Analysis of the CO2 and NH3 reaction in an aqueous solution by 2D IR COS: Formation of bicarbonate and carbamate

Article abstract of DOI:10.1021/jp800991d

The two-dimensional (2D) infrared correlation spectra obtained from the reaction time- and concentrationdependent IR spectra elucidates the reaction of CO₂ and NH₃ in an aqueous solution for CO₂ absorption. In the synchron

Full text of DOI:10.1021/jp800991d

6
558
J. Phys. Chem. A 2008, 112, 6558–6562
Analysis of the CO2 and NH3 Reaction in an Aqueous Solution by 2D IR COS: Formation
of Bicarbonate and Carbamate
†
‡
†
,†
,§
HoSeok Park, Young Mee Jung, Jong Kyun You, Won Hi Hong,* and Jong-Nam Kim*
Department of Chemical and Biomolecular Engineering (BK21 Program), KAIST, 373-1 Guseong-dong,
Yusong-gu, Daejeon, Republic of Korea, Department of Chemistry, Kangwon National UniVersity,
Chunchon 200-701, Republic of Korea, and Chemical Research Center, KIER, 71-2 Jang-dong, Yusong-gu,
Daejeon, Republic of Korea
ReceiVed: February 2, 2008; ReVised Manuscript ReceiVed: May 12, 2008
The two-dimensional (2D) infrared correlation spectra obtained from the reaction time- and concentration-
dependent IR spectra elucidates the reaction of CO and NH in an aqueous solution for CO absorption. In
2 3 2
the synchronous 2D correlation spectra, the interrelation of the proton with carbamate and bicarbonate indicates
that the pH level affected the formation reactions of the two products. Furthermore, the interrelation of carbamate
with bicarbonate confirmed the conversion of carbamate into bicarbonate with the release of protons (or the
decrease of the pH). From the experimental results including the asynchronous 2D correlation spectra, the
reaction of the CO and aqueous ammonia proceeded through the following steps: formation of carbamate,
2
formation of bicarbonate, release of protons, and conversion of carbamate into bicarbonate. The analysis of
the formation of carbamate and bicarbonate by 2D infrared correlation spectroscopy provides useful information
on the reaction mechanism of CO
2
and NH
3
in aqueous solutions.
Introduction
NH COONH (s/aq) + CO (g) +
2
4
2
2
H O(1/g) T 2NH HCO (s/aq) (3)
2 4 3
Carbon dioxide emissions need to be reduced because they
are a major greenhouse gas that causes global warming.
Absorption of carbon dioxide by various chemical solvents is
considered to be the most effective technology to capture CO2.
To date, alkanolamine solutions have been commonly used as
absorbents to remove CO2 from flue gas. Recently, aqueous
ammonia solutions have been recognized as an alternative
+
The formation of ammonium (NH4 ) and carbamate
NH2COO ) ions via reaction 1 is very fast and irreversible at
low temperatures. However, reactions 2 and 3 are reversible,
resulting in the formation of ammonium bicarbonates. Am-
-
(
6
7
monium carbamates can be converted into ammonium bicarbon-
ates via reaction 3. The formation of carbonate as a minor
product is negligible because of its slow formation reaction and
fast conversion reaction into bicarbonate under the experimental
conditions used in this study (298 K, 1 atm). In particular, the
formation of carbamate and bicarbonate is of prime importance
in terms of the effect of the two components on the CO2 working
capacity and regeneration energy. A fundamental understand-
ing of the reaction of CO2 and NH3 in aqueous solutions,
however, has thus far proved elusive, primarily because of the
complexity of these systems and the difficulties in monitoring
1
–4
chemical solvent for CO2 because of their advantages over
amine absorbents in terms of lower costs, higher CO2 absorption
capacities, lower decomposition temperatures of ammonium
bicarbonate, and less corrosive environments for the absorber
5
material.
8,9
Despite the relatively small amount of research conducted
on aqueous ammonia as a CO2 absorbent, there has been a long-
standing interest in the absorption reaction between CO2 and
NH3, especially in connection with industrially relevant pro-
cesses, such as the Solvay process, and academically important
chemistry, such as carbamation. The reaction between CO2 and
NH3 in aqueous media can occur at room temperature (298.25
K) with 1 atm as follows:
1
0
the kinetic reaction changes. Herein, the reaction of CO2 and
NH3 in an aqueous solution is verified by using a two-
dimensional infrared correlation spectroscopy (2D IR COS).
11–13
2
D IR COS is defined by two independent wavenumbers,
generated by a cross-correlation analysis of the dynamic
fluctuations of IR signals induced by an external perturbation,
such as temperature, time, stress, concentration, and so on. Some
of the notable features of the generalized 2D correlation spectra
include the simplification of complex spectra consisting of many
overlapped peaks, the enhancement of the spectral resolution
by spreading the peaks along the second dimension, the
establishment of unambiguous assignments through the cor-
relation of selectively coupled bands by using various interaction
mechanisms, and the determination of the sequence of spectral
+
2
NH (aq) + CO (g) f NH (aq) +
3
2
4
-
2
COONH (aq) f NH COONH (s) (1)
4
2
+
4
NH (1) + CO (g) + H O(1) T NH (aq) +
3
2
2
-
HCO (aq) T NH HCO (s) (2)
3
4
3
1
1–13
*
Corresponding author. Tel: +82-042-869-3919. E-mail: whhong@
peak emergence.
Therefore, 2D COS is particularly suitable
kaist.ac.kr.
to elucidate complex chemical reactions such as CO2 absorption.
In a synchronous 2D correlation spectrum, the auto peaks located
at the diagonal positions represent the overall susceptibility of
†
KAIST.
Kangwon National University.
KIER.
‡
§
1
0.1021/jp800991d CCC: $40.75  2008 American Chemical Society
Published on Web 06/27/2008

Analysis of CO2-NH3-H2O Reaction by 2D IR COS
J. Phys. Chem. A, Vol. 112, No. 29, 2008 6559
SCHEME 1: Diagram of Reactor System for CO2
Scrubbing
the corresponding spectral region to change in the spectral
intensity as an external perturbation is applied to the system,
whereas the cross peaks located at the off-diagonal positions
of a synchronous 2D correlation spectrum reveal simultaneous
or coincidental changes of spectral intensities that are observed
1
1–13
at two different spectral variables (ν1 and ν2).
In contrast,
an asynchronous 2D correlation spectrum consisting of cross
peaks only provides information that is useful to interpret the
kinetics of the chemical/physical reactions: the relative temporal
relationship and the order of the actual sequence of individual
Figure 1. pH variation of the solutions (a) as a function of the initial
concentration of ammonia in the range of 5-25 wt% and (b) after the
2 3
CO and NH reaction in an aqueous solution at 5 wt% as a function
1
1–13
reaction processes.
The intensity of an asynchronous 2D
of the reaction time.
correlation spectrum represents sequential, or successive, changes
of spectral intensities measured at ν1 and ν2. An asynchronous
cross peak develops only if the intensities of two spectral
features change out of phase (i.e., delayed or accelerated) with
each other. If the signs of synchronous and asynchronous cross
peaks are the same, the intensity change at ν1 occurs before ν2.
If the signs of synchronous and asynchronous cross peaks are
different, the intensity change at ν1 occurs after ν2.
all samples to avoid differences caused by the pressure and
penetrating depth. Each spectrum, which was recorded as the
average of 13 scans with a resolution of 1 cm , was taken
-
1
-
1
-1
from 4000 cm to 650 cm . Other regions, except 1900-900
-
1
cm , were omitted because of the independence of the peaks
from the reaction. We collected each sample of 2 mL at the
specific condition through a sampling port and then directly
measured it to block the reaction of CO and NH further.
In this work, the chemical reactions of CO2 and NH3 in an
aqueous solution were analyzed by focusing on the formation
of carbamate and bicarbonate as major components by using a
2
3
The synchronous and asynchronous 2D correlation spectra
were obtained by using the same software as that described
1
4
2D IR COS obtained from the reaction time- and concentration-
previously. In light of recent publications concerning the
14
dependent FT-IR spectra.
normalization procedure in 2D correlation analyses, we tested
various methods and found that the best results were obtained
by using non-normalized data.
Experimental Section
The schematic diagram of the experimental system that
investigates the reaction between CO2 and NH3 is shown in
Scheme 1. The temperature of the solution was kept within (0.5
K of the set point during the experiments by circulating water
through the water jacket around the reactor. The bubble blowing
apparatus, the inlet port of which was attached to a glass filter
for CO2 bubbling and mixing, was a glass bottle with a 45 mm
inner diameter containing 200 mL of ammonia solution. The
CO2 gas was obtained from a CO2 gas cylinder (>99.9%). The
CO2 flow rate of 970 mL/min was controlled by using mass
flow controllers (MFC, MKS). All experiments in this study
were performed at room temperature conditions (298 K). The
ammonia solution was obtained from Junsei (28 wt%) and was
diluted by using deionized water. The concentrations of NH3
in this experiment were 5, 10, 15, 20, and 25 wt%. The dry
powders of ammonium carbamates and ammonium bicarbonates
were used as the standards for product identification, were
supplied by Sigma-Aldrich (99%), and were not purified further.
FT-IR spectra were collected by a JASCO FT-IR 470 plus
as attenuated total reflection mode. The pressure was equal in
Results and Discussion
The pH of reaction media, which is influenced by the
formation of products as well as the consumption of reactants,
is important in controlling the reaction route of CO2-NH3 in
9
an aqueous solution. As the reaction of NH3 with CO2 as acidic
gas in an aqueous solution proceeds, the pH of the solution
8
,9
decreases, and the CO2 loading increases. The pHs of the
solutions linearly increased in the range of 5-25 wt% with the
initial concentration of ammonia, as shown in Figure 1a. The
pHs of standard ammonium carbamate and bicarbonate were
1
5
consistent with those reported in previous work. Figure 1b
shows the pH variation of the solution after the CO2 and NH3
reaction in an aqueous solution at 5 wt% as a function of the
reaction time. The rapid drop of pH until the reaction time of
60 min was associated with the fast reaction of CO2 and NH3
at the initial reaction time. In contrast, the smooth decrease of
pH above 60 min of reaction time was due to the conversion
reaction from ammonium carbamates to ammonium bicarbon-
1
ates.

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560 J. Phys. Chem. A, Vol. 112, No. 29, 2008
Park et al.
Figure 2. IR spectra of standard ammonium carbamate and ammonium
bicarbonate at 1 and 3 M in aqueous solutions. Asterisks were assigned
to peaks of carbamates, whereas diamonds were assigned to those of
bicarbonates.
Figure 3. IR spectra of the solutions after the CO and NH reaction
in an aqueous solution at 5 wt% as a function of the reaction time.
2
3
formation or conversion reactions via reactions 2 and 3.
However, the IR spectra did not provide detailed information
on the kinetic changes of reactions related to the formation of
carbamate and bicarbonate.
FT-IR was used to assign the typical bands of standard
ammonium bicarbonates and carbamates at 1 and 3 M aqueous
solution, shown in Figure 2. The proton band was 1612 cm ,
and the three carbamate bands appeared in the range of
-1 16
In order to clarify the correlation among the reaction routes,
such as the formation of carbamate and bicarbonate and the
conversion of bicarbonate into carbamate, we applied the 2D
COS to the reaction time-dependent FT-IR spectra in Figure
3. Figure 4 shows the synchronous and asynchronous 2D
correlation spectra obtained from the reaction time-dependent
IR spectra of the CO2 and aqueous ammonia at 5 wt%. As
shown in the top section of Figure 4a, the power spectrum
extracted along a diagonal line in the synchronous spectrum
-
1
9
00-1900 cm . In previous results, there were six bands
ascribed to the carbamates ion. However, it is impossible to
differentiate the three carbamate bands below 900 cm because
of the superposition of the bands from other components. The
-1
-
1
bands at approximately 1550 and 1100 cm were ascribed to
the CdO asymmetric and symmetric stretching bands of
-
17
NH2CO2 groups, respectively. In addition, the peak near 1400
-
1
18
cm was attributed to the C-N stretching band. The two
-
1
shoulder peaks of the C-N stretching band at 1450 and 1350
demonstrates that the band around 1354 cm contained an
-
1
-1
cm were attributed to the C-O asymmetric and symmetric
additional band at 1304 cm as a result of the conformational
19,20
bands of bicarbonate, respectively.
However, the ammonium
change of bicarbonate through different reaction routes and
interactions with other components. Positive cross peaks at
carbamate in the aqueous system also had a relatively weak
intensity of bands because of its dissolution and instantaneous
conversion reaction into bicarbonate in water. The intensities
of the shoulder bands at 1450 and 1350 cm- were proportional
to the concentration of ammonium bicarbonate from 1 to 3 M,
indicating that these bands were attributed to ammonium
bicarbonate. For the ammonium carbamate, the intensities of
the three bands also increased with the concentration of
ammonium carbamate. The intensities of the three bands that
are stronger than those of the other peaks in the reaction products
indicate that the formation reaction of carbamates via reaction
-
1
-1
(1612, 1452) cm and (1612, 1354) cm in the synchronous
2D correlation spectrum indicate that the changes in the pH
related to the proton peak were strongly interrelated with
1
-
1
the two bicarbonate bands at 1452 and 1354 cm . When
the absorption reaction of CO2 by NH3 in the aqueous solution
occurred, the band intensity of the proton was enhanced
(Figure 3). Furthermore, bicarbonate was formed via reaction
2 or 3 with a reduction of pH in the reaction solution, as
confirmed by the existence of positive cross peaks and as
shown in Figure 3. In contrast, the negative cross peak at
(1354, 1536) cm elucidates that the change of spectra
intensity of the band assigned to carbamate at 1536 cm
was inversely proportional to that ascribed to bicarbonate at
1354 cm . The reciprocal relation of carbamate and
bicarbonate bands confirmed the conversion reaction of
carbamate into bicarbonate via reaction 3. As shown in the
synchronous 2D correlation spectrum in Figure 5a, obtained
from the concentration-dependent IR spectra (Supporting
Information, Figure S1), the changes in the pH related to
the proton band were also interrelated with bands of
carbamate as a result of the positive cross peak at (1612,
-
1
1
proceeds dominantly, and the content of carbamate ions
increases.
Figure 3 shows the FT-IR spectra of the solutions after the
-
1
-
1
CO2 and NH3 reaction in an aqueous solution at 5 wt% as a
function of the reaction time. After the reaction of CO2 and
NH3 for 10 min, the pH of the solutions reached approximately
9.5, which approached that of the standard ammonium carbam-
ates in the 1 M solution (Figure 1a). The pH of the solutions
dropped gradually to 8.1, which is close to that of ammonium
bicarbonates. Consequently, the typical bands of ammonium
carbamates were gradually weakened and almost disappeared.
When comparing the IR spectra for 20 min of reaction time
with the other IR spectra for reactions longer than 20 min, the
former displayed relatively apparent bands of carbamate. Their
pHs were near the pH of standard ammonium carbamate (Figure
-
1
1409) cm . As the pH of the solution decreased, the proton
concentration increased, and protons reacted with the car-
bamate ions, causing them to become unstable. In particular,
2
1
-
1
the band of carbamate at 1409 cm was split into two bands
-
1
1). After the ammonium carbamates were produced via reaction
at 1411 and 1379 cm because of its interactions with the
proton, as shown in the asynchronous 2D correlation spectrum
in Figure 5b. The unstable carbamate intermediate was
1
at pH 9.5-9.2 during a reaction time of 10-20 min,
ammonium bicarbonates were produced gradually by the

Analysis of CO2-NH3-H2O Reaction by 2D IR COS
J. Phys. Chem. A, Vol. 112, No. 29, 2008 6561
Figure 4. (a) Synchronous and (b) asynchronous 2D correlation spectra
2
obtained from the reaction time-dependent IR spectra of the CO and
aqueous ammonia at 5 wt% in the range of 1900-900 cm- . The solid
1
Figure 5. (a) Synchronous and (b) asynchronous 2D correlation spectra
and dotted lines represent positive and negative cross peaks, respectively.
obtained from the concentration-dependent IR spectra of the CO
2
and
aqueous ammonia at 5, 10, 15, 20, and 25 wt% in the range of
-
1
1
900-900 cm . The solid and dotted lines represent positive and
converted into bicarbonate through conversion reaction 3,
as supported by the reduction of carbamate bands after 20
min of reaction time (Figure 3). The formation reaction of
carbamate via reaction 1 occurred at higher pH levels than
that of the formation reaction of bicarbonate via reaction 2
negative cross peaks, respectively.
of carbamate), and 1536 (CdO asymmetric stretching band
of carbamate). Furthermore, the reaction route of CO2 and
NH3 in an aqueous solution was confirmed by the results of
(
Figures 1 and 3). An analysis of the asynchronous 2D
2
D IR COS of CO2 and aqueous ammonia at 15 wt%
correlation spectrum in Figure 4 revealed the following
sequence of changes in spectral intensities: 1409 (C-N
stretching band of carbamate), 1452 (C-O asymmetric band
of bicarbonate), 1354 (C-O symmetric band of bicarbonate),
(Supporting Information, Figures S2 and S3). These findings
clarify that the reaction of CO2 and aqueous ammonia
proceeded as follows: the formation of carbamate via reaction
1, the formation of bicarbonate via reaction 2, the release of
1
612 (proton band), 1107 (CdO symmetric stretching band

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562 J. Phys. Chem. A, Vol. 112, No. 29, 2008
Park et al.
protons with the pH reduction, and the conversion of
carbamate into bicarbonate via reaction 3.
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We demonstrated that the absorption reaction of CO2
and aqueous ammonia occurred through the formation of
carbamate and bicarbonate, and the conversion of carbamate
into bicarbonate as confirmed by 2D IR COS obtained from
the reaction time- and concentration-dependent IR spectra.
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JP800991D