Interplay of Electronic and Steric Effects to Yield Low-Temperature CO Oxidation at Metal Single Sites in Defect-Engineered HKUST-1

Construction of the Z-Scheme Heterogeneous HKUST-1/BiVO4 Nanorod Composite for Enhanced Piezo-Photocatalytic Reduction Performance of Cr(VI)

In recent years, piezo-photocatalysis has become a promising strategy for solving environmental pollution problems by adding additional mechanical energy to the photocatalysis process. This work reported the effective synthesis of a variety of HKUST-1/BiVO4 heterogeneous materials by combining monoclinic BiVO4 and porous HKUST-1 semiconductors. The piezo-photocatalytic properties of HKUST-1/BiVO4 were studied by the reduction of hexavalent chromium (Cr(VI)) under visible-light irradiation and ultrasonic waves. In the piezo-photocatalysis process, the best reduction rates among as-prepared HKUST-1/BiVO4 composites were up to 96.20% of 10 ppm Cr(VI) solution, which was approximately 1.80 times that under visible light and about 4.13 times that under ultrasound. Under the action of the piezoelectric potential, the availability of free radicals increased the reduction rate of Cr(VI) and reached a synergistic effect of 1.14-fold.

Preparation of Free-Standing Defect-Free ZIF-8/PVA Membranes via Confined Reaction at the Quasi-Interface

Developing a facile strategy to synthesize free-standing defect-free metal-organic framework (MOF) membranes with high separation selectivity and good mechanical stability is very appealing but challenging. Herein, by confining the reaction of metal and ligand at the quasi-interface, a representative membrane composed of a continuous ZIF-8 layer and poly(vinyl alcohol) (PVA) was fabricated. The continuous ZIF-8 layer endowed the membrane with high separation efficiency, while PVA acted as a filler to eliminate the defection, synergistically achieving high selective ion transport and good mechanical stability. The continuous defect-free ZIF-8/PVA membrane showed excellent separation performance of selective ion transport with high Li+ permeance of 17.83 mol·m-2·h-1 as well as decent Li+/Mg2+ and Li+/Ca2+ selectivities of 24.60 and 244.58, respectively. The separation performance of the ZIF-8/PVA membrane remained stable after 10% strain, indicating its good mechanical stability. This work will promote the development of MOF-based membranes in practical applications.

Asymmetric Cu(I)─W Dual-Atomic Sites Enable C─C Coupling for Selective Photocatalytic CO2 Reduction to C2H4

Solar-driven CO2 reduction into value-added C2+ chemical fuels, such as C2H4, is promising in meeting the carbon-neutral future, yet the performance is usually hindered by the high energy barrier of the C─C coupling process. Here, an efficient and stabilized Cu(I) single atoms-modified W18O49 nanowires (Cu1/W18O49) photocatalyst with asymmetric Cu─W dual sites is reported for selective photocatalytic CO2 reduction to C2H4. The interconversion between W(V) and W(VI) in W18O49 ensures the stability of Cu(I) during the photocatalytic process. Under light irradiation, the optimal Cu1/W18O49 (3.6-Cu1/W18O49) catalyst exhibits concurrent high activity and selectivity toward C2H4 production, reaching a corresponding yield rate of 4.9 µmol g-1 h-1 and selectivity as high as 72.8%, respectively. Combined in situ spectroscopies and computational calculations reveal that Cu(I) single atoms stabilize the *CO intermediate, and the asymmetric Cu─W dual sites effectively reduce the energy barrier for the C─C coupling of two neighboring CO intermediates, enabling the highly selective C2H4 generation from CO2 photoreduction. This work demonstrates leveraging stabilized atomically-dispersed Cu(I) in asymmetric dual-sites for selective CO2-to-C2H4 conversion and can provide new insight into photocatalytic CO2 reduction to other targeted C2+ products through rational construction of active sites for C─C coupling.

Heterovalent Metal Pair Sites on Metal-Organic Framework Ordered Macropores for Multimolecular Co-Activation

The precise design of catalytic metal centers with multiple chemical states to facilitate sophisticated reactions involving multimolecular activation is highly desirable but challenging. Herein, we report an ordered macroporous catalyst with heterovalent metal pair (HMP) sites comprising CuII-CuI on the basis of a microporous metal-organic framework (MOF) system. This macroporous HMP catalyst with proximity heterovalent dual copper sites, whose distance is controlled to ∼2.6 Å, on macropore surface exhibits a co-activation behavior of ethanol at CuII and alkyne at CuI, and avoids microporous restriction, thereby promoting additive-free alkyne hydroboration reaction. The desired yield enhances dramatically compared with the pristine MOF and ordered macroporous MOF both with solely isovalent CuII-CuII sites. Density functional theory calculations reveal that the Cu-HMP sites can stabilize the Bpin-CuII-CuI-alkyne intermediate and facilitate C-B bond formation, resulting in a smooth alkyne hydroboration process. This work provides new perspectives to design multimolecular activation catalysts for sophisticated matter transformations.

Ultrastrong Electron-Phonon Coupling in Uranium-Organic Frameworks Leading to Inverse Luminescence Temperature Dependence

Electron-phonon interactions, crucial in condensed matter, are rarely seen in Metal-Organic Frameworks (MOFs). Detecting these interactions typically involves analyzing luminescence in lanthanide- or actinide-based compounds. Prior studies on Ln- and Ac-based MOFs at high temperatures revealed additional peaks, but these were too faint for thorough analysis. In our research, we fabricated a high-quality, crystalline uranium-based MOF (KIT-U-1) thin film using a layer-by-layer method. Under UV light, this film showed two distinct "hot bands," indicating a strong electron-phonon interaction. At 77 K, these bands were absent, but at 300 K, a new emission band appeared with half the intensity of the main luminescence. Surprisingly, a second hot band emerged above 320 K, deviating from previous findings in rare-earth compounds. We conducted a detailed ab-initio analysis employing time-dependent density functional theory to understand this unusual behaviour and to identify the lattice vibration responsible for the strong electron-phonon coupling. The KIT-U-1 film's hot-band emission was then utilized to create a highly sensitive, single-compound optical thermometer. This underscores the potential of high-quality MOF thin films in exploiting the unique luminescence of lanthanides and actinides for advanced applications.

Electrical Conductivity Boost: In Situ Polypyrrole Polymerization in Monolithically Integrated Surface-Supported Metal-Organic Framework Templates

Recent progress in synthesizing and integrating surface-supported metal-organic frameworks (SURMOFs) has highlighted their potential in developing hybrid electronic devices with exceptional mechanical flexibility, film processability, and cost-effectiveness. However, the low electrical conductivity of SURMOFs has limited their use in devices. To address this, researchers have utilized the porosity of SURMOFs to enhance electrical conductivity by incorporating conductive materials. This study introduces a method to improve the electrical conductivity of HKUST-1 templates by in situ polymerization of conductive polypyrrole (PPy) chains within the SURMOF pores (named as PPy@HKUST-1). Nanomembrane-origami technology is employed for integration, allowing a rolled-up metallic nanomembrane to contact the HKUST-1 films without causing damage. After a 24 h loading period, the electrical conductivity at room temperature reaches approximately 5.10-6 S m-1 . The nanomembrane-based contact enables reliable electrical characterization even at low temperatures. Key parameters of PPy@HKUST-1 films, such as trap barrier height, dielectric constant, and tunneling barrier height, are determined using established conduction mechanisms. These findings represent a significant advancement in real-time control of SURMOF conductivity, opening pathways for innovative electronic-optoelectronic device development. This study demonstrates the potential of SURMOFs to revolutionize hybrid electronic devices by enhancing electrical conductivity through intelligent integration strategies.

Atomic-level design of metalloenzyme-like active pockets in metal-organic frameworks for bioinspired catalysis

Natural metalloenzymes with astonishing reaction activity and specificity underpin essential life transformations. Nevertheless, enzymes only operate under mild conditions to keep sophisticated structures active, limiting their potential applications. Artificial metalloenzymes that recapitulate the catalytic activity of enzymes can not only circumvent the enzymatic fragility but also bring versatile functions into practice. Among them, metal-organic frameworks (MOFs) featuring diverse and site-isolated metal sites and supramolecular structures have emerged as promising candidates for metalloenzymes to move toward unparalleled properties and behaviour of enzymes. In this review, we systematically summarize the significant advances in MOF-based metalloenzyme mimics with a special emphasis on active pocket engineering at the atomic level, including primary catalytic sites and secondary coordination spheres. Then, the deep understanding of catalytic mechanisms and their advanced applications are discussed. Finally, a perspective on this emerging frontier research is provided to advance bioinspired catalysis.

Unlocking Solid-State Organometallic Photochemistry with Optically Transparent, Porous Salt Thin Films

Synthetic porous materials continue to garner attention as platforms for solid-state chemistry and as designer heterogeneous catalysts. Applications in photochemistry and photocatalysis, however, are plagued by poor light harvesting efficiency due to light scattering resulting from sample microcrystallinity and poor optical penetration that arises from inner filter effects. Here we demonstrate the layer-by-layer growth of optically transparent, photochemically active thin films of porous salts. Films are grown by sequential deposition of cationic Zr-based porous coordination cages and anionic Mn porphyrins. Photolysis facilitates the efficient reduction of Mn(III) sites to Mn(II) sites, which can be observed in real-time by transmission UV-vis spectroscopy. Film porosity enables substrate access to the Mn(II) sites and facilitates reversible O2 activation in the solid state. These results establish optically transparent, porous salt thin films as versatile platforms for solid-state photochemistry and in operando spectroscopy.

Superstructure-Induced Hierarchical Assemblies for Nanoconfined Photocatalysis

Most attempts to synthesize supramolecular nanosystems are limited to a single mechanism, often resulting in the formation of nanomaterials that lack diversity in properties. Herein, hierarchical assemblies with appropriate variety are fabricated in bulk via a superstructure-induced organic-inorganic hybrid strategy. The dynamic balance between substructures and superstructures is managed using covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) as dual building blocks to regulate the performances of hierarchical assemblies. Significantly, the superstructures resulting from the controlled cascade between COFs and MOFs create highly active photocatalytic systems through multiple topologies. Our designed tandem photocatalysis can precisely and efficiently regulate the conversion rates of bioactive molecules (benzo[d]imidazoles) through competing redox pathways. Furthermore, benzo[d]imidazoles catalyzed by such supramolecular nanosystems can be isolated in yields ranging from 70 % to 93 % within tens of minutes. The multilayered structural states within the supramolecular systems demonstrate the importance of hierarchical assemblies in facilitating photocatalytic propagation and expanding the structural repertoire of supramolecular hybrids.

Switching the Oxygen Evolution Mechanism on Atomically Dispersed Ru for Enhanced Acidic Reaction Kinetics

Designing stable single-atom electrocatalysts with lower energy barriers is urgent for the acidic oxygen evolution reaction. In particular, the atomic catalysts are highly dependent on the kinetically sluggish acid-base mechanism, limiting the reaction paths of intermediates. Herein, we successfully manipulate the steric localization of Ru single atoms at the Co3O4 surface to improve acidic oxygen evolution by precise control of the anchor sites. The delicate structure design can switch the reaction mechanism from the lattice oxygen mechanism (LOM) to the optimized adsorbate evolution mechanism (AEM). In particular, Ru atoms embedded into cation vacancies reveal an optimized mechanism that activates the proton donor-acceptor function (PDAM), demonstrating a new single-atom catalytic pathway to circumvent the classic scaling relationship. Steric interactions with intermediates at the anchored Ru-O-Co interface played a primary role in optimizing the intermediates' conformation and reducing the energy barrier. As a comparison, Ru atoms confined to the surface sites exhibit a lattice oxygen mechanism for the oxygen evolution process. As a result, the delicate atom control of the spatial position presents a 100-fold increase in mass activity from 36.96 A gRu(ads)-1 to 4012.11 A gRu(anc)-1 at 1.50 V. These findings offer new insights into the precise control of single-atom catalytic behavior.

MOF-based catalysts: insights into the chemical transformation of greenhouse and toxic gases

Metal-organic framework (MOF)-based catalysts are outstanding alternative materials for the chemical transformation of greenhouse and toxic gases into high-add-value products. MOF catalysts exhibit remarkable properties to host different active sites. The combination of catalytic properties of MOFs is mentioned in order to understand their application. Furthermore, the main catalytic reactions, which involve the chemical transformation of CH4, CO2, NOx, fluorinated gases, O3, CO, VOCs, and H2S, are highlighted. The main active centers and reaction conditions for these reactions are presented and discussed to understand the reaction mechanisms. Interestingly, implementing MOF materials as catalysts for toxic gas-phase reactions is a great opportunity to provide new alternatives to enhance the air quality of our planet.

Metal-Organic Framework Thin Films as Ideal Matrices for Azide Photolysis in Vacuum

Studies on reactions in solutions are often hampered by solvent effects. In addition, detailed investigation on kinetics is limited to the small temperature regime where the solvent is liquid. Here, we report the in situ spectroscopic observation of UV-induced photochemical reactions of aryl azides within a crystalline matrix in vacuum. The matrices are formed by attaching the reactive moieties to ditopic linkers, which are then assembled to yield metal-organic frameworks (MOFs) and surface-mounted MOFs (SURMOFs). These porous, crystalline frameworks are then used as model systems to study azide-related chemical processes under ultrahigh vacuum (UHV) conditions, where solvent effects can be safely excluded and in a large temperature regime. Infrared reflection absorption spectroscopy (IRRAS) allowed us to monitor the photoreaction of azide in SURMOFs precisely. The in situ IRRAS data, in conjunction with XRD, MS, and XPS, reveal that illumination with UV light first leads to forming a nitrene intermediate. In the second step, an intramolecular rearrangement occurs, yielding an indoloindole derivative. These findings unveil a novel pathway for precisely studying azide-related chemical transformations. Reference experiments carried out for solvent-loaded SURMOFs reveal a huge diversity of other reaction schemes, thus highlighting the need for model systems studied under UHV conditions.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

DDTC-Cu(I) based metal-organic framework (MOF) for targeted melanoma therapy by inducing SLC7A11/GPX4-mediated ferroptosis

Disulfiram (DSF), a drug for alcohol withdrawal, has attracted extensive scientific attention due to its potential to treat cancer. The metabolite of DSF, diethyl dithiocarbamate (DDTC), forms a Cu-DDTC complex in vivo with copper ions, which has been shown to be a proteasome inhibitor with high antitumor activity. However, the in vivo stability of Cu-DDTC complexes remains a challenge. In this study, the nanomedicine Cu-BTC@DDTC with high antitumor activity was prepared by using the nanoscale metal-organic framework (MOF) Cu-BTC as a carrier and loading diethyldithiocarbamate (DDTC) through coordination interaction. The results showed that Cu-BTC@DDTC had high drug loading and adequate stability, and exhibited DDTC-Cu(I) chemical valence characteristics and polycrystalline structure features. In vitro cytocompatibility investigation and animal xenograft tumor model evaluation demonstrated the anti-cancer potential of Cu-BTC@DDTC, especially the combination of Cu-BTC@DDTC with low-dose cisplatin showed significant antitumor effect and biosafety. This study provides a feasible protocol for developing antitumor drugs based on the drug repurposing strategy.

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).

Synergistic Effect of FeII and MnII Ions in Cyano-Bridged Heterometallic Coordination Polymers on Catalytic Selectivity of Benzene Oxygenation to Phenol

A cyano-bridged heterometallic coordination polymer with partial deficiencies of CN^- ligands, [Mn^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)], forms open metal sites both on Mn^II and Fe^II ions by liberation of monodentate ligands such as NH₃ and H₂O. [Mn^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] exhibits high catalytic activity and selectivity of benzene oxygenation to phenol in the presence of m-chloroperoxybenzoic acid as an oxidant. The postcatalytic spectroscopy of [Mn^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] and catalysis comparison with a physical mixture of [Mn^II(H₂O)₃]₂[Fe^II(CN)₆] and [Fe(H₂O)_3/2]_4/3[Fe(CN)₆], which has open metal sites on both Mn^II and Fe ions separately, indicated that the high activity resulted from high oxidation ability and phenol adsorption ability of Fe^II and Mn^II ions, respectively.

Reversible Switching CuII /CuI Single Sites Catalyze High-rate and Selective CO2 Photoreduction

Herein, we first design a model of reversible redox-switching metal-organic framework single-unit-cell sheets, where the abundant metal single sites benefit for highly selective CO₂ reduction, while the reversible redox-switching metal sites can effectively activate CO₂ molecules. Taking the synthetic Cu-MOF single-unit-cell sheets as an example, synchrotron-radiation quasi in situ X-ray photoelectron spectra unravel the reversible switching Cu^II /Cu^I single sites initially accept photoexcited electrons and then donate them to CO₂ molecules, which favors the rate-liming activation into CO₂ ^δ- , verified by in situ FTIR spectra and Gibbs free energy calculations. As an outcome, Cu-MOF single-unit-cell sheets achieve near 100 % selectivity for CO₂ photoreduction to CO with a high rate of 860 μmol g^-1  h^-1 without any sacrifice reagent or photosensitizer, where both the activity and selectivity outperform previously reported photocatalysts evaluated under similar conditions.

Broadband Dielectric Spectroscopic Detection of Ethanol: A Side-by-Side Comparison of ZnO and HKUST-1 MOFs as Sensing Media

The most common gas sensors are based on chemically induced changes in electrical resistivity and necessarily involve making imperfect electrical contacts to the sensing materials, which introduce errors into the measurements. We leverage thermal- and chemical-induced changes in microwave propagation characteristics (i.e., S-parameters) to compare ZnO and surface-anchored metal–organic-framework (HKUST-1 MOF) thin films as sensing materials for detecting ethanol vapor, a typical volatile organic compound (VOC), at low temperatures. We show that the microwave propagation technique can detect ethanol at relatively low temperatures (<100 °C), and afford new mechanistic insights that are inaccessible with the traditional dc-resistance-based measurements. In addition, the metrological technique avoids the inimical measurand distortions due to parasitic electrical effects inherent in the conductometric volatile organic compound detection.

Defects engineering simultaneously enhances activity and recyclability of MOFs in selective hydrogenation of biomass

The development of synthetic methodologies towards enhanced performance in biomass conversion is desirable due to the growing energy demand. Here we design two types of Ru impregnated MIL-100-Cr defect engineered metal-organic frameworks (Ru@DEMOFs) by incorporating defective ligands (DLs), aiming at highly efficient catalysts for biomass hydrogenation. Our results show that Ru@DEMOFs simultaneously exhibit boosted recyclability, selectivity and activity with the turnover frequency being about 10 times higher than the reported values of polymer supported Ru towards D-glucose hydrogenation. This work provides in-depth insights into (i) the evolution of various defects in the cationic framework upon DLs incorporation and Ru impregnation, (ii) the special effect of each type of defects on the electron density of Ru nanoparticles and activation of reactants, and (iii) the respective role of defects, confined Ru particles and metal single active sites in the catalytic performance of Ru@DEMOFs for D-glucose selective hydrogenation as well as their synergistic catalytic mechanism.

The catalytic performance of metal‒organic frameworks can be tuned by introducing defects in their structure. Here, the authors introduce defects and impregnate ruthenium nanoparticles in cationic metal-organic frameworks, which enables enhanced recyclability and catalytic performance in D-glucose hydrogenation.

The Adsorption of Small Molecules on the Copper Paddle-Wheel: Influence of the Multi-Reference Ground State

We report a theoretical study of the adsorption of a set of small molecules (C2H2, CO, CO2, O2, H2O, CH3OH, C2H5OH) on the metal centers of the "copper paddle-wheel"-a key structural motif of many MOFs. A systematic comparison between DFT of different rungs, single-reference post-HF methods (MP2, SOS-MP2, MP3, DLPNO-CCSD(T)), and multi-reference approaches (CASSCF, DCD-CAS(2), NEVPT2) is performed in order to find a methodology that correctly describes the complicated electronic structure of paddle-wheel structure together with a reasonable description of non-covalent interactions. Apart from comparison with literature data (experimental values wherever possible), benchmark calculations with DLPNO-MR-CCSD were also performed. Despite tested methods show qualitative agreement in the majority of cases, we showed and discussed reasons for quantitative differences as well as more fundamental problems of specific cases.

Postsynthetic Defect Formation in Three-Dimensional Hofmann-Type Coordination Polymers and Its Impact on Catalytic Activity

We report a systematic investigation of postsynthetic defect formation in Hofmann-type coordination polymers M(pz)[M'(CN)₄] (M = Fe²⁺, Co²⁺, Ni²⁺; M' = Pd²⁺, Pt²⁺; pz = pyrazine). These compounds readily undergo selective ligand exchange at the pyrazine site when immersed in methanol (MeOH) at ambient temperature. The ligand exchange changes the chemical formula to M(pz)_1-x(MeOH)_2x[M'(CN)₄] (0 < x < 0.3), affording a defective coordination environment around the M ions. The defect concentration is highly dependent on the combination of the metal ions and solvent species, reaching the defect concentration of ca. 30% (x ∼ 0.3) at maximum. The magnetic state of one such coordination polymer gives an additional control of the defect formation, making the compound less susceptible to the ligand exchange at the low-spin state. Structures that form the defects at a high concentration function as catalysts and promote an acetalization reaction heterogeneously by providing Lewis acidic sites. The solvent-dependent character of the defect formation can be used to control the catalytic activity of the active compounds, demonstrating a facile defect engineering for functionalizing solid materials.

In Situ Fabrication of Porous MOF/COF Hybrid Photocatalysts for Visible-Light-Driven Hydrogen Evolution

Construction of porous metal-organic framework (MOF)/covalent organic framework (COF) hybrid photocatalysts for enriched structures and unprecedented properties is still a great challenge but highly desirable. Herein, a new series of Cu₃(HHTP)₂-MOF/Tp-Pa-1-COF hybrids with different MOF content are successfully fabricated. The as-prepared MOF/COF hybrids exhibit intimate interaction based on the coordination of Cu ions with the carbonyl oxygen and enamine nitrogen groups in Tp-Pa-1. The integrated conductive Cu₃(HHTP)₂ is able to act as an excellent electron extractor instead of noble metal cocatalysts to significantly promote the charge transfer and inhibit the recombination of photogenerated electron-hole pairs. As a results, the optimized photocatalyst with Cu₃(HHTP)₂:Tp-Pa-1 ratio of 1:15 achieves the highest hydrogen evolution rate of 1.76 mmol·h^-1·g^-1 under visible-light irradiation, which is about 93 times higher than that of the pure Tp-Pa-1 and even slightly higher than that of the Tp-Pa-1 with Pt (3 wt %) as a cocatalyst.

Isolation of a Side-On V(III)-(η2-O2) through the Intermediacy of a Low-Valent V(II) in a Metal-Organic Framework

We report the isolation of vanadium(II) in a metal-organic framework (MOF) by the reaction of the chloride-capped secondary building unit in the all-vanadium(III) V-MIL-101 (1) with 1,4-bis(trimethylsilyl)-2,3,5,6-tetramethyl-1,4-dihydropyrazine. The reduced material, 2, has a secondary building unit with the formal composition [V^IIV₂^III], with each metal ion presenting one open coordination site. Subsequent reaction with O₂ yields a side-on η² vanadium-superoxo species, 3. The MOF featuring V(III)-superoxo moieties exhibits a mild enhancement in the isosteric enthalpy of adsorption for methane compared to the parent V-MIL-101. We present this synthetic methodology as a potentially broad way to access low-valent open metal sites within MOFs without causing a loss of crystallinity or porosity. The low-valent sites can serve as isolable intermediates to access species otherwise inaccessible by direct synthesis.

Cathodic Electrodeposition of MOF Films Using Hydrogen Peroxide

Metal-organic framework (MOF) films can be made by cathodic electrodeposition, where a Brønsted base is formed electrochemically which deprotonates the MOF linkers that are present in solution as undissociated/partially dissociated weak acids. However, the co-deposition of metal and the narrow range of possible metal nodes limit the scope of this method. In this work, we propose the use of hydrogen peroxide (hydrogen peroxide assisted cathodic deposition or HPACD), to overcome these limitations. Electrochemical measurements indicate that in DMF, hydrogen peroxide is reduced to superoxide anions that deprotonate the carboxylic ligands. This single-electron reduction happens at much higher potentials than all previous reported methods. This prevents the co-deposition of metal and extends the range of possible metal nodes. Various pure MOF films (HKUST-1, MIL-53(Fe) and MOF-5) were prepared via this approach. HPACD was also used for the preparation of patterned MOF films and of flexible Cu-BTC coated paper membranes which reject 99.1 % of Rose Bengal from water with a permeance of 8.4 L m^-2  h^-1  bar^-1 .

Sensing Molecules with Metal-Organic Framework Functionalized Graphene Transistors

Graphene is inherently sensitive to vicinal dielectrics and local charge distributions, a property that can be probed by the position of the Dirac point in graphene field-effect transistors. Exploiting this as a useful sensing principle requires selectivity; however, graphene itself exhibits no molecule-specific interaction. Complementarily, metal-organic frameworks can be tailored to selective adsorption of specific molecular species. Here, a selective ethanol sensor is demonstrated by growing a surface-mounted metal-organic framework (SURMOF) directly onto graphene field-effect transistors (GFETs). Unprecedented shifts of the Dirac point, as large as 15 V, are observed when the SURMOF/GFET is exposed to ethanol, while a vanishingly small response is observed for isopropanol, methanol, and other constituents of the air, including water. The synthesis and conditioning of the hybrid materials sensor with its functional characteristics are described and a model is proposed to explain the origin, magnitude, and direction of the Dirac point voltage shift. Tailoring multiple SURMOFs to adsorb specific gases on an array of such devices thus generates a versatile, selective, and highly sensitive platform for sensing applications.

Catching the Reversible Formation and Reactivity of Surface Defective Sites in Metal-Organic Frameworks: An Operando Ambient Pressure-NEXAFS Investigation

In this work, we apply for the first time ambient pressure operando soft X-ray absorption spectroscopy (XAS) to investigate the location, structural properties, and reactivity of the defective sites present in the prototypical metal-organic framework HKUST-1. We obtained direct evidence that Cu⁺ defective sites form upon temperature treatment of the powdered form of HKUST-1 at 160 °C and that they are largely distributed on the material surface. Further, a thorough structural characterization of the Cu⁺/Cu²⁺ dimeric complexes arising from the temperature-induced dehydration/decarboxylation of the pristine Cu²⁺/Cu²⁺ paddlewheel units is reported. In addition to characterizing the surface defects, we demonstrate that CO₂ may be reversibly adsorbed and desorbed from the surface defective Cu⁺/Cu²⁺ sites. These findings show that ambient pressure soft-XAS, combined with state-of-the-art theoretical calculations, allowed us to shed light on the mechanism involving the decarboxylation of the paddlewheel units on the surface to yield Cu⁺/Cu²⁺ complexes and their reversible restoration upon exposure to gaseous CO_2.

Room-Temperature Negative Differential Resistance in Surface-Supported Metal-Organic Framework Vertical Heterojunctions

The advances of surface-supported metal-organic framework (SURMOF) thin-film synthesis have provided a novel strategy for effectively integrating metal-organic framework (MOF) structures into electronic devices. The considerable potential of SURMOFs for electronics results from their low cost, high versatility, and good mechanical flexibility. Here, the first observation of room-temperature negative differential resistance (NDR) in SURMOF vertical heterojunctions is reported. By employing the rolled-up nanomembrane approach, highly porous sub-15 nm thick HKUST-1 films are integrated into a functional device. The NDR is tailored by precisely controlling the relative humidity (RH) around the device and the applied electric field. The peak-to-valley current ratio (PVCR) of about two is obtained for low voltages (<2 V). A transition from a metastable state to a field emission-like tunneling is responsible for the NDR effect. The results are interpreted through band diagram analysis, density functional theory (DFT) calculations, and ab initio molecular dynamics simulations for quasisaturated water conditions. Furthermore, a low-voltage ternary inverter as a multivalued logic (MVL) application is demonstrated. These findings point out new advances in employing unprecedented physical effects in SURMOF heterojunctions, projecting these hybrid structures toward the future generation of scalable functional devices.

Modeling the Layer-by-Layer Growth of HKUST-1 Metal-Organic Framework Thin Films

Metal organic frameworks have emerged as an important new class of materials with many applications, such as sensing, gas separation, drug delivery. In many cases, their performance is limited by structural defects, including vacancies and domain boundaries. In the case of MOF thin films, surface roughness can also have a pronounced influence on MOF-based device properties. Presently, there is little systematic knowledge about optimal growth conditions with regard to optimal morphologies for specific applications. In this work, we simulate the layer-by-layer (LbL) growth of the HKUST-1 MOF as a function of temperature and reactant concentration using a coarse-grained model that permits detailed insights into the growth mechanism. This model helps to understand the morphological features of HKUST-1 grown under different conditions and can be used to predict and optimize the temperature for the purpose of controlling the crystal quality and yield. It was found that reactant concentration affects the mass deposition rate, while its effect on the crystallinity of the generated HKUST-1 film is less pronounced. In addition, the effect of temperature on the surface roughness of the film can be divided into three regimes. Temperatures in the range from 10 to 129 °C allow better control of surface roughness and film thickness, while film growth in the range of 129 to 182 °C is characterized by a lower mass deposition rate per cycle and rougher surfaces. Finally, for T larger than 182 °C, the film grows slower, but in a smooth fashion. Furthermore, the potential effect of temperature on the crystallinity of LbL-grown HKUST-1 was quantified. To obtain high crystallinity, the operating temperature should preferably not exceed 57 °C, with an optimum around 28 °C, which agrees with experimental observations.

Defect engineering: an effective tool for enhancing the catalytic performance of copper-MOFs for the click reaction and the A3 coupling

A series of Cu(i)-enriched and Lewis basic site-containing defect-engineering MOFs was investigated for significantly enhanced catalytic performance in the click reaction and the A³ coupling.

The efficient separation of N2O/CO2 using unsaturated Fe2+ sites in MIL-100Fe

It is a big challenge to separate N2O from CO2 using adsorption because they have similar physical properties. The Fe3+-F- site in MIL-100Fe transforms to an unsaturated Fe2+ site under high-temperature activation (300 °C), and the target sorbent MIL-100Fe-300 exhibits the biggest difference in CO2 and N2O adsorption capacity, and the selectivity of N2O/CO2 (50%/50%) is up to 3.00 at 298 K. According to DFT calculations, the original Fe3+-F- site has strong interaction with CO2, but the open Fe2+ site has a stronger interaction with N2O. Through a breakthrough experiment, it was confirmed that MIL-100Fe-300 has the best N2O/CO2 separation performance, making it potentially a useful material in industry.

Single Open Sites on FeII Ions Stabilized by Coupled Metal Ions in CN-Deficient Prussian Blue Analogues for High Catalytic Activity in the Hydrolysis of Organophosphates

CN-deficient Prussian blue analogues (PBAs), [M^N(H₂O)_x]_y[Fe^II(CN)₅(NH₃)] (M^N = Cu^II, Co^II, or Ga^III), were synthesized and examined as a new class of heterogeneous catalysts for hydrolytic decomposition of organophosphates often used as pesticides. The active species of the CN-deficient PBAs were mainly C-bound Fe^II ions with only single open sites generated by liberation of the NH₃ ligand during the catalytic reactions. [Cu^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] showed higher catalytic activity than [Co^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] and [Ga^III(H₂O)][Fe^II(CN)₅(NH₃)], although N-bound Cu^II species has been reported as less active than Co^II and Ga^III species in conventional PBAs. IR measurements of a series of the CN-deficient PBAs after the catalytic reactions clarified that a part of the NH₃ ligands remained on [Co^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] and that hydrogen phosphate formed as a product strongly adsorbed on the Fe^II ions of [Ga^III(H₂O)][Fe^II(CN)₅(NH₃)]. Hydrogen phosphate also adsorbed, but weakly, on the Fe^II ions of [Cu^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)]. These results suggest that heterogeneous catalysis of the Fe^II ions with single open sites were tuned by the M^N ions through metal-metal interaction.

In Situ Spectroscopy of Calcium Fluoride Anchored Metal-Organic Framework Thin Films during Gas Sorption

Surface‐mounted metal–organic frameworks (SURMOFs) show promising behavior for a manifold of applications. As MOF thin films are often unsuitable for conventional characterization techniques, understanding their advantageous properties over their bulk counterparts presents a great analytical challenge. In this work, we demonstrate that MOFs can be grown on calcium fluoride (CaF₂) windows after proper functionalization. As CaF₂ is optically (in the IR and UV/Vis range of the spectrum) transparent, this makes it possible to study SURMOFs using conventional spectroscopic tools typically used during catalysis or gas sorption. Hence, we have measured HKUST‐1 during the adsorption of CO and NO. We show that no copper oxide impurities are observed and also confirm that SURMOFs grown by a layer‐by‐layer (LbL) approach possess Cu⁺ species in paddlewheel confirmation, but 1.9 times less than in bulk HKUST‐1. The developed methodology paves the way for studying the interaction of any adsorbed gases with thin films, not limited to MOFs, low temperatures, or these specific probe molecules, pushing the boundaries of our current understanding of functional porous materials.

Thermal Defect Engineering of Precious Group Metal-Organic Frameworks: A Case Study on Ru/Rh-HKUST-1 Analogues

A methodology is introduced for controlled postsynthetic thermal defect engineering (TDE) of precious group metal-organic frameworks (PGM-MOFs). The case study is based on the Ru/Rh analogues of the archetypical structure [Cu₃(BTC)₂] (HKUST-1; BTC = 1,3,5-benzenetricarboxylate). Quantitative monitoring of the TDE process and extensive characterization of the samples employing a complementary set of analytical and spectroscopic techniques reveal that the compositionally very complex TDE-MOF materials result from the elimination and/or fragmentation of ancillary ligands and/or linkers. TDE involves the preferential secession of acetate ligands, intrinsically introduced via coordination modulation during synthesis, and the gradual decarboxylation of ligator sites of the framework linker BTC. Both processes lead to modified Ru/Rh paddlewheel nodes. These nodes exhibit a lowered average oxidation state and more accessible open metal centers, as deduced from surface-ligand IR spectroscopy using CO as a probe and supported by density functional theory (DFT)-based computations. The monometallic and the mixed-metal PGM-MOFs systematically differ in their TDE properties and, in particular in the hydride generation ability (HGA). This latter property is an important indicator for the catalytic activity of PGM-MOFs, as demonstrated by the ethylene dimerization reaction to 1-butene.

Defect Engineering of Copper Paddlewheel-Based Metal-Organic Frameworks of Type NOTT-100: Implementing Truncated Linkers and Its Effect on Catalytic Properties

A series of new defect-engineered metal-organic frameworks (DEMOFs) were synthesized by framework doping with truncated linkers employing the mixed-linker approach. Two tritopic defective (truncated) linkers, biphenyl-3,3',5-tricarboxylates (L_H) lacking a ligating group and 5-(5-carboxypyridin-3-yl)isophthalates (L_Py) bearing a weaker interacting ligator site, were integrated into the framework of Cu₂(BPTC) (NOTT-100, BPTC = biphenyl-3,3',5,5'-tetracarboxylates). Incorporating L_H into the framework mainly generates missing metal node defects, thereby obtaining dangling COOH groups in the framework. However, introducing L_Py forms more modified metal nodes featuring reduced and more accessible Cu sites. In comparison with the pristine NOTT-100, the defect-engineered NOTT-100 (DE-NOTT-100) samples show two unique features: (i) functional groups (the protonated carboxylate groups as the Brønsted acid sites or the pyridyl N atoms as the Lewis basic sites), which can act as second active sites, are incorporated into the MOF frameworks, and (ii) more modified paddlewheels, which provided extra coordinatively unsaturated sites, are generated. The cooperative functioning of the above characteristics enhances the catalytic performance of certain types of reactions. For a proof of concept, two exemplary reactions, namely, the cycloaddition of CO₂ with propylene oxide to propylene carbonate and the cyclopropanation of styrene, were carried out to evaluate the catalytic activities of those DE-NOTT-100 materials depending on the defect structure.

Interplay of Electronic and Steric Effects to Yield Low-Temperature CO Oxidation at Metal Single Sites in Defect-Engineered HKUST-1

In contrast to catalytically active metal single atoms deposited on oxide nanoparticles, the crystalline nature of metal-organic frameworks (MOFs) allows for a thorough characterization of reaction mechanisms. Using defect-free HKUST-1 MOF thin films, we demonstrate that Cu+ /Cu2+ dimer defects, created in a controlled fashion by reducing the pristine Cu2+ /Cu2+ pairs of the intact framework, account for the high catalytic activity in low-temperature CO oxidation. Combining advanced IR spectroscopy and density functional theory we propose a new reaction mechanism where the key intermediate is an uncharged O2 species, weakly bound to Cu+ /Cu2+ . Our results reveal a complex interplay between electronic and steric effects at defect sites in MOFs and provide important guidelines for tailoring and exploiting the catalytic activity of single metal atom sites.