ISOTHERM AND SORPTION MECHANISM OF AMMONIA ADSORPTION IN LTA ZEOLITES

The article presents experimentally obtained values of the adsorption isotherm of ammonia molecules in LTA zeolites at a temperature of 303 K. The isotherm was measured using a DAK-1-1A microcalorimeter connected to a universal high-vacuum system. A consistent relationship between the adsorption capacity and the energetic characteristics of ammonia molecules in LTA zeolites was established. In addition, the sorption mechanism was elucidated over the entire experimental range-from the initial adsorption region to higher loadings-along with the regularities of pore volume filling by ammonia molecules. It was determined that under the given experimental conditions, the adsorption capacity of these zeolites toward ammonia varies significantly and depends on the type and content of extra-framework cations present in the zeolite structure. For the Ca4Na4A zeolite, the adsorption isotherm was successfully described using the three-term equation of the theory of volume filling of micropores (TVFM). The theoretically calculated values were found to be in full agreement with the experimental data. Furthermore, it was established that ammonia molecules form tetrameric ion-molecular complexes of the type 4NH3:Me+ with calcium and sodium cations in the zeolite framework.