The identified classes are described hereafter:
• Class 1: adsorbents with a high adsorbed quantity of orthoxylene (oX) and then
good oX selectivities, such as BaCaY. This class also includes the adsorbents that
are best for separating paraxylene (pX) from ethylbenzene (EB) such as NaBaX
(27–73) or NaBaX [49–51].
• Class 2: adsorbents with a high adsorbed quantity of EB and low para-selectivity
toward EB. They are also characterized by low total adsorbed volumes. Most of
these adsorbents are exchanged with Cs
+ .
• Class 3: adsorbents with strong para-selectivity compared to all isomers. The
classification of the affinity of the xylene isomers for these adsorbents is as
follows: pX > EB > oX > mX. Regarding PDEB (p-diethylbenzene) which is
the usual regeneration solvent for the process, these adsorbents are characterized
by low pX/PDEB selectivities. This is a class of high practical value, because
these materials adsorb the para-xylene preferentially compared to the other isomers in the feed, which can easily be desorbed by the PDEB in the regeneration
step. They are mainly X and Y zeolites exchanged with mixtures of Ba
2+ and K
+
or mono-cationic forms of BaX or KY.
• Class 4: adsorbents characterized by poor para-selectivity, good meta-selectivity,
and a high adsorbed quantity of metaxylene (mX). These adsorbents are primarily
exchanged with small cations such as Na
+ and Ca
2+ .
On the basis of the established experimental library, quantitative structureproperty relationship (QSPR) approaches can be implemented [32]. By means of a
multi-linear predictive model, the separation properties are correlated with a set of
structural descriptors of the zeolitic adsorbents. The selected descriptors (d i ) essentially characterize the nature of the confinement in the faujasite supercage, i.e., the
size of the cations localized in adsorption sites II (d 1 ), as well as the occupancy ratio
of both adsorption sites II (d 2 ) and III (d 3 ). The implementation of such an approach
makes it necessary to (1) set an appropriate design of experiments, (2) prepare an
adsorbent database, and (3) test the adsorbent database for xylene separation. Based
on the experimental measurements, a multiple linear regression model enables
(among others) the prediction of para-/meta-xylene selectivity. A parity plot
between the experimental and the predicted data is shown in Fig. 12.
Based on the identified descriptors, an adsorption mechanism (illustrated in
Fig. 13) can be proposed. At the high fillings corresponding to a liquid-phase
separation, para-selectivity is generated by the occurrence of the 12-ring window
site. This phenomenon takes place when all sites II in the supercage are occupied
with an optimal average cation radius of 1.33 Å at the same time that the occupancy
of sites III is minimized. We note that this mechanism very closely mimics the
behavior of the BaLSX faujasite, which shows a para-selectivity value of about 4. In
BaLSX faujasite, the average radius of SII cations is 1.35 Å, which is comparable to
the optimal value. All sites II are occupied, and the occupancy of sites III is equal to
zero [33].
Industrial Zeolite Applications for Gas Adsorption and Separation Processes
215
• Class 1: adsorbents with a high adsorbed quantity of orthoxylene (oX) and then
good oX selectivities, such as BaCaY. This class also includes the adsorbents that
are best for separating paraxylene (pX) from ethylbenzene (EB) such as NaBaX
(27–73) or NaBaX [49–51].
• Class 2: adsorbents with a high adsorbed quantity of EB and low para-selectivity
toward EB. They are also characterized by low total adsorbed volumes. Most of
these adsorbents are exchanged with Cs
+ .
• Class 3: adsorbents with strong para-selectivity compared to all isomers. The
classification of the affinity of the xylene isomers for these adsorbents is as
follows: pX > EB > oX > mX. Regarding PDEB (p-diethylbenzene) which is
the usual regeneration solvent for the process, these adsorbents are characterized
by low pX/PDEB selectivities. This is a class of high practical value, because
these materials adsorb the para-xylene preferentially compared to the other isomers in the feed, which can easily be desorbed by the PDEB in the regeneration
step. They are mainly X and Y zeolites exchanged with mixtures of Ba
2+ and K
+
or mono-cationic forms of BaX or KY.
• Class 4: adsorbents characterized by poor para-selectivity, good meta-selectivity,
and a high adsorbed quantity of metaxylene (mX). These adsorbents are primarily
exchanged with small cations such as Na
+ and Ca
2+ .
On the basis of the established experimental library, quantitative structureproperty relationship (QSPR) approaches can be implemented [32]. By means of a
multi-linear predictive model, the separation properties are correlated with a set of
structural descriptors of the zeolitic adsorbents. The selected descriptors (d i ) essentially characterize the nature of the confinement in the faujasite supercage, i.e., the
size of the cations localized in adsorption sites II (d 1 ), as well as the occupancy ratio
of both adsorption sites II (d 2 ) and III (d 3 ). The implementation of such an approach
makes it necessary to (1) set an appropriate design of experiments, (2) prepare an
adsorbent database, and (3) test the adsorbent database for xylene separation. Based
on the experimental measurements, a multiple linear regression model enables
(among others) the prediction of para-/meta-xylene selectivity. A parity plot
between the experimental and the predicted data is shown in Fig. 12.
Based on the identified descriptors, an adsorption mechanism (illustrated in
Fig. 13) can be proposed. At the high fillings corresponding to a liquid-phase
separation, para-selectivity is generated by the occurrence of the 12-ring window
site. This phenomenon takes place when all sites II in the supercage are occupied
with an optimal average cation radius of 1.33 Å at the same time that the occupancy
of sites III is minimized. We note that this mechanism very closely mimics the
behavior of the BaLSX faujasite, which shows a para-selectivity value of about 4. In
BaLSX faujasite, the average radius of SII cations is 1.35 Å, which is comparable to
the optimal value. All sites II are occupied, and the occupancy of sites III is equal to
zero [33].
Industrial Zeolite Applications for Gas Adsorption and Separation Processes
215