Donnan Effect-Engineered Covalent Organic Framework Membranes toward Size- and Charge-Based Precise Molecular Sieving

Area of Science:

• Materials Science
• Nanotechnology
• Separation Science

Background:

• Covalent organic frameworks (COFs) offer ordered pores ideal for nanofiltration (NF) membranes, potentially overcoming the permeance/selectivity trade-off.
• Existing COF membranes primarily focus on size-based separation, showing limited selectivity for molecules with similar sizes but different charges.

Purpose of the Study:

• To develop a COF-based nanofiltration membrane capable of separating molecules based on both size and charge.
• To investigate the combined effects of the Donnan effect and size exclusion for enhanced separation performance.

Main Methods:

• Fabrication of a negatively charged COF layer in situ on a microporous support.
• Testing membrane performance using various dyes with differing sizes and charges.
• Analysis of separation selectivity based on molecular size, charge, and the Donnan effect.

Main Results:

• Achieved ultrahigh water permeance (216.56 L m⁻² h⁻¹ bar⁻¹) due to ordered pores and hydrophilicity.
• Demonstrated superior rejection of negatively and neutrally charged dyes (>1.3 nm).
• Successfully separated mixed-charge dyes by allowing positively charged dyes (1.6 nm) to permeate, showcasing charge-based selectivity.

Conclusions:

• The developed COF membrane effectively separates molecules based on both size and charge, addressing limitations of traditional NF membranes.
• The strategy of combining the Donnan effect and size exclusion in nanoporous materials presents a versatile platform for advanced separations.
• This approach holds promise for sophisticated separation applications beyond traditional molecular sieving.