO2 absorption were assessed by using a stopped-flow spectrophotometer, and their heats of absorption and CO2
absorption capacities were measured with a differential reaction calorimeter (DRC) at high concentrations of the tertiary
amine medium. The CO2 hydration rate constants were determined under basic conditions and the catalysts were found to
exhibit higher absorption of CO2 (a highest value of 430 M-1s-1) than the tertiary amine medium (133 M-1s-1). The increased
absorption of CO2 and the low heat absorption energies of the new catalysts suggest that they could be used in post-
combustion processes.
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Introduction
absorbent.16-18Very recently, CO2 absorption through the use of the
carbonic anhydrase (CA) enzyme19-20 and its model complexes21-23 in
Globally, the accumulation of carbon dioxide (CO2) in the
atmosphere is leading to severe environmental problems.1-2 To
minimize the undesirable effects of climate change and the
greenhouse effect, many techniques for controlling CO2 emissions
tertiary amine solution has been demonstrated, and found to
exhibit enhanced absorption of CO2 hydration. However, the use of
CA model complexes in highly concentrated tertiary amine medium
results in anion coordination with the metal center, which inhibits
the fourth catalytic cycle in the generation of bicarbonate, and the
CA enzyme is expensive and unstable under industrial conditions.24
Our approach to this problem is to attempt to improve the
efficiency of CO2 absorption and thus develop a better carbon
dioxide capture system. Enhancing the CO2 absorption capacity of
the tertiary amine by adding a suitable absorbent could also
possibly reduce the overall cost of the system.
3-7
into the atmosphere have been developed. There are four major
CO2 capture technologies available to industry: pre-combustion,
post-combustion, oxy-fuel combustion, and electrochemical
8-9
separation. Post-combustion capture is commonly utilized and is
considered the most effective industrial CO2 capture process.10-13
Aqueous alkanolamine solutions are widely used for the removal of
CO2 in post-combustion processes. The overall process is based on
the reversible chemical absorption of CO2 via an acid–base reaction.
Primary, secondary, and tertiary amines are solvents that are To this end, we synthesized three catalysts that each contains three
frequently used in post-combustion processes. The primary amines tertiary amine nitrogens and a nitrate salt, which were found to
are more effective in the absorption of CO2 than secondary and exhibit enhanced CO2 hydration activity for the generation of
tertiary amines because of the formation of the stable carbamate bicarbonate in a tertiary amine medium. This paper discusses the
species.14-15 However, the regeneration processes of primary and development of these new amine salt based catalysts for the
secondary amines require more energy than that of tertiary amines. absorption of CO2 in basic solution, and our DRC experimental
Hence, the tertiary amine methyldiethanolamine (MDEA) is results confirm their potential in post-combustion processes.
commonly employed in CO2 absorption processes; it has two
Experimental
further advantages: a lower level of solvent degradation, and less
energy must be supplied to the stripper to regenerate the
Material and methods: All chemicals purchased were of analytical
grade and used without further purification unless otherwise
stated. FT-IR spectra were recorded with a KBr disk or the ATR
method by using a Varian 640-IR FT-IR spectrometer. UV-Vis spectra
were recorded on an OPTIZEN 2120UV spectrophotometer. 1H NMR
and 13C NMR spectra were recorded on Varian 500 MHz and Varian
125 MHz instruments respectively.
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X-ray single crystal structural analysis: A single crystal was
mounted at room temperature on the tips of quartz fibers, coated
with Paratone-N oil, and cooled under a stream of cold nitrogen.
Intensity data were collected on a Bruker CCD area diffractometer
running the SMART software package with Mo Kα radiation (λ=
0.71073). The structure was determined with direct methods and
refined on F2 by using the SHELXTL software package. The multi-
scan absorption correction was applied with SADABS, which is part
of the SHELXTL program package, and the structure was checked for
higher symmetry by using the program PLATON. All non-hydrogen
atoms were refined anisotropically. In general, hydrogen atoms
were assigned idealized positions and given thermal parameters
equivalent to 1.2 times the thermal parameter of the carbon atom
to which they were attached. Data collection and experimental
details for catalyst 2 (CCDC - 1454963) are summarized in Table S1,
and the bond angles and distances are summarized in Table S2.
Scheme 1: Synthesis of catalyst 2. Reagents: a. 3-nitrobenzaldehyde, EtOH,
NaBH4, MeOH b. HCHO, NaBH4, CH3CN/AcOH c. Pd/C, Hydrazine hydrate d.
0.1 M HNO3
Synthesis of the catalysts and characterization: To enhance the
absorbance of CO2 by the tertiary amine medium, three amine
ammonium nitrate salt catalysts were designed and a simple
synthetic method was employed to obtain them in good yield.
Initially, N1-(2-aminoethyl)-N1-methylethane-1,2-diamine was
treated with various nitro benzaldehydes to obtain the
corresponding imines. The resulting imines were subjected to
reduction using NaBH4 to produce the amine intermediates. The
amine intermediates were then methylated with formaldehyde and
NaBH4. The nitro groups were hydrogenated with Pd/C and
hydrazine hydrate to obtain the corresponding amines25, which
were finally treated with diluted HNO3 to prepare the desired
nitrate salts in excellent yields. The synthetic scheme is shown in
Scheme 1 and the synthesized amine salts 1-3 are presented in
Chart 1. Pentamethyldiethylenetriamine (4) is a commercially
available tertiary amine that was tested to assess the effects of CO2
absorption in the absence of a primary amine nitrate salt. The
synthesized amine salts were characterized by performing proton
and carbon nuclear magnetic resonance spectroscopy, FT-IR, and
mass spectrometry.
Stopped-Flow Kinetics Spectrophotometry: The experimental rate
constants for the CO2 hydration reactions catalyzed by the amine
nitrate
salts
were
determined
with
stopped-flow
spectrophotometry by using methods similar to those previously
described. Prior to each experiment, a solution of CO2 saturated
water was prepared by purging deionized water with 99.99% CO2
gas at 25°C for at least 60 min. By using Henry’s constant, this
solution was calculated to contain 33.8 mM CO2. The reaction rates
were obtained by rapidly mixing the dissolved CO2 solution and the
MDEA solution (1:1, v/v) in the stopped-flow spectrophotometer
while recording the time dependent absorbance at λ = 596 nm. The
kcat and Km kinetic parameters were calculated by using the
Michaelis−Menten and Lineweaver−Burk equations. The CO2
solution and a solution of MDEA+TB (blank), MDEA+bCA+TB or
(catalyst+MDEA+TB) were injected into two separate syringe ports
in the thermostatic unit and were maintained at 25°C. Each reaction
was repeated three times, and the averages of the results were
obtained and fitted by using the software Pro-Data Viewer.
Differential Reaction Calorimeter: The CO2 absorption heats and
the absorption capacities of MDEA and the amine nitrate salts (1-4)
were determined from DRC data (the DRC was manufactured by
Setaram). The DRC apparatus consisted of two mechanically
agitated glass reactors (250 cm3). The reaction temperature was
maintained by circulating water from a temperature-controlled
bath through the reactor and the jacket. The reactor contained a
Joule effect calibration probe and a thermocouple (±0.01°C) for
measuring the reaction temperature (ΔT). The reactor was filled
with 150 g of each of the amine solvents (10 wt %) containing the
amine nitrate salts (2 mM). To enhance the contact between the
amine nitrate salts and CO2, the 30% CO2 gas mixture (the balance
of which was N2) was supplied in a purge form at 100 mL/min with
stirring at 250 rpm. The CO2 absorption rate and reaction
completion were evaluated by analyzing the discharge gas with a
TCD detector consisting of a Porapak-Q (2 m, Supelco) column
(7890N model GC from Agilent). The experimental temperature was
maintained at 40°C.
Chart 1. Structure of the catalysts 1-4
Description of the crystal structure: Suitable off-white crystals of
catalyst 2 were obtained from the acetonitrile and diethyl ether
diffusion system over four days. The structure of 2 is shown in Fig.
1, and its crystal data as well as selected bond distances and bond
Results and Discussion
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angles are given in Table 1 and Table S1 respectively. This amine Hence, aqueous tertiary alkanolamine MDEA was used as a reaction
salt forms orthorhombic crystals with Pna2(1) space group. The medium for the amine salts, which results in the selective
−
weak hydrogen bond interaction between the primary amine and
production of bicarbonate (HCO3 ). The hydration of CO2 was
determined by rapidly mixing dissolved CO2 solutions (5 mM,
10mM, or 15 mM) and a tertiary amine solution of bovine carbonic
anhydrase (bCA) or one of catalysts 1-4 (1:1, v/v) in a stopped-flow
spectrophotometer. The time dependent absorbance was recorded
at λ = 596 nm for thymol blue (TB), a pH indicator.32,33 The
−
formation of HCO3 during CO2 absorption or hydration is directly
proportional to the decay of the indicator absorbance intensity17.
The rate constants for the CO2 hydration reactions were calculated
by using the Michaelis−Menten and Lineweaver−Burk equations.
34,35
Fig. 1. Single crystal X-ray structure of 2. ORTEP view showing the 50%
probability thermal ellipsoids
the nitrate anion (Fig. S1) suggests that bicarbonate, which is
isoelectronic with the nitrate anion26-28, may also weakly interact
with the primary amine via hydrogen bonding when CO2 is
introduced into a tertiary amine medium containing the amine
nitrate salts. The FT-IR spectra confirm the presence of a primary
amine salt stretching band in the region 3200-3500 cm-1. Further
the –NH stretches of amine salt broad peaks in FT-IR show the
possible weak hydrogen bonds between the amine group with
nitrate anion in 1 (Fig. S2).
Determination of the CO2 hydration rate constants with stopped-
flow kinetics experiments: Highly concentrated amine solutions
have successfully been used in post-combustion processes as
absorbents. Such absorbents are chosen based on their CO2
absorption capacities. Primary29 and secondary amines exhibit
higher CO2 absorption than tertiary amines. However, the
regeneration of primary and secondary amine absorbents requires
more energy than that of tertiary amines. Furthermore, the use of
primary and secondary amines in aqueous solution in post-
combustion processes generates a mixture of carbamate and
carbonate products (equations 1 & 2), whereas aqueous tertiary
amine solutions generate bicarbonate rather than carbamate and
carbonate (equation 3). 30 The absence of protons in tertiary amines
means that bicarbonate is the sole product of the aqueous amine
CO2 hydration reaction. The following reactions occur
simultaneously when CO2 is absorbed into aqueous amine
solutions.31
Fig. 2. Top) Hydration of CO2 by the aqueous MDEA-CO2 system: black (5
mM), red(10 mM), blue(15 mM). Bottom) the corresponding
Lineweaver−Burk plots, where thymol blue(12.5 µM) and MDEA(15 mM).
Initially, the experiments were performed in the MDEA reaction
medium to establish a base level of CO2 consumption; the CO2
hydration rate constant was found to be 133 M-1s-1. Next, the use of
the enzyme bCA was found to provide an excellent rate of CO2
conversion, as expected, with a rate constant of 10.6 ×103 M-1s-1
under our experimental conditions. Under similar conditions, when
catalyst 1 was used, it was found to produce an enhanced rate of
reaction (430 M-1s-1), which indicates that CO2 absorption is
increased by using a tridentate amine based salt in a basic medium.
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The initial rate constants for the catalysts, bCA, and MDEA are given
in Table 1; this table shows that different CO2 absorption rate
constants for the catalysts suggest that adding the same
concentration of catalyst varies in the CO2 absorption behaviour. By
using the amine nitrate salts we achieved CO2 hydration rate
constants ranging from 242 M-1s-1 to 430 M-1s-1, which suggests that
it may be possible to tune the amine nitrate salts so as to provide
efficient CO2 sequestration.
Entry
Catalyst
CO2 loading
(mol/mol
solute)
0.64
Q
ΔH
(-kJ /mol of
CO2)
(-kJ)
1
2
3
4
5
MDEA
5.052
4.854
4.492
5.076
6.550
61.860
54.496
53.975
61.510
85.899
1
2
3
4
0.79
0.73
0.67
0.61
Table 2. DRC measurement of CO2 absorption capacities and heats of
absorption at 40°C. Q - quantity of heat released, ΔH – heat of absorption
per mole of CO2
DRC results for the heats of absorption and CO2 absorption
capacities: We tested the CO2 absorption capacities of the amine
nitrate salts in highly concentrated tertiary amine solutions to
assess the possible use of the amine nitrate salts in post-
combustion processes by performing DRC experiments. It is well
established that when CO2 is exposed to tertiary amines
bicarbonate is formed.36-38 Bicarbonate formation was confirmed in
the presence of the amine nitrate salts with 13C NMR spectra. The
13C NMR spectra of the mixtures of aqueous amine, a tertiary amine
salt, and CO2 contain peaks at 161 ppm for bicarbonate as a single
product. Importantly, the addition of the tertiary amine salts into
the tertiary amine solutions was found to enhance the formation of
bicarbonate (Fig. S3).
Fig. 3. Top) Hydration of CO2 by 1 (0.25 mM) in the aqueous MDEA-CO2
system: black (5 mM), red(10 mM), blue(15 mM). Bottom) the
corresponding Lineweaver−Burk plots, where thymol blue(12.5 µM) and
MDEA(15 mM).
The change in absorbance with respect to hydration of CO2 by
MDEA in tertiary amine solution over a period of 30 seconds was
measured for various concentrations of CO2 [Fig. 2 (left)] and the
corresponding Lineweaver−Burk plot is shown in Fig. 2 (right) and
also for 1 in Fig. 3. The CO2 absorption rate constant for the
commercially available amine 4 is less than those of the amine
nitrate salts 1, 2, and 3 (Table 1). This result confirms that the
presence of amine nitrate salts in absorbents 1-3 further enhances
their absorption of CO2. The low CO2 absorption is one of the
limitations of tertiary amine based CO2 fixation, such a constraint
can be partially overcome by incorporating the catalysts 1-3 into
the tertiary amine medium.
Fig. 4. DRC result: the CO2 absorption rate for absorbents 1-4 by gas
chromatography
The CO2 absorption heats and the absorption capacities of the
tertiary amines and amine nitrate salts (1-3) were determined by
using DRC. Carbon capture and storage technologies are based
upon gas-liquid absorption systems in which heat is exchanged. The
corresponding thermal data for the generated heat can provide
critical information about the amount of absorbed CO2 at a given
temperature and gas pressure. Further, such data also provide
information about the kinetics of the CO2 and amine absorbent
reactions.39-40 The DRC apparatus consisted of two mechanically
agitated glass reactors (250 cm3). Here the reaction temperature
was maintained by using a temperature-controlled bath that
circulates water through the reactor and the jacket. The DRC
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experimental values of MDEA were compared with previously
reported values.40 Initially, the CO2 absorption capacity and the heat
of absorption of the MDEA (10%) solution were measured. The
obtained absorption capacity and heat of absorption at 40oC are
given in Table 1. For MDEA, the CO2 absorption mol/mol absorbent
and the heat of absorption are 0.64 and -64.928 kJ respectively. The
heats of absorption and absorption capacities of the primary and
secondary amines are higher than those of MDEA.40 Next, under
similar conditions absorbent 1 was tested and found to exhibit a
higher CO2 absorption capacity (Table 2, entry 2). Interestingly, the
heat of absorption for 1, -54.496 kJ, is less than that of MDEA (Table
2, entries 1&2) which suggest that addition of 1 in highly
concentrated amine medium could only increase the absorption of
CO2. The heats of absorption for absorbents 2 and 3 are -53.975 and
-61.510 kJ, respectively. However, the heat of absorption for 4 is
higher than catalysts 1-3 because the direct reaction of 4 with CO2
in tertiary amine medium. The absorption rates determined from
the DRC experiments are shown in Fig. 4. The CO2 absorption rates
of 1-3 are higher than that of MDEA. This result clearly confirms
that the addition of the absorbents enhances the absorption of CO2
by tertiary amine solutions. The highest CO2 absorption rate was
obtained for 1 (Table 2). These results confirm that even in high
concentrations of the amine medium the absorbents 1-3 exhibit
improved CO2 absorption.
Notes and references
‡ Experimental procedure, analytical data and crystallographic
information of are available in supporting information.
2
1
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Graphical Abstract
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