chain alcohols was also confirmed via the dynamic separation of ethanol/hexanol,
ethanol/n-propanol, and methanol/ethanol/n-butanol with the shorter-chain alcohol
preferentially adsorbing for each mixture. This selectivity of chabazite zeolites is
remarkable, considering that for channel-type zeolites (such a ZSM-5), the affinity of
the material increases with increasing chain-length (vide supra).
The mechanism of alcohol adsorption in CHA zeolites via Monte Carlo simulations was studied in detail by Krishna and van Baten [181]. They simulated
gas-phase adsorption isotherms of linear n-alcohols with a chain-length ranging
from methanol to hexanol on an all-silica chabazite. As for the liquid phase experimental data, a high amount of ethanol (5.5 molecules/cage) and methanol molecules
are able to adsorb at saturation (four molecules/cage). A sharp decrease is observed
for propanol and n-butanol (two molecules/cage), and for the larger alcohols
(pentanol and hexanol), only one molecule adsorbs per cage. This trend is similar
to the reported liquid phase experiments of Daems et al. [197] and Remy et al. [101];
however the exact values obtained experimentally are somewhat lower, which could
be caused by the difference in chemical composition of the simulated and experimental framework. As is shown in the snapshots presented by Krishna and van Baten
[181], coiling of the higher chain linear alcohols inside the chabazite cages can be
observed at saturation. They also simulated isotherms of binary equimolar alcohol
mixtures, showing the occurrence of selectivity reversal: at low pressures, the longer
chain alcohol is adsorbed preferentially, while at higher adsorbed amounts, the
shorter chain alcohol is preferentially adsorbed.
Most adsorption studies performed on LTA zeolites are limited to the commercial
3A, 4A, and 5A materials [100, 190, 192, 198–200]. These materials are commonly
used to dehydrate linear alcohols, either as adsorbent [100, 190, 192, 198–200] or in
membrane applications [131–133]. While for alcohols, only the larger α supercage is
accessible [87, 128], water is able to enter the smaller β-cages [98, 100, 127, 189–
194]. Typical heats of adsorption of water on 4A and 5A zeolite lie around 54 kJ/mol
and 62 kJ/mol, respectively [189]. Vapor phase adsorption isotherms show typical
adsorption capacities at saturation of between 210 and 250 mg/g [98, 189,
191]. Molecular simulations have shown that on hydrophilic 4A and 5A zeolites,
the associated cations serve as preferential sites for adsorption in the lower vapor
pressure regions [99]. The smaller β-cages serve are the initial preferential adsorption sites at lower vapor pressures [99]. At higher amounts adsorbed, clustering of
water molecules occurs inside the larger α-cages [99]. Interestingly, the adsorption
of water has a profound effect on the position of the cations inside LTA zeolites.
Molecular simulations show that, due to preferential adsorption of water in the
β-cages, Na
+ and Ca
+ ions are “pulled” from a preferential site in the α-cages into
the β-cages [190]. This effect of water on the cation positions also leads to different
observed adsorption isotherms of water, after different adsorption-desorption
cycles [98].
Besides the well-known hydrophilic 3A, 4A, and 5A variants of LTA zeolites, an
all-silica variant, named ITQ-29, was synthesized by Corma et al. [187]. In their
paper reporting the synthesis, the authors demonstrate the hydrophobicity of this
material with only 10 mg/g of water adsorbed at 20 mbar, compared to 240 mg/g on a
Efficient Downstream Processing of Renewable Alcohols Using Zeolite Adsorbents
101
ethanol/n-propanol, and methanol/ethanol/n-butanol with the shorter-chain alcohol
preferentially adsorbing for each mixture. This selectivity of chabazite zeolites is
remarkable, considering that for channel-type zeolites (such a ZSM-5), the affinity of
the material increases with increasing chain-length (vide supra).
The mechanism of alcohol adsorption in CHA zeolites via Monte Carlo simulations was studied in detail by Krishna and van Baten [181]. They simulated
gas-phase adsorption isotherms of linear n-alcohols with a chain-length ranging
from methanol to hexanol on an all-silica chabazite. As for the liquid phase experimental data, a high amount of ethanol (5.5 molecules/cage) and methanol molecules
are able to adsorb at saturation (four molecules/cage). A sharp decrease is observed
for propanol and n-butanol (two molecules/cage), and for the larger alcohols
(pentanol and hexanol), only one molecule adsorbs per cage. This trend is similar
to the reported liquid phase experiments of Daems et al. [197] and Remy et al. [101];
however the exact values obtained experimentally are somewhat lower, which could
be caused by the difference in chemical composition of the simulated and experimental framework. As is shown in the snapshots presented by Krishna and van Baten
[181], coiling of the higher chain linear alcohols inside the chabazite cages can be
observed at saturation. They also simulated isotherms of binary equimolar alcohol
mixtures, showing the occurrence of selectivity reversal: at low pressures, the longer
chain alcohol is adsorbed preferentially, while at higher adsorbed amounts, the
shorter chain alcohol is preferentially adsorbed.
Most adsorption studies performed on LTA zeolites are limited to the commercial
3A, 4A, and 5A materials [100, 190, 192, 198–200]. These materials are commonly
used to dehydrate linear alcohols, either as adsorbent [100, 190, 192, 198–200] or in
membrane applications [131–133]. While for alcohols, only the larger α supercage is
accessible [87, 128], water is able to enter the smaller β-cages [98, 100, 127, 189–
194]. Typical heats of adsorption of water on 4A and 5A zeolite lie around 54 kJ/mol
and 62 kJ/mol, respectively [189]. Vapor phase adsorption isotherms show typical
adsorption capacities at saturation of between 210 and 250 mg/g [98, 189,
191]. Molecular simulations have shown that on hydrophilic 4A and 5A zeolites,
the associated cations serve as preferential sites for adsorption in the lower vapor
pressure regions [99]. The smaller β-cages serve are the initial preferential adsorption sites at lower vapor pressures [99]. At higher amounts adsorbed, clustering of
water molecules occurs inside the larger α-cages [99]. Interestingly, the adsorption
of water has a profound effect on the position of the cations inside LTA zeolites.
Molecular simulations show that, due to preferential adsorption of water in the
β-cages, Na
+ and Ca
+ ions are “pulled” from a preferential site in the α-cages into
the β-cages [190]. This effect of water on the cation positions also leads to different
observed adsorption isotherms of water, after different adsorption-desorption
cycles [98].
Besides the well-known hydrophilic 3A, 4A, and 5A variants of LTA zeolites, an
all-silica variant, named ITQ-29, was synthesized by Corma et al. [187]. In their
paper reporting the synthesis, the authors demonstrate the hydrophobicity of this
material with only 10 mg/g of water adsorbed at 20 mbar, compared to 240 mg/g on a
Efficient Downstream Processing of Renewable Alcohols Using Zeolite Adsorbents
101