Near-Perfect CO₂/CH₄ Selectivity Achieved through Reversible Guest Templating in the Flexible Metal-Organic Framework...

Metal-organic frameworks are among the most promising materials for industrial gas separations, including the removal of carbon dioxide from natural gas, although substantial improvements in adsorption selectivity are still sought. Herein, we use equilibrium adsorption experiments to demonstrate that the flexible metal-organic framework Co(bdp) (bdp^2- = 1,4-benzenedipyrazolate) exhibits a large CO₂ adsorption capacity and approaches complete exclusion of CH₄ under 50:50 mixtures of the two gases, leading to outstanding CO₂/CH₄ selectivity under these conditions. In situ powder X-ray diffraction data indicate that this selectivity arises from reversible guest templating, in which the framework expands to form a CO₂ clathrate and then collapses to the nontemplated phase upon desorption. Under an atmosphere dominated by CH₄, Co(bdp) adsorbs minor amounts of CH₄ along with CO₂, highlighting the importance of studying all relevant pressure and composition ranges via multicomponent measurements when examining mixed-gas selectivity in structurally flexible materials. Altogether, these results show that Co(bdp) may be a promising CO₂/CH₄ separation material and provide insights for the further study of flexible adsorbents for gas separations.