The emergence of metal−organic frameworks (MOFs) as functional ultrahigh surface area materials is one of the most exciting recent developments in solid-state chemistry. Now constituting thousands of ...distinct examples, MOFs are an intriguing class of hybrid materials that exist as infinite crystalline lattices with inorganic vertices and molecular-scale organic connectors. Useful properties such as large internal surface areas, ultralow densities, and the availability of uniformly structured cavities and portals of molecular dimensions characterize functional MOFs. Researchers have effectively exploited these unusual properties in applications such as hydrogen and methane storage, chemical separations, and selective chemical catalysis. In principle, one of the most attractive features of MOFs is the simplicity of their synthesis. Typically they are obtained via one-pot solvothermal preparations. However, with the simplicity come challenges. In particular, MOF materials, especially more complex ones, can be difficult to obtain in pure form and with the optimal degree of catenation, the interpenetration or interweaving of identical independent networks. Once these two issues are satisfied, the removal of the guest molecules (solvent from synthesis) without damaging the structural integrity of the material is often an additional challenge. In this Account, we review recent advances in the synthetic design, purification, and activation of metal−organic framework materials. We describe the rational design of a series of organic struts to limit framework catenation and thereby produce large pores. In addition, we demonstrate the rapid separation of desired MOFs from crystalline and amorphous contaminants cogenerated during synthesis based on their different densities. Finally, we discuss the mild and efficient activation of initially solvent-filled pores with supercritical carbon dioxide, yielding usable channels and high internal surface areas. We expect that the advances in the synthesis, separation, and activation of metal−organic frameworks could lead to MOFs with new structures and functions, better and faster separation and purification of these materials, and processing methods that avoid pore blockage and pore collapse.
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 strategiesespecially 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.
A ZIF-8 thin film-based Fabry-Pérot device has been fabricated as a selective sensor for chemical vapors and gases. The preparation of the ZIF-8 thin film and a series of ZIF-8 thin films of various ...thicknesses grown on silicon substrates are presented.
UiO-66 is an archetypal zirconium-based metal–organic framework (MOF) that is constructed from hexanuclear zirconium oxide clusters as secondary building units (SBUs) and 1,4-benzenedicarboxylate ...(bdc) linkers. For the first time, a room-temperature solution-based synthesis is reported for UiO-66 and several of its derivatives, UiO-66-X (X = NH2, OH, or NO2), resulting in materials that are as porous and crystalline as those made at elevated temperatures. In addition, via modulation of the temperature at which UiO-66 is synthesized, the number of defect sites can be varied. It was found through N2 sorption isotherm analysis and potentiometric acid–base titrations that increasing the synthesis temperature from 25 to 130 °C results in a systematic decrease in the number of defect sites in UiO-66. The results suggest that, with respect to this synthetic procedure, a maximal number of defect sites is achieved ∼1.3 missing linkers per Zr6O4(OH)4(bdc)6 at a temperature of 45 °C.
Organophoshorus nerve agents are among the most toxic chemicals known to humans, and because of their unfortunate recent use despite international bans, there is an urgent need to develop materials ...that can effectively degrade these nerve agents. Within the past decade, zirconium-based metal–organic frameworks (Zr-MOFs) have emerged as a bioinspired class of materials capable of rapidly hydrolyzing these compounds and significantly diminishing their toxicity. Both experimental and computational insights have guided the design of Zr-MOFs, leading to the development of catalysts capable of detoxifying nerve agents and simulants, chemicals with similar functionality but lower toxicity, via hydrolysis within seconds in basic aqueous solutions. While these systems are acceptable for the elimination of stockpile weapons, translating this catalytic performance to filters incorporating Zr-MOFs that can be used in masks or protective clothing is not trivial. As such, a large area of focus recently has been targeted toward integrating these hydrolysis catalysts into protective clothing and gear while retaining the performance from solution-based catalytic systems. This Forum Article provides an overview of the development of Zr-MOFs for the catalytic hydrolysis of organophosphorus substrates, including design principles and mechanistic insights for both solution-based and textile-coated systems. Finally, we highlight the remaining challenges yet to be addressed and offer perspectives on the future directions for this field.
We have examined the methane uptake properties of six of the most promising metal organic framework (MOF) materials: PCN-14, UTSA-20, HKUST-1, Ni-MOF-74 (Ni-CPO-27), NU-111, and NU-125. We discovered ...that HKUST-1, a material that is commercially available in gram scale, exhibits a room-temperature volumetric methane uptake that exceeds any value reported to date. The total uptake is about 230 cc(STP)/cc at 35 bar and 270 cc(STP)/cc at 65 bar, which meets the new volumetric target recently set by the Department of Energy (DOE) if the packing efficiency loss is ignored. We emphasize that MOFs with high surface areas and pore volumes perform better overall. NU-111, for example, reaches ∼75% of both the gravimetric and the volumetric targets. We find that values for gravimetric uptake, pore volume, and inverse density of the MOFs we studied scale essentially linearly with surface area. From this linear dependence, we estimate that a MOF with surface area 7500 m2/g and pore volume 3.2 cc/g could reach the current DOE gravimetric target of 0.5 g/g while simultaneously exhibiting around ∼200 cc/cc volumetric uptake. We note that while values for volumetric uptake are based on ideal single crystal densities, in reality the packing densities of MOFs are much lower. Finally, we show that compacting HKUST-1 into wafer shapes partially collapses the framework, decreasing both volumetric and gravimetric uptake significantly. Hence, one of the important challenges going forward is to find ways to pack MOFs efficiently without serious damage or to synthesize MOFs that can withstand substantial mechanical pressure.
Metal‐organic frameworks (MOFs) have gained considerable attention as hybrid materials—in part because of a multitude of potential useful applications, ranging from gas separation to catalysis and ...light harvesting. Unfortunately, de novo synthesis of MOFs with desirable function–property combinations is not always reliable and may suffer from vagaries such as formation of undesirable topologies, low solubility of precursors, and loss of functionality of the sensitive network components. The recently discovered synthetic approach coined solvent‐assisted linker exchange (SALE) constitutes a simple to implement strategy for circumventing these setbacks; its use has already led to the generation of a variety of MOF materials previously unobtainable by direct synthesis methods. This Review provides a perspective of the achievements in MOF research that have been made possible with SALE and examines the studies that have facilitated the understanding and broadened the scope of use of this invaluable synthetic tool.
Changing pillars: Solvent‐assisted linker exchange (SALE) has gained a lot of attention as a novel synthetic pathway toward metal–organic frameworks (MOFs) that are challenging to access de novo. This Review analyzes the recent advances in the application of SALE and provides a critical assessment of the studies that have facilitated our understanding of this invaluable tool for the development of new MOFs.
Porous organic polymers (POPs), a class of highly cross-linked, amorphous polymers possessing micropores, have recently emerged as a versatile platform for the deployment of catalysts. These ...materials can be divided into three major classes: POPs that incorporate rigid well-defined homogeneous catalysts as building blocks, POPs that can be modified post-synthesis, and POPs that encapsulate metal particles. This perspective article summarizes the recent developments in POP-based catalysis and outlines the potential of POPs as platforms of heterogeneous catalysts along with some of the challenges.
A critical review of the emerging field of MOFs for photon collection and subsequent energy transfer is presented. Discussed are examples involving MOFs for (a) light harvesting, using (i) ...MOF-quantum dots and molecular chromophores, (ii) chromophoric MOFs, and (iii) MOFs with light-harvesting properties, and (b) energy transfer, specifically via the (i) Förster energy transfer and (ii) Dexter exchange mechanism.
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