Formation and local structure of framework Al Lewis sites in beta zeolites

• Published in: The Journal of chemical physics Vol. 156; no. 10; p. 104702
• Main Authors: Kobera, Libor, Dedecek, Jiri, Klein, Petr, Tabor, Edyta, Brus, Jiri, Fishchuk, Anna V, Sklenak, Stepan
• Format: Journal Article
• Language: English
• Published: United States 14.03.2022
• ISSN: 1089-7690
• DOI: 10.1063/5.0083666

Framework Al Lewis sites represent a substantial portion of active sites in H-BEA zeolite catalysts activated at low temperatures. We studied their nature by Al WURST-QCPMG nuclear magnetic resonance (NMR) and proposed a plausible mechanism of their formation based on periodic density functional theory calculations constrained by H MAS, Al WURST-QCPMG, and Si MAS NMR experiments and FTIR measurements. Our results show that the electron-pair acceptor of Al Lewis sites corresponds to an Al atom tricoordinated to the zeolite framework, which adsorbs a water molecule. This Al -OH complex is reflected in Al NMR resonance with δ = 70 ± 5 ppm and C = 13 ± 2 MHz. In addition, the Al atom with adsorbed acetonitrile-d (the probe of Al Lewis sites in FTIR spectroscopy) exhibits a similar Al NMR resonance. We suggest that these Al Lewis sites are formed from Al-OH-Si-O-Si-O-Si-OH-Al sequences located in 12-rings (i.e., close unpaired Al atoms).