2.4 Diffusion of Alcohols and Water in MFI Frameworks
The molecular mobility of alcohols in MFI zeolites has been studied using a variety
of macroscopic and microscopic measurement techniques [170–173]. Caro et al.
studied the diffusion of methanol and water in ZSM-5 zeolites on a microscopic level
using pulse field gradient (PFG) – NMR at 27
C [170]. They observed that the selfdiffusivity of water appeared to be only slightly depend on the amount adsorbed,
while for methanol a strong dependency was observed with a decrease of an order of
magnitude at high-adsorbed amounts (4 Â 10
À9 m
2 /s) compared to low-adsorbed
amounts (0.4 Â 10
À9 m
2 /s).
The molecular mechanism of the diffusion of water inside silicalite-1 pores has
been investigated by different authors using molecular dynamics simulations
[153, 174, 175]. Demontis et al. studied the mechanism of water diffusion inside
silicalite-1 pores at complete saturation. They observed the diffusion mechanism to
be dependent on the temperature: at low temperatures (<À70
C), large solid-like
clusters are formed inside the pores, and transport occurs via the dynamic addition
and removal of water molecules to these large clusters [174, 175]. At higher
temperatures, a transition from these solid-like structures occurs to a more liquidlike state. Starting from 30
C, the adsorbed phase starts to behave as water vapor,
with very few long-living water clusters forming. In a different study, Fleys et al.
also showed the self-diffusion coefficients to be dependent on the amount of water
adsorbed, with a higher amount adsorbed leading to slower diffusion [175]. Overall,
molecular dynamics studies show that water diffuses preferentially along the larger,
straight silicalite channels [174–176]. The effect of cations and Si/Al ratio in ZSM-5
was studied by Ari et al. via molecular simulations, with the overall self-diffusion
coefficient of water to be observed to decrease with decreasing Si/Al ratio, which
could be related to the interaction of the water molecules with the cations present in
the structure [153].
The diffusion of mixtures of alcohols and water in MFI zeolites can lead an
increase or decrease of molecular transport compared to the pure components. For
instance, membrane pervaporation experiments with ethanol/methanol mixtures,
performed by Yu et al., showed that the addition a small amount of methanol (7%)
to ethanol leads to a significant decrease in methanol flux over the membrane. In
contrast, the flux of ethanol is increased compared to the pure liquid when ethanol is
added in a small amount (2%) to ethanol [177]. Similarly, molecular dynamics
studies show that for water/methanol mixtures, the self-diffusion coefficient of
water decreases with increasing amount of methanol adsorbed, while the selfdiffusion coefficient of water stays constant [128]. These effects appear to be related
to the cluster formation of alcohols with each other and water, due to hydrogen
bonding [128].
Efficient Downstream Processing of Renewable Alcohols Using Zeolite Adsorbents
97
The molecular mobility of alcohols in MFI zeolites has been studied using a variety
of macroscopic and microscopic measurement techniques [170–173]. Caro et al.
studied the diffusion of methanol and water in ZSM-5 zeolites on a microscopic level
using pulse field gradient (PFG) – NMR at 27
C [170]. They observed that the selfdiffusivity of water appeared to be only slightly depend on the amount adsorbed,
while for methanol a strong dependency was observed with a decrease of an order of
magnitude at high-adsorbed amounts (4 Â 10
À9 m
2 /s) compared to low-adsorbed
amounts (0.4 Â 10
À9 m
2 /s).
The molecular mechanism of the diffusion of water inside silicalite-1 pores has
been investigated by different authors using molecular dynamics simulations
[153, 174, 175]. Demontis et al. studied the mechanism of water diffusion inside
silicalite-1 pores at complete saturation. They observed the diffusion mechanism to
be dependent on the temperature: at low temperatures (<À70
C), large solid-like
clusters are formed inside the pores, and transport occurs via the dynamic addition
and removal of water molecules to these large clusters [174, 175]. At higher
temperatures, a transition from these solid-like structures occurs to a more liquidlike state. Starting from 30
C, the adsorbed phase starts to behave as water vapor,
with very few long-living water clusters forming. In a different study, Fleys et al.
also showed the self-diffusion coefficients to be dependent on the amount of water
adsorbed, with a higher amount adsorbed leading to slower diffusion [175]. Overall,
molecular dynamics studies show that water diffuses preferentially along the larger,
straight silicalite channels [174–176]. The effect of cations and Si/Al ratio in ZSM-5
was studied by Ari et al. via molecular simulations, with the overall self-diffusion
coefficient of water to be observed to decrease with decreasing Si/Al ratio, which
could be related to the interaction of the water molecules with the cations present in
the structure [153].
The diffusion of mixtures of alcohols and water in MFI zeolites can lead an
increase or decrease of molecular transport compared to the pure components. For
instance, membrane pervaporation experiments with ethanol/methanol mixtures,
performed by Yu et al., showed that the addition a small amount of methanol (7%)
to ethanol leads to a significant decrease in methanol flux over the membrane. In
contrast, the flux of ethanol is increased compared to the pure liquid when ethanol is
added in a small amount (2%) to ethanol [177]. Similarly, molecular dynamics
studies show that for water/methanol mixtures, the self-diffusion coefficient of
water decreases with increasing amount of methanol adsorbed, while the selfdiffusion coefficient of water stays constant [128]. These effects appear to be related
to the cluster formation of alcohols with each other and water, due to hydrogen
bonding [128].
Efficient Downstream Processing of Renewable Alcohols Using Zeolite Adsorbents
97