Site Preferences in the Mixed Cation Zeolite, Li,Na-Chabazite:  A Combined Solid-State NMR and Neutron Diffraction Study

The locations of Li⁺ and Na⁺ cations in dehydrated chabazite were studied by neutron powder diffraction, ⁷Li and ²³Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and ²³Na multiple quantum MAS (MQMAS) NMR spectroscopy. Neutron powder diffraction data were collected on lithium chabazite (space group:  R3̄m, a = 9.3357(5) Å, α = 93.482(4)°, R_wp = 5.83%, R_p = 4.65%, χ² = 1.24) and on a mixed lithium sodium chabazite (space group:  R3̄m, a = 9.3385(5) Å, α = 93.382(4)°, R_wp = 5.94%, R_p = 4.83%, χ² = 1.27). Both neutron diffraction and ⁷Li MAS reveal lithium chabazite to have two cationic sites:  one at the six-ring window of the hexagonal prism (SII) and one in the supercage at the four-ring window of the hexagonal prism (SIII). Mixed lithium−sodium chabazites reveal strong evidence of selective occupancy accompanied by concomitant rearrangement effects. While the introduction of sodium into lithium chabazite reduces occupation primarily at the SIII site, a decrease of the SII site lithium cation population is also observed at sodium levels above 24%. At low sodium content, sodium cations occupy a site in the eight-ring window of the channel (SIII‘). At sodium content around 70% and higher, sodium cations also reside at the SII sites vacated by the lithium cations. The increased population of SII sites by Na⁺ is associated with a marked increase in the lattice constant. The implications of the observed site preferences for noncryogenic air separation are discussed.