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

Libor Kobera; Jiri Dedecek; Petr Klein; Edyta Tabor; Jiri Brus; Anna V Fishchuk; Stepan Sklenak

doi:10.1063/5.0083666

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

J Chem Phys. 2022 Mar 14;156(10):104702. doi: 10.1063/5.0083666.

Authors

Libor Kobera¹, Jiri Dedecek², Petr Klein², Edyta Tabor², Jiri Brus¹, Anna V Fishchuk², Stepan Sklenak²

Affiliations

¹ Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovský nám. 2, CZ 162 06 Prague 6, Czech Republic.
² J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, Dolejškova 3, CZ 182 23 Prague 8, Czech Republic.

PMID: 35291781
DOI: 10.1063/5.0083666

Abstract

Framework Al_FR Lewis sites represent a substantial portion of active sites in H-BEA zeolite catalysts activated at low temperatures. We studied their nature by ²⁷Al WURST-QCPMG nuclear magnetic resonance (NMR) and proposed a plausible mechanism of their formation based on periodic density functional theory calculations constrained by ¹H MAS, ²⁷Al WURST-QCPMG, and ²⁹Si MAS NMR experiments and FTIR measurements. Our results show that the electron-pair acceptor of Al_FR Lewis sites corresponds to an Al_TRI atom tricoordinated to the zeolite framework, which adsorbs a water molecule. This Al_TRI-OH₂ complex is reflected in ²⁷Al NMR resonance with δ_iso = 70 ± 5 ppm and C_Q = 13 ± 2 MHz. In addition, the Al_TRI atom with adsorbed acetonitrile-d₃ (the probe of Al_FR Lewis sites in FTIR spectroscopy) exhibits a similar ²⁷Al NMR resonance. We suggest that these Al_FR 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).