consisting of AlO 4 and SiO 4 tetrahedra linked to each other by sharing oxygens. The
framework is composed of linking sodalite cages through double six-rings (D6R),
which create a large cavity called the supercage accessible by a three-dimensional
12-membered ring with a free diameter of approximately 7.4 Å. Small gases can also
enter the sodalite cage via the six-membered ring window (S6R). Each unit cell
contains 192 (Si/Al)O 4 tetrahedra. The number of aluminum atoms per unit cell
varies from 96 to 77, corresponding to a Si/Al ratio between 1 and 1.5. Exchangeable
extra framework cations balance the negative charges on AlO 4 tetrahedron. In
commercial zeolite NaX (13X), the charge balancing cations are sodium ions.
These cations are located in different sites in the structure. Adsorption and diffusion
properties of the adsorbate molecules in the zeolite NaX depend on the interaction of
the zeolite framework and the extra framework (Na
+
) cations with these
molecules [2].
The most challenging industrial problems in separation in gas phase are light
gases (e.g., H 2 , N 2 , O 2 , CO 2 , and CH 4 ), light olefins and paraffins (e.g., ethane/
ethylene and propane/propylene), harmful gases (e.g., H 2 S, SO 2 , and CO), noble
gases (e.g., Ar, He, and Xe), and vapors (e.g., solvents, xylene isomers, and hexane
isomers) [4]. To explore the adsorption-based technologies and its possible application in the abovementioned challenging industrial separations, cost-effective options
using zeolites were proposed by different studies. However, the first studies
concerning this topic were focused on the adsorption mechanism and properties,
such as adsorption equilibrium, adsorption kinetics, and the adsorption selectivity
for the mixtures.
Regarding the propane/propylene separation, there are many published studies
focused on the selectivity of commercial zeolites such as zeolite 13X [5–10], zeolite
4A [5, 11–17], and zeolite 5A [11, 18]. Other materials such as zeolite 13X
containing Li
+ , Na
+
, K
+
, Rb
+ , and Cs
+ cations [19, 20]; CsY, RbY, and KY [21];
natural zeolites ERI including its K
+ - or Ag
+ - exchanged forms [22]; and binderless
zeolite 13X [23] were also investigated. It is not the goal of this chapter to
extensively present all the materials and separations studied; for that, the readers
should refer to the existing reviews [24, 25]. In recent work, Narin and co-workers
tested a binderless zeolite 13X that presented a higher adsorption capacity for
propylene than for its respective homologous paraffin. Propane and propylene
presented an adsorption capacity of 3.5 and 3.9 molÁkg
À1 , respectively. The authors
suggested that the higher adsorption capacity for propylene could be related to the
π-bond interactions and dipole and quadrupole moments [23].
Also, many zeolites have been proposed in the literature for natural gas upgrading
by carbon dioxide (CO 2 ) removal, showing the stronger surface interactions with
CO 2 , adsorbing more significant amounts of this component when compared to
methane (CH 4 ). These zeolites are equilibrium-based materials and were investigated by several authors [26–31]. Among the zeolites, natural chabazite, zeolite 13X,
zeolite 4A, HMordenite, and a binderless zeolite 13X are the ones with more
available information on the adsorption of CH 4 and CO 2 [28, 29, 32–38]. Palomino
et al. have demonstrated that the polarity of a zeolite can be changed via its Si/Al
ratio and has shown the effect on the adsorption of CO 2 /CH 4 mixtures [39]. Reducing
Perspectives of Scaling Up the Use of Zeolites for Selective Separations from. . .
147
framework is composed of linking sodalite cages through double six-rings (D6R),
which create a large cavity called the supercage accessible by a three-dimensional
12-membered ring with a free diameter of approximately 7.4 Å. Small gases can also
enter the sodalite cage via the six-membered ring window (S6R). Each unit cell
contains 192 (Si/Al)O 4 tetrahedra. The number of aluminum atoms per unit cell
varies from 96 to 77, corresponding to a Si/Al ratio between 1 and 1.5. Exchangeable
extra framework cations balance the negative charges on AlO 4 tetrahedron. In
commercial zeolite NaX (13X), the charge balancing cations are sodium ions.
These cations are located in different sites in the structure. Adsorption and diffusion
properties of the adsorbate molecules in the zeolite NaX depend on the interaction of
the zeolite framework and the extra framework (Na
+
) cations with these
molecules [2].
The most challenging industrial problems in separation in gas phase are light
gases (e.g., H 2 , N 2 , O 2 , CO 2 , and CH 4 ), light olefins and paraffins (e.g., ethane/
ethylene and propane/propylene), harmful gases (e.g., H 2 S, SO 2 , and CO), noble
gases (e.g., Ar, He, and Xe), and vapors (e.g., solvents, xylene isomers, and hexane
isomers) [4]. To explore the adsorption-based technologies and its possible application in the abovementioned challenging industrial separations, cost-effective options
using zeolites were proposed by different studies. However, the first studies
concerning this topic were focused on the adsorption mechanism and properties,
such as adsorption equilibrium, adsorption kinetics, and the adsorption selectivity
for the mixtures.
Regarding the propane/propylene separation, there are many published studies
focused on the selectivity of commercial zeolites such as zeolite 13X [5–10], zeolite
4A [5, 11–17], and zeolite 5A [11, 18]. Other materials such as zeolite 13X
containing Li
+ , Na
+
, K
+
, Rb
+ , and Cs
+ cations [19, 20]; CsY, RbY, and KY [21];
natural zeolites ERI including its K
+ - or Ag
+ - exchanged forms [22]; and binderless
zeolite 13X [23] were also investigated. It is not the goal of this chapter to
extensively present all the materials and separations studied; for that, the readers
should refer to the existing reviews [24, 25]. In recent work, Narin and co-workers
tested a binderless zeolite 13X that presented a higher adsorption capacity for
propylene than for its respective homologous paraffin. Propane and propylene
presented an adsorption capacity of 3.5 and 3.9 molÁkg
À1 , respectively. The authors
suggested that the higher adsorption capacity for propylene could be related to the
π-bond interactions and dipole and quadrupole moments [23].
Also, many zeolites have been proposed in the literature for natural gas upgrading
by carbon dioxide (CO 2 ) removal, showing the stronger surface interactions with
CO 2 , adsorbing more significant amounts of this component when compared to
methane (CH 4 ). These zeolites are equilibrium-based materials and were investigated by several authors [26–31]. Among the zeolites, natural chabazite, zeolite 13X,
zeolite 4A, HMordenite, and a binderless zeolite 13X are the ones with more
available information on the adsorption of CH 4 and CO 2 [28, 29, 32–38]. Palomino
et al. have demonstrated that the polarity of a zeolite can be changed via its Si/Al
ratio and has shown the effect on the adsorption of CO 2 /CH 4 mixtures [39]. Reducing
Perspectives of Scaling Up the Use of Zeolites for Selective Separations from. . .
147