Solvent Choice in Metal-Organic Framework Linker Exchange Permits Microstructural Control

Interfacial Engineering of Heterogeneous Reactions for MOF-on-MOF Heterostructures

Metal-organic frameworks (MOFs), as a subclass of porous crystalline materials with unique structures and multifunctional properties, play a pivotal role in various research domains. In recent years, significant attention has been directed toward composite materials based on MOFs, particularly MOF-on-MOF heterostructures. Compared to individual MOF materials, MOF-on-MOF structures harness the distinctive attributes of two or more different MOFs, enabling synergistic effects and allowing for the tailored design of diverse multilayered architectures to expand their application scope. However, the rational design and facile synthesis of MOF-on-MOF composite materials are in principle challenging due to the structural diversity and the intricate interfaces. Hence, this review primarily focuses on elucidating the factors that influence their interfacial growth, with a specific emphasis on the interfacial engineering of heterogeneous reactions, in which MOF-on-MOF hybrids can be conveniently obtained by using pre-fabricated MOF precursors. These factors are categorized as internal and external elements, encompassing inorganic metals, organic ligands, lattice matching, nucleation kinetics, thermodynamics, etc. Meanwhile, these intriguing MOF-on-MOF materials offer a wide range of advantages in various application fields, such as adsorption, separation, catalysis, and energy-related applications. Finally, this review highlights current complexities and challenges while providing a forward-looking perspective on future research directions.

Continuously Tunable MOFs Enable Precise Mass Transfer for High-Performance Isomer Separation

Modulating mass transfer is crucial for optimizing the catalytic and separation performances of porous materials. Here, we systematically developed a series of continuously tunable MOFs (CTMOFs) that exhibit incessantly increased mass transfer. This was achieved through the strategic blending of ligands with different lengths and ratios in MOFs featuring the fcu topology. By employing a proportional mixture of two ligands in the synthesis of UiO-66, the micropores expanded, facilitating faster mass transfer. The mass transfer rate was evaluated by dye adsorption, dark-field microscopy, and gas chromatography (GC). The GC performance proved that both too-fast and too-slow mass transfer led to low separation performance. The optimized mass transfer in CTMOFs resulted in an exceptionally high separation resolution (5.96) in separating p-xylene and o-xylene. Moreover, this study represents the first successful use of MOFs for high-performance separation of propylene and propane by GC. This strategy provides new inspiration in regulating mass transfer in porous materials.

3D Hierarchical MOF-Derived Defect-Rich NiFe Spinel Ferrite as a Highly Efficient Electrocatalyst for Oxygen Redox Reactions in Zinc-Air Batteries

The strategy of defect engineering is increasingly recognized for its pivotal role in modulating the electronic structure, thereby significantly improving the electrocatalytic performance of materials. In this study, we present defect-enriched nickel and iron oxides as highly active and cost-effective electrocatalysts, denoted as Ni0.6Fe2.4O4@NC, derived from NiFe-based metal-organic frameworks (MOFs) for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). XANES and EXAFS confirm that the crystals have a distorted structure and metal vacancies. The cation defect-rich Ni0.6Fe2.4O4@NC electrocatalyst exhibits exceptional ORR and OER activities (ΔE = 0.68 V). Mechanistic pathways of electrochemical reactions are studied by DFT calculations. Furthermore, a rechargeable zinc-air battery (RZAB) using the Ni0.6Fe2.4O4@NC catalyst demonstrates a peak power density of 187 mW cm-2 and remarkable long-term cycling stability. The flexible solid-state ZAB using the Ni0.6Fe2.4O4@NC catalyst exhibits a power density of 66 mW cm-2. The proposed structural design strategy allows for the rational design of electronic delocalization of cation defect-rich NiFe spinel ferrite attached to ultrathin N-doped graphitic carbon sheets in order to enhance active site availability and facilitate mass and electron transport.

Direct Electrochemical Synthesis of Metal-Organic Frameworks: Cu3 (BTC)2 and Cu(TCPP) on Copper Thin films and Copper-Based Microstructures

Cu thin films and Cu2 O microstructures were partially converted to the Metal-Organic Frameworks (MOFs) Cu3 (BTC)2 or Cu(TCPP) using an electrochemical process with a higher control and at milder conditions compared to the traditional solvothermal MOF synthesis. Initially, either a Cu thin film was sputtered, or different kinds of Cu or Cu2 O microstructures were electrochemically deposited onto a conductive ITO glass substrate. Then, these Cu thin films or Cu-based microstructures were subsequently coated with a thin layer of either Cu3 (BTC)2 or Cu(TCPP) by controlled anodic dissolution of the Cu-based substrate at room temperature and in the presence of the desired organic linker molecules: 1,3,5-benzenetricarboxylic acid (BTC) or photoactive 4,4',4'',4'''-(Porphine-5,10,15,20-tetrayl) tetrakis(benzoic acid) (TCPP) in the electrolyte. An increase in size of the Cu micro cubes with exposed planes [100] of 38,7 % for the Cu2 O@Cu3 (BTC)2 and a 68,9 % increase for the Cu2 O@Cu(TCPP) was roughly estimated. Finally, XRD, Raman spectroscopy and UV-vis absorption spectroscopy were used to characterize the initial Cu films or Cu-based microstructures, and the obtained core-shell Cu2 O@Cu(BTC) and Cu2 O@Cu(TCPP) microstructures.

An Overview of Metal-organic Framework Based Electrocatalysts: Design and Synthesis for Electrochemical Hydrogen Evolution, Oxygen Evolution, and Carbon Dioxide Reduction Reactions

Due to the increasing global energy demands, scarce fossil fuel supplies, and environmental issues, the pursued goals of energy technologies are being sustainable, more efficient, accessible, and produce near zero greenhouse gas emissions. Electrochemical water splitting is considered as a highly viable and eco-friendly energy technology. Further, electrochemical carbon dioxide (CO2 ) reduction reaction (CO2 RR) is a cleaner strategy for CO2 utilization and conversion to stable energy (fuels). One of the critical issues in these cleaner technologies is the development of efficient and economical electrocatalyst. Among various materials, metal-organic frameworks (MOFs) are becoming increasingly popular because of their structural tunability, such as pre- and post- synthetic modifications, flexibility in ligand design and its functional groups, and incorporation of different metal nodes, that allows for the design of suitable MOFs with desired quality required for each process. In this review, the design of MOF was discussed for specific process together with different synthetic methods and their effects on the MOF properties. The MOFs as electrocatalysts were highlighted with their performances from the aspects of hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and electrochemical CO2 RR. Finally, the challenges and opportunities in this field are discussed.

Identifying pathways to metal-organic framework collapse during solvent activation with molecular simulations

Metal-organic framework (MOF) materials are a vast family of nanoporous solids with potential applications ranging from drug delivery to environmental remediation. Application of MOFs in these scenarios is hindered, however, by difficulties in MOF 'activation' after initial synthesis - removal of the synthesis solvent from the pores to make the pore space accessible - often leading to framework collapse if improperly performed. While experimental studies have correlated collapse to specific solvent properties and conditions, the mechanism of activation-collapse is currently unknown. Developing this understanding would enable researchers to create better activation protocols for MOFs, accelerating discovery and process intensification. To achieve this goal, we simulated solvent removal using grand-canonical Monte Carlo and free energy perturbation methods. By framing activation as a fluid desorption problem, we investigated activation processes in the isoreticular metal organic framework (IRMOF) family of MOFs for different solvents. We identified two pathways for solvent activation - the solvent either desorbs uniformly from each individual pore or forms coexisting phases during desorption. These mesophases in turn lead to large capillary stresses within the framework, corroborating experimental hypotheses for the cause of activation-collapse. Finally, we found that the activation energy of solvent removal increased with pore size and connectivity due to the increased stability of solvent mesophases, matching experimental findings. Using these simulations, it is possible to screen MOF activation procedures, enabling rapid identification of ideal solvents and conditions and thus enabling faster development of MOFs for practical applications.

Chemical complexity for targeted function in heterometallic titanium-organic frameworks

Research on metal-organic frameworks is shifting from the principles that control the assembly, structure, and porosity of these reticular solids, already established, into more sophisticated concepts that embrace chemical complexity as a tool for encoding their function or accessing new properties by exploiting the combination of different components (organic and inorganic) into these networks. The possibility of combining multiple linkers into a given network for multivariate solids with tunable properties dictated by the nature and distribution of the organic connectors across the solid has been well demonstrated. However, the combination of different metals remains still comparatively underexplored due to the difficulties in controlling the nucleation of heterometallic metal-oxo clusters during the assembly of the framework or the post-synthetic incorporation of metals with distinct chemistry. This possibility is even more challenging for titanium-organic frameworks due to the additional difficulties intrinsic to controlling the chemistry of titanium in solution. In this perspective article we provide an overview of the synthesis and advanced characterization of mixed-metal frameworks and emphasize the particularities of those based in titanium with particular focus on the use of additional metals to modify their function by controlling their reactivity in the solid state, tailoring their electronic structure and photocatalytic activity, enabling synergistic catalysis, directing the grafting of small molecules or even unlocking the formation of mixed oxides with stoichiometries not accessible to conventional routes.

Uncertainty in Composite Membranes: From Defect Engineering to Film Processing

Composite membranes featuring metal-organic framework (MOF)-dispersed polymers have attracted tremendous attention in recent years. However, evaluating commercial viability is oftentimes obscured by the irreproducibility in both MOF synthesis and film manufacturing protocols. Variability in MOF property sets are typically ascribed to crystal defects resulting from subtle variations in synthesis, but quantitative studies investigating the role of defects on transport properties are exceedingly rare. Likewise, controlled film formation protocols are rarely reported in the open literature, making it difficult to provide substantial and informative structure-property correlations. This study aims to address these uncertainties. To this end, two samples of a prototypical MOF, UiO-66-NH₂, were synthesized to feature similar particle size, morphology, and colloidal stability. However, defect engineering protocols coupled with careful screening experiments were developed to synthesize the two MOFs with maximally different porosities. Composite membranes were prepared for each MOF and a high-performance polymer, 6FDA-Durene, and then tested for light gas permeation measurements, revealing a small and unexpected enhancement in CO₂/CH₄ performance for samples containing low-porosity UiO-66-NH₂. Mechanistic studies on sorption revealed a surprising 50% decrease in sorption capacity for high-porosity UiO-66-NH₂, completely offsetting enhancements from increased gas diffusion. By using multiple replicate experiments, the sample-to-sample variation was large enough to obscure any differences in permeability and selectivity between the two types of MOF composites at low volume fractions. Application of the Maxwell model to extrapolate pure-MOF performance led to significant variations in predicted values, demonstrating the importance of collecting and reporting replicate experiments for membrane preparation and testing.

Surface, Interface and Structure Optimization of Metal-Organic Frameworks: Towards Efficient Resourceful Conversion of Industrial Waste Gases

Industrial waste gas emissions from fossil fuel over-exploitation have aroused great attention in modern society. Recently, metal-organic frameworks (MOFs) have been developed in the capture and catalytic conversion of industrial exhaust gases such as SO₂ , H₂ S, NO_x , CO₂ , CO, etc. Based on these resourceful conversion applications, in this review, we summarize the crucial role of the surface, interface, and structure optimization of MOFs for performance enhancement. The main points include (1) adsorption enhancement of target molecules by surface functional modification, (2) promotion of catalytic reaction kinetics through enhanced coupling in interfaces, and (3) adaptive matching of guest molecules by structural and pore size modulation. We expect that this review will provide valuable references and illumination for the design and development of MOF and related materials with excellent exhaust gas treatment performance.

Directing the Morphology, Packing, and Properties of Chiral Metal–Organic Frameworks by Cation Exchange**

We show that metal–organic frameworks, based on tetrahedral pyridyl ligands, can be used as a morphological and structural template to form a series of isostructural crystals having different metal ions and properties. An iterative crystal‐to‐crystal conversion has been demonstrated by consecutive cation exchanges. The primary manganese‐based crystals are characterized by an uncommon space group (P622). The packing includes chiral channels that can mediate the cation exchange, as indicated by energy‐dispersive X‐ray spectroscopy on microtome‐sectioned crystals. The observed cation exchange is in excellent agreement with the Irving–Williams series (Mn<Fe<Co<Ni< Cu>Zn) associated with the relative stability of the resulting coordination nodes. Furthermore, we demonstrate how the metal cation controls the optical and magnetic properties. The crystals maintain their morphology, allowing a quantitative comparison of their properties at both the ensemble and single‐crystal level.

MIL-53 and its OH-bonded variants for bio-polyol adsorption from aqueous solution

The adsorption of bio-polyols from dilute aqueous solution is important but faces challenges in the sustainable bio-refinery process. One solution to increase adsorption efficiency is to leverage host–guest interactions between the polyols and materials to grant a preference for polyols. In this study, we synthesized MIL-53 and diverse OH-bonded variants, and studied their adsorption properties towards ethanediol, 1,3-propanediol and glycerol in water. Among the four materials, OH–MIL-53 exhibited fast adsorption kinetics and high capacity, and could be completely regenerated through ethanol elution. Hydrophobic interactions between the alkyl chains of the polyols and the organic linkers of OH–MIL-53 and hydrogen bonding interactions between their OH groups were identified. The synergistic effect of the host–guest interactions is responsible for the unique adsorption performances of OH–MIL-53 towards polyols, and particularly for 1,3-propanediol.

Delicate host–guest interaction drives OH-bonded MOF to capture bio-polyols from diluted aqueous solution, with high capacity, fast kinetics and recyclability.

Solvent-controlled synthesis of Ti-based porphyrinic metal–organic frameworks for the selective photocatalytic oxidation of amines

The photocatalytic activity of metal-organic frameworks (MOFs) can be managed by the milieu of synthesis. Herein, N,N'-dimethylacetamide (DMA) and N,N'-diethylformamide (DEF) were employed as solvents for the synthesis of two Ti-based porphyrinic MOFs, namely Ti-PMOF-DMA and Ti-PMOF-DEF, from tetrabutyl orthotitanate and 4,4',4'',4'''-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid). Notably, both DMA and DEF were adsorbed onto the Ti-oxo clusters of the two MOFs to shape their properties. Ti-PMOF-DMA was observed with better optoelectronic response and charge transfer than Ti-PMOF-DEF. Moreover, Ti-PMOF-DMA owned a larger pore volume than Ti-PMOF-DEF, imparting more accessible sites to benzyl amines. Ti-PMOF-DMA exhibited better activity in selective photocatalytic aerobic oxidation of benzylamine than Ti-PMOF-DEF. Irradiated by red light-emitting diodes, outstanding results for selective conversion of  benzyl amines to imines over Ti-PMOF-DMA were attained. Superoxide radical anion, generated by the electron transfer from porphyrin via Ti-oxo clusters to dioxygen, turned out to be the primary reactive oxygen species. There was generality towards aerobic oxidation of amines to imines and considerable stability for Ti-PMOF-DMA. This work provides a new perspective on the altering MOFs to enhance photocatalytic organic transformations.

Acetate promotes the formation of NiRu/NiO towards efficient hydrogen evolution

The introduction of acetate in the precursor of metal-organic frameworks (MOFs) turns the final product from MOFs to NiRu/Ni(OH)₂. Followed by annealing treatment, the obtained NiRu/NiO catalyst exhibits high hydrogen evolution reaction (HER) activity with a low overpotential (18 mV at 10 mA cm^-2), and a small Tafel slope of 43.3 mV dec^-1 in alkaline electrolyte.

Hybrid quantum-classical simulations of magic angle spinning dynamic nuclear polarization in very large spin systems

Solid-state nuclear magnetic resonance can be enhanced using unpaired electron spins with a method known as dynamic nuclear polarization (DNP). Fundamentally, DNP involves ensembles of thousands of spins, a scale that is difficult to match computationally. This scale prevents us from gaining a complete understanding of the spin dynamics and applying simulations to design sample formulations. We recently developed an ab initio model capable of calculating DNP enhancements in systems of up to ∼1000 nuclei; however, this scale is insufficient to accurately simulate the dependence of DNP enhancements on radical concentration or magic angle spinning (MAS) frequency. We build on this work by using ab initio simulations to train a hybrid model that makes use of a rate matrix to treat nuclear spin diffusion. We show that this model can reproduce the MAS rate and concentration dependence of DNP enhancements and build-up time constants. We then apply it to predict the DNP enhancements in core–shell metal-organic-framework nanoparticles and reveal new insights into the composition of the particles’ shells.

Spatial distribution modulation of mixed building blocks in metal-organic frameworks

The placement of mixed building blocks at precise locations in metal–organic frameworks is critical to creating pore environments suitable for advanced applications. Here we show that the spatial distribution of mixed building blocks in metal–organic frameworks can be modulated by exploiting the different temperature sensitivities of the diffusion coefficients and exchange rate constants of the building blocks. By tuning the reaction temperature of the forward linker exchange from one metal–organic framework to another isoreticular metal–organic framework, core–shell microstructural and uniform microstructural metal–organic frameworks are obtained. The strategy can be extended to the fabrication of inverted core–shell microstructures and multi-shell microstructures and applied for the modulation of the spatial distribution of framework metal ions during the post-synthetic metal exchange process of a Zn-based metal–organic framework to an isostructural Ni-based metal–organic framework.

Gaining control over the structure of metal organic–frameworks can be challenging. Here the authors report the modulation of the spatial distribution of mixed building blocks in a metal–organic framework from a uniform to a core–shell distribution; temperature control plays a crucial role.

An X-ray Photoelectron Spectroscopy Study of Postsynthetic Exchange in UiO-66

Postsynthetic exchange (PSE) is a method that is widely used to change the composition of metal-organic frameworks (MOFs) by replacing connecting linkers or metal nodes after the framework has been synthesized. However, few techniques can probe the nature and distribution of exchanged species following PSE. Herein, we show that X-ray photoelectron spectroscopy can be used to compare the relative concentrations of exchanged ligands at the surface and interior regions of MOF particles. Specifically, PSE of iodobenzene dicarboxylate ligands results in a gradient distribution from surface to bulk in UiO-66 nanoparticles that depends on PSE time. X-ray photoelectron spectroscopy also reveals differences between the surface chemistry of the PSE product and that of the direct synthesis product.

Elemental Depth Profiling of Intact Metal-Organic Framework Single Crystals by Scanning Nuclear Microprobe

The growing field of MOF–catalyst composites often relies on postsynthetic modifications for the installation of active sites. In the resulting MOFs, the spatial distribution of the inserted catalysts has far-reaching ramifications for the performance of the system and thus needs to be precisely determined. Herein, we report the application of a scanning nuclear microprobe for accurate and nondestructive depth profiling of individual UiO-66 and UiO-67 (UiO = Universitetet i Oslo) single crystals. Initial optimization work using native UiO-66 crystals yielded a microbeam method which avoided beam damage, while subsequent analysis of Zr/Hf mixed-metal UiO-66 crystals demonstrated the potential of the method to obtain high-resolution depth profiles. The microbeam method was further used to analyze the depth distribution of postsynthetically introduced organic moieties, revealing either core–shell or uniform incorporation can be obtained depending on the size of the introduced molecule, as well as the number of carboxylate binding groups. Finally, the spatial distribution of platinum centers that were postsynthetically installed in the bpy binding pockets of UiO-67-bpy (bpy = 5,5′-dicarboxyy-2,2′-bipyridine) was analyzed by microbeam and contextualized. We expect that the method presented herein will be applicable for characterizing a wide variety of MOFs subjected to postsynthetic modifications and provide information crucial for their optimization as functional materials.

Synthetic approaches for accessing rare-earth analogues of UiO-66

Rare-earth (RE) analogues of UiO-66 with non-functionalised 1,4-benzenedicarboxylate linkers are synthesised for the first time, and a series of synthetic approaches is provided to troubleshoot the synthesis. RE-UiO-66 analogues are fully characterised, and demonstrate a high degree of crystallinity, high surface area and thermal stability, consistent with the UiO-66 archetype.