| Summary: | Efficient CO<sub>2</sub> capture materials must possess a high adsorption capacity, suitable CO<sub>2</sub> adsorption enthalpy and resistance to water vapor. We have recently reported that Ca<sup>2+</sup> cations exchanged in FAU zeolite can attach up to three CO<sub>2</sub> molecules. Here we report the effect of water on the adsorption of CO<sub>2</sub>. Formation of Ca<sup>2+</sup>(H<sub>2</sub>O)(CO<sub>2</sub>), Ca<sup>2+</sup>(H<sub>2</sub>O)(CO<sub>2</sub>)<sub>2</sub> and Ca<sup>2+</sup>(H<sub>2</sub>O)<sub>2</sub>(CO<sub>2</sub>) mixed ligand complexes were established. The Ca<sup>2+</sup>(H<sub>2</sub>O)(CO<sub>2</sub>) species are readily formed even at ambient temperature and are characterized by ν(<sup>12</sup>CO<sub>2</sub>) and ν(<sup>13</sup>CO<sub>2</sub>) infrared bands at 2358 and 2293 cm<sup>−1</sup>, respectively. The Ca<sup>2+</sup>(H<sub>2</sub>O)(CO<sub>2</sub>)<sub>2</sub> species are produced at low temperature and are identified by a ν(<sup>13</sup>CO<sub>2</sub>) band at 2291 cm<sup>−1</sup>. In the presence of large amounts of water, Ca<sup>2+</sup>(H<sub>2</sub>O)<sub>2</sub>(CO<sub>2</sub>) complexes were also evidenced by ν(<sup>12</sup>CO<sub>2</sub>) and ν(<sup>13</sup>CO<sub>2</sub>) bands at 2348 and 2283 cm<sup>−1</sup>, respectively. The results demonstrate that, although it has a negative effect on CO<sub>2</sub> adsorption uptake, water in moderate amounts does not block CO<sub>2</sub> adsorption sites.
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