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

Supramolecular systems and their connection with metal-organic structures

Supramolecular structures with specific applications are a pillar in several areas of science. Thus, from a contemporary point of view, there are several reasons to embrace a systematic order of the supramolecular concept itself. First, the structuring of a supramolecular material seems safer now than it did decades ago. Second, the interactions of metal-organic frameworks (MOFs) and supramolecular chemistry and, conversely, supramolecularity to assemble MOFs and create efficient complex systems in multiple cutting-edge applications are an image to be safeguarded. Third, perhaps we should simply limit ourselves to considering how researchers in these fields have attempted to correlate the notion of supramolecular systems by linking self-assembly considerations. In any case, these topics present advantages to optimize innovative geometries that are useful to highlight significant practical applications. This review covers a general introduction to MOFs and supramolecularity, the key unit of the study presented here, followed by a survey of recent advances in confined space chemistry, the relationships of MOFs with supramolecular structures, and the synthesis electrochemistry of MOFs and switchable MOFs to obtain a greater understanding of structure-property relationships. To conclude, some future perspectives on this promising and plausible field of science will be mentioned.

CH4 Carbonylation to Acetic Acid Using H2O as an Oxidant on a Rh-Functionalized UiO-67 Combined with Oriented External Electric Fields: Selectivity and Mechanistic Insights from DFT Calculations

Acetic acid (CH3COOH), as an industrially important petrochemical product, is predominantly produced via multistep energy-intensive processes. The development of a rhodium single-site heterogeneous catalyst has received considerable attention due to its potential to transform CH4 into CH3COOH in a single step. Herein, the reaction mechanism for the generation of CH3COOH from CH4, CO, and H2O catalyzed by Rh-functionalized metal-organic framework (MOF) UiO-67 and the selectivity of products CH3COOH, formic acid (HCOOH), methanol (CH3OH), and acetaldehyde (CH3CHO) under the oriented external electric fields (OEEFs) were systematically explored by density functional theory (DFT) calculations. The results reveal that the insertion of CO into Rh-CH3 is the rate-determining step with a free energy barrier of 21.0 kcal/mol in CH4 carbonylation to CH3COOH. Upon applying an OEEF of Fx = +0.0050 au along the C-C bond, the rate-determining step shifts toward H2O decomposition with the barrier of 19.6 kcal/mol, significantly improving the selectivity for CH3COOH production, compared to the major competitive HCOOH route. The Brønsted-Evans-Polanyi (BEP) relationships between key transition states, field strength, and NPA charge transfer were established. This study may guide the rational design of atomically dispersed MOF catalysts for the selective coconversion of CH4 and CO to CH3COOH using H2O as the oxidant under the OEEF.

Structural Comparisons, Fluorescence Properties, and Glass-to-Crystal Transformations of Heat-Cooled and Melt-Quenched Zeolitic Imidazolate Framework Glass

As a representative of zeolitic imidazolate framework glass, agZIF-62 has been reported to be synthesized using a melt-quenching method in which the ZIF-62 crystal is heated to a temperature above the melting point. Interestingly, we unexpectedly found that agZIF-62 can also be synthesized by simple heating at temperatures lower than the melting point, which may be assisted by the release of encapsulated solvent molecules. The structural differences between melt-quenched agZIF-62 (MQ-agZIF-62) and heat-cooled agZIF-62 (HC-agZIF-62) were investigated. The results indicated that MQ-agZIF-62 is closer to the liquid state, while HC-agZIF-62 is closer to the crystal state. Interestingly, their luminescent emissions exhibit significant differences. Compared with the ZIF-62 crystal, MQ-agZIF-62 showed a blue-shift of 14 nm, whereas HC-agZIF-62 showed a red-shift of 9 nm. The emission intensity of agZIF-62 is also significantly stronger than that of ZIF-62; thus, rapid semiquantitative detection of the content of the MOF glass in glass and crystal mixtures can be achieved. In addition, HC-agZIF-62 and MQ-agZIF-62 can transform into ZIF-62 crystals via a solvent-media mechanism. This study provides new insights into ZIF-62 glass.

Constructing Isoreticular Metal-Organic Frameworks by Silver-Carbon Bonds

The incorporation of new coordinate bonds and the development of universal methods for new structures have always been of major interest in metal-organic framework (MOF) research. The poor reversibility makes metal-carbon (M-C) bonds a great challenge to adopt as linkages to construct crystalline MOFs. Herein, three isoreticular microcrystalline MOFs connected by silver-carbon (Ag-C) bonds are presented for the first time and named AgC-MOFs. Their structures contain a double coordination mode (σ and π) between Ag(I) and alkynyl. The three AgC-MOFs all exhibit three-dimensional (3D) frameworks with uniform one-dimensional (1D) hexagonal channels, and the pore width could be tuned from 1.1 to 1.8 nm. The construction of crystalline MOFs using poorly reversible Ag-C coordinate bonds extends the nexuses for the MOF structure and lights up more possibilities for the systematic design of MOFs.

Translocation and Confinement of Tetraamines in Adaptable Microporous Cavities

Metal-Organic Frameworks can be grafted with amines by coordination to metal vacancies to create amine-appended solid adsorbents, which are being considered as an alternative to using aqueous amine solutions for CO2 capture. In this study, we propose an alternative mechanism that does not rely on the use of neutral metal vacancies as binding sites but is enabled by the structural adaptability of heterobimetallic Ti2Ca2 clusters. The combination of hard (Ti4+) and soft (Ca2+) metal centers in the inorganic nodes of the framework enables MUV-10 to adapt its pore windows to the presence of triethylenetetramine molecules. This dynamic cluster response facilitates the translocation and binding of tetraamine inside the microporous cavities to enable the formation of bis-coordinate adducts that are stable in water. The extension of this grafting concept from MUV-10 to larger cavities not restrictive to CO2 diffusion will complement other strategies available for the design of molecular sorbents for decarbonization applications.

Synergistically enhancing the selective adsorption of cationic dyes through copper impregnation and amino functionality into iron-based metal-organic frameworks

Dyes contaminating the sewages have seriously threatened the living beings and their separation from wastewater in terms of potential resource recovery is of high value. Herein, both of metal node doping and ligand group grafting were taken into account to enhance the adsorption selectivity of Fe-MOFs towards cationic dyes. The positive correlation between copper doping amount and selective coefficient (∂MOMB) for methylene blue (MB) over methyl orange (MO) within a certain range was mainly attributed to the increased surface negative charges via partial replacement of Fe(III) with Cu(II). Moreover, the amount of surface negative charges was further increased after amino functionalization and there was a synergism between Cu(II) and -NH2 in selectivity enhancement. As a result, Fe0.6Cu0.4-BDC-NH2 exhibited a 22.5-times increase in ∂MOMB and other cationic dyes including malachite green (MG) and rhodamine B (Rh. B) could also be selectively separated from binary and quaternary mixed dye systems. Moreover, Fe0.6Cu0.4-BDC-NH2 showed many superiorities like a wide pH range of 4.0-8.0, strong anti-interference ability over various inorganic ions, good recyclability, and stability. The adsorption kinetics and isotherm suggested that the MB adsorption process was a homogeneous single-layer chemisorption. Additionally, the thermodynamics manifested that the overall process was exothermic and spontaneous. According to the FT-IR and XPS spectra analysis, the electrostatic interaction and hydrogen bonding were determined as the main driving forces, and π-π interaction also contributed to the adsorption process.

Site-Isolated Rhodium(II) Metalloradicals Catalyze Olefin Hydrofunctionalization

Rh(II) porphyrin complexes display pronounced metal-centered radical character and the ability to activate small molecules under mild conditions, but catalysis with Rh(II) porphyrins is extremely rare. In addition to facile dimerization, Rh(II) porphyrins readily engage in kinetically and thermodynamically facile reactions involving two Rh(II) centers to generate stable Rh(III)-X intermediates that obstruct turnover in thermal catalysis. Here we report site isolation of Rh(II) metalloradicals in a MOF host, which not only protects Rh(II) metalloradicals against dimerization, but also allows them to participate in thermal catalysis. Access to PCN-224 or PCN-222 in which the porphyrin linkers are fully metalated by Rh(II) in the absence of any accompanying Rh(0) nanoparticles was achieved via the first direct MOF synthesis with a linker containing a transition-metal alkyl moiety, followed by Rh(III)-C bond photolysis.

Surface-supported metal-organic frameworks with geometric topological diversity via scanning tunneling microscopy

Surface-supported metal-organic frameworks (SMOFs) are long-range ordered periodic 2D lattice layers formed by inorganic metal nodes and organic ligands via coordination bonds on substrate surfaces. The atomic resolution STM lays a solid foundation for the conception and construction of SMOFs with large area, stable structure, and special function. In this review, the cutting-edge research of SMOFs from design strategy, preparation process, and how to accurately achieve structural and functional diversity are reviewed. Furthermore, we focus on the design and construction of novel and fascinating periodic and fractal structures, in which some typical honeycomb structures, Kagome lattice, hexagonal geometry, and Sierpiński triangles are summarized, and the related prospects for designing functional nanoscale systems and architectures are prospected. Finally, the challenges faced in the design and synthesis of SMOFs are denoted, and the application prospect and development trend of SMOFs are forecasted based on the current research status.

Iodine Adsorption-Desorption-Induced Structural Transformation and Improved Ag+ Turn-On Luminescent Sensing Performance of a Nonporous Eu(III) Metal-Organic Framework

Posttreatment of pristine metal-organic frameworks (MOFs) with suitable vapor may be an effective way to regulate their structures and properties but has been less explored. Herein, we report an interesting example in which a crystalline nonporous Eu(III)-MOF was transferred to a porous amorphous MOF (aMOF) via iodine vapor adsorption-desorption posttreatment, and the resulting aMOF showed improved turn-on sensing properties with respect to Ag+ ions. The crystalline Eu-MOF, namely, Eu-IPDA, was assembled from Eu(III) and 4,4'-{4-[4-(1H-imidazol-1-yl)phenyl]pyridine-2,6-diyl}dibenzoic acid (H2IPDA) and exhibited a two-dimensional (2D) coordination network based on one-dimensional secondary building blocks. The close packing of the 2D networks gives rise to a three-dimensional supramolecular framework without any significant pores. Interestingly, the nonporous Eu-IPDA could absorb iodine molecules when Eu-IPDA crystals were placed in iodine vapor at 85 °C, and the adsorption capacity was 1.90 g/g, which is comparable to those of many MOFs with large BET surfaces. The adsorption of iodine is attributed to the strong interactions among the iodine molecule, the carboxy group, and the N-containing group and leads to the amorphization of the framework. After immersion of the iodine-loaded Eu-IPDA in EtOH, approximately 89.7% of the iodine was removed, resulting in a porous amorphous MOF, denoted as a-Eu-IPDA. In addition, the remaining iodine in the a-Eu-IPDA framework causes strong luminescent quenching in the fluorescence emission region of the Eu(III) center when compared with that in Eu-IPDA. The luminescence intensity of a-Eu-IPDA in water suspensions was significantly enhanced when Ag+ ions were added, with a detection limit of 4.76 × 10-6 M, which is 1000 times that of pristine Eu-IPDA. It also showed strong anti-interference ability over many common competitive metal ions and has the potential to sense Ag+ in natural water bodies and traditional Chinese medicine preparations. A mechanistic study showed that the interactions between Ag+ and the absorbed iodine, the carboxylate group, and the N atoms all contribute to the sensing performance of a-Eu-IPDA.

Characterization, Structure, and Reactivity of Hydroxyl Groups on Metal-Oxide Cluster Nodes of Metal-Organic Frameworks: Structural Diversity and Keys to Reactivity and Catalysis

Among the most stable metal-organic frameworks (MOFs) are those incorporating nodes that are metal oxide clusters with frames such as Zr6 O8 . This review is a summary of the structure, bonding, and reactivity of MOF node hydroxyl groups, emphasizing those bonded to nodes containing aluminum and zirconium ions. Hydroxyl groups are often present on these nodes, sometimes balancing the charges of the metal ions. They arise during MOF syntheses in aqueous media or in post-synthesis treatments. They are identified with infrared and 1 H nuclear magnetic resonance spectroscopies and characterized by their reactivities with polar compounds such as alcohols. Terminal OH, paired µ2 -OH, and aqua groups on nodes are catalytic sites in numerous reactions. Relatively unreactive hydroxyl groups (such as isolated µ2 -OH groups) may replace reactive groups and inhibit catalysis; some node hydroxyl groups (e.g., µ3 -OH) are mere spectators in catalysis. There are similarities between MOF node hydroxyl groups and those on the surfaces of bulk metal oxides, zeolites, and enzymes, but the comparisons are mostly inexact, and much remains to be understood about MOF node hydroxyl group chemistry. It is posited that understanding and controlling this chemistry will lead to tailored MOFs and improved adsorbents and catalysts.

Selective Gas-Phase Hydrogenation of CO2 to Methanol Catalysed by Metal-Organic Frameworks

Large scale production of green CH3 OH obtained from CO2 and green H2 is a highly wanted process due to the role of CH3 OH as H2 /energy carrier and for producing chemicals. Starting with a short summary of the advantages of metal-organic frameworks (MOFs) as catalysts in liquid-phase reactions, the present article highlights the opportunities that MOFs may offer also for some gas-phase reactions, particularly for the selective CO2 hydrogenation to CH3 OH. It is commented that there is a temperature compatibility window that combines the thermal stability of some MOFs with the temperature required in the CO2 hydrogenation to CH3 OH that frequently ranges from 250 to 300 °C. The existing literature in this area is briefly organized according to the role of MOF as providing the active sites or as support of active metal nanoparticles (NPs). Emphasis is made to show how the flexibility in design and synthesis of MOFs can be used to enhance the catalytic activity by adjusting the composition of the nodes and the structure of the linkers. The influence of structural defects and material crystallinity, as well as the role that should play theoretical calculations in models have also been highlighted.

Transparent-to-Brown-Black Patterned Electrochromic Metal-Organic Frameworks

Metal-organic frameworks (MOFs) exhibit promising electrochromic (EC) performance owing to their porous structure, regular channel, and tunable component characteristics. However, few reports focus on MOF materials with the EC performance of a transparent to brown-black (neutral colored state) change that is more suitable for smart windows. In this work, we proposed a strategy for synthesizing MOF (named Ni-BPY) EC materials and corresponding films fabricated via a low-cost electrostatic spray deposition technique. The obtained film exhibits excellent EC performance with a neutral color change from transparent to brown-black, a large optical modulation of 70% at 430 nm, and a fast response within 10 s. Benefiting from good electrical and chemical stability, the Ni-BPY film can be cycled over 500 times. Notably, the Ni-BPY MOF film also delivers a stepwise-controlled process during the bleached state due to its porous characteristics. In addition, the unique color variation of the Ni-BPY film derives from the redox reaction of the Ni metal node between Ni2+ and Ni3+, which is verified by the in situ potential-dependent Raman and X-ray photoelectron spectroscopy (XPS) measurement. As a proof of application, the patterned Ni-BPY EC films and devices are additionally constructed to demonstrate their potential application in electronic tags and logo displays.

Enhancing CO2 Hydrogenation Using a Heterogeneous Bimetal NiAl-Deposited Metal-Organic Framework NU-1000: Insights from First-Principles Calculations

The hydrogenation of CO2 to high-value-added liquid fuels is crucial for greenhouse gas emission reduction and optimal utilization of carbon resources. Developing supported heterogeneous catalysts is a key strategy in this context, as they offer well-defined active sites for in-depth mechanistic studies and improved catalyst design. Here, we conducted extensive first-principles calculations to systematically explore the reaction mechanisms for CO2 hydrogenation on a heterogeneous bimetal NiAl-deposited metal-organic framework (MOF) NU-1000 and its catalytic performance as atomically dispersed catalysts for CO2 hydrogenation to formic acid (HCOOH), formaldehyde (H2CO), and methanol (CH3OH). The present results reveal that the presence of the NiAl-oxo cluster deposited on NU-1000 efficiently activates H2, and the facile heterolysis of H2 on Ni and adjacent O sites serves as a precursor to the hydrogenation of CO2 into various C1 products HCOOH, H2CO, and CH3OH. Generally, H2 activation is the rate-determining step in the entire CO2 hydrogenation process, the corresponding relatively low free energy barriers range from 14.5 to 15.9 kcal/mol, and the desorption of products on NiAl-deposited NU-1000 is relatively facile. Although the Al atom does not directly participate in the reaction, its presence provides exposed oxygen sites that facilitate the heterolytic cleavage of H2 and the hydrogenation of C1 intermediates, which plays an important role in enhancing the catalytic activity of the Ni site. The present study demonstrates that the catalytic performance of NU-1000 can be finely tuned by depositing heterometal-oxo clusters, and the porous MOF should be an attractive platform for the construction of atomically dispersed catalysts.

Reactive high-spin iron(IV)-oxo sites through dioxygen activation in a metal-organic framework

In nature, nonheme iron enzymes use dioxygen to generate high-spin iron(IV)=O species for a variety of oxygenation reactions. Although synthetic chemists have long sought to mimic this reactivity, the enzyme-like activation of O2 to form high-spin iron(IV) = O species remains an unrealized goal. Here, we report a metal-organic framework featuring iron(II) sites with a local structure similar to that in α-ketoglutarate-dependent dioxygenases. The framework reacts with O2 at low temperatures to form high-spin iron(IV) = O species that are characterized using in situ diffuse reflectance infrared Fourier transform, in situ and variable-field Mössbauer, Fe Kβ x-ray emission, and nuclear resonance vibrational spectroscopies. In the presence of O2, the framework is competent for catalytic oxygenation of cyclohexane and the stoichiometric conversion of ethane to ethanol.