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 CO2 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.
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