APPLICATION OF INTERFERENCE AND IR MICROSCOPY FOR
STUDIES OF INTRACRYSTALLINE MOLECULAR TRANSPORT IN
AFI TYPE ZEOLITES
C. CHMELIK, E. LEHMANN, S. VASENKOV, B. STAUDTE, J.
KÄRGER
Universität Leipzig, Institut für Physik, Linnéstr. 5, D-04103,
Leipzig, Germany
Abstract
Interference microscopy (IFM) and FTIR microscopy (IRM) are applied
to study intracrystalline concentration profiles in SAPO-5 and CrAPO-5 zeolite
crystals. By using both techniques, the high spatial resolution of interference
microscopy is complemented by the ability of FTIR spectroscopy to pinpoint
adsorbates by their characteristic IR bands.
Intracrystalline concentration profiles of water, adsorbed in large crystals
of CrAPO-5 and SAPO-5 under equilibrium with water vapor at 1 and 20 mbar,
were determined with use of interference microscopy. At lower pressure, the
profiles reveal highly inhomogeneous distributions of intracrystalline water in
both types of crystal. This effect is attributed to structural heterogeneity of the
crystals. The structural heterogeneity has been found to have a low or no
influence on the final uptake level at 20 mbar of the crystals.
Concentration profiles of methanol during its adsorption into the onedimensional channels of CrAPO-5 crystals are reported. The exceptionally high
spatial resolution allowed us to obtain detailed information on the interplay of
intracrystalline diffusion, the permeability of the crystal surface and the role of
the internal structure on molecule uptake.
1. Introduction
Due to their regular structure and a well-defined morphology zeolites are
broadly used as catalysts and molecular sieves in different fields of applied
chemistry and technology.
Considering diffusion, due to simplicity of the framework topology, a
zeolite consisting of a packing of oriented cylinders (viz. AFI type) represents
an ideal model system for investigations of the intracrystalline transport [1].
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While the ideal structure of zeolites is routinely used to elucidate their
adsorption and transport properties it was only recently appreciated that these
properties can be influenced to a great extent by building defects of the crystals
[2,3]. Investigation of intergrowth effects in zeolites is particularly important
since these phenomena may strongly influence molecular uptake and
intracrystalline diffusion.
Such investigations are also important in view of the persistent
differences between intra-crystalline diffusivities obtained for the same zeolites
by various experimental techniques [2,3]. In the present work interference
microscopy (IFM) and IR microscopy (IRM) are applied to study the internal
structure of zeolite crystals as well as the influence of this structure on
intracrystalline molecular transport.
2. Experimental section
The measurements were carried out by applying the only two techniques, which
have been proved to be suitable for studies of intracrystalline concentration
profiles of guests in zeolite crystals [4-8]. The first one is the interference
microscopy technique (IFM), which has been recently introduced in our
laboratory. It is based on following the change of the refractive index of a
zeolite crystal during molecular adsorption or desorption. Due to the
proportionality of the refractive index and the local concentration of guest
molecules in the crystal it is possible to monitor the concentration integrals in
the direction of observation. The concentration integrals were also monitored in
a somewhat more direct way by using the IR microscopy method (IRM).
Despite a poor spatial resolution, IR microscopy presents an extremely useful
tool to study intracrystalline concentration profiles due to its ability to
distinguish between different adsorbates. Thus, it opens the possibility for
tracer-exchange measurements.
For the measurements and activation the zeolite sample was introduced
into a specially made optical or IR cell, which was connected to the vacuum
system. The adsorption, desorption or tracer exchange was achieved by
appropriate changes of adsorbate pressure in the surrounding gas phase.
3. Results and discussion
3.1. METHANOL IN CrAPO-5: EQUILIBRIUM WITH THE VAPOR
PHASE
The IFM equilibrium concentration profiles at a pressure of 1 mbar are shown
in Figure 1a. These non-homogenous profiles can not be explained by using the
ideal textbook structure [1]. In view of the image obtained for an unloaded
2
crystal under crossed Nicols we ascribe these non-homogeneous profiles to the
influence of regular intergrowth effects. They render parts of the channel system
to be inaccessible for methanol molecules. Based upon the concentration
profiles we propose an internal structure schematically shown in Figure 1c [7].
To confirm the IFM results concentration profiles of methanol in CrAPO5 crystals were recorded under the same measurement conditions by IRM. Onedimensional concentration profiles were compared. Figure 1b demonstrates the
good agreement between the results obtained by both techniques [7].
Figure 1. (a) IFM equilibrium intracrystalline concentration profile
of methanol in a CrAPO-5 crystal. The color intensity is
proportional to the integrals of local concentration. (b) Comparison
of mean concentration integrals I recorded by IRM and IFM. (c)
Suggested internal structure. The channel direction coincides with
the z direction.
3.2. METHANOL IN CrAPO-5: UPTAKE KINETICS
The primary aim of the dynamic Monte Carlo simulations was to investigate
quantitatively the influence of the intergrowth structure and the transport
barriers on the crystal surface on the intracrystalline transport (figure 2). The
used simulations are analogous to the numerical solution of Fick’s second-law-
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type equations for diffusion in one-dimensional channels (accessible part only)
with transport barriers at the channel edges. Comparing the simulations and
experimental results we finally obtain the value of D=0.43x10-12 m2/s for the
intracrystalline diffusivity of methanol in the limit of small loadings and can
estimate the rate constant of barrier penetration to α=0.35x10-7 m/s.
(a)
(b)
Assumptions (MC):
1.one-dimensional
random walk in
the lattice
2. probability of diffusion
step is independent of
concentration
Figure 2. (b) Intracrystalline concentration of methanol, integrated
along the y crystallographic direction in CrAPO-5 at different times
after the start of the methanol adsorption. The profiles were obtained
by IFM (black line) and by the dynamic MC method (broken line). (a)
The internal structure of CrAPO-5 crystals (shown only for the lower
part of the crystal). x, y and z are the crystallographic directions. The
broken lines outline the observation plane of the IFM measurements of
the transient profiles with the arrow indicating the direction of
observation.
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The reported results show that the estimated value of the diffusion
coefficient D is around a factor of 2 smaller than that of L*α (L - crystal size in
z-direction). This indicates that the rate of methanol uptake is mainly
determined by the intracrystalline diffusivity of methanol. However, this rate is
somewhat reduced due to existence of transport barriers on the crystal surface
[9].
3.3. WATER IN CrAPO-5 AND SAPO-5: EQUILIBRIUM WITH THE
VAPOR PHASE
Figure 3 shows the intracrystalline concentration profiles of water in the
CrAPO-5 and SAPO-5 measured by IFM. It is seen that the profiles obtained
for low pressure of 1 mbar reveal highly inhomogeneous, but reproducible
patterns [10]. Noteworthy is that the profile in CrAPO-5 (figure 3 a1, a3)
resembles, to some extent, the methanol profile observed for these crystals
(figure 1a). Apparently, the explanation might also be applied to water.
The origin of the intergrowth effects may be derived from the progress of
the crystal growth process. As has been shown in ref. [11] the dumbbell shape is
characteristic of some AFI-type crystals in the intermediate stage of growth.
Figure 3. Intracrystalline concentration profiles of water in the
CrAPO-5 (a1, a2, a3) and SAPO-5 (b1, b2, b3) crystals integrated
along the y direction under equilibrium with water vapor at 1 mbar
(a1, b1, a3, b3) and 20 mbar (a2, b2). The channels run along the z
axis. Darker regions correspond to higher concentration integrals.
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The increase in water vapor pressure to 20 mbar results in an essentially
homogeneous profile in CrAPO-5 (figure 4 a2), i.g. all the crystal constituents
are equally filled with liquid-like water..
Imaginable structural factors which can influence the intracrystalline
water concentration at low water vapor pressure are the content and the
distribution of the Cr atoms and/or the presence of defect sites.
The inhomogeneous profiles of water in SAPO-5 may result from a
structural heterogeneity of the crystals as well. For SAPO-5 has been indicated
by electron microprobe analysis that the silicon content of the central part of the
crystals was lower by a factor of 2 to 3 than that at the crystal margin [12]. By
studying the progress of crystal growth it was found that “pencil-like” crystals
are formed initially, later the tips of the “pencils” flatten out proceeding with a
much higher consumption of silicon than the initial one.
At the high water vapor pressure the condensation of water and the
volume filling of the pores of SAPO-5 occur, which are primarily determined
by the accessible pore volume as justified by the homogeneous concentration
profile observed at 20 mbar (figure 4 b2).
It may be speculated that the crystal components which form at the earlier
stages of the growth process, i.e. dumbbell core (CrAPO-5) or “pencil-like”
(SAPO-5) core, can adsorb more water than the other crystal parts under the
low vapor pressure of water [10].
4. Conclusion
The obtained results show large potentials of the new IRM and IFM methods
for elucidation of structural and transport properties of zeolite crystals,
particularly when these techniques are combined with dynamic MC simulations.
From the experimental evidence provided by these techniques, the real structure
of crystals, which reveal perfect shape characteristic for single crystals, has
been found to deviate decisively from the textbook structure of the given type of
zeolite. Combination of the experimental methods with dynamic MC
simulations allowed us to investigate separately the influences of (i) the
intracrystalline diffusion, (ii) the transport barriers on the external crystal
surface and (iii) the effects of the intergrowth structure on molecular uptake.
Acknowledgement
We acknowledge Prof. F. Schüth and Dr. Ö. Weiss as well as Dr. J.
Kornatowski and Dr. G. Zadrozna for synthesizing and providing us the large
zeolite crystals.
Financial support by Deutsche Forschungsgemeinschaft (SFB 294 and
Graduierten-kolleg “Physikalische Chemie der Grenzflächen”), Fonds der
Chemischen Industrie and Max-Buchner-Forschungsstiftung is gratefully
acknowledged.
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