Mixed hierarchical local structure in a disordered metal-organic framework

A Study on the Ability of Nanomaterials to Adsorb NO and SO2 from Combustion Gases and the Effectiveness of Their Separation

Climate neutrality for the year 2050 is the goal assumed at the level of the EU27+UK. As Romania is no exception, it has assumed the gradual mitigation of pollution generated by the energy sector, and by 2030, according to 'Fit for 55', the share of energy from renewable sources must reach 42.5% from total energy consumption. For the rest of the energy produced from traditional sources, natural gas and/or coal, modern technologies will be used to retain the gaseous noxes. Even if they are not greenhouse gases, NO and SO2, generated from fossil fuel combustion, cause negative effects on the environment and biodiversity. The adsorption capacity of different materials, three nanomaterials developed in-house and three commercial adsorbents, both for NO and SO2, was tackled through gas chromatography, elemental analysis, and Fourier-transform infrared spectroscopy. Fe-BTC has proven to be an excellent material for separation efficiency and adsorption capacity under studied conditions, and is shown to be versatile both in the case of NO (80.00 cm3/g) and SO2 (63.07 cm3/g). All the developed nanomaterials generated superior results in comparison to the commercial adsorbents. The increase in pressure enhanced the performance of the absorption process, while temperature showed an opposite influence, by blocking the active centers on the surface.

High-Level Disordered Metal-Organic Frameworks Synthesized by Interference-Oriented Attachment for Electrochemical Anion Sieve

Disordered MOFs seamlessly amalgamate the robust stability and pore tunability inherent in crystalline MOFs with the advantages derived from abundant defects and active sites present in amorphous structures. This study pioneers the use of the interference-oriented attachment (IOA) mechanism to meticulously craft the morphology and crystal growth of MIL-101(Cr) (Cr-MOF), resulting in the successful synthesis of a high-level disordered Cr-MOF boasting an enhanced array of active sites and exceptional electrochemical properties. The correlation between disordered structures and the electrochemical properties of MOFs are elucidated using the lattice distortion index and fractal dimension. The high-level disordered MOF electrode showcases a remarkable fluoride sieving effect, outperforming conventional fluoride removal materials with a remarkable fluoride adsorption capacity of 41.04 mgNaF gelectrodes -1. First-principles calculations, in conjunction with relevant experiments, provided further validation that the disordered structure significantly enhances the defluorination performance of the material. This study introduces a novel approach for the direct bottom-up synthesis of high-level disordered MOFs, showcasing their potential for applications in electrochemical water treatment.

Defect engineering on electrocatalysts for sustainable nitrate reduction to ammonia: Fundamentals and regulations

Electrocatalytic nitrate (NO3 -) reduction to ammonia (NH3) is a "two birds-one stone" method that targets remediation of NO3 --containing sewage and production of valuable NH3. The exploitation of advanced catalysts with high activity, selectivity, and durability is a key issue for the efficient catalytic performance. Among various strategies for catalyst design, defect engineering has gained increasing attention due to its ability to modulate the electronic properties of electrocatalysts and optimize the adsorption energy of reactive species, thereby enhancing the catalytic performance. Despite previous progress, there remains a lack of mechanistic insights into the regulation of catalyst defects for NO3 - reduction. Herein, this review presents insightful understanding of defect engineering for NO3 - reduction, covering its background, definition, classification, construction, and underlying mechanisms. Moreover, the relationships between regulation of catalyst defects and their catalytic activities are illustrated by investigating the properties of electrocatalysts through the analysis of electronic band structure, charge density distribution, and controllable adsorption energy. Furthermore, challenges and perspectives for future development of defects in NO3RR are also discussed, which can help researchers to better understand the defect engineering in catalysts, and also inspire scientists entering into this promising field.

Transient intermediate in the formation of an amorphous metal-organic framework

Amorphous metal-organic frameworks are rarely formed via direct synthesis. Our limited understanding of their atomic assembly in solution prevents full exploitation of their unique structural complexity. Here, we use in situ synchrotron X-ray absorption spectroscopy with sub-second time resolution to probe the formation of the amorphous Fe-BTC framework. Using a combination of spectral fingerprinting, linear combination analysis, and principal component analysis coupled with kinetic analyses, we reveal a multi-stage formation mechanism that, crucially, proceeds via the generation of a transient intermediate species.

Component regulation on ternary FeCoNi nano-bundles as efficient electrocatalysts for driving water oxidation

Metal organic frameworks (MOFs) are considered as promising electrocatalytic materials due to their tunable porosity, functional organic ligands, and large specific surface area for oxygen evolution reaction (OER). Recently, most reported electrocatalysts focus on the establishing heterogeneous structures by thermal treatments to improve OER performance. However, the thermal treatments are accompanied by the complex synthetic process and destruction of the MOFs structure. Therefore, improving the catalytic performance of pristine MOFs remains a challenge. Here, a series of trimetallic MaMbMc-MOFs (M represents metal element) were synthesized by one-pot method. Modulating the Co/Ni ratio not only adjusts the morphology of FeCoNi-MOFs, but also effectively optimizes the electronic structure. The composition-optimized FeCo0.5Ni2.5-MOF nano-bundles (FeCo0.5Ni2.5-NBs) only required a low overpotential of 273 mV to achieve the current density of 10 mA cm-2 in alkaline solution, with a Tafel slope of 51.1 mV dec-1, lower than other FeCoNi-MOFs and commercial RuO2 catalyst. The two-electrode couple FeCo0.5Ni2.5-NBs || Pt/C achieved the cell voltage of 1.55 V, delivering current density of 10 mA cm-2 for overall water splitting.

Assessing Early Atherosclerosis by Detecting and Imaging of Hypochlorous Acid and Phosphorylation Using Fluorescence Nanoprobe

The assessment of early atherosclerosis (AS) is of great significance for the early diagnosis and mechanism research. Herein, a novel nanoprobe PCN@FL is developed to realize the simultaneous detection and imaging of phosphorylation and hypochlorous acid (HClO). The selective recognition of HClO is achieved through the specific interaction between DMTC and HClO, while the levels of phosphorylation are detected via the specific interaction between Zr (IV) and phosphates. The nanoprobe can be utilized to monitor the fluctuations in HClO and phosphate in early atherosclerosis. It is observed that the levels of HClO and phosphate in the serum of early AS mice are higher than those of the normal mice. Ultimately, the levels of hypochlorous acid and phosphorylation in the inner wall of aortic vessels are imaged by two-photon microscope. The results show that the levels of HClO and phosphorylation in the early atherosclerotic mice are significantly higher than those of in normal mice. The nanoprobe provides a suitable fluorescent tool for simultaneous detection and imaging of HClO and phosphorylation, which holds promise for early atherosclerotic disease assessment.

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

HIGHLIGHTS

The recent development and implementation of metal-organic frameworks (MOFs) and MOF-based materials in electrochemical water applications are reviewed. The critical factors that affect the performances of MOFs in the electrochemical reactions, sensing, and separations are highlighted. Advanced tools, such as pair distribution function analysis, are playing critical roles in unraveling the functioning mechanisms, including local structures and nanoconfined interactions. Metal-organic frameworks (MOFs), a family of highly porous materials possessing huge surface areas and feasible chemical tunability, are emerging as critical functional materials to solve the growing challenges associated with energy-water systems, such as water scarcity issues. In this contribution, the roles of MOFs are highlighted in electrochemical-based water applications (i.e., reactions, sensing, and separations), where MOF-based functional materials exhibit outstanding performances in detecting/removing pollutants, recovering resources, and harvesting energies from different water sources. Compared with the pristine MOFs, the efficiency and/or selectivity can be further enhanced via rational structural modulation of MOFs (e.g., partial metal substitution) or integration of MOFs with other functional materials (e.g., metal clusters and reduced graphene oxide). Several key factors/properties that affect the performances of MOF-based materials are also reviewed, including electronic structures, nanoconfined effects, stability, conductivity, and atomic structures. The advancement in the fundamental understanding of these key factors is expected to shed light on the functioning mechanisms of MOFs (e.g., charge transfer pathways and guest-host interactions), which will subsequently accelerate the integration of precisely designed MOFs into electrochemical architectures to achieve highly effective water remediation with optimized selectivity and long-term stability.

Functions and applications of emerging metal-organic-framework liquids and glasses

Traditional metal-organic-frameworks (MOFs) have been extensively studied and applied in various fields across chemistry, biology and engineering in the past decades. Recently, a family of emerging MOF liquids and glasses have gained ever-growing research interests owing to their fascinating phase transitions and unique functions. To date, a growing number of MOF crystals have been found to be capable of transforming into liquid and glassy states under external stimuli, which overcomes the limitations of MOF crystals by introducing functional disorder in a controlled manner and offering some desirable properties. This review is dedicated to compiling recent advances in the fundamental understanding of the phase and structure evolution during crystal melting and glass formation in order to give insights into the underlying conversion mechanism. Benefiting from the disordered metal-ligand arrangement and free grain boundaries, various functional properties of liquid and glassy MOFs including porosity, ionic conductivity, and optical/mechanical properties are summarized and evaluated in detail, accompanied by the structure-property correlation. At the same time, their potential applications are further assessed from a developmental perspective according to their unique functions. Finally, we summarize the current progress in the development of liquid/glassy MOFs and point out the serious challenges as well as the potential solutions. This work provides perspectives on the functional applications of liquid/glassy MOFs and highlights the future research directions for the advancement of MOF liquids and glasses.

Mapping nanocrystalline disorder within an amorphous metal-organic framework

Intentionally disordered metal-organic frameworks (MOFs) display rich functional behaviour. However, the characterisation of their atomic structures remains incredibly challenging. X-ray pair distribution function techniques have been pivotal in determining their average local structure but are largely insensitive to spatial variations in the structure. Fe-BTC (BTC = 1,3,5-benzenetricarboxylate) is a nanocomposite MOF, known for its catalytic properties, comprising crystalline nanoparticles and an amorphous matrix. Here, we use scanning electron diffraction to first map the crystalline and amorphous components to evaluate domain size and then to carry out electron pair distribution function analysis to probe the spatially separated atomic structure of the amorphous matrix. Further Bragg scattering analysis reveals systematic orientational disorder within Fe-BTC's nanocrystallites, showing over 10° of continuous lattice rotation across single particles. Finally, we identify candidate unit cells for the crystalline component. These independent structural analyses quantify disorder in Fe-BTC at the critical length scale for engineering composite MOF materials.

Phosphorus-Doped directly interconnected networks of amorphous Metal-Organic framework nanowires for efficient methanol oxidation

Developing high-performance and low-cost electrocatalysts toward methanol oxidation reaction (MOR) is essential for fuel cell applications. Herein, we report a defect engineering strategy integrating amorphization and phosphorization to construct directly interconnected networks of amorphous NiCo-based metal-organic framework nanowires (a-NiCo-MOFNWs) with phosphorus (P) doping. The resulting P-doped a-NiCo-MOFNWs (a-NiCo-MOFNWs-P) network displays superior MOR efficiency and long-term durability over 1000 cyclic voltammetry (CV) measurements. The special structure of directly interconnected networks and the synergistic effect between the amorphous MOFs and dispersed phosphorus species give rise to abundant exposed active sites, accelerated electron transport, and increased porosity for mass transfer, thus boosting the reaction kinetics of MOR. This work provides additional insights into the network assembly and structural evolution of one-dimensional (1D) MOFs, and also opens up new avenues for the design of highly reactive and robust non-precious metal-based electrocatalysts.

One-dimensional alignment of defects in a flexible metal-organic framework

Crystalline materials are often considered to have rigid periodic lattices, while soft materials are associated with flexibility and nonperiodicity. The continuous evolution of metal-organic frameworks (MOFs) has erased the boundaries between these two distinct conceptions. Flexibility, disorder, and defects have been found to be abundant in MOF materials with imperfect crystallinity, and their intricate interplay is poorly understood because of the limited strategies for characterizing disordered structures. Here, we apply advanced nuclear magnetic resonance spectroscopy to elucidate the mesoscale structures in a defective MOF with a semicrystalline lattice. We show that engineered defects can tune the degree of lattice flexibility by combining both ordered and disordered compartments. The one-dimensional alignment of correlated defects is the key for the reversible topological transition. The unique matrix is featured with both rigid framework of nanoporosity and flexible linkage of high swellability.

Quantification of gas-accessible microporosity in metal-organic framework glasses

Metal-organic framework (MOF) glasses are a new class of glass materials with immense potential for applications ranging from gas separation to optics and solid electrolytes. Due to the inherent difficulty to determine the atomistic structure of amorphous glasses, the intrinsic structural porosity of MOF glasses is only poorly understood. Here, we investigate the porosity features (pore size and pore limiting diameter) of a series of prototypical MOF glass formers from the family of zeolitic imidazolate frameworks (ZIFs) and their corresponding glasses. CO2 sorption at 195 K allows quantifying the microporosity of these materials in their crystalline and glassy states, also providing excess to the micropore volume and the apparent density of the ZIF glasses. Additional hydrocarbon sorption data together with X-ray total scattering experiments prove that the porosity features of the ZIF glasses depend on the types of organic linkers. This allows formulating design principles for a targeted tuning of the intrinsic microporosity of MOF glasses. These principles are counterintuitive and contrary to those established for crystalline MOFs but show similarities to strategies previously developed for porous polymers.

Computational modeling to assist in the discovery of supramolecular materials

Computational modeling is increasingly used to assist in the discovery of supramolecular materials. Supramolecular materials are typically primarily built from organic components that are self-assembled through noncovalent bonding and have potential applications, including in selective binding, sorption, molecular separations, catalysis, optoelectronics, sensing, and as molecular machines. In this review, the key areas where computational prediction can assist in the discovery of supramolecular materials, including in structure prediction, property prediction, and the prediction of how to synthesize a hypothetical material are discussed, before exploring the potential impact of artificial intelligence techniques on the field. Throughout, the importance of close integration with experimental materials discovery programs will be highlighted. A series of case studies from the author's work across some different supramolecular material classes will be discussed, before finishing with a discussion of the outlook for the field.

Enzyme Immobilization on Metal Organic Frameworks: the Effect of Buffer on the Stability of the Support

Metal organic frameworks (MOFs) have been used to encapsulate an array of enzymes in a rapid and facile manner; however, the stability of MOFs as supports for enzymes has not been examined in detail. This study examines the stability of MOFs with different compositions (Fe-BTC, Co-TMA, Ni-TMA, Cu-TMA, and ZIF-zni) in buffered solutions commonly used in enzyme immobilization and biocatalysis. Stability was assessed via quantification of the release of metals by inductively coupled plasma optical emission spectroscopy. The buffers used had varied effects on different MOF supports, with incubation of all MOFs in buffers resulting in the release of metal ions to varying extents. Fe-BTC was completely dissolved in citrate, a buffer that has a profound destabilizing effect on all MOFs analyzed, precluding its use with MOFs. MOFs were more stable in acetate, potassium phosphate, and Tris HCl buffers. The results obtained provide a guide for the selection of an appropriate buffer with a particular MOF as a support for the immobilization of an enzyme. In addition, these results identify the requirement to develop methods of improving the stability of MOFs in aqueous solutions. The use of polymer coatings was evaluated with polyacrylic acid (PAA) providing an improved level of stability. Lipase was immobilized in Fe-BTC with PAA coating, resulting in a stable biocatalyst with retention of activity in comparison to the free enzyme.

Modeling the Effect of Defects and Disorder in Amorphous Metal-Organic Frameworks

Amorphous metal-organic frameworks (aMOFs) are a class of disordered framework materials with a defined local order given by the connectivity between inorganic nodes and organic linkers, but absent long-range order. The rational development of function for aMOFs is hindered by our limited understanding of the underlying structure-property relationships in these systems, a consequence of the absence of long-range order, which makes experimental characterization particularly challenging. Here, we use a versatile modeling approach to generate in silico structural models for an aMOF based on Fe trimers and 1,3,5-benzenetricarboxylate (BTC) linkers, Fe-BTC. We build a phase space for this material that includes nine amorphous phases with different degrees of defects and local order. These models are analyzed through a combination of structural analysis, pore analysis, and pair distribution functions. Therefore, we are able to systematically explore the effects of the variation of each of these features, both in isolation and combined, for a disordered MOF system, something that would not be possible through experiment alone. We find that the degree of local order has a greater impact on structure and properties than the degree of defects. The approach presented here is versatile and allows for the study of different structural features and MOF chemistries, enabling the derivation of design rules for the rational development of aMOFs.

Investigating the chemical sensitivity of melting in zeolitic imidazolate frameworks

The number of zeolitic imidazolate frameworks (ZIFs) that form melt-quenched glasses remains limited, with most displaying the cag network topology. Here, we expand our studies to zni topology ZIFs, starting with ZIF-zni [Zn(Im)₂] before changing its linker chemistry, by incorporating 2-methylimidazolate and 5-aminobenzimidazolate. ZIF-zni was found to melt and form a glass, with T_m = 576 °C and T_g = 322 °C, although it was not possible to prepare the glass without zinc oxide impurities. The addition of 2-methylimidazolate to the structure gave ZIF-61 [Zn(Im)_1.35(mIm)_0.65], which decomposed without passing through the liquid state. However, incorporating small quantities of 5-aminobenzimidazolate resulted in a ZIF [Zn(Im)_1.995(abIm)_0.005] with a lower melting temperature (T_m = 569 °C) than pure ZIF-zni, and no evidence of zinc oxide growth. This demonstrates the sensitivity of melting behaviour in ZIFs towards linker chemistry, with only a 0.25% variation capable of eliciting a 7 °C change in melting temperature. This study highlights the chemical sensitivity of melting in ZIFs and serves as a promising strategy for tuning their melting behaviour.

Insights into Solid-To-Solid Transformation of MOF Amorphous Phases

Metal-organic frameworks (MOFs) have been recently explored as crystalline solids for conversion into amorphous phases demonstrating non-specific mechanical, catalytic, and optical properties. The real-time control of such structural transformations and their outcomes still remain a challenge. Here, we use in situ high-resolution transmission electron microscopy with 0.01 s time resolution to explore non-thermal (electron induced) amorphization of a MOF single crystal, followed by transformation into an amorphous nanomaterial. By comparing a series of M-BTC (M: Fe³⁺, Co³⁺, Co²⁺, Ni²⁺, and Cu²⁺; BTC: 1,3,5-benzentricarboxylic acid), we demonstrate that the topology of a metal cluster of the parent MOFs determines the rate of formation and the chemistry of the resulting phases containing an intact ligand and metal or metal oxide nanoparticles. Confocal Raman and photoluminescence spectroscopies further confirm the integrity of the BTC ligand and coordination bond breaking, while high-resolution imaging with chemical and structural analysis over time allows for tracking the dynamics of solid-to-solid transformations. The revealed relationship between the initial and resulting structures and the stability of the obtained phase and its photoluminescence over time contribute to the design of new amorphous MOF-based optical nanomaterials.

Transformation of Amorphous Terbium Metal-Organic Framework on Terbium Oxide TbOx(111) Thin Film on Pt(111) Substrate: Structure of TbxOy Film

The present study is focused on the synthesis and structural properties of amorphous terbium metal-organic framework thin film (TbMOF-TF) and its transformation to terbium oxide by pyrolysis at 450 °C in the air. The crystalline (cTbMOF) and amorphous (aTbMOF) films were prepared by solvothermal synthesis using different amounts (0.4 and 0.7 mmol) of the modulator (sodium acetate), respectively. The powders were characterized by differential scanning calorimetry (DSC), thermogravimetry (TG), Fourier transform infrared (FTIR), Raman spectroscopy, and scanning electron microscopy (SEM). The varied chemical composition of the surface of TbMOFs and Tb_xO_y was investigated by X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that aTbMOF had been fully transformed to a Tb₄O₇ phase with a cubic crystal structure at 450 °C. The amorphous aTbMOF-TF film was prepared by dropping a colloidal solution of amorphous precursor nanocrystals on the SiO₂/Si substrates covered with Pt as an interlayer. XPS confirmed the presence of Tb in two states, Tb³⁺ and Tb⁴⁺. The amorphous film has a rough, porous microstructure and is composed of large clusters of worm-like particles, while terbium oxide film consists of fine crystallites of cubic fluorite cF-TbO_x, c-Tb₄O_7, and c-Tb₂O₃ phases. The surface topography was investigated by a combination of confocal (CM) and atomic force microscopy (AFM). The amorphous film is porous and rough, which is contrast to the crystalline terbium oxide film.

A new cotton functionalized with iron(III) trimer-like metal framework as an effective strategy for the adsorption of triarylmethane dye: An insight into the dye adsorption processes

A new Cotton@Fe-BTC composite formed by Fe-BTC (BTC-H3: trimesic acid) metal framework (Fe-BTC MOF loading as high 38 wt %) supported by cellulose fiber is synthesized in aqueous media using a simple and green preparation method, described for the first time in this manuscript. This new strategy relies on the synergetic effect of the pure cellulose and MOFs frameworks resulting in hybrid nanofibers of MOFs@cellulose composite. A complete characterization of the composite material reveals its structural similarity to MIL-100(Fe), a Fe-BTC material. The Cotton@Fe-BTC composite potential use as an eco-friendly and low-cost adsorbent was evaluated for its adsorptive performance for the removal of dye belonging to the triarylmethane dye family (Malachite Green (MQ), Brilliant Green (BG), Pararosaniline (PR), Basic Fuchsine (BF), Crystal Violet (CV), Methyl Green (Met-G), Victoria Blue B (VB), Acid Fuchsin (AF) and Aniline Blue (AB)) in aqueous solution. The fast kinetics and high dye removal efficiencies (>90%) obtained in aqueous solutions. The structure of Cotton@Fe-BTC network, contributed to the remarkable adsorption properties towards a variety of triphenylmethanedye. The interparticle studies showed two main steps in the dye adsorption processes, with the exception of AF and BG. The equilibrium adsorption capacities qe (mg/g) follow the order: AF (3.64) Collapse

Key Words

• Adsorption
• Fe-BTC
• Natural cotton
• Solvation/desolvation penalty processes
• Triphenylmetane dye
• Water

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Affiliation(s)

J.E. Conde-González Departamento de Química, Unidad Departamental de Química Analítica, Facultad de Ciencias, Universidad de La Laguna, Tenerife, Spain
P. Lorenzo-Luis Inorganic Chemistry Area, Section of Chemistry Faculty of Science, Tenerife, Spain Instituto Universitario de Bio-Orgánica “Antonio González”, University of La Laguna, Tenerife, Spain
V. Salvadó Department de Química, Facultat de Ciències, Universitat de Girona, C/ M Aurèlia Capmany, 69, 17003 Girona, Spain
J. Havel Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5/A14, 625 00 Brno, Czech Republic
E.M. Peña-Méndez Departamento de Química, Unidad Departamental de Química Analítica, Facultad de Ciencias, Universidad de La Laguna, Tenerife, Spain

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Sustainable One-Pot Immobilization of Enzymes in/on Metal-Organic Framework Materials

The industrial use of enzymes generally necessitates their immobilization onto solid supports. The well-known high affinity of enzymes for metal-organic framework (MOF) materials, together with the great versatility of MOFs in terms of structure, composition, functionalization and synthetic approaches, has led the scientific community to develop very different strategies for the immobilization of enzymes in/on MOFs. This review focuses on one of these strategies, namely, the one-pot enzyme immobilization within sustainable MOFs, which is particularly enticing as the resultant biocomposite Enzyme@MOFs have the potential to be: (i) prepared in situ, that is, in just one step; (ii) may be synthesized under sustainable conditions: with water as the sole solvent at room temperature with moderate pHs, etc.; (iii) are able to retain high enzyme loading; (iv) have negligible protein leaching; and (v) give enzymatic activities approaching that given by the corresponding free enzymes. Moreover, this methodology seems to be near-universal, as success has been achieved with different MOFs, with different enzymes and for different applications. So far, the metal ions forming the MOF materials have been chosen according to their low price, low toxicity and, of course, their possibility for generating MOFs at room temperature in water, in order to close the cycle of economic, environmental and energy sustainability in the synthesis, application and disposal life cycle.