Hollow ZIF-8 nanoparticles improve the permeability of mixed matrix membranes for CO₂/CH₄ gas separation

Solvothermal surface coating was employed to form hetero-nanostructures consisting of a polystyrene (PS) core and ZIF-8 shell. Upon selective removal of the PS core, hollow zeolite imidazole frameworks (H_ZIF-8) with a particle size of 700nm and surface area of 1528.5m²/g were generated. Synthesis was confirmed by Field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) analysis. Free-standing mixed matrix membranes (MMMs) were prepared by dispersing H_ZIF-8 filler in poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) graft copolymer matrix. PVC-g-POEM worked well as a robust matrix to homogeneously disperse H_ZIF-8 with good interfacial contact due to the microphase-separated, amphiphilic properties of the hydrophobic glassy PVC main chains and the hydrophilic rubbery POEM side chains. Secondary bonding interactions were responsible for the good interfacial properties of the MMMs, as confirmed by Fourier-transform infrared (FT-IR) spectroscopy, differential scanning calorimetry (DSC), FE-SEM, and XRD analysis. MMMs exhibited significantly enhanced CO₂ permeability relative to pure PVC-g-POEM membranes: an 8.9-fold increase in permeability from 70.0 to 623Barrer (1Barrer=1×10^-10cm³ (STP)˙cm/cm²scmHg) with only a small decrease in CO₂/CH₄ selectivity from 13.7 to 11.2 at 35°C, as measured via the time-lag method.