The silicalite/p-xylene system : Part I — Flexibility of the MFI framework and sorption mechanism observed during p-xylene pore-filling by X-ray powder diffraction at room temperature
During p-xylene(XYL) sorption in MFI type materials presenting various framework compositions at room temperature, the M ( monoclinic, centric P2 1 n · 1 · 1 group) ↔ O2 ( orthorhombic, acentric P2 12 12 1 group) transition is never observed : for high Si Al ratios (> ≈ 75) the M → O1 is first ob...
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Published in | Materials research bulletin Vol. 30; no. 3; pp. 373 - 380 |
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Main Authors | , |
Format | Journal Article |
Language | English |
Published |
Elsevier Ltd
1995
|
Subjects | |
Online Access | Get full text |
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Summary: | During
p-xylene(XYL) sorption in MFI type materials presenting various framework compositions at room temperature, the
M (
monoclinic, centric
P2
1
n
· 1 · 1
group) ↔
O2 (
orthorhombic, acentric
P2
12
12
1 group) transition is
never observed : for high
Si
Al
ratios (> ≈ 75) the
M → O1 is
first observed at lower sorbate loadings (
0
to
4
molecules/uc), and the
O1 (
orthorhombic, centric
Pnma group) →
O2 transition is only achieved at
saturation (8 molecules/uc); in the case of low
Si
Al
ratios (< 75), which is the usual situation (genuine
ZSM-5
phases), only
O1 → O2 is observed. On sorbate desorption, both
O2 → O1 and
O1 → M
reversible transitions are observed. In the present work the deformations of the tenring (
10R
) channel-openings in the straight and zig-zag channels are investigated versus
p-xylene loadings in silicalite (Si/Al ≈ 5000). It is shown that during the
O1 → O2 phase transition occuring at higher sorbate loadings the mean/maximum displacements of framework
oxygen
atoms are close to ≈ 0.57/1.03Å, which is significantly higher than the mean displacements calculated for
all
the framework atoms(Si
and O) in a theoretical (
molecular mechanics) study (0.36Å) during the
hypothetical and
unobserved
M → O2 transition. It is shown that during the
threestep p-xylene sorption mechanism the framework is
highly flexible. The
effective pore-openings in silicalite · 4XYL are 6.16 × 4.78 and 5.65 × 5.25Å for the straight and zig-zag channels respectively. |
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ISSN: | 0025-5408 1873-4227 |
DOI: | 10.1016/0025-5408(95)00003-8 |