A 2D flexible cobalt-MOF: reversible solid-state structural transformation, two-step and gate-opening adsorption behaviours, and selective adsorption of C2H2 over CO2 and CH4.

A new 2D flexible cobalt(II) framework (Co-MOF) exhibits a reversible solid-state structural transformation upon guest molecule removal/uptake. After activation, Co-MOF-α with 1D porous channels transformed into Co-MOF-ß (0D voids) accompanied by a shift in metal and carboxylate coordination modes, the rotation of organic linkers and the contraction of interstitial spaces. Gas adsorption experiments reveal that Co-MOF-ß exhibits a two-step CO2 adsorption isotherm and close-to-open (type F-IV) isotherms for C2H2, C2H4 and C2H6 at 195 K. Moreover, it shows typical type I adsorption isotherms for the above gases and the selective uptake of C2H2 over CH4 and CO2 at room temperature.