Site-Directed Synthesis of Cobalt Oxide Clusters in a Metal-Organic Framework

Design and Advanced Manufacturing of NU-1000 Metal-Organic Frameworks with Future Perspectives for Environmental and Renewable Energy Applications

Metal-organic frameworks (MOFs) represent a relatively new family of materials that attract lots of attention thanks to their unique features such as hierarchical porosity, active metal centers, versatility of linkers/metal nodes, and large surface area. Among the extended list of MOFs, Zr-based-MOFs demonstrate comparably superior chemical and thermal stabilities, making them ideal candidates for energy and environmental applications. As a Zr-MOF, NU-1000 is first synthesized at Northwestern University. A comprehensive review of various approaches to the synthesis of NU-1000 MOFs for obtaining unique surface properties (e.g., diverse surface morphologies, large surface area, and particular pore size distribution) and their applications in the catalysis (electro-, and photo-catalysis), CO2 reduction, batteries, hydrogen storage, gas storage/separation, and other environmental fields are presented. The review further outlines the current challenges in the development of NU-1000 MOFs and their derivatives in practical applications, revealing areas for future investigation.

Conceptual and Practical Aspects of Metal-Organic Frameworks for Solid-Gas Reactions

The presence of site-isolated and well-defined metal sites has enabled the use of metal-organic frameworks (MOFs) as catalysts that can be rationally modulated. Because MOFs can be addressed and manipulated through molecular synthetic pathways, they are chemically similar to molecular catalysts. They are, nevertheless, solid-state materials and therefore can be thought of as privileged solid molecular catalysts that excel in applications involving gas-phase reactions. This contrasts with homogeneous catalysts, which are overwhelmingly used in the solution phase. Herein, we review theories dictating gas phase reactivity within porous solids and discuss key catalytic gas-solid reactions. We further treat theoretical aspects of diffusion within confined pores, the enrichment of adsorbates, the types of solvation spheres that a MOF might impart on adsorbates, definitions of acidity/basicity in the absence of solvent, the stabilization of reactive intermediates, and the generation and characterization of defect sites. The key catalytic reactions we discuss broadly include reductive reactions (olefin hydrogenation, semihydrogenation, and selective catalytic reduction), oxidative reactions (oxygenation of hydrocarbons, oxidative dehydrogenation, and carbon monoxide oxidation), and C-C bond forming reactions (olefin dimerization/polymerization, isomerization, and carbonylation reactions).

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization

A defining characteristic of nearly all catalytically functional MOFs is uniform, molecular-scale porosity. MOF pores, linkers and nodes that define them, help regulate reactant and product transport, catalyst siting, catalyst accessibility, catalyst stability, catalyst activity, co-catalyst proximity, composition of the chemical environment at and beyond the catalytic active site, chemical intermediate and transition-state conformations, thermodynamic affinity of molecular guests for MOF interior sites, framework charge and density of charge-compensating ions, pore hydrophobicity/hydrophilicity, pore and channel rigidity vs. flexibility, and other features and properties. Collectively and individually, these properties help define overall catalyst functional behaviour. This review focuses on how porous, catalyst-containing MOFs capitalize on molecular-scale confinement, containment, isolation, environment modulation, energy delivery, and mobility to accomplish desired chemical transformations with potentially superior selectivity or other efficacy, especially in comparison to catalysts in homogeneous solution environments.

Temperature-Dependent Nitrous Oxide/Carbon Dioxide Preferential Adsorption in a Thiazolium-Functionalized NU-1000 Metal-Organic Framework

Solvent-assisted ligand incorporation (SALI) of the ditopic linker 5-carboxy-3-(4-carboxybenzyl)thiazolium bromide [(H₂PhTz)Br] into the zirconium metal-organic framework NU-1000 [Zr₆O₄(OH)₈(H₂O)₄(TBAPy)₂, where NU = Northwestern University and H₄TBAPy = 1,3,6,8-tetrakis(p-benzoic-acid)pyrene], led to the SALIed NU-1000-PhTz material of minimal formula [Zr₆O₄(OH)₆(H₂O)₂(TBAPy)₂(PhTz)]Br. NU-1000-PhTz has been thoroughly characterized in the solid state. As confirmed by powder X-ray diffraction, this material keeps the same three-dimensional architecture of NU-1000 and the dicarboxylic extra linker bridges adjacent [Zr₆] nodes ca. 8 Å far apart along the crystallographic c-axis. The functionalized MOF has a BET specific surface area of 1560 m²/g, and it is featured by a slightly higher thermal stability than its parent material (T_dec = 820 vs. 800 K, respectively). NU-1000-PhTz has been exploited for the capture and separation of two pollutant gases: carbon dioxide (CO₂) and nitrous oxide (N₂O). The high thermodynamic affinity for both gases [isosteric heat of adsorption (Q_st) = 25 and 27 kJ mol^-1 for CO₂ and N₂O, respectively] reasonably stems from the strong interactions between these (polar) "stick-like" molecules and the ionic framework. Intriguingly, NU-1000-PhTz shows an unprecedented temperature-dependent adsorption capacity, loading more N₂O in the 298 K ≤ T ≤ 313 K range but more CO₂ at temperatures falling out of this range. Grand canonical Monte Carlo simulations of the adsorption isotherms confirmed that the preferential adsorption sites of both gases are the triangular channels (micropores) in close proximity to the polar pillar. While CO₂ interacts with the thiazolium ring in an "end-on" fashion through its O atoms, N₂O adopts a "side-on" configuration through its three atoms simultaneously. These findings open new horizons in the discovery of functional materials that may discriminate between polluting gases through selective adsorption at different temperatures.

Ammonia Capture within Zirconium Metal-Organic Frameworks: Reversible and Irreversible Uptake

Ammonia uptake by high-capacity and high-porosity sorbents is a promising approach to its storage and release, capture and mitigation, and chemical separation. Here, we examined the ammonia sorption behavior of several versions of an archetypal zirconium-based metal-organic framework (MOF) material, NU-1000-a meso- and microporous crystalline compound having the empirical formula (1,3,6,8-tetrakis(p-benzoate)pyrene)₂ Zr₆(μ₃-O)₄(μ₃-OH)₄(H₂O)₄(OH)₄ with linkers and nodes arranged to satisfy a csq topology. Depending on the thermal treatment protocol used prior to sorption measurements, ammonia can physisorb to NU-1000 via hydrogen-bonding and London-dispersion interactions and chemisorb via Brønsted acid-base reactions with node-integrated proton donors (μ₃-hydroxos) and node-ligated proton donors (terminal hydroxos), via simple coordination at open Zr(IV) sites, or via dissociative coordination to Zr(IV) as NH₂^- and protonation of a node-based μ₃-oxo. Ammonia adsorption occurs via both reversible and irreversible processes. The latter are of particular interest for protection and mitigation. Notably, the unexpected dissociative adsorption occurs only with nodes that have been fully dehydrated and irreversibly structurally distorted via thermal pre-treatment-a finding that is supported by density functional theory calculations. Differentiating and ranking the relative importance of the many modes of adsorption was facilitated, in part, by the availability of variants of NU-1000 that replace the majority of terminal aqua and hydroxo ligands with nonstructural formate ligands, auxiliary ditopic linkers, or both. The study provides insights into the chemical basis for both reversible and irreversible uptake of ammonia by Zr-MOFs and related compounds. The unexpectedly rich variety of sorption motifs suggest the criteria for designing or choosing MOFs that are optimal for specific ammonia-centric applications.

Pyrene-based metal organic frameworks: from synthesis to applications

Pyrene is one of the most widely investigated aromatic hydrocarbons given to its unique optical and electronic properties. Hence, pyrene-based ligands have been attractive for the synthesis of metal-organic frameworks (MOFs) in the last few years. In this review, we will focus on the most important characteristics of pyrene, in addition to the development and synthesis of pyrene-based molecules as bridging ligands to be used in MOF structures. We will summarize the synthesis attempts, as well as the post-synthetic modifications of pyrene-based MOFs by the incorporation of metals or ligands in the structure. The discussion of promising results of such MOFs in several applications; including luminescence, photocatalysis, adsorption and separation, heterogeneous catalysis, electrochemical applications and bio-medical applications will be highlighted. Finally, some insights and future prospects will be given based on the studies discussed in the review. This review will pave the way for the researchers in the field for the design and development of novel pyrene-based structures and their utilization for different applications.

Advanced applications of Zr-based MOFs in the removal of water pollutants

The global water pollution is caused by the increase of industrial and agricultural activities, which have produced various toxic pollutants. Pollutants in water generally consist of metal ions, pharmaceuticals and personal care products (PPCPs), oil spills, organic dyes, and other organic pollutants. Amongst the adsorbents that have been developed to deal with pollutants in water, Zr-based metal-organic frameworks (MOFs) have drawn scientists' great attention due to their excellent stability and adjustable functionalization. Herein, the present review article introduces the synthetic methods of functionalized Zr-based MOFs and summarizes their applications in water pollution treatment. It also clarifies the interactions and removal mechanisms between pollutants and Zr-based MOFs. The use of these MOFs with eminent adsorption ability and recycling performance have been discussed in detail. Zr-based MOFs also face some challenges such as high cost, lack of real water environment applications, selective removal of pollutants, and low ability to remove composite pollutants. Future research should focus on addressing these issues. Although there is still a blank of the practical utility of Zr-based MOFs on a commercial scale, the research reported to date clearly shows that they are very promising materials for the water treatment.

Propane dehydrogenation: catalyst development, new chemistry, and emerging technologies

This review describes recent advances in the propane dehydrogenation process in terms of emerging technologies, catalyst development and new chemistry.

Node-Accessible Zirconium MOFs

High-stability, zirconium-based metal-organic frameworks are attractive as heterogeneous catalysts and as model supports for uniform arrays of subsequently constructed heterogeneous catalysts-for example, MOF-node-grafted metal-oxy and metal-sulfur clusters. For hexa-Zr(IV)-MOFs characterized by nodes that are less than 12-connected, sites not used for linkers are ideally occupied by reactive and displaceable OH/H₂O pairs. The desired pairs are ideal for grafting the aforementioned catalytic clusters, while aqua-ligand lability renders them effective for exposing highly Lewis-acidic Zr(IV) sites (catalytic sites) to candidate reactants. New single-crystal X-ray studies of an eight-connected Zr-MOF, NU-1000, reveal that conventional activation fully removes modulator ligands, but replaces them with three node-blocking formate ligands (from solvent decomposition) and only one OH/H₂O pair, not four-a largely overlooked complication that now appears to be general for Zr-MOFs. Here we describe an alternative activation protocol that effectively removes modulators, avoids formate, and installs the full complement of terminal OH/H₂O pairs. It does so via an unusual isolatable intermediate featuring eight aqua ligands and four non-ligated chlorides-again as supported by single-crystal X-ray data. We find that complete replacement of node-blocking modulators/formate with the originally envisioned OH/OH₂ pairs has striking consequences; here we touch upon just three. First, elimination of unrecognized formate renders aqua ligands much more thermally labile, enabling open Zr(IV) sites to be obtained at lower temperature. Second, in the absence of formate, which otherwise links and locks pairs of node Zr(IV) ions, reversible removal of aqua ligands engenders reversible contraction of MOF meso- and micropores, as evidenced by X-ray diffraction. Third, formate replacement with OH/OH₂ pairs renders NU-1000 ca.10× more active for catalytic hydrolytic degradation of a representative simulant of G-type chemical warfare agents.

Regioselective Functionalization of the Mesoporous Metal-Organic Framework, NU-1000, with Photo-Active Tris-(2,2'-bipyridine)ruthenium(II)

Solvent-assisted ligand incorporation is an excellent method for the post-synthetic functionalization of Zr-based metal-organic frameworks (MOFs), as carboxylate-derivative functionalities readily coordinate to the Zr6 nodes by displacing node-based aqua and terminal hydroxo ligands. In this study, a photocatalytically active ruthenium complex RuII(bpy)2(dcbpy), that is, bis-(2,2'-bipyridine)-(4,4'-dicarboxy-2,2'-bipyridine)ruthenium, was installed in the mono-protonated (carboxylic acid) form within NU-1000 via SALI. Crystallographic information regarding the siting of the ruthenium complex within the MOF pores is obtained by difference envelope density analysis. The ruthenium-functionalized MOF, termed Ru-NU-1000, shows excellent heterogeneous photocatalytic activity for an oxidative amine coupling reaction.

Maximizing Magnetic Resonance Contrast in Gd(III) Nanoconjugates: Investigation of Proton Relaxation in Zirconium Metal-Organic Frameworks

Gadolinium(III) nanoconjugate contrast agents (CAs) provide significant advantages over small-molecule complexes for magnetic resonance imaging (MRI), namely increased Gd(III) payload and enhanced proton relaxation efficiency (relaxivity, r₁). Previous research has demonstrated that both the structure and surface chemistry of the nanomaterial substantially influence contrast. We hypothesized that inserting Gd(III) complexes in the pores of a metal-organic framework (MOF) might offer a unique strategy to further explore the parameters of nanomaterial structure and composition, which influence relaxivity. Herein, we postsynthetically incorporate Gd(III) complexes into Zr-MOFs using solvent-assisted ligand incorporation (SALI). Through the study of Zr-based MOFs, NU-1000 (nano and micronsize particles) and NU-901, we investigated the impact of particle size and pore shape on proton relaxivity. The SALI-functionalized Gd nano NU-1000 hybrid material displayed the highest loading of the Gd(III) complex (1.9 ± 0.1 complexes per node) and exhibited the most enhanced proton relaxivity (r₁ of 26 ± 1 mM^-1 s^-1 at 1.4 T). Based on nuclear magnetic relaxation dispersion (NMRD) analysis, we can attribute the performance of Gd nano NU-1000 to the nanoscale size of the MOF particles and larger pore size that allows for rapid water exchange. We have demonstrated that SALI is a promising method for incorporating Gd(III) complexes into MOF materials and identified crucial design parameters for the preparation of next generation Gd(III)-functionalized MOF MRI contrast agents.

Metal-Organic Framework-Based Catalysts with Single Metal Sites

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Bimetallic Metal-Organic Framework-Derived Hybrid Nanostructures as High-Performance Catalysts for Methane Dry Reforming

Syngas, consisting of equimolar CO and H₂, is an important feedstock for large-scale production of a wide range of commodity chemicals including aldehyde, methanol, ammonia, and other oxygenated chemicals. Dry reforming of methane (DRM), proceeding by reacting greenhouse gases, CO₂ and CH₄, at high temperatures in the presence of a metal catalyst, is considered one of the most environmentally friendly routes for syngas production. Nevertheless, nonprecious metal-based catalysts, which can operate at relatively low temperatures for high product yields and selectivities, are required to drive the DRM process for industrial applications effectively. Here, we developed NiCo@C nanocomposites from a corresponding NiCo-based bimetallic metal-organic framework (MOF) to serve as high-performance catalysts for the DRM process, achieving high turnover frequencies (TOF) at low temperatures (>5.7 s^-1 at 600 °C) and high product selectivities (H₂/CO = 0.9 at 700 °C). The incorporation of Co in Ni catalysts improves the operation stability and light-off stability. The present development for MOF-derived nanocomposites opens a new horizon for design of DRM catalysts.

New linker installation in metal–organic frameworks

Installation of new organic components (including multitopic anionic and neutral linkers) in MOFs and MOPs for multicomponent materials.

Tuning of the flexibility in metal–organic frameworks based on pendant arm macrocycles

An isostructural series of flexible MOFs based on pendant arm macrocycles was developed to tune flexibility depending on functional groups.