Characterization of sodium cations in zeolite omega by solid-state23Na NMR spectroscopy

Sodium cations localized at crystallographically distinct cation sites in the hydrated zeolite omega were characterized by various solid-state23Na NMR techniques, such as23Na MAS,1H-23Na CP/MAS,23Na quadrupolar nutation and variable temperature NMR spectroscopy. In the23Na NMR spectrum of zeolite omega two signals were identified, a low-field Gaussian line at ca. −3 ppm (its quadrupole coupling constant was estimated approximately on the order of magnitude of 0.2 MHz) and a high-field quadrupole pattern with an isotropic chemical shift of −8 ppm and a quadrupole coupling constant of 2.3 MHz. The former signal is attributed to sodium cations in the 12-ring channels and the latter one to sodium cations in gmelinite cages of zeolite omega. The variable temperature NMR experiments reveal the interactions between the sodium cation and the adsorbed water molecules in zeolite omega. The sodium cations in the 12-ring channels are highly hydrated and so mobile as to average the1H-23Na dipole interaction. Hence, these sodium cations give no contribution to the1H-23Na cross polarization process. By contrast, the sodium cations in the gmelinite cages are less hydrated and more rigid. They are closely bonded not only to the adsorbed water molecules but also to the lattice oxygen of the gmelinite cages, which leads to an effective1H-23Na polarization transfer.