Reducing the Pore Size of Covalent Organic Frameworks in Thin-Film Composite Membranes Enhances Solute Rejection

Abstract

Three imine-linked covalent organic framework (COF) films are incorporated as active layers into separate thin-film composite (TFC) membranes and tested for their ability to reject an organic pollutant surrogate and salt from water. The synthesized membranes consist of a polyacrylonitrile (PAN) support and a TAPB-PDA-H, TAPB-PDA-Me, or TAPB-PDA-Et COF thin film. The latter two COFs direct six methyl and ethyl substituents per tiled hexagon into the pores, respectively, while maintaining the same topology across the series. These substituents decrease the effective pore size of the COF compared to the parent TAPB-PDA-H COF. The TFC TAPB-PDA-Me membrane rejects Rhodamine-WT (R-WT) dye and NaCl better than the TFC TAPB-PDA-H membrane, and the TFC TAPB-PDA-Et membrane exhibits the best rejection overall. The solution-diffusion model used to analyze this permeation behavior indicates that there is a systematic difference in rejection as subsequent pendant groups are added to the interior of the COF pores. These findings demonstrate the concept of tuning the selectivity of COF membranes by systematically reducing the effective pore size within a given topology.

Acid Exfoliation of Imine‐linked Covalent Organic Frameworks Enables Solution Processing into Crystalline Thin Films

“…Furthermore, they retained up to 80 % of the powder surface area as confirmed by nitrogen adsorption measurements (Figure S41), as well as the characteristic FT‐IR imine stretch at approximately 1620 cm −1 (Figure S44). These materials ( S BET =1750 m 2 g −1 ) represent some of the highest quality freestanding COF films reported to date …”

“…The second is interfacial polymerization, in which the monomers or catalyst are dissolved separately in two immiscible solvents such that COF formation occurs at the interface . This strategy provides freestanding films, but thickness control requires significant reoptimization for every network, and films with poor or no crystallinity are often obtained, especially when they are thinner than 50 nm . Finally, a third method, exfoliation and reconstitution of COF powders, has been explored .…”

Acid Exfoliation of Imine-linked Covalent Organic Frameworks Enables Solution Processing into Crystalline Thin Films

“…These materials (S BET = 1750 m 2 g À1 )r epresent some of the highest quality freestanding COF films reported to date. [11,14,24,25,47] NMR spectroscopy measurements of COF solutions in TFA-d/D 2 Or eveal aC OF concentration dependence on the degree of depolymerization. To probe how the roles of the acid and water change as af unction of COF concentration during the film-fabrication process,and establish the generality of the depolymerization/repolymerization mechanism across solvent mixtures,t wo uniform TAPB-PDAC OF solutions (12.5 mg mL À1 and 25 mg mL À1 )w ere prepared in TFA-d (5 % v/v D 2 O) along with a1 ,4-dioxane internal standard and analyzed by 1 HNMR spectroscopy (Figures S51, S52).…”

“…[11,23,24] This strategy provides freestanding films,b ut thickness control requires significant reoptimization for every network, and films with poor or no crystallinity are often obtained, especially when they are thinner than 50 nm. [24,25] Finally,athird method, exfoliation and reconstitution of COF powders,h as been explored. [26][27][28][29] This strategy is highly desirable because it provides ad irect route to the film morphology from abundant microcrystalline powders,w hich are simpler to synthesize,a nd the film dimensions can be precisely tuned by changing the suspension concentration and substrate surface area.…”

Extraction of Ibuprofen from Natural Waters Using a Covalent Organic Framework

“…Recently, COFs have raised increasing interest for the extraction of different organic contaminants from water [12][13][14], such as biotoxins [15,16], perfluoroalkyl compounds [17], polycyclic aromatic hydrocarbons [18], and pharmaceutical pollutants [19][20][21]. In addition, COFs are being explored for membrane-based water treatment [22][23][24][25].…”