Understanding the diversity of the metal-organic framework ecosystem

By combining metal nodes and organic linkers one can make millions of different metal-organic frameworks (MOFs). At present over 90,000 MOFs have been synthesized and there are databases with over 500,000 predicted structures. This raises the question whether a new experimental or predicted structure adds new information. For MOF-chemists the chemical design space is a combination of pore geometry, metal nodes, organic linkers, and functional groups, but at present we do not have a formalism to quantify optimal coverage of chemical design space. In this work, we show how machine learning can be used to quantify similarities of MOFs. This quantification allows us to use techniques from ecology to analyse the chemical diversity of these materials in terms of diversity metrics. In particular, we show that this diversity analysis can identify biases in the databases, and how such bias can lead to incorrect conclusions. This formalism provides us with a simple and powerful practical guideline to see whether a set of new structures will have the potential for new insights, or constitute a relatively small variation of existing structures.

By combining metal nodes and organic linkers, an infinite number of metal organic frameworks (MOFs) can be designed in silico. When making new databases of such hypothetical MOFs, we need to assure that they not only contribute towards the growth of the count of structures but also add different chemistry to existing databases. In this study, we designed a database of ~20,000 hypothetical MOFs which are diverse in terms of their chemical design space—metal nodes, organic linkers, functional groups and pore geometries. Using Machine Learning techniques, we visualized and quantified the diversity of these structures. We find that on adding the structures of our database, the overall diversity metrics of hypothetical databases improve, especially in terms of the chemistry of metal nodes. We then assessed the usefulness of diverse structures by evaluating their performance, using grand-canonical Monte Carlo simulations, in two important environmental applications—post combustion carbon capture and hydrogen storage. We find that many of these structures perform better than widely used benchmark materials such as Zeolite-13X (for post combustion carbon capture) and MOF-5 (for hydrogen storage).

The effects of greenhouse gases, like CO2, are becoming more and more visible, from drastic weather changes to global sea level rise. The resultant global warming has become an environmental issue of great concern in recent years. The potential for CO2 capture through metal-organic frameworks has been widely studied and is currently being explored as a way to reduce greenhouse gas emissions. The strong structural and electronic properties of these frameworks make them excellent candidates for capturing CO2 due to their high porosity, tunable composition and good chemical stability. Functionalized metal organic frameworks (MOFs) are important because it allows for the development of MOF materials with properties that are tunable for many different applications. Many of these functions are gas adsorption, catalysis, and separation. Depending on the composition of the linkers and nodes, various functional groups can be introduced into the network through the organic linkers and metal nodes, resulting in MOFs with different functions. Mixed-metal MOFs are composed of several different metals. The variety of metals in a mixed-metal MOF gives the MOF many options for tailoring its properties. The resulting bimetallic MOFs are not only more thermally and chemically stable, but also can absorb more gases. The other objective of this study was to observe the effect of organic functional groups on CO2 adsorption in MOFs. The study found that organic functional groups have a significant effect on CO2 adsorption in MOFs.

By combining metalnodes and organic linkers, an infinite numberof metal organic frameworks (MOFs) can be designed in silico. Therefore,when making new databases of such hypothetical MOFs, we need to ensurethat they not only contribute toward the growth of the count of structuresbut also add different chemistries to the existing databases. In thisstudy, we designed a database of ∼20,000 hypothetical MOFs,which are diverse in terms of their chemical design space—metalnodes, organic linkers, functional groups, and pore geometries. Usingmachine learning techniques, we visualized and quantified the diversityof these structures. We find that on adding the structures of ourdatabase, the overall diversity metrics of hypothetical databasesimprove, especially in terms of the chemistry of metal nodes. We thenassessed the usefulness of diverse structures by evaluating theirperformance, using grand-canonical Monte Carlo simulations, in twoimportant environmental applications—post-combustion carboncapture and hydrogen storage. We find that many of these structuresperform better than widely used benchmark materials such as Zeolite-13X(for post-combustion carbon capture) and MOF-5 (for hydrogen storage).All the structures developed in this study, and their properties,are provided on the Materials Cloud to encourage further use of thesematerials for other applications.

Metal-organic frameworks (MOFs) are periodic, hybrid, atomically well-defined porous materials that typically form by self-assembly and consist of inorganic nodes (metal ions or clusters) and multitopic organic linkers. MOFs as a whole offer many intriguing properties, including ultrahigh porosity, tunable chemical functionality, and low density. These properties point to numerous potential applications, including gas storage, chemical separations, catalysis, light harvesting, and chemical sensing, to name a few. Reticular chemistry, or the linking of molecular building blocks into predetermined network structures, has been employed to synthesize thousands of MOFs. Given the vast library of candidate nodes and linkers, the number of potentially synthetically accessible MOFs is enormous. Nevertheless, a powerful complementary approach to obtain specific structures with desired chemical functionality is to modify known MOFs after synthesis. This approach is particularly useful when incorporation of particular chemical functionalities via direct synthesis is challenging or impossible. The challenges may stem from limited stability or solubility of precursors, unwanted secondary reactivity of precursors, or incompatibility of functional groups with the conditions needed for direct synthesis. MOFs can be postsynthetically modified by replacing the metal nodes and/or organic linkers or via functionalization of the metal nodes and/or organic linkers. Here we describe some of our efforts toward the development and application of postsynthetic strategies for imparting desired chemical functionalities in MOFs of known topology. The techniques include methods for functionalizing MOF nodes, i.e., solvent-assisted ligand incorporation (SALI) and atomic layer deposition in MOFs (AIM) as well as a method to replace structural linkers, termed solvent-assisted linker exchange (SALE), also known as postsynthethic exchange (PSE). For each functionalization strategy, we first describe its chemical basis along with the requirements for its successful implementation. We then present a small number of examples, with an emphasis on those that (a) convey the underlying concepts and/or (b) lead to functional structures (e.g., catalysts) that would be difficult or impossible to access via direct routes. The examples, however, are only illustrative, and a significant body of work exists from both our lab and others, especially for the SALE/PSE strategy. We refer readers to the papers cited and to the references therein. More exciting, in our view, will be new examples and new applications of the functionalization strategies-especially applications made possible by creatively combining the strategies. Underexplored (again, in our view) are implementations that impart electrical conductivity, enable increasingly selective chemical sensing, or facilitate cascade catalysis. It will be interesting to see where these strategies and others take this compelling field over the next few years.

Strategic engineering of structural complexity in metal–organic frameworks

ConspectusPhotonic functional materials have received mounting attention because of their employment in luminescent sensing, display, white-light-emission (WLE), data storage, bioimaging, etc. Metal–organic frameworks (MOFs), usually fabricated by metal nodes and organic linkers, offer many intriguing properties such as permanent porosity, structural designability, diversity of morphology, excellent crystallinity, multiple luminescent centers, and modifiable chemical functionality. These properties make MOFs versatile hosts in various environments, especially in developing advanced photonic functional materials. Furthermore, the pores within MOFs endow them as hosts to confine diverse guest molecules (e.g., organic dyes, perovskite quantum dots (QDs), photochromic molecules, lanthanide ions, metal complexes and so forth), so plenty of photonic functional guest encapsulation in MOF materials can be successfully fabricated.It is worth noting that the advantages of guest-encapsulated MOF materials for photonics are abundant. First, combining guest units and MOFs can result in host–guest MOFs with multiple photonic centers, which can be successfully used to realize diverse photonics properties. Second, apart from the intrinsic photonic properties of MOFs and guests, the isolation and protection of MOFs could optimize and enhance the photonic functionality of guests, such as enhancing the photoluminescence quantum yield (PLQY), stability, and so on. Third, the confinement and orientation of guest molecules within the ordered pores of MOFs undoubtedly enables a promising strategy for generating novel photonic functionalities. It is precisely due to the enormous combination possibilities, synergistic effects, and controllable and directional assembly of guest photonic units in host–guest MOF materials that provide a new avenue for designing and constructing tailored photonic functional materials and applications.In this Account, we describe some of our efforts to encapsulate the guest molecules within MOFs to construct photonic functional materials. Given the unique intermolecular interactions between guest molecules and MOFs, we have put an emphasis on rational encapsulation design to optimize and generate photonic functionality, such as introducing new photonic units in MOFs and then using them as ratiometric luminescent thermometers, luminescent film sensors, and luminescent sensor array; enhancing photonic performance through the isolation and protection of MOF pores and achieving WLE, multiphoton excited luminescence (MPEL); and generating novel photonic functionality including high-order nonlinear optical (NLO) response, multiphoton-pumped polarized lasing, controllable laser switch performance, photostimuli-responsive dynamic fluorescence, and so on. We also discuss the most important measurement techniques [confocal laser scanning microscope (CLSM) and single crystal X-ray diffraction (SCXRD)] that can be applied for proper determination of orientation and position of guests within MOFs. Various synthetic routes using the confinement strategies to yield the MOFs⊃guest composites were also discussed. Finally, we point out future design principles and research directions of guest-encapsulated photonic MOF materials.

In this work, we introduce a combined DFT and machine learning approach to obtain insights into the chemical design of metal-organic framework (MOF) photocatalysts for hydrogen (HER) and oxygen (OER) evolution reactions. To train our machine learning models, we evaluated a dataset of 314 MOFs using a dedicated DFT workflow that computes a set of five descriptors for both closed and open shell MOFs. Our dataset is composed of a diverse selection of the QMOF database and experimentally reported MOF photocatalysts. In addition, to ensure a balanced dataset, we designed a set of MOFs (CDP-MOF) inspired by insights obtained regarding different types of photocatalytic materials. Our machine-learning approach allowed us to screen the entire QMOF and CDP-MOF databases for promising candidates. Our analysis of the chemical design space shows that we have many materials with a suitable spatial overlap of electron and hole, band gap, band-edge alignment to HER, and charge-carrier effective masses. However, we have identified in the QMOF database only a very small percentage of materials that also have the right band-edge alignment to OER. With the CDP-MOF database, we successfully targeted building blocks that potentially have the correct OER band alignment, and indeed obtained a larger percentage of materials that obey these criteria. Among those, a few motifs stood out, such as Au-pyrazolate, Ti clusters and rod-shaped metal nodes, and a particular MOF designed with the Mn4Ca cluster, which mimics the OER center in the photosystem II of photosynthesis.

To relieve and solve the energy problem, effective methods to use solar energy must be built up. To be more specifically, we need figure out how to utilize sunlight for water splitting reaction, giving rise to hydrogen as clean energy, and photoreduction of carbon dioxide, leading to the formation of useful liquid products (e.g., HCOO−, HCHO and CH3OH) or gaseous products (e.g., CH4 and CO). As a class of distinguished and unique materials, metal-organic frameworks (MOFs) have drawn a lot of attention, considering that they display special physical and chemical properties such as exceedingly high surface areas, designable and controllable cavities, different mechanisms of photo-induced electrons transfer, and moreover photoactive parts can be easily introduced into MOFs by either encapsulating dye molecules into the cavities or constructing the frameworks with optically active bridging ligands or metal nodes. In this review, we have commented on the challenges in this field, summarized the unique advantages and inherent merits of MOFs as the emerging materials, and pointed out the opportunities and development strategies of MOFs for their applications in photocatalysis. Firstly, we have introduced MOFs′ concepts and features, distinguishing them from other porous materials, and their advantages in photocatalysis. MOFs are crystalline porous materials formed from ligands (including metalloligands) and transition-metal nodes. The structures of MOFs are of facile design, and can be further modified through post-synthetic methods. Some MOFs display high thermal and chemical stability, which can be stable up to 500°C and resist a variety of reaction media either organic solvents or aqueous, even in acidic and basic solutions. MOFs can be photoresponsive through light absorption by the organic linker, the metal oxide nodes or the photoactive species entrapped in the voids. Photoexcitation of the light absorbing units in MOFs generates the excited state, which might induce photocatalytic activity. Next, we have classified photocatalytic MOFs into three types, including (1) metal-oxo clusters as semiconductor dots, (2) ligands/metalloligands as photocatalysts, and (3) photocatalytic species (nano- particles, polyoxometalates, nano-composites, and etc.) encapsulated into the pore, and discussed their applications in photocatalysis in details. As for type I, metal-oxo clusters, especially Zr-O or Ti-O clusters, as the nodes have been assembled into MOFs. Upon the absorption of photons with the energy greater than the bandgap of the ligand, a ligand-to-metal charge-separation state was generated, resulting in photocatalytic activity. In type II, a few molecular photocatalysts based on metal-polypyridine complexes, usually being Ru and Ir complexes, metalloporphyrins and organic dyes have been incorporated into MOFs to afford photocatalysts under visible light. Considering type III, photoactive species, including polyoxometalates and metal nanoparticles (e.g., Pt, Pd, Au, and Ag NPs), have been doped into the cavities of MOFs. In addition, integration of an inorganic semiconductor with a MOF gives rise to a composite photocatalyst, which combines the advantages of both materials and then results in higher efficiency, selectivity and stability (especially low metal leaching and recyclability). Finally, we have provided our perspectives for the future of MOFs as photocatalysts. MOFs have displayed the potentials in photocatalysis, but there still exist large improvement spaces. The relatively low stability of MOFs compared to inorganic semiconductors limits their applications for practice. In most reports of MOF photocatalysts, sacrificial agents are required, which isn′t consistent with the sustainable development concept. MOFs with strong absorption of visible light, long lifetime of excited state, high product selectivity and stability are in pursuing.

Metal–organic frameworks (MOFs) are crystalline materials that self-assemble from inorganic nodes and organic linkers, and isoreticular chemistry allows for modular and synthetic reagents of various sizes. In this study, a MOF’s components─metal nodes and organic linkers─are constructed in a coarse-grained model from isotropic beads, retaining the basic symmetries of the molecular components. Lennard-Jones and Weeks–Chandler–Andersen pair potentials are used to model attractive and repulsive particle interactions, respectively. We analyze the crystallinity of the self-assembled products and explore the role of modulators─molecules that compete with the organic linkers in binding to the metal nodes, and which we construct analogously─during the self-assembly process of defect-engineered MOFs. The coarse-grained simulation allows for the uncoupling of experimentally interdependent variables to broadly map and determine essential MOF self-assembly conditions, among which are properties of the modulator: binding strength, size (steric hindrance), and concentration. Of these, the simulated modulator’s binding strength has the most pronounced effect on the resulting MOF’s crystal size.

Green synthesis of metal–organic frameworks: A state-of-the-art review of potential environmental and medical applications

Nanocatalysts encapsulated in metal-organic frameworks: Size control and positive influences.

Precise Construction of Stable Bimetallic Metal–Organic Frameworks with Single-Site Ti(IV) Incorporation in Nodes for Efficient Photocatalytic Oxygen Evolution

Toxicity assessment and underlying mechanisms of multiple metal organic frameworks using the green algae Chlamydomonas reinhardtii model

Metal organic frameworks (MOFs) has been considered as an ideal molecular capture material and has been widely used in many fields, including storage, catalytic, drug delivery and molecular separation due to its unique porous structure. MOFs contain organic linkers and metallic nodes, in which the aromatic ligands of MOFs have a strong π-π interaction with the aromatic structure of many organics, and the metal nodes are conducive to the coordination adsorption of the polarized parts of the molecule. Therefore, MOFs has a broad application prospect in the molecular detection field. Especially in recent years, researchers have continuously reported a series of SERS-active MOFs, which enriches the scope of metal-free semiconductor SERS platforms. In this study, MIL-100(Fe) was synthesized via hydrothermal method following the reported procedure with slight modifications, and then was characterized by scanning electron microscope (SEM), powder X-ray diffraction (PXRD) and porosity analyzer. Upon the adsorption of methyl blue (MB) and methylene orange (MO) onto MIL-100(Fe), different adsorption capability of MIL-100(Fe) to dyes were observed from their adsorption behavior curves. We found for the first time that MIL-100(Fe) can serve as an ideal SERS-active substrate for the capture and recognition of MB and MO but with different detection limits, 10−7 mol/L for MB and 10−5 mol/L for MO, respectively, indicating MIL-100(Fe) presents selective enhancement properties for these two organic dye molecules. In order to further increase the sensitivity of MIL-100(Fe) substrate for MO detection, the detection has been accomplished by depositing concentrated gold nanoparticle colloid onto the MIL-100(Fe) substrate, thus forming SERS “hot spots” which enable amplify the Raman signal of MO due to the electromagnetic enhancement (EM) effect. Density functional theory (DFT) was carried out to evaluate the binding position, molecular distance, binding energy and energy gap between dyes molecules and MIL-100(Fe). The energy gap between the MB and MIL-100(Fe) levels is 1.59 eV, close to the laser excitation energy ( E L=1.58 eV). This resonance suggests that the chemical enhancement (CE) mechanism may be operating, accounting for the high SERS activity of MIL-100(Fe) in MB detection. In addition, the adsorption energy between MB and MIL-100(Fe) is stronger than that of MO, this together with the CE accounts for the selective enhancing effect of MIL-100(Fe) on the two dye molecules.