The coordination of aluminum and silicon in zeolites as studied by x-ray spectrometry

Kühl, Günter H.

doi:10.1016/0022-3697(77)90025-7

Cited by 93 publications

(25 citation statements)

References 12 publications

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“…However, no tricoordinated framework Al was observed by the 27 Al MAS NMR spectra [27,28] and also by the X-ray fluorescence (XRF) [29]. So Uytterhoeven model is not accepted extensively so far.…”

Section: Discussionmentioning

confidence: 99%

Characterization of the acid sites in dealuminated nanosized HZSM-5 zeolite with the probe molecule trimethylphosphine

Zhang

Han

Liu

et al. 2003

Journal of Molecular Catalysis A: Chemical

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Section: Discussionmentioning

confidence: 99%

Characterization of the acid sites in dealuminated nanosized HZSM-5 zeolite with the probe molecule trimethylphosphine

Zhang

Han

Liu

et al. 2003

Journal of Molecular Catalysis A: Chemical

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“…Since the supercages of the 250°C ammonia-treated NH 4 TMA-contained only ammonium ions, the question arises, how methylamine can be desorbed at low temperature after ammonia treatment at 400°C. As mentioned above, the treated zeolite is in the dehydroxylated form, i.e., some aluminum has been removed from the framework [13]. Such a reaction would open a 6-membered ring, perhaps sufficiently to permit methylamine to pass and be desorbed, while trimethylamine is thermally degraded.…”

Section: Nh 4 Tma-mentioning

confidence: 99%

Removal of tetramethylammonium cations from zeolites

Kresnawahjuesa

Olson

Gorte

et al. 2002

Microporous and Mesoporous Materials

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Zeolite α (high-silica LTA), a potential shape-selective catalyst, is synthesized in the presence of tetramethylammonium (TMA) ions. Since TMA + ions are incapable of forming olefins at low temperature, temperatures in excess of 500ºC are required to thermally decompose them and burn off the carbonaceous deposits, frequently causing damage to the structure. In this paper, the thermal decomposition of zeolitic TMA + ions is investigated. This work led to a less severe method for removing TMA + ions by stepwise reaction with ammonia at low temperatures. TMA + ions located in the supercage can easily be removed at a temperature as low as 250ºC, generating mono-and dimethylamine. Sodalite cage TMA + ions require a temperature of not more than 400ºC to be degraded. Although this treatment raises the Si/Al ratio somewhat, damage to the structure is minimal. Since the size of the zeolitic pores defines the type of molecules capable of escaping from the zeolite cavities, decomposition of TMA + ions in NaTMA-Y and NaTMA-high-silica sodalite have been included for comparison. to be degraded. Although this treatment raises the Si/Al ratio somewhat, damage to the structure is minimal. Since the size of the zeolitic pores defines the type of molecules capable of escaping from the zeolite cavities, decomposition of TMA + ions in NaTMA-Y and NaTMA-high-silica sodalite have been included for comparison.

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“…Because of the stronger electrostatic repulsion, Al accumulation can be restrained to a certain degree. The increase of charge on both Al and O atoms leads to stronger electrostatic attraction between Al and O atoms; therefore, the Al-O bond, which has been generally acknowledged as the first step during the process of dealumination, 34 becomes much firmer and thus much harder to rupture. Meanwhile, the charge enrichment on ͓AlO 4 ͔ Ϫ anion will result in its lower capability of charge donation, and consequently the basicity of the ͓AlO 4 ͔ Ϫ anion will weaken, which can definitely bring about an enhancement on the hydrothermal stability.…”

Section: Mechanism Of Enhanced Hydrothermal Stability By Lanthanidmentioning

confidence: 99%

On configuration of exchanged La3+ on ZSM-5: A theoretical approach to the improvement in hydrothermal stability of La-modified ZSM-5 zeolite

Yang

Wang

Zhou

et al. 2003

The Journal of Chemical Physics

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Density functional calculations have been employed to investigate the locating and binding of lanthanum cation, i.e., La͑OH) 2 ϩ , on HZSM-5 zeolite. Through geometry optimization, it was determined that lanthanum ions are favorably accommodated in the two 6-T rings of the straight channels ͑Clusters 1 and 2, see Sec. III A for details͒. Cluster 1 was found to exist in prior to Cluster 2 due to the preference of Al substitution in the T11 site ͑Cluster 1͒ rather than in the T8 site ͑Cluster 2͒. Geometry-optimization of Cluster 1 containing another two lanthanide ions Nd 3ϩ and Yb 3ϩ was also carried out and it was found that a monotonic decrease in Ln-O bond length will take place as the atomic number increases, conforming well to the rule of lanthanide contraction. Some of the optimized parameters are comparable to the corresponding experimental values in Y zeolite, which confirms that the optimized configurations are acceptable. The average frequencies of hydroxyls attached to La 3ϩ or Yb 3ϩ in Cluster 1 fall at 3609.16 and 3579.76 cm Ϫ1 , respectively, with the gap of these two frequencies close to that in the sodalite cage of Y zeolite. Compared to H-form zeolite, the charges on both Al and O atoms in Ln-ZSM-5 zeolite show an obvious increase, which will undoubtedly lead to a stronger mutual interaction and hence enhance the stability of the ͓AlO 4 ͔ Ϫ anion. Moreover, the Ln͑OH) 2 ϩ seem to have thickened the zeolite framework, which can effectively retard the process of dealumination. Through the evaluation of the possibility for dimer formation, it turned out that when the exchange degree arrived to approximately 0.28, lanthanum monomers began to aggregate into dimers, and were completely converted into dimers when the exchange degree approached 0.60.

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The coordination of aluminum and silicon in zeolites as studied by x-ray spectrometry

Cited by 93 publications

References 12 publications

Characterization of the acid sites in dealuminated nanosized HZSM-5 zeolite with the probe molecule trimethylphosphine

Characterization of the acid sites in dealuminated nanosized HZSM-5 zeolite with the probe molecule trimethylphosphine

Removal of tetramethylammonium cations from zeolites

On configuration of exchanged La3+ on ZSM-5: A theoretical approach to the improvement in hydrothermal stability of La-modified ZSM-5 zeolite

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