Microporous Metal-organic Framework Research Articles

Microporous Metal–Organic Framework with Cage-within-Cage Structures for Xenon/Krypton Separation

Separation of Xe/Kr mixtures is of high significance but a daunting task in the chemical industry because of the inert physicochemical properties and similar kinetic diameters of Xe and Kr. Herein, a water-resistant metal–organic framework (MOF) JXNU-11(Fe2Ni) features distinctive cage-within-cage structures with small octahedral cages encapsulated in large cuboctahedral cages. JXNU-11(Fe2Ni) displays a high Xe uptake capacity of 124 cm3 g–1 at 273 K. The breakthrough interval between Xe and Kr for Xe/Kr (20/80) mixtures is 56 min g–1 at 273 K, making JXNU-11(Fe2Ni) one of the highest-performing MOF. Additionally, JXNU-11(Fe2Ni) exhibits high Xe productivity for high-purity Xe, good recycling performance, and low regeneration energy in breakthrough experiments. Computational simulations on JXNU-11(Fe2Ni) indicate that the preferential site for Xe is the interlayered spaces between the octahedral cage and the cuboctahedral cage. The Xe atom is firmly trapped through extensive Xe···framework interactions. Thus, the high water stability, high productivity of highly pure Xe, good recyclability, and low regeneration energy of JXNU-11(Fe2Ni) render it a promising adsorbent for actual Xe/Kr separation.

Enhanced water sorption onto bimetallic MOF-801 for energy conversion applications

Adsorption-assisted energy conversion technologies are receiving extensive attention recently as a sustainable technology for meeting the worldwide energy demand. The advancement of this technology relies on the development of the mass and heat transfer properties of the adsorbent materials. MOF-801, a zirconium-based microporous metal organic framework (MOF), is regarded as a promising adsorbent for adsorption-assisted energy conversion technologies. This study focuses on enhancing the water sorption properties of MOF-801 by introducing different transitional metals, nickel, and cobalt, into the framework. Herein, two novel bimetallic MOFs were synthesized by partial substitution of the metal zirconium in MOF-801 with nickel and cobalt, employing a one-pot solvothermal synthesis method. The bimetallic MOFs, Ni-MOF-801, and Co-MOF-801, were found isostructural with the pristine MOF-801. Porous properties were measured experimentally, and an increment in total surface area and microporous surface area was observed for both samples. The water adsorption isotherm of the samples was measured, and a greater affinity at the lower pressure region was observed compared to the pristine MOF due to the synergistic effects of two metals in the frameworks. This greater affinity towards water vapor resulted in an increase in effective net uptake and specific cooling effect. The thermophysical properties were measured experimentally over a wide range of temperatures, and an improvement was found in the doped MOF-801 samples. Co-MOF-801 outperformed the pristine MOF-801 with a 43% improvement in specific cooling effect for delivering high-grade cooling at 10 °C with the same working conditions. The results will significantly contribute towards the development of high-performance next generation adsorption-based energy conversion systems.

Separation of naphtha on a series of ultramicroporous MOFs: A comparative study with zeolites

Naphtha separation is of significant importance for refineries to make full use of oil and to reduce the cost of refining. In the recent years, scientists have devoted great efforts to developing novel porous materials such as metal-organic frameworks (MOFs) for the separation of naphtha. However, previous investigations have been limited to single-component or 3- to 5-component model systems. Separation of full range hydrocarbon mixture such as naphtha has rarely been explored. In this work, we carry out a comprehensive study on the separation of alkane isomers as well as full range naphtha mixture by a series of robust microporous MOFs, e.g., Zr-abtc, Al-bttotb, Co-fa, in comparison with representative zeolite materials, e.g., ZSM-5 and zeolite 5A. We find that Al-bttotb and Co-fa, which can separate monobranched and dibranched alkanes through complete molecular exclusion, show superior separation performance for naphtha mixtures. This work may shed light on strategies for developing adsorbents with optimal performance for naphtha separation and ultimately realizing the industrialization of MOF-based separation technologies.

Metal-organic framework based on iron and terephthalic acid as a multiporous support for lipase Burkholderia lata LBBIO-BL02 and its potential for biocatalysis

Metal-organic frameworks (MOFs) are versatile materials because they have a large internal surface area and tuneable pores, making them suitable for enzyme immobilization. In this study, we prepared a typical microporous Fe-BDC MOF through a thermal treatment to produce additional meso and macropores interconnected to each other, capable of immobilizing the Burkholderia lata LBBIO-BL02 (BLL) lipase by entrapment and physical adsorption. The immobilization efficiency (E) was 90%, and the activity retention (R) was 400% (pNPP hydrolysis). The immobilized lipase (BLL@BDC) also showed excellent activity in the hydrolysis of vegetable oils in aqueous medium, achieving up to 3,200 U g−1 for olive oil, as well as high stability in organic solvents, especially for polar ones, such as iso-propanol (101.5 ± 2.6%), ethanol (103.0 ± 6.0%) and acetone (107.7 ± 8.3%). The results indicate that the multiporous Fe-BDC MOF is a promising support for lipase immobilization and further application in biocatalysis performed in organic media.

Adsorption and Release of 1-Methylcyclopropene by Metal–Organic Frameworks for Fruit Preservation

Metal–organic frameworks (MOFs) are potentially useful in addressing challenges in food chemistry in light of their tunable pore dimensions and functionalities. Here we demonstrate the adsorption and release of 1-methylcyclopropene (1-MCP) by easily attainable microporous MOFs. The effectiveness of the encapsulation and release system has been validated by a fruit preservation test.

Portable Hyperpolarized Xe-129 Apparatus with Long-Time Stable Polarization Mediated by Adaptable Rb Vapor Density.

The extraordinary sensitivity of 129Xe, hyperpolarized by spin-exchange optical pumping, is essential for magnetic resonance imaging and spectroscopy in life and materials sciences. However, fluctuations of the polarization over time still limit the reproducibility and quantification with which the interconnectivity of pore spaces can be analyzed. Here, we present a polarizer that not only produces a continuous stream of hyperpolarized 129Xe but also maintains stable polarization levels on the order of hours, independent of gas flow rates. The polarizer features excellent magnetization production rates of about 70 mL/h and 129Xe polarization values on the order of 40% at moderate system pressures. Key design features include a vertically oriented, large-capacity two-bodied pumping cell and a separate Rb presaturation chamber having its own temperature control, independent of the main pumping cell oven. The separate presaturation chamber allows for precise control of the Rb vapor density by restricting the Rb load and varying the temperature. The polarizer is both compact and transportable─making it easily storable─and adaptable for use in various sample environments. Time-evolved two-dimensional (2D) exchange spectra of 129Xe absorbed in the microporous metal-organic framework CAU-1-AmMe are presented to highlight the quantitative nature of the device.

Orthogonalization of Polyaryl Linkers as a Route to More Porous Phosphonate Metal-Organic Frameworks.

The coordinative pliancy of the phosphonate functional group means that metal-phosphonate materials often self-assemble as well-packed structures with minimal porosity, as efficient inter-ligand packing is enabled. Here, we report a multistep synthesis of a novel aryl-phosphonate linker with an orthogonalized ligand core, 1,3,5-tris(4'-phosphonophenyl)-2,4,6-trimethylbenzene (H6 L2) designed to form more open structures. A series of crystalline metal-phosphonate frameworks (CALF-35 to -39) have been assembled by coordinating to divalent metals (Ba, Sr, Ca, Mg, Zn). H6 L2 is unable to pack efficiently and, as a consequence, yields several distinct microporous structures. The resulting structures are discussed in detail, with a focus on the solid-state packing of the sterically rigidified linker. Combined with larger cations (Sr, and Ba), H6 L2 packs in a parallel-offset manner, yielding isomorphous and microporous metal-organic frameworks (CALF-35 (Sr), and (Ba)). When coordinated to smaller metals (Ca, Mg, Zn), H6 L2 forms four new structures. Two Ca MOFs of different stoichiometry, (CALF-36 and 37) and a Mg MOF CALF-38 show narrow pores and have high selectivities for CO2 over N2 and CH4 . Finally, in CALF-39 (Zn), H6 L2 linkers pack in a herringbone fashion, resulting in a material with 10.9×10.1 Å2 square channels. The stability of all structures was tested, and the most porous structure, CALF-39 (Zn), was found to retain its structure and gas adsorption after immersion in water over pH 3-11.

Ethylene purification in a metal–organic framework over a wide temperature range via pore confinement

The separation of C2H4 from C2H6/C2H4 mixture is of great importance but difficult and energy intensive. Adsorptive separation provides an alternative approach to ameliorate this situation. Here, we report a microporous metal–organic framework (MOF) BUT-315-a as a C2H6-selective adsorbent for the separation of C2H6/C2H4 gas mixture. BUT-315-a combines good IAST selectivity of 2.35 with high C2H6 uptake of 97.5 cm3 g−1, giving superior high separation potential ΔQ (2226 mmol L−1) for equimolar C2H6/C2H4 at 298 K. Impressively, such excellent performance can be preserved at higher temperatures of 313 and 323 K to accommodate industrial conditions. Efficient dynamic separation performance of BUT-315-a has been demonstrated by column breakthrough experiments under varied temperatures and gas ratios. Theoretical calculations further reveal multiple synergistic interactions between C2H6 and the framework. This work highlights a new benchmark material for C2H6/C2H4 separation and provides guidance for designing adsorbent for separation applications.

Meso/microporous MOF@graphene oxide composite aerogels prepared by generic supercritical CO2 technology

The increasing complexity in composition and structure of modern porous nanocomposite materials requires the development of advanced technologies that allow the simultaneous treatment of dissimilar compounds, not only with unlike composition but also involving different classes of pores, e.g., micro and mesopores. This work shows that supercritical CO 2 (scCO 2 ) technology can be used as generic processing aid to obtain composites involving non-reduced graphene oxide (GO) and metal organic frameworks (MOFs) in the form of aerogels with hierarchical porosity. Hybrid aerogels are formed by either depositing ( ex situ ) or growing ( in situ ) MOF nanocrystals onto the surface of 2D GO nanosheets. The archetypal hydrophilic HKUST-1 and UiO-66 and hydrophobic ZIF-8 microporous MOFs are chosen to exemplify the method possibilities. The ex situ route was adequate to prepare hydrophilic MOFs@GO homogeneous composites, while the in situ approach must be used to prepare hydrophobic MOFs@GO aerogels. Moreover, the scCO 2 methodology should be adjusted for each studied MOF in regard of the organic solvent used to disperse the nanoentities constituting the composite. The end-products are obtained in the form of aerogels mimicking the shape of the recipient in which they are contained. The products are characterized in regard of structure by X-ray diffraction, textural properties by low temperature N 2 adsorption/desorption and morphology by electronic microscopy. • scCO 2 as a synthetic platform for fabrication of meso/microporous MOF@GO aerogels. • MOF hydrophobic/hydrophilic nature defines the choice of ex situ/in situ scCO 2 method. • The ex situ route can be applied to a large number of MOFs with hydrophilic nature. • The in situ method can be also applied to hydrophobic MOFs to reduce time/resources. • MOF@GO composites were homogenous with hierarchical micro/meso porosity.

A robust ethane-selective metal-organic framework with nonpolar pore surface for efficient C2H6/C2H4 separation

A robust chiral chain-based microporous MOF with nonpolar pore surface and suitable cavity space was designed and synthesized for C 2 H 6 /C 2 H 4 separation. The MOF shows simultaneously high C 2 H 6 uptake and inverse C 2 H 6 /C 2 H 4 selectivity, affording the one-step purification of C 2 H 4 from C 2 H 6 -C 2 H 4 mixture under ambient condition. • Adopting the methyl-modified ligands afforded a Zn-MOF with nonpolar channels. • The MOF shows excellent stability and uncommon preferential adsorption for C 2 H 6 . • The methyl groups in MOF are crucial for the separation of C 2 H 6 over C 2 H 4 . • The MOF achieves effective one-step purification of C 2 H 4 from C 2 H 6 -C 2 H 4 mixture. Due to similar physical and chemical properties, the separation of ethane (C 2 H 6 ) and ethylene (C 2 H 4 ) is an indispensable and extremely challenging task in petrochemical process. Although traditional C 2 H 4 -selective porous materials realize the separation of C 2 H 6 -C 2 H 4 mixture, but subsequent multiple separation operations are required to produce high-purity C 2 H 4 . Herein, we designed a hydrophobic methyl group-modified metal-organic framework (MOF) for one-step efficient purification of C 2 H 4 from C 2 H 6 -C 2 H 4 mixture. The as-synthesized MOF constructed from inexpensive carboxylic acid and pyrazole mixed linkers shows high thermal stability and chemical stability (water resistance). Cooperative study on the pure-component sorption isotherm, Ideal Adsorbed Solution Theory method, adsorption heat calculation and grand canonical Monte Carlo (GCMC) simulation supports the unique C 2 H 6 -selective feature of the MOF. Breakthrough experiments revealed the one-step C 2 H 4 purification from C 2 H 6 -C 2 H 4 (1/1, 1/10 and 1/15) mixtures through the MOF around room temperature condition with nice recyclability.

Understanding the Working Mechanism of the Novel HKUST-1@BPS Composite Materials as Stationary Phases for Liquid Chromatography.

Composite materials have been used based on coordination polymers or microporous metal-organic frameworks (MOFs) combined with mesoporous matrices for adsorption-related techniques, which enable outflanking some adverse phenomena manifested during pristine components operation and enhance the performance and selectivity of the resulting materials. In this work, for the first time, the novel HKUST-1@BPS composites synthesized by the microwave-assisted (MW) technique starting from microporous HKUST-1 (Cu3(btc)2) MOF and biporous silica matrix (BPS) with bimodal mesopore size distribution were comparatively studied as materials for liquid-phase adsorption techniques utilizing the high-performance liquid chromatography (HPLC) method and benzene as a model adsorbate. It was established that the studied HKUST-1@BPS composites can function as stationary phases for HPLC, unlike the pristine HKUST-1 and bare BPS materials, due to the synergetic effect of both components based on the preliminary enhanced adsorbate mass transfer throughout the silica mesopores and, subsequently, its penetrating into HKUST-1 micropores. The suggested mechanism involves the initial deactivation of open metal Cu2+ sites in the HKUST-1 framework structure by isopropanol molecules upon adding this polar component into the mobile phase in the region of the isopropanol concentration of 0.0 to 0.2 vol.%. Thereafter, at the medium range of varying the isopropanol concentration in the eluent of 0.2 to 0.3 vol.%, there is an expansion of the previously inaccessible adsorption centers in the HKUST-1@BPS composites. Subsequently, while further increasing the isopropanol volume fraction in the eluent in the region of 0.3 to 5.0 vol.%, the observed behavior of the studied chromatographic systems is similar to the quasi-normal-phase HPLC pattern. According to the obtained thermodynamic data, benzene adsorption into HKUST-1 micropores from solutions with a vol.% of isopropanol in the range of 0.4 to 5.0 follows the unique entropy-driven mechanism previously described for the MIL-53(Al) framework. It was found that HKUST-1 loading in the composites and their preparation conditions have pronounced effects on their physicochemical properties and adsorption performance, including the adsorption mechanism.

Influence of metal-organic framework MOF-76(Gd) activation/carbonization on the cycle performance stability in Li-S battery

Carbon materials are widely used as sulphur support to improve the cycle performance, efficiency, electrode stability and high discharge capacity of lithium-sulphur batteries. Metal-organic frameworks with their unique structure can confine polysulphides and restrain the shuttle effect. Microporous metal-organic framework MOF-76(Gd) was synthesized and applied as a support for sulphur in lithium-sulphur batteries. To achieve better conductivity, MOF-76(Gd) was carbonized and applied as a conductive nest in the sulphur cathode. Activated (AC) and carbonized (C) MOF-76(Gd) in electrode material displayed sulphur encapsulation and polysulphide confinement which was proved by stable capacity during cycling. The carbonized MOF-76(Gd) in cathode material showed an initial discharge capacity of 658 mAh g−1, and after 200 cycles at 0.5 C, the capacity retention was almost 93%.

Role of crystal nucleation and growth conditions on the soft-templated preparation of HKUST-1 MOFs with hierarchical porosity involving gemini surfactants

Utilization of porous materials such as metal-organic frameworks (MOFs) will get enhanced if a connectivity between the micro- and meso-pore structures within such solids can be achieved. Soft-templated synthesis has been a widely used method where surfactant based molecular self-assemblies act as templates to introduce mesopores in the intrinsically microporous MOFs and thereby generate hierarchical porosity (HP). In this work, we present our investigations on the formation of HP HKUST-1 MOFs by using two gemini surfactants, G16 and G14 as soft-templates. Crystal nucleation and growth conditions in the soft-templated synthesis play a significant role in the success of HP formation. Here, we have employed variations in the composition of water:ethanol (W:E) solvent mixtures mainly to understand and to arrive at the favourable templation conditions. Depending on the relative ratio of the W:E compositions and the gemini surfactants, we have employed additional parameters such as temperature, reaction time and reactant compositions that crucially influence the nucleation and growth factors to achieve HP. Gas sorption, DLS, zeta potential, thermogravimetry, SEM and TEM investigations indicate the formation of unique types of HP structures including the ones which have formed via bilayer and vesicular based soft-templation mechanism in our samples. Presence of the mesopores in our HP samples have been demonstrated by correlating the higher values of both the hierarchy factor (HF) and dye-sorption with a bulky Rhodamine molecule. A direct relation found between the HF values and the dye-uptake in our HP samples indicate that HF can be used as a quantitative marker of HP.

Enhancing Perfluorinated electron specialty gases separation selectivity in ultra-microporous metal organic framework

Reducing the anthropogenic emission of electron specialty gases (F-gases, e.g., NF3, CF4 and SF6) in semiconductor industry is vital due to the strong greenhouse effect of F-gases. Herein, a microporous metal-organic framework (SBMOF-1) has proved effective for F-gases trapping owing to the enhanced dispersion force in ultramicropores. The 5–8.5 Å pocket-like pore offers overlapped intermolecular interactions in SBMOF-1 for trapping F-gases. Outstanding F-gases/N2 IAST selectivity and time-dependent adsorption tests confirmed the adsorptive separation performance of SBMOF-1. Moreover, breakthrough experiments also manifest the practical potential of SBMOF-1 for capturing F-gases from semiconductor etching exhaust.

A Microporous Metal-Organic Framework Incorporating Both Primary and Secondary Building Units for Splitting Alkane Isomers.

We demonstrate the assembly of a mononuclear metal center, a hexanuclear cluster, and a V-shaped, trapezoidal tetracarboxylate linker into a microporous metal-organic framework featuring an unprecedented 3-nodal (4,4,8)-c lyu topology. The compound, HIAM-302, represents the first example that incorporates both a primary building unit and a hexanuclear secondary building unit in one structure, which should be attributed to the desymmetrized geometry of the organic linker. HIAM-302 possesses optimal pore dimensions and can separate monobranched and dibranched alkanes through selective molecular sieving, which is of significant value in the petrochemical industry.

Biomimetic Growth of Metal-Organic Frameworks for the Stabilization of the Dentin Matrix and Control of Collagenolysis.

The dentin matrix is a collagenous scaffold structurally involved in anchoring resin-based materials to the tooth. Time-dependent degradation of this scaffold at the resin-dentin interface remains a core problem in adhesive dentistry, limiting the service life of dental fillings. This study explored the use of emergent materials termed metal-organic frameworks (MOFs)─formed by the self-assembly of metal ions and organic building blocks─to safeguard the collagen integrity in the functional dentin matrix. We demonstrate that collagen fibrils (from demineralized human dentin) can induce the biomimetic growth of MOF crystals as protective coatings to strengthen and stabilize the fibrils. Zeolitic imidazolate framework-8 (ZIF-8), a zinc-based microporous MOF, was used to fabricate the MOF composites via a "one-pot" reaction in water. The ZIF-modified dentin matrix presented superior mechanical strength and resistance to proteolysis, which can positively affect the longevity of collagen as an anchoring substrate. This work identifies a potential biomedical application of biomimetically synthesized MOFs in repairing dental tissues critical to restorative therapies.

Efficient adsorption of cesium using a novel composite inorganic ion-exchanger based on metal organic framework (Ni[(BDC)(TED)]) modified matal hexacyanoferrate

The surfaces of the microporous metal-organic frameworks (Ni[(BDC)(TED)]) were modified by [KCu(Fe(CN)6)] and then it was employed as a nanocomposite ion exchanger to remove cesium ions from aqueous solutions. Various techniques such as infrared (IR) spectroscopy, X-ray diffraction (XRD), nitrogen adsorption/desorption (BET), field emission scanning electron microscopy (FESEM), Transmission Electron Microscopy (TEM), energy-dispersive X-ray analysis (EDAX), and thermogravimetry analysis were used to study the structure, morphology, composition and thermal stability of the synthesized compounds. The effects of parameters influencing adsorption, such as initial concentration of the solution, temperature, contact time, adsorbent dosage, and pH, were determined. The equilibrium data were fitted into Langmuir, Freundlich, and Temkin isotherm models. In addition, the pseudo-first-order, second-order, Elovich and the intraparticle diffusion model were applied to describe the adsorption kinetics of Cs. The results of the assessments indicated the adsorption experiments reached equilibrium very rapidly in 45 min. Furthermore, a maximum equilibrium adsorption capacity of 206 mgg−1 was obtained. The equilibrium data was fitted well with the Freundlich model, indicating that the Cs adsorption process was multilayered. Moreover, the pseudo-second-order kinetic model suitably described the kinetics of Cs adsorption. Desirable selectivity of Cs in the presence of the other interfering ions was confirmed. In addition, adsorption efficiency by the synthesized nanocomposite after five cycles reached 88% and determination of the thermodynamic parameters showed that the Cs adsorption process was endothermic and spontaneous. In addition, easy synthesis, high adsorption at ambient temperature, and wide ranges of pH are the other factors that make it a promising candidate for cesium removal.

A Microporous Metal-Organic Framework with Channels Constructed from Nonpolar Aromatic Rings for the Selective Separation of Ethane/Ethylene Mixtures.

The separation of ethane and ethylene is an important segment in the purification of chemical raw materials in industrial production. However, due to their similar physical and chemical properties, the separation of C2 H6 /C2 H4 is challenging. Herein, we report the selective adsorption of ethane over ethylene by a microporous metal-organic framework with nonpolar aromatic rings constructed channels, [Co1.5 (TATB)(H2 O)0.5 ] ⋅ 5DMA ⋅ 3H2 O (Co-TATB, H3 TATB=4,4',4''-(s-triazine-2,4,6-triyl) tribenzoic acid). This compound showed a higher ethane capacity than that of ethylene, and a low adsorption enthalpy of ethane only of 19.4 kJ mol-1 . Further, the dynamic breakthrough experimental confirmed that Co-TATB can selectively adsorb ethane from ethane/ethylene separation.

Recent advances in porous MOFs and their hybrids for photothermal cancer therapy.

Cancer is still one of the most life-threatening diseases in the world. Among the various cancer therapeutic strategies, photothermal therapy (PTT) has attracted considerable attention due to its high treatment efficacy, low invasive burden, and minor side effects. Microporous metal-organic frameworks (MOFs) are potential materials for photothermal tumor treatment thanks to their high surface areas, suitable pore geometry, and easy functionalization. Through designating organic linkers, encapsulating PTT agents and fabricating MOF hybrids, MOF-based treatment platforms have great potential in PTT. In this review, we mainly summarize the recent advances of MOFs in photothermal combined cancer therapy. The present challenges and possible future prospects in this field are also explored.

Recent advances in microporous metal–organic frameworks as promising adsorbents for gas separation

We herein not only provide a comprehensive overview of the most important advances in the development of MOFs as separating materials for gas separation over the last five years, but also provide an update of the current challenges in this active field.