Adsorption of CO2 on Sodium-Exchanged Ferrierites: The Bridged CO2 Complexes Formed between Two Extraframework Cations

Isosteric heats of adsorption of CO2 on sodium-exchanged ferrierites (Na-FER)-with different Si/Al ratios were obtained from the adsorption isotherms recorded in the temperature range between 273 and 333 K. The isosteric heats of adsorption significantly depend on the content of the Na+ cation in the zeolite. Large isosteric heats of adsorption were obtained for Na-FER when Si/Al = 8.7. On the basis of calculations employing the periodic density functional theory (DFT) model, these large isosteric heats are attributed to the formation of the linearly bridged CO2 adsorption complexes formed between a pair of Na+ cations located in cationic positions. The CO2 adsorption complexes formed on such dual-cation sites are ca. 10 kJ/mol more stable than the CO2 adsorption complexes where CO2 interacts only with a single Na cation.