Functional Metal-Organic Frameworks. Ali MorsaliЧитать онлайн книгу.
absorption band and generation of long-lived excited electron-holes.
Jinhua Ye and coworkers synthesized MIL-88(Fe) and MIL-88(Fe)-NH2 and applied for photo-reduction of dichromate anion (Figure 2.9) [47]. The mechanism of photo-reduction is based on generation of electron-hole pairs upon light irradiation. It is essential to tune the band gap of MOFs to optimize the photocatalytic activity of MOFs. In parent framework, MIL88(Fe), Fe3-µ3-oxo clusters are directly excited and reduction of Cr(VI) and oxidation of water take place at the Fe-based oxo clusters. But in case of MIL88(Fe)-NH2, photo-sensitizer amine group enables organic ligand to adsorb the visible light photons, excited with generation of long-lived electronhole pairs and then transferring photoexcited electron to Fe3-µ3-oxo clusters. This secondary excitation mechanism is the reason of improved photocatalytic activity of MIL-88(Fe)-NH2 than its parent framework. Diffuse-reflectance UV/vis spectrum of these materials show that the introduction of amine group in the organic linker of iron(III)-based MOF can enhance its light absorption in the visible region. This observation indicates that the more amine group incorporated into the iron(III)-based MOF, the more electron–hole pairs can be generated via excitation of amine functionality under visible-light irradiation, which might lead to enhance photocatalytic activity. Transient photocurrent spectroscopy reveals that incorporation of amine group in the iron(III)-based MOF can enhance the photocurrent significantly indicating the fact that the separation efficiency of photoinduced electron-hole pairs and the lifetime of the photogenerated charge carriers are improved, and this can be explained by the excitation of amine-functionalized organic linker and then the excited electrons transfer to Fe 3 -μ 3 -oxo clusters. Combination of Electron spin resonance (ESR) for pure 2-aminoterohthalic acid ligand and MIL-88(Fe)-NH2 is an effective method to gain evidence about LMCT process. Pure ligand shows an ESR signal of g = 2.004 during the irradiation of visible light which is originated from amine group while such signal is not detected for the MIL-88(Fe)-NH2. ESR signal for MIL-88(Fe)-NH2 at g = 1.994 is attributed to Fe(III) species in which decreases upon light irradiation and recovered when irradiation is stopped. The decrease of Fe(III) ESR signal intensity could be attributed to the trapping of electrons by Fe(III) site in Fe3-μ3-oxo clusters. The disappearance of the ESR signal at g value of 2.004 and the decrease of the ESR signal at g value of 1.996 suggest the electron transfer from excited amine group to Fe3-μ3-oxo clusters in NH2-MIL-88B(Fe) irradiated with visible light.
Figure 2.9 Application of MIL-88(Fe)-NH2 in photocatalytic degradation of Cr(VI). (a) dual pass mechanism in presence of amine functionalized ligand. (b) Diffuse-reflectance UV/vis spectrum of NH2-MIL-88B(Fe) and MIL-88B(Fe). (c) Transient photocurrent spectroscopy of NH2-MIL-88B(Fe) and MIL-88B(Fe) [47].
2.3 Function–Structure Properties
The majority of chemistry of an amine functionalized MOF is around their capability as guest-interactive sites in host-guest chemistry. Structural features of amine FMOFs depend on the amine type as primary, secondary or tertiary.
Primary amines (−NH2) mainly applied as guest interactive site in the side chain of MOFs in tethered form [21, 22]. Anyway, sometimes primary amines are able to coordinate to metal ions during self-assembly synthesis.
Secondary amines could be applied in both main chain and side chain (tethered form) of the MOFs. K. Mark Thomas and coworkers synthesized an amine FMOFs with secondary amine motifs (−NH−) in the main chain [51]. The freedom and rotation around C−N−C might give rise to flexibility of the MOF. Also, organic ligands based on secondary amines applied as V-shaped ligand for development of desired MOF [52]. In most of the time secondary amines in side chain applied for study about the Lewis basicity of the MOF for different applications [53–55].
Similar to secondary amines, tertiary amines could apply in side chain for study about the Lewis basicity of the MOF for different applications. Specific type of tritopic ligands based on tertiary ligands applied for construction of MOFs.
As we saw, amine function extensively applied with the aim of improvement in efficiency of MOFs in different type of applications. This attention is due to the fact that amine decorated MOFs benefits from several chemical features as well as easy synthesis method. Particularly, amine decorated MOFs show very good results in some application like carbon dioxide capture-release in post-combustion process. Also, they successfully applied for oil denitrogenation and detection of picric acid in aqueous solution. Though, amine decorated MOFs applied for energy and environmental purposes, but these achievements have to develop and tailor for real-life applications.
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