Absorption - Introduction to Carbon Capture and Sequestration - page 193

characterized. Take, for example, a particular cation shown in Figure 5.3.5
with three different R-groups, and an anion with one R-group. Between
these two structures, you can make thousands of different ionic liquids
[5.10]. The question therefore becomes, which ionic liquid do we want to
use as a solvent for carbon capture? Figure 5.3.5 shows an example
from the literature [5.10] of subtle changes in these particular functional
groups. This example is related to solubility. We have the possibility of
varying CO 2 solubility in, and reactivity with, ILs based on their geometry.
In Box 5.3.1 we further illustrate how ILs can be chemically functionalized
to enhance the CO 2 solubility. However, at this stage we do not know
whether the increased solubility will indeed lead to a better carbon cap-
ture material [5.11].

Commercial solvents

At the beginning of this chapter we mentioned that there is nothing magi-
cal about carbon capture. If we would like to add a capture unit next to
our power plant, existing commercial processes are available. We will
briefl y touch on a few of these processes.

Box 5.3.1 Designing ionic liquids

Most ionic liquids (IL) selectively absorb CO 2 . This physical solubility, however, is limited.
One way of enhancing the solubility is to use an IL that can react with CO 2 . As amine
groups effi ciently react with CO 2 , an obvious choice is an amine-functionalized IL:

2 IL-NH 2 + CO 2 IL-NH 2 CO 2 + IL-NH 2 (1)

IL-NH 2 CO 2 + IL-NH 2 IL-NHCO 2 − + IL-NH 3 +

(2)

This is the typical carbamate chemistry that we have already seen in the case of aque-
ous amines. Ideally one would like to suppress step (2), the formation of the zwitterion,
so as to have an IL/CO 2 capture ratio of one. Ionic liquids present us with the interest-
ing option to place the amine group on either the cation (3) or the anion (4):

[IL-Cat -NH 2 ] + [IL-An] − + CO 2 [IL-Cat-NHCO 2 H] + [IL-An] −

(3)

[IL-Cat] + [IL-An-NH 2 ] − + CO 2 [IL-Cat] + [IL-An-NHCO 2 H] −

(4)

( Continued )