Enhancement of the CO 2 Retention Capacity of Y Zeolites by Na and Cs Treatments: Effect of Adsorption Temperature and Water Treatment Eva Dı ´az, Emilio Mun ˜ oz, Aurelio Vega, and Salvador Ordo ´ n ˜ ez* Department of Chemical Engineering and EnVironmental Technology, UniVersity of OViedo, Julia ´ n ClaVerı ´a s/n, 33006 OViedo, Spain Adsorption of carbon dioxide on parent and alkaline-modified Y zeolites was investigated by temperature- programmed desorption (TPD) analysis of these materials previously saturated with CO 2 at different temperatures (50-200 °C). Parent zeolite was treated with different sodium and cesium aqueous solutions, using both carbonates and hydroxides as precursors. Morphological, crystallographic, and chemical properties of these materials were determined by nitrogen physisorption, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS), and NH 3 -TPD. Slight desilication phenomena were observed when hydroxides were used as precursors, whereas the treatment with Cs salts leads to higher crystallinity losses. Several successive adsorption-desorption cycles were carried out in order to check the stability of the adsorbents. Both Cs treatments lead to an enhancement of the retention capacity when adsorption is carried out at the highest temperatures (100-200 °C), whereas Na treatments hardly affect adsorption properties of the parent material. The evolution of the retention capacity of carbon dioxide after water adsorption-desorption was also studied. It was observed that water pretreatment enhances the adsorption capacity of the Cs-treated zeolites. 1. Introduction Carbon dioxide, mainly emitted from combustion processes and industrial plants, is the major contributor to global warming. Carbon dioxide sequestration is considered the only real possibility for a tight control of the CO 2 emissions. This sequestration involves three different steps: separation or capture, transportation, and storage. Among these steps, the separation processes are the most expensive. 1 Therefore, the development of CO 2 separation technologies is currently a key issue in environmental and chemical engineering. Nowadays, the most common method for carbon dioxide capture is gas absorption, monoethanolamine (MEA) being the most usual solvent. However, this operation has important drawbacks: degradation of the solvent at process conditions and the large amounts of energy required for regeneration (energy demands are estimated to be 330-340 kWh/ton CO 2 recovered). 2,3 Adsorption processes can overcome these draw- backs, because of its low energy requirement, cost advantage, and easiness of applicability over a relatively wide range of temperatures and pressures. 4 Activated carbons, and especially activated carbons with increased basic character, have been successfully used for carbon dioxide adsorption due to their high surface area and pore volume. 5 They present high adsorption capacity at low CO 2 concentration and fast intraparticle mass transfer. 6 However, activated carbons are not stable at the relatively high temperatures (>200 °C) 7 at which the adsorption of CO 2 from off-gases should be carried out. Zeolitic materials could be an interesting alternative for these purposes. Thus, several works in the literature dealt with CO 2 adsorption over different types of zeolites: X, 8-13 Y, 9,14,15 A, 8,9 and other natural zeolites. Na-Y zeolite has been suggested because of its easier regenerability. 15 In a previous work, 16 we have found that, at low temperatures (50 °C), X zeolite performs better than Y zeolite, but the retention capacity strongly decreases as tem- perature increases, and the presence of water severely inhibits the adsorption of CO 2 . It must be also considered that the cation incorporated to the zeolite structure can largely modify the chemical properties of the material. The addition of Cs to other materials, such as calcium oxides, has shown to largely increase the CO 2 adsorp- tion capacity, 17 whereas the exchange of Na-X zeolite with this metal has shown to increase the basic character of these zeolites for base-catalyzed reactions. 18 On the other hand, the treatment of zeolites in aqueous alkaline media (such as solutions of group I metal hydroxides) can lead to a partial desilication of the zeolites, altering their morphology (development of additional mesoporosity, increasing the accessibility of the active sites) and surface reactivity (increasing their hydrophilic character). This effect is widely described in the literature for high Si/Al ratio zeolites, 19,20 but not for zeolites with lower Si/ Al ratio, such as the Y zeolites. So, the scope of this work was to carry out a systematic study of the effect of the modification of Y zeolites by Cs addition on the adsorption properties, testing two of the most conven- tional precursors: hydroxide and carbonate. 17 To separate the potential effects of the change of the electronegative of the countercation and the potential desilication, parallel treatment of the Y zeolite with sodium hydroxide and carbonate was also carried out. Comparison of the materials was made in terms of total adsorption capacity after two adsorption-desorption cycles at 50, 100, and 200 °C and performance after water desorption. 2. Experimental Section Materials Preparation. The commercial zeolite (Na-Y) used in this study was purchased from Zeolyst Corp. The alkaline treatment of the zeolites was carried out at 70 °C for 2 h, followed by drying at 100 °C for 12 h and calcination at 650 °C for 4 h (Cs 2 CO 3 decomposes at 612 °C, and Na 2 CO 3 , at 400 °C). Alkali metal solutions (0.5 M) were prepared by dissolving CsOH or Cs 2 CO 3 (both from Avocado) into distilled water. In each case, 2 g of zeolite was suspended into 100 mL * To whom correspondence should be addressed. Tel.: +34 985 10 34 37. Fax: +34 985 103 434. E-mail: sordonez@uniovi.es. 412 Ind. Eng. Chem. Res. 2008, 47, 412-418 10.1021/ie070685c CCC: $40.75 © 2008 American Chemical Society Published on Web 12/15/2007