In situ conversion of ZnO into zeolitic imidazolate framework-8 in polyamide layers for well-structured high-permeance thin-film nanocomposite nanofiltration membranes

Abstract

The incorporation of zeolitic imidazolate framework-8 (ZIF-8) into interfacial polymerized thin film nanocomposites (TFNs) has been extensively reported to enhance their membrane performance. However, the traditional membrane synthesis strategy suffers from the poor dispersity of hydrophobic ZIF-8 in an aqueous phase. Moreover, the exact structure of this TFN membrane is still a subject of scientific debate since only few studies of the visualization of ZIF-8 in PA layers by TEM or SEM characterization have been reported. Herein, a novel strategy for the synthesis of TFN membranes containing ZIF-8 (TFN–ZIF-8) was developed via an in situ conversion of ZnO in a polyamide layer into ZIF-8. Due to the hydrophilic nature of ZnO, it can be well dispersed in the aqueous phase of interfacial polymerization. More importantly, SEM and TEM characterizations clearly revealed the presence of ZIF-8 crystals in the polyamide layer. The as-prepared TFN–ZIF-8 membrane shows a pure water permeance of 20.4 L m⁻² h⁻¹ bar⁻¹, which is 423% higher than that of the pristine TFC membrane, and meanwhile, has 97.4% Na₂SO₄ rejection. The novel TFN–ZIF-8 membrane synthesis strategy provided in this study can provide a new incentive to develop advanced ZIF-8 membranes applicable to various separation processes for further sustainable use.