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

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).