gas mixture through the interactions between the unsaturated outer most s orbitals of
transition metal ions like Cu
+ and Ag
+ and the C¼C double bond of the olefin [68].
However, both technologies described above have drawbacks. In the case of
kinetic separation, the paraffin remains as an impurity in the adsorbent employed.
Thus, additional energy consumption steps, including high-pressure feeds, highpressure purge, and co-current blow down, are required to purify the olefin [69]. In
the case of π-complexation-driven separation, in addition, the working capacity can
gradually decrease with repeated regeneration because olefin oligomerization usually occurs due to the polarity of the adsorbents [70]. Up to date, as a result, only a
very limited number of zeolites and MOFs have shown potential for kinetically
separating olefins and paraffin. Given their high selective nature, however, the use of
these two classes of crystalline porous materials is expected to be more dominant in
future practical applications [71].
4.1 Ethylene and Ethane
There are many examples where zeolites and molecular sieves have been used for
ethylene/ethane (C 2
¼
/C 2 ) separation, and the zeolites known to be potentially useful
for this separation include Na-A, Ca-A, and Na-X [72–79]. However, even these
zeolites show relatively low C 2
¼ /C 2 selectivities ( 5) [77–79]. Thus, to further
enhance the uptake and selectivity of C 2
¼ , Cu
+ or Ag
+ ions have been introduced
to induce the π-complexation between the metal cation and C 2
¼ . Miltenburg et al.
dispersed CuCl into Na-X and then measured the C 2
¼ and C 2 adsorption isotherms
on the resulting CuCl/Na-X physical mixture [80]. They found an enhanced C 2
¼
selectivity compared to Na-X. However, there was also a decrease in adsorption
capacity due to a decrease in surface area of the CuCl/Na-X mixture [72, 80]. In
another work Aguado and co-workers reported that the introduction of Ag
+ as a
charge balancing cation into Ca-A leads to infinite C 2
¼ selectivity at 1.0 bar and
303 K, i.e., total C 2 exclusion, which may be a result of a combination of
π-complexation and size exclusion effects (Fig. 9) [81].
One way to prevent olefin polymerization during olefin/paraffin separation is to
use pure-silica zeolites with no acid sites or transition metals in order to ensure that
only size selection takes place. Researchers at ITQ and ExxonMobil have recently
reported that the small-pore pure-silica zeolite ITQ-55 is very selective for C 2
¼ /C 2
separation [82]. ITQ-55 was synthesized using the N
2 ,N
2 ,N
2 ,N
5 ,N
5 ,N
5 ,3a6aoctamethyloctahydropentalene-2,5-diammonium dication as an organic SDA in
either OH
À or F
À media. This sophisticated SDA was supposed to be responsible
for the formation of 48 T heart-shaped cages interconnected through an 8-ring
window (Fig. 10). While ITQ-55 showed an exceedingly high selectivity of
100 due to its unique pore topology and framework flexibility, the intracrystalline
C 2
¼ diffusion rate was relatively slow compared to other known small-pore zeolites
because of its narrow pores.
16
K. C. Kemp et al.
transition metal ions like Cu
+ and Ag
+ and the C¼C double bond of the olefin [68].
However, both technologies described above have drawbacks. In the case of
kinetic separation, the paraffin remains as an impurity in the adsorbent employed.
Thus, additional energy consumption steps, including high-pressure feeds, highpressure purge, and co-current blow down, are required to purify the olefin [69]. In
the case of π-complexation-driven separation, in addition, the working capacity can
gradually decrease with repeated regeneration because olefin oligomerization usually occurs due to the polarity of the adsorbents [70]. Up to date, as a result, only a
very limited number of zeolites and MOFs have shown potential for kinetically
separating olefins and paraffin. Given their high selective nature, however, the use of
these two classes of crystalline porous materials is expected to be more dominant in
future practical applications [71].
4.1 Ethylene and Ethane
There are many examples where zeolites and molecular sieves have been used for
ethylene/ethane (C 2
¼
/C 2 ) separation, and the zeolites known to be potentially useful
for this separation include Na-A, Ca-A, and Na-X [72–79]. However, even these
zeolites show relatively low C 2
¼ /C 2 selectivities ( 5) [77–79]. Thus, to further
enhance the uptake and selectivity of C 2
¼ , Cu
+ or Ag
+ ions have been introduced
to induce the π-complexation between the metal cation and C 2
¼ . Miltenburg et al.
dispersed CuCl into Na-X and then measured the C 2
¼ and C 2 adsorption isotherms
on the resulting CuCl/Na-X physical mixture [80]. They found an enhanced C 2
¼
selectivity compared to Na-X. However, there was also a decrease in adsorption
capacity due to a decrease in surface area of the CuCl/Na-X mixture [72, 80]. In
another work Aguado and co-workers reported that the introduction of Ag
+ as a
charge balancing cation into Ca-A leads to infinite C 2
¼ selectivity at 1.0 bar and
303 K, i.e., total C 2 exclusion, which may be a result of a combination of
π-complexation and size exclusion effects (Fig. 9) [81].
One way to prevent olefin polymerization during olefin/paraffin separation is to
use pure-silica zeolites with no acid sites or transition metals in order to ensure that
only size selection takes place. Researchers at ITQ and ExxonMobil have recently
reported that the small-pore pure-silica zeolite ITQ-55 is very selective for C 2
¼ /C 2
separation [82]. ITQ-55 was synthesized using the N
2 ,N
2 ,N
2 ,N
5 ,N
5 ,N
5 ,3a6aoctamethyloctahydropentalene-2,5-diammonium dication as an organic SDA in
either OH
À or F
À media. This sophisticated SDA was supposed to be responsible
for the formation of 48 T heart-shaped cages interconnected through an 8-ring
window (Fig. 10). While ITQ-55 showed an exceedingly high selectivity of
100 due to its unique pore topology and framework flexibility, the intracrystalline
C 2
¼ diffusion rate was relatively slow compared to other known small-pore zeolites
because of its narrow pores.
16
K. C. Kemp et al.