Material Exhibiting Efficient CO₂ Adsorption at Room Temperature for Concentrations Lower Than 1000 ppm: Elucidation of the State of Barium Ion Exchanged in an MFI-Type Zeolite

Carbon dioxide (CO₂) gas is well-known as a greenhouse gas that leads to global warming. Many efforts have been made to capture CO₂ from coal-fired power plants, as well as to reduce the amounts of excess CO₂ in the atmosphere to around 400 ppm. However, this is not a simple task, particularly in the lower pressure region than 1000 ppm. This is because the CO₂ molecule is chemically stable and has a relatively low reactivity. In the present study, the CO₂ adsorption at room temperature on MFI-type zeolites exchanged with alkaline-earth-metal ions, with focus on CO₂ concentrations <1000 ppm, was investigated both experimentally and by calculation. These materials exhibited a particularly efficient adsorption capability for CO₂, compared with other presented samples, such as the sodium-form and transition-metal ion-exchanged MFI-type zeolites. Ethyne (C₂H₂) was used as a probe molecule. Analyses were carried out with IR spectroscopy and X-ray absorption, and provided significant information regarding the presence of the M²⁺-O^2--M²⁺ (M²⁺: alkaline-earth-metal ion) species formed in the samples. It was subsequently determined that this species acts as a highly efficient site for CO₂ adsorption at room temperature under very low pressure, compared to a single M²⁺ species. A further advantage is that this material can be easily regenerated by a treatment, e.g., through the application of the temperature swing adsorption process, at relatively low temperatures (300-473 K).