Cation selectivity in alkali-exchanged chabazite:: An ab initio periodic study

Ab initio periodic calculations based on local Gaussian basis sets have been performed to investigate the site preference and cation selectivity of H, Li, Na, and K in Al-substituted high-silica dehydrated chabazite. Two cation sittings were considered: the 6-membered ring (SII) and the 8-membered ring (SIII') pores. Structures have been fully relaxed at the Hartree-Fock level of theory, and the relative stabilities were verified with density functional single point calculations employing the B3-LYP hybrid functional. Computed ion site preferences for Li and K are in agreement with experimental evidence: Li is more stable at SII whereas K prefers the SHY site. For Na, our calculations show that the more stable location is the SII site, at variance with the experimentally observed site preference which depends on the Si/Al ratio. Computed zeolite framework selectivity toward hydrated cations follows the sequence K > Na > Li in line with observed trends. Computed electric field gradients have also been used to evaluate the quadrupolar parameters as a further characterization of the cation positions.