Structure of Framework Aluminum Lewis Sites and Perturbed Aluminum Atoms in Zeolites as Determined by Al-27\H-1\ REDOR (3Q) MAS NMR Spectroscopy and DFT/Molecular Mechanics
Sprache des Titels:
Zeolites are highly important heterogeneous catalysts. Besides BrOnsted
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
precursorsperturbed Al-FR atoms in hydrated zeoliteswere 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 Al-27 NMR resonance ((i)=59-62ppm, C-Q=5MHz,
and =0.3-0.4) with a shoulder at 40ppm in the (27)AlMASNMR 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 (AlNMR)-Al-27 resonance ((i)approximate to 67ppm, C-Q approximate
to 20MHz, and approximate to 0.1).