Potassium Cation Effects on Site Preferences in the Mixed Cation Zeolite Li,Na−Chabazite

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Abstract

The influence of potassium ions on the locations of Li+ and Na+ cations in dehydrated chabazites was studied by 7Li and 23Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and 23Na multiple quantum MAS (MQMAS) NMR spectroscopy. 7Li MAS NMR reveals 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 for lithium cations accompanied by concomitant rearrangement effects. While the introduction of sodium and potassium into lithium chabazite reduces the Li site occupancies, no changes in lithium site preferences are observed: the SII site is favored over the SIII site. At low sodium content, sodium cations occupy a site in the eight-ring window of the channel (SIII‘) and at the center of the hexagonal prism (SI). In the presence of potassium, sodium cations reside at the SII sites vacated by the lithium cations at much lower sodium content than is observed in the lithium−sodium binary system. The introduction of potassium is associated with a marked increase in the lattice constant, resulting in the elimination of sodium cation site preferences.

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