High-flux polyamide thin-film composite membranes consisting of Tröger's base motif with enhanced microporosity for nanofiltration

Abstract

High-flux and selective membranes are key components to improving the energy efficiency of nanofiltration processes for water purification. Herein, we report high-flux polyamide thin-film composite (TFC) membranes consisting of Tröger's base for nanofiltration. Tröger's base diamine (TBD) was synthesized as an aqueous phase monomer for interfacial polymerization. Detailed characterization of TBD-based polyamides was performed using thermal, spectroscopic, and microscopic analyses. Notably, the V-shaped and rigid Tröger's base motif rendered TBD-based polyamide (named TBD-TMC) features with enhanced microporosity as well as an enlarged pore size compared to conventional polyamide chemistry (i.e., MPD-TMC). As a result, the TBD-TMC membrane exhibited a 570 % improvement in water permeance compared to MPD-TMC membranes while exhibiting moderate salt rejection up to 91 %, outperforming most reported nanofiltration membranes. Also, the TBD-TMC membrane exhibited high monovalent/divalent ion selectivity (∼7.0 for NaCl/Na₂SO₄ separation), which may have resulted from the combined effects of size exclusion and charge repulsion. This work highlights the potential of Tröger's base motif as a new diamine monomer for interfacially polymerized membranes to tune their microporous structures.