(71g) Adsorption of Carbon Dioxide on Alkali Metal Exchanged Zeolites

The increasing atmospheric CO2 concentration, mainly caused by fossil fuel combustion, has led to concerns about global warming. Most of the emissions of CO2 to the atmosphere from the electricity generation and industrial sectors are currently in the form of flue gas from combustion, in which the CO2 concentration is typically 4-14 % by volume, although CO2 is produced at high concentrations by a few industrial processes. The high flow rates of these flue gases lead to a fast filling of the storage reservoirs. For these reasons it is preferred to produce a relatively pure stream of CO2 for transport and storage; this process is called CO2 capture.

Currently, the most common method for carbon dioxide capture is gas absorption, being monoethanolamine (MEA) the most used solvent. Amine-based absorption technology has been established for over 60 years in the chemical and oil industries, for removal of hydrogen sulphide and other acid gases from gas streams. However, the main concerns with MEA and other amine solvents are corrosion in the presence of O2 and other impurities, high solvent degradation rates because of its reaction with SOX and NO2 and the large amounts of energy required for regeneration [1].

In this work we study the adsorption as alternative for CO2 removal from flue gases. Some solid materials with high surface areas, such as zeolites and activated carbons, can be used to separate CO2 from gas mixtures by adsorption. Gas is fed to a bed of solids that adsorbs CO2 and allows the other gases to pass through. When a bed becomes fully saturated with CO2, the feed gas is switched to another clean adsorption bed and the fully loaded bed is regenerated to remove the CO2. In pressure swing adsorption (PSA), the adsorbent is regenerated by reducing the pressure. In temperature swing adsorption (TSA), the adsorbent is regenerated by raising its temperature and in electric swing adsorption (ESA) regeneration takes place by passing a low-voltage electric current through the adsorbent. Adsorption is not yet considered attractive for large-scale separation of CO2 from flue gas because the capacity and CO2 selectivity of available adsorbents is low. However, it may be successful in combination with another capture technology, and it could be really successfully if adsorbents could operate at higher temperatures in the presence of steam with increased capacity and improved selectivity.

Activated carbons have been widely used as carbon dioxide adsorbents due to their high surface area, which confers them high adsorption capacity. However, this high capacity of adsorption is limited at room temperatures. Przepiorski et al. [2] have tested activated carbons in the capture of CO2 at 25 and 36 ºC, observing an important decrease in the capacity of adsorption in only 9 ºC. For this reason, in this work, we have selected zeolites as adsorbents for carbon dioxide capture. High aluminium content zeolites have been extensively used for separation of gases including carbon dioxide from gas mixtures. Inui et al. [3] studied the relation between the properties of various zeolites and their CO2 adsorption behaviours, concluding that 13X zeolites were the most proper choice. Likewise, Kumar et al. [4] established that NaY zeolite could be a substitute of 13X zeolite due to its easier regenerability. Furthermore, Al-rich zeolites can only be used to purify mixtures of gases which are less polar than CO2, a characteristic that limits its use for cases where the gas steam contains other compounds such as H2O and SO2 [5].

Hence, the present study focuses in the preparation of hydrophobic zeolites which will be able to selectively adsorb carbon dioxide. 13X (NaX) and NaY and those resulting of ion exchanged with Cs, since it is the most electropositive metal of the periodic table, will be tested in the adsorption of CO2. Zeolite NaX (Alltech) and NaY (Zeolyst Corporation) are used as received. The exchange to obtain the alkali exchanged zeolites was carried out at 70 ºC for 2 h, followed by drying at 100 ºC 12 h and calcination at 600 ºC for 4 h. Alkali metal solutions (0.5M) were prepared disolving CsOH (Avocado) or CsCO3 (Avocado) into distilled water. Ion exchange between zeolites and Cs+ solutions was allowed to take place by adding 2 g of zeolite into 100 mL of the metal solution. The ion exchanged zeolites were recovered by filtration and repeatedly washed with distillate water to remove the impurities completely. The resulting zeolites were pretreated at 600 ºC in an oven for 4h in order to remove the moisture and other contaminants prior to the experiments. The unit cell chemical composition of all samples was determined by ICP-MS (percentage of Cs between 16 and 19 %), the zeolitic structure by XRD and the surface area and pore volume by N2 adsorption.

CO2 adsorption was characterised by TPD (temperature-programmed) experiments. 50 mg of sample was introduced in a quartz tube and outgassed in a He flow of 30 mL/min by thermal treatment at 500 ºC for 1 h, with a heating rate of 10 ºC/min from room temperature. After being cooled to 50 ºC, the adsorbent material was contacted with the gaseous feed (pure CO2) for 20 min. The reversibly adsorbed carbon dioxide was the removed by treatment of the sample in He flow for 1 h at 50 ºC. The completion of this desorption process was confirmed by the recovery of the baseline of the mass spectrometer (outgoing gases from the sample were analyzed on-line using a Glaslab 300 quadrupole mass spectrometer). The TPD tests were carried out by heating the sample with a ramp of 10 ºC/min between 50 ºC and 600 ºC with constant He flow. In order to study the regenerability of the adsorbents, after keeping the latter temperature constant for 60 min, the sample is cooled to 50 ºC and the adsorption process repeated. The selectivity for CO2 adsorption in presence of water vapor is studied saturating the sample at 50 ºC with water, by successive injection of water pulses, and then the desorption process is carried out according to the previous described method. Once the sample is cooled to 50 ºC, it is saturated with CO2 in order to evaluate its adsorption after the water adsorption.

From the TPD curves obtained, it is observed that the temperature corresponding to the major peak of the desorption curve is displaced towards higher temperatures, both over NaX and NaY zeolites, after Cs incorporation. The following increasing order for the temperature of the major peak is observed: NaY (157 ºC)