Carbon Dioxide Capture by Amines: Increasing the Efficiency by ...

CARBON DIOXIDE CAPTURE BY AMINES:
INCREASING THE EFFICIENCY BY AMINE
STRUCTURE MODIFICATION
Angela Dibenedetto*, Michele Aresta and Marcella Narracci
University of Bari and METEA Research Center
Department of Chemistry, Campus Universitario, 70126 - Bari, Italy,
fax +39 080 544 2083, e-mail: a.dibenedetto@chimica.uniba.it
Introduction
Carbon dioxide is the largest contributor to the greenhouse
effect, being produced in the combustion of fossil fuels as oil, natural
gas or coal, for energy production or heating, and in industrial
processes.
Carbon dioxide can be recovered by using several technologies
such as the absorption into an appropriate solution 1 or the membrane
separation. 2 The former is used in several industrial applications such
as CO2 separation from power plants flue gases, natural gas treatment
and landfill gas upgrading. The capacity of the amine to uptake/bind
CO2 is of fundamental importance for sizing the separation tower.
In this paper we consider the advantage of using new amines for
capturing carbon dioxide from gas mixtures. The kinetics of CO2
uptake using four new different amines is compared with that of
absorption by MEA.
Experimental
All data were recorded under the same conditions: 1 atm of CO2 and
298 K. The kinetics was followed in tetrahydrofuran (THF) as
solvent. 8 mL of THF were saturated with CO2 in the absorption
flask, then the amine (0.90 mmol) was added. The uptake of CO2 was
followed using a gas burette.
Neat amines have been also used.
Results and Discussion
Aliphatic amines are known to react promptly with CO2 [Eq 1] to
afford an ammonium carbamate. 3
2 RNH2 + CO2 RNH3 +- O2CNHR
(1)
We have investigated the behaviour of mono- (A, B, C) and di-amines
(D, E) 4 towards CO2 at various temperatures. Mono-amines A, B and
MEA (C) react rapidly with CO2 at 298 K in THF. Figure 1a-b-c
show that the reaction is almost complete in 15 minutes and the
amount of CO2 taken up by the amine (ca. 0.55 mol/mol amine) says
that one mol of CO2 reacts with two moles of amine, as required by
the stoichiometry of reaction 1, to afford ammonium carbamates.
mol CO2/mol amine
0,6
0,5
0,4
0,3
0,2
0,1
0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Time (min)
amine A
amine B
MEA
Figure 1. Kinetics of reactions of mono-amines with carbon dioxide
Fuel Chemistry Division Preprints 2002, 47(1), 53
Amines A and B are as performing as MEA. In all cases, the solution
at the end of the reaction is cloudy due to a fine suspension of a white
solid. Interestingly, if the reaction is carried out at 273 K, the amount
of CO2 fixed by amines A and B, after a fast initial uptake, of ca. 0.50
mol/mol, increases slowly to 1 mol/mol within three hours. This
feature can be explained assuming that at low temperature any of the
ammonium carbamates RNHCOO -+ H3NR slowly converts into the
dimeric form of the relevant carbamic (RNHCOOH)2 acid. We have
already shown that, in the same conditions, the carbamate/carbamic
acid conversion takes place with benzylamine 5 and other Ncompounds.
(Eq.2) In the case of R=benzyl, the dimeric carbamic acid
has been isolated and characterized by X-ray 5a .
RNHCOO -+ H3NR + CO2 → (RNHCOOH)2
R = Benzyl (2)
Carbamates of amines (A and B) do not release CO2 easily upon
heating. Therefore, they are not good candidates for the separation of
carbon dioxide from other gases (e.g., from flue gases) as they can not
advantageously operate in a cyclic system implying the uptake and
release of carbon dioxide.
When diamine D or E is used, the CO2 uptake curve (Fig. 2) at 298 K
has a different shape with respect to that characteristic of
monoamines.
mol CO2/mol amine
1
0,8
0,6
0,4
0,2
0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Time (min)
amine D
amine E
Figure 2. Kinetics of reactions of di-amines with carbon dioxide
Diamines D, E take up CO2 with a 0.8-1 molar ratio, to afford a
glassy material that occludes solvent. The reaction quickly takes place
also in absence of solvent. The resulting carbamates have been
isolated and characterised. Diamines are thus by far more efficient
than monoamines for CO2-capturing. This aspect is quite important as
by using diamines it would be possible to reduce to half the amount of
amine necessary for the separation of a given amount of CO2, and thus
the volume of the absorption tower. Interestingly, the carbamates of
diamines easily release carbon dioxide at moderate temperatures,
making such systems quite interesting candidates for CO2 separation.
We are now working at some improvement of the amine
molecular structure in order to further increase the separation
efficiency.
Acknowledgement. This work was supported by MURST,
Project no. 9803026360 and MM03027791
References
(1) Rangwala H.A. J. Memb. Science 1986, 112, 229.
(2) RUCADI Project, BRRT-CT98-5089.
(3) a) Aresta, M. and Quaranta, E. Tetrahedron, 1991, 47, 9489; b) Aresta,
M. and Quaranta, E. Ital. Pat. 1237207, 1993.

(4) The structure of the amines A, B, C, D will be discussed at the symposium.
(5) a) Aresta, M.; Ballivet-Tkatchenko, D.; Belli Dell’Amico, D.; Bonnet, M.
C.; Boschi, D.; Calderazzo, F.; Faure, R.; Labella, L. and Marchetti, F.
Chem. Commun. 2000, 1099; b) Aresta, M.; Ballivet-Tkatchenko, D.;
Bonnet, M. C.; Faure, R.; and Loiseleur, H. J. Am. Chem. Soc. 1985, 107,
2994
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