10864 J. Phys. Chem. B, Vol. 103, No. 49, 1999
Shantz et al.
combination of using SDAs with different charge distributions
and a basic knowledge of the SDA's organization in the zeolite
micropores should be a useful starting point for a more rational
approach to altering the geometric arrangement of framework
heteroatoms via synthesis. We are studying this problem using
samples of ZSM-12 synthesized with different SDAs but the
same framework Si/Al ratio.
-
Figure 13. Proposed model for as-made Al-ZSM-12 made with
benzyltrimethylammonium cations.
Summary
methylene protons are preferentially located near the (Si1Al)
silicons, which are adjacent to the framework aluminum. The
27Al{1H} REDOR results demonstrate there is a strong dipolar
coupling between the organic protons and the framework
aluminum, which, however, cannot be described in terms of a
specific spatial relationship between the SDA and the framework
aluminum. Attempts were made to model the 27Al{1H} REDOR
curve of the d14 sample first by assuming the sample consisted
of isolated spin pairs. The data could not be simulated accurately
by assuming an isolated spin pair, suggesting that more advanced
modeling would be necessary to describe the local structure of
the as-made materials. Subsequently, more rigorous simulations
were undertaken that explicitly accounted for the presence of
three spins (one aluminum, two hydrogens). The simulated
REDOR curves were again in poor agreement with the
experimental curves. One possible explanation for the discrep-
Samples of Al-ZSM-12 have been prepared using selectively
deuterated benzyltrimethylammonium cations where the organic
to framework aluminum ratio is approximately 1. The tri-
methylammonium segment of the SDA undergoes at least two
rapid rotations, while the benzyl segment is essentially immobile,
undergoing only small wobbling motions. Cross-polarization and
rotational echo double resonance NMR experiments have been
performed to study the relative orientation and spatial proximity
of the SDAs with respect to the zeolite framework. Both
CPMAS and REDOR experiments are consistent with the
methylene protons of the SDA being spatially located near the
silicon atoms adjacent to the framework aluminum. The results
are consistent with a model where the framework aluminum is
directly associated with the charge center of the SDA. These
results demonstrate that it should be possible to alter the spatial
arrangement of trivalent atoms in the zeolite framework based
on the charge distribution of the SDA used.
5
ancy is due to 27Al being a quadrupolar nucleus (I ) /2). The
nutation behavior of the aluminum causes ambiguity in deter-
mining the 180° pulse length, which can lead to incomplete
refocusing in the spin echo sequence. Another possible explana-
tion is that the effect of the homonuclear dipolar coupling
between the methylene protons is neglected.
Acknowledgment. This work was funded from NSF grant
CTS-9713516. R.F.L. and D.F.S. acknowledge an NSF Inter-
national Travel Award to support collaborative work at West-
fa¨lische Wilhelms-Universita¨t Mu¨nster in the group of H. Eckert
(INT-9725941). We thank H. Eckert for use of his laboratory
facilities and helpful discussions. We are grateful to the
Department of Chemistry and Biochemistry at the University
of Delaware for use of the NMR facilities and C. Dybowski
for useful discussions.
On the basis of the NMR results we propose a model for
charge ordering in high-silica zeolites. We have shown that the
methylene protons of the SDA are closer to the (Si1Al) silicons
than the (Si0Al) silicons, which implies they are in close
proximity to the framework aluminum. The aromatic protons
do not exhibit a selective dipolar interaction with the (Si1Al)
silicons, and so we can subsequently conclude they are not
spatially located near these silicons. In the case of the methyl
protons, the situation is complex due to the multiple rotations
of the methyl groups, which will influence both the CPMAS
and REDOR behavior and render our results inconclusive.
However, the aromatic protons are essentially rigid like the
methylene protons and the results show these protons are not
in close spatial proximity to the (Si1Al) silicons. Taking these
facts into account we propose a model (Figure 13) where the
nitrogen of the structure-directing agent is in close proximity
to the framework aluminum, and as a result of the SDA’s bent
geometry, the methyl groups and the aromatic ring are further
from the pore wall while the methylene protons are in close
proximity to the silicons adjacent to the aluminum.
References and Notes
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