having a larger charge difference between Na
+ and O
À than between O and H). They
noticed a shift in onset pressure of the “step” in the vapor water isotherm (Fig. 5)
depending on the strength of the defect which was simulated (silanol or silanol
associated with Na
+
), with stronger defects leading to a “step” at lower pressure.
Interestingly, at low water vapor pressures, they observed approximately four water
molecules clustered around a silanol defect, which is similar to the experimental
values obtained for clusters around Al sites in H-ZSM-5 by Olson et al. (vide supra)
[160]. Similar results from GC-MC simulations were also obtained by other authors
[149, 150, 152, 164, 167].
Besides silanol defects, extra-framework cations might also serve as a site for the
adsorption of water. While in the case of silanol defects, four molecules of water
cluster around the defect, in Na-ZSM-5, a cluster of only two water molecules
appears to be formed around the Na
+ ion [167]. Overall, cations are observed to
serve as seeds for water adsorption, with cations having a stronger interaction with
water [149, 150, 152, 164, 167].
The effect of water on the adsorption of alcohols from binary water/alcohol
mixtures is difficult to study from an experimental point of view. In a traditional
liquid phase batch measurement, the concentration in a liquid mixture before and
after contact with an adsorbent sample is measured. However, it is very difficult to
reconstruct the absolute adsorbed amounts from these measurements, since the
change in mixture volume is usually not measured [168]. Farhadpour and Bono
developed a volumetric technique using a calibrated pycnometer glass tube, allowing
the determination of the change in volume as well as the change in mass fraction of
the mixture upon adsorption [115, 116]. They observed a decreasing amount of
water adsorbed with increasing amount of ethanol adsorbed on silicalite-1.
The liquid-phase isotherms measured by Farhadpour and Bono were simulated by
Gómez-Álvarez et al. via GC-MC [156]. The results of this study show a similar
trend as the experimental results: in the case of methanol and ethanol, the amount of
water adsorbed decreases with an increased amount of methanol/ethanol adsorbed.
However in the case of n-butanol, the amount adsorbed of n-butanol stays very high
over the whole liquid phase butanol concentration range, with almost no water
adsorbing in the structure [156].
Looking at the distribution of n-butanol and water molecules inside the framework via GC-MC simulations, Dejaco et al. observed the displacement of water in
the framework depending on the concentration of n-butanol in the binary liquid
isotherm [169]. At low concentrations (<0.0009 wt%) and low-adsorbed amounts,
n-butanol preferentially adsorbs in the zigzag channels, leading to a higher concentration of water in the straight channels. With increasing n-butanol concentration
(>5.7 wt%), the straight channels become more occupied, leading to displacement
of water toward the larger intersections [169]. Due to their hydroxyl functional
group, hydrogen bonding between alcohols and water is possible. Therefore, the
adsorbed alcohol molecules can serve as seeds for water adsorption, as is observed
by different authors by molecular simulations [120, 123, 169]. However, the opposite effect is not observed: the presence of water in high-adsorbed amounts does not
lead to an increase in the alcohol equilibrium capacity [120, 123, 169].
96
B. Claessens et al.
+ and O
À than between O and H). They
noticed a shift in onset pressure of the “step” in the vapor water isotherm (Fig. 5)
depending on the strength of the defect which was simulated (silanol or silanol
associated with Na
+
), with stronger defects leading to a “step” at lower pressure.
Interestingly, at low water vapor pressures, they observed approximately four water
molecules clustered around a silanol defect, which is similar to the experimental
values obtained for clusters around Al sites in H-ZSM-5 by Olson et al. (vide supra)
[160]. Similar results from GC-MC simulations were also obtained by other authors
[149, 150, 152, 164, 167].
Besides silanol defects, extra-framework cations might also serve as a site for the
adsorption of water. While in the case of silanol defects, four molecules of water
cluster around the defect, in Na-ZSM-5, a cluster of only two water molecules
appears to be formed around the Na
+ ion [167]. Overall, cations are observed to
serve as seeds for water adsorption, with cations having a stronger interaction with
water [149, 150, 152, 164, 167].
The effect of water on the adsorption of alcohols from binary water/alcohol
mixtures is difficult to study from an experimental point of view. In a traditional
liquid phase batch measurement, the concentration in a liquid mixture before and
after contact with an adsorbent sample is measured. However, it is very difficult to
reconstruct the absolute adsorbed amounts from these measurements, since the
change in mixture volume is usually not measured [168]. Farhadpour and Bono
developed a volumetric technique using a calibrated pycnometer glass tube, allowing
the determination of the change in volume as well as the change in mass fraction of
the mixture upon adsorption [115, 116]. They observed a decreasing amount of
water adsorbed with increasing amount of ethanol adsorbed on silicalite-1.
The liquid-phase isotherms measured by Farhadpour and Bono were simulated by
Gómez-Álvarez et al. via GC-MC [156]. The results of this study show a similar
trend as the experimental results: in the case of methanol and ethanol, the amount of
water adsorbed decreases with an increased amount of methanol/ethanol adsorbed.
However in the case of n-butanol, the amount adsorbed of n-butanol stays very high
over the whole liquid phase butanol concentration range, with almost no water
adsorbing in the structure [156].
Looking at the distribution of n-butanol and water molecules inside the framework via GC-MC simulations, Dejaco et al. observed the displacement of water in
the framework depending on the concentration of n-butanol in the binary liquid
isotherm [169]. At low concentrations (<0.0009 wt%) and low-adsorbed amounts,
n-butanol preferentially adsorbs in the zigzag channels, leading to a higher concentration of water in the straight channels. With increasing n-butanol concentration
(>5.7 wt%), the straight channels become more occupied, leading to displacement
of water toward the larger intersections [169]. Due to their hydroxyl functional
group, hydrogen bonding between alcohols and water is possible. Therefore, the
adsorbed alcohol molecules can serve as seeds for water adsorption, as is observed
by different authors by molecular simulations [120, 123, 169]. However, the opposite effect is not observed: the presence of water in high-adsorbed amounts does not
lead to an increase in the alcohol equilibrium capacity [120, 123, 169].
96
B. Claessens et al.