ABSTRACT
We show that flexible metal-organic frameworks (MOFs) exhibiting "gate openings/closings" for CO2 can intrinsically suppress the exothermic heat released by adsorption and the endothermic heat gained by desorption, both of which reduce the working capacity of CO2 in a separation process under near-adiabatic conditions. We use the elastic layer-structured metal-organic framework-11 (ELM-11) [Cu(4,4'-bipyridine)2(BF4)2], which exhibits a two-step gate-adsorption isotherm, as a model system for flexible MOFs, and perform free energy analyses with the aid of grand canonical Monte Carlo simulations for ELM-11 structures that were determined by the Rietveld method using in situ synchrotron X-ray powder diffraction data. We demonstrate that the thermal management capabilities of ELM-11 showing the two-step gating for CO2 at lower and higher pressures are nearly identical and quite effective (41% and 44% at 298 K, respectively). Moreover, we show that ELM-11 has an extremely high CO2 selectivity for both CO2/N2 and CO2/CH4 mixtures at 298 K that, in addition to the intrinsic thermal management capability, is a crucial factor for application to carbon capture and storage (CCS). The multigate closing pressures of ELM-11 are not necessarily matched to the operating pressures used in CCS; however, our findings, and perspectives based on free energy analyses regarding modification of the host framework structure to tune the gating pressure, suggest that flexible MOFs exhibiting multigate openings/closings are promising materials for further development into systems with intrinsic thermal management mechanisms for CCS applications.
