Adsorption and separation of CO2 from N2-rich gas on zeolites: Na-X faujasite vs Na-mordenite

The CO2 adsorption capacity of Na-MOR and Na-X have been analyzed. The effects of CO2 concentration, activation temperature, water vapor in the feed, and the desorption/regeneration temperature have been considered. CO2 adsorption experiments on Na-X and Na-MOR zeolites were made in static conditions with pure CO2 and in dynamic conditions with N2/CO2 mixtures. CO2 desorption experiments were also made in pure N2 flow. IR experiments of CO2 adsorption on dry and wet surfaces were performed. Over both zeolites, when dry, CO2 is adsorbed reversibly at r.t. in a linear way, as molecular CO2 interacting with Na+ ions in a 1:1 or 2:1 CO2:Na+ ratio. Over both zeolites, non-linear species also form, i.e. either carbonate-like species or bent CO2 molecules, or even both. These species are held more strongly and are only desorbed by flowing nitrogen at 373 K. The faujasite zeolite Na-X displays a higher adsorption capacity at high CO2 partial pressures with respect to Na-MOR, essentially due to the greater amount of active Na+ ions associated to the lower Si/Al ratio in Na-X. CO2 adsorption on Na-MOR is strongly diminished when the feed contains 3% water, while on Na-X CO2 adsorption is slightly enhanced at high PCO2 (60-80%) in wet feed. The IR spectra show that in Na-X CO2 is absorbed as free hydrogencarbonate ion in water clusters adsorbed into the faujasite cavities. This is not found for Na-MOR.