Congo red stitched covalent organic framework membrane for dye separation

Abstract

Covalent organic framework (COF) membranes have shown great potential in dyes’ separation. However, COFs are difficult to form large-area and highly crystalline membranes, therefore there normally exists crystal and stacking defects in COF membranes, impairing their rejection performance. Herein, we discovered the prepared COF-TATP membranes have exceptional high rejection to Congo red (CR) solution. Further study showed the presence of small proportion of CR in other dye solutions also could enhance the rejection to those dyes. 3:100 CR/Brilliant blue R250 (BB) solution could obtain 99.81% rejection to BB. Characterizations indicated CR molecules could well merged into the COF layer and strongly bonded with its N/O groups via plentiful hydrogen bonds. Inspired by the finding, we proposed a strategy of using CR to stitch the defects in COF layer. Specifically, CR was embedded into and then chemically crosslinked with COF-TATP layer by glutaraldehyde to prepare LCR-TATP-NH2CS/nylon membranes. The membranes showed obviously enhanced rejection to all dyes tested, typically BB rejection of 99.75%. It also could selectively separate BB/Methyl orange mixed solutions. This study reveals some insights into dyes’ behavior on COF membranes, and provides a simple and feasible strategy for stitching COF membranes’ defects to enhance their performance.