Molecular identification and quantification of defect sites in metal-organic frameworks with NMR probe molecules

Promoted electrochemical performance by MOF on MOF composite catalyst of microbial fuel cell: CuCo-MOF@ZIF-8 and the comparison between two-step hydrothermal method and dual-solution method

The microbial fuel cell (MFC) is a device that simultaneously achieves electricity generation and sewage degradation. In this study, a novel cathode catalyst metal-organic frameworks (MOFs) have been fabricated by two-step hydrothermal and dual-solution method (CuCo-MOF@ZIF-8). The synthesized trimetal MOFs exhibited a 3D badminton-like structure morphology and porosity. The results of the characterizations showed that CuCo-MOF@ZIF-8 possesses greater surface area porosity and novel functional groups. The Trimetal MOF-on-MOF mode not only demonstrated the stability of the structure but also enhanced its mechanism. Molecular mechanism analysis revealed changes in the organic ligand and metal binding site due to the transformation of Cu2+ to Cu+, Co2+ to Co3+, and Zn-N to Zn-O organic connection. Furthermore, differences between the two fabrication methods were compared. The solid-state single preparation (CuCo-MOF@ZIF-8-1), was synthesized using the two-step hydrothermal method; the liquid mixed preparation material (CuCo-MOF@ZIF-8-2), was synthesized using the dual-solution method; they exhibited completely different chemical structures and morphologies during material testing and characterization. The maximum output power density of CuCo-MOF@ZIF-8-2-MFC was 246.38 mW/m2, about 2.49 times of ZIF-8 (98.72 mW/m2). The output voltage of CuCo-MOF@ZIF-8-1-MFC was measured at 357 mV over 10 d, while that of CuCo-MOF@ZIF-8-2-MFC reached 365 mV over 12 d.

Defect Induced Structural Transition and Lipase Immobilization in Mesoporous Aluminum Metal-Organic Frameworks

The transition from disorder to order and structural transformation are distinctive metal-organic framework (MOF) features. How to adapt or control both behaviors in MOF has rarely been studied. In this case, we demonstrate that our successful synthesis of [Al(OH)(PDA)]n (AlPDA-53-DEF, AlPDA-53-H, and AlPDA-68) with H2PDA=4,4'-[1,4-phenylenebis(ethyne-2,1-diyl)]-di benzoic acid has shown the intricate world of Aluminum Metal-Organic Frameworks (Al-MOFs). It offers profound insights into defect structures to order and transformations. AlPDA-53-DEF, in particular, revealed a fascinating interplay of various pore sizes within both micro and mesoporous regions, unveiling a unique lattice rearrangement phenomenon upon solvent desorption. Defects and disorders emerged as crucial impacts of transforming AlPDA-53-DEF, with its initially imperfect crystallinity, into the highly crystalline, hierarchically porous AlPDA-53-H.

A Drop-and-Drain Method for Convenient and Efficient Fabrication of MOF/Fiber Composites

The fabrication of flexible composites by integrating metal-organic frameworks (MOFs) with flexible substrates is a critical strategy for developing advanced materials with excellent feasibility and processability. These flexible MOF-based composites play a particularly important role in the separation and purification processes. However, several drawbacks remain challenge to overcome such as long processing time, high-cost, complicated processes, or harsh reaction conditions. In this paper, a convenient and efficient method is reported for fabricating MOF/fiber composites using a simple drop-and-drain (D&D) process. By exploiting the electrostatic interactions between the positively charged MOF particles and negatively charged fiber-based flexible substrates, a uniform coating of MOF on flexible fibers are achieved. This is accomplished by allowing the MOF ink to drop and drain through a substrate using a custom-made Teflon cell. Additionally, the D&D method enables the production of multiple layers of composites in a single-step process. UiO-66 and ZIF-8 submicroparticles and various substrates such as cotton-pad, cotton-fabric, nylon-fabric, PET-fabric, and filter-paper are employed to create flexible MOF/fiber composites. These composites demonstrate outstanding capacities for capturing negatively charged organic dyes, including methyl orange and indigo carmine. Furthermore, the MOF/fiber composites can be reused for dye capture after a simple washing process.

Understanding Cu(i) local environments in MOFs via63/65Cu NMR spectroscopy

The field of metal-organic frameworks (MOFs) includes a vast number of hybrid organic and inorganic porous materials with wide-ranging applications. In particular, the Cu(i) ion exhibits rich coordination chemistry in MOFs and can exist in two-, three-, and four-coordinate environments, which gives rise to many structural motifs and potential applications. Direct characterization of the structurally and chemically important Cu(i) local environments is essential for understanding the sources of specific MOF properties. For the first time, 63/65Cu solid-state NMR has been used to investigate a variety of Cu(i) sites and local coordination geometries in Cu MOFs. This approach is a sensitive probe of the local Cu environment, particularly when combined with density functional theory calculations. A wide range of structurally-dependent 63/65Cu NMR parameters have been observed, including 65Cu quadrupolar coupling constants ranging from 18.8 to 74.8 MHz. Using the data from this and prior studies, a correlation between Cu quadrupolar coupling constants, Cu coordination number, and local Cu coordination geometry has been established. Links between DFT-calculated and experimental Cu NMR parameters are also presented. Several case studies illustrate the feasibility of 63/65Cu NMR for investigating and resolving inequivalent Cu sites, monitoring MOF phase changes, interrogating the Cu oxidation number, and characterizing the product of a MOF chemical reaction involving Cu(ii) reduction to Cu(i). A convenient avenue to acquire accurate 65Cu NMR spectra and NMR parameters from Cu(i) MOFs at a widely accessible magnetic field of 9.4 T is described, with a demonstrated practical application for tracking Cu(i) coordination evolution during MOF anion exchange. This work showcases the power of 63/65Cu solid-state NMR spectroscopy and DFT calculations for molecular-level characterization of Cu(i) centers in MOFs, along with the potential of this protocol for investigating a wide variety of MOF structural changes and processes important for practical applications. This approach has broad applications for examining Cu(i) centers in other weight-dilute systems.

Scaling-Up Microwave-Assisted Synthesis of Highly Defective Pd@UiO-66-NH2 Catalysts for Selective Olefin Hydrogenation under Ambient Conditions

The need to develop green and cost-effective industrial catalytic processes has led to growing interest in preparing more robust, efficient, and selective heterogeneous catalysts at a large scale. In this regard, microwave-assisted synthesis is a fast method for fabricating heterogeneous catalysts (including metal oxides, zeolites, metal-organic frameworks, and supported metal nanoparticles) with enhanced catalytic properties, enabling synthesis scale-up. Herein, the synthesis of nanosized UiO-66-NH2 was optimized via a microwave-assisted hydrothermal method to obtain defective matrices essential for the stabilization of metal nanoparticles, promoting catalytically active sites for hydrogenation reactions (760 kg·m-3·day-1 space time yield, STY). Then, this protocol was scaled up in a multimodal microwave reactor, reaching 86% yield (ca. 1 g, 1450 kg·m-3·day-1 STY) in only 30 min. Afterward, Pd nanoparticles were formed in situ decorating the nanoMOF by an effective and fast microwave-assisted hydrothermal method, resulting in the formation of Pd@UiO-66-NH2 composites. Both the localization and oxidation states of Pd nanoparticles (NPs) in the MOF were achieved using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray photoelectron spectroscopy (XPS), respectively. The optimal composite, loaded with 1.7 wt % Pd, exhibited an extraordinary catalytic activity (>95% yield, 100% selectivity) under mild conditions (1 bar H2, 25 °C, 1 h reaction time), not only in the selective hydrogenation of a variety of single alkenes (1-hexene, 1-octene, 1-tridecene, cyclohexene, and tetraphenyl ethylene) but also in the conversion of a complex mixture of alkenes (i.e., 1-hexene, 1-tridecene, and anethole). The results showed a powerful interaction and synergy between the active phase (Pd NPs) and the catalytic porous scaffold (UiO-66-NH2), which are essential for the selectivity and recyclability.

Analysis and Refinement of Host-Guest Interactions in Metal-Organic Frameworks

ConspectusMetal-organic frameworks (MOFs) are a class of hybrid porous materials characterized by their periodic assembly using metal ions and organic ligands through coordination bonds. Their high crystallinity, extensive surface area, and adjustable pore sizes make them promising candidates for a wide array of applications. These include gas adsorption and separation, substrate binding, and catalysis, of relevance to tackling pressing global issues such as climate change, energy challenges, and pollution. In comparison to traditional porous materials such as zeolites and activated carbons, the design flexibility of organic ligands in MOFs, coupled with their orderly arrangement with associated metal centers, allows for the precise engineering of uniform pore environments. This unique feature enables a rich variety of interactions between the MOF host and adsorbed gas molecules, which are fundamental to understanding the observed uptake capacity and selectivity for target gas molecules and thus the overall performance of the material.In this Account, a data set for three-dimensional MOFs has been constructed based upon the structural analysis of host-guest interactions using the largest experimental database, the Cambridge Structural Database (CSD). A full screening was performed on structures with guest molecules of H2, C2H2, CO2, and SO2, and the relationship between the primary binding site, the isosteric heats of adsorption (Qst), and the adsorption uptake was extracted and established. We review the methodologies to refine host-guest interactions based primarily on our studies on the host-guest chemistry of MOFs. The methods include ligand functionalization, variation of metal centers, formation of defects, addition of single atom sites, and control of pore size and structure. In situ structural and dynamic investigations using diffraction and spectroscopic techniques are powerful tools to visualize the details of host-guest interactions upon the above modifications, affording key insights into functional performance at a molecular level. Finally, we give an outlook of future research priorities in the study of host-guest chemistry in MOF materials. We hope this Account will encourage the rational development and improvement of future MOF-based sorbents for applications in challenging gas adsorption, separations, and catalysis.

Nanoanalytical Insights into the Stability, Intracellular Fate, and Biotransformation of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have found increasing applications in the biomedical field due to their unique properties and high modularity. Although the limited stability of MOFs in biological environments is increasingly recognized, analytical techniques have not yet been harnessed to their full potential to assess the biological fate of MOFs. Here, we investigate the environment-dependent biochemical transformations of widely researched nanosized MOFs (nMOFs) under conditions relevant to their medical application. We assess the chemical stability of antimicrobial zinc-based drug delivery nMOFs (Zn-ZIF-8 and Zn-ZIF-8:Ce) and radio-enhancer candidate nMOFs (Hf-DBA, Ti-MIL-125, and TiZr-PCN-415) containing biologically nonessential group IV metal ions. We reveal that even a moderate decrease in pH to values encountered in lysosomes (pH 4.5-5) leads to significant dissolution of ZIF-8 and partial dissolution of Ti-MIL-125, whereas no substantial dissolution was observed for TiZr-PCN-415 and Hf-DBA nMOFs. Exposure to phosphate-rich buffers led to phosphate incorporation in all nMOFs, resulting in amorphization and morphological changes. Interestingly, long-term cell culture studies revealed that nMOF (bio)transformations of, e.g., Ti-MIL-125 were cellular compartment-dependent and that the phosphate content in the nMOF varied significantly between nMOFs localized in lysosomes and those in the cytoplasm. These results illustrate the delicate nature and environment-dependent properties of nMOFs across all stages of their life cycle, including storage, formulation, and application, and the need for in-depth analyses of biotransformations for an improved understanding of structure-function relationships. The findings encourage the considerate choice of suspension buffers for MOFs because these media may lead to significant material alterations prior to application.

Solvent-derived defects suppress adsorption in MOF-74

Defects in metal-organic frameworks (MOFs) have great impact on their nano-scale structure and physiochemical properties. However, isolated defects are easily concealed when the frameworks are interrogated by typical characterization methods. In this work, we unveil the presence of solvent-derived formate defects in MOF-74, an important class of MOFs with open metal sites. With multi-dimensional solid-state nuclear magnetic resonance (NMR) investigations, we uncover the ligand substitution role of formate and its chemical origin from decomposed N,N-dimethylformamide (DMF) solvent. The placement and coordination structure of formate defects are determined by ¹³C NMR and density functional theory (DFT) calculations. The extra metal-oxygen bonds with formates partially eliminate open metal sites and lead to a quantitative decrease of N₂ and CO₂ adsorption with respect to the defect concentration. In-situ NMR analysis and molecular simulations of CO₂ dynamics elaborate the adsorption mechanisms in defective MOF-74. Our study establishes comprehensive strategies to search, elucidate and manipulate defects in MOFs.

Structural and Acidic Characteristics of Multiple Zr Defect Sites in UiO-66 Metal-Organic Frameworks

Although defects are prevalent in metal-organic frameworks (MOFs) and usually play a crucial role in modulating their performance in various applications, detailed structural characterizations of various defects remain a challenging task mainly due to their disordered, heterogeneous, and local nature. In this work, by using solid-state nuclear magnetic resonance spectroscopy (SSNMR) techniques in conjunction with density functional theory (DFT) calculations, it is clearly elucidated that the trimethylphosphine (TMP)-assisted ³¹P NMR strategy is capable of greatly facilitating the qualitative and quantitative description of the detailed structural and acidic characteristics as well as the evolution process of various Zr defects with subtle distinctions in UiO-66 upon moderate thermal treatment, hence surpassing most conventional analytical techniques. These results offer a fundamental understanding of the defect chemistry in MOFs.