Metal–organic polyhedra (MOP) are a promising class of crystalline porous materials with multifarious potential applications. Although MOPs and metal–organic frameworks (MOFs) have similar potential ...in terms of their intrinsic porosities and physicochemical properties, the exploitation of carboxylate MOPs is still rudimentary because of the lack of systematic development addressing their chemical stability. Herein we describe the fabrication of chemically robust carboxylate MOPs via outer‐surface functionalization as an a priori methodology, to stabilize those MOPs system where metal–ligand bond is not so strong. Fine‐tuning of hydrophobic shielding is key to attaining chemical inertness with retention of the framework integrity over a wide range of pH values, in strong acidic conditions, and in oxidizing and reducing media. These results are further corroborated by molecular modelling studies. Owing to the unprecedented transition from instability to a chemically ultra‐stable regime using a rapid ambient‐temperature gram‐scale synthesis (within seconds), a prototype strategy towards chemically stable MOPs is reported.
Systematic stability: A correlation between the hydrophobicity and chemical stability of metal–organic polyhedra (MOP) was uncovered. Enhancement of the chemical stability is a consequence of the incremental hydrophobicity of the surfaces for systems where the internal core of each cage is similar. Structural characterizations and simulation data are presented.
Simple and efficient regeneration of MOF-5 and HKUST-1 is demonstrated via acid–base treatment. The reactants are recovered simply by dissolving the deteriorated MOFs in strong acid. Pristine MOFs ...are regenerated in high yields by adjusting the solution pH via either in situ base formation or ex situ base addition. Both regeneration protocols are environmentally benign and cost-effective because the reactants in the deteriorated MOFs are recycled. Especially the ex situ base addition protocol can be done in a cheap and environmentally friendly ethanol/water mixed solvent at ambient condition and is suitable for large-scale batch regeneration owing to the simple procedure and short reaction time.
A metal–organic framework (MOF) having superprotonic conductivity, MOF‐808, is prepared by modulating the binding mode of the sulfamate (SA) moieties grafted onto the metal clusters. The activation ...of the SA‐grafted MOF‐808 at 150 °C changes the binding mode of the grafted SA from monodentate to bridging bidentate, thus converting the neutral amido (‐S−NH2) moiety of the grafted SA to the more acidic cationic sulfiliminium (‐S=NH2+) moiety. Further, the acidic sulfiliminium moiety of MOF‐808‐4SA‐150 results in more efficient proton conduction than the amido moiety of MOF‐808‐4SA‐60. At 60 °C and 95 % relative humidity, MOF‐808‐4SA‐150 is found to have a proton conductivity of 7.89×10−2 S cm−1, which is more than 30‐times higher than that of MOF‐808‐4SA‐60. Moreover, this superprotonic conductivity is well maintained over 1000 cycles of conductivity measurements and for similar cyclic measurements each day for seven days.
Superprotonic conductivity in a metal–organic framework is achieved by modulating the binding mode of sulfamate‐grafted MOF‐808 to produce the more acidic cationic sulfiliminium group (−S=NH2+), which results in more efficient proton conduction.
MOF‐74 is one of the most explored metal–organic frameworks (MOFs), but its functionalization is limited to the dative post‐synthetic modification (PSM) of the monodentate solvent site. Owing to the ...nature of the organic ligand and framework structure of MOF‐74, the covalent PSM of MOF‐74 is very demanding. Herein, we report, for the first time, the covalent PSM of amine‐tagged defective Ni‐MOF‐74, which is prepared by de novo solvothermal synthesis by using aminosalicylic acid as a functionalized fragmented organic ligand. The covalent PSM of the amino group generates metal binding sites, and subsequent post‐synthetic metalation with PdII ions affords the PdII‐incorporated Ni‐MOF‐74 catalyst. This catalyst exhibits highly efficient, size‐selective, and recyclable catalytic activity for the Suzuki–Miyaura cross‐coupling reaction. This strategy is also useful for the covalent modification of amine‐tagged defective Ni2(DOBPDC), an expanded analogue of MOF‐74.
Aminosalicylic acid has been used as a functionalized fragment to generate an amine‐tagged defect‐engineered Ni‐MOF‐74, which could be covalently modified post‐synthetically to generate metal‐binding sites. Subsequent metalation with PdII ions afforded PdII‐incorporated Ni‐MOF‐74, which is a highly efficient, size‐selective, and recyclable catalyst for the Suzuki–Miyaura cross‐coupling reaction.
The formation of 2D polyaniline (PANI) has attracted considerable interest due to its expected electronic and optoelectronic properties. Although PANIwas discovered over 150 y ago, obtaining an ...atomically well-defined 2D PANI framework has been a longstanding challenge. Here, we describe the synthesis of 2D PANI via the direct pyrolysis of hexaaminobenzene trihydrochloride single crystals in solid state. The 2D PANI consists of three phenyl rings sharing six nitrogen atoms, and its structural unit has the empirical formula of C₃N. The topological and electronic structures of the 2D PANI were revealed by scanning tunneling microscopy and scanning tunneling spectroscopy combined with a first-principle density functional theory calculation. The electronic properties of pristine 2D PANI films (undoped) showed ambipolar behaviors with a Dirac point of −37 V and an average conductivity of 0.72 S/cm. After doping with hydrochloric acid, the conductivity jumped to 1.41 × 10³ S/cm, which is the highest value for doped PANI reported to date. Although the structure of 2D PANI is analogous to graphene, it contains uniformly distributed nitrogen atoms for multifunctionality; hence, we anticipate that 2D PANI has strong potential, from wet chemistry to device applications, beyond linear PANI and other 2D materials.
The transmetalation (the replacement of metal ions) of a family of highly porous isostructural metal–organic frameworks (MOFs), M6(BTB)4(BP)3 (where M = Zn(II) (1), Co(II) (2), Cu(II) (3), and Ni(II) ...(4), BTB = 1,3,5-benzenetribenzoate, and BP = 4,4′-dipyridyl) with an ith-d net topology has been investigated. These compounds have different framework stabilities depending on the framework metal ions. The transmetalation and the reverse transmetalation reactions of the framework metal ions were observed between the MOFs, 1 and 2, having a similar thermodynamic stability. While the transmetalation from thermodynamically less stable 1 and 2 to more stable 3 and 4 were achieved by soaking single crystals of 1 and 2 in a solution of N,N′-dimethylformamide (DMF) containing Cu(II) and Ni(II) ions, respectively, no reverse transmetalation was observed. By simply controlling the soaking time, not only could homogeneously transmetalated crystalline framework structures be prepared via the thermodynamically controlled complete replacement of the framework metal ions but also selectively transmetalated core–shell heterostructures were formed via kinetically controlled replacement that was mainly restricted to the external shell region of the crystal. The fully transmetalated MOFs showed significantly improved framework stabilities compared with the parent MOFs. A marked improvement in the framework stability was observed, even in the selectively transmetalated Co(II)/Cu(II)- and Co(II)/Ni(II)-core–shell heterostructures. Although the frameworks are partially transmetalated, the framework stability of not only the external shell region but also of the internal core region was significantly affected.
The placement of mixed building blocks at precise locations in metal-organic frameworks is critical to creating pore environments suitable for advanced applications. Here we show that the spatial ...distribution of mixed building blocks in metal-organic frameworks can be modulated by exploiting the different temperature sensitivities of the diffusion coefficients and exchange rate constants of the building blocks. By tuning the reaction temperature of the forward linker exchange from one metal-organic framework to another isoreticular metal-organic framework, core-shell microstructural and uniform microstructural metal-organic frameworks are obtained. The strategy can be extended to the fabrication of inverted core-shell microstructures and multi-shell microstructures and applied for the modulation of the spatial distribution of framework metal ions during the post-synthetic metal exchange process of a Zn-based metal-organic framework to an isostructural Ni-based metal-organic framework.
A few metal–organic frameworks (MOFs), which typically use strong acids as proton sources, display superprotonic conductivity (≈10−1 S cm−1); however, they are rare due to the instability of MOFs in ...highly acidic conditions. For the first time, we report superprotonic conductivity using a moderately acidic guest, zwitterionic sulfamic acid (HSA), which is encapsulated in MOF‐808 and MIL‐101. HSA acts not only as a proton source but also as a proton‐conducting medium due to its extensive hydrogen bonding ability and zwitterion effect. A new sustained concentration gradient method results in higher HSA encapsulation compared to conventional methods, producing 10HSA@MOF‐808‐(bSA)2 and 8HSA@MIL‐101. These MOFs show impressive superprotonic conductivity of 2.47×10−1 and 3.06×10−1 S cm−1, respectively, at 85 °C and 98 % relative humidity, and maintain stability for 7 days.
Strategic loading of dual‐functioning zwitterionic sulfamic acid in MOFs leads to proton conductivity of the order of 10−1 S cm−1. This high conductivity is the result of higher loading of sulfamic acid, which can act as both a proton source due to its acidity and a conducting medium due to its extensive hydrogen‐bonding ability and zwitterion effect.
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