Solvent-free technologies based on solid sorbents and membranes have been recognized as a potential solution for reducing operating costs of carbon capture technologies. In principle, solid sorbent and membrane materials can be tailored with specific physico-chemical characteristics to meet the CO2 capture performance targets specific to a particular separation process. However, systematic approaches for designing carbon capture materials with cost-effective capture efficiency, selectivity, adsorption/desorption cycle life, and scale-up potential require identification of the ‘active’ material structure and establishment of the CO2 sorption or separation mechanism. Promising CO2 sorbent materials present an interesting challenge due to the dynamic nature of the material structure that may respond in a complex fashion to various stimuli (e.g., temperature, pressure, and/or gas composition), making it difficult to establish the nature of the interaction between CO2 and the sorbent structure 'in action'.
To design new sorbent materials that meet such performance goals, we must have a detailed understanding of CO2/sorbent interactions. In particular, an important feature in the development of novel CO2 capture materials with engineered flexible architectures and pore sizes is the phenomenon of gas adsorption hysteresis, where the volume of gas molecules adsorbed by the porous host is larger on the desorption branch than on the adsorption. High pressure hysteresis in gas sorption by rigid porous materials has been traditionally attributed to capillary condensation. Sorption hysteresis that persists down to very low pressures can be ascribed to a failure of the adsorptive system to reach equilibrium and is a reflection of specific dynamic that depend on temperature and maximum pressure achieved in the adsorption/desorption cycle.
We have evaluated the CO2 sorption properties of octahedral molecular sieves with manganese oxide frameworks in which the nanopores (prior to contact with CO2) appear to be of comparable size to the kinetic diameter of CO2. We observe time dependant low pressure hysteretic CO2 adsorption and desorption behavior at temperatures slightly below and above the supercritical temperature of CO2. Our results open up the possibility of hysteresis due to a combination of time dependent events involving physico-chemical interactions, structure dynamics, and kinetic CO2 trapping, which may pave the way towards a comprehensive interpretation of hysteretic gas adsorption phenomena.
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