infinite number of hypothetical structures have been proposed so far [6]. However,
only a limited number (ca. 250) of these structures have been discovered or synthesized. Furthermore, an even more limited number (<10) of them are being used as
commercial adsorbents [7]. Zeolites can be broadly classified based on the size of
their cage windows or channels: those with the largest pore openings limited by rings
of 8, 10, 12, and more than 12 T-atoms are defined as small-, medium-, large-, and
extra-large-pore zeolites, respectively [8]. According to this classification, smallpore zeolites, especially the cage-based materials, have recently received much
attention due to their large pore volume and preferential guest molecule size exclusion, which enables them to show excellent performance in gas adsorption and
separation [9].
From the adsorption point of view, there are several factors that should be taken
into account: adsorption capacity, adsorption kinetics, selectivity, and regeneration
efficiency. It is now well established that such properties of zeolites can be tuned by
a variety of methods such as varying the framework Si/Al ratio from 1 to infinity,
incorporation of heteroatoms into the zeolite framework, and ion exchange,
affording them with a greater degree of framework flexibility, control of pore size,
increased acidity/basicity, temperature/water tolerance, etc. Adsorption of an adsorbate by a zeolite adsorbent can take place through either (1) the molecular sieving
effect, exclusion of certain components of a gas mixture based on size/shape; (2) the
thermodynamic equilibrium effect, preferential interactions between adsorbate and
adsorbent surface or adsorbate packing interactions; or (3) the kinetic effect, differences in the diffusion rate of various components of a gas mixture [10].
In consideration of the three possible adsorption mechanisms described above, a
zeolite can be synthesized to satisfy a more favorable adsorption mechanism. For
example, it may be preferable to separate CO 2 and NO x based on a thermodynamic
effect rather than on a molecular sieving effect. Depending on the adsorption
mechanism, separations are then performed using specific industry technologies,
such as pressure swing adsorption (PSA), vacuum swing adsorption (VSA), temperature swing adsorption (TSA), or a combination thereof. TSA involves the
adsorption of the adsorbate at a low temperature and then desorption at a higher
temperature, when the pressure is preferably kept constant at atmospheric pressure.
PSA includes the adsorption of the adsorbate at high pressure followed by desorption
at a lower pressure, where the lower pressure is preferably atmospheric pressure,
while the temperature is kept constant. VSA is similar to the PSA process, but the
difference is that the lower pressure is the application of vacuum, which allows the
adsorbate to desorb more effectively.
Figure 1 compares the adsorption isotherms and potential working capacities of
these three processes. It can be seen that the working capacity depends strongly on
the shape of the adsorption isotherm. If we consider the example of the TSA process,
application of the PSA process will lead to a large decrease in working capacity
based purely on the shape of the isotherm. In contrast, VSA application to either the
PSA or TSA example will lead to an increase in working capacity, but at a large
energy cost. This chapter aims to introduce adsorption/separation using small-pore
zeolites, as well as some of their interesting synthesis approaches. We have grouped
Small Gas Adsorption and Separation in Small-Pore Zeolites
3
only a limited number (ca. 250) of these structures have been discovered or synthesized. Furthermore, an even more limited number (<10) of them are being used as
commercial adsorbents [7]. Zeolites can be broadly classified based on the size of
their cage windows or channels: those with the largest pore openings limited by rings
of 8, 10, 12, and more than 12 T-atoms are defined as small-, medium-, large-, and
extra-large-pore zeolites, respectively [8]. According to this classification, smallpore zeolites, especially the cage-based materials, have recently received much
attention due to their large pore volume and preferential guest molecule size exclusion, which enables them to show excellent performance in gas adsorption and
separation [9].
From the adsorption point of view, there are several factors that should be taken
into account: adsorption capacity, adsorption kinetics, selectivity, and regeneration
efficiency. It is now well established that such properties of zeolites can be tuned by
a variety of methods such as varying the framework Si/Al ratio from 1 to infinity,
incorporation of heteroatoms into the zeolite framework, and ion exchange,
affording them with a greater degree of framework flexibility, control of pore size,
increased acidity/basicity, temperature/water tolerance, etc. Adsorption of an adsorbate by a zeolite adsorbent can take place through either (1) the molecular sieving
effect, exclusion of certain components of a gas mixture based on size/shape; (2) the
thermodynamic equilibrium effect, preferential interactions between adsorbate and
adsorbent surface or adsorbate packing interactions; or (3) the kinetic effect, differences in the diffusion rate of various components of a gas mixture [10].
In consideration of the three possible adsorption mechanisms described above, a
zeolite can be synthesized to satisfy a more favorable adsorption mechanism. For
example, it may be preferable to separate CO 2 and NO x based on a thermodynamic
effect rather than on a molecular sieving effect. Depending on the adsorption
mechanism, separations are then performed using specific industry technologies,
such as pressure swing adsorption (PSA), vacuum swing adsorption (VSA), temperature swing adsorption (TSA), or a combination thereof. TSA involves the
adsorption of the adsorbate at a low temperature and then desorption at a higher
temperature, when the pressure is preferably kept constant at atmospheric pressure.
PSA includes the adsorption of the adsorbate at high pressure followed by desorption
at a lower pressure, where the lower pressure is preferably atmospheric pressure,
while the temperature is kept constant. VSA is similar to the PSA process, but the
difference is that the lower pressure is the application of vacuum, which allows the
adsorbate to desorb more effectively.
Figure 1 compares the adsorption isotherms and potential working capacities of
these three processes. It can be seen that the working capacity depends strongly on
the shape of the adsorption isotherm. If we consider the example of the TSA process,
application of the PSA process will lead to a large decrease in working capacity
based purely on the shape of the isotherm. In contrast, VSA application to either the
PSA or TSA example will lead to an increase in working capacity, but at a large
energy cost. This chapter aims to introduce adsorption/separation using small-pore
zeolites, as well as some of their interesting synthesis approaches. We have grouped
Small Gas Adsorption and Separation in Small-Pore Zeolites
3