This two-step mechanism, originally proposed by Caplow [2] and later reintroduced by Danckwerts [3], suggests that CO2 forms a bond to the amine functionality in the first step. In the second step an amine-proton is transferred to a second molecule. In Caplow’s article the second molecule was water, but this can be any base-molecule. The intermediate species in
the reaction is a zwitterion. Caplow [2] assumed (as shown in Figure 2.3) that a hydrogen bond is formed between the amine and a water molecule before the amine reacts with the CO2 molecule. This feature has however been omitted in the later published literature, as can be seen in the work by Danckwerts [3], Versteeg et al. [4] and Kumar et al. [5]. So, according to this mechanism reaction between CO2 and the amine (denoted here as Am) proceeds through the formation of a zwitterion as an intermediate:
2
2 1
CO Am k Am COO
k−
+ −
+ YZZZZZZX (2.1)
This zwitterion undergoes deprotonation by a base (or bases) b, thereby resulting in carbamate formation:
Am COO+ −+ ⎯⎯→b kb AmCOO−+bH+ (2.2)
The base, b, can be an amine, OH- or H2O although the contribution of OH- can be neglected as its concentration is very low compared with those of amine and H2O [6].
Applying the steady-state principle to the intermediate zwitterion, the rate of reaction of CO2 in aqueous solutions can be expressed as:
[ ]
2 2
b 1
[CO ][Am]
1 1
b r k
k k−
=
+
∑
⎛⎜⎝ ⎞⎟⎠(2.3)
The derivation of Eq. (2.3) is given in Appendix I. For two asymptotic situations Eq. (2.3) may be simplified as follows:
i. The termk−1
(
kb[ ]
b)
1. This results in simple second order kinetics indicating that zwitterion deprotonation reaction is fast in comparison with the reversion rate of CO2 and amine. Then Eq. (2.3) is simplified to:[ ][ ]
2 CO2 Am
r=k (2.4)
ii. The termk−1
(
kb[ ]
b)
1. This results in a more complex expressions for the kinetics:[ ][ ] [ ] ( )
2 2 b
1
CO Am b
k k
r = k− (2.5)
It can be seen from Eq. (2.5) that the overall reaction order is three if the contribution of water towards the deprotonation of the zwitterion is neglected. In the transition region between the
two asymptotic cases the reaction order changes between two and three. In the limiting case when the contribution of amine to zwitterion deprotonation is much more significant than that of other bases, such as H2O and OH–, the overall reaction is of the second order with respect to amine. The kinetics of the reaction of CO2 with monoethanolamine (MEA) has been widely studied and adequately described by zwitterionic mechanism [4, 7, 8]. The reaction is of the first order with respect to both CO2 and MEA in aqueous systems thereby suggesting that zwitterion deprotonation is instantaneous. Another primary amine, Diglycolamine® (DGA), was recently studied by Al-Juaied and Rochelle [9]. Previous studies on the absorption of CO2
by an aqueous DGA solution, which are outlined by Versteeg et al. [4], suggest that the overall reaction order has a value of two. Diethanolamine (DEA) is the most popular secondary alkanolamine used for CO2 removal. The literature on the absorption of CO2 by an aqueous DEA solution, which exhibits complex kinetic behavior, was reviewed earlier [10].
Depending on the rate-limiting step (zwitterion formation or deprotonation) and the amine concentration, the reaction order with respect to DEA lies between one and two. The kinetics of the reaction of CO2 with another secondary amine, diisopropanolamine (DIPA), in aqueous solutions was studied by Camacho et al. [11]. The reaction was found to be of the second order with respect to DIPA, and hence, totally of the third order.
If the base, b, in the reaction described by Eq. (2.2) is the amine itself, the carbamate formation can be represented as follows:
Am COO+ −+Am→AmCOO−+AmH+ (2.6)
In this case, the overall reaction, which accounts for carbamate formation in a solution, is given by the sum of reactions represented by Eqs. (2.1) and (2.6):
+
CO2+2AmUAmCOO−+AmH (2.7)
If the amine is sterically hindered, the zwitterion reacts more easily with water than with Am and bicarbonate formation takes place:
- +
2 3
Am COO+ −+H O→HCO +AmH (2.8)
In this case, the reaction, which accounts for bicarbonate formation, is given by the sum of reactions represented by Eqs. (2.1) and (2.8):
- +
2 2 3
CO +Am+H OUHCO +AmH (2.9)
Due to the low stability, the carbamates of sterically hindered amines may also readily
undergo hydrolysis, forming bicarbonates and releasing free amine molecules. This can be represented as:
-
2 3
AmCOO−+H O→HCO +Am (2.10)
These free amine molecules will again react with CO2. Thus, bicarbonate ions will be present in larger amounts than carbamate ions. However, a certain amount of carbamate hydrolysis (reaction (2.10)) occurs with all amines, particularly at high pressures [1]. The carbamate stability constants at 313 K for aqueous MEA, DEA and AMP have been measured by 13C NMR [12]. These are presented in Table 2.1. Substantial differences in carbamate stability between hindered and conventional amines are evident from the values of KC presented in Table 2.1.
A discussion of CO2 absorption into aqueous amine solutions is not complete without including the reactions of water and its dissociation products with the gases and the amines.
The most important reaction in aqueous chemistry is the water dissociation reaction.
+ -
H O2 UH +OH (2.11)
The following reactions may also take place simultaneously in an aqueous amine solution:
+ -
Am+H O2 UAmH +OH (2.12)
- +
2 2 3
CO +H OUHCO +H (2.13)
- -
2 3
CO +OH UHCO (2.14)
The reaction (2.13) is very slow (k = 0.026 s-1 at 298 K) [13] and may usually be neglected [14]. The reaction (2.14) has a large influence on the overall reaction rate even when the concentration of hydroxyl ion is low. The forward reaction rate for reaction (2.14) can be described as [13]:
- -
2
* -
CO -OH OH[CO ][OH ]2
r =k (2.15)
( )
* -10 OH
log k 13.635 2895
= − T (2.16)
The hydroxyl ion concentration can be estimated from the relations given by Astarita et al.
[15].
[ ]
- 3
3
[OH ] 1 , 10
Am , 10
W P
W P
K K
K K
αα α α
−
−
⎛ − ⎞
= ⎜⎝ ⎟⎠ ≥
= ≤
(2.17)
KW and KP are the water and amine protonation constants, respectively.
The total rate of all CO2 reactions in an aqueous solution is given by the sum of the reaction rates given by Eqs. (2.15) and (2.3):
[ ]
-
* -
2 2
ov OH 2
1
[CO ][Am]
[CO ][OH ] 1 1
b
b
r k k
k k−
= +
+
∑
⎛⎜⎝ ⎞⎟⎠(2.18)
[ ]
ov ov CO2
r =k (2.19)
kov denotes the observed overall reaction rate constant which can be measured and is given by:
[ ]
-
* -
2
ov OH
b 1
[Am] [OH ]
1 1
b
k k k
k k−
= +
+⎛⎜⎝ ⎞⎟⎠
(2.20)
The apparent reaction rate constant (kapp), which is used for the analysis of experimental data, is given by:
[ ]
2 app
b 1
[Am]
1 1
b k k
k k−
=
+⎛⎜⎝ ⎞⎟⎠
(2.21)
kapp can be obtained from kov as follows:
-
* -
app ov OH[OH ]
k =k −k (2.22)