Viologen-Derived Covalent Organic Frameworks: Advancing PFAS Removal Technology with High Adsorption Capacity.

The escalating presence of per- and polyfluoroalkyl substances (PFAS) in drinking water poses urgent public health concerns, necessitating effective removal. This study presents a groundbreaking approach, using viologen to synthesize covalent organic framework nanospheres: MELEM-COF and MEL-COF. Characterized by highly crystalline features, these nanospheres exhibit exceptional affinity for diverse anionic PFAS compounds, achieving simultaneous removal of multiple contaminants within 30 min. Investigating six anionic PFAS compounds, MEL- and MELEM-COFs achieved 90.0-99.0% removal efficiency. The integrated analysis unveils the synergistic contributions of COF morphology and functional properties to PFAS adsorption. Notably, MELEM-COF, with cationic surfaces, exploits electrostatic and dipole interactions, with a 2500 mg g-1 adsorption capacity-surpassing all reported COFs to date. MELEM-COF exhibits rapid exchange kinetics, reaching equilibrium within 30 min. These findings deepen the understanding of COF materials and promise avenues for refining COF-based adsorption strategies.

Tetrazine-Linked Covalent Organic Frameworks With Acid Sensing and Photocatalytic Activity.

The first synthesis and comprehensive characterization of two vinyl tetrazine-linked covalent organic frameworks (COF), TA-COF-1 and TA-COF-2, are reported. These materials exhibit high crystallinity and high specific surface areas of 1323 and 1114 m2 g-1. The COFs demonstrate favorable band positions and narrow band gaps suitable for light-driven applications. These advantages enable TA-COFs to act as reusable metal-free photocatalysts in the arylboronic acids oxidation and light-induced coupling of benzylamines. In addition, these TA-COFs show acid sensing capabilities, exhibiting visible and reversible color changes upon exposure to HCl solution, HCl vapor, and NH3 vapor. Further, the TA-COFs outperform a wide range of previously reported COF photocathodes. The tetrazine linker in the COF skeleton represents a significant advancement in the field of COF synthesis, enhancing the separation efficiency of charge carriers during the photoreaction and contributing to their photocathodic properties. TA-COFs can also degrade 5-nitro-1,2,4-triazol-3-one (NTO), an insensitive explosive present in industrial wastewater, in 20 min in a sunlight-driven photocatalytic process; thus, revealing dual functionality of the protonated TA-COFs as both photodegradation and Brønsted acid catalysts. This pioneering work opens new avenues for harnessing the potential of the tetrazine linker in COF-based materials, facilitating advances in catalysis, sensing, and other related fields.

Substitution and Orientation Effects on the Crystallinity and PFAS Adsorption of Olefin-Linked 2D COFs.

Solid phase adsorbents with high removal affinity for per- and polyfluoroalkyl substances (PFAS) in aqueous environments are sought. We report the synthesis and investigation of COF-I, a new covalent organic framework (COF) with a good affinity for PFAS adsorption. COF-I was synthesized by the condensation reaction between 2,4,6-trimethyl-1,3,5-triazine and 2,3-dimethoxyterephthaldehyde and fully characterized. In addition to the high crystallinity and surface area, COF-I showed high hydrolytic and thermal stability. Further, we converted its hydrophobic surface to a hydrophilic surface by converting the ortho-methoxy groups to hydroxyl derivatives and produced a new hydrophilic olefin-linked two-dimensional (2D) COF. We experimentally measured the crystallinity of both COFs by X-ray diffraction and used atomistic simulations coupled with cross-polarization/magic angle spinning solid-state nuclear magnetic resonance (CP/MAS ssNMR) to determine the relative amounts of AA-stacking and AB-stacking present. COF-I, with its hydrophobic surface and methoxy groups in the ortho positions, showed the best PFAS adsorption. COF-I reduced the concentration of perfluorooctanoic acid from 20 to 0.069 µg L-1 and to 0.052 µg L-1 for perfluorooctanesulfonic acid. These amounts are lower than the U.S. Environmental Protection Agency advisory level (0.070 µg L-1). High efficiency, fast kinetic adsorption, and reusability of COF-I are advantages of COF-I for PFAS removal from water.

Lithium Pyrene Squarate Covalent Organic Frameworks for Efficient Lithium and Magnesium Separation from Salt Water.

The increasing pressure for lithium resources from the electric vehicle and nuclear energy industries means that new technologies to separate Mg2+ from Li+ from salt water are in demand. To address this need, we fabricated lithium pyrene squarate covalent organic frameworks (Li-SQCOFs) to separate Mg2+/Li+ mixtures from salt water. We optimized the effect of the electrolyte and the amount of the adsorbent and then carried out a kinetics study on the adsorbent recovery at various pH levels using both batch and continuous flow adsorption methods. Li-SQCOF was found to have excellent selectivity for solutions containing a mixture of Mg2+/Li+ ions. This work represents a unique path for the separation of Mg2+/Li+ through direct adsorption using a covalent organic framework (COF). The COF-supported ultrafiltration bed made in this study gave a Mg2+ separation flux of 60.5 h-1 m-2.

Microwave irradiation as a versatile tool for increasing reaction rates and yields in synthesis of optically active polyamides containing flexible L-leucine amino acid.

In this investigation, a series of thermally stable and optically active polyamides (PA)s containing bulky pendant chiral functionality from polymerization of a diacid monomer containing rigid phthalimide and flexible L-leucine groups, (2S)-5-[4-(4-methyl-2-phthalimidylpentanoylamino)benzoylamino]isophthalic acid with several aromatic and aliphatic diisocyanates such as 4,4'-methylenebis(phenyl isocyanate), toluylene-2,4-diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate under gradual heating method were prepared and compared with microwave-assisted polycondensation method. The polymerization reactions occurred rapidly under microwave irradiation and produced a series of PAs with good yields and moderate inherent viscosities of 0.26-0.68 dL/g. All of the new PAs showed good solubility and were readily dissolved in aprotic organic solvents. The resulting polymers were characterized by FT-IR, (1)H NMR spectroscopy, and elemental analysis technique. Thermal stability and thermal properties of PAs were evaluated by thermogravimetric analysis and differential scanning calorimetry. The interpretation of kinetic parameters (E, Delta H, Delta S, and Delta G) of thermal decomposition stages have been evaluated using Coats-Redfern equations.

Molten salt-supported polycondensation of optically active diacid monomers with an aromatic thiazole-bearing diamine using microwave irradiation.

Microwave heating was used to prepare optically active thiazole-bearing poly(amide-imide)s. Polymerization reactions were carried out in the molten tetrabutylammonium bromide as a green molten salt medium and triphenyl phosphite as the homogenizer. Structural elucidation of the compounds was performed by Fourier transform infrared and NMR spectroscopic data and elemental analysis results. The polymeric samples were readily soluble in various organic solvents, forming low-colored and flexible thin films via solution casting. They showed high thermal stability with decomposition temperature being above 360 °C. They were assembled randomly in a nanoscale size.