# A Crown Ether-Based Covalent Organic Polymer Composite Membrane and Its Application in Molecular Separation

## TL;DR

A new membrane made with a crown ether-based polymer efficiently separates dyes and salts from water, showing high performance and stability.

## Contribution

A covalent organic polymer composite membrane with exceptional dye rejection and desalination selectivity is developed for wastewater treatment.

## Key findings

- The membrane achieved over 99% rejection of Congo red dye for 48 hours with a water flux of 103.2 LMH/bar.
- It demonstrated a high separation selectivity of 186.8 for Congo red over potassium chloride with a flux of 138.2 LMH/bar.
- The COP interlayer provided effective molecular sieving and stability in long-term tests.

## Abstract

Organic dyes are critical components in industries ranging from textiles, plastics, and paper to food, cosmetics, and pharmaceuticals. However, their widespread use leads to significant environmental pollution. Consequently, developing efficient methods to treat dye wastewater is urgently needed. In this work, a high-performance composite membrane was developed with a poly(dibenzo-18-crown-6) covalent organic polymer (COP) interlayer. The chemical structure of the COP was verified by FT-IR, and BET analysis indicated that the as-synthesized material possesses a predominantly mesoporous structure with a minor microporous contribution. Subsequently, the membrane was fabricated by depositing a COP colloid on a nylon-66 support via vacuum filtration, followed by the formation of a dense polyamide (PA) active layer through interfacial polymerization (IP) between amine and acyl chloride monomers. Systematic evaluation of dye separation performance using a cross-flow filtration setup identified optimal operating conditions. Under these conditions, the membrane demonstrated effective molecular sieving behavior, achieving both high dye rejection and favorable solvent permeability. In long-term stability tests, the membrane maintained a rejection rate of over 99% for Congo red over 48 h, while sustaining a water flux of 103.2 L m−2 h−1 bar−1 (LMH/bar). Furthermore, the membrane exhibited promising potential for dye desalination applications, achieving a high Congo red/potassium chloride separation selectivity of 186.8 with a flux of 138.2 LMH/bar. This study confirms that the poly(dibenzo-18-crown-6)-based composite membrane is a reliable and efficient material for molecular separation in wastewater treatment.