Post-synthetic modification of MOFs to enhance interfacial compatibility and selectivity of thin-film nanocomposite (TFN) membranes for water purification

Efficient discrimination of ions and small neutral contaminants remains a challenge for polyamide thin-film nanocomposite (TFN) water separation membranes due to the presence of interfacial defects and particle-agglomeration-induced nonselective voids in the polyamide layer. In this work, highly selective TFN membranes were successfully fabricated by leveraging post-synthetic modifications of (3-glycidyloxypropyl)triethoxysilane (GPTES) and trimesoyl chloride (TMC) on MIL-101(Cr)–NH2 MOF filler nanoparticles. GPTES modification suppresses particle agglomeration during interfacial polymerization for membrane formation by forming a stable particle suspension in the organic monomer solution, while TMC-induced in situ chemical crosslinking between filler particles and polyamide addresses the phase compatibility and interfacial issues at the filler–polyamide interface. The resulting TFN membranes show high NaCl, MgCl2, Na2SO4, and MgSO4 rejections of 99.0–99.6% at 150 psi with a water permeance of 0.9 L m−2 h−1 bar−1. Compared to the thin-film composite (TFC) control membrane, the incorporation of the MOF particles yields a 53.0% and 24.5% increase in water permeance and NaCl rejection. Importantly, the TFN membranes also exhibit excellent rejections to small neutral contaminants such as PEG200 (99.2% rejection) and boric acid (89.0% rejection at a pH value of 7.5) at a relatively low pressure of 150 psi, which is higher than the respective value obtained by the commercial Dow SW30XLE TFC control membrane at the same conditions. The outstanding long-term performance stability revealed the robust structure of the MOF TFN membranes. Our results demonstrate a facile strategy to effectively control particle agglomeration and interfacial defects in polyamide TFN membranes by manipulating the surface chemistry of the filler nanoparticles, which can be applied to the fabrication of effective TFN membranes with molecular level size-exclusion selectivity.