Two‐dimensional (2D) materials and ultrathin nanosheets are advantageous for elevating the catalysis performance and elucidating the catalysis mechanism of heterogeneous catalysts, but they are ...mostly restricted to inorganic or organic materials based on covalent bonds. We report an electrochemical/chemical exfoliation strategy for synthesizing metal–organic 2D materials based on coordination bonds. A catechol functionalized ligand is used as the redox active pillar to construct a pillared‐layer framework. When the 3D pillared‐layer MOF serves as an electrocatalyst for water oxidation (pH 13), the pillar ligands can be oxidized in situ and removed. The remaining ultrathin (2 nm) nanosheets of the metal–organic layers are an efficient catalyst with overpotentials as low as 211 mV at 10 mA cm−2 and a turnover frequency as high as 30 s−1 at an overpotential of 300 mV.
MOF slicing: A pillared‐layer metal–organic framework (MOF), in which the catechol functionalized pillars can be oxidized and removed in an electrochemical process, gives ultrathin nanosheets (2 nm). These are efficient electrocatalysts for water oxidation at pH 13 with a low overpotential and high turnover frequency (TOF).
Efficient adsorptive separation of propylene/propane (C3H6/C3H8) is highly desired and challenging. Known strategies focus on either the thermodynamic or the kinetic mechanism. Here, we report an ...interesting reactivity of a metal–organic framework that improves thermodynamic and kinetic adsorption selectivity simultaneously. When the metal–organic framework is heated under oxygen flow, half of the soft methylene bridges of the organic ligands are selectively oxidized to form the more polar and rigid carbonyl bridges. Mixture breakthrough experiments showed drastic increase of C3H6/C3H8 selectivity from 1.5 to 15. For comparison, the C3H6/C3H8 selectivities of the best‐performing metal–organic frameworks Co‐MOF‐74 and KAUST‐7 were experimentally determined to be 6.5 and 12, respectively. Gas adsorption isotherms/kinetics, single‐crystal X‐ray diffraction, and computational simulations revealed that the oxidation gives additional guest recognition sites, which improve thermodynamic selectivity, and reduces the framework flexibility, which generate kinetic selectivity.
Tailoring with O2: When a flexible metal–organic framework is heated in a flow of oxygen, half of the organic ligands are selectively oxidized, thereby adding guest recognition sites and reducing the flexibility of the pores. As a result, the thermodynamic and kinetic selectivity for propylene/propane separation improve simultaneously.
The changeable molecular dynamics of flexible polar cations in the variable confined space between inorganic chains brings about a new type of two‐step nonlinear optical (NLO) switch with genuine ...“off–on–off” second harmonic generation (SHG) conversion between one NLO‐active state and two NLO‐inactive states.
Cobalt imidazolate frameworks are classical electrocatalysts for the oxygen evolution reaction (OER) but suffer from the relatively low activity. Here, a non‐3d metal modulation strategy is presented ...for enhancing the OER activity of cobalt imidazolate frameworks. Two isomorphous frameworks Co4(MO4)(eim)6 (M=Mo or W, Heim=2‐ethylimidazole) having Co(eim)3(MO4) units and high water stabilities were designed and synthesized. In different neutral media, the Mo‐modulated framework coated on a glassy carbon electrode shows the best OER performances (1 mA cm−2 at an overpotential of 210 mV in CO2‐saturated 0.5 m KHCO3 electrolyte and 2/10/22 mA cm−2 at overpotential of 388/490/570 mV in phosphate buffer solution) among non‐precious metal catalysts and even outperforms RuO2. Spectroscopic measurements and computational simulations revealed that the non‐3d metals modulate the electronic structure of Co for optimum reactant/product adsorption and tailor the energy of rate‐determining step to a more moderate value.
Non‐3D for 3D: Introducing non‐3d metal oxide units into a cobalt imidazolate framework results in the drastic enhancement of electrocatalytic performance of the oxygen evolution reaction in neutral media.
A rigid hydrogen‐bonded organic framework (HOF) was constructed from a C3‐symmetric hexatopic carboxylic acid with a hydrophilic 18‐crown‐6‐ether (18C6) component. Despite the flexible macrocyclic ...structure with many conformations, the derivative with three 4,4’‐dicarboxy‐o‐terphenyl moieties in the periphery yielded a rigid layered porous framework through directional intermolecular hydrogen bonding. Interestingly, the HOF possesses 1D channels with bottleneck composed of 18C6 rings. The HOF shows proton conductivity (1.12×10−7 S cm−1) through Grotthuss mechanism (Ea=0.27 eV) under 98 %RH. The present unique water channel structure provides an inspiration to create molecular porous materials.
A porous hydrogen‐bonded framework (HOF) was constructed from a 18‐crown‐6‐ether (18C6) derivative. Although a 18C6 macrocycle is flexible and has many possible conformations, directional intermolecular hydrogen bonds of 4,4′‐dicarboxy‐o‐terphenyl modules in the periphery of the 18C6 allowed to form a rigid HOF with 1D channels with a bottleneck composed of 18C6 rings. The wet HOF shows proton conductivity (1.12×10−7 S cm−1) through a Grotthuss mechanism (Ea=0.27 eV) under 98 %RH.
Molecular sieving can lead to ultrahigh selectivity and low regeneration energy because it completely excludes all larger molecules via a size restriction mechanism. However, it allows adsorption of ...all molecules smaller than the pore aperture and so separations of complicated mixtures can be hindered. Here, we report an intermediate-sized molecular sieving (iSMS) effect in a metal-organic framework (MAF-41) designed with restricted flexibility, which also exhibits superhydrophobicity and ultrahigh thermal/chemical stabilities. Single-component isotherms and computational simulations show adsorption of styrene but complete exclusion of the larger analogue ethylbenzene (because it exceeds the maximal aperture size) and smaller toluene/benzene molecules that have insufficient adsorption energy to open the cavity. Mixture adsorption experiments show a high styrene selectivity of 1,250 for an ethylbenzene/styrene mixture and 3,300 for an ethylbenzene/styrene/toluene/benzene mixture (orders of magnitude higher than previous reports). This produces styrene with a purity of 99.9%+ in a single adsorption-desorption cycle. Controlling/restricting flexibility is the key for iSMS and can be a promising strategy for discovering other exceptional properties.
Manufacturing two‐dimensional (2D) materials or nanosheets with desired crystal faces originating from an established intrinsic 3D structure is a significant challenge. Herein, an intrinsic t3D ...metal–organic frameworks (MOFs) can be exfoliated into ultrathin metal–organic nanosheets with (001) and (101) crystal faces, which is regulated by the solute's shard and soft acid−base and pH value in the exfoliating solution. The inorganic ligands (with different charges) possessing the similar hardness of Lewis acid‐base of metal center can replace some of the organic ligands along the established crystal faces of the 3D MOFs with different electrostatic potentials, resulting in the exfoliating of the 3D MOFs into metal–organic nanosheets with different crystal faces. Using the Knoevenagel reaction as the catalytic reaction model, the catalytic activity of (001) crystal faces is three times that of (101) crystal faces, which is determined by the different locations of open metal sits in the 3D lamellar structure, further confirming the occurrence of oriented exfoliation.
An intrinsic three–dimensional metal–organic framework can be exfoliated into metal–organic nanosheets exposed different crystal facets by the solute's hard and soft acid‐base and charge of Lewis bases in exfoliating solution. The catalytic activity of (001) crystal faces is much higher than that of (101) crystal faces, attributed to more catalytic active sites expose on the former.
MnCr(oxalate)
3
−
possesses a two-dimensional ferromagnetic network that is an ideal system for the construction of multifunctional molecular materials based on ferromagnetism. This is because ...additional functions, such as ferroelectricity, can be hybridised by incorporating functional cations between the layers. However, the majority of MnCr(oxalate)
3
−
networks readily incorporate solvent molecules upon crystallisation, and it is sometimes difficult to measure the crystal physical properties because of the collapse associated with desolvation. Upon desolvation, the polar crystal (CBA
+
)(18crown-6)MnCr(oxalate)
3
−
(CH
3
OH) (
1
·CH
3
OH) (CBA
+
= 4-carboxybutan-1-aminium) underwent a crystal-to-crystal transformation to form (CBA
+
)(18crown-6)MnCr(oxalate)
3
−
,
1
. Furthermore, this change was accompanied by hydrogen bond reorganisation in the (CBA
+
)(18crown-6) supramolecular assembly. Both crystals exhibited ferromagnetic ordering at approximately 5 K. In crystal
1
, a "merry-go-round" motion of 18crown-6 was observed, with an activation energy of 41.41 kJ mol
−1
, which resulted in dielectric relaxation. This crystal-to-crystal structural transformation provides a strategy for designing multifunctional hybrid materials, in which an additional function arises from molecular motion.
The orientation of (4-carboxybutan-1-aminium)(18crown-6), as well as the rotational behaviour of 18crown-6, is triggered by guest CH
3
OH through hydrogen bond reorganisation.
Abstract
The flexibility and guest-responsive behavior of some metal-organic frameworks (MOFs) indicate their potential in the fields of sensors and molecular recognition. As a subfamily of MOFs, the ...flexible zeolitic imidazolate frameworks (ZIFs) typically feature a small displacive transition due to the rigid zeolite topology. Herein, an atypical reversible displacive transition (6.4 Å) is observed for the sodalite (SOD) cage in flexible ZIF-65(Zn), which represents an unusually large breathing effect compared to other ZIFs. ZIF-65(Zn) exhibits a stepwise II → III → I expansion between an unusual ellipsoidal SOD cage (8.6 Å × 15.9 Å for II) and a spherical SOD cage (15.0 Å for I). The breathing behavior of ZIF-65(Zn) varies depending on the nature of the guest molecules (polarity and shape). Computational simulations are employed to rationalize the differences in the breathing behavior depending on the structure of the ZIF-65(Zn) cage and the nature of the guest-associated host–guest and guest–guest interactions.
Spin-reorientation-induced magnetodielectric coupling effects were discovered in two layered perovskite magnets, C
H
CH
CH
NH
MCl
(M = Mn
and Cu
),
isothermal magnetodielectric measurements on ...single-crystal samples. Specifically, peak-like dielectric anomalies and spin-flop transitions appeared simultaneously at around ±34 kOe for the canted antiferromagnet (M = Mn
) at below 44.3 K, while a low-field (1 kOe) controlled magnetodielectric effect was observed in the "soft" ferromagnet (M = Cu
) at below 9.5 K. These isothermal magnetodielectric effects are highly reproducible and synchronous with the field-induced magnetization at different temperatures, well confirming the essential role of spin reorientation on inducing magnetodielectric coupling effects. These findings strongly imply that the layered perovskite magnets are new promising organic-inorganic hybrid systems to host magnetodielectric coupling effects.
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