various contaminants (e.g., SO x , NO x , etc.) while having a relatively low production
cost, especially compared to MOFs [18–20].
The CO 2 uptake of zeolites is strongly influenced by the framework structure and
composition, as well as by the extraframework cation composition and location, i.e.,
thermodynamic effects. Also, their CO 2 separation capability increases with increasing electrostatic field in zeolite pores, which is mostly affected by the nature of the
charge-compensating cations. Among the various families of zeolites classified
above, small-pore materials have received much attention as effective and highly
selective adsorbents for CO 2 capture from flue gas (CO 2 /N 2 ) and natural gas (CO 2 /
CH 4 ), due to the molecular sieving effect arising from the tunable pore architecture
of adsorbents and the kinetic diameters of adsorbates (CO 2 , 3.3 Å; N 2 , 3.64 Å; CH 4 ,
3.8 Å). Thus, it is not difficult to see that small-pore zeolites have higher selectivity
for CO 2 separation than medium- and larger-pore ones.
3.1 LTA-Type Zeolites
The pore size of LTA-type zeolites can vary between 3 and 5 Å, depending on the
type of the extraframework cation occluded. For example, Na-A (4A; Si/Al ¼ 1) has
a pore window size of 3.8 Å, but this pore size can be tuned by ion exchanging the
Na
+ ions in Na-A with K
+ (K-A; 3 Å) or Ca
2+ (Ca-A; 5 Å), respectively, yielding
notable differences in the CO 2 adsorption capacity and selectivity. Hedin and
co-workers have measured the CO 2 and N 2 uptakes of a series of NaK-A (LTA)
zeolites with different Na/K ratios and found that an optimized sample with
Na/K ¼ 4.9 adsorbs a significant quantity of CO 2 but almost no N 2 [21]. As a result,
its CO 2 /N 2 selectivity (172) was much higher than the value (12) of Na-A, although
the CO 2 uptakes (3.4 vs 3.9 mmol g
À1 ) of these two materials are not quite different
from each other. They observed a quite similar trend in the separation of CO 2 from
CH 4 , when the Na/K ratio of NaK-A zeolites was varied between 2.8 Na/K 5.7
[22]. To estimate the effect of the Si/Al ratio on their CO 2 adsorption properties, on
the other hand, Palomino et al. have synthesized five LTA zeolites with different
Si/Al ratios (1.0 - 1) [23]. They found that the CO 2 adsorption capacity of LTA
zeolites increases as the Al content increases, reaching a maximum of 5.0 mmol g
À1
at Si/Al ¼ 2.0, 1.0 bar, and 303 K. These authors also showed that the CO 2 /CH 4
selectivity decreases with increasing Si content in the low pressure range. Therefore,
it is likely that CO 2 adsorption requires a zeolite adsorbent with an optimal negative
framework charge.
The Hedin group also examined the CO 2 capacity and CO 2 /N 2 selectivity of
different cation forms of ZK-4 with Si/Al ¼ 1.3 and showed that Na-ZK-4 exhibits a
CO 2 capacity of 4.85 mmol g
À1 at 273 K and 1.0 bar [24]. When approximately 26%
of its Na
+ ions were exchanged for K
+
, the resulting NaK-ZK-4 material was found
to still adsorb a large amount of CO 2 , but a negligible amount of N 2 (4.35 vs
<0.03 mmol g
À1 at 273 K and 1.0 bar). Hedin and co-workers further demonstrated
that the multiple cation form of zeolite A (i.e., Na 10.2 K 1.0 Cs 0.8 A) shows very
6
K. C. Kemp et al.
cost, especially compared to MOFs [18–20].
The CO 2 uptake of zeolites is strongly influenced by the framework structure and
composition, as well as by the extraframework cation composition and location, i.e.,
thermodynamic effects. Also, their CO 2 separation capability increases with increasing electrostatic field in zeolite pores, which is mostly affected by the nature of the
charge-compensating cations. Among the various families of zeolites classified
above, small-pore materials have received much attention as effective and highly
selective adsorbents for CO 2 capture from flue gas (CO 2 /N 2 ) and natural gas (CO 2 /
CH 4 ), due to the molecular sieving effect arising from the tunable pore architecture
of adsorbents and the kinetic diameters of adsorbates (CO 2 , 3.3 Å; N 2 , 3.64 Å; CH 4 ,
3.8 Å). Thus, it is not difficult to see that small-pore zeolites have higher selectivity
for CO 2 separation than medium- and larger-pore ones.
3.1 LTA-Type Zeolites
The pore size of LTA-type zeolites can vary between 3 and 5 Å, depending on the
type of the extraframework cation occluded. For example, Na-A (4A; Si/Al ¼ 1) has
a pore window size of 3.8 Å, but this pore size can be tuned by ion exchanging the
Na
+ ions in Na-A with K
+ (K-A; 3 Å) or Ca
2+ (Ca-A; 5 Å), respectively, yielding
notable differences in the CO 2 adsorption capacity and selectivity. Hedin and
co-workers have measured the CO 2 and N 2 uptakes of a series of NaK-A (LTA)
zeolites with different Na/K ratios and found that an optimized sample with
Na/K ¼ 4.9 adsorbs a significant quantity of CO 2 but almost no N 2 [21]. As a result,
its CO 2 /N 2 selectivity (172) was much higher than the value (12) of Na-A, although
the CO 2 uptakes (3.4 vs 3.9 mmol g
À1 ) of these two materials are not quite different
from each other. They observed a quite similar trend in the separation of CO 2 from
CH 4 , when the Na/K ratio of NaK-A zeolites was varied between 2.8 Na/K 5.7
[22]. To estimate the effect of the Si/Al ratio on their CO 2 adsorption properties, on
the other hand, Palomino et al. have synthesized five LTA zeolites with different
Si/Al ratios (1.0 - 1) [23]. They found that the CO 2 adsorption capacity of LTA
zeolites increases as the Al content increases, reaching a maximum of 5.0 mmol g
À1
at Si/Al ¼ 2.0, 1.0 bar, and 303 K. These authors also showed that the CO 2 /CH 4
selectivity decreases with increasing Si content in the low pressure range. Therefore,
it is likely that CO 2 adsorption requires a zeolite adsorbent with an optimal negative
framework charge.
The Hedin group also examined the CO 2 capacity and CO 2 /N 2 selectivity of
different cation forms of ZK-4 with Si/Al ¼ 1.3 and showed that Na-ZK-4 exhibits a
CO 2 capacity of 4.85 mmol g
À1 at 273 K and 1.0 bar [24]. When approximately 26%
of its Na
+ ions were exchanged for K
+
, the resulting NaK-ZK-4 material was found
to still adsorb a large amount of CO 2 , but a negligible amount of N 2 (4.35 vs
<0.03 mmol g
À1 at 273 K and 1.0 bar). Hedin and co-workers further demonstrated
that the multiple cation form of zeolite A (i.e., Na 10.2 K 1.0 Cs 0.8 A) shows very
6
K. C. Kemp et al.