Effect of Counter Cations on the Hydrothermal Conversion of FAU-Type Zeolites into ABW or ANA and Their Potential Applicability for CO2/N2 Separation

In this study, the hydrothermal conversion of FAU-type zeolites using an aqueous ammonia solution was investigated to understand the solid-phase transformation mechanism of the selective synthesis of target zeolites. In addition, the CO2/N-2 separation ability of prepared materials was estimated from single-component adsorption isotherms by the ideal adsorbed solution theory. ABW and ANA-type zeolites were successfully synthesized from the FAU-type zeolites such as Li-LSX and Na-X, respectively, by a hydrothermal treatment using an aqueous ammonia solution. Li-LSX was completely converted into ABW by treating at a temperature above 150 degrees C for 24h, and ANA was mainly produced from Na-X at 250 degrees C after treating for over 72 h. The pellet shape of the raw zeolites was maintained even after the hydrothermal treatment. Considering the transformation mechanisms, the results suggest that the transformation of the zeolite topology was caused by the ion exchange between the counter cation of the zeolite and ammonium ion in the solvent and the cation structure-directing effect under hydrothermal conditions. It can be concluded that the counter cations of the raw zeolites played an important role in the structural transformation. The simple synthesis method presented in this study enables the conversion of FAU-type zeolites, particularly X-type zeolite, into other types of zeolites without using strong bases, such as alkali hydroxides. In terms of the CO2/N-2 separation ability, CO2/N-2 selectivity for post-combustion CO2 capture conditions drastically increased upon the hydrothermal treatment. In particular, the ABW-type zeolite exhibited extremely high CO2/N-2 selectivity over 10000.