zeolites. In the following paragraphs, the adsorption of alcohols in the cage-andwindow-type zeolites LTA and CHA will be discussed.
3.2 Equilibrium: Chain-Length and Entropic Effects
The first article on alcohol adsorption in chabazite frameworks was published in
1924 [195], showing that ethanol and methanol vapors are adsorbed on natural
chabazite zeolites but that larger acetone molecules are excluded. This makes it
one of the earliest studies on the remarkable molecular sieving properties of zeolites.
A first report on the adsorption of larger linear alcohols on synthetic chabazite
zeolites was published in 1976 by Skazyvaev et al. [196]. The authors studied the
adsorption of linear alcohol vapors on both natural and synthetic chabazites with a
Si/Al ratio of around 4 and Ca
+ and K
+ as extra-framework cations. Methanol and
ethanol in a high capacity adsorbed (0.2–0.3 g/g at saturation) on these materials.
However, the saturation capacity of propanol (0.15 g/g) and n-butanol (0.08 g/g) was
observed to be much lower.
In liquid phase, single solute isooctane/alcohol adsorption equilibria on a K-CHA
were published by Daems et al. [197]. They noticed a significant chain-length
dependency in the adsorption of different n-alcohols on K-CHA: small alcohols,
such as methanol and ethanol, were readily adsorbed in high amounts. However, for
longer chain alcohols, the equilibrium amounts were observed to be much lower. A
clear cut-off exists between ethanol and n-propanol: for ethanol and methanol, on
average 10 and 5.5 molecules are adsorbed per cage, respectively. Starting from
n-propanol, this number decreases to 1 molecule/cage. According to Daems et al.,
this chain-length dependence of the adsorption equilibrium is linked to the shape of
the CHA cages: molecules of the size of ethanol and methanol have a length smaller
than the width of the cages (6.8 Å), while molecules with a larger chain-length only
fit in the length of the cage.
A more detailed study of the vapor phase and liquid phase adsorption of linear
alcohols on the chabazite analogue SAPO-34 was performed by Remy et al.
[101]. For the vapor phase measurements, extremely slow uptake was observed for
alcohols with a chain larger than or equal to n-propanol. As in the work of Daems
et al., single solute alcohol/isooctane isotherms were measured using the batch
technique. Compared to the work on K-CHA, a slightly lower amount of molecules/cage was observed to be adsorbed at equilibrium: 7.9 molecules of methanol
and 5.3 molecules of ethanol per cage. A second cut-off region was reported between
n-butanol and n-pentanol, with the number of molecules adsorbed per cage dropping
from 1 to 0. This second cut-off region could be linked to the length of an n-butanol
molecule compared to the cage size of SAPO-34, with n-butanol being able to nestle
in a stretched conformation inside the cage, while molecules with the size of
pentanol have to adsorb in a coiled conformation. It should be noted that, also in
liquid phase, very slow uptake for the alcohols larger than ethanol was observed. The
applicability of SAPO-34 to preferentially adsorb shorter chain alcohols over larger
100
B. Claessens et al.
3.2 Equilibrium: Chain-Length and Entropic Effects
The first article on alcohol adsorption in chabazite frameworks was published in
1924 [195], showing that ethanol and methanol vapors are adsorbed on natural
chabazite zeolites but that larger acetone molecules are excluded. This makes it
one of the earliest studies on the remarkable molecular sieving properties of zeolites.
A first report on the adsorption of larger linear alcohols on synthetic chabazite
zeolites was published in 1976 by Skazyvaev et al. [196]. The authors studied the
adsorption of linear alcohol vapors on both natural and synthetic chabazites with a
Si/Al ratio of around 4 and Ca
+ and K
+ as extra-framework cations. Methanol and
ethanol in a high capacity adsorbed (0.2–0.3 g/g at saturation) on these materials.
However, the saturation capacity of propanol (0.15 g/g) and n-butanol (0.08 g/g) was
observed to be much lower.
In liquid phase, single solute isooctane/alcohol adsorption equilibria on a K-CHA
were published by Daems et al. [197]. They noticed a significant chain-length
dependency in the adsorption of different n-alcohols on K-CHA: small alcohols,
such as methanol and ethanol, were readily adsorbed in high amounts. However, for
longer chain alcohols, the equilibrium amounts were observed to be much lower. A
clear cut-off exists between ethanol and n-propanol: for ethanol and methanol, on
average 10 and 5.5 molecules are adsorbed per cage, respectively. Starting from
n-propanol, this number decreases to 1 molecule/cage. According to Daems et al.,
this chain-length dependence of the adsorption equilibrium is linked to the shape of
the CHA cages: molecules of the size of ethanol and methanol have a length smaller
than the width of the cages (6.8 Å), while molecules with a larger chain-length only
fit in the length of the cage.
A more detailed study of the vapor phase and liquid phase adsorption of linear
alcohols on the chabazite analogue SAPO-34 was performed by Remy et al.
[101]. For the vapor phase measurements, extremely slow uptake was observed for
alcohols with a chain larger than or equal to n-propanol. As in the work of Daems
et al., single solute alcohol/isooctane isotherms were measured using the batch
technique. Compared to the work on K-CHA, a slightly lower amount of molecules/cage was observed to be adsorbed at equilibrium: 7.9 molecules of methanol
and 5.3 molecules of ethanol per cage. A second cut-off region was reported between
n-butanol and n-pentanol, with the number of molecules adsorbed per cage dropping
from 1 to 0. This second cut-off region could be linked to the length of an n-butanol
molecule compared to the cage size of SAPO-34, with n-butanol being able to nestle
in a stretched conformation inside the cage, while molecules with the size of
pentanol have to adsorb in a coiled conformation. It should be noted that, also in
liquid phase, very slow uptake for the alcohols larger than ethanol was observed. The
applicability of SAPO-34 to preferentially adsorb shorter chain alcohols over larger
100
B. Claessens et al.