(269b) Probing the Site-Pairing Preference of Framework Aluminum in ZSM-5 with Ga(CH3)3
Site-Pairing Preference of Framework Aluminum in ZSM-5 with Ga(CH3)3
Kyle Grodena, Zachary
Jonesb, Renqin Zhanga, Susannah Scottb,
State University, WA, 99164;
of California, Santa Barbara, CA, 93106;
Figure 1. Comparison of the computed and the simulated XANES for methylene bridged Ga and how the Al distribution influences the Ga-Ga distance within a zeolite framework. The computed Ga-Ga distance is 2.96 Å, in strong agreement with experimental findings.
ZSM-5 is a zeolite catalyst marked with great versatility
in catalytic application as a result of its flexibility in cation
incorporation. Due to this property, the structure of ZSM-5 has been under
question for its ability to accept divalent cations, implying that the
substituted aluminum sites exist in paired configurations at specific locations
within the zeolite framework. Trimethylgallium was used to probe these sites,
as the dimethyl gallium fragments that exist in these locations following
impregnation were thought to bind to nearby fragments via terminal methyl
groups. Experimental extended x-ray absorption fine structure (EXAFS)
spectroscopy data supports this bridging configuration due to the presence of
potential gallium-gallium scattering signals. Zeolite cluster models were
created containing potential sites for aluminum pairings with dimethyl gallium
fragments, with optimized geometrical conformations obtained from first
principles calculations based on density functional theory (DFT) implemented in
the VASP program.1,2 As methyl-bridged structures were unfavorable, additional
cluster and periodic models were constructed to explore the possibility of
carbon-hydrogen bond cleavage due to the reactivity of the methyl groups bound
to the gallium centers. This reaction results in a methylene (CH2)
group joining the gallium centers in contrast with the methyl bridges
previously hypothesized. X-ray absorption near edge spectra (XANES) were
computed using CASTEP3, another DFT program, for many of these
configurations examined, which allows for a direct comparison with the
synchrotron-based experimental results. As can be seen from Figure 1, the
agreement between the gallium-gallium bond distances and spectral results from
our calculations and those measured experimentally imply that paired aluminum
sites are indeed present within the ZSM-5 architecture.
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