Fabrication and characterization of a defect-free mixed matrix membrane by facile mixing PPSU with ZIF-8 core–shell microspheres for solvent-resistant nanofiltration

Metal–organic frameworks (MOFs) nanoparticles can act as microporous fillers to fabricate high-performance mixed matrix membranes (MMMs) for solvent-resistant nanofiltration (SRNF) application. However, a high content of MOFs nanoparticles mixed in a polymeric membrane may be easily to form MOFs aggregates and MOFs/polymer interface defects, attributing to the decrease in membrane selectivity. In this study, ZIF-8 core–shell microspheres were synthesized by growing microporous ZIF-8 nanoparticles on the outer surface of macroporous resin microspheres (RMs), and then polyphenylsulfone (PPSU)/ZIF-8@RMs membranes were prepared in order to avoid the nonselective PPSU/ZIF-8 defects and then to improve SRNF performance of PPSU membranes. The effects of different type of fillers and ZIF-8@RMs content in PPSU MMMs were evaluated with respect to structural properties and SRNF performance. The obtained PPSU/ZIF-8@RMs membranes exhibited higher selectivity compared with the PPSU/ZIF-8 and PPSU/RMs membranes, due to the combination of the affinity of ZIF-8 nanoparticles, the microporous and microsphere structure of the ZIF-8 shells is benefit to reduce the defects between ZIF-8@RMs and PPSU. Compared with the pure PPSU membrane (with a rejection of 53% and a methanol flux of 35.2 kg m−2 h−1) for Methyl red (MR) removal, the optimal PPSU/ZIF-8@RMs membrane with filler content of 5.0 wt% significantly exhibited the improved rejection of 88.8% and the methanol flux slightly decreased to 33.0 kg m−2 h−1 at 1.0 MPa. Moreover, the PPSU/5.0 wt% ZIF-8@RMs membrane remained stable in 12-h SRNF process, and the MR rejection exceeding 86.2% was maintained. This method provides a good prospect for the combination of polymeric membranes and more types of MOF core–shell composite materials.