Adsorption - Introduction to Carbon Capture and Sequestration - page 263

Chemisorbents

Metal oxides

Materials that have basic groups are a natural choice for CO 2 capture, as
we know from our analysis of absorption. Examples of “basic” materials
include alkaline metal oxides (Na 2 O, K 2 O) and alkaline earth metal oxides
(CaO, MgO) [6.20]. Calcium minerals are the most abundant among alka-
line earth metal oxides. Limestone or dolomite are commonly found
mineral forms of calcium carbonate.
These materials interact with CO 2 , following the reaction:

() () ()

MO s + CO g ' MCO s

2 3

Heating at high temperatures (>900 K) removes the CO 2 and regenerates
the metal oxide [6.21]. However, the kinetics of this reaction are slower
than for zeolites or activated carbon. One interesting research direction
involves additives or different metals altogether. For these materials, the
energy for regeneration is generally larger compared to that for the phy-
sisorbed materials. This effect may be partially mitigated because at the
higher regeneration temperatures, recovery of some of the process heat
becomes easier [6.22].

Amines on supports

We have seen that amine solutions are promising solvents for absorption
because they selectively bind the CO 2 (see the previous chapter). Another
popular strategy for CO 2 capture is to develop materials in which these
amine groups are immobilized on a solid support. One example of such
materials would be amine-impregnated silicas, such as the mesoporous
molecular sieve MCM-41. The CO 2 adsorption capacity is signifi cantly
increased if the material is loaded with branched polyethylenimine [6.23].
In these materials the amines are physically adsorbed, which may limit
the conditions in which the material can be regenerated. An alternative
strategy is to covalently bind these amines to the silica support, which
prevents leaching of amines. These materials are called covalently teth-
ered amine adsorbents [6.24, 6.25].
Amines can also be bound to organic supports. Examples of such
materials include carbon-supported amines [6.26], polymer-supported
amines [6.27], and solid resins [6.28].