Carbon dioxide (CO2) absorption behavior of mixed matrix polymer composites containing a flexible coordination polymer

Abstract

Mixed matrix membranes (MMMs) comprised of metal organic frameworks (MOFs) dispersed in organic polymers are popular materials under study for potential applications in gas separations. However, research on MMMs containing structurally dynamic sorbents known as flexible MOFs has only very recently appeared in the literature. The thermodynamic requirements of the structure transition between the low porosity and high porosity phases of flexible MOFs may provide a mechanism for high adsorption selectivity in these materials. A fundamental question in MMMs containing flexible MOFs is how the constraint of the polymer matrix on the intrinsic expansion of the flexible MOF particles that occurs during gas adsorption might affect the thermodynamics of this structural phase transition and influence the gas adsorption properties of the embedded MOF. To investigate the fundamental nature of this flexible MOF–polymer interface, thin films of ∼20um thickness were prepared using the flexible linear chain coordination polymer catena-bis(dibenzoylmethanato)-(4,4′bipyridyl)nickel(II) “Ni(Bpy)(DBM)2” embedded as 35wt% dispersions in Matrimid®, polystyrene, and polysulfone. The adsorption of CO2 in the polymers and embedded particles was studied using in situ ATR–FTIR spectroscopy and variable temperature volumetric CO2 adsorption/desorption isotherms. Interestingly, no effect of the polymer matrix on the gas adsorption behavior of the embedded Ni(Bpy)(DBM)2 particles was observed. The composite samples all showed the same threshold pressures for CO2 absorption and desorption hysteresis associated with the structural phase change in the polymer embedded Ni(Bpy)(DBM)2 particles as was observed in the pristine polycrystalline sample. The current results contrast those recently reported for a MMM containing the flexible MOF “NH2-MIL-53” where a significant increase in the threshold pressure for CO2 adsorption associated with the structural phase change of the MOF was observed in the MMM as compared to the isolated MOF. The conflicting behaviors in these two systems are rationalized from the large differences in unit cell expansions between the two MOFs during the CO2 adsorption process.