Facile synthesis of cationic covalent organic frameworks with abundant protonated pyridine nitrogen groups for selective absorption of organic dyes†

The indiscriminate use of organic dyes without adequate wastewater treatment can result in severe water pollution and pose significant threats to biological health. This underscores the urgent need for the development of advanced materials capable of selectively removing contaminant organic dyes from water systems. In this study, highly crystalline covalent organic framework (COF) materials with pyridine structures were synthesized via a molten polymerization method, employing a Knoevenagel condensation reaction between 2,4,6-trimethylpyridine and 1,4-phthalaldehyde. Following modification with hydrochloric acid, cationic COF materials (S-iCOF) were obtained. These positively charged COF exhibited excellent crystallinity and a high specific surface area of up to 354.1 m² g⁻¹. They demonstrated exceptional efficiency in adsorbing anionic dyes, with maximum adsorption capacities of 481.7 mg g⁻¹ for methyl orange (MO) and 460.4 mg g⁻¹ for orange II (O II). The adsorption behavior adhered to a pseudo-second-order kinetic model. Furthermore, due to the difference in charge, S-iCOF exhibited significantly lower adsorption capacities for cationic dyes such as methylene blue (MB) and crystal violet (CV), enabling its use as a selective adsorbent for separating dyes with different ionic properties. By changing the 1,4-phthalaldehyde to 4,4′-biphenyldicarbaldehyde, the pore size of the COFs could be precisely expanded. It was demonstrated that large-pore cationic COFs (L-iCOF) are more advantageous for the removal of medium- to large-sized dyes. The molten polymerization method used for the synthesis of iCOF is both cost-effective and efficient. This study highlights the immense potential of cationic COFs for removing and separating organic dye pollutants from wastewater, offering promising applications in the remediation of organic contamination.