Tailoring Gas Transport Properties in MOF-Based Mixed Matrix Membranes: The Role of Filler Morphology and Surface Functionalization

The development of high-performance mixed-matrix membranes (MMMs) for gas separations relies on precise control of filler morphology and optimized polymer–filler interactions. In this study, we elucidate how crystal size, morphology, and surface functionalization of the perfluorinated metal–organic framework (MOF) F4_MIL-140A(Ce) influence the gas transport properties of MMMs based on Pebax 1657 and Matrimid 9725 matrices. Through systematic modulation of reaction conditions, we synthesized submicrometric F4_MIL-140A(Ce) particles with tailored size and aspect ratio, achieving improved dispersion and interfacial contact within the polymer phases. A postsynthetic grafting strategy using stearic acid further enhanced the compatibility of the MOF with the polymer matrix without altering crystallinity, pore accessibility, or CO2 adsorption behavior, as confirmed by powder X-ray diffraction, scanning electron microscopy, and sorption analyses. MMMs prepared with the surface-functionalized MOF displayed improved polymer–filler adhesion, minimized interfacial voids, and significantly enhanced gas transport properties. In particular, the Pebax 1657-based MMMs approached the 2008 Robeson upper bound for CO2/N2 separation, primarily driven by increased CO2 solubility, which increased the CO2 permeability by 62% (from 76 to 123 Barrer) and the CO2/N2 selectivity by 28% (from 58 to 74). In contrast, Matrimid 9725 MMMs exhibited substantial improvements in H2/N2 and O2/N2 selectivity (i.e., from 93 to 108 and from 6.6 to 7.7, respectively), because of enhanced diffusivity. The insights presented in this work provide a rational framework for engineering MOF–polymer interfaces and advancing membrane-based gas separation and CO2 capture technologies.