ZSM-5 zeolites, Ga modified via different
methods (in situ hydrothermal
synthesis, mechanical mixing, incipient wetness impregnation, solid-state
ion exchange, and liquid phase ion exchange), were systematically
investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy
(XPS), 29Si, 27Al, and 71Ga magic-angle
spinning (MAS) NMR, and H2 temperature-programmed reduction
(H2-TPR). It is important to prove that both impregnation
and liquid phase ion exchange could facilitate the incorporation of
Ga species into the framework in addition to in situ hydrothermal
synthesis. The liquid phase ion exchange method drove part of the
Ga species into the framework, and it was further migrated into the
framework by drying, while the incipient wetness impregnation method
promoted part of the Ga species into the framework only during the
calcination process. In the n-heptane catalytic aromatization
procedure on the fixed bed, Ga modified ZSM-5 by in situ hydrothermal
synthesis showed the highest benzene, toluene, ethylbenzene and xylene
(BTEX) selectivity, owing to the increased strong Lewis acidic sites
and mesopore volumes induced by the framework Ga species.
Titanium silicalite-1 (TS-1) has
been shown to be a heterogeneous
catalyst with remarkable efficiency and selectivity; however, the
nature of the active Ti site in the MFI framework remains elusive.
Here we report combined experimental and theoretical research on Ti
distribution in the 12 crystallographically distinct T sites of the
MFI framework in high-Ti-loaded TS-1 (2.7 wt % in TiO2).
Using a multishell fit to extended X-ray absorption fine structure,
we show that T4 is the most populated site, in marked contrast to
the preferential substitution sites and the definitely excluded sites
assumed hitherto by diffraction studies. The identification is supported
by a good agreement between calculated and experimental X-ray absorption
near-edge structure studies and by full periodic density functional
theory (DFT) computation. In spite of having the identical most favored
site, the preference order for the remaining sites predicted by DFT
does not fully match the experimental results. This suggests that
Ti distribution in the resulting TS-1 framework is positively correlated
with the thermodynamic stability of pure material but can be affected
by other factors such as interdefects. These new insights may facilitate
the bottom-up design of new zeolites with tailored catalytic performance
and studies on mechanisms of various oxidation reactions.
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