Study of the CO2/Sorbent Interaction in Sorbents Prepared with Mesoporous Supports and Calcium Compounds

CO2 chemisorption experiments were used to study the behavior of sorbents prepared from MCM-41 and SBA-15 mesoporous supports and Ca(OH)(2) at different ratios in two main aspects: to determine the capacity of the sorbents to capture CO2 and to identify the products synthesized in the process. Two different ways of preparation of the sorbents were used: Type I sorbents were prepared by mixing CaO, water, and the support at room temperature, and Type II were prepared by impregnation of the support with calcium nitrate followed by calcination and hydration to form Ca(OH)(2). Static chemisorption experiments at low temperature (323 K) and low CO2 pressure showed that Type I yielded higher values of CO2 retention than Type II. MCM-41 support, with a higher specific Surface area than SBA-15, achieved the more effective sorbent, 10% Ca(OH)(2)/MCM-41. The moles of CO2 per mole of calcium retained by this sorbent was approximately five times that obtained with unsupported commercial Ca(OH)(2). To determine the form in which CO2 was retained at this low temperature dynamic chemisorption experiments with CO2 were performed with commercial Ca(OH)(2) and with 10% Ca(OH)(2)/MCM-41 sorbent. The results showed that CO2 was chemisorbed on both solids as a complex CaO center dot CO2, which was reversibly desorbed at 473 K, and as CaCO3. The 10% Ca(OH)(2)/MCM-41 sorbent can be used as a regenerable sorbent at temperatures up to 823 K. At those conditions, only the CO2 retained as CaO center dot CO2 is described, with no relevant loss of capture in the five cycles tested. Experiments varying the adsorption time proved that, in a first step, the CO2 retention is fast and probably kinetically controlled, making the further uptake much slower in a process controlled by diffusion; the maximum amount of CO2 retained (56.6% mol CO2/mol Ca) was obtained at 18 h of exposure with the sorbent prepared at the lowest Ca(OH)(2)/MCM-41 ratio tested (2.5%), showing that the reaction at low temperature is favored with a good calcium distribution.