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

0555736 - ÚMCH 2023 RIV US eng J - Journal Article
Kobera, Libor - Dědeček, Jiří - Klein, Petr - Tabor, Edyta - Brus, Jiří - Fishchuk, Anna V. - Sklenák, Štěpán
Formation and local structure of framework Al Lewis sites in beta zeolites.
Journal of Chemical Physics. Roč. 156, č. 10 (2022), č. článku 104702. ISSN 0021-9606. E-ISSN 1089-7690
R&D Projects: GA ČR(CZ) GA19-05259S
Research Infrastructure: e-INFRA CZ - 90140
Institutional support: RVO:61389013 ; RVO:61388955
Keywords : Al Lewis sites * beta zeolites * MAS NMR
OECD category: Analytical chemistry; Physical chemistry (UFCH-W)
Impact factor: 4.4, year: 2022
Method of publishing: Limited access
https://aip.scitation.org/doi/10.1063/5.0083666

Framework AlFR 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 AlFR Lewis sites corresponds to an AlTRI atom tricoordinated to the zeolite framework, which adsorbs a water molecule. This AlTRI–OH2 complex is reflected in 27Al NMR resonance with δiso = 70 ± 5 ppm and CQ = 13 ± 2 MHz. In addition, the AlTRI atom with adsorbed acetonitrile-d3 (the probe of AlFR Lewis sites in FTIR spectroscopy) exhibits a similar 27Al NMR resonance. We suggest that these AlFR 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).
Permanent Link: http://hdl.handle.net/11104/0330576