acid) for selective vapor sorption and nanomolar sensing of TNP (Figure 2.2) [14]. The pore walls of Cd-ATAIA are decorated with two primary amino groups and one secondary amino group. Activated Cd-ATAIA displays UV–Vis absorption peaks centered at 278 and 323 nm and fluorescence emission at 376 nm upon excitation at 310 nm. Application of Cd-ATAIA in aqueous solution detection of nitroaromatic compounds (NACs) reveals that activated Cd-ATAIA represents the largest quenching efficiency (98%, 60 µl) in presence of TNP as well as a significant color change (KSV = 1.59 × 10+7 mol−1 , detection limit = 0.94 nM). DFT calculation performed to investigate about possible detection mechanism. The results show that there is strong electrostatic interactions and hydrogen bonding between two types of Lewis basic amine groups of the ATAIA ligand and the highly acidic hydroxyl group of TNP. Also UV–Vis experiments reveal that there is large overlap between ATAIA and TNP. As a result of such overlap and host-guest interactions, activated Cd-ATAIA could detect TNP in presence of other NACs. Also, Cd-ATAIA could detect TNP in vapor phase. The results reveal that a visual color change is observable after 30 while a significant quenching in photoluminescence (PL) emission spectra of CD-ATAIA is distinguishable.
Figure 2.2 Application of [Cd(ATAIA)].4H2O in detection of TNP. (a) Schematic representation view showing coordination environment around Cd(II) center. (b) Perspective view of packing along c-axis (free guests and hydrogen have been omitted for clarity) (Color code; carbon: grey, oxygen: red, nitrogen: blue, Cadmium: green). (c) spectral overlap of absorption spectra of different nitro-analytes and emission spectrum of activated Cd-ATAIA. (d) Energy minimized structures of H2ATAIA and TNP complex. (e) Photograph of Whatman filter paper strips coated with activated Cd-ATAIA at different concentration of TNP [14].
Similarly in another work, Bio-MOF-1 can selectively detect 2,4,6-trinitrophenol (TNP) through hydrogen bonding interaction between free amine functions of adenine and acidic hydroxyl group of TNP (Figure 2.3) [37].
Owing to their Lewis basicity amine incorporated FMOFs could apply in detection of metal ions. Since metal ions are Lewis acid, they can interact with amine group inside pores of FMOFs as Lewis basic guest-interactive site. Through this kind of interaction, some changes could be induced in energy level of N atom of amine or even the metal ion. These changes in energy levels of metal ion∙(N)amine interactions could be distinguished using some analysis like XPS (X-ray photoelectron spectroscopy). Sometime FT-IR analysis through observation of some changes in characteristic peaks of amine or generation of new peak related to metal ion ∙(N)amine bond is useful.
Xin Liu and coworkers applied a amine decorated FMOF, formulated as [Zn2(TPOM)(NH2‒BDC)2]∙4H2O (Zn-NH2-MOF, TPOM = tetrakis(4-pyridyloxymethylene)methane and NH2-BDC = 2-aminoterphthalic acid) for detection of metal ions (Figure 2.4) [10]. The PL emission peak (λex = 353 nm,
Figure 2.3 3 dimensional framework of bio-MOF-1 along the c crystallographic direction (above) and desirable H-bonding interaction between free primary amine of adenine and hydroxyl group of TNP (down) [37].
Figure 2.4 Application of [Zn2(TPOM)(NH2–BDC)2]∙4H2O in detection of Cr(III) ions. (a) Coordination environment of NH2-Zn-MOF representing the free amine and carboxylate groups. (b) Proposed detection mechanism. (c) Change in emission peak of NH2-Zn-MOF in presence of different metal ions [10].
Host–guest chemistry of amine FMOFs in removal of detrimental analytes from aqueous and other media resembles to detection of metal ions or small organic molecules. So, Lewis basicity and capability to participate in hydrogen bonding are most practical features of amine function in removal of hazardous materials.
Nitrogen containing molecules like pyridine and quinoline (with N sites), pyrrole and indole (with NH sites) are among the well-known oil pollutants. So, they are potentially able to interact with the adsorbent through hydrogen bonding; pyridine and quinoline as hydrogen bond acceptor and pyrrole and indole as hydrogen bond donor. Since amine function is able to interact as both H-donor and H-acceptor site in hydrogen bonding, they could be applied for denitrogenation of oil. In this regard, Sung Hwa Jhung and coworkers synthesized some amine decorated MOFs, MIL-125-NH2 (Figure 2.5) and UiO-66-NH2, to purify the liquid model oil [15, 16]. Owing to mentioned interactions, host–guest chemistry of NH2-MIL-125(Ti) is enriched and adsorption capacity toward quinoline and indole is improved in comparison of MIL-125(Ti), 103 mg·g−1 vs. 460 mg·g−1 for quinoline and 264 mg·g−1 vs. 502 mg·g−1 for indole [15]. Clearly, this improvement is attributed to presence of amine site. In next move, they protonated amine sites from –NH2 to
Amine decorated FMOFs applied for Lewis basic catalyzed reactions like Henry and Knoevenagel reactions [38] and some of other reactions like transesterification of triglycerides [39]. Also, Amine decorated FMOFs with open metal sites applied in those kinds of tandem reactions needing both Lewis basic and Lewis acidic catalytic sites like cycloaddition reaction of CO2 with various epoxides [20].
In Henry or Knoevenagel reactions, benzaldehyde should be activated and then react with nitromethane or malonitrile, respectively. Since, benzaldehyde contains positively-charged C-atom which is Lewis acid site, Lewis basic sites can active benzaldehyde and catalyze Henry or Knoevenagel
