In situ synchrotron X-ray powder diffraction and Cs-133 and Na-23 MAS NMR have been used to investigate the cation migration and ordering in samples of cesium-exchanged zeolite NaY as a function of temperature and cesium cation-exchange level and during dehydration. Samples were prepared with cesium-exchange levels varying from 68 to 83% of the total cation content by carrying out repeated ion-exchange and calcination steps. Lower cesium content samples contain sodium cations in the sodalite cages and cesium cations in the supercages (SII and SIII) directly after the ion-exchange process. After dehydration at 350 degrees C and above, sodium cations are observed in the supercages (in SII) and in the double B-rings (SI). A maximum of 8 cesium cations/unit cell (1/sodalite cage) migrate from the super to the sodalite cages, occupying SII' and SI' positions. The supercage sodium cations are then exchanged for cesium in the subsequent ion-exchange steps, increasing the cesium content. In situ X-ray data, collected during dehydration, showed that the sodium migration from SI' to SI occurs initially (at > 180 degrees C). This migration appears to be accompanied by, or drives, the migration of Cs+ from the supercages to the sodalite cages, which occurs at approximately the same temperature (> 200 degrees C). It is not until > 300 degrees C that the migration of SI' sodium cations to SII is observed. Significant variations in the cation occupancies within the cages are seen at different temperatures in the dehydrated samples. For example, at 500 degrees C, there are an equal number of SII and SIII' cesium cations in the lower cesium content sample. On cooling, the cesium cations order in the SII position, the SIII' occupancy dropping from 12.5 to 7. An ordering scheme for the cations in the supercage is suggested to explain these observations. A number of resonances are seen in the Cs-133 MAS NMR, which are assigned, making use of the occupancies obtained from the Rietveld refinements, to the various cesium positions. The lack of spinning sidebands associated with some of the resonances indicates that the some cations in the supercages are mobile, even at room temperature. When the temperature is raised, a number of SII and Sm resonances coalesce, as the cation mobility in the supercages increases.

Cation migration in zeolites: An in situ powder diffraction and MAS NMR study of the structure of zeolite Cs(Na)-Y during dehydration / Norby, P; Poshni, Fi; Gualtieri, Alessandro; Hanson, Jc; Grey, Cp. - In: JOURNAL OF PHYSICAL CHEMISTRY. B, CONDENSED MATTER, MATERIALS, SURFACES, INTERFACES & BIOPHYSICAL. - ISSN 1520-6106. - 102:(1998), pp. 839-856. [10.1021/jp9730398]

Cation migration in zeolites: An in situ powder diffraction and MAS NMR study of the structure of zeolite Cs(Na)-Y during dehydration

GUALTIERI, Alessandro;
1998

Abstract

In situ synchrotron X-ray powder diffraction and Cs-133 and Na-23 MAS NMR have been used to investigate the cation migration and ordering in samples of cesium-exchanged zeolite NaY as a function of temperature and cesium cation-exchange level and during dehydration. Samples were prepared with cesium-exchange levels varying from 68 to 83% of the total cation content by carrying out repeated ion-exchange and calcination steps. Lower cesium content samples contain sodium cations in the sodalite cages and cesium cations in the supercages (SII and SIII) directly after the ion-exchange process. After dehydration at 350 degrees C and above, sodium cations are observed in the supercages (in SII) and in the double B-rings (SI). A maximum of 8 cesium cations/unit cell (1/sodalite cage) migrate from the super to the sodalite cages, occupying SII' and SI' positions. The supercage sodium cations are then exchanged for cesium in the subsequent ion-exchange steps, increasing the cesium content. In situ X-ray data, collected during dehydration, showed that the sodium migration from SI' to SI occurs initially (at > 180 degrees C). This migration appears to be accompanied by, or drives, the migration of Cs+ from the supercages to the sodalite cages, which occurs at approximately the same temperature (> 200 degrees C). It is not until > 300 degrees C that the migration of SI' sodium cations to SII is observed. Significant variations in the cation occupancies within the cages are seen at different temperatures in the dehydrated samples. For example, at 500 degrees C, there are an equal number of SII and SIII' cesium cations in the lower cesium content sample. On cooling, the cesium cations order in the SII position, the SIII' occupancy dropping from 12.5 to 7. An ordering scheme for the cations in the supercage is suggested to explain these observations. A number of resonances are seen in the Cs-133 MAS NMR, which are assigned, making use of the occupancies obtained from the Rietveld refinements, to the various cesium positions. The lack of spinning sidebands associated with some of the resonances indicates that the some cations in the supercages are mobile, even at room temperature. When the temperature is raised, a number of SII and Sm resonances coalesce, as the cation mobility in the supercages increases.
102
839
856
Cation migration in zeolites: An in situ powder diffraction and MAS NMR study of the structure of zeolite Cs(Na)-Y during dehydration / Norby, P; Poshni, Fi; Gualtieri, Alessandro; Hanson, Jc; Grey, Cp. - In: JOURNAL OF PHYSICAL CHEMISTRY. B, CONDENSED MATTER, MATERIALS, SURFACES, INTERFACES & BIOPHYSICAL. - ISSN 1520-6106. - 102:(1998), pp. 839-856. [10.1021/jp9730398]
Norby, P; Poshni, Fi; Gualtieri, Alessandro; Hanson, Jc; Grey, Cp
File in questo prodotto:
Non ci sono file associati a questo prodotto.
Pubblicazioni consigliate

Caricamento pubblicazioni consigliate

Licenza Creative Commons
I metadati presenti in IRIS UNIMORE sono rilasciati con licenza Creative Commons CC0 1.0 Universal, mentre i file delle pubblicazioni sono rilasciati con licenza Attribuzione 4.0 Internazionale (CC BY 4.0), salvo diversa indicazione.
In caso di violazione di copyright, contattare Supporto Iris

Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11380/8623
Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus 113
  • ???jsp.display-item.citation.isi??? 114
social impact