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Structure of Framework Aluminum Lewis Sites and Perturbed Aluminum Atoms in Zeolites as Determined by 27 Al{ 1 H} REDOR (3Q) MAS NMR Spectroscopy and DFT/Molecular Mechanics
Author(s) -
Brus Jiří,
Kobera Libor,
Schoefberger Wolfgang,
Urbanová Martina,
Klein Petr,
Sazama Petr,
Tabor Edyta,
Sklenak Stepan,
Fishchuk Anna V.,
Dědeček Jiří
Publication year - 2015
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201409635
Subject(s) - lewis acids and bases , chemistry , zeolite , nuclear magnetic resonance spectroscopy , spectroscopy , aluminium , molecular sieve , brønsted–lowry acid–base theory , catalysis , resonance (particle physics) , crystallography , inorganic chemistry , stereochemistry , organic chemistry , physics , quantum mechanics , particle physics
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).