Unprecedented CO2 adsorption behaviour by 5A-type zeolite discovered in lower pressure region and at 300 K

Abstract

Mitigation of the amounts of CO₂ in the environment is one of the most urgent problems requiring a solution. To fulfil this demand, efficient adsorbents for CO₂ are required that work at room temperature (RT) and in a lower pressure region of not more than 5000 ppm under ambient conditions. In the present work, specific and selective adsorption of CO₂ onto NaCaA-85 (A-type zeolite with an ion-exchange capacity of 85%) was observed under the required conditions; the amounts of adsorbed CO₂ on the NaCaA-85 sample were far larger than amounts reported for other materials in the pressure range from 400 to 5000 ppm. The characteristic adsorption mechanism induced by this material was verified directly through methods combining synchrotron-based far-infrared (far-IR) measurements with a computational technique. The resultant Ca²⁺-framework vibration modes for the NaCaA-85 sample were observed at 266 and 246 cm⁻¹, which shifted toward the lower wave-numbers, i.e., 225 and 203 cm⁻¹, after CO₂ adsorption at RT, respectively. The observed characteristic property was explained by a model consisting of a CO₂ molecule simultaneously pinned by two Ca²⁺ ions positioned on two types of exchangeable sites composed of 8- and 6-membered rings, which was well supported by the density functional theory calculation method. This characteristically adsorbed CO₂ species was completely desorbed, and the original state easily recovered through evacuation around 400 K. In addition, the selective adsorption behaviour of CO₂ from other gases, such as H₂, CH₄, O₂ and N₂, was found at RT. On the basis of these data, the separation properties of CO₂ were examined by measuring the breakthrough curve using a model gas composed of 0.04% CO₂, 20% O₂ and 79.96% N₂, which mimicked ambient air, indicating the superior separation feature. These findings may pave a new way for the use of the NaCaA-85 material as an efficient adsorbent for selective CO₂ adsorption functioning at RT and in the lower pressure region of up to 5000 ppm.