Absorption - Introduction to Carbon Capture and Sequestration - page 191

Enzymes
Now we know that tertiary amines or binary amines with steric hindrance
can uptake more CO 2 than other amines. The disadvantage of these
molecules is that they are much slower to react with CO 2 . The reaction
can be so slow, in fact, that it limits the transfer of CO 2 from the gas
phase to the liquid phase. Additionally, the dissolution can be limited by
the formation of bicarbonate (see Figure 5.3.4 ).
In nature, the enzyme carbonic anhydrase catalyzes the reaction of
carbon dioxide and water to bicarbonate and protons, and is very adept
at helping with CO 2 uptake. This reaction occurs with astonishingly fast
rates in our tissues every time we exhale. Carbonic anhydrase is part of
a family of enzymes that are classifi ed as metalloenzymes due the fact
that they contain a zinc atom at their center. The mechanism of the cata-
lytic process of carbonic anhydrase enzymes is well known and it works
very effi ciently. One idea is that we could use a similar enzyme to
increase the uptake of CO 2 in amine solutions that have an intrinsically
low reaction rate. From a research point of view this is a very interesting
topic, but the natural enzyme has a very poor stability under the harsh
conditions of the carbon capture processes. The temperatures range
from 40º to over 120ºC, and high concentrations of organic amine and
trace contaminants of fl ue gasses are not typical of a normal biological
environment. To make enzymes work for carbon capture, researchers are
studying the carbonic anhydrases that can be found in organisms that
live in very harsh conditions, as well as looking at protein engineering
techniques to create thermo-tolerant enzymes [5.8].
Ionic liquids
So far we have focused on changing the properties of water in order to
improve the uptake of CO 2 . A different strategy is to replace the water
with a completely different solvent. In this context, there is a great deal
of interest in ionic liquids (IL) [5.9]. Recall that ions have either a negative
(anion) or positive (cation) charge, and that positive and negatively
charged ions are attracted to each other. When they come together, ions
form solid crystalline salts. These salts are very stable — to melt sodium
chloride one has to heat the crystals to 801ºC. In an ionic liquid, how-
ever, the cations and anions are both large, asymmetric, bulky mole-
cules, preventing them from forming the crystalline lattice of a typical