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

Abstract

Abstract The location of Li + and Na + cations in a series of dehydrated low-silica LiNaX zeolites (LiNaLSX, framework Si/Al ratio=1.0) were characterized by 7 Li and 23 Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Depending on the Li + content, up to three lines were observed in the 7 Li MAS NMR spectra attributed to Li + cations on SI′, SII and SIII sites. The 23 Na 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 + cations located on SI, SI′, SII, and two different SIII′ sites. LiNaLSX zeolites containing more than 40% of Li + show only a single narrow line in the 23 Na MAS NMR spectra attributed to mobile sodium cations. The populations of the 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 sites are fully occupied by Li + , and the population of site SIII by Li + increases. It was found that the nitrogen-adsorption capacity correlates well with the occupation of Li + at site SIII.