Absorption - Introduction to Carbon Capture and Sequestration - page 194

Box 5.3.1 ( Continued )

Because the anion possesses a negative charge, it has a higher affi nity for protons,
and thus is much less willing to give up a proton for a reaction of the type shown in (2).
Schneider, Brennecke and co-workers used this idea to synthesize amino acid based
ILs with the amine functional group on the anion [5.11]. The fi gure shows two examples
of these ILs with methioninate (Met, top structure) and prolinate (Pro, bottom structure)
as amino acid-based anions and the structure known as P 66614 for the cation.

The absorption isotherms confi rm that a 1:1 chemistry can indeed be obtained.

1.0

0.8

0.6

[P 66614 ][Met]
[P 66614 ][Met] React-IR
[P 66614 ][Met] calc.
[P 66614 ][Pro]
[P 66614 ][Pro]-React-IR
[P 66614 ][Met] calc.

+

0.4

0.2

0.0

0.0 0.2 0.4 0.6 0.8 1.0

Pressure (bar)

An important advantage of liquid absorption processes is that it is easy to trans-
port fl uids. However, if the viscosity of a fl uid gets too high it will become increasingly
diffi cult to pump. In practice one would like to ensure that the viscosity is below
100-200 centi-Poise (cP). One of the challenges of IL is that the viscosity increases
once CO 2 is absorbed [5.9]. The reason for this viscosity increase is the formation of
a hydrogen bonded network within the ionic liquid. By using steric hinderance
Brennecke and co-workers were able to develop ILs in which the formation of such a
network is suppressed [5.11]. Figures reprinted with permission from Brennecke and
Gurkan [5.9]. Copyright (2010) American Chemical Society .