Site preferences in the mixed cation zeolite Li,Na-chabazite: A combined solid-state NMR and neutron diffraction study

Journal Article · Wed Mar 01 04:00:00 EST 2000 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/ja992882b· OSTI ID:20017334

Smith, L J; Eckert, H; Cheetham, A K

The locations of Li{sup +} and Na{sup +} cations in dehydrated chabazite were studied by neutron powder diffraction, {sup 7}Li and {sup 23}Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and {sup 23}Na multiple quantum MAS (MQMAS) NMR spectroscopy. Neutron powder diffraction data were collected on lithium chabazite (space group: R{bar 3}m, a = 9.3357(5){angstrom}, {alpha} = 93.482(4){degree}, R{sub wp} = 5.83%, R{sub p} = 4.65%, {sub {chi}}{sup 2} = 1.24) and on a mixed lithium sodium chabazite (space group: R{bar 3}m, a = 9.3385(5){angstrom}, {alpha} = 93.382(4){degree}, R{sub wp} = 5.94%, R{sub p} = 4.83%, {sub {chi}}{sup 2} = 1.27). Both neutron diffraction and {sup 7}Li 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 hexagonalprism (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{prime}). 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{sup +} is associated with a marked increase in the lattice constant. The implications of the observed site preferences for noncryogenic air separation are discussed.

• Research Organization: Univ. of California, Santa Barbara, CA (US)
• Sponsoring Organization: US Department of Energy; Wissenschaftministerium Nordrhein-Westfalen
• OSTI ID: 20017334
• Journal Information: Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 8 Vol. 122; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CRYSTAL STRUCTURE
LITHIUM
NEUTRON DIFFRACTION
NUCLEAR MAGNETIC RESONANCE
SODIUM
ZEOLITES