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

摘要

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 Al-27 WURST-QCPMG nuclear magnetic resonance (NMR) and proposed a plausible mechanism of their formation based on periodic density functional theory calculations constrained by H-1 MAS, Al-27 WURST-QCPMG, and Si-29 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 Al-27 NMR resonance with delta(iso) = 70 +/- 5 ppm and C-Q = 13 +/- 2 MHz. In addition, the Al-TRI atom with adsorbed acetonitrile-d(3) (the probe of Al-FR Lewis sites in FTIR spectroscopy) exhibits a similar Al-27 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). Published under an exclusive license by AIP Publishing.