CO2 capture on natural zeolite clinoptilolite: Effect of temperature and role of the adsorption sites

In this study, the adsorption capacity of the low-cost zeolite clinoptilolite was investigated for capturing carbon dioxide (CO2) emitted from industrial processes at moderate temperature. The CO2 adsorption capacity of clinoptilolite (a commercial natural zeolite) and ion-exchanged (with Na+ and Ca2+) clinoptilolite were tested under both dynamic (using a fixed-bed reactor operating with 10% vol. CO2 in N2) and equilibrium conditions (measuring single component adsorption isotherms). The dynamic CO2 adsorption capacity of bare clinoptilolite and ion-exchanged clinoptilolite were evaluated in the temperature range from 293 K to 338 K and the obtained breakthrough curves were compared with those of the commercial zeolite 13X (Z13X). Although the adsorption capacity of Z13X exceeded those of bare clinoptilolite and ion-exchanged clinoptilolite at 293 K, the clinoptilolite exhibited the highest CO2 uptake at a moderate temperature of 338 K (i.e. 25 % higher than Z13X). This feature appears in agreement with the lower isosteric heat of CO2 adsorption on clinoptilolite compared to the other samples. The surface species affecting the qiso and adsorption capacity were investigated through the FTIR spectroscopy using CO2 as probe molecule. As a whole, it has been observed that CO2 forms linear adducts onto K+ and Mg2+ cations of the bare clinoptilolite, and carbonate-like species onto its basic sites. With the Na-exchanged clinoptilolite, Na+ ions led to a decrease in surface basicity and to the formation of both single (Na+···O[dbnd]C[dbnd]O) and dual (Na+···O[dbnd]C[dbnd]O⋯Na+) cationic sites available for the formation of linear adducts. As a result of the remarkable adsorption capacity of clinoptilolite at 338 K, this material appears to be a promising adsorbent for the direct CO2 removal from different flue gases sources operating at such temperatures.