Influence of casting solvents on sedimentation and performance in metal–organic framework mixed-matrix membranes

Mixed matrix membranes (MMMs) for gas separation is a promising application and has received much attention. The construction of membranes ultimately decides the performance of the membrane, with the underlying interaction between fillers and polymer playing the most important role concerning pathways for gas molecules. In this study, we compared the interactions between the metal–organic framework MIL-53(Al) and polysulfone (PSF) in MMMs with various concentrations of PSF in two different casting solvents chloroform and tetrahydrofuran (THF). We detail the preparation of membranes with varying degrees of sedimentation, subsequent analysis, and gas permeation performance. The morphology of MMMs prepared in this work exhibit filler sedimentation at low precursor solution concentrations, which were characterized by scanning electron microscopy (SEM) with positions of filler further detected with energy-dispersive X-ray spectroscopy (EDS). The results of X-ray diffraction (XRD) confirmed obvious discrepancy of intensity caused by sedimentation. Moreover, we investigated the relationship between viscosity and thermal analysis, measured by rheometer, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), where the highest T_g and best thermal stability in moderate viscosity of 60–70 Pa s also matched with a homogeneous morphology with stronger interaction between fillers and polymer. Further gas permeation measurements showed a promising CO₂/N₂ selectivity of 22.71 for homogenous membrane. Permeation properties of various preparations were also studied, revealing the influence of fillers distribution on gas permeation relating to different diffusion paths for gas molecules in sedimentation, aggregation, and homogeneous morphologies.