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Synthesis, characterization, and framework heteroatom localization in ITQ-21.
Teresa Blasco1, Avelino Corma, Maria José Díaz-Cabañas
1Instituto de Tecnología Química, UPV-CSIC, Universidad Politécnica de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain.
Journal of the American Chemical Society
|October 14, 2004
Summary
Researchers precisely controlled the synthesis of ITQ-21 zeolite, tuning crystal size and preventing unwanted phases. Germanium incorporation was localized, revealing preferences and the formation of Ge-O-Ge pairs at high loadings.
Area of Science:
- Materials Science
- Inorganic Chemistry
- Catalysis
Background:
- ITQ-21 is a zeolite with potential applications in catalysis and separation.
- Controlling synthesis parameters is crucial for obtaining pure zeolite phases and desired properties.
- Understanding the incorporation of heteroatoms like Germanium (Ge) is key to tailoring zeolite functionality.
Purpose of the Study:
- To investigate the synthesis of ITQ-21 with controlled crystallite size and phase purity.
- To determine the crystallographic positions of Germanium (Ge) and Silicon (Si) in the ITQ-21 framework.
- To understand the impact of Ge loading on the zeolite structure and the formation of Ge-O-Ge bonds.
Main Methods:
- Synthesis of ITQ-21 under varied conditions to control crystallite size and avoid competing phases (CIT-5, SSZ-24, laminar).
- X-ray Diffraction (XRD) for phase identification and crystallite size analysis.
- Fluorine-19 Magic Angle Spinning Nuclear Magnetic Resonance ((19)F MAS NMR) spectroscopy to localize Ge and Si.
- Theoretical calculations to determine energetic preferences for Ge substitution.
Main Results:
- Synthesis variables were optimized to achieve tunable crystallite sizes (nanocrystals to microns) and high phase purity of ITQ-21.
- Ge preferentially occupies T1 crystallographic positions within the D4R cages, avoiding Ge-O-Ge pair formation.
- At high Ge loadings (Si/Ge = 1.7), Ge-O-Ge pairs form within Ge-rich D4R cages, evidenced by a new (19)F MAS NMR signal.
- Theoretical calculations confirmed the experimental findings, establishing the energy order for Ge substitution: T1 < T2 < Ge-O-Ge in T1 < T3.
Conclusions:
- Rational control over ITQ-21 synthesis enables fine-tuning of crystallite size and phase purity.
- Ge incorporation in ITQ-21 shows a preference for specific crystallographic sites, influencing structural integrity.
- High Ge loadings can lead to the formation of Ge-O-Ge linkages, impacting the zeolite's local environment.