H
+ adsorbed less water and the isosteric heat of water were also observed to be
lower. Thus, showing that the nature of the cation influences the adsorption of water.
A series of molecular simulation studies have been conducted to gain a better
understanding of how water molecules behave inside hydrophobic pores of highsilica ZSM-5 and silicalite-1 zeolites [151, 161–164]. However, GC-MC simulations
of water molecules inside the pores of these materials are not trivial. First, the results
are highly sensitive to the used force field model for water and the zeolite, as well as
the exact crystal model which is used [151, 161, 162]. Secondly, the confinement of
water in these hydrophobic pores appears to also lead to a change in the dipole
moment of water molecules [163, 164]. Especially the characteristic step in the vapor
phase isotherm (Fig. 5) is difficult to describe correctly, with the onset pressure
differing orders of magnitudes in some cases [151, 161, 162].
To understand the physical properties of water inside the hydrophobic silicalite-1
pores, Desbiens et al. combine GC-MC simulations with an experimental approach.
In their adsorption experiments, the authors employed a technique similar to Hg
intrusion, by “forcing” liquid water into the silicalite-1 pores using a high external
pressure (up to 350 MPa). The results of these studies show that condensed water at
high capacity appears to exist in three distinct phases within the MFI framework
[161, 162]. At the channel intersections, the density becomes very similar to that of
bulk water, while in the side and zigzag channels, a phase with a much lower density
is present [161, 162]. Due to the smaller size of these channels, chains of water
molecules are formed, whereas the intersections are big enough to allow the water
clusters to reach a state which is liquid-like [161, 162].
The presence of silanol defects, either internal or external on the crystal surface,
can significantly influence the adsorption of water, especially in silicalite-1, where
no extra-framework cations are present. An important factor governing the amount
of silanol defects in silicalite-1 is the used synthesis medium. Typically, this medium
contains a source of silica, water, a tetrapropylammonium (TPA) salt as structure
directing agent, and a so-called mineralizing agent (which serves a role as “catalyst”
in the crystallization process) [165]. The mineralizing agent, in the case of silicalite1, can be either hydroxide ions (OH
À
) or fluoride ions (F
À ), added in solution as
TPAOH or NH 4 F. Typically, zeolites synthesized in a hydroxide medium contain
more silanol defects [166].
To investigate the effect of internal defects on the adsorption of water, Trzpit et al.
studied the water intrusion on two silicalite-1 samples, synthesized in fluoride and
hydroxide medium, using the same technique as Desbiens et al. (vide supra). Besides
the self-synthesized materials, the authors also used a commercial silicalite-1 sample
synthesized in hydroxide medium. They also performed GC-MC simulations, with
the silicalite-1 framework containing silanol “nests,” consisting of a removed Si
atom on a T-site and balancing the charge on the surrounding oxygens by adding a
hydrogen atom [150]. From NMR experiments, the authors showed that the sample
synthesized in hydroxide medium indeed contained more silanol defects, while in
the commercial material, defects of Si-O
À coordinated with Na
+ ions were observed
to be present. In their simulations, both silanol (SiOH) and Si-O
À
-Na
+ defects were
incorporated, where the authors assumed the Na
+ defect to be “stronger” (e.g.,
Efficient Downstream Processing of Renewable Alcohols Using Zeolite Adsorbents
95
+ adsorbed less water and the isosteric heat of water were also observed to be
lower. Thus, showing that the nature of the cation influences the adsorption of water.
A series of molecular simulation studies have been conducted to gain a better
understanding of how water molecules behave inside hydrophobic pores of highsilica ZSM-5 and silicalite-1 zeolites [151, 161–164]. However, GC-MC simulations
of water molecules inside the pores of these materials are not trivial. First, the results
are highly sensitive to the used force field model for water and the zeolite, as well as
the exact crystal model which is used [151, 161, 162]. Secondly, the confinement of
water in these hydrophobic pores appears to also lead to a change in the dipole
moment of water molecules [163, 164]. Especially the characteristic step in the vapor
phase isotherm (Fig. 5) is difficult to describe correctly, with the onset pressure
differing orders of magnitudes in some cases [151, 161, 162].
To understand the physical properties of water inside the hydrophobic silicalite-1
pores, Desbiens et al. combine GC-MC simulations with an experimental approach.
In their adsorption experiments, the authors employed a technique similar to Hg
intrusion, by “forcing” liquid water into the silicalite-1 pores using a high external
pressure (up to 350 MPa). The results of these studies show that condensed water at
high capacity appears to exist in three distinct phases within the MFI framework
[161, 162]. At the channel intersections, the density becomes very similar to that of
bulk water, while in the side and zigzag channels, a phase with a much lower density
is present [161, 162]. Due to the smaller size of these channels, chains of water
molecules are formed, whereas the intersections are big enough to allow the water
clusters to reach a state which is liquid-like [161, 162].
The presence of silanol defects, either internal or external on the crystal surface,
can significantly influence the adsorption of water, especially in silicalite-1, where
no extra-framework cations are present. An important factor governing the amount
of silanol defects in silicalite-1 is the used synthesis medium. Typically, this medium
contains a source of silica, water, a tetrapropylammonium (TPA) salt as structure
directing agent, and a so-called mineralizing agent (which serves a role as “catalyst”
in the crystallization process) [165]. The mineralizing agent, in the case of silicalite1, can be either hydroxide ions (OH
À
) or fluoride ions (F
À ), added in solution as
TPAOH or NH 4 F. Typically, zeolites synthesized in a hydroxide medium contain
more silanol defects [166].
To investigate the effect of internal defects on the adsorption of water, Trzpit et al.
studied the water intrusion on two silicalite-1 samples, synthesized in fluoride and
hydroxide medium, using the same technique as Desbiens et al. (vide supra). Besides
the self-synthesized materials, the authors also used a commercial silicalite-1 sample
synthesized in hydroxide medium. They also performed GC-MC simulations, with
the silicalite-1 framework containing silanol “nests,” consisting of a removed Si
atom on a T-site and balancing the charge on the surrounding oxygens by adding a
hydrogen atom [150]. From NMR experiments, the authors showed that the sample
synthesized in hydroxide medium indeed contained more silanol defects, while in
the commercial material, defects of Si-O
À coordinated with Na
+ ions were observed
to be present. In their simulations, both silanol (SiOH) and Si-O
À
-Na
+ defects were
incorporated, where the authors assumed the Na
+ defect to be “stronger” (e.g.,
Efficient Downstream Processing of Renewable Alcohols Using Zeolite Adsorbents
95