A Study of Structural Defects in X- and Y-Type Zeolites and Their Effect on Their Transformation to Aluminum-Rich Chabazite

From extensive synthetic studies to prepare aluminum-rich (Al-rich) chabazite (CHA), using the interzeolite conversion (IZC) of NaY, it was found that defects in the FAU precursor are crucial for obtaining phase pure CHA. This discovery led to extending the IZC approach to the conversion of treated NaX to Al-rich CHA, which positively effects process economics. Studies showed that deamination was most effective at producing defects and therefore directing the synthesis to phase pure CHA. Characterization of the defects included N2 physisorption, 29Si MAS NMR, 29Si CPMAS NMR, and 27Al MAS NMR. Steam alone was effective for producing a self-bound NaX precursor that, after K+ exchange, could be converted in situ to CHA without any macroscopic change in the extrudate. The O2 and Ar isotherms at 175 K were used to probe eight-ring access and showed a drastic change in O2/Ar selectivity caused by adding a single monovalent cation to the composition. Kinetic and equilibrium ion exchange studies were used to optimize the batch ion exchange conditions for a single contact Na+ exchange to obtain the desired potassium level. The adsorption properties at 175 K for the single contact Na-exchanged CHAs were consistent with those of 3KNaCHA(1.5) prepared from a stoichiometric K+ back exchange of NaCHA(1.5), validating the mixed cation batch exchange method.