Potential of metal-organic frameworks for separation of xenon and krypton

Tunable Porous Coordination Polymers for the Capture, Recovery and Storage of Inhalation Anesthetics

The uptake of inhalation anesthetics by three topologically identical frameworks is described. The 3D network materials, which possess square channels of different dimensions, are formed from the relatively simple combination of Zn^II centres and dianionic ligands that contain a phenolate and a carboxylate group at opposite ends. All three framework materials are able to adsorb N₂ O, Xe and isoflurane. Whereas the framework with the widest channels is able to adsorb large quantities of the various guests from the gas phase, the frameworks with the narrower channels have superior binding enthalpies and exhibit higher levels of retention. The use of ligands in which substituents are bound to the aromatic rings of the bridging ligands offers great scope for tuning the adsorption properties of the framework materials.

Metal-like Boronic-Organic Frameworks: A Design and Computation

Because of the lack of strong π-interaction in their bonds connecting building units, most of the metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) achieved so far are insulators or wide-bandgap semiconductors. The design of metal-like frameworks based on known chemical components is a challenge. This work reports that aryl borons can be linked together through isocyanides to form stable and easily accessible low-dimensional boronic-organic frameworks (BOFs). Particularly, the boron atoms in the BOFs behave like transition metals, forming the combined σ-donation and π-backdonation bonds instead of the usual electron-sharing bonds with the isocyanide linkers. This peculiar bonding endows BOFs with semimetal and narrow-bandgap semiconductor features, which are different from MOFs and COFs and may be found to be useful in future nanoelectronics. The results open a door to integrating the knowledge of the donor-acceptor chemistry in the main group into materials science.

Effect of ring rotation upon gas adsorption in SIFSIX-3-M (M = Fe, Ni) pillared square grid networks

Dynamic and flexible metal-organic frameworks (MOFs) that respond to external stimuli, such as stress, light, heat, and the presence of guest molecules, hold promise for applications in chemical sensing, drug delivery, gas separations, and catalysis. A greater understanding of the relationship between flexible constituents in MOFs and gas adsorption may enable the rational design of MOFs with dynamic moieties and stimuli-responsive behavior. Here, we detail the effect of subtle structural changes upon the gas sorption behavior of two "SIFSIX" pillared square grid frameworks, namely SIFSIX-3-M (M = Ni, Fe). We observe a pronounced inflection in the Xe adsorption isotherm in the Ni variant. With evidence from X-ray diffraction studies, density functional theory, and molecular simulations, we attribute the inflection to a disordered to ordered transition of the rotational configurations of the pyrazine rings induced by sorbate-sorbent interactions. We also address the effect of cage size, temperature, and sorbate on the guest-induced ring rotation and the adsorption isotherms. The absence of an inflection in the Xe adsorption isotherm in SIFSIX-3-Fe and in the Kr, N2, and CO2 adsorption isotherms in SIFSIX-3-Ni suggest that the inflection is highly sensitive to the match between the size of the cage and the guest molecule.

Adsorbate-induced lattice deformation in IRMOF-74 series

IRMOF-74 analogues are among the most widely studied metal-organic frameworks (MOFs) for adsorption applications because of their one-dimensional channels and high metal density. Most studies involving the IRMOF-74 series assume that the crystal lattice is rigid. This assumption guides the interpretation of experimental data, as changes in the crystal symmetry have so far been ignored as a possibility in the literature. Here, we report a deformation pattern, induced by the adsorption of argon, for IRMOF-74-V. This work has two main implications. First, we use molecular simulations to demonstrate that the IRMOF-74 series undergoes a deformation that is similar to the mechanism behind breathing MOFs, but is unique because the deformation pattern extends beyond a single unit cell of the original structure. Second, we provide an alternative interpretation of experimental small-angle X-ray scattering profiles of these systems, which changes how we view the fundamentals of adsorption in this MOF series.

Enhanced xenon adsorption and separation with an anionic indium–organic framework by ion exchange with Co2+

Ion-exchanged Co²⁺-CPM-6 exhibits a distinctly higher Xe/Kr separating ability than organic cation analogues, suggesting a promising candidate material for Xe/Kr separation.

Syntheses, structures and magnetic properties of nine coordination polymers based on terphenyl-tetracarboxylic acid ligands

Nine coordination polymers based on dinuclear metal units/metal-carboxylate chains and 1,1′:4′,1′′-terphenyl-2′,4,4′′,5′-tetracarboxylic acid have been synthesized and structurally characterized. Complexes 2 and 9 show the weak ferromagnetic interactions.

Biomimetic mineralization of metal–organic frameworks around polysaccharides

Biomimetic mineralization exploits natural biomineralization processes for the design and fabrication of synthetic functional materials.

Structural diversity and catalytic properties of five Co2(COO)4cluster-based coordination polymers modified with R-isophthalic acid (R = H, NO2, CH3, OH andtBu)

By regulation the substituents on 5-position of isophthalic acid, we isolated five Co-based CPs with structural diversity, which all constructed from the Co₂(COO)₄unit. The catalytic properties for Knoevenagel condensation and selective epoxidation of styrene have also been investigated.

Directed assembly of a high surface area 2D metal–organic framework displaying the augmented “kagomé dual” (kgd-a) layered topology with high H2and CO2uptake

A high surface area layered MOF withkgd-atopology, based on a nanosized and highly aromatic hexagonal linker, shows high H₂and CO₂uptake.

Screening metal–organic frameworks for mixture separations in fixed-bed adsorbers using a combined selectivity/capacity metric

For screening purposes, mixture separations with MOFs are evaluated on the basis of a combined selectivity/capacity metric.

Capture of actinides (Th4+, [UO2]2+) and surrogating lanthanide (Nd3+) in porous metal–organic framework MIL-100(Al) from water: selectivity and imaging of embedded nanoparticles

Aluminium-based metal–organic framework MIL-100 was utilized for the capture of actinide ([UO₂]²⁺, Th⁴⁺) and lanthanide (Nd³⁺) cations.

Efficiently mapping structure–property relationships of gas adsorption in porous materials: application to Xe adsorption

Designing better porous materials for gas storage or separations applications frequently leverages known structure–property relationships. Reliable structure–property relationships, however, only reveal themselves when adsorption data on many porous materials are aggregated and compared. Gathering enough data experimentally is prohibitively time consuming, and even approaches based on large-scale computer simulations face challenges. Brute force computational screening approaches that do not efficiently sample the space of porous materials may be ineffective when the number of possible materials is too large. Here we describe a general and efficient computational method for mapping structure–property spaces of porous materials that can be useful for adsorption related applications. We describe an algorithm that generates random porous “pseudomaterials”, for which we calculate structural characteristics (e.g., surface area, pore size and void fraction) and also gas adsorption properties via molecular simulations. Here we chose to focus on void fraction and Xe adsorption at 1 bar, 5 bar, and 10 bar. The algorithm then identifies pseudomaterials with rare combinations of void fraction and Xe adsorption and mutates them to generate new pseudomaterials, thereby selectively adding data only to those parts of the structure–property map that are the least explored. Use of this method can help guide the design of new porous materials for gas storage and separations applications in the future.

Tuning the iron redox state inside a microporous porphyrinic metal organic framework

Tuning the iron redox state inside a microporous porphyrinic metal organic framework.

Adsorptive separation of xenon/krypton mixtures using a zirconium-based metal-organic framework with high hydrothermal and radioactive stabilities

The separation of xenon/krypton mixtures is important for both environmental and industrial purposes. The potential of three hydrothermally stable MOFs (MIL-100(Fe), MIL-101(Cr), and UiO-66(Zr)) for use in Xe/Kr separation has been experimentally investigated. From the observed single-component Xe and Kr isotherms, isosteric heat of adsorption (Q_st^o), and IAST-predicted Xe/Kr selectivities, we observed that UiO-66(Zr) has the most potential as an adsorbent among the three candidate MOFs. We performed dynamic breakthrough experiments with an adsorption bed filled with UiO-66(Zr) to evaluate further the potential of UiO-66(Zr) for Xe/Kr separation under mixture flow conditions. Remarkably, the experimental breakthrough curves show that UiO-66(Zr) can efficiently separate the Xe/Kr mixture. Furthermore, UiO-66(Zr) maintains most of its Xe and Kr uptake capacity, as well as its crystallinity and internal surface area, even after exposure to gamma radiation (2kGy) for 7h and aging for 16 months under ambient conditions. This result indicates that UiO-66(Zr) can be considered to be a potential adsorbent for Xe/Kr mixtures under both ambient and radioactive conditions.

Direct in Situ Conversion of Metals into Metal-Organic Frameworks: A Strategy for the Rapid Growth of MOF Films on Metal Substrates

The fabrication of metal-organic framework (MOF) films on conducting substrates has demonstrated great potential in applications such as electronic conduction and sensing. For these applications, direct contact of the film to the conducting substrate without a self-assembled monolayer (SAM) is a desired step that must be achieved prior to the use of MOF films. In this report, we propose an in situ strategy for the rapid one-step conversion of Cu metal into HKUST-1 films on conducting Cu substrates. The Cu substrate acts both as a conducting substrate and a source of Cu²⁺ ions during the synthesis of HKUST-1. This synthesis is possible because of the simultaneous reaction of an oxidizing agent and a deprotonating agent, in which the former agent dissolves the metal substrate to form Cu²⁺ ions while the latter agent deprotonates the ligand. Using this strategy, the HKUST-1 film could not only be rapidly synthesized within 5 min but also be directly attached to the Cu substrate. Based on microscopic studies, we propose a plausible mechanism for the growth reaction. Furthermore, we show the versatility of this in situ conversion methodology, applying it to ZIF-8, which comprises Zn²⁺ ions and imidazole-based ligands. Using an I₂-filled HKUST-1 film, we further demonstrate that the direct contact of the MOF film to the conducting substrate makes the material more suitable for use as a sensor or electronic conductor.

Structures of Metal-Organic Frameworks with Rod Secondary Building Units

Rod MOFs are metal-organic frameworks in which the metal-containing secondary building units consist of infinite rods of linked metal-centered polyhedra. For such materials, we identify the points of extension, often atoms, which define the interface between the organic and inorganic components of the structure. The pattern of points of extension defines a shape such as a helix, ladder, helical ribbon, or cylinder tiling. The linkage of these shapes into a three-dimensional framework in turn defines a net characteristic of the original structure. Some scores of rod MOF structures are illustrated and deconstructed into their underlying nets in this way. Crystallographic data for all nets in their maximum symmetry embeddings are provided.

Neon-Bearing Ammonium Metal Formates: Formation and Behaviour under Pressure

The incorporation of noble gas atoms, in particular neon, into the pores of network structures is very challenging due to the weak interactions they experience with the network solid. Using high-pressure single-crystal X-ray diffraction, we demonstrate that neon atoms enter into the extended network of ammonium metal formates, thus forming compounds Ne_x [NH₄ ][M(HCOO)₃ ]. This phenomenon modifies the compressional and structural behaviours of the ammonium metal formates under pressure. The neon atoms can be clearly localised within the centre of [M(HCOO)₃ ]₅ cages and the total saturation of this site is achieved after ∼1.5 GPa. We find that by using argon as the pressure-transmitting medium, the inclusion inside [NH₄ ][M(HCOO)₃ ] is inhibited due to the larger size of the argon. This study illustrates the size selectivity of [NH₄ ][M(HCOO)₃ ] compounds between neon and argon insertion under pressure, and the effect of inclusion on the high-pressure behaviour of neon-bearing ammonium metal formates.

PIM-1 mixed matrix membranes for gas separations using cost-effective hypercrosslinked nanoparticle fillers

High-free-volume glassy polymers, such as polymers of intrinsic microporosity (PIMs) and poly(trimethylsilylpropyne), have attracted attention as membrane materials due to their high permeability. However, loss of free volume over time, or aging, limits their applicability. Introduction of a secondary filler phase can reduce this aging but either cost or instability rules out scale up for many fillers. Here, we report a cheap, acid-tolerant, nanoparticulate hypercrosslinked polymer 'sponge' as an alternative filler. On adding the filler, permeability is enhanced and aging is strongly retarded. This is accompanied by a CO2/N2 selectivity that increases over time, surpassing the Robeson upper bound.

An Efficient Synthesis Strategy for Metal-Organic Frameworks: Dry-Gel Synthesis of MOF-74 Framework with High Yield and Improved Performance

Vapor-assisted dry-gel synthesis of the metal-organic framework-74 (MOF-74) structure, specifically Ni-MOF-74 produced from synthetic precursors using an organic-water hybrid solvent system, showed a very high yield (>90% with respect to 2,5-dihydroxyterepthalic acid) and enhanced performance. The Ni-MOF-74 obtained showed improved sorption characteristics towards CO2 and the refrigerant fluorocarbon dichlorodifluoromethane. Unlike conventional synthesis, which takes 72 hours using the tetrahydrofuran-water system, this kinetic study showed that Ni-MOF-74 forms within 12 hours under dry-gel conditions with similar performance characteristics, and exhibits its best performance characteristics even after 24 hours of heating. In the dry-gel conversion method, the physical separation of the solvent and precursor mixture allows for recycling of the solvent. We demonstrated efficient solvent recycling (up to three times) that resulted in significant cost benefits. The scaled-up manufacturing cost of Ni-MOF-74 synthesized via our dry-gel method is 45% of conventional synthesis cost. Thus, for bulk production of the MOFs, the proposed vapor-assisted, dry-gel method is efficient, simple, and inexpensive when compared to the conventional synthesis method.

Metal-organic framework with optimally selective xenon adsorption and separation

Nuclear energy is among the most viable alternatives to our current fossil fuel-based energy economy. The mass deployment of nuclear energy as a low-emissions source requires the reprocessing of used nuclear fuel to recover fissile materials and mitigate radioactive waste. A major concern with reprocessing used nuclear fuel is the release of volatile radionuclides such as xenon and krypton that evolve into reprocessing facility off-gas in parts per million concentrations. The existing technology to remove these radioactive noble gases is a costly cryogenic distillation; alternatively, porous materials such as metal–organic frameworks have demonstrated the ability to selectively adsorb xenon and krypton at ambient conditions. Here we carry out a high-throughput computational screening of large databases of metal–organic frameworks and identify SBMOF-1 as the most selective for xenon. We affirm this prediction and report that SBMOF-1 exhibits by far the highest reported xenon adsorption capacity and a remarkable Xe/Kr selectivity under conditions pertinent to nuclear fuel reprocessing.

Increased nuclear energy usage requires the reprocessing of used nuclear fuel to recover radioactive waste, including xenon. Here, the authors perform high-throughput computational screening to identify a metal-organic framework with high xenon selectivity, and demonstrate this with performance analysis.

Redox-Active Metal-Organic Composites for Highly Selective Oxygen Separation Applications

A redox-active metal-organic composite material shows improved and selective O2 adsorption over N2 with respect to individual components (MIL-101 and ferrocene). The O2 sensitivity of the composite material arises due to the formation of maghemite nanoparticles with the pore of the metal-organic framework material.

Single-Crystal-to-Single-Crystal Anion Exchange in a Gadolinium MOF: Incorporation of POMs and [AuCl₄]. Polymers (Basel) 2016;8:E171. [PMID: 30979261 PMCID: PMC6431858 DOI: 10.3390/polym8050171]

The encapsulation of functional molecules inside porous coordination polymers (also known as metal-organic frameworks, MOFs) has become of great interest in recent years at the field of multifunctional materials. In this article, we present a study of the effects of size and charge in the anion exchange process of a Gd based MOF, involving molecular species like polyoxometalates (POMs), and [AuCl₄]-. This post-synthetic modification has been characterized by IR, EDAX, and single crystal diffraction, which have provided unequivocal evidence of the location of the anion molecules in the framework.

ZIF-Derived Nitrogen-Doped Porous Carbons for Xe Adsorption and Separation

Currently, finding high capacity adsorbents with large selectivity to capture Xe is still a great challenge. In this work, nitrogen-doped porous carbons were prepared by programmable temperature carbonization of zeolitic imidazolate framework-8 (ZIF-8) and ZIF-8/xylitol composite precursors and the resultant samples are marked as Carbon-Z and Carbon-ZX, respectively. Further adsorption measurements indicate that ZIF-derived nitrogen-doped Carbon-ZX exhibits extremely high Xe capacity of 4.42 mmol g⁻¹ at 298 K and 1 bar, which is higher than almost all other pristine MOFs such as CuBTC, Ni/DOBDC, MOF-5 and Al-MIL-53, and even more than three times of the matrix ZIF-8 at similar conditions. Moreover, Carbon-ZX also shows the highest Xe/N₂ selectivity about ~120, which is much larger than all other reported MOFs. These remarkable features illustrate that ZIF-derived nitrogen-doped porous carbon is an excellent adsorbent for Xe adsorption and separation at room temperature.

Stability of metal–organic frameworks under gamma irradiation

We report the study of the resistance of archetypal MOFs (MILs, HKUST-1, UiO-66, and ZIF-8) under gamma irradiation.

Efficient HPLC enantiomer separation using a pillared homochiral metal–organic framework as a novel chiral stationary phase

HPLC enantioseparation of various racemates using novel pillared homochiral MOF–silica composite as chiral stationary phase has been successfully demonstrated.

The moisture-triggered controlled release of a natural food preservative from a microporous metal–organic framework

In this work we demonstrate that allyl isothiocyanate (AITC), a common food flavoring agent and food preservative, can be effectively captured by and released in a controlled manner from a microporous metal–organic framework (MOF).

Enantioselective Diels–Alder reaction in the confined space of homochiral metal–organic frameworks

A novel homochiral porous MOF synthesized using (R)-2,2′-dihydroxy-1,1′-binaphthyl-4,4′-dibenzoic acid was shown to be an effective heterogeneous catalyst for asymmetric Diels–Alder reaction between isoprene and N-ethyl maleimide.

The dynamic response of a flexible indium based metal–organic framework to gas sorption

This work presents a flexible indium based MOF which exhibits a novel dynamic response to N₂, Ar and CO₂ sorption.