Computationally Guided Study of Cross-Linked Covalent Organic Frameworks for Membrane Applications

Two-dimensional covalent organic frameworks (2D-COFs) exhibit characteristics that are favorable for membrane applications including, but not limited to, their high stability and well-ordered nanopores. A challenge with fabricating these materials into membranes, however, is that membrane wetting leads to flake dispersion and increased interstitial flow of particles. Cross-linking, or “chemically stitching” two layers of the COF together has been shown to significantly improve the membrane performance. This work seeks to investigate the stability of cross-linked 2D-COFs via computational modelling coupled with synthesis, characterization, and membrane permeance testing. A quinoxaline-based COF was computationally modeled and cross-linked with oxalyl chloride (OC) and hexafluoroglutaryl chloride (HFG). Cohesive energy calculations indicate that both cross-linking moieties increase the intramolecular bond strength of the COF, thereby increasing the stability of the material. Based on enthalpy of formation, it was concluded that HFG is a marginally more favorable cross-linking moiety. This was attributed to the length of HFG which allows it to cross-link with the COF in more configurations than OC. Cross-linking with HFG was synthesized and characterized with Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and water contact angles. The presence of HFG cross-linking was confirmed and it was concluded via membrane permeance testing that varying the amount of cross-linking reagent during synthesis had not changed the amount of cross-linking occurring. This was also supported by thermogravimetric analysis with differential scanning calorimetry (TGA-DSC). Enthalpy of formation and cohesive energy calculations suggest that reduced cross-linking has a negligible effect on framework stability, and <i>ab-initio </i>molecular dynamics also agreed with the temperature range of stability from TGA-DSC.