29Si and 27Al MAS/3Q-MAS NMR Studies of High Silica USY Zeolites

High-resolution solid state 29 Si, 27 Al magic angle spinning (MAS) and 27 Al triple quantum magic angle spinning (3Q-MAS) NMR spectroscopic techniques were used to characterize highly crystalline ultrastable Y (USY) zeolites having different framework Si/Al ratios, obtained by multistep-temperature-programmed (MSTP) steaming at different temperatures. The 29 Si MAS NMR spectra of high silica USY show, in addition to the regularly ordered Q 4 (Si) framework sites (δ ≈ −105 ppm), the presence of distorted tetrahedral environments (δ ≈ −111 ppm), generated by the healing of the framework vacancies created by dealumination. Further, 27 Al MAS experiments show the presence of aluminum in four (Al IV ), five (Al V ), and six (Al VI ) coordinations, whereas the multiplicity within Al IV and Al VI is revealed by 27 Al 3Q-MAS experiments. Two different tetrahedral and octahedral Al environments are resolved in the samples subjected to increasing steaming severity. Quantification of all the Al environments is provided based on isotropic chemical shifts (δ CS ) and second-order quadrupole interaction parameters (P Q ), which were derived by a graphical analysis of 3Q-MAS data. In high silica USY materials prepared by MSTP method, the two Al IV species resolved by 3Q-MAS experiments could be assigned to Al environments which are tetrahedrally connected to silicons in the original and healed regions of faujasite framework.