In:
Angewandte Chemie International Edition, Wiley, Vol. 54, No. 2 ( 2015-01-07), p. 541-545
Abstract:
Zeolites are highly important heterogeneous catalysts. Besides Brønsted SiOHAl acid sites, also framework Al FR Lewis acid sites are often found in their H‐forms. The formation of Al FR Lewis sites in zeolites is a key issue regarding their selectivity in acid‐catalyzed reactions. The local structures of Al FR Lewis sites in dehydrated zeolites and their precursors—“perturbed” Al FR atoms in hydrated zeolites—were studied by high‐resolution MAS NMR and FTIR spectroscopy and DFT/MM calculations. Perturbed framework Al atoms correspond to (SiO) 3 AlOH groups and are characterized by a broad 27 Al NMR resonance ( δ i =59–62 ppm, C Q =5 MHz, and η =0.3–0.4) with a shoulder at 40 ppm in the 27 Al MAS NMR spectrum. Dehydroxylation of (SiO) 3 AlOH occurs at mild temperatures and leads to the formation of Al FR Lewis sites tricoordinated to the zeolite framework. Al atoms of these (SiO) 3 Al Lewis sites exhibit an extremely broad 27 Al NMR resonance ( δ i ≈67 ppm, C Q ≈20 MHz, and η ≈0.1).
Type of Medium:
Online Resource
ISSN:
1433-7851
,
1521-3773
DOI:
10.1002/anie.201409635
Language:
English
Publisher:
Wiley
Publication Date:
2015
detail.hit.zdb_id:
2011836-3
detail.hit.zdb_id:
123227-7
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