Solid-state 23Na, 139La, 27Al and 29Si nuclear magnetic resonance spectroscopic investigations of cation location and migration in zeolites LaNaY

Abstract

23Na Magic-angle spinning (MAS), double rotation (DOR) and two-dimensional nutation nuclear magnetic resonance (NMR) and static 139La NMR spectroscopy were applied to study the location and migration of sodium and lanthanum cations in faujasites. Generally, 23Na 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 23Na 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 23Na MAS NMR signal at about −13 ppm was found to be caused by two coonents. The component that is characterized by a quadrupolar interaction causing a field-dependent shift and a signal at v 1 = 2 v rf in the two-dimensional quadrupolar nutation spectra is attributed to Na + enclosed in the sodalite cages. The 23Na 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-field 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 139La 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 29Si and 27Al MAS NMR high-field shifts, respectively.