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Magnetic resonance studies on V-containing, and V,Mg-containing AFI aluminophosphates

Publicat a:Microporous And Mesoporous Materials. 39 (1-2): 219-228 - 2000-09-01 39(1-2), DOI: 10.1016/S1387-1811(00)00199-2

Autors: Blasco, T; Fernández, L; Martínez-Arias, A; Sánchez-Sánchez, M; Concepción, P; Nieto, JML

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Resum

As-synthesised V-containing and V,Mg-containing aluminophosphates of the AlPO4-5 type (AFI structure) with different compositions were characterised with the use of NMR and ESR spectroscopic techniques, with the aim of determining if V and/or Mg have been successfully incorporated into the framework sites. The identification of different structural P (n Al, 4 - n Mg) units by P-31-MAS NMR is a clear evidence of the substitution of framework Al3+ by Mg2+ in V,Mg-containing samples. The relative site population shows that Mg2+ is randomly distributed and that above ca. 2 wt.% a part is outside the framework. The ESR spectra of V- and V,Mg-containing samples indicate the presence of isolated (VO)(2+) ions in square pyramidal or distorted octahedral environment, and the spin density accounts for 25-35% of the total. The contribution of a broad resonance of associated V4+ species in the spectra of the samples with Mg is attributed to the presence of an extra-framework oxidic phase. The location of the well dispersed vanadyl species has been deduced from the P-31 T-1 longitudinal relaxation times measured by NMR. The shorter P-31 T-1 in V-containing aluminophosphates is consistent with the presence of vanadyl cations at framework sites, whereas in V,Mg-containing aluminophosphates, V4+ occurs preferentially outside the framework as aggregated and dispersed species. The V-51-NMR spectra indicate the presence of both tetrahedral and octahedral V5+ in V-containing and octahedral V5+ in V,Mg-containing aluminophosphates, although nothing can be concluded about its incorporation into the framework sites. (C) 2000 Elsevier Science B.V. All rights reserved.

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