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

Abstract

High-resolution solid state 29Si, 27Al magic angle spinning (MAS) and 27Al 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 29Si MAS NMR spectra of high silica USY show, in addition to the regularly ordered Q4(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, 27Al MAS experiments show the presence of aluminum in four (AlIV), five (AlV), and six (AlVI) coordinations, whereas the multiplicity within AlIV and AlVI is revealed by 27Al 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 (δCS) and second-order quadrupole interaction parameters (PQ), which were derived by a graphical analysis of 3Q-MAS data. In high silica USY materials prepared by MSTP method, the two AlIV 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.