Chemical stability of secondary-alkanolamine-based co2 solvents under stripping conditions

Full text of DOI:10.1246/cl.130688

doi:10.1246/cl.130688
Published on the web September 11, 2013
1559
Chemical Stability of Secondary-alkanolamine-based CO2 Solvents
under Stripping Conditions
Shin Yamamoto* and Takayuki Higashii
Chemical Research Group, Research Institute of Innovative Technology for the Earth (RITE),
9-2 Kizugawadai, Kizugawa, Kyoto 619-0292
(Received July 25, 2013; CL-130688; E-mail: yamashin@rite.or.jp)
Back Pressure Valve
Chemical stabilities of secondary alkanolamine substrates in
CO2 solvents under stripping conditions and generation path-
ways of thermal degradation products have been studied. The
two products generated in each amine solution have been proven
to be oxazolidone and dimer of each amine. The thermal
degradation of secondary alkanolamine proceeds in two steps as
consecutive reactions. The carbamate compound, which is CO2-
absorbing amine, cyclodehydrates to generate oxazolidone, and
the oxazolidone further reacts with another amine to generate
dimer through decarbonation condensation.
Condenser
CO2/N2
CO2 Meter
Vent
MFC
Mixture
P
N2
Mixer
Amine Solution
P
Chiller
MFC
T
CO2
Sampling Port
Blast-furnace gas (BFG), which is generated from blast
furnaces as by-products through the reduction of iron ore with
coke to metallic iron, is known to be a large-scale CO2 emission
source, as well as flue gases from coal-fired power plants and
cement plants. These CO2 emission sources contribute to
increasing global warming. Carbon dioxide capture and storage
(CCS) have been currently brought to international attention as
one of the most feasible CO2 reduction technologies. The cost
for CO2 capture accounts for more than 60% of the CCS total
cost,1 and development of CO2 capture technologies to lower the
energy costs is promoted worldwide. Amine-based chemical
CO2 solvent process is one of the most practical methods to
capture CO2 from large-scale CO2 emission sources; these
solvents are called “chemical solvents” by reason that CO2 is
absorbed and desorbed in the following chemical reactions (1)
and/or (2) through a temperature swing process.
Reactor
Humidifier
Figure 1. Chemical CO2 solvent stability test apparatus (CAT-DEG).
þ
2RnNH3ꢀn þ CO2 ꢀ RnNH2ꢀnCOOꢀ þ RnNH4ꢀn
ð1Þ
ð2Þ
þ
RmNH3ꢀm þ CO2 þ H2O ꢀ HCO3ꢀ þ RmNH4ꢀm
where, RnNH3¹n and RmNH3¹m are amine compounds (n = 1 or
2; m = 1, 2, or 3). Thus, the development of chemical CO2
solvents that have high efficiency and high stability is desired to
realize low-cost CO2 capture.
Figure 2. GC-FID chromatogram of tested solutions (CAT-DEG,
120 °C, 32 h later).
Three secondary alkanolamines, 2-(methylamino)ethanol
(MAE), 2-(ethylamino)ethanol (EAE), and 2-(butylamino)etha-
nol (BAE), were employed as each substrate of chemical CO2
solvent examined in this study. To examine the chemical
stabilities of each amine and the generation of degradation
products, CO2 solvent thermal stability was examined at 120 °C,
which is the temperature generally employed as CO2-stripping
conditions, with a chemical CO2 solvent stability test apparatus
(CAT-DEG) shown in Figure 1 and a heat-pressure-resistant
autoclave. Experimental details are described in the section of
References and Notes.5 The tested solution was sampled and
analyzed with gas chromatography-flame ionization detector
(GC-FID), liquid chromatography-mass spectrometer (LC-
MS), and gas chromatography-mass spectrometer (GC-MS).
GC-FID chromatograms at 32 h after the beginning of the
thermal stability tests with CAT-DEG are shown in Figure 2.
Currently, we have been developing unique secondary
amine solutions as novel chemical CO2 solvents for the CCS
process from BFG.2 Amine-based CO2 solvents are gradually
deteriorated by means of heating through the temperature swing
process, as well as effects of coexisting materials contained in
gases from the CO2 emission sources. We reported the effect of
CO contained in BFG on secondary-alkanolamine-based CO2
solvents in a previous paper.3 In this paper, we report chemical
stabilities of secondary alkanolamine substrates in chemical CO2
solvents under CO2-stripping conditions (120 °C) and generation
pathways of thermal degradation products. Already, degradation
mechanisms of primary amines and tertiary amines have been
proposed in some articles.4 However, there is no detailed report
on thermal degradation pathways and products of secondary
amines to the best of our knowledge.
Chem. Lett. 2013, 42, 1559-1561
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1560
Table 1. Identified structures of products
(a)
20
15
10
5
100
95
90
85
80
75
Substrate
amine
Convex-upward
typea
Convex-downward
typea
HN
N H
MAE
O
N-(2-hydroxyethyl)-
N,N′-dimethylethylenediamine
O
RT131
RT129
N
N
H O
3-methyl-2-oxazolidone
HO
MAE
RT131
RT129
0
0
20
20
20
40
60
80
100
120
120
120
140
Test Time / h
(b)
O
HN
N H
8
6
4
2
0
O
100
95
90
85
80
75
N-(2-hydroxyethyl)-
N,N′-diethylethylenediamine
N
EAE
3-ethyl-2-oxazolidone
N
H O
HO
EAE
RT144
RT150
RT144
RT150
N-(2-hydroxyethyl)-
N,N′-dibutylethylenediamine
O
O
0
40
60
80
100
140
HN
N H
Test Time / h
N
(c)
3-butyl-2-oxazolidone
3
2
1
0
100
95
90
85
80
75
N
H O
HO
BAE
RT166
RT182
BAE
aConvex-upward type is to show convex-upward-increasing
tendency, and convex-downward type is to show convex-down-
ward increasing tendency, in temporal changes in concentrations
(Figures 3a, 3b, and 3c).
RT166
RT182
upward-increasing tendency; RT131 from MAE, RT144 from
EAE, and RT166 from BAE fall under this type, and the other
type is to show convex-downward-increasing tendency; RT129
from MAE, RT150 from EAE, and RT182 from BAE fall under
this type. These tendencies indicate that the two products from
each amine are generated through consecutive reactions, that is,
the products of convex-downward type would be generated from
those of convex-upward type.
0
40
60
80
100
140
Test Time / h
Figure 3. Temporal changes in concentrations of products; (a) MAE,
120 °C, 0-32 h: CAT-DEG, 32-128 h: autoclave; (b) EAE, 120 °C,
0-32 h: CAT-DEG, 32-128 h: autoclave; (c) BAE, 120 °C, 0-32 h: CAT-
DEG, 32-128 h: autoclave.
Analyses to identify the molecular structures of the products
have been carried out with LC-MS and GC-MS; this equipment
has been used for accurate mass analyses, structural estimation,
and confirmatory analysis. The analysis results are supplied as
Supporting Information.6 The identified structures of the
products are shown in Table 1. The products classified into
convex-upward type, RT131, RT144, and RT166, have been
proven to be oxazolidones of each amine, which are presumed to
be generated through cyclodehydration reactions of the carba-
mate compounds that are CO2-absorbing amines (1). The
products of convex-downward type, RT129, RT150, and
RT182, have been proven to be dimers of each amine. We can
think of the following three pathways as the dimer generation
reaction. One pathway is dehydration condensation between two
By the GC-FID analyses, two peaks of novel products that are
unobservable before the tests have been detected for each
solution; these two peaks are RT129 and RT131 generated from
MAE, RT144 and RT150 generated from EAE, and RT166 and
RT182 generated from BAE. All of the other detected peaks are
impurities contained in the solutions initially.
Figures 3a, 3b, and 3c show temporal changes in generated
concentrations of the products from MEA, EAE, and BAE,
respectively, where [Int./(Int. ¹ Amine0)] of the ordinate in the
graphs means intensities of the products obtained as GC peak
areas divided by that of each substrate amine at initial
conditions. The products generated in each solution show two
types of generation behavior. One type is to show convex-
Chem. Lett. 2013, 42, 1559-1561
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1561
MAE
RT131
RT129
H
N
1.00
0.95
0.90
0.85
0.80
105
100
95
HO
R
Cyclodehydration
of Carbamate
CO2
O
90
O
85
0.05
0.00
N
80
R
0
10
20
30
Test Time / h
H
N
Decarbonation
Condensation
CO2
HO
R
Figure 4. Temporal changes in solute components (Verification test of
dimer generation pathway, 120 °C).
R
HN
R
amines, another pathway is decarbonation condensation between
amine and oxazolidone, and the third pathway is decarbonation
condensation between two oxazolidones.
Thus, the following test has been carried out to verify the
dimer generation pathway. A mixed aqueous solution of MAE
(45 wt %) and RT131 (5 wt %), which is the oxazolidone of
N
HO
Figure 5. Thermal degradation pathways of secondary alkanolamine
substrates in chemical CO2 solvents.
¹1
MAE, was aerated with pure N2 gas at 500 mL min for 32 h
with CAT-DEG. The test temperature was 120 °C. The result of
the verification test is shown in Figure 4, where molar ratio
of the ordinate in the graph means molar numbers of solute
components in the solution divided by the sum of those at initial
condition. A similar amount of decrease is observed in molar
ratios of MAE and RT131, and a similar amount of production
occurs in RT129. The decrease observed in carbon balance
means that CO2 equimolal to the consumed oxazolidone,
RT131, is decarbonated during production in the dimer,
RT129, thus the carbon balance reaches 100% by addition of
the released CO2. This result supports the idea that the dimer of
each amine is generated through decarbonation condensation
between the amine and the oxazolidone generated from another
amine.
Therefore, thermal degradation of secondary alkanolamine
substrates in chemical CO2 solvents proceeds in the following
two steps as consecutive reactions (Figure 5). The first step is
that the carbamate compound, which is CO2-absorbing amine
(1), cyclodehydrates to generate oxazolidone. The second step is
that the oxazolidone reacts with another amine to generate dimer
through decarbonation condensation. These pathways are con-
sistent with the results shown in Figures 3a, 3b, and 3c.
The generation behaviors of each product shown in
Figures 3a, 3b, and 3c show that steric hindrance of the amine
molecule lowers total thermal degradation and the conversion
from oxazolidone to dimer. As the reason for these results, it is
suggested that the cyclodehydration and the decarbonation
condensation are prevented by the steric hindrance of amine
substrate. Besides, trace amounts of trimers have been observed
in the solutions of MAE and EAE. Each trimer is considered to
be generated through decarbonation condensation between each
dimer and the oxazolidone generated from each amine.
References and Notes
1
2
Y. Fijioka, Kagaku Kogaku 2007, 71, 747.
The chemical CO2 solvents have been developed in cooperation
with Nippon Steel & Sumitomo Metal Corporation in Japan as
parts of “Cost-Saving CO2 Capture System (COCS)” Project
(2001-2008) and “CO2 Ultimate Reduction in Steelmaking
Process by Innovative Technology for Cool Earth 50
(COURSE50)” Project (2008-); these projects have been promot-
ed under the initiative by the Japanese government. Supporting
Information is available electronically on the Japan Iron and Steel
Federation Web site, http://www.jisf.or.jp/course50/index.html.
S. Yamamoto, T. Higashii, Chem. Lett. 2013, 42, 532.
a) A. Bello, R. O. Idem, Ind. Eng. Chem. Res. 2005, 44, 945. b)
A. O. Lawal, R. O. Idem, Ind. Eng. Chem. Res. 2005, 44, 986. c)
J. Davis, G. Rochelle, Energy Procedia 2009, 1, 327. d) H.
Lepaumier, D. Picq, P.-L. Carrette, Ind. Eng. Chem. Res. 2009,
48, 9061.
3
4
5
Three secondary alkanolamines, 2-(methylamino)ethanol, 2-(eth-
ylamino)ethanol, and 2-(isopropylamino)ethanol, were employed
as each substrate of chemical CO2 solvents examined in this
study. CO2 solvent thermal stability tests were carried out with
chemical CO2 solvent stability test apparatus (CAT-DEG) and
heat-pressure-resistant autoclave. With CAT-DEG, each aqueous
solution of these amines (3 mol L¹1, 700 mL) was aerated with
¹1
CO2/N2 mixture (CO2: 20 vol %) at 500 mL min for 32 h. The
test temperature was set to 120 °C, and the test pressure was the
water vapor pressure at the test temperature plus one atm. A part
of each tested solution was sampled from CAT-DEG at 8-h
intervals. After the test with CAT-DEG, another part of each
tested solution was sampled into the heat-pressure-resistant
autoclave and then maintained at 120 °C for 96 h. Each sampled
solution was analyzed with gas chromatography-flame ionization
detector (GC-2010, SIMADZU), liquid chromatography-mass
spectrometer (LTQ Orbitrap Discovery, Thermo Fisher Scientif-
ic), gas chromatography-mass spectrometer (GCMS-QP2010
plus, SIMADZU).
This work was financially supported by the COURSE 50
project funded by New Energy and Industrial Technology
Development Organization, Japan.
6
Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
Chem. Lett. 2013, 42, 1559-1561
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/