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

摘要

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).