| Apr 22, 2022 |
Employing melt polymerization to fabricate robust covalent organic framework foams
(Nanowerk News) Adsorption-based (e.g., gas or liquid molecules) separation technologies have shown unique economic and environmental advantages in specific applications. In industrial applications, ideal high-efficiency adsorbents require not only high adsorption capacity/selectivity, but also good machinability, cycling, and mechanical stability. Thus, it is necessary to assemble the adsorbents into high-stability monoliths (e.g., spheres, membranes, aerogels, etc.).
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Melt polymerization methods are often used
Recently, COFs, as an emerging class of advanced adsorbents, have demonstrated many heartening performances in numerous separation fields. However, most of the state-of-the-art COF adsorbents still suffer from issues such as low processability (mostly existing as microcrystalline powders), lack of stability (mostly built by the reversible bond linkages), and difficulty for scalable synthesis.
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Melt polymerization methods are often used
Therefore, it is of great significance to create new strategies to produce highly robust COF monoliths (e.g., porous foams) for practical applications.
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| Construct covalent organic frameworks foam separators by melt polymerization. (© Science China Press)
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Melt polymerization methods are often used to prepare linear polymers. Since both monomers and polymers are in a molten state, it is easy to directly process and shape. Inspired by this, Zhang's group from Nankai University first introduced the "melt polymerization" strategy into COFs synthesis (Science China Chemistry, "Melt polymerization Synthesis of A Class of Robust Self-shaped Olefin-linked COF Foams as High-efficiency Separators").
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Melt polymerization methods are often used
By adding benzoic anhydride as a flux, a "one-step thermoforming" method was developed to prepare a series of olefin-linked COF foams. This is because the flux promoted the monomers to form a eutectic, which slowly crystallized upon heating, and finally solidified to form highly crystalline COF foams (density as low as 0.23 g/cm3).
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| (a-d) Characterization of COF foams by PXRD. (e) Nitrogen adsorption curves of COF foams at 77 K. (f) Comparison of BET surface areas for the four COFs (gray columns for reported COFs; colored columns for COF foams in this study). (© Science China Press)
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This method not only can enhance crystallinity and porosity of the reported COFs (Figure 2), but also can produce new COFs that cannot be obtained by the traditional solvothermal methods. For example, a novel olefin-linked COF (NKCOF-12) with ultra-micropores (0.58 nm) was synthesized for the first time by this method.
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Melt polymerization methods are often used
These obtained foams demonstrate excellent processability and mechanical properties which are suitable for adsorption and separation (Figure 3).
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| (a) COF foams with different shapes. (b) Scheme of the one-step synthetic procedure for gram-scale preparation of NKCOF-12. (c) Left: A photograph of NKCOF-12 foam standing on a leaf. Right: SEM image of NKCOF-12 foam. (d) The image of different shapes processed from NKCOF-12 foam. (e) A photograph of NKCOF-12 foam under compression. (© Science China Press)
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The oil-water selective adsorption experiments showed that these foams achieved highly efficient oil-water separation (up to 99% removal efficiency) with facile recycling and ultrahigh reusability (more than 100 cycles). Moreover, NKCOF-12 with the smallest pore size among all reported eclipsed stacking 2D COFs was constructed.
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Melt polymerization methods are often used
Attributed to its regular ultramicroporous channel (0.58 nm) and enriched binding sites, NKCOF-12 possesses excellent C2H2/CO2 separation performance with higher C2H2 purity (99.3%) than the benchmark materials.
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Melt polymerization methods are often used
This work not only provides an opportunity for the construction of COF foams via melt polymerization but also significantly advances the development of COFs for practical applications.
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