Potassium cation effects on site preferences in the mixed cation zeolite Li,Na-chabazite

The influence of potassium ions on the locations of Li+ and Naf cations in dehydrated chabazites was studied by Li-7 and Na-23 magic-angle spinning (M AS) nuclear magnetic resonance (NMR) spectroscopy and Na-23 multiple quantu m MAS (MQMAS) NMR spectroscopy. 7Li MAS NMR reveals lithium chabazite to ha ve two cationic sites: one at the six-ring window of the hexagonal prism (S II) and one in the supercage at the four-ring window of the hexagonal prism (SIII). Mixed lithium-sodium chabazites reveal strong evidence of selectiv e occupancy for lithium cations accompanied by concomitant rearrangement ef fects. While the introduction of sodium and potassium into lithium chabazit e reduces the Li site occupancies, no changes in lithium site preferences a re observed: the SII site is favored over the SIII site. At low sodium cont ent, 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 po tassium, sodium cations reside at the SII sites vacated by the lithium cati ons at much lower sodium content than is observed in the lithium-sodium bin ary system. The introduction of potassium is associated with a marked incre ase in the lattice constant, resulting in the elimination of sodium cation site preferences.