Functional Metal-Organic Frameworks. Ali MorsaliЧитать онлайн книгу.
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Amine Decorated Metal–Organic Frameworks
Abstract
In this chapter, chemical properties of amine functions discussed and then these chemical features have been extended in the synthesis of amine decorated metalorganic frameworks. Application of amine decorated metal-organic frameworks reveals that, they applied successfully in deferent applications especially CO2 post-combustion capture and release, metal ion and picric acid detection and removal of some pollutants like indole and quinoline from oil.
Keywords: Amine, functional metal-organic frameworks, CO2 capture, picric acid detection, oil denitrogenation, metal ion detection, electrophilic substitution reactions, Lewis basic catalysis
2.1 General Chemical Properties of Amine Function
Amine is among the frequently applied organic functional groups in the structure of functional MOFs (FMOFs). Although, the chemistry of amine is simple, it is useful and effective. The chemistry of amine is dominated by the non-bonding electron pair on nitrogen atom and positively charged hydrogen atom.
The electrostatic potential map for the van der Waals surface of amine function reveals localization of negative charge on N atom due to high electronegativity of N atom engaged in N‒H bond and its non-bonding electrons. In this view, amine can (as Lewis basic site) interact with Lewis acid species. Also, it can accept one proton from Brønsted acid to form ammonium cations. Moreover, through nitrogen atom, amine group can act as hydrogen bond acceptor site. In addition, accumulation of negative charge on nitrogen atom enables it to engage with polar and quadruple molecules.
Another characteristic of amine function is revolved around H atom(s). Since these H atoms are positively charged, they can participate in a hydrogen bonding. Overally, amine function can interact as both hydrogen bond donor and hydrogen bond acceptor sites.
2.2 Function–Application Properties
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 Qst 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