SOLID-STATE NA-23, LA-139, AL-27 AND SI-29 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPIC INVESTIGATIONS OF CATION LOCATION AND MIGRATION IN ZEOLITES LANAY

Na-23 Magic-angle spinning (MAS), double rotation (DOR) and two-dimensional nutation nuclear magnetic resonance (NMR) and static La-139 NMR spectroscopy were applied to study the location and migration of sodium and lanthanum cations in faujasites. Generally, Na-23 MAS NMR spectroscopy of as-exchanged and hydrated zeolites LaNaY was used for the quantitative determination of non-localized Na+ in the large cavities at a Na-23 NMR shift of -9 ppm and of sodium cations observed at -13 ppm. The latter originate from Na+ ions located on position SII in the large cavities, on position SI in the hexagonal prisms and on positions SII' and/or SI' in the sodalite cages. The Na-23 MAS NMR signal at about -13 ppm was found to be caused by two components. The component that is characterized by a quadrupolar interaction causing a field-dependent shift and a signal at nu(1) = 2 nu(rf) in the two-dimensional quadrupolar nutation spectra is attributed to Na+ enclosed in the sodalite cages. The Na-23 MAS NMR spectra of dehydrated lanthanum-exchanged faujasites are characterized by a low-field Gaussian line of Na+ located on SI positions in the hexagonal prisms and a high-held quadrupole pattern of Na+ located on positions SII and SI'. The migration of lanthanum cations from the large cavities to position SI' in the sodalite cages was monitored by La-139 NMR spectroscopy and verified by a theoretical estimation of the electric field gradient. The lanthanum migration was found to be coupled with a strain of Si-O-T and Al-O-T angles observed by Si-29 and Al-27 MAS NMR high-field shifts, respectively.