Modelling a Linker Mix‐and‐Match Approach for Controlling the Optical Excitation Gaps and Band Alignment of Zeolitic Imidazolate Frameworks

Tuning the electronic structure of metal–organic frameworks is the key to extending their functionality to the photocatalytic conversion of absorbed gases. Herein we discuss how the band edge positions in zeolitic imidazolate frameworks (ZIFs) can be tuned by mixing different imidazole‐based linkers within the same structure. We present the band alignment for a number of known and hypothetical Zn‐based ZIFs with respect to the vacuum level. Structures with a single type of linker exhibit relatively wide band gaps; however, by mixing linkers of a low‐lying conduction edge with linkers of a high‐lying valence edge, we can predict materials with ideal band positions for visible‐light water splitting and CO₂ reduction photocatalysis. By introducing copper in the tetrahedral position of the mixed‐linker ZIFs, it would be possible to increase both photo‐absorption and the electron–hole recombination times.     

Insight into Ion Transfer through the Sub‐Nanometer Channels in Zeolitic Imidazolate Frameworks

A crack‐free sub‐nanometer composite structure for the study of ion transfer was constructed by in situ growth of ZIF‐90 [Zn(ICA)₂, ICA=Imidazole‐2‐carboxaldehyde] on the tip of a glass nanopipette. The potential‐driven ion transfer through the sub‐nanometer channels in ZIF‐90 is strongly influenced by the pH of the solution. A rectification ratio over 500 is observed in 1 m KCl solution under alkaline conditions (pH 11.58), which is the highest value reported under such a high salt concentration. Fluorescence experiments show the super‐high rectification ratio under alkaline conditions results from the strong electrostatic interaction between ions and the sub‐nanometer channels of ZIF‐90. In addition to providing a general pathway for further study of mass‐transfer process through sub‐nanometer channels, the approach enable all kinds of metal–organic frameworks (MOFs) to be used as ionic permselectivity materials in nanopore‐based analysis.     

Entropy‐Driven Mechanochemical Synthesis of Polymetallic Zeolitic Imidazolate Frameworks for CO2 Fixation

High‐entropy materials refer to a kind of materials in which five or more metal species were incorporated deliberately into a single lattice with random occupancy. Up to now, such a concept has been only restricted to hard materials, such as high‐entropy alloys and ceramics. Herein we report the synthesis of hybrid high‐entropy materials, polymetallic zeolitic imidazolate framework (also named as high‐entropy zeolitic imidazolate framework, HE‐ZIF), via entropy‐driven room‐temperature mechanochemistry. HE‐ZIF contains five metals including Zn^II, Co^II, Cd^II, Ni^II, and Cu^II which are dispersed in the ZIF structure randomly. Moreover, HE‐ZIF shows enhanced catalytic conversion of CO₂ into carbonate compared with ZIF‐8 presumably a result of the synergistic effect of the five metal ions as Lewis acid in epoxide activation.     

Encapsulating Perovskite Quantum Dots in Iron‐Based Metal–Organic Frameworks (MOFs) for Efficient Photocatalytic CO2 Reduction

Improving the stability of lead halide perovskite quantum dots (QDs) in a system containing water is the key for their practical application in artificial photosynthesis. Herein, we encapsulate low‐cost CH₃NH₃PbI₃ (MAPbI₃) perovskite QDs in the pores of earth‐abundant Fe‐porphyrin based metal organic framework (MOF) PCN‐221(Fe_x) by a sequential deposition route, to construct a series of composite photocatalysts of MAPbI₃@PCN‐221(Fe_x) (x=0–1). Protected by the MOF the composite photocatalysts exhibit much improved stability in reaction systems containing water. The close contact of QDs to the Fe catalytic site in the MOF, allows the photogenerated electrons in the QDs to transfer rapidly the Fe catalytic sites to enhance the photocatalytic activity for CO₂ reduction. Using water as an electron source, MAPbI₃@PCN‐221(Fe_0.2) exhibits a record‐high total yield of 1559 μmol g^?1 for photocatalytic CO₂ reduction to CO (34 %) and CH₄ (66 %), 38 times higher than that of PCN‐221(Fe_0.2) in the absence of perovskite QDs.     

Switching on the Photocatalysis of Metal–Organic Frameworks by Engineering Structural Defects

Defect engineering is a versatile approach to modulate band and electronic structures as well as materials performance. Herein, metal–organic frameworks (MOFs) featuring controlled structural defects, namely UiO‐66‐NH₂‐X (X represents the molar equivalents of the modulator, acetic acid, with respect to the linker in synthesis), were synthesized to systematically investigate the effect of structural defects on photocatalytic properties. Remarkably, structural defects in MOFs are able to switch on the photocatalysis. The photocatalytic H₂ production rate presents a volcano‐type trend with increasing structural defects, where Pt@UiO‐66‐NH₂‐100 exhibits the highest activity. Ultrafast transient absorption spectroscopy unveils that UiO‐66‐NH₂‐100 with moderate structural defects possesses the fastest relaxation kinetics and the highest charge separation efficiency, while excessive defects retard the relaxation and reduce charge separation efficiency.     

Surface Modification of Two‐Dimensional Metal–Organic Layers Creates Biomimetic Catalytic Microenvironments for Selective Oxidation

Microenvironments in enzymes play crucial roles in controlling the activities and selectivities of reaction centers. Herein we report the tuning of the catalytic microenvironments of metal–organic layers (MOLs), a two‐dimensional version of metal–organic frameworks (MOFs) with thickness down to a monolayer, to control product selectivities. By modifying the secondary building units (SBUs) of MOLs with monocarboxylic acids, such as gluconic acid, we changed the hydrophobicity/hydrophilicity around the active sites and fine‐tuned the selectivity in photocatalytic oxidation of tetrahydrofuran (THF) to exclusively afford butyrolactone (BTL), likely a result of prolonging the residence time of reaction intermediates in the hydrophilic microenvironment of catalytic centers. Our work highlights new opportunities in using functional MOLs as highly tunable and selective two‐dimensional catalytic materials.     

Mechanical Control of the Kinetic Propylene/Propane Separation by Zeolitic Imidazolate Framework‐8

One intriguing feature of many porous MOFs is their stimulus‐induced flexibility, which makes them unique compared to standard adsorbents. Here we propose an innovative concept to achieve an efficient kinetic separation of species with similar properties by the mechanical fine‐tuning of the pore architecture of the flexible zeolitic imidazolate framework ZIF‐8. This unprecedented approach was applied to one of the most challenging societally relevant separations: the separation of propylene and propane, which is of vital importance in the petrochemical industry. It was demonstrated that the application of an external pressure creates a gradual enhancement in the propylene/propane diffusion selectivity to more than one order of magnitude at 1 GPa pressure. A detailed analysis of the molecular simulations was further able to unravel the origin of this unusual behavior at the atomistic level.     

Transformation of Metal‐Organic Frameworks into Stable Organic Frameworks with Inherited Skeletons and Catalytic Properties

Metal‐organic frameworks (MOFs) are an emerging class of porous materials with attractive properties, however, their practical applications are heavily hindered by their fragile nature. We report herein an effective strategy to transform fragile coordination bonds in MOFs into stable covalent organic bonds under mild annealing decarboxylative coupling reaction conditions, which results in highly stable organic framework materials. This strategy successfully endows intrinsic framework skeletons, porosity and properties of the parent MOFs in the daughter organic framework materials, which exhibit excellent chemical stability under harsh catalytic conditions. Therefore, this work opens a new avenue to synthesize stable organic framework materials derived from MOFs for applications in different fields.     

Ultrathin Metal–Organic Framework Nanosheets with Ultrahigh Loading of Single Pt Atoms for Efficient Visible‐Light‐Driven Photocatalytic H2 Evolution

A surfactant‐stabilized coordination strategy is used to make two‐dimensional (2D) single‐atom catalysts (SACs) with an ultrahigh Pt loading of 12.0 wt %, by assembly of pre‐formed single Pt atom coordinated porphyrin precursors into free‐standing metal–organic framework (MOF) nanosheets with an ultrathin thickness of 2.4±0.9 nm. This is the first example of 2D MOF‐based SACs. Remarkably, the 2D SACs exhibit a record‐high photocatalytic H₂ evolution rate of 11 320 μmol g^?1 h^?1 via water splitting under visible light irradiation (λ>420 nm) compared with those of reported MOF‐based photocatalysts. Moreover, the MOF nanosheets can be readily drop‐casted onto solid substrates, forming thin films while still retaining their photocatalytic activity, which is highly desirable for practical solar H₂ production.     

Non‐3d Metal Modulation of a Cobalt Imidazolate Framework for Excellent Electrocatalytic Oxygen Evolution in Neutral Media

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.     

Lock‐and‐Key and Shape‐Memory Effects in an Unconventional Synthetic Path to Magnesium Metal–Organic Frameworks

We report a new magnesium metal–organic framework (MOF) (CPM‐107) with a special interaction with CO₂. CPM‐107 contains Mg₂‐acetate chains crosslinked into a 3D net by terephthalate. It has an anionic framework encapsulating ordered extra‐framework cations and solvent molecules. The desolvated form is closed and unresponsive to common gasses, such as N₂, H₂, and CH₄. Yet, with CO₂ at 195 K, it abruptly opens and turns into a rigid porous form that is irreversible via desorption. Once opened by CO₂, CPM‐107 remains in the stable porous state accessible to additional gas types over multiple cycles or CO₂ itself at different temperatures. The porous phase can be re‐locked to return to the initial closed phase via re‐solvation and desolvation. Such peculiar properties of CPM‐107 are apparently linked to a convergence of factors related to both framework and extra‐framework features. The unusual CO₂ effect is currently the only available path to porous CPM‐107 which shows efficient C₂H₂/CO₂ separation.     

Two‐Phase System Utilizing Hydrophobic Metal–Organic Frameworks (MOFs) for Photocatalytic Synthesis of Hydrogen Peroxide

Much effort has been devoted to photocatalytic production of hydrogen peroxide (H₂O₂) as an alternative to fossil fuels. From an economic point of view, reductive synthesis of H₂O₂ from O₂ coupled with the oxidative synthesis of value‐added products is particularly interesting. We herein report application of MIL‐125‐NH₂, a photoactive metal–organic framework (MOF), to a benzylalcohol/water two‐phase system that realized photocatalytic production and spontaneous separation of H₂O₂ and benzaldehyde. Hydrophobization of the MOF enabled its separation from the aqueous phase. This resulted in enhanced photocatalytic efficiency and enabled application of various aqueous solutions including extremely low pH solution which is favorable for H₂O₂ production but fatal to MOF structure. In addition, a highly concentrated H₂O₂ solution was obtained by simply reducing the volume of the aqueous phase.     

Fulleretic Well‐Defined Scaffolds: Donor–Fullerene Alignment Through Metal Coordination and Its Effect on Photophysics

Herein, we report the first example of a crystalline metal–donor–fullerene framework, in which control of the donor–fullerene mutual orientation was achieved through chemical bond formation, in particular, by metal coordination. The ¹³C cross‐polarization magic‐angle spinning NMR spectroscopy, X‐ray diffraction, and time‐resolved fluorescence spectroscopy were performed for comprehensive structural analysis and energy‐transfer (ET) studies of the fulleretic donor–acceptor scaffold. Furthermore, in combination with photoluminescence measurements, the theoretical calculations of the spectral overlap function, Förster radius, excitation energies, and band structure were employed to elucidate the photophysical and ET processes in the prepared fulleretic material. We envision that the well‐defined fulleretic donor–acceptor materials could contribute not only to the basic science of fullerene chemistry but would also be used towards effective development of organic photovoltaics and molecular electronics.