Unraveling Topology-Driven Flexibility in Chlorinated Zeolitic Imidazolate Frameworks under CO 2 Pressure Using Experimental and Simulations Study

Gas-framework interaction-induced flexibility of zeolitic imidazolate frameworks (ZIFs) is of paramount interest as it enhances the gas adsorption capacity and can also be exploited to fine-tune the gas separation selectivity of ZIFs. The gas-framework interaction-induced lattice expansion or linker-twisting leading to pore volume enhanement, a signature of flexibility, observed in ZIFs is primarily attributed to the metal-linker composition. Thus, observed flexible behavior of ZIFs was found to be dependent on crystal size, gas type and pressure, and temperature. In the present study, we report a contrasting flexible behavior of two polymorphs of a chlorinated ZIF (ZIF-71 and COK-17 having RHO and SOD topologies, respectively) having the same chemical composition, Zn(4,5-dichloroimidazolate)2. Using in situ positron annihilation lifetime spectroscopy (PALS) under high pressure (≤55 bar) of CO2 gas, structural dynamics at cavity pores of these ZIFs has been investigated. It is observed that ZIF-71 behaves as a rigid framework, whereas COK-17 shows the characteristics of a highly flexible framework through the enlargement of pores as a result of the gas-framework interaction. It is also inferred from the PALS study that the filling mechanism of CO2 molecules in the pore networks of these frameworks is distinctly different. The cavity pores of ZIF-71 are directly filled by CO2 molecules starting from the lower pressure, whereas CO2 molecules migrate to the cavity pores of COK-17 at higher pressure, which remain initially localized at the surrounding ring apertures. Grand canonical Monte Carlo and density functional theory simulations have been performed to get better insights about the host-guest interaction-induced flexibility in these frameworks. It is observed that different adsorption sites are energetically favored in these polymorphs due to their distinct topologies. CO2 adsorption at particular sites in COK-17 leads to cavity pore volume expansion due to ring distortions and outward linker twisting. The present study confirms that the gas pressure-induced flexibility of ZIFs can be exclusively controlled by the topology for their enhanced applications in gas storage and separation.