In one possible mechanism, it is reported that benzaldehyde can be activated through the interaction with an amine group and the formation of imine through a new (amine)N= C(benzaldehyde) covalent bond that can be followed by the rearrangement and addition of malonitrile (Figure 2.6) [18]. In other mechanism it is mentioned that benzaldehyde activation in Henry reaction is done through a noncovalent Lewis base–acid interactions between Lewis basic catalytic site with the carbonyl C atom of benzaldehyde [40].
Figure 2.5 Application of NH2-MIL-125 (Ti) and
Lewis basicity and hydrogen-bond donation/accepting are common and well-known chemical properties of amine functions which applied extensively in development of functional MOFs. Anyway, there are some of other chemical properties which are interesting for fabrication of FMOFs. For example, amine function is able to interact with donor acceptor interactions. Through this mechanism, amine decorated MOFs applied for improved Li-storage capacity [41] through host-guest interactions between Li and amine groups (N atoms) and accelerated I2 removal [42] through
Figure 2.6 Possible catalytic mechanism by amine function in Knoevenagel reactions. In this mechanism, amine site could active benzaldehyde through formation of imine. Reaction is progressed after addition of ethyl cyanoacetate to the imine complex [18].
In synthetic organic chemistry it is known that aromatic rings with electron donor groups like amine could participate in electrophilic substitution reactions. Also, aromatic amine groups could be converted to diazonium or other products like reduction to hydrogen. These principal roles in chemistry of arylamines applied in construction of highly efficient removal and sensing of Cl2 [43], NO [44] and NO2 [45] gases with amine decorated MOFs.
Gregory W. Peterson and coworkers synthesized UiO-6-NH2 and applied in for removal of chlorine gas [43]. The material could remove 1.24g·g−1 Cl2(g). Using different characterization methods they proposed that there are two predominately mechanisms engaged in removal process including the loss of one carboxylate group of ligand and reaction between Cl2(g) and Zr6O6 nodes as well as reaction between organic linker (2-aminoterephthlic acid) and chlorine gas through electrophilic substitution reaction in ortho and para positions. Also, produced HCl molecules are neutralized by amine functions (Figure 2.7a).
In another work by the same group, UiO-66-NH2 applied for removal of 1.4g·g−1 NO2(g) [45]. Experimental analyses show that NO2(g) is adsorbed through different types removal mechanisms (Figure 2.7b). At low loading, NO2(g) first adsorbs within the pores of the MOF and loading increases with decreasing the temperature indicating that physical adsorption has a major impact on removal. At higher loading, the organic ligand react with oxidant NO2(g) molecules in multiple locations.
Figure 2.7 Application of UiO-66-NH2 in removal of harmful gases. Removal and degradation mechanism of chlorine (a) [43] and nitrogen dioxide (b) [45] gases on 2-aminoterphthalate linker of UiO-66-NH2.
Sujit K. Ghosh and coworkers applied UiO-66-NH2 for aqueous phase detection of nitric acid gas (Figure 2.8) [44]. After exposure to NO(g) and deamination process, UiO-66-NH2 is transformed to UiO-66. Considering this mechanism, fluorescent UiO-66-NH2 is converted to non-fluorescent UiO-66. PL measurements reveal that UiO-66-NH2 could detect NO(g) gas with detection limit equal to 0.575 µM and a quenching constant of 4.15 × 10+5 M−1. The UiO-66 framework do not show any change is PL emission peak which clarifies the role of the primary amine group in the NO(g) detection. Competitive experiments also show that there is no substantial change in presence of similar species while there is considerable quenching in presence of NO(g).
Figure 2.8 Application of UiO-66-NH2 in aqueous media detection of NO(g). (a) Proposed mechanism for detection of NO(g) molecules. (b) quenching in PL emission of UiO-66-NH2 after addition of NO(g). (c) Quenching efficiency in presence of other analytes. (d) Competitive experiment for detection of NO(g) in presence of other analytes [44].
Photoactive MOFs could be developed by immobilizing photoactive catalytic sites in MOF materials. Especially, practical adsorption of solar light could be easily attained by functionalization of the metal ions or the organic ligands. Amine function is recognized as a photosensitizer group in the structure of MOFs for the improvement of solar-light photocatalytic activity in MOFs [46–49]. 2-aminoterphthalic acid is well-known linker for construction of amine decorated MOFs like NH2-UiO-66, NH2-MIL-125 and other MOFs.
The amine function has a substantial role in the modification of optical band gap of MOFs constructed based on 2-aminoterphthalic acid ligand. In this case, HOMO of amine decorated MOFs based on 2-aminoterphthalic acid ligand composed of O, C and N 2p orbitals [50]. The insertion of N character in HOMO, or valance band, of MOFs induces the band-gap narrowing to shift the photo-absorption and lower band gap will shift the band gap to the visible light region [49]. So, the material shows an extended absorption band in the visible light region with enhanced visible-light absorption owing to introduction of photosensitizer amine function. Moreover, upon light irradiation a ligand-to-metal charge transfer with long-lived excited charge separation could be observed [47]. This charge transfer is effectual for oxidizing of reactive substrates adsorbed on the amine site by photogenerated holes on organic ligand and reducing other reactive substrates adsorbed on the inorganic building blocks by transferring of photogenerated electrons. So amine function could intensify the photocatalytic activity of MOFs through extending in 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
