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Adsorption, Separation, and Catalytic Properties of Densified Metal-Organic Frameworks
Abstract
Metal-organic frameworks (MOFs) are one of the widely investigated materials of 21st century due to their unique properties such as structural tailorability, controlled porosity, and crystallinity. These exceptional properties make them promising candidates for various applications including gas adsorption and storage, separation, and catalysis. However, commercial applications of MOFs produced by conventional methods including solvothermal or hydrothermal synthesis are rather limited or restricted because they often produce fine powders. The use of MOF powders for industrial applications often results in pressure drop problems similar to the case with zeolites and limited robustness against water. To realize these materials for practical applications, densification of MOFs (by increasing pellet density) is routinely employed to form pellets, extrudates or beads to improve the overall density, volumetric adsorption, mechanical and thermal properties. However, the improvements come with some drawbacks such as reduction in overall porosity, surface area, and gravimetric adsorption capacity. Thus, optimizing the properties of densified MOF's by tuning the pellet density is very crucial for realizing these materials for industrial applications. Methods that increase the packing density in MOFs (for example by intentional interpenetration, etc.), which is different from pellet density, is not the scope of this review. In this review, the properties and applications of densified MOFs with different metal clusters and organic linkers are discussed.
... Metal-organic frameworks (MOFs) are porous crystalline materials formed by linking metal ions or metal oxide clusters with organic ligands. These materials are characterized by an ultra-high specific surface area, abundant pores, a tunable surface functional group, and a highly ordered structure [37,[65][66][67]. They have a wide range of applications in many fields, such as gas adsorption [37,66] and capture [68], catalysis [69,70], gas stor- ...
... These materials are characterized by an ultra-high specific surface area, abundant pores, a tunable surface functional group, and a highly ordered structure [37,[65][66][67]. They have a wide range of applications in many fields, such as gas adsorption [37,66] and capture [68], catalysis [69,70], gas stor- ...
... Metal-organic frameworks (MOFs) are porous crystalline materials formed by linking metal ions or metal oxide clusters with organic ligands. These materials are characterized by an ultra-high specific surface area, abundant pores, a tunable surface functional group, and a highly ordered structure [37,[65][66][67]. They have a wide range of applications in many fields, such as gas adsorption [37,66] and capture [68], catalysis [69,70], gas storage [71], batteries [72], sensing [73], and sustained drug release [74]. ...
Cyanide gas is highly toxic and volatile and is among the most typical toxic and harmful pollutants to human health and the environment found in industrial waste gas. In the military context, cyanide gas has been used as a systemic toxic agent. In this paper, we review cyanide gas elimination methods, focusing on adsorption and catalysis approaches. The research progress on materials capable of affecting cyanide gas adsorption and catalytic degradation is discussed in depth, and the advantages and disadvantages of various materials are summarized. Finally, suggestions are provided for future research directions with respect to cyanide gas elimination materials.
... This method, known as chemical absorption, is typically used in plants with low levels of CO 2 in the exhaust gas, up to 15%, including coal dust and natural gas fired combined cycle plants. Despite the widespread use and understanding of this technology, its key limitations are due to its high energy consumption and significant costs in the carbon dioxide capture stage [14]. ...
... When using the "oxygen assisted combustion" method, where the process takes place in an environment of pure oxygen rather than air, a more environmentally friendly combustion process can be observed, with CO 2 emissions at 80% of the volume. This is due to the absence of nitrogen in the mixture, and the result is the generation of water vapour, which is not difficult to remove [1,14]. Nevertheless, this combustion approach, despite its effectiveness in CO 2 accumulation, faces a number of challenges when integrated into existing power plant processes. ...
The article discusses the importance of finding effective technological solutions for CO2 capture and subsequent utilisation in the context of underground storage or underground injection of carbon dioxide in order to implement water-gas effects to enhance hydrocarbon recovery. The authors also focus on current and promising carbon capture methods, comparing their technological features, advantages and disadvantages. Conclusions are drawn about which technological developments may be the most promising for solving the problem of reducing the carbon footprint of energy industry enterprises, and where there is a need for further research, despite the high energy costs and technological complexities.
... Since capture technologies vary significantly depending on the industry where they are applied, more common sectors, like the power sector (coal and gas-fired power plants), are often chosen for analyzing the techno-economic aspects of projects. The three main technological methods for carbon dioxide capture in major industries are precombustion, post-combustion, and oxyfuel combustion [11][12][13]. ...
... Despite being well-researched and prevalent, the primary drawbacks of this technology include significant energy losses and considerable resource costs during the capture phase. This method is frequently employed in natural gas combined cycle power plants and in power plants operating on coal dust [13]. ...
The work examines current methods for the development and study of environmental, social, and governance aspects (ESG factors) in connection with international and governmental measures for sustainable development. It covers the UN Sustainable Development Goals and the Paris Agreement, which incentivize the consideration of ESG factors, as well as the impact of ESG on the industry and investors, particularly in the oil and gas sector. The authors delve into CO2 utilization technologies (CCS, CCUS, CCU) and the challenges of their implementation in various sectors. The role of oil and gas companies in sustainable development through the implementation of CCU technologies is analyzed; methods for capturing, transporting, and utilizing CO2 are discussed, along with technologies for producing chemicals from CO2 and their efficiency. The influence of CCU technologies on Scope 1, 2, 3 emissions, defining greenhouse gas emissions, is also examined. The challenges of transitioning to sustainable development and the importance of implementing CCU projects to enhance the ESG-rating of companies are highlighted. Sound implementation of CCU projects can determine successful industrial development, especially in the oil and gas sector, by reducing carbon dioxide emissions and creating competitive products.
... The promised features of the metal-organic frameworks, such as their porosity and high adsorption capability, have led to efficient microextraction applications [45,46,[69][70][71][72][73]. Herein, a microextraction method for the separation of arsenic(III) traces was established using the prepared SSC-MOFs. ...
... The promised features of the metal-organic frameworks, such as their porosity a adsorption capability, have led to efficient microextraction applications [45,46,[69][70][71][72][73]. a microextraction method for the separation of arsenic(III) traces was established u prepared SSC-MOFs. ...
Spiny-like spherical copper metal–organic frameworks (SSC-MOFs) were prepared and characterized via SEM, TEM, EDS, XRD, FTIR and the BET surface area. The fabricated SSC-MOFs were applied to develop a procedure for the microextraction of trace arsenic(III) for preconcentration. The results show that a copper- and imidazole-derived metal–organic framework was formed in a sphere with a spiny surface and a surface area of 120.7 m2/g. The TEM confirmed the perforated network structures of the SSC-MOFs, which were prepared at room temperature. The surface functional groups were found to contain NH and C=N groups. The XRD analysis confirmed the crystalline structure of the prepared SSC-MOFs. The application for the process of microextracting the arsenic(III) for preconcentration was achieved with superior efficiency. The optimum conditions for the recovery of the arsenic(III) were a pH of 7 and the use of a sample volume up to 40 mL. The developed SSC-MOF-derived microextraction process has an LOD of 0.554 µg·L−1 and an LOQ of 1.66 µg·L−10. The developed SSC-MOF-derived microextraction process was applied for the accurate preconcentration of arsenic(III) from real samples, including food and water, with the promised performance efficiency.
... In laboratory experiments, the performance of these materials is excellent, but most of them are in powder form, and the application of these materials is limited. Over the past decades, metal-organic frameworks (MOFs) have been widely studied and applied in the field of gas adsorption [17], separation [17], and catalysis [18]. MOFs have the advantages of high specific surface area, high porosity, and adjustable pore size and have broad application prospects [19]. ...
... In laboratory experiments, the performance of these materials is excellent, but most of them are in powder form, and the application of these materials is limited. Over the past decades, metal-organic frameworks (MOFs) have been widely studied and applied in the field of gas adsorption [17], separation [17], and catalysis [18]. MOFs have the advantages of high specific surface area, high porosity, and adjustable pore size and have broad application prospects [19]. ...
Cyanogen chloride (CNCl) is highly toxic and volatile, and it is difficult to effectively remove via porous substances such as activated carbon due to the weak interaction between CNCl and the adsorbent surface. Developing a highly effective elimination material against CNCl is of great importance in military chemical protection. In this work, a new metal-organic framework (MOF) CuBTC@PA-PEI (polyacrylate-polyethyleneimine) composite was prepared and exhibited excellent CNCl elimination performance in the breakthrough tests. PEI was used for the functionalization of PA with amino groups, which is beneficial to anchor with metal ions of MOF. Afterward, the growth of MOF occurred on the surface and in the pores of the matrix by molecular self-assembly via our newly proposed stepwise impregnation layer-by-layer growth method. Breakthrough tests were performed to evaluate the elimination performance of the composites against CNCl. Compared with the pristine CuBTC powder, the CuBTC@PA-PEI composite exhibited better adsorption capacity and a longer breakthrough time. By compounding with the PA matrix, a hierarchically porous structure of CuBTC@PA-PEI composite was constructed, which provides a solution to the mass transfer problem of pure microporous MOF materials. It also solves the problems of MOF molding and lays a foundation for the practical application of MOF.
... MOFs and their derivatives have been widely used in the fields of catalysis, hydrogen storage, and drug delivery. [55][56][57][58][59] Recently, researchers have been actively exploring various MOF-based catalysts to optimize the biomass conversion process. [60][61][62] Fang et al. sintered a MOF to form a carbon material with a specific structure and loaded it with a composite oxide of Co and Fe to catalyse the oxidation of 5-HMF to generate 2,5diformylfuran(DFF). ...
Biomass is a renewable energy source abundantly available in nature. Their degradation by chemical or biological means produces various transition compounds. These transition compounds can be further transformed into valuable chemicals and biofuels that exhibit a wide range of applications and benefits. Recently, there has been a significant focus on this research area. Among the transition compounds, 5‐hydroxymethylfurfural (HMF), derived from the degradation of hexose, is an important biomass‐based platform compound that has hydroxymethyl and aldehyde functional groups in its molecular structure. This study reviews the recent progress in the transition metal‐catalysed conversion of HMF using Co and Mn to generate high‐value‐added downstream products. Furthermore, the active components, the carbon supporter, the reaction mechanism, the oxidants and the reaction conditions of the catalysts are discussed in detail. The oxidation of HMF deserves further research as an important and complete value‐added process for generating biomass‐based valuable chemicals.
... [1,32] Nevertheless, conventional synthesis methods like solvothermal, microwave-assisted, and mechanochemical processes often yield nano/microcrystalline MOF powder, which are less amenable to the assembly, shaping, and processing required for preparing MOF monoliths and/or aerogels. [33] In contrast, MOF sols exhibit remarkable rheological properties, simplifying the preparation of monoliths, xerogels, and/or aerogels with outstanding overall performance. [17c] MOF sols offer the flexibility to produce monolithic xerogels or aerogels through various drying techniques. ...
The intrinsic lack of processability in the conventional nano/microcrystalline powder form of metal‐organic frameworks (MOFs) greatly limits their application in various fields. Synthesis of MOFs with certain flowability make them promising for multitudinous applications. The direct synthesis strategy represents one of the simplest and efficient method for synthesizing solution processable MOF sols/suspensions, compared with other approaches, for instance, the post‐synthesis surface modification, the direct dispersion of MOFs in hindered ionic liquids, as well as the calcination method toward a few MOFs with melting behavior. This article reviews the recent direct synthesis strategies of solution processable MOF sols and their typical applications in different fields. The direct synthesis strategies of MOF sols can be classified into two categories: particle size reduction strategy, and selective coordination strategy. The synthesis mechanism of different strategies and the factors affecting the formation of sols are summarized. The application of solution processable MOF sols in different fields are introduced, showing great application potentials. Furthermore, the challenges faced by the direct synthesis of MOF sols and the main methods to deal with the challenges are emphasized, and the future development trend is prospected.
... 6 While conserving many of the intrinsic advantages of their crystalline counterparts, these states yield the potential of increased mechanical robustness and would allow for greater ease of processing, notably by circumventing the performance drop due to the necessary densification of the MOF powders. 7,8 While a series of experimental techniques have successfully been employed to determine the mechanical properties of crystalline MOFs, there is a lack of studies on the amorphous phases. 9 Some methods such as those relying on high-pressure X-ray diffraction 10 are not straightforwardly applicable to disordered materials, and many others require large bulk glass samples which are challenging to prepare. ...
Mechanical properties of amorphous phases of metal-organic frameworks (MOF), such as MOF glasses, are difficult to determine experimentally. Moreover, computational characterization is limited by the level of theory chosen for the description of interatomic interactions and is often computationally expensive. In this work, we have extensively investigated the computation of finite temperature mechanical properties of ZIF-4 in the crystal and glass phases. We critically assessed computational methodologies including ab initio molecular dynamics, reactive force fields, and classical force fields, based on a variety of glass models. We find that ZIF-4 glasses have a larger bulk modulus than the crystal and confirm previous studies that the density is larger for the glass phases. Moreover, we confirm in the case of zeolitic imidazolate framework (ZIF) glasses the relationship between density and bulk modulus, showing that obtaining models of correct density is key to the prediction of physical properties for these systems.
... 6 While conserving many of the intrinsic advantages of their crystalline counterparts, these states yield the potential of increased mechanical robustness and would allow for greater ease of processing, notably by circumventing the performance drop due to the necessary densification of the MOF powders. 7,8 While a series of experimental techniques have successfully been employed to determine the mechanical properties of crystalline MOFs, there is a lack of studies on the amorphous phases. 9 Some methods such as those relying on high-pressure X-ray diffraction 10 are not straightforwardly applicable to disordered materials, and many others require large bulk glass samples which are challenging to prepare. ...
Mechanical properties of amorphous phases of metal-organic frameworks, such as MOF glasses, are difficult to determine experimentally. Moreover, computational characterization is limited by the level of theory chosen for the description of interatomic interactions and is often computationally expensive. In this work, we have extensively investigated the computation of finite temperature mechanical properties of ZIF-4 in the crystal and glass phases. We critically assessed computational methodologies including ab initio molecular dynamics, reactive force fields, and classical force fields, based on a variety of glass models. We find that ZIF-4 glasses have a larger bulk modulus than the crystal and confirm previous studies that the density is larger for the glass phases. Moreover, we confirm in the case of ZIF glasses the relationship between density and bulk modulus, showing that obtaining models of correct density is key to the prediction of physical properties for these systems.
... It is pertinent to mention that metal−organic frameworks (MOFs; can be defined as porous materials consisting of metal clusters and organic ligands 21 ) have attracted significant attention from researchers in the last couple of decades due to their exclusive features, such as controllable pore sizes, high specific surface area, and unique surface chemistry. 22 As the result of these special features, various applications have been reported using MOFs, including catalysis, 23 gas storage and separation, 24 sensing, 25 luminescence, 26 and drug delivery. 27 Because of its nontoxic nature and perfect stability, 28 ironbased MOF (MOF-Fe) showed excellent catalytic properties 29 and sorption/separation 30 among the various MOFs. ...
... 6 While conserving the intrinsic advantages of their crystalline counterparts, these states yield the potential of increased mechanical robustness and would allow for greater ease of processing, notably by circumventing the performance drop due to the necessary densification of the MOF powders. 7,8 While a series of experimental techniques have successfully been employed to determine the mechanical properties of crystalline MOFs, there is a lack of studies on the amorphous phases. 9 Some methods such as those relying on high-pressure X-ray diffraction 10 are not straightforwardly applicable to disordered materials, and many others require large bulk glass samples which are challenging to prepare. ...
Mechanical properties of amorphous phases of metal-organic frameworks, such as MOF glasses, are difficult to determine experimentally. Moreover, computational characterization is limited by the level of theory chosen for the description of interatomic interactions and is often computationally expensive. In this work, we have extensively investigated the computation of finite temperature mechanical properties of ZIF-4 in the crystal and glass phases. We critically assessed computational methodologies including ab initio molecular dynamics, reactive force fields, and classical force fields, based on a variety of glass models. We find that ZIF-4 glasses have a larger bulk modulus than the crystal and confirm previous studies that the density is larger for the glass phases. Moreover, we confirm in the case of ZIF glasses the relationship between density and bulk modulus, showing that obtaining models of correct density is key to the prediction of physical properties for these systems.
... Metal−organic frameworks (MOFs) have attracted significant attention from researchers in the last couple of decades due to their exclusive features, such as controllable pore sizes, high specific surface area, and unique surface chemistry. 1 As a result of these special features, various applications have been reported using MOFs, including catalysis, 2 gas storage and separation, 3 sensing, 4 luminescence, 5 drug delivery, 6 and 4nitrophenol (4-NP) reduction. 7−10 MOF-5 is among the most widely used MOFs due to its remarkable properties, including an open skeleton structure, high porosity, high surface-tovolume ratio, and exceptional thermal stability. ...
... Electrochemical sensor model is presented in Fig. 5. A sensor is usually made up of a transduction and a detecting electrode which converts the detected information to an electrical or visual sign (Nandasiri et al., 2016). The critical features of a sensor depend on the sensitivity, selectivity, response time, sustainability, limit of detection (LOD) and cost. ...
Water bodies are being polluted rapidly by disposal of toxic chemicals with their huge entrance into drinking water supply chain. Among these pollutants, heavy metal ions (HMIs) are the most challenging one due to their non-biodegradability, toxicity, and ability to biologically hoard in ecological systems, thus posing a foremost danger to human health. This can be addressed by robust, sensitive, selective, and reliable sensing of metal ions which can be achieved by Metal organic frameworks (MOF) based electrochemical sensors. In the present era, MOFs have caught greater interest in a variety of applications including sensing of hazardous pollutants such as heavy metal ions. So, in this review article, types, synthesis and working mechanism of MOF based sensors is explained to give general overview with updated literature. First time, detailed study is done for sensing of metal ions such as chromium, mercury, zinc, copper, manganese, palladium, lead, iron, cadmium and lanthanide by MOFs based electrochemical sensors. The use of MOFs as electrochemical sensors has attractive success story along with some challenges of the area. Considering these challenges, we attempted to highlight the milestone achieved and shortcomings along with future prospective of the MOFs for employing it in electrochemical sensing devices for HMIs. Finally, challenges and future prospects have been discussed to promote the development of MOFs-based sensors in future.
... Metal−organic frameworks (MOFs) have attracted significant attention from researchers in the last couple of decades due to their exclusive features, such as controllable pore sizes, high specific surface area, and unique surface chemistry. 1 As a result of these special features, various applications have been reported using MOFs, including catalysis, 2 gas storage and separation, 3 sensing, 4 luminescence, 5 drug delivery, 6 and 4nitrophenol (4-NP) reduction. 7−10 MOF-5 is among the most widely used MOFs due to its remarkable properties, including an open skeleton structure, high porosity, high surface-tovolume ratio, and exceptional thermal stability. ...
A novel, unique, highly effective, and recyclable heterogeneous
6 catalyst, diethyl imidazolium hexafluorophosphate ionic liquid supported metal−
7 organic framework ([DEIm][PF6]@MOF-5), has been synthesized using a simple
8 impregnation method at ambient temperature. Characterization of the catalyst was
9 done through various techniques such as Fourier transform infrared (FTIR),
10 energy dispersive X-ray, X-ray diffraction (XRD), transmission electron
11 microscopy, scanning electron microscopy (SEM), elemental mapping, Raman
12 spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis
13 (TGA) analyses. The kinetic study has shown the high catalytic performance of
14 [DEIm][PF6]@MOF-5 for the reduction of 4-nitrophenol (NP) compared to
15 other catalysts. The catalyst also exhibited efficient electrochemical activity toward
16 4-NP reduction. The catalyst was recyclable for more than seven cycles without
17 any significant loss in its catalytic performance. The recycled catalyst was further
18 studied using XRD, FTIR, SEM, and TGA analyses to investigate the structural
19 changes that occurred during the reaction. The catalyst maintained its structural integrity even after seven cycles
... Much effort has been devoted to developing practical MOFbased devices that can modify the weaknesses of MOFs. Traditional techniques like pelletization and granulation [33,34] are quick and convenient in operation, but they have obvious lim-itation when building complex and delicate structures. Recently, researchers have focused on employing printing technologies to design MOFs as functional devices with controllable geometry structures [35]. ...
Metal-organic frameworks (MOFs) are achieving unprecedented progress in scientific research for their attractive properties. However, MOFs’ disadvantages, including poor processibility, low chemical stability and limited mechanical properties, have prevented their widespread application. Printing techniques with advantages such as geometric controllability, rapid prototyping, and sustainable manufacturing can effectively address these problems of MOFs. Up to now, MOF-integrated printed devices have made significant breakthroughs and progress. This article provides the latest progress in MOF-integrated printed devices. First, the commonly used printing techniques for MOF-based device fabrication are summarized. Then, the important achievements made in MOF-based printed device fabrication and applications of these fabricated devices are highlighted. Finally, major challenges and potential research frontiers of MOF-integrated printed devices are proposed.
... Metal organic frameworks [18][19][20][21] (MOFs) assembled by inorganic metal ions or clusters and organic building blocks, which are also called nodes and linkers, respectively, are versatile coordination polymers [22], an unprecedented class of nanocrystalline materials widely used in separation [23,24], storage [25,26], catalysis [27], and sensing [28,29]. In the past decade, with high-throughput computing screening (HTCS) based on molecular simulation [30][31][32] rapidly developed and ameliorated, especially in the adsorptive separation of mixtures [33][34][35][36], adsorbents suitable for capturing CO 2 can be precisely obtained from hundreds of thousands of MOFs. ...
There is a significant challenge to discover porous materials that can effectively capture and separate CO2 from natural gas, refining gas, and flue gas, which has attracted attention in dealing with climate warming and energy purification. Recently, hybrid microporous materials with narrow pores and containing fluorine have been rapidly used in gas separation based on physisorption. In this contribution, molecular simulations combined with high-throughput calculations were performed to calculate structural parameters and performance evaluation metrics of 1015 promising adsorbents to rank and screen out the top candidates for CO2/CH4, CO2/H2 and CO2/N2 separation. To the best of our knowledge, this is the first time to unlock this kind of fluorinated material database in CO2 separation, in which statistical information indicates that a large number of interpenetrating structures lead to 64% of ultra-microporous materials and the number of particles per unit volume of fluorine we defined also has a positive effect on the heat of adsorption of CO2. The structural performance relationship reveals a clear picture of strong CO2 capture but poor energy gas (CH4 and H2) storage. The mathematical model established from the geometry and energy descriptors has a strong correlation with the mixture adsorption selectivity in CO2/N2 separation, and the ideal selectivity can be applied to save computing resources in CO2/H2 separation. Cadmium and vanadium with high frequency may represent the new characteristics of the next generation adsorbent for capturing CO2, among the 18 high-performance materials selected according to the adsorption performance score and mixture adsorption selectivity. The centroid density distribution and radial distribution function manifest that CO2 is preferentially close to fluorine atoms and metal atoms. Here, we established an online high-throughput calculation code (https://github.com/oddthinker/HTCS) for adsorption and separation. All of this will provide guidelines for experimental synthesis and large-scale screening of target materials.
... For understanding the detailed working procedure of electrochemical sensor, the electrochemical sensor works on the principle of redox reaction. A sensor is usually made up of a sensing electrode and a transduction electrode which converts the sensed information into an electrical or optical signal [41]. The critical features of a sensor depend on the sensitivity, selectivity, response time, sustainability, limit of detection (LOD) and cost. ...
In the present era, organic–inorganic nanohybrids based sensor has attained interests of researches due to their outstanding applications in the detection of volatile organic compounds (VOCs). The remarkable features of organic–inorganic nanohybrids include enhanced surface area, tunable porosity, good stability and flexible structure. These characteristics features reflect organic–inorganic nanohybrids to be a potential candidate for detection of VOCs. In this chapter, we focused on (i) sources of VOCs, (ii) adverse effects of VOCs on environments, (iii) various sensor used to detect VOCs, (iv) organic–inorganic nanohybrids sensor for various VOCs detection. For obtaining high performance in organic–inorganic nanohybrids sensors, challenges and future prospects have been discussed to promote the development of organic–inorganic nanohybrids based sensors in the future.
... Porous matrixes based on metal-organic frameworks (MOFs) are promising systems for preparation of various functional nanocomposites [15][16][17][18][19]. Typical representatives of MOFs are chromium (III) terephthalate Cr3O(H2O)2(C6H4(COO)2)3X·nG, where X = OH − , F − ; G are guest molecules, mainly H2O and other solvents, known as MIL-101(Cr). ...
Experimental data on nitrogen adsorption, pellets density and ionic conductivity of nanocomposite solid electrolytes (1−x)LiClO4–xMIL-101(Cr) were interpreted in frames of the model of the composite in which the lithium salt fills the pores of a metal-organic framework MIL-101(Cr). According to the model, the concentration of lithium salt located in the pores reaches a maximum at the concentration x = xmax which is defined by a ratio of the molar volume of LiClO4 and the total volume of accessible pores in the MIL-101(Cr) framework. The model allows one to describe the dependences of pore volume and pellet density on the concentration of MIL-101(Cr). Conductivity of the composites were successfully described by two separate mixing equations for concentration ranges x < xmax and x > xmax. In the first concentration region x < xmax, the composite may be regarded as a mixture of LiClO4 and MIL-101(Cr) with completely filled pores accessible for LiClO4. At x > xmax, the total amount of lithium perchlorate is located in the pores of MIL-101(Cr) and occupies only part of the volume of the accessible pores. It was found that xmax value determined from the concentration dependence of conductivity (xmax = 0.06) is noticeably lower than the corresponding value estimated from adsorption data (xmax = 0.085) indicating a practically complete filling the pores of MIL-101(Cr) in the composite pellets heated before conductivity measurements.
The poor hydrostability of most reported metal-organic frameworks (MOFs) has become a daunting challenge in their practical applications. Recently, MOFs combined with multifunctional polymers can act as a functional platform and exhibit unique catalytic performance; they can not only inherit the outstanding properties of the two components but also offer unique synergistic effects. Herein, an original porous polymer-confined strategy has been developed to prepare a superhydrophobic MOF composite to significantly enhance its moisture or water resistance. The selective nucleation and growth of MOF nanocrystals confined in the pore of PDVB-vim are closely related to the structure-directing and coordination-modulating properties of PDVB-vim. The resultant MOF/PDVB-vim composite not only produces superior superhydrophobicity without significantly disturbing the original features but also exhibits a novel catalytic activity in the Friedel-Crafts alkylation reaction of indoles with trans-β-nitrostyrene because of the accessible sites and synergistic effects.
Fine particles possess remarkable characteristics including extensive surface‐to‐weight ratios and diverse morphologies. Consequently, through the use of fluidization techniques, they have become favoured in various industrial processes, especially with continuous production. This review paper offers a comprehensive exploration of the integration of fine particle applications with fluidization technologies, with a specific focus on the Geldart Group C particles sized <25–40 μm. Although there are challenges with processing fine particles such as the strong cohesion in fluidized beds, recent progress, including the nanoparticle modulation method, has demonstrated potential solutions. These advancements render these cohesive particles applicable to industrial applications in different fields, including gas‐phase catalytic reactions, gas–solid fluidized bed coal beneficiation, ultrafine powder coating (UPC), pharmaceuticals, environmental sustainability, energy storage, and food processing. However, further research is needed to obtain a better understanding of fine particle fluidization in industrial settings in order to achieve larger‐scale implementation. In summary, this review provides a comprehensive overview of fine particle utilization integrated with fluidization technologies, demonstrating the potential in large‐scale industrial processes, and enabling significant advancements in practical applications.
Conductometric gas sensors (CGS) have been extensively explored in recent decades owing to easy fabrication and miniaturization, low cost and distributable detectability. Among numerous performance parameters, selectivity is a critical one to evaluate the operation quality of CGS in diverse application scenarios such as environment monitoring, food quality assessment, individual healthcare, etc. Nevertheless, in most preceding work either the underlying mechanism for the shown selectivity is not explained clearly or the strategies to improve the selectivity are not detailed, which necessitates an urgent need to address these. Also, there still lacks comprehensive summaries reported in this aspect thus far. A favorable selectivity always means a stronger sensitivity toward a specific gas than that toward other interference gases. Thus, it is very essential to conduct a comprehensive overview to understand the sensitivity, selectivity and their relationships from both quantitative and qualitative perspectives. In this review, the sensing mechanism of CGS is first studied according to the selectivity coefficient simultaneously concluding the factors that influence the detection selectivity. Subsequently, chemical and physical modification strategies to improve the selectivity are discussed. Finally, challenges and perspectives of the selectivity optimization methods are proposed for future research.
Superhydrophobic metal-organic frameworks (MOFs) exhibit excellent application prospects in many fields, such as catalysis, water pollution treatment, self-cleaning, and so on. In this study, two stable salen-based three-dimensional (3D) MOFs,...
Glassy states of network‐forming coordination polymers (CPs) and metal‐organic frameworks (MOFs) represent a novel category of amorphous materials. Recent years have seen substantial progress in expanding the compound library and advancing the structural design of CP/MOF glasses. This review examines the current status and future potential of multi‐scale hierarchical CP/MOF glass materials, covering synthesis, shape adaptability, and synergistic materials. Extensive experimental and theoretical investigations of structure‐property relationships have shed light on their diverse utility, spanning areas such as permanent porosity and gas permeability, ionic and electronic conductivity, optics, catalysis, and battery technology. Additionally, the inherent properties of these glasses, coupled with their phase transformation capability, enable a range of morphological architectures. Current challenges are also addressed and offer insights into future research, with the goal of bringing CP/MOF glasses to the industry.
In this study, a magnetic core–shell metal–organic framework (MOF) nanocomposite, Fe3O4-COOH@UiO-66-NH2, was synthesized for tumor-targeting drug delivery by incorporating carboxylate groups as functional groups onto ferrite nanoparticle surfaces, followed by fabrication of the UiO-66-NH2 shell using a facile self-assembly approach. The anticancer drug quercetin (QU) was loaded into the magnetic core–shell nanoparticles. The synthesized magnetic nanoparticles were comprehensively evaluated through multiple techniques, including FT-IR, PXRD, FE-SEM, TEM, EDX, BET, UV–vis, ZP, and VSM. Drug release investigations were conducted to investigate the release behavior of QU from the nanocomposite at two different pH values (7.4 and 5.4). The results revealed that QU@Fe3O4-COOH@UiO-66-NH2 exhibited a high loading capacity of 43.1% and pH-dependent release behavior, maintaining sustained release characteristics over a prolonged duration of 11 days. Furthermore, cytotoxicity assays using the human breast cancer cell line MDA-MB-231 and the normal cell line HEK-293 were performed to evaluate the cytotoxic effects of QU, UiO-66-NH2, Fe3O4-COOH, Fe3O4-COOH@UiO-66-NH2, and QU@Fe3O4-COOH@UiO-66-NH2. Treatment with QU@Fe3O4-COOH@UiO-66-NH2 substantially reduced the cell viability in cancerous MDA-MB-231 cells. Cellular uptake and cell death mechanisms were further investigated, demonstrating the internalization of QU@Fe3O4-COOH@UiO-66-NH2 by cancer cells and the induction of cancer cell death through the apoptosis pathway. These findings highlight the considerable potential of Fe3O4-COOH@UiO-66-NH2 as a targeted nanocarrier for the delivery of anticancer drugs.
A new Cu(II) paddle-wheel-like complex with 4-vinylbenzoate was synthesized using acetonitrile as the solvent. The complex was characterized by X-ray crystal diffraction, FT-IR, diffuse reflectance spectroscopy, thermogravimetric, differential scanning calorimetric, magnetic susceptibility, and electronic paramagnetic resonance analyses. The X-ray crystal diffraction analysis indicated that each copper ion was bound at an equatorial position to four oxygen atoms from the carboxylate groups of the 4-vinylbenzoate ligand in a square-based pyramidal geometry. The distance between the copper ions was 2.640(9) Å. The acetonitrile molecules were coordinated at the axial position to the copper ions. Exposure of the Cu(II) complex to humid air promoted the gradual replacement of the coordinated acetonitrile by water molecules, but the complex structure integrity remained. The EPR spectra exhibited signals attributed to the presence of a mixture of the monomeric (S = ½) and dimeric (S = 1) copper species in a possible 3:1 ratio. The magnetic studies revealed a peak at 50–100 K, which could be associated with the oxygen absorption capacity of the Cu(II)–vba complex.
Metal-organic frameworks (MOFs) have attracted considerable attention owing to their large surface area and high structural tunability, which lead to a wide range of applications. Owing to excellent properties of MOFs in various applications, the synthesis process of MOFs has been gradually developed with high yields and stable shapes to fulfill industrial requirements. However, the industrial usage of traditional powder-formed MOFs is limited because powder may lead to safety issues and disperse easily. Therefore, MOFs with different shapes and standard properties are required for applications. This review has summarized currently reported methods for shaping MOFs, e.g., binder-based, high-pressure, surfactant-assisted, supercritical CO2 (scCO2), and sol-gel processes. The industrial applications based on shaped MOFs are summarized to support future studies on the formation of MOFs with stable shapes.
The purpose of this review is to summarize recent advances in inorganic and organometallic polymers as electrode materials for supercapacitors. The review discusses the advantages and disadvantages of various electrode materials, including activated metal oxides and hydroxides, coordination polymers, and their composites. Inorganic and coordination polymers have attracted high interests as electrode materials for electrochemical capacitors, because of their electrical conductivity, high surface area, low cost and chemical stability. At the end of this review, future trends for construction new types of electrochemical capacitor are proposed.
A series of MIL-101-L compounds (L = 4,4′-bipyridyl, pyrazine, piperazine, 1,4-diaza[2.2.2]bicyclooctane, and ethylenediamine) was synthesized using the post-synthetic coordination modification of mesoporous chromium(iii) terephthalate MIL-101. These compounds contain basic sites and are capable of exhibiting catalytic activity in the Henry nitroaldol condensation reaction with high selectivity. Compounds MIL-101-Sal-M (M = Ni, Cr, Zr, Co) containing the salen complexes grafted onto the framework surface were prepared using the covalent post-synthetic modification methods. These compounds exhibit catalytic activity in the addition of carbon dioxide to propylene oxide in quantitative yield in the presence of a cocatalyst. The results obtained confirm that the post-synthetic modification of porous metal-organic frameworks is an efficient approach for preparing heterogeneous catalysts based on the known homogeneous catalysts.
In this work, a nickel-cobalt bimetallic organic framework (NiCo-MOF) was introduced as an intermediate layer to synthesize a Pd/NiCo-MOF/Nickel foam composite electrode, which was applied for electrocatalytic hydrodechlorination (ECH) of chlorinated pharmaceuticals and personal care products (PPCPs). The introduction of the NiCo-MOF structure dramatically improved the catalytic activity compared with the common Pd/Nickel foam electrode. Combined with transmission electron microscope and X-ray absorption fine structure analysis, catalyst on Pd/NiCo-MOF/Nickel foam surface had smaller particle size. The removal of the Pd/NiCo-MOF/Nickel foam electrode for chloramphenicol (CAP) reached more than 95 % at 40 min, while the removal of the Pd/Nickel foam electrode was only approximately 55 %. In this study, the active atomic hydrogen (H*) adsorbed on the Pd and NiCo-MOF structures is the critical active species to remove chlorinated PPCPs by Pd/NiCo-MOF/Nickel foam electrode. Density functional theory (DFT) was used to calculate and investigate the adsorption strength of the composite electrode structure for CAP, the product and H*. This work provided a new method to achieve higher catalytic performance and reactivity operability and longevity of catalytic materials through the synergism of a NiCo-MOF interlayer with Pd particles.
Metal–organic frameworks (MOFs)/coordination polymers are promising materials for gas separation, fuel storage, catalysis, and biopharmaceuticals. However, most applied research on MOFs is limited to these functional materials thus far. This study focuses on the potential of MOFs as structural adhesives. A sintering technique is applied to a zeolitic imidazolate framework‐67 (ZIF‐67) gel that enables the joining of Cu substrates, resulting in a shear strength of over 30 MPa, which is comparable to that of conventional structural adhesives. Additionally, systematic experiments are performed to evaluate the effects of temperature and pressure on adhesion, indicating that the removal of excess 2‐methylimidazole and the by‐product (acetic acid) from the sintered material by vaporization results in a microstructure composed of large spherical ZIF‐67 crystals that are densely aggregated, which is essential for achieving a high shear strength.
Crystalline metal-organic frameworks (MOFs) with high porosity have high sorption capacities for various gases. Their fragile and pulverulent characteristics have prompted significant efforts to prepare shaped bodies e.g., pellets, granules for use in adsorbers. A hollow fiber membrane-based strategy is adopted since hollow fiber membrane (HFM) modules are highly preferred for industrial separation processes due to very high surface area provided per unit device volume and their easy scalability. We report herein a solvothermal synthesis method whereby nanocrystals of the MOF, UiO-66-NH2, are synthesized directly inside submicron pores of hydrophilic hollow fiber membranes of Nylon 6 as well as in the bores of the HFMs. Nanocrystals of around 100 nm populate HFM pores. Cylindrical modules containing such HFMs and MOF nanocrystals and microcrystals in membrane pores, HFM bores and the extra capillary space were studied for adsorption of ammonia from a dilute gas stream. High values of ammonia breakthrough time were achieved. The corresponding behaviors of three MOF configurations namely, MOF in membrane pores, MOF in membrane pores and the HFM bores and MOF present in membrane pores, HFM bores and in extra capillary space were studied. The values of time/MOF weight achieved were very high. The MOFs synthesized were characterized by Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), powder X-ray diffractometer (PXRD), Brunauer-Emmett-Teller (BET) adsorption isotherms, surface area, and pore size distribution. High performance of HFM-supported MOF-based scalable devices for gas/vapor adsorption has been demonstrated with values of 20,000 min/g of MOF for trace ammonia breakthrough from humid ammonia feed gas stream employed. Other potential uses of such devices for adsorbing 2 to 3 gases and liquid phase adsorption have also been discussed.
Volatile organic compounds (VOCs) are an important indicator for fungal-infected wheat identification. This work proposes a novel approach for toxigenic Aspergillus flavus infected wheat identification through characteristic VOCs analyzed by nano-composite colorimetric sensors. Nanoparticles of poly styrene-co-acrylic acid (PSA), porous silica nanoparticles (PSN), and metal-organic framework (MOF) were combined with boron dipyrromethene (BODIPY) to fabricate nano-composite colorimetric sensors. The combination mechanisms for nanoparticles and the information extracted from nano-colorimetric sensors by digital images were analyzed in the current work. Furthermore, linear discriminant analysis (LDA) and k-nearest neighbor (KNN) were used comparatively to analyze the data from images, and toxigenic Aspergillus flavus infected wheat samples could be 100.00% correctly identified when using the optimal KNN model. This research contributes to the practical analysis of VOCs and the detection of toxigenic Aspergillus flavus infected wheat.
Metal-oxide-based chemiresistive hydrogen sensors exhibit high sensitivity, long-term stability, and low cost and have been extensively applied in safety monitoring of H2. However, the sensing performances are dramatically affected by the water vapor, resulting in reduced response value and increased response/recovery time. To improve the anti-humidity property of sensors, coating the breathable and hydrophobic membrane on the surface of the sensing film is an effective strategy. In this work, the poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene] (Teflon AF-2400) was dip-coated on the surface of SnO2 in a commercial hydrogen sensor (TGS2615) as a breathable and hydrophobic membrane. For safety, He instead of H2 was used to test the gas permeability of membranes. The Teflon membrane shows a high He permeability of up to 40,700 Barrer and an excellent He/H2O selectivity of 99. Moreover, Teflon shows high processability to form a defect-free coating on the rough surface of the sensing film and high chemical stability under the operando condition of the sensor. Thus, the Teflon-modified sensor possesses excellent selectivity with a value of 5, and the resistance is stable at 10,554 ± 3% Ω for 20 days in 80% RH. The modified sensor shows an improved anti-humidity property with a 75% response to 200 ppm H2 at 80% RH and has a low coefficient of variation value of 7.23% that shows advances than other reported sensors modified by coatings. The commercially available Teflon and the simple coating technology make the strategy easily scale up and show promising applications.
Adsorptive separation based on porous solid adsorbents has emerged as an excellent effective alternative to energy-intensive conventional separation methods in a low energy cost and high working capacity manner. However, there are few stable mesoporous metal-organic frameworks (MOFs) for efficient purification of methane from other light hydrocarbons in natural gas. Herein, we report a series of stable mesoporous MOFs, MIL-101-Cr/Fe/Fe-NH2, for efficient separation of CH4 and C3H8 from a ternary mixture CH4/C2H6/C3H8. Experimental results show that all three MOFs possess excellent thermal, acid/basic, and hydrothermal stability. Single-component adsorption suggested that they have high C3H8 adsorption capacity and commendable selectivity for C3H8 and C2H6 over CH4. Transient breakthrough experiments further certified the ability of direct separation of CH4 from simulated natural gas and indirect recovery of C3H8 from the packing column. Theoretical calculations illustrated that the van der Waals force proportional to the molecular weight is the key factor and that the structural integrity and defect can impact separation performances.
Nanoscale porous coordination polymers were synthesized using simple wet chemical method. The effect of various polymer surfactants on colloidal stability and shape selectivity was investigated. Our results suggest that the nanoparticles exhibited significantly improved adsorption kinetics compared to bulk crystals due to decreased diffusion path lengths and preferred crystal plane interaction.
Development of safe and effective hydrogen storage systems becomes a critical factor for further implementation of fuel cell and hydrogen technologies. Among new approaches aimed at improving the performance of such systems, the concept of porous materials-based adsorptive hydrogen storage is now considered as a long-term solution due to the reversibility, good kinetics and absence of thermal management issues. However, the low packing densities associated with the porous materials such as carbon structure materials, zeolites, metal-organic frameworks lead to the compromised volumetric capacity, potential pipe contaminations and difficulties in handling, when introducing the powdered adsorbents into hydrogen storage systems. Some efforts have been devoted to solve this problem by shaping the porous materials into beads, pellets or monoliths and achieve higher storage densities at more moderate temperatures and pressures. This review will firstly state the essential properties of shaped structures for hydrogen adsorption, and then highlight the recent attributes that potentially can be utilized to shape porous materials into specific configurations for hydrogen storage applications. Later, several testing techniques on structured porous material will be also discussed.
We present the in situ synthesis of Au nanoparticles within the Zr based Metal Organic Framework, UiO-66. The resulting Au@UiO-66 materials were characterized by means of N2 sorption, XRPD, UV-Vis, XRF, XPS and TEM analysis. The Au nanoparticles (NP) are homogeneously distributed along the UiO-66 host matrix when using NaBH4 or H2 as reducing agents. The Au@UiO-66 materials were evaluated as catalysts in the oxidation of benzyl alcohol and benzyl amine employing O2 as oxidant. The Au@MOF materials exhibit a very high selectivity towards the ketone (up to 100 %). Regenerability and stability tests demonstrate that the Au@UiO-66 catalyst can be recycled with a negligible loss of Au species and no loss of crystallinity. In situ IR measurements of UiO-66 and Au@UiO-66-NaBH4, before and after treatment with alcohol, showed an increase in IR bands that can be assigned to a combination of physisorbed and chemisorbed alcohol species. This was confirmed by velocity power spectra obtained from the molecular dynamics simulations. Active peroxo and oxo species on Au could be visualized with Raman analysis.
We report an easy synthetic procedure to produce large, crystalline, mechanically and chemically robust ZIF-8 monoliths without using any binders or high pressures. Gas adsorption studies show that the monolithic structures retain the characteristic porosity of ZIF-8 without any damage to the micropore system, while the bulk densities and volumetric BET areas are 3 times higher than the conventional, powder material. In addition, these structures are substantially more rigid than single crystals of the material.
Metal-organic framework (MOF) materials, which are constructed from metal ions or metal ion clusters and bridging organic linkers, exhibit regular crystalline lattices with relatively well-defined pore structures and interesting properties. As a new class of porous solid materials, MOFs are attractive for a variety of industrial applications including separation membranes - a rapidly developing research area. Many reports have discussed the synthesis and applications of MOFs and MOF thin films, but relatively few have addressed MOF membranes. This critical review provides an overview of the diverse MOF membranes that have been prepared, beginning with a brief introduction to the current techniques for the fabrication of MOF membranes. Gas and liquid separation applications with different MOF membranes are also included (175 references).
This review article presents the fundamental and practical aspects of water adsorption in Metal-Organic Frameworks (MOFs). The state of the art of MOF stability in water, a crucial issue to many applications in which MOFs are promising candidates, is discussed here. Stability in both gaseous (such as humid gases) and aqueous media is considered. By considering a non-exhaustive yet representative set of MOFs, the different mechanisms of water adsorption in this class of materials are presented: reversible and continuous pore filling, irreversible and discontinuous pore filling through capillary condensation, and irreversibility arising from the flexibility and possible structural modifications of the host material. Water adsorption properties of more than 60 MOF samples are reported. The applications of MOFs as materials for heat-pumps and adsorbent-based chillers and proton conductors are also reviewed. Some directions for future work are suggested as concluding remarks.
Advances in flexible and functional metal-organic frameworks (MOFs), also called soft porous crystals, are reviewed by covering the literature of the five years period 2009-2013 with reference to the early pertinent work since the late 1990s. Flexible MOFs combine the crystalline order of the underlying coordination network with cooperative structural transformability. These materials can respond to physical and chemical stimuli of various kinds in a tunable fashion by molecular design, which does not exist for other known solid-state materials. Among the fascinating properties are so-called breathing and swelling phenomena as a function of host-guest interactions. Phase transitions are triggered by guest adsorption/desorption, photochemical, thermal, and mechanical stimuli. Other important flexible properties of MOFs, such as linker rotation and sub-net sliding, which are not necessarily accompanied by crystallographic phase transitions, are briefly mentioned as well. Emphasis is given on reviewing the recent progress in application of in situ characterization techniques and the results of theoretical approaches to characterize and understand the breathing mechanisms and phase transitions. The flexible MOF systems, which are discussed, are categorized by the type of metal-nodes involved and how their coordination chemistry with the linker molecules controls the framework dynamics. Aspects of tailoring the flexible and responsive properties by the mixed component solid-solution concept are included, and as well examples of possible applications of flexible metal-organic frameworks for separation, catalysis, sensing, and biomedicine.
This review summarizes the use of metal-organic frameworks (MOFs) as a versatile supramolecular platform to develop heterogeneous catalysts for a variety of organic reactions, especially for liquid-phase reactions. Following a background introduction about catalytic relevance to various metal-organic materials, crystal engineering of MOFs, characterization and evaluation methods of MOF catalysis, we categorize catalytic MOFs based on the types of active sites, including coordinatively unsaturated metal sites (CUMs), metalloligands, functional organic sites (FOS), as well as metal nanoparticles (MNPs) embedded in the cavities. Throughout the review, we emphasize the incidental or deliberate formation of active sites, the stability, heterogeneity and shape/size selectivity for MOF catalysis. Finally, we briefly introduce their relevance into photo- and biomimetic catalysis, and compare MOFs with other typical porous solids such as zeolites and mesoporous silica with regard to their different attributes, and provide our view on future trends and developments in MOF-based catalysis.
The release of anthropogenic toxic pollutants into the atmosphere is a worldwide threat of growing concern. In this regard, it is possible to take advantage of the high versatility of MOFs materials in order to develop new technologies for environmental remediation purposes. Consequently, one of the main scientific challenges to be achieved in the field of MOF research should be to maximize the performance of these solids towards the sensing, capture and catalytic degradation of harmful gases and vapors by means of a rational control of size and reactivity of the pore walls that are directly accessible to guest molecules.
While much attention of the MOF community has been devoted to adsorption and purification of gases, there is now also a vast body of data on the capability of MOFs to separate and purify liquid mixtures. Initial studies focused on separation of petrochemicals in apolar backgrounds, but the attention has moved now to the separation of complex, e.g. chiral compounds, and to the isolation of biobased compounds from aqueous media. We here give an overview of most of the existing literature, with an accent on separation mechanisms and structure-selectivity relationships.
Metal–organic frameworks (MOFs) are constructed from metal ions/clusters coordinated by organic linkers (or bridging-ligands). The hallmark of MOFs is their permanent porosity, which is frequently found in MOFs constructed from metal-clusters. These clusters are often formed in situ, whereas the linkers are generally pre-formed. The geometry and connectivity of a linker dictate the structure of the resulting MOF. Adjustments of linker geometry, length, ratio, and functional-group can tune the size, shape, and internal surface property of a MOF for a targeted application. In this critical review, we highlight advances in MOF synthesis focusing on linker design. Examples of building MOFs to reach unique properties, such as unprecedented surface area, pore aperture, molecular recognition, stability, and catalysis, through linker design are described. Further search for application-oriented MOFs through judicious selection of metal clusters and organic linkers is desirable. In this review, linkers are categorized as ditopic (Section 1), tritopic (Section 2), tetratopic (Section 3), hexatopic (Section 4), octatopic (Section 5), mixed (Section 6), desymmetrized (Section 7), metallo (Section 8), and N-heterocyclic linkers (Section 9).
Microporous materials such as zeolites, metal organic frameworks, activated carbons and aluminum phosphates are suitable for catalysis and separation applications. These high surface area materials are invariably produced in particulate forms and need to be transformed into hierarchically porous structures for high performance adsorbents or catalysts. Structuring of porous powders enables an optimized structure with high mass transfer, low pressure drop, good heat management, and high mechanical and chemical stability. The requirements and important properties of hierarchically porous structures are reviewed with a focus on applications in gas separation and catalysis. Versatile powder processing routes to process porous powders into hierarchically porous structures like extrusion, coatings of scaffolds and honeycombs, colloidal processing and direct casting, and sacrificial approaches are presented and discussed. The use and limitations of the use of inorganic binders for increasing the mechanical strength is reviewed, and the most important binder systems, e.g. clays and silica, are described in detail. Recent advances to produce binder-free and complex shaped hierarchically porous monoliths are described and their performance is compared with traditional binder-containing structured adsorbents. Needs related to better thermal management and improved kinetics and volume efficiency are discussed and an outlook on future research is also given.
The physical upper limit of hydrogen uptake for powder and compressed pellet MIL-101 has been experimentally investigated. Maximum uptake in pellets at 20 K achieves 9.6 wt% and 42 g L-1. Moreover, cryo-adsorption of hydrogen on pellets compared to liquid H2 possesses a larger temperature window for operation without boil-off loss, which will be beneficial for industrial applications.
Metal-organic frameworks (MOFs) are a new class of porous, crystalline materials with applications in the capture, storage, and separation of gasses. Although much effort has been devoted to understanding the properties of MOFs in powder form, in a realistic system the MOF media will likely be employed as dense compacts, such as pucks or pellets, to maximize volumetric efficiency. In these applications efficient transport of the heat of adsorption/desorption is an important design consideration. Consequently, densified composites consisting of a physical mixture of a MOF and expanded natural graphite (ENG) have been proposed as a means to enhance the intrinsically low thermal conductivity of these materials. Here we demonstrate that the high-aspect ratio of ENG particles, combined with uni-axial compression, results in anisotropic microstructural and thermal transport properties in composite MOF-5/ENG pellets. Microscopy of pellet cross-sections revealed a textured microstructure with MOF particle boundaries and ENG orientations aligned perpendicular to the pressing direction. This anisotropy is manifested in the thermal conductivity, which is two to four times higher in directions perpendicular to the pressing direction. We further demonstrate that this anisotropy can be exploited using two processing techniques. First, a custom die and densification process allows for reorientation of the preferred heat flow pathway. Second, a layered MOF-5/ENG microstructure increases the thermal conductivity by an order of magnitude, with only minor ENG additions (5 wt.%). These results reveal that anisotropic thermal transport in MOF composites can be tailored using a judicious combination of second phase additions and processing techniques.
An important class of novel mesoporous and microporous adsorbents like metal-organic frameworks (MOFs) are normally produced in powder form. This paper presents a generic method of manufacturing and characterisation of these materials into low pressure drop and energy saving monolithic structures for industrial applications. One of the MOF candidates that was considered in this study was MIL-101 (Cr) ([Cr3O(OH)(H2O)2(bdc)3].xH2O; bdc = 1,4-benzenedicarboxylate), and the model contaminant gas tested was carbon dioxide (CO2). MIL-101 (Cr) monoliths were manufactured by paste extrusion techniques from the synthesized MIL-101 (Cr) powder. These MIL-101 (Cr) monoliths were then characterised using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), radial compression tests and intelligent gravimetric analysis (IGA). Adsorption properties of the prepared MIL-101 (Cr) powder and monoliths were determined from their pure CO2 sorption isotherms and dynamic adsorption breakthrough curves, that were carried out using high concentration (40% v/v) CO2 challenge. Results have demonstrated that the resulting MIL-101 (Cr) monoliths were highly porous, mechanically strong on compressive loading, thermally regenerable with comparable CO2 adsorption capacity to the synthesized MIL-101 (Cr) powder. From breakthrough curves, mass transfer characteristics such as mass transfer zone velocity and length of the prepared MIL-101 (Cr) monoliths have also been evaluated in this study.
The separation of propylene/propane mixtures is one of the most important but challenging processes in the petrochemical industry. A novel zeolitic imidazole framework (ZIF-8) membrane prepared by a facile hydrothermal seeded growth method showed excellent separation performances for a wide range of propylene/propane mixtures. The membrane showed a permeability of propylene up to 200 barrers and a propylene to propane separation factor up to 50 at optimal separation conditions, well surpassing the "upper-bound trade-off" lines of existing polymer and carbon membranes. The experimental data also showed that the membranes had excellent reproducibility, long-term stability and thermal stability.
Metal-organic frameworks (MOFs) are an emerging class of microporous, crystalline materials with potential applications in the capture, storage, and separation of gases. Of the many known MOFs, MOF-5 has attracted con-siderable attention due to its ability to store gaseous fuels at high densities. Nevertheless, MOF-5 and several other MOFs are known to exhibit limited stability upon exposure to reactive species such as water. The present study quantifies the impact of humid air exposure on the properties of MOF-5 as a function of exposure time, humidity level, and morphology (i.e., powders vs. pellets). Properties examined include hydrogen storage capacity, surface area, and crystallinity. Water adsorption/desorption isotherms are measured using a gravimetric technique; the first uptake exhibits a type V isotherm with a sudden increase in uptake at ~50% relative humidity. For humidity levels below this threshold only minor degradation is observed for exposure times up to several hours, suggesting that MOF-5 is more stable than generally assumed under moderately humid conditions. In contrast, irreversible degradation occurs in a matter of minutes for exposures above the 50% threshold. FTIR spectroscopy indicates that molecular and/or dissociated water is inserted into the skeletal framework after long exposure times. Densification into pellets can slow the degradation of MOF-5 significantly, and may present a pathway to enhance the stability of some MOFs.
Nanofluids, dispersions of metal or oxide nanoparticles in a base working fluid, are being intensively studied due to improvements they offer in thermal properties of the working fluid. However, these benefits have been erratically demonstrated and proven impacts on thermal conductivity are modest and well described from long-established effective medium theory. In this paper, we describe a new class of metal-organic heat carrier (MOHC) nanofluid that offers potential for a larger performance boost in thermal vapor-liquid compression cycles. MOHCs are nanophase porous coordination solids designed to reversibly uptake the working fluid molecules in which the MOHCs are suspended. Additional heat can be extracted in a heat exchanger or solar collector from the endothermic enthalpy of desorption, which is then released as the nanofluid transits through a power generating device such as a turboexpander. Calculations for an R123 MOHC nanofluid indicated potential for up to 15% increase in power output. Capillary tube experiments show that liquid-vapor transitions occur without nanoparticle deposition on the tube walls provided entrance Reynolds number exceeds approximately 100.
Shaping of Zr-MOF powder material into spherical pellets with diameters of 0.5–15 mm in the presence of 10 wt.% sucrose as a binder was successfully demonstrated using a granulator. Zr-MOF pellets were produced in a kilogram batch within 30 min operation time. This granulation approach is a more efficient way to shape MOF-type powder materials into application-specific configurations compared to the mechanical pressing method. The pellets could be conveniently packed in a small hydrogen storage tank. The physical degradation characteristics of the Zr-MOF pellets were studied by drop test and simulated tumbler drum test. The results showed zero breakage of the pellets after 70 consecutive drops at a height of 0.5 m and 5% breakage after 60 min of tumbling time at a speed of 25 rpm. Although the compromised value of the surface area led to a decreased hydrogen storage capacity, this shaping approach still holds promise given an appropriate choice of binder.
Synthesis of a zirconium-benzenedicarboxylate, UiO-66, was carried out in increasing production scales of 250 mL, 1 L and 5 L at 393 K. Investigation on the effect of synthesis parameters on the product yield and textural properties was carried out using autoclaves of 250 mL capacity, whereas systematic scale up works were conducted using glass reactors fitted with condensers under ambient pressure conditions. Finally, 100 L pilot scale synthesis of UiO-66 was carried out using commercial terephthalic acid as ligand with industrial-grade dimethylformamide solvent. The products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and N2 adsorption–desorption measurements. The pelletized UiO-66 using polyvinyl alcohol as a binder demonstrated excellent CO2/N2 separation (36:1) with moderately high CO2 adsorption capacity (191 mg/g) at 20 bar room temperature conditions. UiO-66 also showed excellent catalytic activity in cycloaddition of CO2 to styrene oxide at 2.0 MPa and 373 K with close to 100% selectivity to carbonate. No appreciable particle size effect on the catalytic activity was observed, and UiO-66 could be reused 3 times without loss of catalytic activity.
The main polluting compound in natural gas and biogas is CO2. Therefore, the removal of CO2 from these fuels is a major process in the industry for upgrading their energy content. The separation by Pressure Swing Adsorption (PSA) is energy-wise efficient and the porous aluminium terephthalate - MIL-53(Al) MOF has been pointed out as a promising adsorbent to carry out this separation. In this work, MIL-53(Al) tablets (Basolite® A100) provided by BASF are evaluated to carry out CO2/CH4 separation by adsorption. The adsorption capacity of CO2 and CH4 was assessed by dynamic experiments in a fixed-bed reactor, carried out at 303 K and pressures up to 3.5 bar. The evaluated material presents an adsorption capacity of 4.3 mol kg−1 at 3.5 bar for CO2. Fixed-bed experiments adsorption and desorption in helium flow revealed high selectivity of MIL-53(Al) material for CO2, with a separation factor of 4.1 at 303 K and pressures of 0.1-3.5 bar, thus showing to be promising for a PSA process. The measured single and binary breakthrough curves were simulated with a mathematical model for fixed bed column. Two VSA cycles, both with 4-steps but with different pressurization types were designed to produce 96.5% CH4 from a 40:60 CO2/CH4 mixture; experimental validation confirms a good model prediction. Two industrial-scale PSA processes were designed and optimized by simulations, a case similar to natural gas upgrade (but lower inlet pressure) and biogas upgrade. The CH4 recoveries were determined as 92.8% and 72.9%. The productivities were estimated as 2.09 and 2.78 mol kgads−1 h−1 and the power consumptions as 17.0 and 5.1 W h mol−1CH4. The obtained purity values allow the distribution via pipelines of upgraded CH4.
Metal-organic frameworks (MOFs) are constructed by linking inorganic units with organic linkers to make extended networks. Though more than 20 000 MOF structures have been reported most of these are ordered and largely composed of a limited number of different kinds building units, and very few have multiple different building units (heterogeneous). Although heterogeneity and multiplicity is a fundamental characteristic of biological systems, very few synthetic materials incorporate heterogeneity without losing crystalline order. Thus, the question arises: how do we introduce heterogeneity into MOFs without losing their ordered structure? This Review outlines strategies for varying the building units within both the backbone of the MOF and its pores to produce the heterogeneity that is sought after. The impact this heterogeneity imparts on the properties of a MOF is highlighted. We also provide an update on the MOF industry as part of this themed issue for the 150th anniversary of BASF.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Computational studies on nanofluids composed of metal organic frameworks (MOFs) were performed using molecular modeling techniques. Grand Canonical Monte Carlo (GCMC) simulations were used to study adsorption behavior of 1,1,1,3,3-pentafluoropropane (R-245fa) in a MIL-101 MOF at various temperatures. To understand the stability of the nanofluid composed of MIL-101 particles, we performed molecular dynamics simulations to compute potentials of mean force between hypothetical MIL-101 fragments terminated with two different kinds of modulators in R-245fa and water. Our computed potentials of mean force results indicate that the MOF particles tend to disperse better in water than in R-245fa. The reasons for this observation were analyzed and discussed. Our results agree with experimental results indicating that the employed potential models and modeling approaches provide good description of molecular interactions and the reliabilities.
Objective
To determine whether supplemental screening ultrasound (US) to mammography could improve cancer detection rate of the contralateral breast in patients with a personal history of breast cancer and dense breasts.
Materials and methods
During a one-year study period, 1314 screening patients with a personal history of breast cancer and dense breasts simultaneously underwent mammography and breast US. BI-RADS categories were given for mammography or US-detected lesions in the contralateral breast. The reference standard was histology and/or 1-year imaging follow-up, and the cancer rate according to BI-RADS categories and cancer detection rate and positive biopsy rate according to detection modality were analyzed.
Results
Of 1314 patients, 84 patients (6.4%) were categorized as category 3 with one interval cancer and one cancer which was upgraded to category 4A after 6-month follow-up US (2.5% cancer rate, 95% CIs 1.5-9.1%). Fifteen patients (1.1%) had category 4A or 4B lesions in the contralateral breast. Four lesions were detected on mammography (two lesions were also visible on US) and 11 lesions were detected on US and 5 cancers were confirmed (33.3%, 95% CIs 15.0-58.5%). Six patients (0.5%) had category 4 C lesions, 2 detected on mammography and 4 on US and 4 cancers were confirmed (66.7%, 95% CIs 29.6-90.8%). No lesions were categorized as category 5 in the contralateral breast. Cancer detection rate by mammography was 3.3 per 1000 patients and that by US was 5.0 per 1000 patients, therefore overall cancer detection rate by mammography plus US was 8.3 per 1000 patients. Positive biopsy rate of mammography-detected lesions was 66.7% (4 of 6) and that of US-detected lesions was 40.0% (6 of 15).
Conclusion
US can be helpful to detect mammographically occult breast cancer in the contralateral breast with high positive biopsy rate and low category 3 rate in patients with a previous history of breast cancer and dense breasts.
Hexagonal prism shaped monoliths of envelope density 0.40-0.467 g/cm3 and remarkable mechanical stability were obtained from MIL-101 powder. The hydrogen adsorption isotherms within an extended pressure range show that the excess adsorption decreases with the increasing density of the pellets. At 77 K and 150 bar, the total volumetric capacity is 46.5 g/L; the discharge to 159 K and 5 bar leads to 45 g/L (38.8 g/L referring to the outer tank volume) supporting MIL-101 as a promising candidate for applications in the 77-160 K range of interest for cryo-adsorption hydrogen storage method. The isosteric adsorption enthalpy evaluated from the experimental data with the van't Hoff equation, using fugacity, is in agreement with the calorimetric heat of adsorption reported in literature. Monoliths of this shape allow the best possible packing density of any sorbent in a container and the primary data reported here on MIL-101 could serve as material engineering properties required for modeling hydrogen storage tanks. Copyright (c) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Hierarchical porous crystalline metal-organic framework (MOF) monoliths are prepared by powder-packing synthesis. The resulting MOF monolithic column shows fast and efficient chromatographical separation.
Development of safe and effective hydrogen storage systems is critical for further implementation of hydrogen in fuel cell technologies. Amongst the various approaches to improve the performance of such systems, porous materials-based adsorptive hydrogen storage is envisaged as a long-term solution because of the excellent reversibility, good kinetics and the possibility to store hydrogen at low pressures. Metal–organic frameworks (MOFs) have attracted much attention as porous hydrogen storage materials in the transition from the laboratory to commercial applications. However, MOF materials are often obtained as loose powders with low packing densities and low thermal conductivities. Therefore, to facilitate this transition and enable the MOF materials to form part of a practical hydrogen storage system, knowledge of the ‘processing’ techniques to improve the properties of the powders is essential. However, the processing routes of MOF materials towards system integration are rarely reviewed in the literature although this is of great significance in their proper assessment and potential use for hydrogen storage on a commercial scale. In this review, we begin by introducing the general requirements of an MOF materials-based hydrogen storage system and present how these requirements translate into desired characteristics for further processing. Then, an overview of MOF materials processing towards system integration is provided with an emphasis on improving selected properties including (i) structural stability, (ii) thermal conductivity and (iii) hydrogen storage properties. Copyright © 2014 John Wiley & Sons, Ltd.
Competitive sorption of molecular iodine gas from a mixed stream containing iodine and water vapor is identified and characterized for the hydrophilic Cu-BTC metal–organic framework. By combining simulation (Grand Canonical Monte Carlo and molecular dynamics simulations) with crystallography (high-energy synchrotron-based powder X-ray diffraction data and pair distribution function analyses), we show that I2 substantially adsorbs, in preference to water vapor, into two principal areas. First, it adsorbs in the smallest cage close to the copper paddle wheel. Second, it adsorbs within the main pore with close interactions with the benzene tricarboxylate organic linker. Analysis suggests that I2 forms an effective hydrophobic barrier to minimize water sorption. The finding is relevant to mixed gas streams in nuclear energy industrial processes and accident remediation. This also represents the highest reported I2 sorption by a metal–organic framework (175 wt % I2 or 3 I/Cu).
The separation of paraffin isomers is a very important topic in the petrochemical industry. Zeolite 5A is industrially used to sieve alkane isomers, but its pore size does not allow the separation of monobranched and dibranched alkanes by a kinetic mechanism. In this publication, we compare three ZIF materials in the separation of C6-paraffin isomers: ZIF-8, ZIF-76, and a new material called IM-22. The performance of the materials is evaluated by a breakthrough curve of binary mixtures of n-hexane, 3-methylpentane, and 2,2-dimethylbutane. We show that ZIF-8 is a very attractive alternative to zeolite 5A because it exhibits a high (kinetic) selectivity for the adsorption of linear alkanes and at the same time a high adsorption capacity. The new material IM-22, a ZIF with CHA topology, seems to be particularly suited for the separation of mono- and dibranched paraffin isomers.
Separation of styrene (ST) from ethylbenzene (EB) remains an industrially relevant challenge in the production of polystyrene. Adsorptive separation with metal–organic frameworks (MOFs) is a potential alternative for the conventional vacuum distillation process. Adsorption and separation of ST and EB on the MOFs MIL-47 and MIL-53(Al) were studied under vapor-phase conditions. ST and EB show traditional type I isotherms on MIL-47. Contrarily, ST adsorption isotherms show steep steps on MIL-53(Al) as a result of the breathing of the flexible MOF upon increased adsorbate pressure. The separation potential of both MOFs was investigated by performing vapor-phase breakthrough experiments at total hydrocarbon partial pressures between 1.14 and 16.4 mbar and temperatures between 35 and 90 °C. ST is preferentially adsorbed on both MOFs. Although the MOFs are isostructural, the evolution of selectivity with temperature and pressure is different for both materials due to the different interaction and separation mechanisms.
A hierarchically structured nitrogen-doped porous carbon is prepared from a nitrogen-containing isoreticular metal-organic framework (IRMOF-3) using a self-sacrificial templating method. IRMOF-3 itself provides the carbon and nitrogen content as well as the porous structure. For high carbonization temperatures (950 C), the carbonized MOF required no further purification steps, thus eliminating the need for solvents or acid. Nitrogen content and surface area are easily controlled by the carbonization temperature. The nitrogen content decreases from 7 to 3.3 at% as carbonization temperature increases from 600 to 950 C. There is a distinct tradeoff between nitrogen content, porosity, and defects in the carbon structure. Carbonized IRMOFs are evaluated as supercapacitor electrodes. For a carbonization temperature of 950C, the nitrogen-doped porous carbon has an exceptionally high capacitance of 239 F g-1. In comparison, an analogous nitrogen-free carbon bears a low capacitance of 24 F g-1, demonstrating the importance of nitrogen dopants in the charge storage process. The route is scalable in that multi-gram quantities of nitrogen-doped porous carbons are easily produced.
Solar energy is an alternative, sustainable energy source for mankind. Finding a convenient way to convert sunlight energy into chemical energy is a key step towards realizing large-scale solar energy utilization. Owing to their structural regularity and synthetic tunability, metal-organic frameworks (MOFs) provide an interesting platform to hierarchically organize light-harvesting antennae and catalytic centers to achieve solar energy conversion. Such photo-driven catalytic processes not only play a critical role in the solar to chemical energy conversion scheme, but also provide a novel methodology for the synthesis of fine chemicals. In this review, we summarize the fundamental principles of energy transfer and photocatalysis and provide an overview of the latest progress in energy transfer, light-harvesting, photocatalytic proton and CO2 reduction, and water oxidation using MOFs. The applications of MOFs in organic photocatalysis and degradation of model organic pollutants are also discussed.
Natural gas (NG), whose main component is methane, is an attractive fuel for vehicular applications. Realization of safe, cheap and convenient means and materials for high-capacity methane storage can significantly facilitate the implementation of natural gas fuelled vehicles. The physisorption based process involving porous materials offers an efficient storage methodology and the emerging porous metal-organic frameworks have been explored as potential candidates because of their extraordinarily high porosities, tunable pore/cage sizes and easily immobilized functional sites. In this view, we provide an overview of the current status of metal-organic frameworks for methane storage.
Metal-organic frameworks (MOFs) are finding increasing application as solid catalysts for liquid phase reactions leading to the synthesis of fine chemicals. In the present review we have focused on those reports describing the use of MOFs as catalysts for the synthesis of N-containing heterocycles that is a class of organic compounds with high added value due to their therapeutic use as drugs and their remarkable biological activities. After an introduction describing relevant structural features of MOFs and the nature of their active sites, this manuscript is organized according to the type of N-containing heterocycle synthesized employing MOFs as catalysts including pyrimidines, N-substituted piperidines, quinolines, indoles, N-substituted imidazoles, triazoles and heterocyclic amides. Special attention has been paid to the structural stability of MOFs under the reaction conditions, to the occurrence of metal leaching and reusability. The final section of this review provides some concluding remarks and future prospects for the field, with emphasis on showing the superiority of MOFs with respect to other solid catalysts for this type of liquid phase organic reactions and pointing out that the final goal in this research would be the use of these materials as catalysts in real industrial synthesis.
Metal-organic frameworks (MOFs) are a unique class of crystalline solids comprised of metal cations (or metal clusters) and organic ligands that have shown promise for a wide variety of applications. Over the past 15 years, research and development of these materials have become one of the most intensely and extensively pursued areas. A very interesting and well-investigated topic is their optical emission properties and related applications. Several reviews have provided a comprehensive overview covering many aspects of the subject up to 2011. This review intends to provide an update of work published since then and focuses on the photoluminescence (PL) properties of MOFs and their possible utility in chemical and biological sensing and detection. The spectrum of this review includes the origin of luminescence in MOFs, the advantages of luminescent MOF (LMOF) based sensors, general strategies in designing sensory materials, and examples of various applications in sensing and detection.
Metal Organic Frameworks (MOF) or coordination polymers are porous crystalline materials made up of metal ions and organic ligands that emerged in the 1980s. While their synthesis is well documented, their shaping and formulation are rarely studied in the literature although this step conditions their proper assessment and their potential use in catalysis and separation at an industrial scale. This question is crucial as the properties of these materials require the development of new ways of shaping appropriate to their limited resistance, especially towards heat treatments. This work proposes, for a few representative MOF (ZIF-8, HKUST-1 and SIM-1), to assess the feasibility and impact of their shaping by compression – process most often mentioned in literature – on their structural, textural and catalytic properties.
The metal-organic framework UiO-66-NH2 was synthesized in a scaled batch of approximately 100 grams. The material was then pressed into small pellets at pressures ranging from 5000 – 100,000 psi to determine the effects on porosity and crystal structure. Nitrogen isotherm data and powder x-ray diffraction data indicate that the structure remains intact up to 25,000 psi, with only a slight decrease in surface area. The structure exhibits significant degradation at pressures above 25000 psi. Subsequently, the powder was pressed at 5000 psi and then crushed and sieved into 20x40 mesh size granules to evaluate against ammonia and cyanogen chloride in a breakthrough system simulating individual protection filters. The MOF showed similar capacity to a broad spectrum carbon for both ammonia and cyanogen chloride; however, the breakthrough times, especially for cyanogen chloride, were drastically reduced, likely due to mass transfer limitations from the completely microporous MOF.
Porous adsorbents such as MOF-5 have low thermal conductivities which can limit the performance of adsorption-based hydrogen storage systems. To improve the thermal properties of these materials, we have prepared a series of high-density MOP-5 composites containing 0-10 wt % expanded natural graphite (ENG), which serves as a thermal conduction enhancer. The addition of 10 wt % ENG to MOF-5 and compaction to 0.5 g/cm(3) was previously found to increase the thermal conductivity relative to neat MOP-5 of the same density by a factor of 5. In this study, detailed measurements of the hydrogen storage behavior of MOF-5/ENG composites between 77 and 295 K are reported. We find that MOF-5 pellets with 0 wt % ENG and a density of 0.5 g/cm(3) have a total volumetric hydrogen storage density at 77 K and 100 bar that is 23% larger than powder MOF-5 and 41% larger than cryo-compressed hydrogen. The addition of 10% ENG to 0.5 g/cm(3) MOF-5 pellets produces only a small decrease (6%) in the total volumetric hydrogen storage compared to neat MOF-5 pellets of equal density. The excess, absolute, total, and deliverable hydrogen storage amounts by the MOF-5 composites are compared for ENG additions of 0-10 wt % and pellet densities of 0.3-0.7 g/cm(3). Three adsorption models (Unilan, Toth, Dubinin-Astakhov) are compared for their effectiveness in describing hydrogen adsorption isotherms of MOF-5 and MOF-5/ENG composites. The Unilan model provides the most accurate description of the experimental data, requiring only five temperature-invariant parameters to accurately fit the data across a wide temperature range.
The catalytic activity of ZIF-8 in the synthesis of styrene carbonate from carbon dioxide and styrene oxide is presented. ZIF-8 crystals displayed catalytic activity even at temperatures as low as 50 °C, with styrene carbonate yields as high as ~ 54% at 100 °C. In contrast to many prior-art catalysts, solvents or co-catalysts were not required. Pyridine and ammonia were used as probe molecules to estimate the type and density of acid sites in fresh and reused ZIF-8 catalysts. DRIFT spectroscopy of adsorbed pyridine revealed the presence of both Brönsted (B) and Lewis (L) acid sites. The B-sites have nearly vanished in the case of recycled ZIF-8 catalysts. The simultaneous presence of both the acid sites and the nitrogen basic moieties from the imidazole linker in ZIF-8 promoted the adsorption of the CO2 on the solid surface and its further conversion to the cyclic carbonate. The ZIF-8 catalysts could be recycled and reused without significant loss in catalytic activity.
The metal–organic framework (MOF) ZIF-8 (ZIF-8 = zeolitic imidazolate framework-8) was evaluated as molecular sieve membrane in the pervaporation of the two liquid mixtures n-hexane/benzene and n-hexane/mesitylene. Though it is known from permeation studies of light gases that ZIF-8 membranes show no sharp separation cut-off at the estimated crystallographic pore size of 3.4 Å, highly branched or aromatic hydrocarbons > C5 could be expected therefore to become rejected by the ZIF-8 pores thus remaining in the retentate. However, the ZIF-8 membrane shows for n-hexane and benzene remarkable fluxes. Under consideration of the leakage of the apparatus, we can state that n-hexane and benzene can pass the ZIF-8 membrane. Benzene had a lower flux than n-hexane, whereas for mesitylene we could only observe a very small leakage rate through the O-ring gasket. Correspondingly, medium mixture separation factors have been found for the pervaporation separation of a liquid n-hexane/benzene mixture. Additional mixed gas hydrogen/methane separations, adsorption experiments and leak rate measurements were carried out to evaluate the results.
The present work describes the adsorption and separation of xylene isomers by ZIF-8. Although the formal pore diameter of ZIF-8 is much smaller than the molecular diameter of the xylene isomers, ZIF-8 is able to separate the isomers by molecular sieving. A structural study indicates that the diffusion of the xylenes into the pore structure of ZIF-8 happens via a transitory deformation of the pore aperture which is based on a tilt of the imidazolate linkers, followed by a return to the initial conformation. The rate of adsorption depends on the size of the isomer, i.e. it decreases from para- to meta- and to ortho-xylene. The separation of the xylene isomers is good in the gas phase. In liquid phase breakthrough experiments, the quality of the separation is deteriorated. Moreover, as expected for a separation based on molecular sieving, para-xylene cannot be well separated from ethyl-benzene.
A metal organic framework, Cu3(BTC)2 was prepared in a 1 L-capacity Pyrex reactor by ethanol reflux method. H3BTC (1,3,5-benzenetricarboxylic acid) ligand concentration of 1.01 M with Cu salt/H3BTC mole ratio of 9:5 led to a product with excellent textural properties (>1700 m2/g) in yield in the range of 40–76% − depending on the stirring rate − after 24 h synthesis. Structural integrity was maintained after pelletizing the powder with a PVA binder, but textural properties deteriorated to some extent. The Cu3(BTC)2 powder was highly hydrophilic and water-sorption capacity decreased after pelletization. CO2 adsorption using the ground/sieved Cu3(BTC)2 pellets produced an improved breakthrough pattern despite a small reduction in saturation capacity compared with that of the powders. Tetralin oxidation reaction was then carried out for the Cu3(BTC)2 powder catalysts using TBHP as an oxidant. Tetralin conversions for three repeated runs remained almost constant and the heterogeneity of the Cu3(BTC)2 catalyst in the liquid phase oxidation was examined by hot-filtering experiment. Due to increased pore diffusion resistance, the pellets and its ground/sieved form showed lower catalytic properties than did the powder form of Cu3(BTC)2.
Adsorptive separation of CH4/CO2 mixtures was studied using a fixed-bed packed with MIL-53(Al) MOF pellets. Such pellets of MIL-53(Al) were produced using a polyvinyl alcohol binder. As revealed by N2 adsorption isotherms, the use of polyvinyl alcohol as binder results in a loss in overall capacity of 32%. Separations of binary mixtures in breakthrough experiments were successfully performed at pressures varying between 1 and 8 bar and different mixture compositions. The binary adsorption isotherms reveal a preferential adsorption of CO2 compared to CH4 over the whole pressure and concentration range. The separation selectivity was affected by total pressure; below 5 bar, a constant selectivity, with an average separation factor of about 7 was observed. Above 5 bar, the average separation factor decreases to about 4. The adsorption selectivity is affected by breathing of the framework and specific interaction of CO2 with framework hydroxyl groups. CO2 desorption can be realised by mild thermal treatment.
The hydrogen adsorption isotherms of MIL-101 compressed pellets at 77.3 K are reported. The specific surface area and micropore volume decrease rather sharply when the pellet density approaches the crystal density. Optimum volumetric storage capacity of 40 g L−1 is obtained for monoliths of remarkable mechanical integrity. The X-ray diffraction patterns do not exhibit notable changes with compression up to densities close to the crystal density. However, the infrared spectra show significant modification of the band structure in the range of vibration frequencies characteristic to the carboxylate and phenylene groups, due to the pressure-induced changes in the coordination environment of the metal, close to the adsorption sites. The compression effect on hydrogen adsorption can be correlated with the changes in the nitrogen adsorption isotherms. The results are discussed and compared with the literature results on volumetric hydrogen storage capacity of MOF-5 and MOF-177 monoliths.
The metal-organic framework Zn4O (BDC)3 (BDC = 1,4-bezene dicarboxlate), also known as MOF-5, has demonstrated considerable adsorption of hydrogen, up to 7 excess wt.% at 77 K. Consequently, it has attracted significant attention for vehicular hydrogen storage applications. To improve the volumetric hydrogen density and thermal conductivity of MOF-5, prior studies have examined the hydrogen storage capacities of dense MOF-5 pellets and the impact of thermally conductive additives such as expanded natural graphite (ENG). However, the performance of a storage system based on densified MOF-5 powders will also hinge upon the rate of hydrogen mass transport through the storage medium. In this study, we further characterize MOF-5 compacts by measuring their hydrogen transport properties as a function of pellet density (ρ = 0.3–0.5 g cm−3) and the presence/absence of ENG additions. More specifically, the Darcy permeability and diffusivity of hydrogen in pellets of neat MOF-5, and composite pellets consisting of MOF-5 with 5 and 10 wt.% ENG additions, have been measured at ambient (296 K) and liquid nitrogen (77 K) temperatures. The experimental data suggest that the H2 transport in densified MOF-5 is strongly related to the MOF-5 pellet density ρ.
The low thermal conductivity of the prototype hydrogen storage adsorbent, metal-organic framework 5 (MOF-5), can limit performance in applications requiring rapid gas uptake and release, such as in hydrogen storage for fuel cell vehicles. As a means to improve thermal conductivity, we have synthesized MOF-5-based composites containing 1–10 wt.% of expanded natural graphite (ENG) and evaluated their properties. Cylindrical pellets of neat MOF-5 and MOF-5/ENG composites with densities of 0.3, 0.5, and 0.7 g/cm3 are prepared and assessed with regard to thermal conductivity, specific heat capacity, surface area, and crystallinity. For pellets of density ∼0.5 g/cm3, we find that ENG additions of 10 wt.% result in a factor of five improvement in thermal conductivity relative to neat MOF-5, increasing from 0.10 to 0.56 W/mK at room temperature. Based on the relatively higher densities, surface areas, and enhanced crystallinity exhibited by the composites, ENG additions appear to partially protect MOF-5 crystallites from plastic deformation and/or amorphization during mechanical compaction; this suggests that thermal conductivity can be improved while maintaining the favorable hydrogen storage properties of this material.