Adsorption and separation of light gases on an amino-functionalized metal-organic framework: An adsorption and in situ XRD study

The NH₂-MIL-53(Al) metal-organic framework was studied for its use in the separation of CO₂ from CH₄, H₂, N₂ C₂H₆ and C₃H₈ mixtures. Isotherms of methane, ethane, propane, hydrogen, nitrogen, and CO ₂ were measured. The atypical shape of these isotherms is attributed to the breathing properties of the material, in which a transition from a very narrow pore form to a narrow pore form and from a narrow pore form to a large pore form occurs, depending on the total pressure and the nature of the adsorbate, as demonstrated by in situ XRD patterns measured during adsorption. Apart from CO₂, all tested gases interacted weakly with the adsorbent. As a result, they are excluded from adsorption in the narrow pore form of the material at low pressure. CO₂ interacted much more strongly and was adsorbed in significant amounts at low pressure. This gives the material excellent properties to separate CO₂ from other gases. The separation of CO₂ from methane, nitrogen, hydrogen, or a combination of these gases has been demonstrated by breakthrough experiments using pellets of NH₂-MIL-53(Al). The effect of total pressure (1-30 bar), gas composition, temperature (303-403 K) and contact time has been examined. In all cases, CO₂ was selectively adsorbed, whereas methane, nitrogen, and hydrogen nearly did not adsorb at all. Regeneration of the adsorbent by thermal treatment, inert purge gas stripping, and pressure swing has been demonstrated. The NH₂-MIL-53(Al) pellets retained their selectivity and capacity for more than two years.