893 research outputs found
Compressibility behavior and pressure-induced over-hydration of zeolite K-AlSi-L
This paper reports a study, performed by in-situ synchrotron X-ray powder diffraction, of the high-pressure
behavior of zeolite L. The experiments were performed using both penetrating (methanol: ethanol: water mixture,
m.e.w.) and non-penetrating (silicon oil, s.o.) pressure transmitting media (PTM) to study the compressibility
and the possible pressure-induced hydration (PIH) of this synthetic zeolite, technologically relevant as
host-guest system exploited in numerous application fields. The experiments were performed from Pamb to 6.2
and 6.3 GPa in s.o. and m.e.w., respectively. The crystal structure refinements were performed up to 6.3 GPa and
3.1 GPa for the patterns collected in m.e.w. and s.o., respectively, while the unit cell parameters were determined
in the whole pressure range for both media. A strong PIH effect is evident when zeolite L is compressed
in m.e.w. and the over-hydration is essentially ascribable to the filling of most the H2O sites, to the appearance of
a new H2O site and to the partially filling of the K sites. The over-hydration starts at a very low pressure
(0.5 GPa) and the maximum H2O content can be estimated in 31.1 H2O molecules, against the original value of
18. The PIH is completely reversible upon P release. The main difference between the compression behavior of
zeolite L in the two media is the higher compressibility in the non-penetrating one, evidenced by ΔV=− 6.3%
and −9.9% in m.e.w. and s.o, respectively. Our data are consistent with the general behavior of zeolites
compressed with penetrating media, when the intrusion of H2O molecules hinders the effects of the applied
pressure. The results of this work are compared with those obtained on a K-gallosilicate with LTL topology,
where PIH induces the formation of H2O nanotubes inside the zeolite channel
Amorphization induced by pressure: results for zeolites and general implications
We report an {\sl ab initio} study of pressure-induced amorphization (PIA) in
zeolites, which are model systems for this phenomenon. We confirm the
occurrence of low-density amorphous phases like the one reported by Greaves
{\sl et al.} [Science {\bf 308}, 1299 (2005)], which preserves the crystalline
topology and might constitute a new type of glass. The role of the zeolite
composition regarding PIA is explained. Our results support the correctness of
existing models for the basic PIA mechanim, but suggest that energetic, rather
than kinetic, factors determine the irreversibility of the transition.Comment: 4 pages with 3 figures embedded. More information at
http://www.icmab.es/dmmis/leem/jorg
Compressibility behaviour of as-synthesized high-silica Ferrierite.
Ferrierite (FER framework topology) is a well-known aluminosilicate zeolite
mineral. An understanding of the structure and properties of FER remains
important because of its role as a catalyst in commercial reactions. For example,
it is important in the petrochemical industry, where it has been used as a shape
selective catalyst for the production of isobutene. The thermal behavior of this
phase (in its high silica form) was recently studied by Bull et al [1], while its
compressibilty has never been investigated before.
The high pressure (HP) behavior of synthetic high silica zeolite ferrierite (FER)
was investigated by means of in-situ synchrotron X-ray powder diffraction, with
the aim to understand the P-induced deformation mechanism. The microporous
material was synthesized starting from pure silica and pyridine and propyl-amine
as structure directing agents. Here we report the preliminary results on the
compressibility of the as-synthesized phase. The study of the compressibility of
the calcinated one will be carried out in the following steps of the project.
The crystal structure of ferrierite is built up of rings of fivecorner-shared SiO4
tetrahedra (known as five-membered ringsor 5MRs) building units, which form
layers in the ab plane. The layers are connected to form a matrix of 10MR
channels running parallel to the c axis, which are intersected by 8MR channels
running parallel to the b axis. Six-membered rings connect the 10MRs alongthe c
axis direction.
The HP diffraction experiments were performed at BM01a beamline (ESRF), at
the fixed wavelength of 0.71 Å, using a modified Merril-Basset DAC and a
mixture of methanol:ethanol:water (16:3:1) as P-transmitting medium. The
powder patterns were collected from Pamb to 6.2 GPa. Some patterns were also
measured upon pressure release up to Pamb, to check the reversibility of the
compression effects. The unit cell parameters were refined by means of Rietveld
method.
The main results of this study are:
1) No complete X-ray amorphization is observed up to about 6.6 GPa;
2) No abrupt change of the elastic behavior is observed in the whole pressure
range. Between Pamb and 6.2 GPa the reduction of the cell parameter are 4%, 5%
and 6% for a, b and c respectively, accounting for a volume reduction of about
14 %.
3) The P-induced effects on the as-synthesized Si-ferrierite cell parameters are
completely reversible.
4) The bulk modulus obtained using a second order Birch-Murnaghan equation
of state and data weighted by the uncertainties in P and V was K0 = 30.1(3) GPa.
This compressibility is one of the highest when compared with the other natural
and synthetic zeolites studied with “penetrating” aqueous media [2, 3] and is very
similar to that of SAPO-34 [4], another microporous material studied at HP in
its as-synthesized form containing the organic template
High-pressure behavior of high silica ferrierite
Ferrierite (FER framework topology) is a well-known
aluminosilicate zeolite mineral. An understanding of the structure and
properties of FER remains important because of its role as a catalyst in
commercial reactions. For example, it is important in the petrochemical
industry, where it has been used as a shape selective catalyst for
the production of isobutene. The thermal behavior of this phase (in
its high silica form) was recently studied by Bull et al [1], while its
compressibilty has never been investigated before.
The high pressure (HP) behavior of synthetic high silica zeolite
ferrierite (FER) was investigated by means of in-situ synchrotron X-ray
powder diffraction, with the aim to understand the P-induced deformation
mechanism. The microporous material was sinthesized starting from
pure silica and pyridine and propyl-amine as structure directing agents.
Here we report the preliminary results on the compressibility of the as
synthesized phase. The study of the compressibility of the calcined one
will be carried out in the following steps of the project.The crystal structure of ferrierite is built up of rings of fivecornershared
SiO4 tetrahedra (known as five-membered ringsor 5MRs)
building units, which form layers in the ab plane.The layers are
connected to form a matrix of 10MR channels running parallel to the
c axis, which are intersected by 8MR channels running parallel to
the b axis. Six-membered rings connect the 10MRs alongthe c axis
direction.
The HP diffraction experiments were performed at BM01a beamline
(ESRF), at the fixed wavelength of 0.71 Å, using a modified Merril-
Basset DAC and a mixture of methanol- ethanol and water (16:3:1)
as penetrating P-transmitting medium. The powder patterns were
collected from Pamb to 6.2 GPa. Some patterns were also measured
upon pressure release up to Pamb, to check the reversibility of the
compression effects. The unit cell parameters were refined by means
of Rietveld method.
The main results of this study are:
1) No complete X-ray amorphization is observed up to about 6.6
GPa;
2) No abrupt change of the elastic behavior is observed in the whole
pressure range. Between Pamb and 6.2 GPa the reduction of the
cell parameter are 4%, 5% and 6% for a, b and c respectively,
accounting for a volume reduction of about 14 %.
3) The bulk modulus obtained using a second order Birch-Murnaghan
equation of state and data weighted by the uncertainties in P and
V was K0 = 30.1(3) GPa. This compressibility determined in
m.e.w. is one of the highest when compared with other natural and
synthetic zeolites studied with “penetrating” aqueous media [2],
[3].
4) The P-induced effects on as-syntehsized ferrierite structure are
completely reversible
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