Assignment of ²³Na MAS NMR signals of ammonium- and lanthanum-exchanged sodium Y zeolites

Two distinct 23Na MAS NMR lines in spectra of hydrated Na,NH4-Y zeolites at about -9 and -13 ppm (referenced to crystalline NaCl) were assigned to sodium lattice cations located in the large cavities (SIII) and inside the truncated octahedra (SI′ or SII′). This assignment was supported by the appearance of these two signals in spectra of air-dried and heat-treated La,Na-Y zeolites (obtained by conventional or contact-induced solid-state ion exchange), the more so as the intensity ratios corresponded to the cation distribution known to arise after the distinctly different pretreatments. It is concluded that slowly tumbling sodium cations are located in SIII whereas Na cations in truncated octahedra show stronger quadrupole interactions. A 23Na signal at -5.5 ppm sometimes observed in spectra of La,Na-Y zeolites was tentatively assigned to sodium cations located in the hexagonal prisms (SI)