Impact of framework topologies on CO2 adsorption and transport properties in Zr-fumarate metal–organic frameworks and their mixed-matrix membranes

Herein, we present a comparative study aimed at disclosing the role of framework topologies and pore chemical environments in governing the CO2 adsorption, dynamics and transport properties of the two most widely used fumarate-based Zr-MOFs, namely, MOF-801 and MIP-203. The two MOFs were synthesized under the conditions reported in the literature, and their CO2 adsorption properties were determined via volumetric gas sorption. The CO2-loaded samples were studied using ex situ powder X-ray diffraction (PXRD) and multinuclear solid-state NMR (SSNMR) spectroscopy, revealing different interactions and dynamics of the adsorbed molecules within the two distinct frameworks, with MIP-203 exhibiting stronger confinement effects and restricted CO2 motion, in contrast to the isotropic dynamics observed in MOF-801. To bridge molecular-scale behaviour with macroscopic performance in CO2 capture and separation, both MOFs were employed as fillers for the preparation of mixed-matrix membranes (MMMs) based on a polymer of intrinsic microporosity, PIM-1. The CO2 transport properties of the two MMMs were evaluated by combining permeation/diffusivity measurements with the SSNMR analysis. The distinct CO2 dynamics in the pristine MOFs were preserved within the MMMs. Notably, the incorporation of MIP-203 into PIM-1 led to enhanced permeability, primarily driven by an increase in the diffusion coefficient with respect to MOF-801. These findings highlight how different connectivities and pore environments in MOFs affect CO2 behaviour, from its interaction with the framework to its transport in membranes.