Probe metal binding mode of imine covalent organic frameworks: cycloiridation for (photo)catalytic hydrogen evolution from formate.
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ABSTRACT:
SUBMITTER: Hu J
PROVIDER: S-EPMC8188469 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Covalent organic frameworks (COFs) have emerged as an important class of organic semiconductors and photocatalysts for the hydrogen evolution reaction (HER)from water. To optimize their photocatalytic activity, typically the organic moieties constituting the frameworks are considered and the most suitable combinations of them are searched for. However, the effect of the covalent linkage between these moieties on the photocatalytic performance has rarely been studied. Herein, we demonstrate that donor-acceptor (D-A) type imine-linked COFs can produce hydrogen with a rate as high as 20.7 mmol g-1 h-1 under visible light irradiation, upon protonation of their imine linkages. A significant red-shift in light absorbance, largely improved charge separation efficiency, and an increase in hydrophilicity triggered by protonation of the Schiff-base moieties in the imine-linked COFs, are responsible for the improved photocatalytic performance.
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Project description:The growing interest in two-dimensional imine-based covalent organic frameworks (COFs) is inspired by their crystalline porous structures and the potential for extensive ?-electron delocalization. The intrinsic reversibility and strong polarization of imine linkages, however, leads to insufficient chemical stability and optoelectronic properties. Developing COFs with improved robustness and ?-delocalization is highly desirable but remains an unsettled challenge. Here we report a facile strategy that transforms imine-linked COFs into ultrastable porous aromatic frameworks by kinetically fixing the reversible imine linkage via an aza-Diels-Alder cycloaddition reaction. The as-formed, quinoline-linked COFs not only retain crystallinity and porosity, but also display dramatically enhanced chemical stability over their imine-based COF precursors, rendering them among the most robust COFs up-to-date that can withstand strong acidic, basic and redox environment. Owing to the chemical diversity of the cycloaddition reaction and structural tunability of COFs, the pores of COFs can be readily engineered to realize pre-designed surface functionality.
| S-EPMC6068140 | biostudies-literature
Project description:Mechanistic understanding into the formation and growth of imine-linked two-dimensional (2D) covalent organic frameworks (COFs) is needed to improve their materials quality and access larger crystallite sizes, both of which limit the promise of 2D COFs and 2D polymerization techniques. Here we report a previously unknown temperature-dependent depolymerization of colloidal 2D imine-linked COFs, which offers a new means to improve their crystallinity. 2D COF colloids form at room temperature but then depolymerize when their reaction mixtures are heated to 90 °C. As the solutions are cooled back to room temperature, the 2D COFs repolymerize and crystallize with improved crystallinity and porosity, as characterized by X-ray diffraction, infrared spectroscopy and N2 porosimetry. The evolution of COF crystallinity during the solvothermal depolymerization and repolymerization processes was characterized by in situ wide angle X-ray scattering, and the concentrations of free COF monomers as a function of temperature were quantified by variable temperature 1H NMR spectroscopy. The ability of a 2D COF to depolymerize under these conditions depends on both the identity of the COF and its initial materials quality. For one network formed at room temperature (TAPB-PDA COF), a first depolymerization process is nearly complete, and the repolymerization yields materials with dramatically enhanced crystallinity and surface area. Already recrystallized materials partially depolymerize upon heating their reaction mixtures a second time. A related 2D COF (TAPB-DMTA COF) forms initially with improved crystallinity compared to TAPB-PDA COF and then partially depolymerizes upon heating. These results suggest that both high materials quality and network-dependent properties, such as interlayer interaction strength, influence the extent to which 2D COFs resist depolymerization. These findings offer a new means to recrystallize or solvent anneal 2D COFs and may ultimately inform crystallization conditions for obtaining large-area imine-linked two-dimensional polymers from solution.
| S-EPMC8672728 | biostudies-literature
Project description:Synthesizing H2 O2 from water and air via a photocatalytic approach is ideal for efficient production of this chemical at small-scale. However, the poor activity and selectivity of the 2 e- water oxidation reaction (WOR) greatly restricts the efficiency of photocatalytic H2 O2 production. Herein we prepare a bipyridine-based covalent organic framework photocatalyst (denoted as COF-TfpBpy) for H2 O2 production from water and air. The solar-to-chemical conversion (SCC) efficiency at 298 K and 333 K is 0.57 % and 1.08 %, respectively, which are higher than the current reported highest value. The resulting H2 O2 solution is capable of degrading pollutants. A mechanistic study revealed that the excellent photocatalytic activity of COF-TfpBpy is due to the protonation of bipyridine monomer, which promotes the rate-determining reaction (2 e- WOR) and then enhances Yeager-type oxygen adsorption to accelerate 2 e- one-step oxygen reduction. This work demonstrates, for the first time, the COF-catalyzed photosynthesis of H2 O2 from water and air; and paves the way for wastewater treatment using photocatalytic H2 O2 solution.
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Project description:Constructing photocatalytically active and stable covalent organic frameworks containing both oxidative and reductive reaction centers remain a challenge. In this study, benzotrithiophene-based covalent organic frameworks with spatially separated redox centers are rationally designed for the photocatalytic production of hydrogen peroxide from water and oxygen without sacrificial agents. The triazine-containing framework demonstrates high selectivity for H2O2 photogeneration, with a yield rate of 2111 μM h-1 (21.11 μmol h-1 and 1407 μmol g-1 h-1) and a solar-to-chemical conversion efficiency of 0.296%. Codirectional charge transfer and large energetic differences between linkages and linkers are verified in the double donor-acceptor structures of periodic frameworks. The active sites are mainly concentrated on the electron-acceptor fragments near the imine bond, which regulate the electron distribution of adjacent carbon atoms to optimally reduce the Gibbs free energy of O2* and OOH* intermediates during the formation of H2O2.
| S-EPMC10462664 | biostudies-literature
Project description:Covalent Organic Frameworks (COFs) are thermally and chemically stable, nanoporous materials with high surface areas, making them interesting for a large variety of applications including energy storage, gas separation, catalysis and chemical sensing. However, pore blocking and pore collapse may limit their performance. Reducing the capillary forces by using solvents with low surface tension, like supercritical CO2, for activation, and the introduction of bulky isopropyl/methoxy groups were found to reduce pore collapse. Herein, we present an easy-to-use alternative that involves the combination of a new, methylated building block (2,4,6-trimethylbenzene-1,3,5-tricarbaldehyde, Me3TFB) with vacuum drying. Condensation of Me3TFB with 1,4-phenylenediamine (PA) or benzidine (BD) resulted in imine-linked 2D COFs (Me3TFB-PA and Me3TFB-BD) with higher degrees of crystallinity and higher BET surface areas compared to their non-methylated counterparts (TFB-PA and TFB-BD). This was rationalized by density functional theory computations. Additionally, the methylated COFs are less prone to pore collapse when subjected to vacuum drying and their BET surface area was found to remain stable for at least four weeks. Within the context of their applicability as sensors, we also studied the influence of hydrochloric acid vapour on the optical and structural properties of all COFs. Upon acid exposure their colour and absorbance spectra changed, making them indeed suitable for acid detection. Infrared spectroscopy revealed that the colour change is likely attributed to the cleavage of imine bonds, which are only partially restored after ammonia exposure. While this limits their application as reusable sensors, our work presents a facile method to increase the robustness of commonly known COFs.
| S-EPMC8638808 | biostudies-literature
Project description:Isoreticular chemically stable two-dimensional imine covalent organic frameworks (COFs), further denoted as DUT-175 and DUT-176, are obtained in a reaction of 4,4'-bis(9H-carbazol-9-yl)biphenyl tetraaldehyde with phenyldiamine and benzidine. The crystal structures, solved and refined from the powder X-ray diffraction data and confirmed by high-resolution transmission electron microscopy, indicate AA-stacked layer structures. Both structures feature distorted hexagonal channel pores, assuring remarkable porosity (SBET = 1071 m2 g-1 for DUT-175 and SBET = 1062 m2 g-1 for DUT-176), as confirmed by adsorption of gases and vapors. The complex conjugated π system of the COFs involves electron-rich carbazole building units, which in combination with the imine groups allow reversible pH-dependent protonation of the frameworks, accompanied by charge transfer and shift of the absorption bands in the UV-vis spectrum. The sigmoidal shape of the water vapor adsorption and desorption isotherms with a steep adsorption step at p/p0 = 0.4-0.6 in combination with excellent stability over dozens of adsorption and desorption cycles ranks these COFs among the best materials for indoor humidity control applications.
| S-EPMC8587605 | biostudies-literature
Project description:This work focuses on the impact of covalent organic frameworks' (COFs) pore flexibility in the adsorption and separation of benzene and cyclohexane. With this aim, we have selected the imine-linked 3D COFs COF-300 and LZU-111 as examples of flexible and rigid frameworks, respectively. Optimized syntheses at room temperature or in solvothermal conditions enabled us to selectively isolate the narrow-pore form of COF-300 (COF-300-rt) or a mixture of the narrow-pore and a larger-pore form (COF-300-st), respectively, with different textural properties (BET specific surface area = 39 or 1270 m2/g, respectively, from N2 adsorption at 77 K). In the case of LZU-111, only the room temperature route was successful, leading to the known microporous framework. COF-300-rt, COF-300-st, and LZU-111 were studied for benzene and cyclohexane adsorption and separation in static and dynamic conditions. At 298 K and 1 bar, these COFs adsorb more benzene (251, 221, and 214 cm3/g STP, respectively) than cyclohexane (175, 133, and 164 cm3/g STP, respectively). Moreover, the benzene and cyclohexane isotherms of COF-300-rt and COF-300-st are characterized by steps, as expected with a flexible material. Indeed, in situ powder X-ray diffraction experiments on benzene- and cyclohexane-impregnated batches enabled us to trap, for the first time, a sequence of forms of COF-300 with different pore aperture, rationalizing the stepped hysteretic isotherms. Finally, benzene/cyclohexane separation was evaluated using a benzene/cyclohexane 50:50 v/v flow at different temperatures (T = 298, 323, and 348 K): LZU-111 does not selectively retain any of the two components, while COF-300 exhibits stronger benzene-COF interactions also in dynamic conditions.
| S-EPMC9931175 | biostudies-literature
Project description:A series of imine-based covalent organic frameworks decorated in their cavities with different alkynyl, pyrrolidine, and N-methylpyrrolidine functional groups have been synthetized. These materials exhibit catalytic activity in aqueous media for the hydrolytic detoxification of nerve agents, as exemplified with nerve gas simulant diisopropylfluorophosphate (DIFP). These preliminary results suggest imine-based covalent organic frameworks (COFs) as promising materials for detoxification of highly toxic molecules.
| S-EPMC6631658 | biostudies-literature
Project description:Covalent organic frameworks (COFs) are two- or three-dimensional (2D or 3D) polymer networks with designed topology and chemical functionality, permanent porosity, and high surface areas. These features are potentially useful for a broad range of applications, including catalysis, optoelectronics, and energy storage devices. But current COF syntheses offer poor control over the material's morphology and final form, generally providing insoluble and unprocessable microcrystalline powder aggregates. COF polymerizations are often performed under conditions in which the monomers are only partially soluble in the reaction solvent, and this heterogeneity has hindered understanding of their polymerization or crystallization processes. Here we report homogeneous polymerization conditions for boronate ester-linked, 2D COFs that inhibit crystallite precipitation, resulting in stable colloidal suspensions of 2D COF nanoparticles. The hexagonal, layered structures of the colloids are confirmed by small-angle and wide-angle X-ray scattering, and kinetic characterization provides insight into the growth process. The colloid size is modulated by solvent conditions, and the technique is demonstrated for four 2D boronate ester-linked COFs. The diameter of individual COF nanoparticles in solution is monitored and quantified during COF growth and stabilization at elevated temperature using in situ variable-temperature liquid cell transmission electron microscopy imaging, a new characterization technique that complements conventional bulk scattering techniques. Solution casting of the colloids yields a free-standing transparent COF film with retained crystallinity and porosity, as well as preferential crystallite orientation. Collectively this structural control provides new opportunities for understanding COF formation and designing morphologies for device applications.
| S-EPMC5269657 | biostudies-literature