Investigation of sodium cations in dehydrated zeolites LSX, X, and Y by 23Na off-resonance RIACT triple-quantum and high-speed MAS NMR spectroscopy

We investigated by two-dimensional Na-23 ORIACT MQMAS NMR and one-dimensional Na-23 highspeed MAS NMR spectroscopy a homologous series of dehydrated zeolites with faujasite structure. The framework silicon to aluminum ratios varied between 1.06 and 2.60. In the case of zeolites Y (n(Si)/n(Al) = 2.60), we studied materials with sodium exchange degrees between 0 and 95%. The recently introduced ORIACT method (Caldarelli, S.; Ziarelli, F. J. Am. Chem. Sec. 2000, 122, 12015) significantly improved the resolution of the MQMAS spectra, in comparison with earlier studies. It was thus possible to extract meaningful quadrupole parameters by MQMAS NMR, which were used as a starting point for the simulation of 1D MAS NMR spectra to obtain more accurate values of the NMR parameters and site occupancy. We were able to show by this approach that in zeolite NaLSX the SI positions in the hexagonal prisms are occupied by sodium cations. For the homologous series of zeolites Y, it was found that sodium cations located at SII positions ore the easiest to be substituted by ammonium ions through the exchange process.