Elucidating the Role of Embedded Metal–Organic Frameworks in Water and Ion Transport Properties in Polymer Nanocomposite Membranes

Abstract

Metal–organic frameworks (MOFs) have been extensively studied as promising nanofillers in developing high-performance polymer nanocomposite membranes (PNMs) for efficient water/ion separation applications. However, given the ambiguous role of embedded MOFs, achieving simultaneous improvement in both water permeability and water/ion selectivity of PNMs remains challenging. Here, we elucidates fundamental water and ion transport properties of MOF/PNMs to better understand the role of embedded MOFs in polymer matrices. We prepared freestanding PNMs consisting of a cross-linked poly(ethylene glycol) (XPEG)-based hydrogel and nanoporous zeolitic imidazole framework-8 (ZIF-8) exhibiting high diffusivity selectivity. The transport studies and material characterizations, especially with Raman mapping analysis showing a homogeneous distribution of permeating water molecules throughout ZIF-8/XPEG PNM, revealed that the incorporated ZIF-8 acts as an additional water-permselective channel inside the polymeric matrix, which leads to an unusual “reverse-selective” ion transport behavior. Ultimately, 20 wt % of ZIF-8 loading could significantly enhance both water permeability (∼240%) and water/NaCl selectivity (∼160%) compared to a pure polymer membrane by overcoming the conventional permeability–selectivity trade-off limitation. Our finding provides new insights for developing advanced PNMs for water/ion separation.