The diffusion of n-alcohols in SAPO-34 was also studied with single crystal
uptake measurements using interference microscopy (IFM) [183]. In this paper, it
was shown that the diffusion mechanism is very dependent on the size of the used
probe molecule. For smaller alcohols (methanol and ethanol), the diffusional time
constants for surface barrier permeation and intracrystalline diffusion were observed
to be similar. In contrast, for n-propanol and n-butanol, the mechanism appeared to
be dominated by the surface barrier. Interestingly, when studying different crystals,
the authors found the contribution of the surface permeability to be very dependent
on the used crystal, while the intracrystalline diffusion coefficient essentially
remained constant. Hence, the effect of the so-called surface barrier appeared to be
very dependent on the investigated crystal (Fig. 9). Most strikingly, the authors were
able to show that this distribution of surface permeabilities of single crystals leads to
batch uptake curves that mimic intracrystalline diffusion limitations. In other words,
it shows that the heterogeneity of zeolite crystals can lead to wrong conclusions in
batch studies using macroscopic measurements, where an average uptake curve is
obtained from a broad distribution of crystals, having a different size, external
surface, or slightly different chemical composition.
In LTA zeolites, the self-diffusion of ethanol and n-butanol was studied for
different ion-exchanged materials by Dyer and Amin, using a liquid batch uptake
technique with a radioactively labeled tracer [201]. Depending on the cations present
in the structure, for ethanol self-diffusion coefficients of 0.26–2.4 Â 10
À18 m
2 /s and
for n-butanol 0.58–4.91 Â 10
À19 m
2 /s were obtained. Their results show a complex
relationship between alcohol diffusion and the cations present in the structure. For
instance, the fastest self-diffusion coefficient of ethanol was observed on the
Ca-exchanged sample and not on the H-exchanged sample, while the latter cation
is smaller. However, in the case of Li- and K-exchanged LTA, the self-diffusion
coefficient of ethanol decreased proportional to the cation size. For n-butanol, the
Fig. 9 Normalized uptake curves of methanol at 25
C measured by IFM on single crystals of the
SAPO-34 zeolite with same size and shape [183]
104
B. Claessens et al.
uptake measurements using interference microscopy (IFM) [183]. In this paper, it
was shown that the diffusion mechanism is very dependent on the size of the used
probe molecule. For smaller alcohols (methanol and ethanol), the diffusional time
constants for surface barrier permeation and intracrystalline diffusion were observed
to be similar. In contrast, for n-propanol and n-butanol, the mechanism appeared to
be dominated by the surface barrier. Interestingly, when studying different crystals,
the authors found the contribution of the surface permeability to be very dependent
on the used crystal, while the intracrystalline diffusion coefficient essentially
remained constant. Hence, the effect of the so-called surface barrier appeared to be
very dependent on the investigated crystal (Fig. 9). Most strikingly, the authors were
able to show that this distribution of surface permeabilities of single crystals leads to
batch uptake curves that mimic intracrystalline diffusion limitations. In other words,
it shows that the heterogeneity of zeolite crystals can lead to wrong conclusions in
batch studies using macroscopic measurements, where an average uptake curve is
obtained from a broad distribution of crystals, having a different size, external
surface, or slightly different chemical composition.
In LTA zeolites, the self-diffusion of ethanol and n-butanol was studied for
different ion-exchanged materials by Dyer and Amin, using a liquid batch uptake
technique with a radioactively labeled tracer [201]. Depending on the cations present
in the structure, for ethanol self-diffusion coefficients of 0.26–2.4 Â 10
À18 m
2 /s and
for n-butanol 0.58–4.91 Â 10
À19 m
2 /s were obtained. Their results show a complex
relationship between alcohol diffusion and the cations present in the structure. For
instance, the fastest self-diffusion coefficient of ethanol was observed on the
Ca-exchanged sample and not on the H-exchanged sample, while the latter cation
is smaller. However, in the case of Li- and K-exchanged LTA, the self-diffusion
coefficient of ethanol decreased proportional to the cation size. For n-butanol, the
Fig. 9 Normalized uptake curves of methanol at 25
C measured by IFM on single crystals of the
SAPO-34 zeolite with same size and shape [183]
104
B. Claessens et al.