Characterisation of sodium cations in dehydrated zeolite NaX by Na-23 NMR spectroscopy

Na-23 MAS, 2D nutation MAS, and DOR NMR spectroscopy has been applied to characterise the location of sodium cations in dehydrated zeolite NaX (Si/Al = 1.23). The Na-23 MAS NMR spectra recorded at three different magnetic field strengths were decomposed by computer simulation into five lines, which were attributed to five crystallographically distinct cation sites known from X-ray diffraction studies. The assignments of the lines follow from electric field gradient calculations at the Na-23 nuclei applying a simple point charge model based on crystal structure data. A weak Gaussian line at low field (delta(iso) = -6 ppm) is assigned to sodium cations at site I, two broad quadrupole patterns at the high-field side of the spectra are attributed to site I' (delta(iso) = -19 ppm, QCC = 5.2 MHz, eta = 0) and site II cations (delta(iso) = -15 ppm, QCC = 4.6 MHz, eta = 0), and two quadrupolar lines dominating the central region of the spectra originate from Na+ at two different III' sites (delta(iso) = -13 and -29 ppm, QCC = 2.6 and 1.6 MHz, eta = 0.7 and 0.9, respectively). Na+ ions located on a second I' site could be identified from the DOR NMR spectra. The line assignment is further corroborated by the reasonable agreement of the site occupancies estimated from the line intensities with those determined by X-ray diffraction. In addition, sodium site populations of five dehydrated zeolites NaX and NaY with Si/Al ratios between 1.09 and 4.0 were derived from the Na-23 MAS NMR spectra.