The Molecular Pathway to ZIF-7 Microrods Revealed by In Situ Time-Resolved Small- and Wide-Angle X-Ray Scattering, Quick-Scanning Extended X-Ray Absorption Spectroscopy, and DFT Calculations

Prospects and challenges for autonomous catalyst discovery viewed from an experimental perspective

Autonomous catalysis research requires elaborate integration of operando experiments into automated workflows. Suitable experimental data for analysis by artificial intelligence can be measured more readily according to standard operating procedures.

MOFs in the time domain

Many of the proposed applications of metal-organic framework (MOF) materials may fail to materialize if the community does not fully address the difficult fundamental work needed to map out the 'time gap' in the literature - that is, the lack of investigation into the time-dependent behaviours of MOFs as opposed to equilibrium or steady-state properties. Although there are a range of excellent investigations into MOF dynamics and time-dependent phenomena, these works represent only a tiny fraction of the vast number of MOF studies. This Review provides an overview of current research into the temporal evolution of MOF structures and properties by analysing the time-resolved experimental techniques that can be used to monitor such behaviours. We focus on innovative techniques, while also discussing older methods often used in other chemical systems. Four areas are examined: MOF formation, guest motion, electron motion and framework motion. In each area, we highlight the disparity between the relatively small amount of (published) research on key time-dependent phenomena and the enormous scope for acquiring the wider and deeper understanding that is essential for the future of the field.

Evolution of Zn(II) single atom catalyst sites during the pyrolysis-induced transformation of ZIF-8 to N-doped carbons

The pyrolysis of zeolitic imidazolate frameworks (ZIFs) is becoming a popular approach for the synthesis of catalysts comprising porphyrin-like metal single atom catalysts (SACs) on N-doped carbons (M-N-C). Understanding the structural evolution of M-N-C as a function of ZIF pyrolysis temperature is important for realizing high performance catalysts. Herein, we report a detailed investigation of the evolution of Zn single atom catalyst sites during the pyrolysis of ZIF-8 at temperatures ranging from 500 to 900 °C. Results from Zn L-edge and Zn K-edge X-ray absorption spectroscopy studies reveal that tetrahedral ZnN₄ centers in ZIF-8 transform to porphyrin-like ZnN₄ centers supported on N-doped carbon at temperatures as low as 600 °C. As the pyrolysis temperature increased in the range 600-900 °C, the Zn atoms moved closer to the N₄ coordination plane. This subtle geometry change in the ZnN₄ sites alters the electron density on the Zn atoms (formally Zn²⁺), strongly impacting the catalytic performance for the peroxidase-like decomposition of H₂O₂. The catalyst obtained at 800 °C (Zn-N-C-800) offered the best performance for H₂O₂ decomposition. This work provides valuable new insights about the evolution of porphyrin-like single metal sites on N-doped carbons from ZIF precursors and the factors influencing SAC activity.

Modulated self-assembly of metal-organic frameworks

Exercising fine control over the synthesis of metal-organic frameworks (MOFs) is key to ensuring reproducibility of physical properties such as crystallinity, particle size, morphology, porosity, defectivity, and surface chemistry. The principle of modulated self-assembly - incorporation of modulator molecules into synthetic mixtures - has emerged as the primary means to this end. This perspective article will detail the development of modulated synthesis, focusing primarily on coordination modulation, from a technique initially intended to cap the growth of MOF crystals to one that is now used regularly to enhance crystallinity, control particle size, induce defectivity and select specific phases. The various mechanistic driving forces will be discussed, as well as the influence of modulation on physical properties and how this can facilitate potential applications. Modulation is also increasingly being used to exert kinetic control over self-assembly; examples of phase selection and the development of new protocols to induce this will be provided. Finally, the application of modulated self-assembly to alternative materials will be discussed, and future perspectives on the area given.

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

A comprehensive overview of characterization tools for the analysis of well-known metal–organic frameworks and physico-chemical phenomena associated to their applications.

From IR to x-rays: gaining molecular level insights on metal-organic frameworks through spectroscopy

This topical review focuses on the application of several types of spectroscopy methods to a class of solid state materials called metal organic frameworks (MOFs). MOFs are self-assembled, porous crystalline materials composed of metal cluster nodes linked through coordination bonds with organic or organometallic molecular constituents. Their unique host-guest properties make them attractive for many adsorption-based applications such as gas storage and separation, catalysis, sensing and others. While much research focuses on the development and application of these materials, fundamental studies of MOF properties and molecular level host-guest interactions behind their functionality have become a significant research direction on its own. Spectroscopy methods are now ubiquitous tools in this pursuit. This review focuses on the application of three classes of spectroscopy methods to MOF materials: vibrational, optical electronic and x-ray spectroscopies. Following brief introductions to each method that include pertinent theory and experimental considerations, we present a broad overview of the types of MOF systems that have been studied, with specific examples and important new molecular level insights highlighted along the way. The current status of spectroscopic studies of MOFs is presented at the end along with some perspectives on the future directions in this area of research.

Packaging and delivering enzymes by amorphous metal-organic frameworks

Enzymatic catalysis in living cells enables the in-situ detection of cellular metabolites in single cells, which could contribute to early diagnosis of diseases. In this study, enzyme is packaged in amorphous metal-organic frameworks (MOFs) via a one-pot co-precipitation process under ambient conditions, exhibiting 5–20 times higher apparent activity than when the enzyme is encapsulated in corresponding crystalline MOFs. Molecular simulation and cryo-electron tomography (Cryo-ET) combined with other techniques demonstrate that the mesopores generated in this disordered and fuzzy structure endow the packaged enzyme with high enzyme activity. The highly active glucose oxidase delivered by the amorphous MOF nanoparticles allows the noninvasive and facile measurement of glucose in single living cells, which can be used to distinguish between cancerous and normal cells.

For packaging enzymes into metal–organic frameworks (MOFs), crystalline MOFs are usually used. Here, the authors encapsulated enzymes in amorphous MOFs a via one-pot co-precipitation process under ambient condition, which led to higher enzymatic activity than in a corresponding crystalline MOF composite.

In Situ X-ray Absorption Spectroscopy Studies of Nanoscale Electrocatalysts

Nanoscale electrocatalysts have exhibited promising activity and stability, improving the kinetics of numerous electrochemical reactions in renewable energy systems such as electrolyzers, fuel cells, and metal-air batteries. Due to the size effect, nano particles with extreme small size have high surface areas, complicated morphology, and various surface terminations, which make them different from their bulk phases and often undergo restructuring during the reactions. These restructured materials are hard to probe by conventional ex-situ characterizations, thus leaving the true reaction centers and/or active sites difficult to determine. Nowadays, in situ techniques, particularly X-ray absorption spectroscopy (XAS), have become an important tool to obtain oxidation states, electronic structure, and local bonding environments, which are critical to investigate the electrocatalysts under real reaction conditions. In this review, we go over the basic principles of XAS and highlight recent applications of in situ XAS in studies of nanoscale electrocatalysts.

In Situ Time-Resolved Attenuated Total Reflectance Infrared Spectroscopy for Probing Metal-Organic Framework Thin Film Growth

In situ chemical measurements of solution/surface reactions during metal-organic framework (MOF) thin film growth can provide valuable information about the mechanistic and kinetic aspects of key reaction steps, and allow control over crystal quality and material properties. Here, we report a new approach to study the growth of MOF thin films in a flow cell using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Real-time spectra recorded during continuous flow synthesis were used to investigate the mechanism and kinetics that govern the formation of (Zn, Cu) hydroxy double salts (HDSs) from ZnO thin films and the subsequent conversion of HDS to HKUST-1. We found that both reactions follow pseudo-first order kinetics. Real-time measurements also revealed that the limited mass transport of reactants may lead to partial conversion of ZnO to HDS and therefore leaves an interfacial ZnO layer beneath the HDS film providing strong adhesion of the HKUST-1 coating to the substrate. This in situ flow-cell ATR-FTIR method is generalizable for studying the dynamic processes of MOF thin film growth, and could be used for other solid/liquid reaction systems involving thin films.

The surface chemistry of metal-organic frameworks

Metal-organic frameworks (MOFs) have received particular attention over the last 20 years as a result of their attractive properties offering potential applications in a number of areas. Typically, these characteristics are tuned by functionalisation of the bulk of the MOF material itself. This Feature Article focuses instead on modification of MOF particles at their surfaces only, which can also offer control over the bulk properties of the material. The differing surface modification techniques available to the synthetic chemist will be discussed, with a focus on the effect of surface modification of MOFs on their fundamental properties and application in adsorption, catalysis, drug delivery and other areas.

Impact of the Nature of the Organic Spacer on the Crystallization Kinetics of UiO-66(Zr)-Type MOFs

The influence of the constitutive dicarboxylate linkers (size, functional group) over the crystallization kinetics of a series of porous Zr metal-organic frameworks with the UiO-66 topology has been investigated by in situ time-resolved energy dispersive X-ray diffraction (EDXRD). Both large aromatic spacers (2,6-naphthalene-, 4,4'-biphenyl- and 3,3'-dichloro-4,4'-azobenzene-dicarboxylates) and a series of X-functionalized terephthalates (X=NH2 , NO2 , Br, CH3 ) were investigated in dimethylformamide (DMF) at different temperatures and compared with the parent UiO-66. Using different crystallization models, rate constants and further kinetic parameters (such as activation energy) have been extracted. Finally, the impact of the replacement of the toxic DMF by water on the crystallization kinetics was studied through the synthesis of the functionalized UiO-66-NO2 solid.

Facile formation of ZIF-8 thin films on ZnO nanorods

Thin films of ZIF-8 on ZnO nanorods were synthesized by casting a thin linker film followed by heating for less than an hour.

Zeolitic imidazolate frameworks: next-generation materials for energy-efficient gas separations

Industrial separation processes comprise approximately 10% of the global energy demand, driven largely by the utilization of thermal separation methods (e.g., distillation). Significant energy and cost savings can be realized using advanced separation techniques such as membranes and sorbents. One of the major barriers to acceptance of these techniques remains creating materials that are efficient and productive in the presence of aggressive industrial feeds. One promising class of emerging materials is zeolitic imidazolate frameworks (ZIFs), an important thermally and chemically stable subclass of metal organic frameworks (MOFs). The objectives of this paper are (i) to provide a current understanding of the synthetic methods that enable the immense tunability of ZIFs, (ii) to identify areas of success and areas for improvement when ZIFs are used as adsorbents, (iii) to identify areas of success and areas for improvement in ZIF membranes. A review is given of the state-of-the-art in ZIF synthesis procedures and novel ZIF formation pathways as well as their application in energy efficient separations.