Michio Matsumoto

Rapid, Low Temperature Formation of Imine-Linked Covalent Organic Frameworks Catalyzed by Metal Triflates

Imine-linked two-dimensional covalent organic frameworks (2D COFs) are crystalline polymer networks with enhanced stability compared to boronate ester-linked systems and with broad monomer scope. They are traditionally prepared by condensing polyfunctional aldehydes and amines at elevated temperature in a mixture of organic solvents and aqueous CH3CO2H, which catalyzes imine formation and exchange. Here we employ metal triflates, which are water-tolerant Lewis acids, to accelerate 2D imine-linked COF synthesis and improve their materials quality. Low catalyst loadings provide crystalline polymer networks in nearly quantitative yields. These conditions are demonstrated for several COFs, including heteroatom-containing systems of interest for optoelectronic applications.

Development and Performance Characterization of a Polyimine Covalent Organic Framework Thin-Film Composite Nanofiltration Membrane

Two-dimensional covalent organic frameworks (COFs) were used to create the first asymmetric, thin-film composite (TFC) nanofiltration (NF) membrane with a COF active layer. NF membrane active layers of polyimine COF were synthesized via the interfacial polymerization (IP) of terephthalaldehyde and tris(4-aminophenyl)benzene monomers on top of a poly(ether sulfone) (PES) ultrafiltration membrane support. Rutherford backscattering spectrometry and Fourier transform infrared spectroscopy analyses confirmed the presence of an imine-linked film with a thickness of ∼10 nm that was formed reproducibly. The rejection efficiencies of the COF NF membrane for a model organic compound, Rhodamine-WT, and a background electrolyte, NaCl, were higher than those of the PES support without the COF film. This enhanced solute rejection is the first successful demonstration of a TFC membrane with a thin COF active layer. However, this work also demonstrates the need for COF NF membranes with smaller active layer pores and alternative support materials. The former should result in greater solute rejection, and the latter is key because the PES used for support in the COF membranes is incompatible with the organic solvents used for the COF IP process.

Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids

Electric double layer (EDL) capacitors are promising as next-generation energy accumulators if their capacitances and operation voltages are both high. However, only few electrolytes can simultaneously fulfill these two requisites. Here we report that an oligomeric ionic liquid such as IL4TFSI with four imidazolium ion units in its structure provides a wide electrochemical window of ∼5.0 V, similar to monomeric ionic liquids. Furthermore, electrochemical impedance measurements using Au working electrodes demonstrated that IL4TFSI exhibits an exceptionally high EDL capacitance of ∼66 μF/cm2, which is ∼6 times as high as those of monomeric ionic liquids so far reported. We also found that an EDL-based field effect transistor (FET) using IL4TFSI as a gate dielectric material and SrTiO3 as a channel material displays a very sharp transfer curve with an enhanced carrier accumulation capability of ∼64 μF/cm2, as determined by Hall-effect measurements.

Removal of GenX and Perfluorinated Alkyl Substances from Water by Amine-Functionalized Covalent Organic Frameworks

Per- and polyfluorinated alkyl substances (PFAS), such as perfluorooctanoic acid (PFOA), perfluorooctanesulfonate (PFOS), and ammonium perfluoro-2-propoxypropionate (GenX), contaminate ground and surface waters throughout the world. The cost and performance limitations of current PFAS removal technologies motivate efforts to develop selective and high-affinity adsorbents. Covalent organic frameworks (COFs) are unexplored yet promising adsorbents because of their high surface area and tunable pore sizes. Here we show that imine-linked two-dimensional (2D) COFs bearing primary amines adsorb GenX rapidly at environmentally relevant concentrations. COFs with partial amine incorporation showed the highest capacity and fastest removal, suggesting that the synergistic combination of the polar group and hydrophobic surfaces are responsible for GenX binding. A COF with 28% amine loading also removed more than 90% of 12 out of 13 PFAS. These results demonstrate the promise of COFs for PFAS removal and suggest design criteria for maximizing adsorbent performance.