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O
R'
(a)
R'
C
N
+
H
2
+
N
HO
R
carbamate
R
O
R'
H
2
O
+
2
C
HN
R
R'
O
R
O
+
+
N
+
H
R'
HN
C
HO
O
R
H
bicarbonate
(b)
O
R
R'
O
+
H
2
O
+
2
+
C
R
N
R'
N
+
H
C
HO
O
R''
O
R''
Figure 5.3.4
General reaction schemes for the chemical absorption of CO
2
Reaction scheme by (a) primary or secondary and (b) tertiary amine-containing solvents.
More complete reactions schemes can be found in the literature.
formation of a carbamate bond between the carbon of the CO
2
molecule
and the nitrogen atom of the amine [5.4, 5.7]. A fraction of the formed
carbamate species is subsequently hydrolyzed to form bicarbonates. As
a result, the CO
2
loading capacity for primary and secondary amines lies
in the range 0.5-1 moles of CO
2
per mole of amine. For tertiary amines,
the additional R-group gets in the way of the carbamation reaction
(which does not proceed), giving a base-catalyzed hydration of CO
2
to
Question 5.3.2 Concentration of MEA
We measure that 1 molar MEA solution has a pH of approximately 11.7. What
is the OH
-
concentration in the solution? What is the percentage of MEA pre-
sent in its un-ionized (i.e., molecular) form?
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