polarizing power and can produce a higher local electrostatic field and field gradient
but also because this zeolite has a slightly higher effective micropore volume than its
other cation forms [37]. The Lobo group also evaluated the CO 2 capture and
separation ability of ZK-5 in simulated PSA/VSA processes. Mg-ZK-5 was shown
to be the most promising adsorbent for PSA applications with the highest working
capacity (△CO 2 ¼ 2.1 mmol g
À1 ) and excellent selectivity (121) over N 2 . While for
the VSA process, Li-, Na-, and K-ZK-5 zeolites were found to have impressive
working capacities (△CO 2 ¼ 1.6–2.2 mmol g
À1 ) and excellent selectivities
(103–128) [37]. In another study the same group reported that Li-ZK-5 with
Si/Al ¼ 1.7 shows an improved adsorption capacity of 4.8 mmol g
À1 at 303 K and
40 bar. This indicates that a decrease in adsorption capacity with increasing Si/Al
ratio was caused by an increase in extraframework content, which in turn leads to a
smaller accessible pore volume [38].
3.4 The RHO Family of Embedded Isoreticular Zeolites
In 2015, Zou, Wright, and our group discovered a novel zeolite family with
increasing structural complexity and embedded isoreticular structures, denoted the
RHO family (Fig. 4) [39–42]. This zeolite family starts from rho (RHO) consisting
of 10-hedral ([4
8 8
2 ]) double 8-rings (d8rs) and 26-hedral ([4
12 6
8 8
6 ]) lta cages and
expands as follows: (1) the scaffolds are extended by inserting an extra pair of d8r
and 18-hedral ([4
12 8
6 ]) pau cages between the lta cages along each unit-cell edge,
which increases the isoreticular dimension by approximately 10 Å per generation
and (2) the space between the scaffolds is completely filled up with the four types of
embedded cages, i.e., 14-hedral([4
6 6
2 8
6 ]) t-plg, 8-hedral ([4
5 8
3 ]) t-oto, 10-hedral
([4
6 8
4 ]) t-gsm, and 12-hedral ([4
7 8
5 ]) t-phi cages, to form fully tetrahedrally
connected frameworks. As described above, rho is the first (RHO-G1) generation
of this RHO family, and the natural zeolite paulingite (PAU) and the synthetic zeolite
ZSM-25 (MWF) are the third (RHO-G3) and fourth (RHO-G4) generations, respectively. It is remarkable that ECR-18, the synthetic version of paulingite, has crystallized using Na
+ and K
+
, Rb
+
, or Ba
2+ , together with tetraethylammonium (TEA
+
)
ions, the same organic SDA used in ZSM-25 synthesis [43].
A series of more complex members of the RHO family of embedded isoreticular
zeolites were predicted by extending the approach given above and synthesized via
the so-called multiple inorganic cation approach in the presence of TEA
+ as an
organic SDA [5]. The intentional use of a small amount of alkaline earth cations,
specifically Ca
2+ and Sr
2+ , allowed us to crystallize four more complex higher
generations: PST-20 (RHO-G5), PST-25 (RHO-G6), PST-26 (RHO-G7), and
PST-28 (RHO-G8) [39–41]. One member of this family, which has remained
synthetically elusive, is the RHO-G2 structure, first proposed by Gordon et al. in
1966, with one pau and two d8r cages per unit-cell edge [44]. Very recently, we have
also been able to synthesize this long-term missing generation in the Na
+
-K
+
-N,
Small Gas Adsorption and Separation in Small-Pore Zeolites
9
but also because this zeolite has a slightly higher effective micropore volume than its
other cation forms [37]. The Lobo group also evaluated the CO 2 capture and
separation ability of ZK-5 in simulated PSA/VSA processes. Mg-ZK-5 was shown
to be the most promising adsorbent for PSA applications with the highest working
capacity (△CO 2 ¼ 2.1 mmol g
À1 ) and excellent selectivity (121) over N 2 . While for
the VSA process, Li-, Na-, and K-ZK-5 zeolites were found to have impressive
working capacities (△CO 2 ¼ 1.6–2.2 mmol g
À1 ) and excellent selectivities
(103–128) [37]. In another study the same group reported that Li-ZK-5 with
Si/Al ¼ 1.7 shows an improved adsorption capacity of 4.8 mmol g
À1 at 303 K and
40 bar. This indicates that a decrease in adsorption capacity with increasing Si/Al
ratio was caused by an increase in extraframework content, which in turn leads to a
smaller accessible pore volume [38].
3.4 The RHO Family of Embedded Isoreticular Zeolites
In 2015, Zou, Wright, and our group discovered a novel zeolite family with
increasing structural complexity and embedded isoreticular structures, denoted the
RHO family (Fig. 4) [39–42]. This zeolite family starts from rho (RHO) consisting
of 10-hedral ([4
8 8
2 ]) double 8-rings (d8rs) and 26-hedral ([4
12 6
8 8
6 ]) lta cages and
expands as follows: (1) the scaffolds are extended by inserting an extra pair of d8r
and 18-hedral ([4
12 8
6 ]) pau cages between the lta cages along each unit-cell edge,
which increases the isoreticular dimension by approximately 10 Å per generation
and (2) the space between the scaffolds is completely filled up with the four types of
embedded cages, i.e., 14-hedral([4
6 6
2 8
6 ]) t-plg, 8-hedral ([4
5 8
3 ]) t-oto, 10-hedral
([4
6 8
4 ]) t-gsm, and 12-hedral ([4
7 8
5 ]) t-phi cages, to form fully tetrahedrally
connected frameworks. As described above, rho is the first (RHO-G1) generation
of this RHO family, and the natural zeolite paulingite (PAU) and the synthetic zeolite
ZSM-25 (MWF) are the third (RHO-G3) and fourth (RHO-G4) generations, respectively. It is remarkable that ECR-18, the synthetic version of paulingite, has crystallized using Na
+ and K
+
, Rb
+
, or Ba
2+ , together with tetraethylammonium (TEA
+
)
ions, the same organic SDA used in ZSM-25 synthesis [43].
A series of more complex members of the RHO family of embedded isoreticular
zeolites were predicted by extending the approach given above and synthesized via
the so-called multiple inorganic cation approach in the presence of TEA
+ as an
organic SDA [5]. The intentional use of a small amount of alkaline earth cations,
specifically Ca
2+ and Sr
2+ , allowed us to crystallize four more complex higher
generations: PST-20 (RHO-G5), PST-25 (RHO-G6), PST-26 (RHO-G7), and
PST-28 (RHO-G8) [39–41]. One member of this family, which has remained
synthetically elusive, is the RHO-G2 structure, first proposed by Gordon et al. in
1966, with one pau and two d8r cages per unit-cell edge [44]. Very recently, we have
also been able to synthesize this long-term missing generation in the Na
+
-K
+
-N,
Small Gas Adsorption and Separation in Small-Pore Zeolites
9