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Catalysis by Metal Organic Frameworks: Perspective and Suggestions for Future Research
Abstract
Metal organic frameworks (MOFs) have drawn wide attention as potential catalysts, offering high densities of catalytic sites in high-area porous solids, some with stabilities at high temperatures. The field is at an early stage, characterized by numerous discoveries and novel demonstrations of catalytic properties associated with the unique structures of MOFs, but applications of MOFs as catalysts are still lacking. In this perspective we summarize advantages and limitations of MOFs as catalysts and fundamental issues to be addressed about their potential applications. MOF framework compositions and pore structures can strongly influence catalytic performance, allowing, for example, shape-selective and bifunctional catalysis, but research is needed to quantify reaction/transport processes in MOFs; identify catalytic sites; and determine intrinsic catalytic reaction rates. Progress is hindered by the lack of understanding of the heterogeneity of MOFs, with catalytic sites sometimes being in structures such as defects not determined by X-ray diffraction crystallography. Determination of the dynamics of MOFs and their catalytic sites, as well as the intrinsic kinetics of catalytic reactions, will help to advance guidelines for synthesizing optimum catalysts. Further, MOFs present challenges related to stability and regeneration as catalysts, some associated with the crystalline nature of MOFs, such as the node-linker bonds, which can break during catalysis. There are opportunities to understand these matters in depth and to find conditions of catalytic operation that minimize the processes leading to deactivation.
... MOFs are organic−inorganic hybrids assembled from metal ions or clusters and organic ligands [143]. As shown in Fig. 14, we know various species of MOFs [141]. Metal organic frameworks (MOFs) are reticular solids consisting of inorganic nodes (such as metal atoms) and organic linkers. ...
... When this is combined with an organic linker, benzene-1,4-dicarboxylic acid, it is named as UiO-66. For other nodes and linkers with the names appeared in Fig. 14, it is recommended to refer Table 1 of ref. [141]. They are developed in the course of improving heterogenous catalysis starting from basic solids, due to their unique properties like high thermal stability, discrete ordered structure, ultra-low densities and large internal surface area [144]. ...
... Crystal structures of MOFs[141] ...
... In catalysis applications, MOF surface area can affect the number of active sites and their accessibility to reactant molecules and, thus, their catalytic activity 5 . The quantity and accessibility of active sites are also relevant in sensing applications, as more guest interactions with active sites lead to improved sensitivity (and potentially selectivity) 6 The pore volume of a MOF is a parameter that quantifies the amount of void space within the porous structure. ...
... Over the past few decades, an emergent class of hybrid extremely ordered crystalline porous materials, metal-organic frameworks (MOFs) have gained significant attention for their prospective applications in various fields such as ion exchange, [1][2][3][4] molecular recognition, [5,6] heterogeneous catalysis, [7][8][9][10][11][12][13][14] as well as in delivering medications, [15][16][17] separating substances, and storing gases. [18][19][20][21] Although the utilization of MOFs in catalysis is a relatively new field of study, numerous organic reactions have already been conducted using MOF catalysts or as supports for catalysis, spanning from carboncarbon to carbon-heteroatom synthesis methods. ...
Within the extensive array of metal‐organic frameworks (MOFs), Zr‐based MOFs stand out as particularly promising materials due to their diverse structures, exceptional stability, and intriguing properties. These MOFs are currently in the early stages of development, but noteworthy strides have been achieved in recent years. This overview focuses on the progress in Zr‐MOFs since 2012, covering design, synthesis, and applications in various reactions such as C−H activation, oxidation, C−C, and C−N coupling approaches. Different Zr‐MOF structural variants are explained in terms of Zr‐based secondary building units and a variety of organic ligands. The review highlights the roles that Zr‐MOFs play in catalysis by highlighting their uses as porous carriers, molecule adsorption, and separation. It is anticipated that this kind of concentrated review on Zr‐MOFs would offer important direction for next studies investigating MOFs for catalytic applications.
... Additionally, they can operate under pressure without collapsing and in humid environments. On the other hand, other porous materials such as MOFs exhibit interesting characteristics such as easy tunability of acid sites strength and the richness of the organic structure in MOFs may tune the properties that may pose this material appealing for the tandem formulation [109]. Their high versatility in terms of structural design allows for the fabrication of MOFs with a very selective reticular chemistry towards a specific product. ...
Fischer-Tropsch Synthesis (FTS) allows the conversion of syngas to high-density liquid fuels, playing a key role in the petrochemical and global energy sectors over the last century. However, the current Global Challenges impose the need to recycle CO2 and foster green fuels, opening new opportunities to adapt traditional processes like FTS to become a key player in future bioenergy scenarios. This review discusses the implementation of CO2-rich streams and in tandem catalysis to produce sustainable fuels via the next generation of FTS. Departing from a brief revision of the past, present, and future of FTS, we analyse a disruptive approach coupling FTS to upstream and downstream processes to illustrate the advantages of process intensification in the context of biofuel production via FTS. We showcase a smart tandem catalyst design strategy addressing the challenges to gather mechanistic insights in sequential transformations of reagents in complex reaction schemes, the precise control of structure-activity parameters, catalysts aging-deactivation, optimization of reaction parameters, as well as reaction engineering aspects such as catalytic bed arrangements and non-conventional reactor configurations to enhance the overall performance. Our review analysis includes technoeconomic elements on synthetic aviation fuels as a case of study for FTS applications in the biofuel context discussing the challenges in market penetration and potential profitability of synthetic biofuels. This comprehensive overview provides a fresh angle of FTS and its enormous potential when combined with CO2 upgrading and tandem catalysis to become a front-runner technology in the transition towards a low-carbon future.
... Metal-organic frameworks are a class of crystalline porous materials containing metal centers and organic linkers. Due to their structural and functional tunability, they have a wide variety of applications including separation and purification, catalysis and/or drug delivery, among others [22][23][24]. Within the family of MOFs, some of them stand out due to the accessibility of the metallic centers (open metal site, OMS), which may affect some of their applications. For instance, the capture of CO 2 using Mg-MOF-74 is favored by the interaction of the molecule with the open metal site [25]. ...
... Combining them with highly conductive carbon to form a composite support is a popular solution to this problem [24,25]. Metal organic frameworks (MOF) have advantages of adjustable structure, large specific surface area, high porosity, and easy functionalization, making them a self-sacrificial precursor suitable for porous carbon materials [26,27]. Among them, zeolitic imidazolate framework (ZIF-8) is one of the most common precursors, in which the central metal ion Zn 2+ can be reduced to metallic Zn and evaporated during high-temperature carbonization and it is a commonly used precursor for synthesizing nitrogen-doped carbon (NC) materials. ...
In this work, an advanced hybrid material was constructed by incorporating niobium pentoxide (Nb2O5) nanocrystals with nitrogen-doped carbon (NC) derived from ZIF-8 dodecahedrons, serving as a support, referred to as Nb2O5/NC. Pt nanocrystals were dispersed onto Nb2O5/NC using a simple impregnation reduction method. The obtained Pt/Nb2O5/NC electrocatalyst showed high oxygen reduction reaction (ORR) activity due to three-phase mutual contacting structure with well-dispersed Pt and Nb2O5 NPs. In addition, the conductive NC benefits electron transfer, while the induced Nb2O5 can regulate the electronic structure of Pt element and anchor Pt nanocrystals, thereby enhancing the ORR activity and stability. The half-wave potential (E 1/2) for Pt/Nb2O5/NC is 0.886 V, which is higher than that of Pt/NC (E 1/2 = 0.826 V). The stability examinations demonstrated that Pt/Nb2O5/NC exhibited higher electrocatalytic durability than Pt/NC. Our work provides a new direction for synthesis and structural design of precious metal/oxides hybrid electrocatalysts.
... Zeolites and MOFs are two common materials with porous structures. They have gained significant attention in recent decades due to their versatile properties and wide-ranging applications in gas separation, water treatment, and catalysis [43][44][45]. One advantage of these 3D structures as catalysts is their tunability. ...
Although density functional theory (DFT) has aided in accelerating the discovery of new materials, such calculations are computationally expensive, especially for high-throughput efforts. This has prompted an explosion in exploration of machine learning (ML) assisted techniques to improve the computational efficiency of DFT. In this study, we present a comprehensive investigation of the broader application of Finetuna, an active learning framework to accelerate structural relaxation in DFT with prior information from Open Catalyst Project pretrained graph neural networks. We explore the challenges associated with out-of-domain systems: alcohol ( C>2 ) on metal surfaces as larger adsorbates, metal oxides with spin polarization, and three-dimensional (3D) structures like zeolites and metal organic frameworks. By pre-training ML models on large datasets and fine-tuning the model along the simulation, we demonstrate the framework’s ability to conduct relaxations with fewer DFT calculations. Depending on the similarity of the test systems to the training systems, a more conservative querying strategy is applied. Our best-performing Finetuna strategy reduces the number of DFT single-point calculations by 80% for alcohols and 3D structures, and 42% for oxide systems.
... As such, the number of possible combination of MOFs is enormous [26]. Various kinds of MOF application are under intensive studies and those include drug delivery systems for biomedical use [27], gas storage and separation [28,29], catalysis [30], water treatment [31], sensor [32], degradation of organic pollutant [33] and electrochemistry such as batteries and fuel cells [34,35]. Moreover, MOFs have demonstrated to be promising materials for CO 2 photocatalytic reduction [36,37]. ...
Climate change and global warming problem are becoming the serious issue and some action is necessary in order to mitigate the rising temperature. CO 2 increase is one of the reason for temperature rise, and the technology to convert CO 2 to beneficial energy or chemical substance could be one of the key solution (CO 2 photocatalytic reduction). Metal–organic frameworks (MOFs) have gained much attention owing to their extremely large surface areas, tunable fine structures, and potential applications in many areas. Recently, MOFs have been demonstrated to be promising materials for CO 2 photocatalytic reduction. This review summarized recent research progresses in photocatalytic reduction using MOFs. MOFs were classified mainly by the type of metal center, and the feature and tendency against their functions towards CO 2 photocatalytic activity will be explained.
... The effect of solvent has become a major concern because it regulates the coordination environment of metal ions and organic linkers, controlling organic linkers deprotonation, act as a directing agent and medium for the MOF crystal growth [24]. Conventional solvothermal method commonly uses polar organic solvents such as dimethylformamide or diethylformamide which may generate hazardous amines substance upon heating, contributing to MOF toxicity [25]. Green and environmentally friendly solvents have been explored in the solvothermal synthesis of MOF such as water, ethanol, and mixed solvent between water and ethanol [26][27][28][29]. ...
In this study, the synthesis of FeBTC (BTC = 1,3,5-benzenetricarboxylate) also known as MIL-100 (Fe) metal organic framework (MOF) has been carried out successfully using green mechanochemical method (neat grinding and liquid assisted grinding). The effect of solvent used in the synthesis was investigated for the first time to elucidate the physicochemical properties of FeBTC including crystal structure, thermal stability, pore size and specific surface area. The physicochemical properties of all FeBTC obtained in this study were compared to commercial FeBTC (Basolite F-300), characterized using powder X-Ray Diffraction (XRD), Thermogravimetric Analysis (TGA) and nitrogen physisorption isotherms. All Fe-BTC MOF synthesized in this study showed improved textural properties compared to commercial Basolite F-300 such as higher crystallinity, higher surface area and larger pore size. It was found that the best synthesis method was by using the mixture of ethanol and water with equal volume ratio as solvent. The highest BET surface area of FeBTC synthesized using this method was 972 m2/g for FeBTC-EtOH/H2O. This value is 2.3 times higher than the surface area of commercial Basolite F-300 (418 m2/g). FeBTC with higher surface area is expected to have higher catalytic activity which makes this FeBTC an excellent candidate as a heterogenous catalyst for many reactions such as aldol condensation or esterification reaction.
The presence of shape‐ and size‐selective catalysts in various catalytic reactions is of paramount importance. Metal‐organic frameworks (MOFs) possess a distinctive characteristic of lacking in‐accessible dead spaces, owing to their well‐structured nature. The effective separation of active sites within MOFs is facilitated by their exceptionally high surface area, which allows for a high density of active sites per unit volume of the catalyst. In this comprehensive review article, we delve into one of the most critical and practical features of MOFs: their ability to modify and engineer the structure of these materials. This structural engineering approach enables the attainment of desired physical, chemical, and surface properties, particularly in the realm of heterogeneous catalysts. The article encompasses several key areas, including surface functionalization within MOFs, synthesis of novel enzyme‐inspired MOFs, creation of mesoporous MOFs, development of porous structures utilizing MOFs, and engineering of structural limitations in MOFs. These rapidly advancing and highly applicable topics, especially in the field of heterogeneous catalysts, are thoroughly investigated and analyzed within the purview of this comprehensive review article.
The electrocatalytic carbon dioxide reduction reaction (ECO2RR) is a promising way to realize the transformation of waste into valuable material, which can not only meet the environmental goal of reducing carbon emissions, but also obtain clean energy and valuable industrial products simultaneously. Herein, we first introduce the complex CO2RR mechanisms based on the number of carbons in the product. Since the coupling of C–C bonds is unanimously recognized as the key mechanism step in the ECO2RR for the generation of high-value products, the structural–activity relationship of electrocatalysts is systematically reviewed. Next, we comprehensively classify the latest developments, both experimental and theoretical, in different categories of cutting-edge electrocatalysts and provide theoretical insights on various aspects. Finally, challenges are discussed from the perspectives of both materials and devices to inspire researchers to promote the industrial application of the ECO2RR at the earliest.
Beta-cyclodextrin Metal-Organic Framework (β-CD-MOF) is a unique class of porous materials that merges the inherent properties of cyclodextrins with the structural advantages of metal-organic frameworks (MOFs). When combined with the concept of MOFs, which are crystalline structures composed of metal ions or clusters linked by organic ligands, the resulting β-CD-MOF holds immense potential for various applications, especially in the field of drug delivery. In this study, biocompatible metal-organic frameworks (MOFs) synthesized using β-Cyclodextrin (β-CD) and potassium enabled drug delivery of curcumin (CCM) to cancerous cells. Functionalizing β-CD-MOF with l-glutamine (glutamine-β-CD-MOF) enhanced cancer cell-specific targeting due to glutamine's essential role in cancer cell proliferation and energy pathways. Amino group functionalization provided further functionalization opportunities. Gelatin coating (gelatin@β-CD-MOF) facilitated controlled drug release in an acidic medium. High drug loading capacities (52.38–55.63 %) were achieved for β-CD-MOF@CCM and glutamine-β-CD-MOF@CCM, leveraging the high porosity and affinity of amine and phenol groups of curcumin. The MTT assay highlighted the specificity and differentiation of glutamine-β-CD-MOF in targeting cancerous over normal cells. These functionalized β-CD MOFs efficiently encapsulate curcumin, ensuring controlled drug release and enhanced therapeutic efficacy, particularly in cancer therapy.
A novel electrically conducting cubic mesoporous metal–organic framework (MOF), consisting of hexahydroxy- cata -hexabenzocoronene ( c -HBC) and Fe III ions is presented.
RE-UiO-66 analogues are synthesized using RE acetates as precursors for the first time. These MOFs are fully characterized and the influence of the precursor on the materials obtained is studied....
We simulated XAS with LR-TDDFT for Cu ²⁺ /Cu ⁺ in MOF Cu I -MFU-4 l and revealed a larger 2p core-exciton binding energy for Cu ²⁺ , finding that corrections with self-consistent excited-state total energy differences provide accurate XAS peak alignment.
The movement of molecules (i.e. diffusion) within angstrom-scale pores of porous materials such as metal-organic frameworks (MOFs) and zeolites is influenced by multiple complex factors that can be challenging to assess and manipulate. Nevertheless, understanding and controlling this diffusion phenomenon is crucial for advancing energy-economic membrane-based chemical separation technologies, as well as for heterogeneous catalysis and sensing applications. Through precise assessment of the factors influencing diffusion within a porous metal-organic framework (MOF) thin film, we have developed a chemical strategy to manipulate and reverse chemical isomer diffusion selectivity. In the process of cognizing the molecular diffusion within oriented, angstrom-scale channels of MOF thin film, we have unveiled a dynamic chemical interaction between the adsorbate (chemical isomers) and the MOF using a combination of kinetic mass uptake experiments and molecular simulation. Leveraging the dynamic chemical interactions, we have reversed the haloalkane (positional) isomer diffusion selectivity, forging a novel chemical pathway to elevate the overall efficacy of membrane-based chemical separation and selective catalytic reactions.
Direct laser writing (DLW), being a universal tool for fast creating colorless/color images on different substrates, still suffers from simultaneous writing grayscale and color images inside the transparent media. Here, it is discovered that a unique set of porosity, coordination bonding between organic and inorganic building blocks, and the lack of inversion symmetry of the label‐free metal‐organic frameworks (MOFs), on the one hand, provides the possibility of laser writing the grayscale images through the amorphization/carbonization. On the other hand, the reduction of the laser writing power leads to controllable creation of color images via defect formation with sub‐diffraction resolution inside the MOF crystals. The latter is due to the processes of self‐absorption of generated optical harmonics by nonlinear MOFs within the visible spectral range. As a result, simultaneous grayscale and multicolor writing of QR codes and images are demonstrated with up to 400 nm resolution inside optically transparent MOF crystals, thereby discovering a new family of functional materials for DLW.
A series of 2D {RE 9 }-cluster-based rare-earth MOFs were built by dimensional reduction and active site addition strategies. They exhibit excellent catalytic activity in CO 2 fixation and Knoevenagel condensation under mild conditions.
Metal–organic frameworks (MOFs) draw increasing attention as nanoenvironments for chemical reactions, especially in the field of catalysis. Knowing the specifics of MOF cavities is decisive in many of these cases; yet, obtaining them in situ remains very challenging. We report the first direct assessment of the apparent polarity and solvent organization inside MOF cavities using a dedicated structurally flexible spin probe. A stable β-phosphorylated nitroxide radical was incorporated into the cavities of a prospective MOF ZIF-8 in trace amounts. The spectroscopic properties of this probe depend on local polarity, structuredness, stiffness and cohesive pressure and can be precisely monitored by Electron Paramagnetic Resonance (EPR) spectroscopy. Using this approach, we have demonstrated experimentally that the cavities of bare ZIF-8 are sensed by guest molecules as highly non-polar inside. When various alcohols fill the cavities, remarkable self-organization of solvent molecules is observed leading to a higher apparent polarity in MOFs compared to the corresponding bulk alcohols. Accounting for such nanoorganization phenomena can be crucial for optimization of chemical reactions in MOFs, and the proposed methodology provides unique routes to study MOF cavities inside in situ, thus aiding in their various applications.
Selective oxidation of hydrocarbons using molecular oxygen as sole oxidant under mild conditions remains a challenging task. In this context, metal‐organic frameworks (MOFs) have been widely used in various oxidation reactions due to their porosity, high surface area and designability. However, the slow diffusion of substrates/products in micropores of three‐dimensional (3D) bulk MOFs hinders the efficient catalytic performance of such materials. Herein an ultrathin two‐dimensional (2D) porphyrin‐based Zr‐MOF nanosheet (Zr‐TCPP) is synthesized through modulator‐control strategy. Subsequently, various metal ions are anchored into the porphyrin ring by post synthesis modification to afford a series of 2D Zr‐TCPP(M) (M=Mn, Fe, Co, Ni, Cu and Zn). Various structural characterization techniques indicate Zr‐TCPP(M) is nanoflower structure with ultrathin nanoplate petals which provides fully exposed accessible active sites. Among them, Zr‐TCPP(Fe) shows excellent catalytic performance in styrene epoxidation reactions and benzylic C‐H oxidation reactions using O2 as sole oxidant under ambient temperature and pressure. The remarkable activity arises from high density of exposed porphyrin‐Fe active sites, low diffusion barriers for substrates and products, as well as a similar homogeneous reaction space. Furthermore, Zr‐TCPP(Fe) nanosheet is easily recycled by centrifugation and reused at least five times without significant loss of catalytic activity.
One of the greatest environmental threats in recent years is exposure to varioussulfur-containing compounds.Fossil fuels contain various sulfur-containing compounds that produce sulfur oxides when burned.Consequently, the desulfurization of fuels is in high demand.Metal-organic frameworks (MOFs) and MOF-derived materials are the most promising materials for a wide range of applications. This review summarizes the use of pristine MOFs, MOFs immobilized with functional groups, and MOF-derived materials in the desulfurization of fuels by oxidative desulfurization (ODS) and adsorptive desulfurization (ADS), and also highlights the recent advances in the field of application of MOFs for fossil fuel desulfurization.
The conversion of the biomass into eco‐friendly fuels and chemicals has been extensively recognized as the essential pathway to achieve the sustainable economy and carbon neutral society. Lignin, as a kind of promising biomass energy, has been certified to produce the high‐valued chemicals and fuels. Numerous efforts have been made to develop various catalysts for lignin catalytic conversion. Both metal‐organic frameworks (MOFs) and covalent organic frameworks (COFs) belong to very important heterogeneous porous catalysts due to their regular porous structures, high specific surface area, and precisely tailored diversities. In the review, the first part focused on the catalytic conversion of lignin, lignin model compounds, and lignin derivatives using the pristine MOFs, functional MOF composites, and MOF‐derived materials. The second part summarized the catalytic conversion of lignin model compounds using pristine COFs and functional COF composites. The review here mainly concentrated on the design of the materials, screening of catalytic conditions, and explorations of the corresponded mechanisms. Specifically, (1) we summarized the MOF‐ and COF‐based materials for the effects on the catalytic transformation of lignin‐related substances; (2) we emphasized the catalytic mechanism of C–C and C–O bonds cleavage together with the structure–activity relationships; (3) we in‐depth realized the relationship between the chemical/electronic/structural properties of the MOF‐ and COF‐based catalysts and their catalytic performance for lignin‐related substances. Finally, the challenges and future perspectives were also discussed on the catalytic conversion of lignin‐related substances by MOF‐ and COF‐based catalysts.
Enantioselective production of lactic acid from xylose sugar in constrained pore space of Ni-triazole MOF is demonstrated. Robust hydrophilic structure with confined nano-pocket is constructed from {Ni3(µ3-OH)(Tz)3(OH)2(H2O)4}n secondary building unit (SBU). At elevated temperature, the de-coordination of water and hydroxide species on Ni-node can generate accessible open-metal site (OMS) possessing Lewis acidicity, acting as catalytic center for the catalysis of xylose to lactic acid. DFT calculation suggests that enantiospecific yield of lactic acid is realizable by a preferential interaction between any pair of SBUs and the interlocked trans-pyruvaldehyde intermediate. This work highlights the unique spatial and chemical environment of MOFs as the advantageous platform for specific process in catalysis.
Metal-based catalysts play an essential role in organic chemistry and the chemical industry. In this research, a pillar-layered metal-organic framework (MOF) with the urea linkers, namely Basu-HDI, as a novel and efficient heterogeneous catalyst was designed and successfully synthesized. Various techniques such as FT-IR, EDX, elemental mapping, SEM, XRD, BET, and TGA/DTA studied its structure and morphology. Then, we investigated the synthesis of new 1,8-naphthy-ridines utilizing Basu-HDI in mild conditions via a one‐pot three‐component tandem Knoevenagel/ Michael/cyclization/anomeric-based oxidation reaction. Final products were achieved by anomeric-based oxidation without employing an oxidation agent. Remarkably, this tandem process gave a good range of new 1,8-naphthyridines with high yields in a short reaction time. The pure products were confirmed by FT-IR, ¹H NMR, ¹³C NMR, and mass spectrometry techniques. Moreover, the introduced catalyst showed good efficiency and stability and can be reused four times without significantly reducing efficiency.
Multivariate metal–organic frameworks (MTV‐MOFs) provide a handle over programming MOF's physicochemical properties for use in functional applications. In this review, the state‐of‐art design features of MTV‐MOFs are summarized, including multivariate functionalities, mixed metals, and post‐synthetic modification, and highlight their resulting fine‐tuned performance in practical applications, such as gas storage, gas separation, water harvesting, luminescence sensing, heterogeneous catalysis, and drug delivery. Furthermore, a vision is provided for future research avenues, in which MTV‐MOFs enable the incorporation of a variety of functionalities, metals, linkers, secondary building units (SBUs), and pore metrics along a specific crystallographic direction with respect to the ordered backbone. Future directions focus on mechanistic and kinetics investigations into the formation of MTV‐MOFs in addition to high‐throughput screening with the aim of automation and artificial intelligence.
To date, metal–organic frameworks (MOFs) with diverse topological structures and functionalities have proven to be versatile materials with applications in catalysis, drug delivery, and more. Constructing robust MOFs against harsh stimuli poses a significant challenge due to fragile chemical bonds and dynamic behavior that may result in structural collapse. Researchers have actively worked to enhance the stability of MOFs via proposing various approaches for both academic and industrial applications. This review extensively explores the crucial role of organic ligands in contributing to MOFs' stabilities together with exhaustively illustrates the entire stability mechanisms. Additionally, from a synthesis strategy perspective, four types of methods—strong coordination donors, ligand rigidification, ligands modified with free-standing groups, and ligands with cross-linkable side chains or polymerization via post-treatment—are introduced to underscore their significance. Finally, the review presents typical applications of stable MOFs along with challenges and future perspectives, aiming to provide researchers with valuable insights into fabricating robust MOFs to address energy crises and environmental issues.
Immunotherapy harnesses the inherent immune system in the body to generate systemic antitumor immunity, offering a promising modality for defending against cancer. However, tumor immunosuppression and evasion seriously restrict the immune response rates in clinical settings. Catalytic nanomedicines can transform tumoral substances/metabolites into therapeutic products in situ, offering unique advantages in antitumor immunotherapy. Through catalytic reactions, both tumor eradication and immune regulation can be simultaneously achieved, favoring the development of systemic antitumor immunity. In recent years, with advancements in catalytic chemistry and nanotechnology, catalytic nanomedicines based on nanozymes, photocatalysts, sonocatalysts, Fenton catalysts, electrocatalysts, piezocatalysts, thermocatalysts and radiocatalysts have been rapidly developed with vast applications in cancer immunotherapy. This review provides an introduction to the fabrication of catalytic nanomedicines with an emphasis on their structures and engineering strategies. Furthermore, the catalytic substrates and state-of-the-art applications of nanocatalysts in cancer immunotherapy have also been outlined and discussed. The relationships between nanostructures and immune regulating performance of catalytic nanomedicines are highlighted to provide a deep understanding of their working mechanisms in the tumor microenvironment. Finally, the challenges and development trends are revealed, aiming to provide new insights for the future development of nanocatalysts in catalytic immunotherapy.
It remains difficult to understand the surface of solid acid catalysts at the molecular level, despite their importance for industrial catalytic applications. A sulfated zirconium-based metal–organic framework, MOF-808-SO4, was previously shown to be a strong solid Brønsted acid material. In this report, we probe the origin of its acidity through an array of spectroscopic, crystallographic and computational characterization techniques. The strongest Brønsted acid site is shown to consist of a specific arrangement of adsorbed water and sulfate moieties on the zirconium clusters. When a water molecule adsorbs to one zirconium atom, it participates in a hydrogen bond with a sulfate moiety that is chelated to a neighbouring zirconium atom; this motif, in turn, results in the presence of a strongly acidic proton. On dehydration, the material loses its acidity. The hydrated sulfated MOF exhibits a good catalytic performance for the dimerization of isobutene (2-methyl-1-propene), and achieves a 100% selectivity for C8 products with a good conversion efficiency. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
Methane constitutes the largest fraction of natural gas reserves and is a low-cost abundant starting material for the synthesis of value-added chemicals and fuel. Selective catalytic functionalization of methane remains a vital goal in the chemical sciences due to its low intrinsic reactivity. Borylation has recently emerged as a promising route for the catalytic functionalization of methane. A major challenge in this regard is selective borylation towards the monoborylated product that is more active than methane and can easily lead to over-functionalization. Herein, we report a highly selective microporous metal−organic-framework-supported iridium(iii) catalyst for methane borylation that exhibits a chemoselectivity of >99% (mono versus bis at 19.5% yield; turnover number = 67) for monoborylated methane, with bis(pinacolborane) as the borylation reagent in dodecane, at 150 °C and 34 atm of methane. The preference for the monoborylated product is ascribed to the shape-selective effect of the metal−organic framework pore structures. Methane borylation allows for the functionalization of an otherwise unreactive compound, enabling its use as a one-carbon building block; however, competing diborylation presents a selectivity issue. Now, a metal–organic-framework-based catalyst highly selective for monoborylation is reported. The selectivity is due to the reaction taking place within the catalyst pores, which excludes the formation of the larger diborlyated product.
Metal–organic frameworks (MOFs) are an emerging class of porous materials with potential applications in gas storage, separations, catalysis, and chemical sensing. Despite numerous advantages, applications of many MOFs are ultimately limited by their stability under harsh conditions. Herein, the recent advances in the field of stable MOFs, covering the fundamental mechanisms of MOF stability, design, and synthesis of stable MOF architectures, and their latest applications are reviewed. First, key factors that affect MOF stability under certain chemical environments are introduced to guide the design of robust structures. This is followed by a short review of synthetic strategies of stable MOFs including modulated synthesis and postsynthetic modifications. Based on the fundamentals of MOF stability, stable MOFs are classified into two categories: high-valency metal–carboxylate frameworks and low-valency metal–azolate frameworks. Along this line, some representative stable MOFs are introduced, their structures are described, and their properties are briefly discussed. The expanded applications of stable MOFs in Lewis/Brønsted acid catalysis, redox catalysis, photocatalysis, electrocatalysis, gas storage, and sensing are highlighted. Overall, this review is expected to guide the design of stable MOFs by providing insights into existing structures, which could lead to the discovery and development of more advanced functional materials.
Gas storage and separation are closely associated with the alleviation of greenhouse effect, the widespread use of clean energy, the control of toxic gases, and various other aspects in human society. In this review, we highlight the recent advances in gas storage and separation using metal-organic frameworks (MOFs). In addition to summarizing the gas uptakes of some benchmark MOFs, we emphasize on the desired chemical properties of MOFs for different gas storage/separation scenarios. Greenhouse gases (CO2), energy-related gases (H2 and CH4), and toxic gases (CO and NH3) are covered in the review.
An X-ray absorption spectroscopy study of the UiO-67 Pt functionalized metal organic frameworks (MOFs) demonstrates that in proper conditions, at least two types of catalytically active sites can be formed in the cavities of the MOF: isolated Pt-complexes and Pt nanoparticles (Pt-NPs). Both pre-made linker synthesis (PMLS) and post-synthesis functionalization (PSF) methods were adopted. XAS was used to monitor the temperature-dependent behaviour of UiO-67-Pt while heating from RT to 623 K, in different gas feeds (pure He, 3%H2/He and 10%H2/He). We collected static in situ Pt LIII XANES and EXAFS spectra at RT before and after the thermal treatment, as well as spectra acquired under operando conditions upon heating. Under 10% H2/He thermal treatment, we unambiguously detected Pt-NPs formation which it has been followed with a parametric EXAFS analysis of the data collected during temperature programmed H2-reduction (TPR), using the Einstein model to predict the temperature dependence of the Debye-Waller factors. Conversely, in pure He flow, the only significant change observed during TPR is the progressive decrease of the Pt-Cl single scattering contribution, leading to the conclusion that the Pt grafted to the bpydc-linkers remains naked. Advanced EXAFS/TEM analysis allowed us to quantify the fraction of the Pt in form of Pt NPs, values that have been quantitatively confirmed by linear combination analysis of the XANES spectra. In situ XANES/EXAFS study were supported by ex-situ XRPD and BET analyses, confirming the framework stability and testifying a loss of the internal volume after TPR due to the formation of Pt NPs insides the MOF pores, more relevant in the sample where smaller Pt NPs were formed.
Conjugated microporous polymers (CMPs) are a class of organic porous polymers that combine p-conjugated skeletons with permanent nanopores, in sharp contrast to other porous materials that are not p-conjugated and with conventional conjugated polymers that are nonporous. As an emerging material platform, CMPs offer a high flexibility for the molecular design of conjugated skeletons and nanopores. Various chemical reactions, building blocks and synthetic methods have been developed and a broad variety of CMPs with different structures and specific properties have been synthesized, driving the rapid growth of the field. CMPs are unique in that they allow the complementary utilization of p-conjugated skeletons and nanopores for functional exploration; they have shown great potential for challenging energy and environmental issues, as exemplified by their excellent performance in gas adsorption, heterogeneous catalysis, light emitting, light harvesting and electrical energy storage. This review describes the molecular design principles of CMPs, advancements in synthetic and structural studies and the frontiers of functional exploration and potential applications.
The carbon dioxide challenge is one of the most pressing problems facing our planet. Each stage in the carbon cycle — capture, regeneration and conversion — has its own materials requirements. Recent work on metal–organic frameworks (MOFs) demonstrated the potential and effectiveness of these materials in addressing this challenge. In this Review, we identify the specific structural and chemical properties of MOFs that have led to the highest capture capacities, the most efficient separations and regeneration processes, and the most effective catalytic conversions. The interior of MOFs can be designed to have coordinatively unsaturated metal sites, specific heteroatoms, covalent functionalization, other building unit interactions, hydrophobicity, porosity, defects and embedded nanoscale metal catalysts with a level of precision that is crucial for the development of higher-performance MOFs. To realize a total solution, it is necessary to use the precision of MOF chemistry to build more complex materials to address selectivity, capacity and conversion together in one material.
Metal-organic frameworks (MOFs), with their well-ordered pore networks and tunable surface chemistries, offer a versatile platform for preparing well-defined nanostructures wherein functionality such as catalysis can be incorporated. Notably, atomic layer deposition (ALD) in MOFs has recently emerged as a versatile approach to functionalize MOF surfaces with a wide variety of catalytic metal-oxo species. Understanding the structure of newly deposited species and how they are tethered within the MOF is critical to understanding how these components couple to govern the active material properties. By combining local and long-range structure probes, including X-ray absorption spectroscopy, pair distribution function analysis and difference envelope density analysis, with electron microscopy imaging and computational modeling, we resolve the precise atomic structure of metal-oxo species deposited in the MOF NU-1000 through ALD. These analyses demonstrate that deposition of NiOxHy clusters occurs selectively within the smallest pores of NU-1000, between the zirconia nodes, serving to connect these nodes along the c-direction to yield hetero-bimetallic metal-oxo nanowires. This bridging motif perturbs the NU-1000 framework structure, drawing the zirconia nodes closer together, and also underlies the sintering-resistance of these clusters during the hydrogenation of light olefins.
Zirconium terephthalate UiO-66 type metal organic frameworks (MOFs) are known to be highly active, stable and reusable catalysts for the esterification of carboxylic acids with alcohols. Moreover, when defects are present in the structure of these MOFs, coordinatively unsaturated Zr ions with Lewis acid properties are created, which increase the catalytic activity of the resulting defective solids. In the present work, molecular modeling techniques combined with new experimental data on various defective hydrated and dehydrated materials allow to unravel the nature and role of defective active sites in the Fischer esterification and the role of coordinated water molecules to provide additional Brønsted sites. Periodic models of UiO-66 and UiO-66-NH2 catalysts have been used to unravel the reaction mechanism on hydrated and dehydrated materials. Various adsorption modes of water and methanol are investigated. The proposed mechanisms are in line with experimental observations that amino groups yield a reduction in the reaction barriers, although they have a passive role in modulating the electronic structure of the material. Water has a beneficial role on the reaction cycle by providing extra Brønsted sites and by providing stabilization for various intermediates through hydrogen bonds.
Crystalline porous materials are important in the development of catalytic systems with high scientific and industrial impact. Zeolites, ordered mesoporous silica, and metal-organic frameworks (MOFs) are three types of porous materials that can be used as heterogeneous catalysts. This review focuses on a comparison of the catalytic activities of zeolites, mesoporous silica, and MOFs. In the first part of the review, the distinctive properties of these porous materials relevant to catalysis are discussed, and the corresponding catalytic reactions are highlighted. In the second part, the catalytic behaviors of zeolites, mesoporous silica, and MOFs in four types of general organic reactions (acid, base, oxidation, and hydrogenation) are compared. The advantages and disadvantages of each porous material for catalytic reactions are summarized. Conclusions and prospects for future development of these porous materials in this field are provided in the last section. This review aims to highlight recent research advancements in zeolites, ordered mesoporous silica, and MOFs for heterogeneous catalysis, and inspire further studies in this rapidly developing field.
Metal–organic frameworks (MOFs), established as a relatively new class of crystalline porous materials with high surface area, structural diversity, and tailorability, attract extensive interest and exhibit a variety of applications, especially in catalysis. Their permanent porosity enables their inherent superiority in confining guest species, particularly small metal nanoparticles (MNPs), for improved catalytic performance and/or the expansion of reaction scope. This is a rapidly developing interdisciplinary research field. In this review, we provide an overview of significant progress in the development of MNP/MOF composites, including various preparation strategies and characterization methods as well as catalytic applications. Special emphasis is placed on synergistic effects between the two components that result in an enhanced performance in heterogeneous catalysis. Finally, the prospects of MNP/MOF composites in catalysis and remaining issues in this field have been indicated.
Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially separated in a given solid and, ideally, structurally identical. In this review, the potential of metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modelling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.
The metal-organic framework NU-1000, with Zr6-oxo, hydroxo, and aqua nodes, was modified by incorporation of hydroxylated Al(iii) ions by ALD-like chemistry with [Al(CH3)2(iso-propoxide)]2 followed by steam (ALD = atomic layer deposition). Al ions were installed to the extent of approximately 7 per node. Single-site iridium diethylene complexes were anchored to the nodes of the modified and unmodified MOFs by reaction with Ir(C2H4)2(acac) (acac = acetylacetonate) and converted to Ir(CO)2 complexes by treatment with CO. Infrared spectra of these supported complexes show that incorporation of Al weakened the electron donor tendency of the MOF. Correspondingly, the catalytic activity of the initial supported iridium complexes for ethylene hydrogenation increased, as did the selectivity for ethylene dimerization. The results of density functional theory calculations with a simplified model of the nodes incorporating Al(iii) ions are in qualitative agreement with some catalyst performance data.
Promoters are ubiquitous in industrial heterogeneous catalysts. The wider roles of promoters in accelerating catalysis and/or controlling selectivity are, however, not well understood. A model system has been developed where a heterobimetallic active site comprising an active metal (Rh) and a promoter ion (Ga) are preassembled and delivered onto a metal-organic framework (MOF) support, NU-1000. The Rh-Ga sites in NU-1000 selectively catalyze the hydrogenation of acyclic alkynes to E-alkenes. The overall stereoselectivity is complementary to the well-known Lindlar’s catalyst, which generates Z-alkenes. The role of the Ga in promoting this unusual selectivity is evidenced by the lack of semi-hydrogenation selectivity when Ga is absent and only Rh is present in the active site.
We describe herein the highly selective semihydrogenation of acetylene to ethylene using a Zr-based metal–organic framework (MOF; NU-1000) with Cu–oxo clusters present on the nodes as precatalysts. The active form of the catalyst is MOF-supported Cu nanoparticles generated in situ upon a brief H2 reduction treatment at 200 °C. This composite material is stable for many catalytic cycles and effectively avoids overreduction to ethane, a common problem for many semihydrogenation catalysts.
Dual ligand MOFs {[Zn(ADA)(L)]·2H2O}n (1), {[Cd(ADA)(L)]·2H2O}n (2) involving Zn(II)/Cd(II) metal centres and flexible 1,3-adamantanediacetic acid (H2ADA)/ pyridyl based Schiff base ligand, 4-pyridylcarboxaldehydeisonicotinoylhydrazone (L) as linkers have been efficiently synthesized including green mechanochemical methods and characterised by various analytical methods. Crystal structures of both MOFs, luminescence and semiconducting properties have also been investigated. Both materials were exploited for efficient catalytic activity in Knoevenagel condensation reaction for variety of substrates with good yields and recyclability under ambient reaction conditions in aqueous media. Probably the Lewis acidic metal centre and presence of amide functionality in the L of the MOFs cooperatively involve in the efficient condensation reaction by these catalytic materials.
Comprising periodically repeating inorganic nodes and organic linkers, metal–organic frameworks (MOFs) represent a novel class of porous molecular solids with well-defined pores and channels. Over the past two decades, a large array of organic linkers have been combined with many inorganic nodes to afford a vast library of MOFs. The synthetic tunability of MOFs distinguishes them from traditional porous inorganic materials and has allowed the rational design of many interesting properties, such as porosity, chirality, and chemical functionality, for potential applications in diverse areas including gas storage and separation, catalysis, light harvesting, chiral separation, and chemical sensing. In particular, the molecular functionality and intrinsic porosity of MOFs have rendered them attractive candidates as porous single-site solid catalysts for a large number of organic transformations. MOF catalysts offer several advantages over their homogeneous counterparts, including enhanced stability, recyclability and reusability, and facile removal of the toxic catalyst components from the organic products. Additionally, the highly ordered nature of MOFs leads to the generation of single-site solid catalysts, allowing for precise characterization of the catalytic sites through X-ray diffraction, X-ray absorption, and other spectroscopic interrogations and facilitating the elucidation of reaction mechanisms. Thus, MOF catalysis represents a fertile research area that is expected to witness continued growth in the foreseeable future. In this Account, we present our recent research progress in developing ligand-supported single-site MOF catalysts for challenging organic reactions. We present two complementary approaches to the design of ligand-supported MOF catalysts: direct incorporation of prefunctionalized organic linkers into MOFs and postsynthetic functionalization of orthogonal secondary functional groups of the organic linkers in MOFs. Monophosphine-, bipyridine-, β-diketimine-, and salicylaldimine-based ligands have been used to support both precious (Pd, Pt, Ir, Ru) and earth-abundant (Cu, Co, Fe) metals for a number of interesting catalytic reactions. The resulting MOF catalysts feature stable low-coordination species with minimum steric bulk around the active site—a feat that remains a challenge for homogeneous catalysts. For each ligand, we describe types of reactions catalyzed by the MOF in comparison with its homogeneous counterpart. In all cases, MOF catalysts outperformed their homogeneous counterparts in terms of catalyst stability, catalytic activity, and recyclability and reusability. Interestingly, several bipyridine- and salicylaldimine-ligated earth-abundant-metal-based MOF catalysts do not have homogeneous counterparts because the molecular compounds disproportionate or oligomerize to form inactive species in solution. This Account not only presents several interesting designs of ligand-supported single-site MOF catalysts but also provides illustrative examples of how site isolation in MOF catalysts shuts down deactivation pathways experienced by homogeneous systems. With precise knowledge of MOF structures and catalytically active sites, we envision the development of practically useful MOF catalysts comprising tailor-made building blocks that rationally optimize catalytic activities and selectivities.
We report the syntheses, structures, and oxidation catalytic activities of a single-atom-based vanadium oxide incorporated in two highly crystalline MOFs, Hf-MOF-808 and Zr-NU-1000. These vanadium catalysts were introduced by a postsynthetic metalation, and the resulting materials (Hf-MOF-808-V and Zr-NU-1000-V) were thoroughly characterized through a combina-tion of analytic and spectroscopic techniques including single-crystal X-ray crystallography. Their catalytic properties were investigated using the oxidation of 4-methoxybenzyl alcohol under an oxygen atmosphere as a model reaction. Crystallo-graphic and variable-temperature spectroscopic studies revealed that the incorporated vanadium in Hf-MOF-808-V changes position with heat, which led to improved catalytic activity.
Acid-catalyzed skeletal C-C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, o-xylene isomerization/disproportionation is a probe reaction for strong Brønsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal–organic framework (MOF), as a catalyst for o-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), 31P NMR, N2 physisorption, and XRD show that the catalyst is structurally stable with time-on-stream, and that WOx clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000-supported catalyst were comparable to a control WOx-ZrO2, but the NU-1000 composite material was unusually active toward the transmethylation pathway that requires two adjacent active sites in a confined pore, as created when PTA is confined in NU-1000. This work shows the promise of metal–organic framework topologies in giving access to unique reactivity, even for aggressive reactions such as hydrocarbon isomerization.
The highly studied metal-organic framework (MOF), UiO-66, has been touted for its high surface area, stability, and tunability of structural defects for particular applications in catalysis and gas storage. However, atomic-level characterization and quantification of defects remains challenging for traditional methods. Motivated by this knowledge gap, coordinatively unsaturated zirconium (Zrcus) defect sites within UiO-66 have been determined through infrared spectroscopic studies of the highly sensitive probe molecule, carbon monoxide. A characteristic blue-shift of the vibrational frequency for CO adsorbed at nodes within the MOF was used to identify the presence of Zrcus defects. The identification of these coordinatively unsaturated metal sites was further corroborated via exposure of the MOF to the Lewis base, D2O, which was shown to block CO binding to the Lewis acidic Zrcus sites. The infrared spectroscopic signature for CO binding at the Zrcus sites was further found to track the emergence of missing linker defects that develop upon thermal treatment of the MOF. The concentration of missing linker defects could be characterized over a range from less than 1% (nominally a defect-free MOF) to over 30% with this relatively simple spectroscopic probe.
The gram‐scale synthesis, stabilization and characterization of well‐defined ultrasmall subnanometric catalytic clusters on solids represent a challenge. Here, we report the chemical synthesis and X‐ray snapshots of Pt20 clusters, homogenously distributed and densely‐packaged within the channels of a metal‐organic framework. This novel hybrid material catalyzes efficiently, and even most important from an economic and environmental viewpoint, at low temperature (25 to 140 ºC), energetically‐costly industrial reactions in the gas phase such as hydrogen cyanide (HCN) production, carbon dioxide (CO2) methanation and alkene hydrogenations. These results open the way for the design of precisely‐defined catalytically active ultrasmall metal clusters in solids for technically easier, cheaper and dramatically‐less dangerous industrial reactions.
Some metal-organic frameworks (MOFs) incorporate nodes that are metal oxide clusters such as Zr6O8. Vacancies on the node surfaces, accidental or by design, act as catalytic sites. Here we report elucidation of the chemistry of Zr6O8 nodes in the MOFs UiO-66 and UiO-67, having used infrared and nuclear magnetic resonance spectroscopies to determine the ligands on the node surfaces originating from the solvents and modifiers used in the syntheses and having elucidated the catalytic properties of the nodes for ethanol dehydration, which takes place selectively to make diethyl ether but not ethylene at 473–523 K. Density functional theory calculations show that the key to the selective catalysis is the breaking of node-linker bonds (or the accidental adjacency of open/defect sites) that allows catalytically fruitful bonding of the reactant ethanol to neighboring sites on the nodes, facilitating the bimolecular ether formation through an SN2 mechanism.
The targeted incorporation of defects into crystalline matter allows for the manipulation of many properties and has led to relevant discoveries for optimized and even novel technological applications of materials. It is therefore exciting to see that defects are now recognized to be similarly useful in tailoring properties of metal-organic frameworks (MOFs). For instance, heterogeneous catalysis crucially depends on the number of active catalytic sites as well as on diffusion limitations. By the incorporation of missing linker and missing node defects into MOFs, both parameters can be accessed, improving the catalytic properties. Furthermore, the creation of defects allows for adding properties such as electronic conductivity, which are inherently absent in the parent MOFs. Herein, progress of the rapidly evolving field of the past two years is overviewed, putting a focus on properties that are altered by the incorporation and even tailoring of defects in MOFs. A brief account is also given on the emerging quantitative understanding of defects and heterogeneity in MOFs based on scale-bridging computational modeling and simulations.
Functionalization of metal-organic frameworks with metal nanoparticles (NPs) is a promising way for producing advanced materials for catalytic applications. We present synthesis and in situ characterization of palladium NPs encapsulated inside functionalized UiO-67 metal-organic framework. The initial structure was synthesized with 10% of PdCl2bpydc moieties with grafted Pd ions replacing standard 4,4'-biphenyldicarboxylate linkers. This material exhibits the same high crystallinity and thermal stability of standard UiO-67. Formation of palladium NPs was initiated by sample activation in hydrogen and monitored by in situ X-ray powder diffraction and X-ray absorption spectroscopy (XAS). The reduction of PdII ions to Pd⁰ occurs above 200 °C in 6% H2/He flow. The formed palladium NPs have the average size of 2.1 nm as limited by the cavities of UiO-67 structure. The resulting material showed high activity towards ethylene hydrogenation. In reaction conditions, palladium was found to form carbide structure indicated by operando XAS, while formation of ethane was monitored by mass spectroscopy and infra-red spectroscopy.
Sufficient pore size, appropriate stability and hierarchical porosity are three prerequisites for open frameworks designed for drug delivery, enzyme immobilization and catalysis involving large molecules. Herein, we report a powerful and general strate-gy, linker thermolysis, to construct ultra-stable hierarchically porous metal−organic frameworks (HP-MOFs) with tunable pore size distribution. Linker instability, usually an undesirable trait of MOFs, was exploited to create mesopores by generating crystal defects throughout a microporous MOF crystal via thermolysis. The crystallinity and stability of HP-MOFs remain after thermolabile linkers are selectively removed from multivariate metal-organic frameworks (MTV-MOFs) through a decarboxyla-tion process. A domain-based linker spatial distribution was found to be critical for creating hierarchical pores inside MTV-MOFs. Furthermore, linker thermolysis promotes the formation of ultra-small metal oxide (MO) nanoparticles immobilized in an open framework that exhibits high catalytic activity for Lewis acid catalyzed reactions. Most importantly, this work pro-vides fresh insights into the connection between linker apportionment and vacancy distribution, which may shed light on prob-ing the disordered linker apportionment in multivariate systems, a long-standing challenge in the study of MTV-MOFs.
Solvothermal method was used to synthesize MIL-53 (Al) and MIL-53 (Al-Li) at 120 °C. Synthesized catalysts were characterized using X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Field Emission Scanning Electron Microscope (FESEM), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Thermogravimetric Analysis (TGA), Brunauer–Emmett–Teller (BET), and NH3-TPD. Synthesized MIL-53 (Al) and MIL-53 (Al-Li) were used to perform alkylation of benzene with ethanol in a gas phase laboratory reactor. Activity tests revealed that MIL-53 (Al-Li) has better performance compared to MIL-53 (Al) with 73 % conversion of benzene, complete conversion of ethanol, and 75% selectivity for ethylbenzene at 200 °C. The main byproduct is identified as toluene and other byproducts detected in very small amount. Both of catalysts show good stability after 14 hours time on stream at 200°C. The high activity and product selectivity of MIL-53 (Al-Li) are attributed to proper acidity strength of catalytic sites. Finally, the new MIL-53 (Al-Li) catalyst is proposed as a more environmentally friendly and economically favorable alternative for commercial zeolites in the benzene alkylation process.
Single atoms and few-atom clusters of platinum are uniformly installed on the zirconia nodes of a metal-organic framework (MOF) NU-1000 via targeted vapour-phase synthesis. The catalytic Pt clusters, site-isolated by organic linkers, are shown to exhibit high catalytic activity for ethylene hydrogenation while exhibiting resistance to sintering up to 200 °C. In situ IR spectroscopy reveals the presence of both single atoms and few-atom clusters that depend upon synthesis conditions. Operando X-ray absorption spectroscopy and X-ray pair distribution analyses reveal unique changes in chemical bonding environment and cluster size stability while on stream. Density functional theory calculations elucidate a favourable reaction pathway for ethylene hydrogenation with the novel catalyst. These results provide evidence that atomic layer deposition (ALD) in MOFs is a versatile approach to the rational synthesis of size-selected clusters, including noble metals, on a high surface area support.
Metal-organic frameworks (MOFs) have been under development over the past 20 years. Similar to other technologies, research on MOFs in the upcoming 30 years will move towards the direction where MOF materials can deliver societal benefits by solving real-world problems. Taking technology from laboratory to applications is always a challenge. Analysis of the current MOFs research efforts indicates that the high cost, limited availability of MOF products and the knowledge gap for cost-effective production technologies account for the slow progression towards the development of envisioned MOF products at pilot-scale level. This short review brings together the scattered literature that addresses pilot-scale production of MOF materials. An additional aspect focuses on the progress on the development of pilot-scale synthetic strategies with green and sustainable features for MOF materials, which is an imperative to promote MOF-enabled products into the real world.
Partial substitution of ZnII by MnII in Zn4O(terephthalate)3 (MOF-5) leads to a distorted all-oxygen ligand field supporting a single MnII site, whose structure was confirmed by Mn K-edge X-ray absorption spectroscopy. The MnII ion at the MOF-5 node engages in redox chemistry with a variety of oxidants. With tBuSO2PhIO, it produces a putative MnIV-oxo intermediate, which upon further reaction with adventitious hydrogen is trapped as a MnIII-OH species. Most intriguingly, the intermediacy of the high-spin MnIV-oxo species is likely responsible for catalytic activity of the MnII-MOF-5 pre-catalyst, which in the presence tBuSO2PhIO catalyzes oxygen atom transfer reactivity to form epoxides from cyclic alkenes with >99% selectivity. These results demonstrate that MOF secondary building units serve as competent platforms for accessing terminal high-valent metal-oxo species that consequently engage in catalytic oxygen atom transfer chemistry owing to the relatively weak ligand fields provided by the SBU.
Metal-organic organic frameworks of general composition [M6(OH)4(O)4(PDC)6-x(Cl)2x(H2O)2x] with M = Zr, Ce, Hf, PDC2- = 2,5-pyridinedicarboxylate and 0 < x < 2 were obtained under reflux using acetic acid as the solvent. Rietveld refinements carried out fixing the occupancy of the linker molecules according to the results of the thermogravimetric measurements confirmed that the MOFs crystallize in the UiO-66 type structure and demonstrate that the structural models describe the data well. Further characterization was carried out by NMR-spectroscopy, thermogravimetric analysis, Zr K-edge EXAFS- and Ce L3-edge XANES measurements. To highlight the influence of the additional nitrogen atom of the pyridine ring, luminescence and vapour sorption measurements were carried out. The hydrophilisation of the MOFs was shown by the adsorption of water at lower p/p0 (< 0.2) values compared to the corresponding BDC-MOFs (0.3). For the water and methanol stability cycling vapour adsorption experiments were carried out to evaluate the MOFs as potential adsorbents in heat transformation applications.
Molecular iridium catalysts immobilized in metal-organic frame-works (MOFs) were positioned in the condensing chamber of a Soxhlet extractor for efficient CO2 hydrogenation. Droplets of hot water seeped through the MOF catalyst to create dynamic gas/liquid interfaces which maximize the contact of CO2, H2, H2O, and the catalyst to achieve a high turnover frequency of 410 h⁻¹ under atmospheric pressure and at 85 oC. H/D kinetic isotope effect measurements and density functional theory calculations revealed concerted proton-hydride transfer in the rate-determining step of CO2 hydrogenation, which was difficult to unravel in homogeneous reactions due to base-catalyzed H/D exchange.
The introduction of Ce4+ as a structural cation has been shown to be a promising route to redox active metal-organic frameworks (MOFs). However, the mechanism by which these MOFs act as redox catalysts remains unclear. Herein, we present a detailed study of the active site in [Ce6O4(OH)4]-based MOFs such as Ce-UiO-66, involved in the aerobic oxidation of benzyl alcohol, chosen as a model redox reaction. X-ray absorption spectroscopy (XAS) data confirms the reduction of up to one Ce4+ ion per Ce6-cluster with a corresponding outwards radial shift, due to the larger radius of the Ce3+ cation, while not compromising the structural integrity of the framework, as evidenced by powder X-ray diffraction. This unambiguously demonstrates the involvement of the metal node in the catalytic cycle and explains the need for 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a redox mediator to bridge the gap between the one-electron oxidation of the Ce4+/Ce3+ couple and the two-electron alcohol oxidation. Finally, an improved catalytic system with Ce-MOF-808 and TEMPO was developed which outperformed all other tested Ce4+-MOFs.
The interactions of CO with amorphous, monoclinic, and tetragonal zirconia were investigated by diffuse reflectance infrared (DRIFT) and transmission FT-IR spectroscopy to determine the active sites for CO adsorption. The results showed that zirconia polymorphs exerted a great influence on CO bands and the formation of surface intermediates. Meanwhile, differences of CO bands and surface intermediates were observed between DRIFT and transmission FT-IR spectra. Two ways were proposed to produce active sites for CO adsorption, one way was thermal dehydroxylation in vacuum and the other was CO reaction with hydroxyls at high temperature. The former was the main way to produce active sites for CO adsorption in transmission FT-IR spectra, and the latter was the main way to produce active sites for CO adsorption in DRIFT spectra.
A zirconium metal organic framework with UiO-66 topology was synthesized using 1,4-naphthalenedicarboxylic acid linker (UiO-66-NDC). From synchrotron XRPD we found that the naphthalene rings of the NDC linker is out of (a,c)-plane equilibrium by 30 °, similar to the situation found by single crystal XRD for the benzene rings in UiO-67 [Cryst. Growth Des. 2014, 14, 5370]. Different fraction of benzene-carboxylic (BC) acid modulator vs. NDC were used to tune structural properties of the final product. Modulator increases both the crystal size and the surface area of the product, but reduces its thermal stability owing to insertion of defects: missing linkers and Zr6(OH)4O4 clusters. This study proves that the defect density (fraction of BC incorporation) can be tuned in UiO-66-NDC materials up to almost 50 %. At that stage, the crystal is characterized also by high density of missing Zr6(OH)4O4 inorganic cornerstones. Notwithstanding such structural defectivity, even the most defective material is stable after thermal activation at 200 °C (able to fully remove the solvent) and in water conditions, opening possibilities for application in the fields of catalysis and molecule sorption.
Composites incorporating nanoparticles (NPs) within metal-organic frameworks (MOFs) find applications in many different fields. In particular, using MOF layers as molecular sieves built on the NPs could enable selectivity in heterogeneous catalysis. However, such composites typically exhibit low catalytic efficiency, due to the slow diffusion of the reactants in the long and narrow channels of the MOF shell. In order to improve the catalytic efficiency of these systems, here we report the fabrication of NPs incorporated in nanosized MOFs (NPs@nano-MOFs), obtained by reducing the size of the MOF crystals grown around the NPs. The crystal size of the composites was controlled by modulating the nucleation rate of the MOFs during the encapsulation of pre-synthesized and catalytically active NPs; in this way, NPs@MOF crystals smaller than 50 nm were synthesized and subsequently used as highly efficient catalysts. Due to the shorter path from the MOF surface to the active sites, the obtained Pt@nano-MOFs composites showed a higher conversion rate than their larger-sized counterparts in the synthesis of imines via cascade reaction of nitrobenzene and in the hydrogenation of olefins, while retaining the excellent size and shape selectivity associated with the molecular sieving effect of the MOF layer. The present strategy can also be applied to prepare other encapsulated nanostructures combining various types of NPs and nano-MOFs, thus highlighting the broad potential of this approach for developing optimized catalysts with high reactivity and selectivity.
CO2 hydrogenation was carried out over Pt-containing UiO-67 Zr-MOFs at T = 220–280 °C and ambient pressure, with H2/CO2 = 0.2–9 and contact times, τ = 0.004–0.01 gcat×min×ml−1. The catalysts were characterized by XRD, N2 adsorption, FESEM, TEM and HRTEM, dissolution-NMR, CO chemisorption, IR spectroscopy and TGA. A positive correlation was observed between the degree of Pt reduction and CO2 conversion. Contact time variation experiments showed that CO is a primary product of reaction, while CH4 is a secondary product. Testing of catalyst crystals with 0.15 and 2.0 micron crystal size, respectively, revealed no influence of diffusion on the reaction rate. Comparison to a conventional Pt/SiO2 catalyst showed very similar activation energy, with Eapp = 50±3 kJ×mol−1. However, the turn-over frequency over Pt/SiO2 was significantly lower, and Pt/SiO2 did not yield methane as product. The Pt-containing UiO-67 Zr-MOF catalyst showed stable activity during 60 hours testing.
Copper oxide clusters synthesized via atomic layer deposition on the nodes of the metal-organic framework NU-1000 are active for oxidation of methane to methanol under mild reaction conditions. Analysis of chemical reactivity, in situ X-ray absorption spectroscopy, and density functional theory calculations are used to determine structure/activity relations in the Cu-NU-1000 catalytic system. The Cu-loaded MOF contained Cu as clusters of a few atoms each. The Cu was present at ambient conditions as a mixture of ~15 % Cu⁺ and ~ 85 % Cu²⁺. The oxidation of methane on Cu-NU-1000 was accompanied by the reduction of 9 % Cu in the catalyst from Cu²⁺ to Cu⁺. The products, methanol, dimethyl ether and CO2, were desorbed with the passage of 10% water/He at 135 °C, giving a carbon selectivity for methane to methanol of 45–60 %. Cu-NU-1000 is an interesting first-generation of MOF-stabilized Cu oxo-clusters with activity for selective methane oxidation.
The development of catalysts able to assist industrially important chemical processes is a topic of high importance. In view of the catalytic capabilities of small metal clusters, research efforts are being focused on the synthesis of novel catalysts bearing such active sites. Here we report a heterogeneous catalyst consisting of Pd4 clusters with mixed-valence 0/+1 oxidation states, stabilized and homogeneously organized within the walls of a metal-organic framework (MOF). The resulting solid catalyst outperforms state-of-the-art metal catalysts in carbene-mediated reactions of diazoacetates, with high yields (>90%) and turnover numbers (up to 100,000). In addition, the MOF-supported Pd4 clusters retain their catalytic activity in repeated batch and flow reactions (>20 cycles). Our findings demonstrate how this synthetic approach may now instruct the future design of heterogeneous catalysts with advantageous reaction capabilities for other important processes.
The exceptional thermal and chemical stability of UiO-66, -67 and -68 class of isostructural MOFs [J. Am. Chem. Soc., 2008, 130, 13850] makes them ideal materials for functionalization purposes aimed in introducing active centres for potential application in heterogeneous catalysis. We previously demonstrated that a small fraction (up to 10%) of the linkers in the UiO-67 MOF can be replaced by bipyridine-dicarboxylate (bpydc) moieties exhibiting metal-chelating ability and enabling the grafting of Pt(II) and Pt(IV) ions in the MOF framework [Chem. Mater., 2015, 27, 1042] upon interaction with PtCl2 or PtCl4 precursors. Herein we extend this functionalization approach into two directions. First, we show that controlling the activation of the UiO-67-Pt we can move from a material hosting isolated Pt(II) sites anchored to the MOF framework exhibiting two coordination vacancies (potentially interesting for C−H bond activation) to the formation of very small Pt nanparticles hosted inside the MOF cavities (potentially interesting for hydrogenation reactions). The second direction consists in the extension of the approach to the insertion of Cu(II), obtained via interaction with CuCl2, and exhibiting interesting red-ox properties. All materials have been characterized by in situ X-ray absorption spectroscopy at the Pt L3- and Cu K-edges.
Metal–organic frameworks (MOFs) with nodes consisting of zirconium oxide clusters (Zr6) offer new opportunities as supports for catalysts with well-defined, essentially molecular, structures. We used the precursor Rh(C2H4)2(acac) (acac is acetylacetonate) to anchor Rh(I) complexes to the nodes of the MOF UiO-67 and, for comparison, to the zeolite dealuminated HY (DAY). These were characterized experimentally by measurement of catalytic activities and selectivities for ethylene hydrogenation and dimerization in a once-through flow reactor at 298 K and 1 bar. The catalyst performance data are complemented with structural information determined by infrared and extended X-ray absorption fine structure spectroscopies and by calculations at the level of density functional theory, the latter carried out also to extend the investigation to a related MOF, NU-1000. The agreement between the experimental and calculated structural metrics is good, and the calculations have led to predictions of reaction mechanisms and associated energetics. The data demonstrate a correlation between the catalytic activity and selectivity and the electron-donor tendency of the supported rhodium (as measured by the frequencies of CO ligands bonded as probes to the Rh(I) centers), which is itself a measure of the electron-donor tendency of the support.
The potential commercial applications for metal organic frameworks (MOFs) are tantalizing. To address the opportunity, many novel approaches for their synthesis have been developed recently. These strategies present a critical step towards harnessing the myriad of potential applications of MOFs by enabling larger scale production and hence real-world applications. This review provides an up-to-date survey ( references) of the most promising novel synthetic routes, i.e., electrochemical, microwave, mechanochemical, spray drying and flow chemistry synthesis. Additionally, the essential topic of downstream processes, especially for large scale synthesis, is critically reviewed. Lastly we present the current state of MOF commercialization with direct feedback from commercial players.
Iron based metal organic frameworks as novel catalysts were developed for ethylbenzene synthesis by alkylation of benzene with ethanol in low temperature gas phase conditions employing MIL-101 (Fe) and MIL-88 (Fe) catalysts. Both of catalysts were synthesized with solvothermal method. Catalysts characterization were performed using X-Ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FT-IR), Differential Scanning Calorimetry (DSC), and Field Emission Scanning Electron Microscope (FE-SEM). Nitrogen adsorption measurements were done to determining the catalyst surface area and pore volume and pore size distribution. Also the acidity of catalysts was measured using NH3-TPD. The catalysts activity and product selectivity for ethylbenzene production have been investigated in a specially designed flow microreactor in gas phase. Effects of temperature and space velocity at constant benzene to ethanol ratio (3:1 volumetric ratio) have been studied for catalyst performance and ethylbenzene selectivity and yield. In comparison of two synthesized catalysts, MIL-88 showed a higher performance (100% ethanol conversion, 72% benzene conversion and 76% ethylbenzene selectivity at 175 °C). The main byproduct was recognized as toluene. Long reaction time for both catalysts showed no decline in conversion and selectivity of desired product. The BET measurement and the FTIR spectra of used catalysts revealed these Fe-based metal organic frameworks are stable catalysts for alkylation of benzene by ethanol.
The identification and quantification of defects is undoubtedly a thorough challenge in the characterization of “defect engineered” metal-organic frameworks (MOFs). UiO-66, known for its exceptional stability and defect tolerance, has been a popular target for defect engineering studies. Herein, we show that synthesising UiO-66 in the presence of an excess of benzoic acid is a reliable method for obtaining UiO-66 samples with a very high concentration of missing cluster defects, allowing to modulate specific properties (i.e. surface area and hydrophobicity). This was elucidated by a multitechnique marriage of experimental and computational methods: a combination of PXRD, dissolution/1H NMR spectroscopy, and N2 sorption measurements were used to quantify the defect loading, while vibrational spectroscopies (FTIR and Raman) allowed us to unequivocally identify the defect structure by comparison with DFT simulated spectra and the graphical analysis of the vibrational modes.
Heterogeneous catalysts have been widely used for photocatalysis, which is a highly important process for energy conversion, owing to their merits such as easy separation of catalysts from the reaction products and applicability to continuous chemical industry and recyclability. Yet, homogenous photocatalysis receives tremendous attention as it can offer a higher activity and selectivity with atomically dispersed catalytic sites and tunable light absorption. For this reason, there is a major trend to combine the advantages of both homogeneous and heterogeneous photocatalysts, in which coordination chemistry plays a role as the bridge. In this article, we aim to provide the first systematic review to give a clear picture of the recent progress from taking advantage of coordination chemistry. We specifically summarize the role of coordination chemistry as a versatile tool to engineer catalytically active sites, tune light harvesting and maneuver charge kinetics in heterogeneous photocatalysis. We then elaborate on the common fundamentals behind various materials systems, together with key spectroscopic characterization techniques and remaining challenges in this field. The typical applications of coordination chemistry in heterogeneous photocatalysis, including proton reduction, water oxidation, carbon dioxide reduction and organic reactions, are highlighted.
Owing to the vast diversity of linkers, nodes, and topologies, metal-organic frameworks can be tailored for specific tasks, such as chemical separations or catalysis. Accordingly, these materials have attracted significant interest for capture and/or detoxification of toxic industrial chemicals and chemical warfare agents. In this paper, we review recent experimental and computational work pertaining to the capture of several industrially-relevant toxic chemicals, including NH3, SO2, NO2, H2S, and some volatile organic compounds, with particular emphasis on the challenging issue of designing materials that selectively adsorb these chemicals in the presence of water. We also examine recent research on the capture and catalytic degradation of chemical warfare agents such as sarin and sulfur mustard using metal-organic frameworks.
The present review describes the use of metal organic framework (MOF) encapsulated Au nanoparticles (NPs) as heterogeneous catalysts. The purpose is to show that catalysts with very high performance can be obtained by incorporation of Au NPs inside MOFs. The available data indicate that the high catalytic activity of MOF encapsulated Au NPs derives from i) small particle size, ii) high dispersion and homogeneous distribution inside MOFs crystals, iii) stabilization of particle size by confinement of Au NPs inside MOFs cages, and iv) the synergy that can arise by the combination of the activity of Au NPs and MOFs. This mini review covers reactions using Au@MOFs as catalysts for oxidations, reductions, tandem process and photocatalysis with the emphasis in providing a comparison with the performance other alternative Au containing catalysts. In the final section, we summarize in our view the current achievements and which are the next targets in this area.
The interfaces of Cu/ZnO and Cu/ZrO2 play vital roles in the hydrogenation of CO2 to methanol by these composite catalysts. Surface structural reorganization and particle growth during catalysis deleteriously reduce these active interfaces, diminishing both catalytic activities and MeOH selectivities. Here we report the use of pre-assembled bpy and Zr6(μ3-O)4(μ3-OH)4 sites in UiO-bpy metal-organic frameworks (MOFs) to anchor ultrasmall Cu/ZnOx nanoparticles, thus preventing the agglomeration of Cu NPs and phase separation between Cu and ZnOx in MOF cavity-confined Cu/ZnOx nanoparticles. The resultant Cu/ZnOx@MOF catalysts show very high activity with a space-time yield of up to 2.59 gMeOH kgCu⁻¹ h⁻¹ and 100% selectivity for CO2 hydrogenation to methanol and high stability over 100 hours. These new types of strong metal-support interactions between metallic nanoparticles and organic chelates/metal-oxo clusters offer new opportunities in fine-tuning catalytic activities and selectivities of metal nanoparticles@MOFs.
Metal organic frameworks (MOFs) are hybrid crystalline materials, exhibiting high specific surface area, controllable pore sizes and surface chemistry. These properties have made MOFs attractive for a wide range of applications including gas separation, gas storage, sensing, drug delivery and catalysis. This review focuses on recent progress in the application of MOF materials as catalyst for CO2 conversion through chemical fixation, photocatalysis and electrocatalysis. In particular, the review discusses the co-relation between the physicochemical properties of MOF materials including with their catalytic performance as well as their stability and recyclability under different reaction conditions, relevant to CO2 conversion. Current modification techniques for improving MOF performance are highlighted as well as recent understanding in their electronic properties. The limitations facing MOF based catalysts are also discussed and potential routes for improvement suggested.