Читать книгу Functional Metal-Organic Frameworks - Lida Hashemi, Ali Morsali - Страница 19

Such simple but useful chemistry of amine is practical for development of FMOFs for certain applications. For example, they can apply as a polarizing site for interaction with quadruple gas molecules like carbon dioxide [1–8] and H-bond donor/acceptor gases [9], in sensing of metal ions as Lewis acid sites [10, 11], removal or sensing of hydrogen bond donor or hydrogen bond acceptor chemicals [12–17] or in heterogeneous catalysis as Lewis basic sites [18–20]. In this chapter, we deeply discuss about application of amine FMOFs in different fields [21].

Gas adsorption is a field that amine FMOFs extensively applied owing to delicate host–guest chemistry of amine function with polar or quadrupolar gas molecules [22]. Owing to environmental issues, selective CO₂ capture is a concern that extensively studied by scientists [23]. In this field, amine FMOFs show very high efficiency. Possible (which is proved experimentally and theoretically) CO₂(C)·(N)amine and CO₂(O)·(H)amine interactions give rise to high affinity between quadrupole carbon dioxide molecules and polar amine site. However, a mark difference exists between interactions of CO₂ molecule with aliphatic or aromatic amines. This observation is being caused by different basicity of arylamine and alkylamine functional groups. Due to delocalization of non-bonding lone pairs of N atom within aromatic ring in arylamine groups, they carry lower amount of negative charge on N atoms rather aliphatic amines. So, there is a stronger interaction between N atom of aliphatic amine with partially positive carbon of carbon dioxide molecule. The use of arylamines could favor strong physisorption (30–50 kJ·mol⁻¹ ) with CO² while in case of alkylamines host–guest interaction is based on chemisorption process. Although, stronger interaction in case of CO₂ capture by alkylamine could lead to higher selectivity, but it is necessary to mention that release of carbon dioxide molecules are not energy conservative in this case while strong physisorption between CO₂ and arylamine decorated FMOFs is very favorable in case of carbon dioxide release. So, to attain maximum level of CO₂ release and optimized CO₂–amine interaction, it is absolutely essential to engineer the structural features of MOFs as well as their Lewis basicity.

2-aminoterphthalic acid is favorite amine functionalized ligand for synthesis of amine decorated FMOFs for different purposes especially CO₂ separation. UiO-66-NH₂ and CAU-1 are two amine decorated based on 2-aminoterphthalate ligand. Since UiO-66-NH₂ and CAU-1 are decorated with arylamine functions, they represent higher affinity to CO₂ molecules rather non-functional parent frameworks through strong physisorption. UiO-66-NH₂ shows higher adsorption capacity (CO₂ uptake (mmol·g^–1) = ≈ 8.5 vs ≈ 7), isosteric heat of CO₂ adsorption at low loading (−32 vs −25.5 kJ·mol⁻¹ ) and CO₂/N₂ 15:85 selectivity (66.5 vs 37.5) rather UiO-66. CAU-1 has improved zero coverage enthalpy (−48 vs −32 kJ.mol⁻¹) and CO₂/N₂ 15:85 selectivity (101 vs 66.5) rather UiO-66-NH₂ [24, 25]. In these FMOFs, both CAU-1 and Uio-66-NH₂ synthesized using 2-aminoterphthalic acid ligand and the difference between CO₂∙(−NH₂) interaction (which is understood through zero coverage enthalpy) is attributed to the structural differences of these FMOFs.

One applied strategy for stabilization of alkylamine in the structure of MOFs is grafting alkylamine ligands on open metal sites (OMSs) through post-synthesis procedure. Different types of alkylamine ligands like tetraethylenepentamine, N,N’-dimethylethylenediamine, 1-methylethylenediamine, 1,1-dimethylethylenediamine, ethylenediamine, piperazine, 3 and 4-picolylamine, N,N’-dimethylethylenediamine were grafted through this strategy to prove higher Q_st and selectivity compared to arylamine functionalized MOFs [5–8, 26–33].

Immobilization of alkylamine ligands on open metal sites of MOFs is an ideal strategy to improve the affinity of the frameworks to CO₂ molecules through altering the Lewis basic open metal sites to the Lewis basic alkylamine functionalized nodes. Based on this approach, the favorable CO₂ physisorption by open metal site∙(O)carbon dioxide interaction change into strong CO₂ chemisorption by alkylamine (N)∙(C)carbon dioxide interaction which leads to high CO₂ adsorption enthalpy and selectivity. Homodiamine N,N’-dimethylethylenediamine (mmen) ligand applied in the structure of CuBTTri and Mg(dobpdc)₂ to develop post-synthetically modified materials with high affinity to carbon dioxide molecules , −96, −47 and −71 for Cu-BTTri, mmen-Cu-BTTri, Mg(dobpdc)₂ and mmen-Mg(dobpdc)₂ respectively) [6, 34].

Originally, Mg-MOF-74 could adsorb exceptional amount of carbon dioxide (20.6 wt%) under relevant post-combustion flue gas conditions while it display only about 16% recovery of its initial amount of CO₂ in breakthrough experiment with 70% humidity [35]. To remove this barrier and improvement of CO₂ capacities at lower CO₂ partial pressures and presence of humidity, Mg(dobpdc)₂ post-synthetically modified with homodiamine ligands like ethylenediamine and dimethylethylenediamine [31]. Anyway, the working capacity of these alkylamine grafted FMOFs is not high at low temperatures. This is another issue that must be eliminated for efficient release of CO₂ and increase in carbon dioxide working capacity.

Chang Seop Hong and coworkers synthesized a diamine-grafted FMOF (with the name of dmen-Mg₂(dobpdc) where dmen = N,N’ dimethylethylenediamine and H₄dobpdc = 4,4’-dihydroxy-(1,10-biphenyl)-3,3’dicarboxylic acid) and applied it for carbon dioxide capture and evaluation of working capacity at post-combustion conditions (Figure 2.1) [8]. dmen is a heterodiamine with both primary and tertiary amines and incorporated into framework through post-synthesis modification. The results reveal that dmen-Mg₂(dobpdc) represent high CO₂/N₂ selectivity (S = 554 at 25 °C and p(CO₂) = 0.15 bar) which is higher that mmen-Mg₂(dobpdc) (S = 200) and en-Mg₂(dobpdc) (S = 230). At 25 °C and 0.15 bar, activated dmen-Mg₂(dobpdc) adsorb 3.77 mmol·g⁻¹ of CO₂ which is higher than en-Mg (dobpdc) (3.62 mmol·g⁻¹) and mmen-Mg (dobpdc) (3.13 mmol·g⁻¹). Efficiency of an adsorbent in post-combustion process is assessed by measuring the working capacity which is defined as difference between the adsorbed quantities at P_ads = 0.15 bar CO₂/T_ads = 40 °C and P_des = 1 bar CO₂/T_des. The higher working capacity at lower desorption temperatures, the lower energy consumption. dmen-Mg₂(dobpdc) could adsorb 18.8 wt% for pure CO₂, and 14.1 wt% for N₂/CO₂(85/15) which is close to that at the corresponding CO₂ partial pressure in the isotherm at 40 °C, while no obvious adsorption was observed for pure N₂ at 40 °C. Regeneration of the material is evaluated via vacuum-swing adsorption (VSA) and temperature-swing adsorption (TSA) methods. In TSA method, CO₂ was adsorbed at 40 °C for 1 h and desorbed at 75 °C for 1 h under Ar. After 24 cycles, no capacity loss was observed, revealing that the dmen-Mg₂(dobpdc) is thermally stable under these experimental conditions as well as maintenance in its adsorption capacity. In VSA method, at 25 °C, material was saturated with CO₂ at 1.2 bar and then placed under high vacuum. The removal of adsorbed CO₂ from the solid was performed repeatedly by applying a vacuum to the adsorbent. Based on observed results, such a large amount of adsorbed CO₂ (4.5 mmol·g⁻¹) at 1.2 bar can be completely desorbed only under vacuum, without heating. The working capacities of dmen-Mg₂(dobpdc) at 130 to 90 °C desorption temperature is in the range of 11.7–13.5 wt%. Notably, experimental results reveal that at T_des = 75 °C working capacity is 11.6 wt% which is higher that top performing MOFs such as Mg-MOF-74 (3.7 wt%), Mg₂(dobpdc) (4.5 wt%), en-Mg₂(dobpdc) (2.9 wt%), mmen-Mg₂(dobpdc) (2.1 wt%), and tmen-Mg₂(dobpdc) (3.9 wt%) (tmen = N,N,N’,N’-tetramethylethylenediamine). However, the working capacity of dmen-Mg₂(dobpdc) sharply reduced to almost zero at 70 °C. To evaluate the reusability of dmen-Mg₂(dobpdc) in the presence of water vapor, the material exposed to water vapor (100% RH for 10 min). Since the solid sample can be fully saturated with CO₂ within the exposure time (10 min), it exposed to water vapors for 10 min. Then, dmen-Mg₂(dobpdc) reactivated under a pure CO₂ flow at 130 °C for 4 h, followed by CO₂ adsorption at 40 °C. After 5 cycles, a capacity loss of 5% is observed which is markedly higher than working capacities of the other MOFs during the first cycle (lower than 4.5 wt%). Clausius–Clapeyron equation applied for estimation of heat of adsorption (−Q_st ) so that increases to 75 kJ·mol⁻¹ at a loading of 0.25 mmol.g⁻¹, and remains almost invariant in the range of 71–75 kJ·mol⁻¹ below loadings of 2.6 mmol.g⁻¹. Based on DFT calculations, the open metals site are mostly occupied by the primary amine end of dmen-Mg₂(dobpdc), although some tertiary amine ends are probably grafted onto the exposed metal site as well. Possible mechanism based on DFT calculations and in-situ FT-IR analysis possible CO₂ adsorption mechanism is illustrated in Figure 2.1f.

Figure 2.1 Application of dmen-Mg₂(dobpdc) in selective capture-release of carbon dioxide. (a) TSA process: Adsorption (40 °C)-desorption (75 °C) cycling of CO₂ under Ar. (b) Vacuum-swing adsorption at 25 °C. (c) The working capacities of 1-dmen and the other porous solids obtained under the same conditions. (d) CO₂ adsorption of 1-dmen in flue gas using the sequence (adsorption at 40 °C, desorption at 130 °C under pure CO₂-10 min exposure to 100% RH). (e) Estimated working capacity from q_ads (P_ads = 0.15 bar CO₂, T_ads = 40 °C)-q_des (P_des = 1 bar CO₂, T_des = 75). (f) Framework structure of 1 with open metal sites, grafting modes of dmen onto the open metal sites, and subsequent CO₂ adsorption. The schematic diagram (bottom) indicates the arrangement of ammonium carbamates running along the c-axis [8].

Although extensive studies conducted on application of amine decorated MOFs is carbon dioxide capture and release, but there is an urgent need to clarify the effective conditions of arylamine or alkylamine groups in practical CO₂ capture and release for real-life applications.

Christian Serre and coworkers applied amine decorated MIL-125(Ti) (MIL-125 formula is (Ti₈O₈(OH)₄(BDC)₆) where BDC is benzenedicarboxylate), denoted as NH₂-MIL-125(Ti) for separation of CO₂ and H₂S [9]. They mentioned this material could improve separation of these acid gases from biogas or natural gas markedly. Based on in-situ FT-IR analysis, they mentioned that –NH₂ function interact weakly with CO₂ through lone pair of relatively negative N-atom of amine and relatively positively charged C-atom of CO₂. Also, hydrogen bonding is the main reason for improved H₂S separation in a way that H₂S acts as hydrogen bond donor and amine, through its N atom, acts as hydrogen bond acceptor.

Amine function is an ideal gust-adsorptive site to interact with different type of guests. Since it contains positively charged H atoms, it can interact as hydrogen bond donor site. Also, it can interact as hydrogen acceptor or hydrogen bond acceptor site as well as Lewis basic site through its N atom. These multiple chemical features enable amine decorated MOFs to apply as a host for different type of analytes. Their Lewis basicity is potentially suitable to interact with metal ions and their ability to participate in hydrogen bonding is ideal to interact with small organic molecules.

It is reported that MOFs based on central d-metal ions (especially Zn(II) or Cd(II)) functionalized with Lewis basic sites are best sensors toward 2,4,6-trinitrophenol (TNP) with highest K_SV (Stern–Volmer constant) and best detection limits [36]. Especially adenine decorated MOFs are best candidate in this field because adenine molecule contain different kind of Lewis basic site especially free primary amine groups which is able to interact with TNP.

Mandal and coworker(s) synthesized [Cd(ATAIA)].4H₂O (where H₂ATAIA = 5-((4,6-diamino-1,3,5-triazin-2-yl)amino)isophthalic 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 (K_SV = 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 (λ_ex = 353 nm, ) of this amine decorated MOF enhanced in presence of Cr(III) meal ions owing chelation of N atoms of amine groups and O atoms of free carboxylate groups. Application of isostructure framework without ‒NH₂ group, Zn-MOF, reveal that the PL emission of this MOF did not change significantly in presence of Cr(III) ions. This blank experiment confirm the vital role of amine group in detection of Cr(III) ions (detection limit = 4.9 µM) [11]. In other works, Lili and coworkers applied a amine decorated MOF for detection of Hg(II) metal ions (detection limit = 4.2 ×10⁻⁸ M, K_SV = 4550 mol⁻¹). XPS analysis of this materials after exposure to Hg(II) metal ions reveal a new N(1 s) peak at 406.38 eV indicative of chelation interaction between N atom of amino group.

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 for additional improved adsorption capacity of quinoline (546 mg·g⁻¹) and indole (583 mg·g ⁻¹).

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 reactions. 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 -MIL-125 (Ti) in oil denitrogenation. (a) Adsorption mechanism for removal of indole. (b) Adsorption mechanism for removal of quinoline [15].

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 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].