CO 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 (CO) emitted from industrial processes at moderate temperature. The CO adsorption capacity of clinoptilolite (a commercial natural zeolite) and ion-exchanged (with Na and Ca) clinoptilolite were tested under both dynamic (using a fixed-bed reactor operating with 10% vol. CO in N) and equilibrium conditions (measuring single component adsorption isotherms). The dynamic CO 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 CO 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 CO adsorption on clinoptilolite compared to the other samples. The surface species affecting the q and adsorption capacity were investigated through the FTIR spectroscopy using CO as probe molecule. As a whole, it has been observed that CO forms linear adducts onto K and Mg 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···OCO) and dual (Na···OCO⋯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 CO removal from different flue gases sources operating at such temperatures.