Solid-state nuclear magnetic resonance investigation of cation siting in LiNaLSX zeolites

The location of Li+ and Na+ cations in a series of dehydrated low-silica Li NaX zeolites (LiNaLSX, framework Si/Al ratio=1.0) were characterized by Li- 7 and Na-23 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spe ctroscopy. Depending on the Li+ content, up to three lines were observed in the 7Li MAS NMR spectra attributed to Li+ cations on SI', SII and SIII sit es. The Na-23 MAS NMR spectra of the pure sodium form NaLSX and of LiNaLSX samples with low Li+ contents contain up to five lines belong to Na+ cation s located on SI, SI', SII, and two different SIII' sites. LiNaLSX zeolites containing more than 30% of Li+ show only a single narrow line in the Na-23 MAS NMR spectra attributed to mobile sodium cations. The populations of th e different cation sites were determined from the relative line intensities of the MAS NMR spectra. Below about 70% Li+ exchange, lithium cations are located only on sites SI' and SII. Between 70% and 100% Li+ content these s ites are fully occupied by Li+, and the population of site SIII by Li+ incr eases. It was found that the nitrogen-adsorption capacity correlates well w ith the occupation of Li+ at site SIII. (C) 1998 Elsevier Science B.V. All rights reserved.