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

We investigated by two-dimensional Na-23 ORIACT MQMAS NMR and one-dimension al Na-23 highspeed MAS NMR spectroscopy a homologous series of dehydrated z eolites 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. Che m. Sec. 2000, 122, 12015) significantly improved the resolution of the MQMA S spectra, in comparison with earlier studies. It was thus possible to extr act meaningful quadrupole parameters by MQMAS NMR, which were used as a sta rting point for the simulation of 1D MAS NMR spectra to obtain more accurat e 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 prism s 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 t o be substituted by ammonium ions through the exchange process.