NMR study of the composition of aqueous 2-aminoethanol solution used for absorption of carbon dioxide from fuel gases

Article abstract of DOI:10.1134/S1070428016070010

The compositions of aqueous 2-aminoethanol solutions used in industry for absorption of carbon dioxide resulting from combustion of natural gas have been determined by¹H and¹³C NMR spectroscopy. The absorption process does not involve generally accepted paths of thermal decomposition of the absorbent in the reaction with carbon dioxide, but the main path is non-oxidative decomposition of 2-aminoethanol into ammonia and ethylene oxide. Splitting of the NMR signals of carbamate anion formed by reaction of 2-aminoethanol with carbon dioxide has been rationalized by specific structure of the anion due to intramolecular hydrogen bonding.

Full text of DOI:10.1134/S1070428016070010

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2016, Vol. 52, No. 7, pp. 927–931. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © V.P. Talzi, 2016, published in Zhurnal Organicheskoi Khimii, 2016, Vol. 52, No. 7, pp. 935–939.
NMR Study of the Composition of Aqueous 2-Aminoethanol
Solution Used for Absorption of Carbon Dioxide
from Fuel Gases
V. P. Talzi
Institute of Hydrocarbon Processing Problems, Siberian Branch, Russian Academy of Sciences,
ul. Neftezavodskaya 54, Omsk, 644040 Russia
e-mail: vtalsi@ihcp.ru
Received October 7, 2015
Abstract—The compositions of aqueous 2-aminoethanol solutions used in industry for absorption of carbon
dioxide resulting from combustion of natural gas have been determined by 1H and 13C NMR spectroscopy. The
absorption process does not involve generally accepted paths of thermal decomposition of the absorbent in the
reaction with carbon dioxide, but the main path is non-oxidative decomposition of 2-aminoethanol into
ammonia and ethylene oxide. Splitting of the NMR signals of carbamate anion formed by reaction of 2-amino-
ethanol with carbon dioxide has been rationalized by specific structure of the anion due to intramolecular
hydrogen bonding.
DOI: 10.1134/S1070428016070010
Purification of gases from acid components such as
H2S, CO2, COS, CS2, and SO2 by absorption has been
used in industry over many decades. Aqueous
2-aminoethanol solutions are most frequently used as
absorbent. Decomposition of 2-aminoethanol in the
course of gas purification process is responsible for
its consumption, reduction of the capacity, and
foaming, as well as for accelerated corrosion of the
equipment [1, 2].
Decomposition of 2-aminoethanol during the ab-
sorption of carbon dioxide has been studied in detail
under both laboratory and industrial conditions. In
recent time, the importance of studies in this field is
justified by the global greenhouse gases reduction
problem and potential use of 2-aminoethanol for
binding CO2 [2, 3].
[1, 2] (Scheme 1). Equilibrium 3 shifts toward car-
bonate 3 as the pH increases, i.e., as the amount of
absorbed CO2 decreases [3]. In the absorbent regenera-
tion step (90–120°C) carbamate 1 decomposes into the
initial reactants, but this process is accompanied by
formation of impurities which reduce the absorption
efficiency.
The products of 2-aminoethanol decomposition and
their effect on the absorbent performance have been
studied in detail. Thermal and oxidative decomposition
paths of 2-aminoethanol are commonly considered
[2, 4, 5]. The major product of oxidative degradation
of 2-aminoethanol is ammonia. Various methods, in-
cluding NMR spectroscopy, revealed the following
acids in 2-aminoethanol solutions used for absorption
of carbon dioxide: formic acid (4), acetic acid (5),
glycolic acid (6), oxalic acid (7), 2-hydroxypropanoic
acid (8), and 2-aminopropanoic acid (9); the corre-
sponding N-(2-hydroxyethyl) amides were detected by
GC/MS [2, 6–8].
Saturation of an aqueous solution of 2-amino-
ethanol (~15%) with carbon dioxide below 50°C gives
mainly carbamate 1 which may be partially hydrolyzed
to a mixture of hydrogen carbonate 2 and carbonate 3
Scheme 1.
CO2
+
2H2NCH2CH2OH
HOCH2CH2NHCOO– H3NCH2CH2OH
1
1
HCO3 H3NCH2CH2OH + H2NCH2CH2OH
CO23– 2H3NCH2CH2OH
2
3
2
3
927

TALZI
928
HO
O
O
O
N
HN
H
N
N
N
HO
OH
O
OH
OH
10
11
12
More specific paths have also been proposed for
the transformation of 2-aminoethanol. One of these
involves carboxymethylation of 2-aminoethanol with
glycolic acid (6), which rationalizes formation of com-
pounds 10–12 (identified by both NMR and GC/MS in
amounts comparable with the amounts of 4–9 [6, 8].
The formation of 12 was also explained assuming
various redox reactions catalyzed by iron ions which
appear in the absorbent as a result of corrosion [6, 8].
of 1. As far as I know, this fact has never been
discussed. The observed signal splitting may be ac-
counted for by cyclic structure of carbamate ion due to
formation of intramolecular hydrogen bond. This in-
teresting structural feature of carbamic acid derivatives
has been postulated more than 40 years ago; however,
since that time it has not been confirmed directly by
experimental data.
Figures 1–3 show the 13C and H NMR spectra of
1
Relatively little attention has been given to
2-aminoethanol decomposition path involving its dis-
sociation into ammonia and ethylene oxide and subse-
quent reactions of the latter with water and 2-amino-
ethanol to give ethylene glycol (13) and 2,2′-imino-
diethanol (14) [8]. On the average, the concentration of
particular compounds 4–14 in the absorbent did not
exceed 0.1 wt %.
saturated and regenerated 2-aminoethanol solutions,
and 13C signal assignment for compounds 1–14 ac-
cording to [8, 9] is given in table. The spectra given in
Figs. 1 and 2 contain no signals assignable to com-
pounds 15–18 which were identified by 13C NMR in
2-aminoethanol solutions used for absorption of COS
and CS2 [9]. As found previously [8], compounds 13
and 14 were the major impurities in the absorbent.
Thus, the only path of non-oxidative degradation of
2-aminoethanol under the given conditions is its dis-
sociation into ammonia and ethylene oxide.
Thermal degradation paths 4–7 [1–7] (Scheme 2) so
far assumed to be the main ones were proposed as
a rule on the basis of only GC/MS data. As repeatedly
noted [8, 9], these paths are inconsistent with the NMR
data obtained under both laboratory and industrial
conditions. Compounds 15–18 were not detected by
13C NMR in a 2-aminoethanol solution used for ab-
sorption of CO2 in the absence of COS and CS2 [8, 9].
Two relatively strong signals at δC 48.57 and
59.54 ppm (Fig. 2) belong to piperazine 19 provided
that the remaining two less intense signals of the
2-hydroxyethyl substituent in 19, supposedly at δC 60
and 62 ppm (calculated values) are overlapped by
those of other components.
Over years, I analyzed compositions of saturated
and regenerated 2-aminoethanol solutions (10–15%)
from an industrial absorber in Omsk where they were
used to absorb carbon dioxide formed by methane
combustion. It was preliminarily concluded that reac-
tions 4–7 do not occur under the given conditions
[8, 9]. Herein, I report the results of analysis of the
NMR spectra of commercial absorbents, which con-
firm the above conclusions and explain characteristic
splitting of NMR signals belonging to carbamate anion
HN
N
OH
19
According to the GC/MS data, piperazine 19
(m/z 130 [M]+) is one of the most characteristic com-
ponents of the absorbent [9]. Presumably, compound
19 was previously erroneously identified as imidazoli-
Scheme 2.
O
O
H2O
HOCH2CH2NHCOO– H3NCH2CH2OH
O
+
HOCH2CH2NH2
HO
OH
5
N
H
N
4
N
H
1
H
15
16
H
H2O
OH
HN
N
N
15
+
H2NCH2CH2OH
+
CO2
H2N
OH
–H2O
7
6
O
17
18
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 7 2016

NMR STUDY OF THE COMPOSITION OF AQUEOUS 2-AMINOETHANOL SOLUTION
929
70
68
66
64
62
60
58
56
54
52
50
48
46
44
δC, ppm
185 180 175 170 165 160 δC, ppm
75
70
65
60
55
50
45
40
35
30
25
20
15
10
δC, ppm
Fig. 1. 13C NMR spectrum of a 12% aqueous solution of 2-aminoethanol saturated with CO2; UDEFT pulse sequence. For signal
assignment, see table.
(a)
(b)
(c)
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3.0
2.9
2.8
δ, ppm
Fig. 2. Fragments of the 1H NMR spectra of 2-aminoethanol solutions saturated with carbon dioxide by (a) 100, (b) 70, and (c) 40%.
Signal assignment in the spectrum of 3a, δ, ppm: 3.75 t and 2.90 t (1, cation); 3.15 and 3.16 d.t and 3.58 t (1, anion).
dinone 17 on the basis of the GC/MS data since these
compounds are difficult to distinguish by mass spectra.
Compound 19 is the reduction product of 12. Condi-
tions for the reduction of 12 are likely to be created as
a result of iron-catalyzed redox reactions during opera-
tion of absorbers.
The major signals at δC 59.85, 42.01 and 164.7,
61.70, 43.59 ppm in the 13C NMR spectrum (Fig. 1)
correspond to N-(2-hydroxyethyl)ammonium cation
and carbamate anion of 1 [8]. The signal at
δC ~163 ppm with an intensity comparable with that of
the signal of 1 at δC 164.7 ppm arises from HCO3– and
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 7 2016

TALZI
930
(a)
(b)
180
175
170
165
64
60
56
52
48
δC, ppm
(c)
(d)
180
175
170
165
64
60
56
52
48
δC, ppm
Fig. 3. 13C NMR spectrum of a 12% aqueous solution of 2-aminoethanol (a, b) saturated with CO2 and (c, d) after regeneration. For
signal assignment, see text and table.
Signal assignment in the 13C NMR spectra (Fig. 1) of
2-aminoethanol solutions used for absorption of carbon
dioxide under industrial conditions
CO32– ions of 2 and 3. Shift of equilibrium 1 upon
regeneration of the absorbent is reflected in the 1H and
13C NMR spectra by reduction of the intensity of
signals of the carbamate anion of 1 relative to the com-
mon signal of the cation and free amine, as well as by
considerable shift of the latter to the region typical of
the free amine (Figs. 2, 3). Simultaneously, the com-
mon signal of the anions of 2 and 3 is displaced down-
field, from the position typical of HCO3– (δC 163 ppm)
to the position typical of CO32– (δC 167 ppm), whereas
the signal of 1 does not change its position. Analogous
variations in the NMR spectra were observed while
studying CO2 absorption with a solution of NH3 [3].
δC, ppm
other carbons
Compound
no.
C=O
1a
165.0
162.2
171.1
182.5
179.9
173.4
180.7
176.5
61.74, 59.88, 43.59, 42.01
61.74, 42.01
2, 3a
4
5
23.63
61.61
6
7
1
8
68.75, 20.47
In the H NMR spectrum of 1, the NCH2 signal of
58.61, 17.29
the anion characteristically appears as a quartet formed
by two overlapped triplets (Fig. 2); however, this fact
was not rationalized in the literature. Furthermore, I
previously noted that the 13C NMR spectrum of 1 also
contains two NCH2 signals with equal or comparable
intensities (Fig. 1), but the observed splitting was
erroneously assigned to the presence of hydrogen car-
bonate 2 [8]. In fact, it is quite obvious that the NCH2
signal of 2 should be the same as the signal of 2-hy-
droxyethylammonium cation rather than of anion of 1.
The most common reason for splitting of NMR
signals in compounds structurally related to carbamate
1 is restricted rotation about the amide C–N bond.
However, taking into account published data for car-
bamates, thiocarbamates, and dithiocarbamtes derived
from a number of secondary amines [10], this factor
9
10b
11b
12b
13
14
15c
16c
17b, c
18b, c
60.62, 52.60, 50.62
59.29, 51.24, 49.91
58.42, 57.84, 55.36, 47.90, 40.13
62.88
59.88, 49.82
163.3
161.8
66.71, 40.52
62.41, 43.59
60.02, 47.03, 46.82, 38.34
60.30, 51.23, 50.75, 39.05
a
For saturated absorbent.
b
The intensity of the carbonyl carbon signal was lower than the
intensity of the corresponding methylene carbon signals; there-
fore, it was not detected at a given concentration.
c
Not detected; δC values were taken from [9].
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 7 2016

NMR STUDY OF THE COMPOSITION OF AQUEOUS 2-AMINOETHANOL SOLUTION
931
can be completely excluded. Separate signals of rota-
tional isomers were generally observed at low tem-
perature only for thiocarbamates in which the COS–
fragment is unsymmetrical.
In other cases, splitting of methylene proton signals
may be related to their diastereotopicity due to the
presence of a neighboring asymmetric center. Even
when the tetrahedral nitrogen atom of the anion of 1
were considered to be an asymmetric center, it would
be impossible to rationalize splitting of the NCH2
carbon signal of that anion.
There is one more, also well known, factor respon-
sible for NMR signal splitting, which implies the
existence of isomers with axial and equatorial orienta-
tion of alkyl substituent on the nitrogen atom in hetero-
cycles like N-methylaziridine [11].
EXPERIMENTAL
Samples of the absorbent (a 12% aqueous solution
of 2-aminoethanol) were withdrawn from the absorber
and desorber lines of a setup for carbon dioxide
production by combustion of natural gas.
1
The 13C and H NMR spectra were recorded on
a Bruker Avance 400 spectrometer equipped with
a PABBO 400S1 broad-band probe using D2O for
deuterium stabilization and as external standard;
Bruker pulse sequences ZG, JMODE, and UDEFT
were applied with a relaxation time of 6 s. Accumula-
tion over ~12 h was necessary to detect weak signals in
the 13C NMR spectra.
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bamic acid can be represented as cyclic structure with
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HO
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