Designer Metal–Organic Frameworks for Size-Exclusion-Based Hydrocarbon Separations: Progress and Challenges

The separation of hydrocarbons is of primary importance in the petrochemical industry but remains a challenging process. Hydrocarbon separations have traditionally relied predominantly on costly and energy-intensive heat-driven procedures such as low-temperature distillations. Adsorptive separation based on porous solids represents an alternative technology that is potentially more energy efficient for the separation of some hydrocarbons. Great efforts have been made recently not only in the development of adsorbents with optimal separation performance but also toward the subsequent implementation of adsorption-based separation technology. Emerging as a relatively new class of multifunctional porous materials, metal–organic frameworks (MOFs) hold substantial promise as adsorbents for highly efficient separation of hydrocarbons. This is because of their exceptional and intrinsic porosity tunability, which enables size-exclusion-based separations that render the highest possible separation selectivity. In this review, recent advances in the development of MOFs for separation of selected groups of hydrocarbons are reviewed, including methane/C₂ hydrocarbons, normal alkanes, alkane isomers, and alkane/alkene/alkyne and C₈ alkylaromatics, with a particular focus on separations based on the size-exclusion mechanism. Insights into tailor-made structures, material design strategies, and structure–property relationships will be elucidated. In addition, the existing challenges and possible future directions of this important research field will be discussed.     

Integration of Metal–Organic Frameworks and Metals: Synergy for Electrocatalysis

Electrocatalysis is a highly promising technology widely used in clean energy conversion. There is a continuing need to develop advanced electrocatalysts to catalyze the critical electrochemical reactions. Integrating metal active species, including various metal nanostructures (NSs) and atomically dispersed metal sites (ADMSs), into metal–organic frameworks (MOFs) leads to the formation of promising heterogeneous electrocatalysts that take advantage of both components. Among them, MOFs can provide support and protection for the active sites on guest metals, and the resulting host-guest interactions can synergistically enhance the electrocatalytic performance. In this review, three key concerns on MOF-metal heterogeneous electrocatalysts regarding the catalytic sites, conductivity, and catalytic stability are first presented. Then, rational integration strategies of MOFs and metals, including the integration of metal NSs via surface anchoring, space confining, and MOF coating, as well as the integration of ADMSs either with the metal nodes/linkers or within the pores of MOFs, along with their recent progress on synergistic cooperation for specific electrochemical reactions are summarized. Finally, current challenges and possible solutions in applying these increasingly concerned electrocatalysts are also provided.     

Metal–Organic Frameworks Based on Group 3 and 4 Metals

Metal–organic frameworks (MOFs) based on group 3 and 4 metals are considered as the most promising MOFs for varying practical applications including water adsorption, carbon conversion, and biomedical applications. The relatively strong coordination bonds and versatile coordination modes within these MOFs endow the framework with high chemical stability, diverse structures and topologies, and interesting properties and functions. Herein, the significant progress made on this series of MOFs since 2018 is summarized and an update on the current status and future trends on the structural design of robust MOFs with high connectivity is provided. Cluster chemistry involving Y, lanthanides (Ln, from La to Lu), actinides (An, from Ac to Lr), Ti, and Zr is initially introduced. This is followed by a review of recently developed MOFs based on group 3 and 4 metals with their structures discussed based on the types of inorganic or organic building blocks. The novel properties and arising applications of these MOFs in catalysis, adsorption and separation, delivery, and sensing are highlighted. Overall, this review is expected to provide a timely summary on MOFs based on group 3 and 4 metals, which shall guide the future discovery and development of stable and functional MOFs for practical applications.     

Fundamental Perspectives on the Electrochemical Water Applications of Metal–Organic Frameworks

Metal–organic frameworks(MOFs), a family of highly porous materials possessing huge surface areas and feasible chemical tunability, are emerging as critical functional materials to solve the growing challenges associated with energy–water systems, such as water scarcity issues. In this contribution, the roles of MOFs are highlighted in electrochemical-based water applications(i.e., reactions, sensing, and separations), where MOF-based functional materials exhibit outstanding performances in dete...     

Phosphonate Metal–Organic Frameworks: A Novel Family of Semiconductors

Herein, the first semiconducting and magnetic phosphonate metal–organic framework (MOF), TUB75, is reported, which contains a 1D inorganic building unit composed of a zigzag chain of corner-sharing copper dimers. The solid-state UV–vis spectrum of TUB75 reveals the existence of a narrow bandgap of 1.4 eV, which agrees well with the density functional theory (DFT)-calculated bandgap of 1.77 eV. Single-crystal conductivity measurements for different orientations of the individual crystals yield a range of conductances from 10⁻³ to 10³ S m⁻¹ at room temperature, pointing to the directional nature of the electrical conductivity in TUB75. Magnetization measurements show that TUB75 is composed of antiferromagnetically coupled copper dimer chains. Due to their rich structural chemistry and exceptionally high thermal/chemical stabilities, phosphonate MOFs like TUB75 may open new vistas in engineerable electrodes for supercapacitors.     

Photo-Induced Construction and Recovery of Cu+ Sites in Metal–Organic Frameworks

The adjustment of the valence state of metal ions is crucial for various applications because peculiar activity originates from metal ions with specific valence. Cu⁺ can interact with molecules possessing unsaturated bonds like CO via π-complexation, while Cu²⁺ doesn't have such ability. Meanwhile, Cu⁺ sites are easily oxidized to Cu²⁺, leading to the loss of activity. Despite great efforts, the development of a facile method to construct and recover Cu⁺ sites remains a pronounced challenge. Here, for the first time a facile photo-induced strategy is reported to fabricate Cu⁺ sites in metal–organic frameworks (MOFs) and recover Cu⁺ after oxidation. The Cu²⁺ precursor was loaded on NH₂-MIL-125, a typical visible-light responsive Ti-based MOF. Visible light irradiation triggers the formation of Ti³⁺ from Ti⁴⁺ in framework, which reduces the supported Cu²⁺ in the absence of any additional reducing agent, thus simplifying the process for Cu⁺ generation significantly. Due to π-complexation interaction, the presence of Cu⁺ results in remarkably enhanced CO capture capacity (1.16 mmol g⁻¹) compared to NH₂-MIL-125 (0.49 mmol g⁻¹). More importantly, Cu⁺ can be recovered conveniently via re-irradiation when it is oxidized to Cu²⁺, and the oxidation-recovery process is reversible.     

Metal–Organic Framework Materials for the Separation and Purification of Light Hydrocarbons

The separation and purification of light hydrocarbons (LHs) mixtures is one of the most significantly important but energy demanding processes in the petrochemical industry. As an alternative technology to energy intensive traditional separation methods, such as distillation, absorption, extraction, etc., adsorptive separation using selective solid adsorbents could potentially not only lower energy cost but also offer higher efficiency. The need to develop solid materials for the efficiently selective adsorption of LHs molecules, under mild conditions, is therefore of paramount importance and urgency. Metal–organic frameworks (MOFs), emerging as a relatively new class of porous organic–inorganic hybrid materials, have shown promise for addressing this challenging task due to their unparalleled features. Herein, recent advances of using MOFs as separating agents for the separation and purification of LHs, including the purification of CH₄, and the separations of alkynes/alkenes, alkanes/alkenes, C₅–C₆–C₇ normal/isoalkanes, and C₈ alkylaromatics, are summarized. The relationships among the structural and compositional features of the newly synthesized MOF materials and their separation properties and mechanisms are highlighted. Finally, the existing challenges and possible research directions related to the further exploration of porous MOFs in this very active field are also discussed.     

An Overview of Metal–Organic Frameworks for Green Chemical Engineering

Given the current global energy and environmental issues resulting from the fast pace of industrialization, the discovery of new functional materials has become increasingly imperative in order to advance science and technology and address the associated challenges. The boom in metal–organic frameworks (MOFs) and MOF-derived materials in recent years has stimulated profound interest in exploring their structures and applications. The preparation, characterization, and processing of MOF materials are the basis of their full engagement in industrial implementation. With intensive research in these topics, it is time to promote the practical utilization of MOFs on an industrial scale, such as for green chemical engineering, by taking advantage of their superior functions. Many famous MOFs have already demonstrated superiority over traditional materials in solving real-world problems. This review starts with the basic concept of MOF chemistry and ends with a discussion of the industrial production and exploitation of MOFs in several fields. Its goal is to provide a general scope of application to inspire MOF researchers to convert their focus on academic research to one on practical applications. After the obstacles of cost, scale-up preparation, processability, and stability have been overcome, MOFs and MOF-based devices will gradually enter the factory, become a part of our daily lives, and help to create a future based on green production and green living.     

A Self-Templated Design Approach toward Multivariate Metal–Organic Frameworks for Enhanced Oxygen Evolution

Multivariate metal–organic framework (MOF) is an ideal electrocatalytic material due to the synergistic effect of multiple metal active sites. In this study, a series of ternary M-NiMOF (M = Co, Cu) through a simple self-templated strategy that the Co/Cu MOF isomorphically grows in situ on the surface of NiMOF is designed. Owing to the electron rearrange of adjacent metals, the ternary CoCu-NiMOFs demonstrate the improved intrinsic electrocatalytic activity. At optimized conditions, the ternary Co₃Cu-Ni₂MOFs nanosheets give the excellent oxygen evolution reaction (OER) performance of current density of 10 mA cm⁻² at low overpotential of 288 mV with a Tafel slope of 87 mV dec⁻¹, which is superior to that of bimetallic nanosheet and ternary microflowers. The low free energy change of potential-determining step identifies that the OER process is favorable at Cu–Co concerted sites along with strong synergistic effect of Ni nodes. Partially oxidized metal sites also reduce the electron density, thus accelerating the OER catalytic rate. The self-templated strategy provides a universal tool to design multivariate MOF electrocatalysts for highly efficient energy transduction.     

Rare-Earth–Modified Metal–Organic Frameworks and Derivatives for Photo/Electrocatalysis

Metal–organic frameworks (MOFs) and their derivatives have attracted much attention in the field of photo/electrocatalysis owing to their ultrahigh porosity, tunable properties, and superior coordination ability. Regulating the valence electronic structure and coordination environment of MOFs is an effective way to enhance their intrinsic catalytic performance. Rare earth (RE) elements with 4f orbital occupancy provide an opportunity to evoke electron rearrangement, accelerate charged carrier transport, and synergize the surface adsorption of catalysts. Therefore, the integration of RE with MOFs makes it possible to optimize their electronic structure and coordination environment, resulting in enhanced catalytic performance. In this review, progress in current research on the use of RE-modified MOFs and their derivatives for photo/electrocatalysis is summarized and discussed. First, the theoretical advantages of RE in MOF modification are introduced, with a focus on the roles of 4f orbital occupancy and RE ion organic coordination ligands. Then, the application of RE-modified MOFs and their derivatives in photo/electrocatalysis is systematically discussed. Finally, research challenges, future opportunities, and prospects for RE-MOFs are also discussed.     

Structure Engineering of a Lanthanide-Based Metal–Organic Framework for the Regulation of Dynamic Ranges and Sensitivities for Pheochromocytoma Diagnosis

Exploring innovative technologies to precisely quantify biomolecules is crucial but remains a great challenge for disease diagnosis. Unfortunately, the humoral concentrations of most biotargets generally vary within rather limited scopes between normal and pathological states, while most literature-reported biosensors can detect large spans of targets concentrations, but are less sensitive to small concentration changes, which consequently make them mostly unsatisfactory or even unreliable in distinguishing positives from negatives. Herein, a novel strategy of precisely quantifying the small concentration changes of a certain biotarget by editing the dynamic ranges and sensitivities of a lanthanide-based metal–organic framework (Eu-ZnMOF) biosensor is reported. By elaborately tailoring the biosensor's structure and surface areas, the tunable Eu-ZnMOF is developed with remarkably enhanced response slope within the “optimized useful detection window,” enabling it to serve as a powerful signal amplifier (87.2-fold increase) for discriminating the small concentration variation of urinary vanillylmandelic acid (an early pathological signature of pheochromocytoma) within only three times between healthy and diseased subjects. This study provides a facile approach to edit the biosensors' performances through structure engineering, and exhibits promising perspectives for future clinical application in the non-invasive and accurate diagnosis of severe diseases.     

Energetic Metal–Organic Frameworks: Thermal Behaviors and Combustion of Nickel Oxide (II) Based on Activated Carbon Compositions

Journal of Engineering Physics and Thermophysics - Ammonium nitrate (AN) extensively used as an oxidizer in energetic compositions is a promising compound as a propellant and gas generator. It is...     

Near-Infrared Light-Mediated Cyclodextrin Metal–Organic Frameworks for Synergistic Antibacterial and Anti-Biofilm Therapies

Bacterial infections pose a significant threat to global public health; therefore, the development of novel therapeutics is urgently needed. Herein, a controllable antibacterial nanoplatform utilizing cyclodextrin metal–organic frameworks (CD-MOFs) as a template to synthesize ultrafine silver nanoparticles (Ag NPs) in their porous structure is constructed. Subsequently, polydopamine (PDA) is encapsulated on the CD-MOFs’ surface via dopamine polymerization to enhance the water stability and enable hyperthermia capacity. The resulting Ag@MOF@PDA generates localized hyperthermia and gradually releases Ag⁺ to achieve long-term photothermal-chemical bactericidal capability. The release rate of Ag⁺ can be accelerated by NIR-mediated heating in a controllable manner, quickly reaching the effective concentration and reducing the frequency of medication to avoid potential toxicity. In vitro experiments demonstrate that the combined antibacterial strategy can not only effectively kill both gram-negative and gram-positive bacteria, but also directly eradicate mature biofilms. In vivo results confirm that both bacterial- and biofilm-infected wounds treated with a combination of Ag@MOF@PDA and laser exhibit satisfactory recovery with minimal toxicity, displaying a superior therapeutic effect compared to other groups. Together, the results warrant that the Ag@MOF@PDA realizes synergistic antibacterial capacity and controllable release of Ag⁺ to combat bacterial and biofilm infections, providing a potential antibiotic-free alternative in the “post-antibiotic era.”     

Tailoring the Nanoporosity and Photoactivity of Metal–Organic Frameworks With Rigid Dye Modulators for Toluene Purification

Facile synthesis of hierarchically porous metal–organic frameworks (MOFs) with adjustable porosity and high crystallinity attracts great attention yet remains challenging. Herein, a micromolar amount of dye-based modulator (Rhodamine B (RhB)) is employed to easily and controllably tailor the pore size of a Ti-based metal–organic framework (MIL-125-NH₂). The RhB used in this method is easily removed by washing or photodegradation, avoiding secondary posttreatment. It is demonstrated that the carboxyl functional group and the steric effects of RhB are indispensable for enlarging the pore size of the MIL-125-NH₂. The resulting hierarchically porous MIL-125-NH₂ (RH-MIL-125-NH₂) exhibits optimized adsorption and photocatalytic activity because the newly formed mesopore with defects concurrently facilitates mass transport of guest molecules (toluene) and photogenerated charge separation. This work offers a meaningful basis for the construction of hierarchically porous MOFs and demonstrates the superiority of the hierarchical pore structure for adsorption and heterogeneous catalysis.     

Super Proton Conductivity Through Control of Hydrogen-Bonding Networks in Flexible Metal–Organic Frameworks

Metal–organic frameworks (MOFs) have received much attention as a solid-state electrolyte in proton exchange membrane fuel cells. The introduction of proton carriers and functional groups into MOFs can improve the proton conductivity attributed to the formation of hydrogen-bonding networks, while the underlying synergistic mechanism is still unclear. Here, a series of flexible MOFs (MIL-88B, [Fe₃O(OH)(H₂O)₂(O₂C-C₆H₄-CO₂)₃] with imidazole) is designed to modify the hydrogen-bonding networks and investigate the resulting proton-conducting characteristics by controlling the breathing behaviors. The breathing behavior is tuned by varying the amount of adsorbed imidazole into pore (small breathing (SB) and large breathing (LB)) and introducing functional groups onto ligands (-NH₂, -SO₃H), resulting in four kinds of imidazole-loaded MOFs−Im@MIL-88B-SB, Im@MIL-88B-LB, Im@MIL-88B-NH₂, and Im@MIL-88B-SO₃H. Im@MIL-88B-LB without functional groups exhibits the highest proton conductivity of 8.93 × 10⁻² S cm⁻¹ at 60 °C and 95% relative humidity among imidazole-loaded proton conductors despite the mild condition, indicating that functional groups may not be always required to enhance proton conductivity. The elaborately controlled pore size and host–guest interaction in flexible MOFs through imidazole-dependent structural transformation are translated into the high proton concentration without the limitation of proton mobility, contributing to the formation of effective hydrogen-bonding networks in imidazole conducting media.     

Water-Stable Fluorous Metal–Organic Frameworks with Open Metal Sites and Amine Groups for Efficient Urea Electrocatalytic Oxidation

Urea oxidation reaction (UOR) is one of the promising alternative anodic reactions to water oxidation that has attracted extensive attention in green hydrogen production. The application of specifically designed electrocatalysts capable of declining energy consumption and environmental consequences is one of the major challenges in this field. Therefore, the goal is to achieve a resistant, low-cost, and environmentally friendly electrocatalyst. Herein, a water-stable fluorinated Cu(II) metalorganic framework (MOF) {[Cu₂(L)(H₂O)₂]·(5DMF)(4H₂O)}_n (Cu-FMOF-NH₂; H₄L = 3,5-bis(2,4-dicarboxylic acid)-4-(trifluoromethyl)aniline) is developed utilizing an angular tetracarboxylic acid ligand that incorporates both trifluoromethyl (–CF₃) and amine (–NH₂) groups. The tailored structure of Cu-FMOF-NH₂ where linkers are connected by fluoride bridges and surrounded by dicopper nodes reveals a 4,24T1 topology. When employed as electrocatalyst, Cu-FMOF-NH₂ requires only 1.31 V versus reversible hydrogen electrode (RHE) to deliver 10 mA cm⁻² current density in 1.0 m KOH with 0.33 m urea electrolyte and delivered an even higher current density (50 mA cm⁻²) at 1.47 V versus RHE. This performance is superior to several reported catalysts including commercial RuO₂ catalyst with overpotential of 1.52 V versus RHE. This investigation opens new opportunities to develop and utilize pristine MOFs as a potential electrocatalyst for various catalytic reactions.     

The Effect of Linker-to-Metal Energy Transfer on the Photooxidation Performance of an Isostructural Series of Pyrene-Based Rare-Earth Metal–Organic Frameworks

The tetratopic linker, 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H₄TBAPy) along with rare-earth (RE) ions is used for the synthesis of 9 isostructures of a metal–organic framework (MOF) with shp topology, named RE-CU-10 (RE = Y(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), and Lu(III)). The synthesis of each RE-CU-10 analogue requires different reaction conditions to achieve phase pure products. Single crystal X-ray diffraction indicates the presence of a RE₉-cluster in Y- to Tm-CU-10, while a RE₁₁-cluster is observed for Yb- and Lu-CU-10. The photooxidation performance of RE-CU-10 analogues is evaluated, observing competition between linker-to-metal energy transfer versus the generation of singlet oxygen. The singlet oxygen produced is used to detoxify a mustard gas simulant 2-chloroethylethyl sulfide, with half-lives ranging from 4.0 to 5.8 min, some of the fastest reported to date using UV-irradiation and < 1 mol% catalyst, in methanol under O₂ saturation.     

2D Metal–Organic Frameworks as Competent Electrocatalysts for Water Splitting

Hydrogen, a clean and flexible energy carrier, can be efficiently produced by electrocatalytic water splitting. To accelerate the sluggish hydrogen evolution reaction and oxygen evolution reaction kinetics in the splitting process, highly active electrocatalysts are essential for lowering the energy barriers, thereby improving the efficiency of overall water splitting. Combining the distinctive advantages of metal–organic frameworks (MOFs) with the physicochemical properties of 2D materials such as large surface area, tunable structure, accessible active sites, and enhanced conductivity, 2D MOFs have attracted intensive attention in the field of electrocatalysis. Different strategies, such as improving the conductivities of MOFs, reducing the thicknesses of MOF nanosheets, and integrating MOFs with conductive particles or substrates, are developed to promote the catalytic performances of pristine MOFs. This review summarizes the recent advances of pristine 2D MOF-based electrocatalysts for water electrolysis. In particular, their intrinsic electrocatalytic properties are detailly analyzed to reveal important roles of inherent MOF active centers, or other in situ generated active phases from MOFs responsible for the catalytic reactions. Finally, the challenges and development prospects of pristine 2D MOFs for the future applications in overall water splitting are discussed.     

Development and Prospect on Optimum Design of Metal Forming Process

1.IntroductionThe research of metal forming theoryand technology always takes an importantplace in materials science and engineering.However,in a long period,the developmentof technology goes ahead of the theoreticalstudy.Most design principles used in prac-