Combination of X-ray powder diffraction and adsorption calorimetry for the characterization of calcium exchanged LTA zeolites

Abstract

While structural and chemical properties of adsorbents are largely known, energetic properties are mostly not known yet. In cation exchanged zeolites, the type, number, and position of cations have a major influence on energetic properties. Therefore, the energetic values of cations were investigated systematically on LTA zeolites. In these CaNaA materials, Na+-cations were selectively replaced by Ca2+-cations with calcium exchange rates between 8.7% and 92.1%. The cation sites were determined by crystal structure analysis of X-ray powder diffraction data (XRPD). Rietveld analysis showed the sequence of emptying Na+-cations from the various cation sites and the occupancy sequence of Ca2+-cations as a function of the exchange rate. In addition, sensor gas calorimetry was used to measure the heat of adsorption using methane as a sample molecule. The results show an increase in capacity and heat of adsorption up to an exchange rate of 68.6%. At higher exchange rates, the capacity and heat of adsorption decrease again. Characteristic plateaus of the heat of adsorption were found for all materials. By combining XRPD and calorimetry, the plateaus of the heat of adsorption were assigned to the different cation positions inside the zeolite cages and the respective energetic contributions were determined. Ca2+-cations represent energetically more valuable adsorption sites compared to Na+-cations. Furthermore, differences in the energetic values of the various cation sites were identified. It was also found, that the interactions with the zeolite framework provide the largest contribution.