Bimetallic Organic Frameworks via In Situ Solvothermal Sol-Gel-Crystal and Sol-Crystal Transformation as Durable Electrocatalysts for Oxygen Reduction Reaction

The in situ solvothermal conversion of metal-organic gels (MOGs) to crystalline metal-organic frameworks (MOFs) represents a versatile and ingenious strategy that has been employed for the synthesis of MOF materials with specific morphologies, high yield, and improved functional properties. Herein, we have adopted an in situ solvothermal conversion of bimetallic MOGs to crystalline bimetallic MOFs with the aim of introducing a redox-active metal heterogeneity into the monometallic counterpart. The formation of bimetallic NiZn-MOF and CoZn-MOF via in situ solvothermal sol-gel-crystal and sol-crystal transformation is found to depend on the solvent systems used. The sol-to-gel-to-crystal transformation of NiZn-MOF via the formation of NiZn-MOG is found to occur through the gradual disruption of gel fibers leading to subsequent formation of microcrystals and single crystals of NiZn-MOF. These bimetallic MOFs and MOGs serve as promising electrocatalysts for oxygen reduction reaction (ORR) with an excellent methanol tolerance property, which can be attributed to the enhanced mass and charge transfer, higher oxygen vacancies, and bimetallic synergistic interactions among the heterometals. This work demonstrates a convenient strategy for producing bimetallic MOGs to MOFs through the introduction of a redox-active metal heterogeneity in the inorganic hybrid functional materials for fundamental and applied research. Our results connect MOGs and MOFs which have been regarded as having opposite physical states, that is, soft vs hard, and provide promising structural correlation between MOGs and MOFs at the molecular level.

Nickel-Tin Nanoalloy Supported ZnO Catalysts from Mixed-Metal Zeolitic Imidazolate Frameworks for Selective Conversion of Glycerol to 1,2-Propanediol

The successful synthesis of finely tuned Ni1.5 Sn nanoalloy phases containing ZnO catalyst with a small particle size (6.7 nm) from a mixed-metal zeolitic imidazolate framework (MM-ZIF) is investigated. The catalyst was evaluated for the efficient production of 1,2-propanediol (1,2-PDO) from crude glycerol and comprehensively characterized using several analytical techniques. Among the catalysts, 3Ni1Sn/ZnO (Ni/Sn=3/1) showed the best catalytic performance and produced the highest yield (94.2 %) of 1,2-PDO at ~100 % conversion of glycerol; it also showed low apparent activation energy (15.4 kJ/mol) and excellent stability. The results demonstrated that the synergy between Ni-Sn alloy, finely dispersed Ni metallic sites, and the Lewis acidity of SnOx species-loaded ZnO played a pivotal role in the high activity and selectivity of the catalyst. The confirmation of acetol intermediate and theoretical calculations verify the Ni1.5 Sn phases provide the least energetic pathway for the formation of 1,2-PDO selectively. The reusability of solvent for successive ZIF synthesis, along with the excellent recyclability of the ZIF-derived catalyst, enables an overall sustainable process. We believe that the present synthetic protocol that uses MM-ZIF for the conversion of various biomass-derived platform chemicals into valuable products can be applied to various nanoalloy preparations.

The Properties of Microwave-Assisted Synthesis of Metal-Organic Frameworks and Their Applications

Metal-organic frameworks (MOF) are a class of porous materials with various functions based on their host-guest chemistry. Their selectivity, diffusion kinetics, and catalytic activity are influenced by their design and synthetic procedure. The synthesis of different MOFs has been of considerable interest during the past decade thanks to their various applications in the arena of sensors, catalysts, adsorption, and electronic devices. Among the different techniques for the synthesis of MOFs, such as the solvothermal, sonochemical, ionothermal, and mechanochemical processes, microwave-assisted synthesis has clinched a significant place in MOF synthesis. The main assets of microwave-assisted synthesis are the short reaction time, the fast rate of nucleation, and the modified properties of MOFs. The review encompasses the development of the microwave-assisted synthesis of MOFs, their properties, and their applications in various fields.

In situ conversion of a MOG to a crystalline MOF: a case study on solvent-dependent gelation and crystallization

Herein, we report in situ transformation of a metal-organic gel (MOG) to a crystalline metal-organic framework (MOF) and solvent-dependent gelation/crystallization via solvothermal reactions of a tetracarboxylic acid, namely 4,4'-dinitro-2,2',6,6'-tetracarboxybiphenyl, and ZnSO₄. The results provide structural insights into MOGs at the molecular level and also help in the synthesis of crystalline MOFs that are otherwise difficult to obtain.

Applications of water-stable metal-organic frameworks in the removal of water pollutants: A review

Because the pollutants produced by human activities have destroyed the ecological balance of natural water environment, and caused severe impact on human life safety and environmental security. Hence the task of water environment restoration is imminent. Metal-organic frameworks (MOFs), structured from organic ligands and inorganic metal ions, are notable for their outstanding crystallinity, diverse structures, large surface areas, adsorption performance, and excellent component tunability. The water stability of MOFs is a key requisite for their possible actual applications in separation, catalysis, adsorption, and other water environment remediation areas because it is necessary to safeguard the integrity of the material structure during utilization. In this article, we comprehensively review state-of-the-art research progress on the promising potential of MOFs as excellent nanomaterials to remove contaminants from the water environment. Firstly, the fundamental characteristics and preparation methods of several typical water-stable MOFs include UiO, MIL, and ZIF are introduced. Then, the removal property and mechanism of heavy metal ions, radionuclide contaminants, drugs, and organic dyes by different MOFs were compared. Finally, the application prospect of MOFs in pollutant remediation prospected. In this review, the synthesis methods and application in water pollutant removal are explored, which provide ways toward the effective use of water-stable MOFs in materials design and environmental remediation.

Advances in Microwave Synthesis of Nanoporous Materials

Usually, porous materials are synthesized by using conventional electric heating, which can be energy- and time-consuming. Microwave heating is commonly used in many households to quickly heat food. Microwave ovens can also be used as powerful devices in the synthesis of various porous materials. The microwave-assisted synthesis offers a simple, fast, efficient, and economic way to obtain many of the advanced nanomaterials. This review summarizes the recent achievements in the microwave-assisted synthesis of diverse groups of nanoporous materials including silicas, carbons, metal-organic frameworks, and metal oxides. Microwave-assisted methods afford highly porous materials with high specific surface areas (SSAs), e.g., activated carbons with SSA ≈3100 m² g^-1 , metal-organic frameworks with SSA ≈4200 m² g^-1 , covalent organic frameworks with SSA ≈2900 m² g^-1 , and metal oxides with relatively small SSA ≈300 m² g^-1 . These methods are also successfully implemented for the preparation of ordered mesoporous silicas and carbons as well as spherically shaped nanomaterials. Most of the nanoporous materials obtained under microwave irradiation show potential applications in gas adsorption, water treatment, catalysis, energy storage, and drug delivery, among others.

Iron-based metal-organic framework: Synthesis, structure and current technologies for water reclamation with deep insight into framework integrity

Water is a supreme requirement for the existence of life, the contamination from the point and non-point sources are creating a great threat to the water ecosystem. Advance tools and techniques are required to restore the water quality and metal-organic framework (MOFs) with a tunable porous structure, striking physical and chemical properties are an excellent candidate for it. Fe-based MOFs, which developed rapidly in recent years, are foreseen as most promising to overcome the disadvantages of traditional water depolluting practices. Fe-MOFs with low toxicity and preferable stability possess excellent performance potential for almost all water remedying techniques in contrast to other MOF structures, especially visible light photocatalysis, Fenton, and Fenton-like heterogeneous catalysis. Fe-MOFs become essential tool for water treatment due to their high catalytic activity, abundant active site and pollutant-specific adsorption. However, the structural degradation under external chemical, photolytic, mechanical, and thermal stimuli is impeding Fe-MOFs from further improvement in activity and their commercialization. Understanding the shortcomings of structural integrity is crucial for large-scale synthesis and commercial implementation of Fe-MOFs-based water treatment techniques. Herein we summarize the synthesis, structure and recent advancements in water remediation methods using Fe-MOFs in particular more attention is paid for adsorption, heterogeneous catalysis and photocatalysis with clear insight into the mechanisms involved. For ease of analysis, the pollutants have been classified into two major classes; inorganic pollutants and organic pollutants. In this review, we present for the first time a detailed insight into the challenges in employing Fe-MOFs for water remediation due to structural instability.

Aqueous Flow Reactor and Vapour-Assisted Synthesis of Aluminium Dicarboxylate Metal-Organic Frameworks with Tuneable Water Sorption Properties

Energy-efficient indoors temperature and humidity control can be realised by using the reversible adsorption and desorption of water in porous materials. Stable microporous aluminium-based metal-organic frameworks (MOFs) present promising water sorption properties for this goal. The development of synthesis routes that make use of available and affordable building blocks and avoid the use of organic solvents is crucial to advance this field. In this work, two scalable synthesis routes under mild reaction conditions were developed for aluminium-based MOFs: (1) in aqueous solutions using a continuous-flow reactor and (2) through the vapour-assisted conversion of solid precursors. Fumaric acid, its methylated analogue mesaconic acid, as well as mixtures of the two were used as linkers to obtain polymorph materials with tuneable water sorption properties. The synthesis conditions determine the crystal structure and either the MIL-53 or MIL-68 type structure with square-grid or kagome-grid topology, respectively, is formed. Fine-tuning resulted in new MOF materials thus far inaccessible through conventional synthesis routes. Furthermore, by varying the linker ratio, the water sorption properties can be continuously adjusted while retaining the sigmoidal isotherm shape advantageous for heat transformation and room climatisation applications.

Anthracene removal from water samples using a composite based on metal-organic-frameworks (MIL-101) and magnetic nanoparticles (Fe3O4)

Polycyclic hydrocarbons constitute an important source of very dangerous pollutants. Different materials have been used as adsorbent for their removal, but they present difficulties in the separation process. The use of a material based on metal-organic framework (MOF) with large pores and high surface area and magnetic nanoparticles with superparamagnetic properties is an interesting strategy. In this work a magnetic composite based on MOF (MIL-101) and Fe₃O₄ magnetic nanoparticles (Fe₃O₄/MIL-101) was obtained by a simple synthesis method and used as adsorbent for the removal of anthracene. The composite was characterized by transmission electron microscopy, x-ray powder diffraction and vibrating sample magnetometer. The results showed that kinetic data followed a first-order model and equilibrium data were well fitted by the Langmuir model. The maximum adsorption capacity was 12.7 mg g^-1 at pH 6 in 60 min of exposure. The composite was applied for the adsorption of anthracene in water samples reaching more than 95% of anthracene removal in 1 h of contact. The composite material was effectively separated using an external magnet, and no further centrifugation or filtration processes were needed. This composite is a great alternative to remove polycyclic hydrocarbons from water samples and has potential to extend to the removal of other contaminants.

Sized-Controlled ZIF-8 Nanoparticle Synthesis from Recycled Mother Liquors: Environmental Impact Assessment

The effect of different deprotonators as well as washing steps and drying procedure on the synthesis of ZIF-8 from the mother liquor was investigated. The morphology, thermal stability, crystallinity, and surface area of the synthesized MOF were investigated. In addition, life-cycle assessment (LCA) or, in other words, eco-balance, was implemented as well. LCA compares the full range of environmental effects associated with the product by evaluating all inputs and outputs of material flows and predicting how such flow will affect the environment. ZIF-8 nanocrystals were synthesized from the recycled mother liquors using NaOH or NH₄OH thus preserving the main characteristics of the ZIF-8 nanoparticles derived from the initial synthesis. The rest of the characterization methods confirmed the suitability of the synthesis methodology considering the phase purity of the obtained ZIF-8 and nanometer size particles. This procedure enabled us not only to obtain phase pure ZIF-8 but also to substantially decrease the amount of solvent used for washing making it a sustainable process.

MIL-100(Fe) and its derivatives: from synthesis to application for wastewater decontamination

MIL-100(Fe), an environmental-friendly and water-stable metal-organic framework (MOF), has caught increasing research and application attention in the recent decade. Thanks to its mesoporous structure and eximious surface area, MIL-100(Fe) has been utilized as precursors for synthesizing various porous materials under high thermolysis temperature, which makes the derivatives of MIL-100(Fe) pretty promising candidates for the decontamination of wastewater. Herein, this review systematically summarizes the versatile synthetic methods and conditions for optimizing the properties of MIL-100(Fe) and its derivatives. Then, diverse environmental applications (i.e., adsorption, photocatalysis, and Fenton-like reaction) of MIL-100(Fe) and its derivatives and the corresponding removal mechanisms are detailed in the discussion. Finally, existing knowledge gaps related to fabrications and applications are discussed to close and promote the future development of MIL-100(Fe) and its derivatives toward environmental applications. Graphical abstract.

Introducing a dual-step procedure comprising microwave and electrical heating stages for the morphology-controlled synthesis of chromium-benzene dicarboxylate, MIL-101(Cr), applicable for CO2 adsorption

MIL-101(Cr) crystals were synthesized through a dual-step procedure consisting of a short term nucleation step in which the solution mixture was heated using microwave irradiation (MW phase) followed by a long-term growth step in which conventional electrical heating (CE phase) was used as a source of energy. The primary objective of such segregation is to increase the nuclei population at the end of the nucleation step. As a result of the high population density of nuclei, it is expected that the total time required to grow crystals in the CE phase decreases. The results showed that using the dual-step procedure led to a significant reduction in total synthesis time. The results also revealed that increasing pH from around 1.5 to 3 at the beginning of the CE phase resulted in producing octahedral crystals instead of a multifaceted sphere. Octahedral crystals exhibit higher CO₂ adsorption capacity than the multifaceted ones. Using the dual step procedure, one can not only control the morphology but also reduce the total synthesis time of MIL-101(Cr) crystals.

Regeneration, degradation, and toxicity effect of MOFs: Opportunities and challenges

Metal organic frameworks (MOFs) have been investigated extensively for separation, storage, catalysis, and sensing applications. Nonetheless, problems associated with their toxicity, recycling/reuse/regeneration, and degradation have yet to be addressed as one criterion to satisfy their commercialization. Here, the challenges associated with MOF-based technology have been explored to further expand their practical utility in various applications. We start a brief description of challenges associated with MOF-based technology followed by a critical evaluation of toxicity and need of technical options for regeneration of MOFs. Importantly, diverse techniques/process for reuse and regeneration of MOFs like activation of MOFs by heat, vacuum, solvent exchange, supercritical carbon dioxide (SCCO₂) and other miscellaneous options have been discussed with recent examples. Afterward, we also present an economical aspect and future perspectives of MOFs for real world applications. All in all, we aimed to present opportunities and critical review of the current status of MOF technology with respect to their recycling/reuse/regeneration to consider their environmental impact.

Robust synthesis routes and porosity of the Al-based metal–organic frameworks Al-fumarate, CAU-10-H and MIL-160

We confirm that the investigated Al-MOFs are robust with respect to reproducible synthesis and concomitant porosity as a prerequisite for applications.