Adsorption - Introduction to Carbon Capture and Sequestration - page 255

component of some laundry powders, where their role is to soften water.
A class of zeolites can be synthesized such that some of the Si 4+ is
replaced by Al 3+ and the charge is compensated by a cation, e.g., Na + .
Competitive adsorption makes Ca 2+ ions adsorb much more strongly
than Na + ions, which makes these zeolites very effective in reducing the
water hardness; the calcium in the water is adsorbed in the pores of the
material and washed away in the rinse cycle. Other important applica-
tions include catalysis in petrochemical applications and gas separa-
tions, where one uses the difference in adsorption in these materials to
selectively retain one gas over another.
So now the question we would like to ask is what is the best zeolite
for carbon capture? For this purpose, we consider materials composed
of pure SiO 2 . This implies that every material has the same chemical
composition and only the pore topology is changing. Hence, the question
we are asking is what is the optimal pore topology for carbon capture?
We note that in addition to the known zeolite structures, we should
include in our screening process the database of all computationally
predicted zeolite structures [6.6, 6.7].
The diffi culty with such a screening study is that we need the experi-
mental adsorption isotherms to compute the parasitic energy.
Unfortunately, the corresponding CO 2 and N 2 isotherms have been meas-
ured only for a very small number of zeolites. This number is too small to
enable any sensible screening, but it is suffi cient to develop a quantita-
tive model that describes the interactions between gas molecules and
zeolites. This model can then be used in a molecular simulation to predict
the isotherms of all gasses. These simulations give a very reasonable
description of the experimental isotherms (see Figure 6.4.2 ). As we can
use the same model to predict the isotherms in a material with a different
crystal structure, we can use these molecular simulations to predict the
mixture adsorption isotherms, and for each of these materials we can
estimate the optimal parasitic energy. Figure 6.4.3 shows the optimized
parasitic energy as a function of the CO 2 Henry coeffi cient for all known
zeolite structures. Figure 6.4.4 shows some of the structures that have
an optimal parasitic energy. Figure 6.4.3 also shows the parasitic energy
for a different class of materials: Zeolitic Imidazolate Frameworks
(ZIFs). ZIFs are a special kind of Metal Organic Frameworks (see
Section 6.5) in which the size of the linker and angle between linkers are
tuned in such a way that they mimic the Si—O—Si bond in zeolites and
hence have similar pore topologies to zeolites.