under simulated PSA conditions, it show irreversible desorption isotherms at low
pressures [92], which is not the case of pure-silica, small-pore zeolites ITQ-12
(ITW), ITQ-29, and ITQ-32 (IHW) [96–100].
The ITQ-12 structure consists of interconnected double 4-rings, where the interconnectedness occurs around the organic SDA employed, creating a cage. Intriguingly, although this zeolite has a 2D channel system, both C 3
¼ and C 3 molecules
cannot pass through its slit-shaped 8-ring windows (2.4 Â 5.3 Å) [96]. Therefore,
ITQ-12 can be considered a 1D channel system for thermodynamic C 3
¼ /C 3 separation. However, it shows a high diffusion ratio (2.8 Â 10
6 ) and fast diffusion rate
(5.6 Â 10
À2 s
À1 ) for C 3
¼ at 353 K and 0.8 bar, together with a moderate uptake
(1.3 mmol g
À1 ) [96, 97]. On the other hand, ITQ-29 has a low selectivity (1.1), but it
shows a high C 3
¼ uptake of 2.5 mmol g
À1 at 298 K and 1.0 bar. Then again, ITQ-32
was reported to show a faster C 3
¼ diffusion rate (3.86 Â 10
À5 s
À1 at 298 K and
0.04 bar) than even ITQ-12, probably due to its 2D structure [100]. Other small-pore
zeolites such as pure-silica chabazite, ITQ-3 (ITE), and high-silica ZSM-58 (DDR)
have also been studied for C 3
¼
/C 3 separations. Among them, pure-silica chabazite
presents the highest sorption capacity (2.7 mmol g
À1 at 303 K and 0.8 bar) and the
highest ratio of C 3
¼ to C 3 diffusion constants (>40,000). Its outstanding separation
behavior can be attributed to the finely tuned small pore dimension, allowing C 3
¼ to
flow freely while impeding C 3
¼ diffusion [101].
Recently, our group has examined the C 3
¼
/C 3 separation properties of
ferroaluminosilicate levyne (FeAl-LEV) zeolites with different framework Si/Al
and Fe/Al ratios [102]. Among the LEV-type zeolites tested, the mixed Ca
2+ NH 4
+
form of an FeAl-LEV with Si/Al ¼ 15.5 and Fe/Al ¼ 0.27 showed the highest C 3
¼
/
C 3 selectivity (11 at 298 K and 1.0 bar). More interestingly, this zeolite was
characterized by a higher C 3
¼ uptake (ca. 1.0 vs 0.7 mmol g
À1 ) under VSA mode
at 298 K and reservoir pressure 1.2 bar than its aluminosilicate version, together with
better regenerability due to the relatively weaker acidity [102]. This suggests that in
contrast to the general belief, the zeolite adsorbent selective for C 3
¼ /C 3 separation is
not necessarily a pure-silica composition.
4.3 Butenes and Butane
Pure-silica zeolites have also been studied for C 4 paraffin/olefin separation. Bart
et al. reported that RUB-41 (RRO) with two intersecting 8- and 10-ring channels,
which was obtained by calcination of the layered silicate RUB-39, can separate
trans-2-butene and cis-2-butene from 1-butene in the liquid phase [103]. Another
example is ITQ-32 which has been applied in C 3
¼
/C 3 separations. This small-pore
zeolite is an excellent adsorbent for the separation of linear C 4 olefins, with diffusion
rates of 4.73 Â 10
À5 s
À1 at 298 K and 0.04 bar and 6.26 Â 10
À8 at 333 K and
0.04 bar for trans-2-butene and 1-butene, respectively. Therefore, ITQ-32 appears to
be applicable to the separation of linear C 4 hydrocarbons through a kinetic separation process. DDR also exhibits inherently good properties for the separation of C 4
Small Gas Adsorption and Separation in Small-Pore Zeolites
19
pressures [92], which is not the case of pure-silica, small-pore zeolites ITQ-12
(ITW), ITQ-29, and ITQ-32 (IHW) [96–100].
The ITQ-12 structure consists of interconnected double 4-rings, where the interconnectedness occurs around the organic SDA employed, creating a cage. Intriguingly, although this zeolite has a 2D channel system, both C 3
¼ and C 3 molecules
cannot pass through its slit-shaped 8-ring windows (2.4 Â 5.3 Å) [96]. Therefore,
ITQ-12 can be considered a 1D channel system for thermodynamic C 3
¼ /C 3 separation. However, it shows a high diffusion ratio (2.8 Â 10
6 ) and fast diffusion rate
(5.6 Â 10
À2 s
À1 ) for C 3
¼ at 353 K and 0.8 bar, together with a moderate uptake
(1.3 mmol g
À1 ) [96, 97]. On the other hand, ITQ-29 has a low selectivity (1.1), but it
shows a high C 3
¼ uptake of 2.5 mmol g
À1 at 298 K and 1.0 bar. Then again, ITQ-32
was reported to show a faster C 3
¼ diffusion rate (3.86 Â 10
À5 s
À1 at 298 K and
0.04 bar) than even ITQ-12, probably due to its 2D structure [100]. Other small-pore
zeolites such as pure-silica chabazite, ITQ-3 (ITE), and high-silica ZSM-58 (DDR)
have also been studied for C 3
¼
/C 3 separations. Among them, pure-silica chabazite
presents the highest sorption capacity (2.7 mmol g
À1 at 303 K and 0.8 bar) and the
highest ratio of C 3
¼ to C 3 diffusion constants (>40,000). Its outstanding separation
behavior can be attributed to the finely tuned small pore dimension, allowing C 3
¼ to
flow freely while impeding C 3
¼ diffusion [101].
Recently, our group has examined the C 3
¼
/C 3 separation properties of
ferroaluminosilicate levyne (FeAl-LEV) zeolites with different framework Si/Al
and Fe/Al ratios [102]. Among the LEV-type zeolites tested, the mixed Ca
2+ NH 4
+
form of an FeAl-LEV with Si/Al ¼ 15.5 and Fe/Al ¼ 0.27 showed the highest C 3
¼
/
C 3 selectivity (11 at 298 K and 1.0 bar). More interestingly, this zeolite was
characterized by a higher C 3
¼ uptake (ca. 1.0 vs 0.7 mmol g
À1 ) under VSA mode
at 298 K and reservoir pressure 1.2 bar than its aluminosilicate version, together with
better regenerability due to the relatively weaker acidity [102]. This suggests that in
contrast to the general belief, the zeolite adsorbent selective for C 3
¼ /C 3 separation is
not necessarily a pure-silica composition.
4.3 Butenes and Butane
Pure-silica zeolites have also been studied for C 4 paraffin/olefin separation. Bart
et al. reported that RUB-41 (RRO) with two intersecting 8- and 10-ring channels,
which was obtained by calcination of the layered silicate RUB-39, can separate
trans-2-butene and cis-2-butene from 1-butene in the liquid phase [103]. Another
example is ITQ-32 which has been applied in C 3
¼
/C 3 separations. This small-pore
zeolite is an excellent adsorbent for the separation of linear C 4 olefins, with diffusion
rates of 4.73 Â 10
À5 s
À1 at 298 K and 0.04 bar and 6.26 Â 10
À8 at 333 K and
0.04 bar for trans-2-butene and 1-butene, respectively. Therefore, ITQ-32 appears to
be applicable to the separation of linear C 4 hydrocarbons through a kinetic separation process. DDR also exhibits inherently good properties for the separation of C 4
Small Gas Adsorption and Separation in Small-Pore Zeolites
19