CO₂ Adsorption Behavior of Nanosized CHA Zeolites Synthesized in the Presence of Barium or Calcium Cations

Alkaline-earth metal cations impact the rate of crystallization and the adsorption of N-2 and CO2 on nanosized chabazite zeolites. In this study, either calcium or barium is added to precursor mixtures containing alkali-metal cations (Na+, K+, Cs+) to prepare chabazite (labeled Ca-CHA and Ba-CHA), and are compared to a reference sample (Ref-CHA) synthesized using exclusively Na+, K+, and Cs+. Partial substitution of the Na+ and the pore-blocking Cs+ extra-framework cations is observed for Ca-CHA depending on the molar ratio of K2O used in the synthesis, while a change to the amount of Na+ cations only is observed for Ba-CHA. The type of alkaline-earth metal cation affects the crystallization rate; slower in the presence of Ca2+ (10 h to full crystallinity) and similar rates in the presence of Ba2+ (4 h to full crystallinity); the crystallite size and morphology remained similar. The presence of Ca2+ or Ba2+ extra-framework cations leads to N-2 uptake values of 290 and 169 mmol g(-1) (-196 degrees C, 100 kPa), respectively, while at low CO2 pressure (<1 kPa, 25 degrees C), the physisorbed CO2 capacity for Ref-CHA, Ca-CHA, and Ba-CHA zeolites is 0.63, 0.66, and 0.59 mmol g(-1), respectively. Interestingly, an opposite effect is observed for the amount of chemisorbed CO2 species.