Structure of Framework Aluminum Lewis Sites and Perturbed Aluminum Atoms in Zeolites as Determined by ²⁷Al{¹H} REDOR (3Q) MAS NMR Spectroscopy and DFT/Molecular Mechanics

Abstract

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).