1 research outputs found
Probing Ge Distribution in Zeolite Frameworks by Post-Synthesis Introduction of Fluoride in As-Made Materials
A new method has been developed to introduce fluoride
in the structure
of as-made germanium-containing zeolites prepared under pure alkaline
media. Incorporation of fluoride species occurs without modification
of the framework composition (Si/Ge ratio) and of the crystallinity,
as evidenced by X-ray diffraction and electron microscopy. After incorporation, <sup>19</sup>F solid-state NMR has been used to probe the location and
distribution of Ge atoms in the framework. In the case of ITQ-13 and
ITQ-17, which can be prepared from both hydroxide and fluoride routes,
incorporated F anions are located in the same structural units as
those occupied when zeolites are prepared in the presence of fluoride.
In the case of ITQ-22 and ITQ-24, fluoride goes mainly in D4R units,
which appear to be in the most energetically favorable positions for
these zeolites. All experiments clearly show that zeolites prepared
in the absence of fluoride in the synthesis medium are enriched in
germanium, compared to the same materials obtained from F-containing
gels. Moreover, Ge plays a strong structure-directing role by replacing
Si atoms preferentially in D4R, leading to zeolites with mainly [4Si,
4Ge] units in the framework. In the particular case of ITQ-22, a new
line observed around −2 ppm in <sup>19</sup>F NMR spectra has
been tentatively assigned to [3Si, 5Ge] D4R units, which corroborates
the structural data obtained via X-ray diffraction