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 Mechanics1. (13th November 2014)

• Record Type: Journal Article
• Authors: Brus, Jiří; Kobera, Libor; Schoefberger, Wolfgang; Urbanová, Martina; Klein, Petr; Sazama, Petr; Tabor, Edyta; Sklenak, Stepan; Fishchuk, Anna V.; Dědeček, Jiří
• Is Part Of: Angewandte Chemie. Volume 127:Number 2(2015)
• Page Start: 551
• Page End: 555
• Publication Date: 2014-11-13
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/ange.201409635
• Languages: English
• ISSNs: 0044-8249

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>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)₃AlOH groups and are characterized by a broad ²⁷Al NMR resonance (<italic>δ</italic>_i=59–62 ppm, <italic>C</italic>_Q=5 MHz, and <italic>η</italic>=0.3–0.4) with a shoulder at 40 ppm in the ²⁷Al MAS NMR spectrum. Dehydroxylation of (SiO)₃AlOH occurs at mild temperatures and leads to the formation of Al_FR Lewis sites tricoordinated to the zeolite framework. Al atoms of these (SiO)₃Al Lewis sites exhibit an extremely broad ²⁷Al NMR resonance (<italic>δ</italic>_i≈67 ppm, <italic>C</italic>_Q≈20 MHz, and <italic>η</italic>≈0.1).</p> </abstract>