Metal-Organic Frameworks (MOFs) As Hydrogen Storage Materials At Near-Ambient Temperature.

Hydrogen may play a critical role in our efforts to de-carbonize by 2050. However, there remain technical challenges in the storage and transport of hydrogen. Metal-organic frameworks (MOFs) have shown significant promise for hydrogen storage at cryogenic temperatures. A material that can meet the US department of energy (DOE) ultimate goal of 6.5 wt. % for gravimetric performance and 50g/L for volumetric storage at near-ambient temperatures would unlock hydrogen as a future fuel source for on-board applications. Metal-organic frameworks typically have low heat of adsorptions (i.e. 4-7 kJ/mol), whereas for storing significant quantities of hydrogen at near-ambient temperatures, 15-25 kJ/mol is likely required. In this review we explore the current methods used (i.e., open-metal sites, alkali dopants and hydrogen spillover) for promoting strong adsorption within MOFs. Further we discuss MOF-based materials with respect to the technical aspects of deliverable capacity, kinetics and stability.

A Homochiral Poly(2-oxazoline)-based Membrane for Efficient Enantioselective Separation.

Chiral separation membranes have shown great potential for the efficient separation of racemic mixtures into enantiopure components for many applications, such as in the food and pharmaceutical industries; however, scalable fabrication of membranes with both high enantioselectivity and flux remains a challenge. Herein, enantiopure S-poly(2,4-dimethyl-2-oxazoline) (S-PdMeOx) macromonomers were synthesized and used to prepare a new type of enantioselective membrane consisting of a chiral S-PdMeOx network scaffolded by graphene oxide (GO) nanosheets. The S-PdMeOx-based membrane showed a near-quantitative enantiomeric excess (ee) (98.3±1.7 %) of S-(-)-limonene over R-(+)-limonene and a flux of 0.32 mmol m-2 h-1 . This work demonstrates the potential of homochiral poly(2,4-disubstituted-2-oxazoline)s in chiral discrimination and provides a new route to the development of highly efficient enantioselective membranes using synthetic homochiral polymer networks.

Capture, Storage, and Release of Oxygen by Metal-Organic Frameworks (MOFs).

Oxygen is a critical gas for medical and industrial settings. Much of today's global oxygen supply is via inefficient technologies such as cryogenic distillation, membranes or zeolites. Metal-organic frameworks (MOFs) promise a superior alternative for oxygen separation, as their fundamental chemistry can in principle be tailored for reversible and selective oxygen capture. We evaluate the characteristics for reversible and selective uptake of oxygen by MOFs, focussing on redox-active sites. Key characteristics for separation can also be seen in MOFs for oxygen storage roles. Engineering solutions to release adsorbed oxygen from the MOFs are discussed including Temperature Swing Adsorption (TSA), Pressure Swing Adsorption (PSA) and the highly efficient Magnetic Induction Swing Adsorption (MISA). We conclude with the applications and outlooks for oxygen capture, storage and release, and the likely impacts the next generation of MOFs will have on industry and the broader community.

Efficient metal ion sieving in rectifying subnanochannels enabled by metal-organic frameworks.

Biological ion channels have remarkable ion selectivity, permeability and rectification properties, but it is challenging to develop artificial analogues. Here, we report a metal-organic framework-based subnanochannel (MOFSNC) with heterogeneous structure and surface chemistry to achieve these properties. The asymmetrically structured MOFSNC can rapidly conduct K+, Na+ and Li+ in the subnanometre-to-nanometre channel direction, with conductivities up to three orders of magnitude higher than those of Ca2+ and Mg2+, equivalent to a mono/divalent ion selectivity of 103. Moreover, by varying the pH from 3 to 8 the ion selectivity can be tuned further by a factor of 102 to 104. Theoretical simulations indicate that ion-carboxyl interactions substantially reduce the energy barrier for monovalent cations to pass through the MOFSNC, and thus lead to ultrahigh ion selectivity. These findings suggest ways to develop ion selective devices for efficient ion separation, energy reservation and power generation.

Control of Physical Aging in Super-Glassy Polymer Mixed Matrix Membranes.

ConspectusSince the discovery of polymers of intrinsic microporosity (PIMs) in 2004, the fast size-selective interconnected pore cavities of the polymers have caused the upper bound of membrane performance to be revised, twice. Simultaneously, porous materials have meant that mixed matrix membranes (MMMs) are now a relatively simple method of enhancing transport properties. While there are now reliable routes with mixed matrices to improve the fundamental transport properties of membrane materials, many of the other properties crucial for separation applications remain largely unaddressed. Physical aging severely affects membrane performance over time, especially for those prepared from high fractional free volume polymers. Gradual densification of the glassy polymer chains causes the connected pore channels present in these materials to constrict. Studies now suggest that aging of superglassy polymer materials is a two-step process; a rapid densification occurs within the first few days, followed by a gradual rearrangement of packed chains over longer time frames toward a theoretical equilibrium state. Although advantageous in terms of size selectivity, the considerable drop in permeation over the days and weeks after manufacture greatly impacts material applicability. While often still permeating faster than traditional membrane materials, the continuous gradual collapse of cavities in these polymers are a significant challenge in the application of high free volume polymer membranes. In 2014, we discovered that the porous aromatic framework PAF-1 not only greatly improved the membrane's void space and speed of gas transport but also seemingly froze several glassy polymers in a low-density state, holding the polymer's pore channels open, a process termed as Porosity Induced Side chain Adsorption (PISA).This discovery of PISA fundamentally challenged the conventional wisdom at the time that the aging rate could only be addressed by densification of the polymer. Unlike other high-performance glassy polymers, membranes containing PAF-1 can retain their high permeability for more than a year. Several other examples of antiaging behavior have been subsequently reported by the team, where control of aging rate as a function of gas penetrant, selectivity increases, and stability at higher pressures was reported. These works also demonstrate that these mixed matrix systems had applicability for several other separations, including pervaporation, solvent nanofiltration, and as separators for energy applications. In our subsequent studies, the antiaging mechanism has been elucidated as an effect of the interaction between the polymer's accessible pendant methyl group and the aromatic pore surface of PAF-1 or other antiaging additives. In otherwise identical MMMs, where this hypothesized methyl-π interaction is either absent or interrupted, we find that the antiaging behavior expected by the fixation of the polymer chains to the pore surface and PAF-1 does not occur. As a design approach for mixed matrix membranes, targeted interfacial interactions are a promising pathway for developing other stable membranes, enabling the exciting class of PIM materials to improve industrial separation efficiency.

Solvent-induced enantioselectivity reversal in a chiral metal organic framework.

Solvent-induced enantioselectivity reversal is a rarely reported phenomenon in porous homochiral materials. Similar behavior has been studied in chiral high performance liquid chromatography, where minor modifications to the mobile phase can induce elution order reversal of two enantiomers on a chiral stationary phase column. We report the first instance of solvent-induced enantioselectivity reversal in a homochiral metal organic framework. Further, we highlight the complex enantioselectivity behavior of homochiral metal organic frameworks toward racemic mixtures in the presence of solvents through racemate-solvent enantioselectivity and loading experiments as well as enantiopure-solvent loading experiments. We hypothesize that this interesting selectivity reversal behavior is likely to be observed in other competitive adsorption, nonchiral selective processes involving a solvent.

Highly-Efficient Sulfonated UiO-66(Zr) Optical Fiber for Rapid Detection of Trace Levels of Pb2.

Lead detection for biological environments, aqueous resources, and medicinal compounds, rely mainly on either utilizing bulky lab equipment such as ICP-OES or ready-made sensors, which are based on colorimetry with some limitations including selectivity and low interference. Remote, rapid and efficient detection of heavy metals in aqueous solutions at ppm and sub-ppm levels have faced significant challenges that requires novel compounds with such ability. Here, a UiO-66(Zr) metal-organic framework (MOF) functionalized with SO3H group (SO3H-UiO-66(Zr)) is deposited on the end-face of an optical fiber to detect lead cations (Pb2+) in water at 25.2, 43.5 and 64.0 ppm levels. The SO3H-UiO-66(Zr) system provides a Fabry-Perot sensor by which the lead ions are detected rapidly (milliseconds) at 25.2 ppm aqueous solution reflecting in the wavelength shifts in interference spectrum. The proposed removal mechanism is based on the adsorption of [Pb(OH2)6]2+ in water on SO3H-UiO-66(Zr) due to a strong affinity between functionalized MOF and lead. This is the first work that advances a multi-purpose optical fiber-coated functional MOF as an on-site remote chemical sensor for rapid detection of lead cations at extremely low concentrations in an aqueous system.

Sulfonated Sub-1-nm Metal-Organic Framework Channels with Ultrahigh Proton Selectivity.

Biological proton channels are sub-1-nm protein pores with ultrahigh proton (H+) selectivity over other ions. Inspired by biological proton channels, developing artificial proton channels with biological-level selectivity is of fundamental significance for separation science. Herein we report synthetic proton channels fabrication based on sulfonated metal-organic frameworks (MOFs), UiO-66-X, X = SAG, NH-SAG, (NH-SAG)2 (SAG: sulfonic acid groups), which have sub-1-nm windows and a high density of sulfonic acid groups mimicking natural proton channels. The ion conductance of UiO-66-X channels follows the sequence: H+ ≫ K+ > Na+> Li+, and the sulfonated UiO-66 derivative channels show proton selectivity much higher than that of the pristine UiO-66 channels. Particularly, the UiO-66-(NH-SAG)2 channels exhibit ultrahigh proton selectivities, H+/Li+ up to ∼100, H+/Na+ of ∼80, and H+/K+ of ∼70, which are ∼3 times of that of UiO-66-NH-SAG channels, and ∼15 times of that of UiO-66@SAG channels. The ultrahigh proton selectivity in the sulfonated sub-1-nm MOF channels is mainly attributed to the narrow window-cavity pore structure functionalized with nanoconfined high-density sulfonic acid groups that facilitate fast proton transport and simultaneously exclude other cations. Our work opens an avenue to develop functional MOF channels for selective ion conduction and efficient ion separation.

Enhancing Multicomponent Metal-Organic Frameworks for Low Pressure Liquid Organic Hydrogen Carrier Separations.

The resurgence of interest in the hydrogen economy could hinge on the distribution of hydrogen in a safe and efficient manner. Whilst great progress has been made with cryogenic hydrogen storage or liquefied ammonia, liquid organic hydrogen carriers (LOHCs) remain attractive due to their lack of need for cryogenic temperatures or high pressures, most commonly a cycle between methylcyclohexane and toluene. Oxidation of methylcyclohexane to release hydrogen will be more efficient if the equilibrium limitations can be removed by separating the mixture. This report describes a family of six ternary and quaternary multicomponent metal-organic frameworks (MOFs) that contain the three-dimensional cubane-1,4-dicarboxylate (cdc) ligand. Of these MOFs, the most promising is a quaternary MOF (CUB-30), comprising cdc, 4,4'-biphenyldicarboxylate (bpdc) and tritopic truxene linkers. Contrary to conventional wisdom that adsorptive interactions with larger, hydrocarbon guests are dominated by π-π interactions, here we report that contoured aliphatic pore environments can exhibit high selectivity and capacity for LOHC separations at low pressures. This is the first time, to the best of our knowledge, where selective adsorption for cyclohexane over benzene is witnessed, underlining the unique adsorptive behavior afforded by the unconventional cubane moiety.

CUB-5: A Contoured Aliphatic Pore Environment in a Cubic Framework with Potential for Benzene Separation Applications.

One prominent aspect of metal organic frameworks (MOFs) is the ability to tune the size, shape, and chemical characteristics of their pores. MOF-5, with its open cubic connectivity of Zn4O clusters joined by two-dimensional, terephthalate linkers, is the archetypal example: both functionalized and elongated linkers produce isoreticular frameworks that define pores with new shapes and chemical environments. The recent scalable synthesis of cubane-1,4-dicarboxylic acid (1,4-H2cdc) allows the first opportunity to explore its application in leading reticular architectures. Herein we describe the use of 1,4-H2cdc to construct [Zn4O(1,4-cdc)3], referred to as CUB-5. Isoreticular with MOF-5, CUB-5 adopts a cubic architecture but features aliphatic, rather than aromatic, pore surfaces. Methine units point directly into the pores, delivering new and unconventional adsorption locations. Our results show that CUB-5 is capable of selectively adsorbing high amounts of benzene at low partial pressures, promising for future investigations into the industrial separation of benzene from gasoline using aliphatic MOF materials. These results present an effective design strategy for the generation of new MOF materials with aliphatic pore environments and properties previously unattainable in conventional frameworks.

Continuous Flow Synthesis of a Zr Magnetic Framework Composite for Post-Combustion CO2 Capture.

Metal-organic frameworks (MOFs) have an unprecedented ability to store gas molecules, however energy efficient regeneration remains challenging. Use of magnetic induction to aid this shows promise, but economical synthesis of the requisite composites is unresolved. Continuous flow chemistry has been reported as a rapid and reliable method of MOF synthesis, delivering step-change improvements in space time yields (STY). Here the scalable production of nanomaterials suitable for regeneration by magnetic induction is demonstrated. The zirconium MOF composite, MgFe2 O4 @UiO-66-NH2 is prepared using continuous flow chemistry resulting in a material of comparable performance to its batch counterpart. Upscaling using flow chemistry gave STY >25 times that of batch synthesis. Magnetic induced regeneration using this mass produced MFC for carbon capture is then demonstrated.

A Multifunctional, Charge-Neutral, Chiral Octahedral M12 L12 Cage.

A chiral, octahedral M12 L12 cage, which is charge neutral and contains an internal void of about 2000 Å3 , is reported. The cage was synthesised as an enantiopure complex by virtue of amino-acid-based dicarboxylate ligands, which assemble around copper paddlewheels at the vertices of the octahedron. The cage persists in solution with retention of the fluorescence properties of the parent acid. The solid-state structure contains large pores both within and between the cages, and displays permanent porosity for the sorption of gases with retention of crystallinity. Initial tests show some enantioselectivity of the cage towards guests in solution.

Low-Energy CO2 Release from Metal-Organic Frameworks Triggered by External Stimuli.

Groundbreaking research over the past 15 years has established metal-organic frameworks (MOFs) as adsorbents capable of unprecedented gas adsorption capacity. This has encouraged the contemplation of their use in applications such as increasing the storage capacity in natural gas fuel tanks, or the capture of carbon dioxide from coal-fired flue gas streams. However, while the gas adsorption capacity of MOFs is large, not all stored gas can be readily released to realize the efficient regeneration of MOF adsorbents. This leads to an increase in energy requirements, or working capacities significantly lower than the amount of gas adsorbed. This requirement for low energy means to efficiently release more stored gas has motivated the research in our group toward the triggered release of the stored gas from MOFs. Using CO2 as a typical gas adsorbate, we have developed three new methods of releasing stored gas with external stimuli that include light induction swing adsorption, magnetic induction swing adsorption, and their combination, denoted as LISA, MISA and MaLISA, respectively. LISA: Light, being naturally abundant, is particularly interesting for reducing the parasitic energy load on coal-fired power stations for regenerating the CO2 adsorbent. We showed that, by incorporating light-responsive organic linkers, exposure of light to a gas-loaded MOF promoted localized movement in the linkers, expelling around 80% of the adsorbed gas, just from the use of concentrated sunlight. Variation of the light-responsive components such as silver nanoparticles in MOFs allowed the response to be moved from UV to visible wavelengths, improving safety and light penetration depth. MISA: In order to expand this discovery to larger scales, more penetrating forms of radiation were sought. MOFs incorporated with magnetic nanoparticles (Magnetic Framework Composites, MFCs) were developed, and absorb the alternating magnetic fields exceptionally efficiently. The rapid heating of magnetic particles delivers local temperature increases to the otherwise thermally insulating MOF material, and in optimized conditions release all adsorbed gas in a matter of minutes. MaLISA: The triggered release methods of LISA and MISA may be combined in MFCs that also contain light-responsive groups. Both stimuli were employed and cooperative enhancement of gas releasing efficiency were found, minimizing the overall energy requirement even further. Initial calculations of the energy costs for these processes have shown them to have the potential to exceed any other reported method, following optimization. Encouragingly, the efficiency of the process was found to increase at larger scales, prompting further research in this area toward widespread deployment.

New synthetic routes towards MOF production at scale.

The potential commercial applications for metal organic frameworks (MOFs) are tantalizing. To address the opportunity, many novel approaches for their synthesis have been developed recently. These strategies present a critical step towards harnessing the myriad of potential applications of MOFs by enabling larger scale production and hence real-world applications. This review provides an up-to-date survey ( references) of the most promising novel synthetic routes, i.e., electrochemical, microwave, mechanochemical, spray drying and flow chemistry synthesis. Additionally, the essential topic of downstream processes, especially for large scale synthesis, is critically reviewed. Lastly we present the current state of MOF commercialization with direct feedback from commercial players.

Missing Linker Defects in a Homochiral Metal-Organic Framework: Tuning the Chiral Separation Capacity.

Efficient chiral separation remains a very challenging task due to the identical physical and chemical properties of the enantiomers of a molecule. Enantiomers only behave differently from each other in the presence of other chiral species. Homochiral metal-organic frameworks (MOFs) have received much attention for their promising enantioseparation properties. However, there are still challenges to overcome in this field such as high enantiomeric separation. Structural defects play an important role in the properties of MOFs and can significantly change the pore architecture. In this work, we introduced missing linker defects into a homochiral metal-organic framework [Zn2(bdc)(l-lac)(dmf)] (ZnBLD; bdc = 1,4-benzenedicarboxylic acid, l-lac = l-lactic acid, dmf = N,N'-dimethylformamide) and observed an increase in enantiomeric excess for 1-phenylethanol of 35% with the defective frameworks. We adjusted the concentration of monocarboxylic acid ligand l-lactic acid by varying the ratio of Zn2+ to ligand from 0.5 to 0.85 mmol. Additionally, a defective framework was synthesized with propanoic acid as modulator. In order to elucidate the correlation between defects and enantiomeric excess, five characterization techniques (FTIR, TGA, 1H NMR, ICP, and PXRD) were employed. Full width at half-maximum analysis (fwhm) was performed on the powder X-ray diffraction traces and showed that the higher concentration of monocarboxylic acid MOFs were isostructural but suffered from increased fwhm values.

UiO-66 MOF end-face-coated optical fiber in aqueous contaminant detection.

Optical quality metal organic framework (MOF) thin films were integrated, for the first time, to the best of our knowledge, with structured optical fiber substrates to develop MOF-fiber sensors. The MOF-fiber structure, UiO-66 (Zr-based MOF is well known for its water stability), is a thin film that acts as an effective analyte collector. This provided a Fabry-Perot sensor in which concentrations of up to 15 mM Rhodamine-B were detected via wavelength shifts in the interference spectrum.

A Robust Metal-Organic Framework for Dynamic Light-Induced Swing Adsorption of Carbon Dioxide.

Adsorbents for CO2 capture need to demonstrate efficient release. Light-induced swing adsorption (LISA) is an attractive new method to release captured CO2 that utilizes solar energy rather than electricity. MOFs, which can be tailored for use in LISA owing to their chemical functionality, are often unstable in moist atmospheres, precluding their use. A MOF is used that can release large quantities of CO2 via LISA and is resistant to moisture across a large pH range. PCN-250 undergoes LISA, with UV flux regulating the CO2 desorption capacity. Furthermore, under UV light, the azo residues within PCN-250 have constrained, local, structural flexibility. This is dynamic, rapidly switching back to the native state. Reusability tests demonstrate a 7.3 % and 4.9 % loss in both adsorption and LISA capacity after exposure to water for five cycles. These minimal changes confirm the structural robustness of PCN-250 and its great potential for triggered release applications.

Hypercrosslinked Additives for Ageless Gas-Separation Membranes.

The loss of internal pores, a process known as physical aging, inhibits the long-term use of the most promising gas-separation polymers. Previously we reported that a porous aromatic framework (PAF-1) could form a remarkable nanocomposite with gas-separation polymers to stop aging. However, PAF-1 synthesis is very onerous both from a reagent and reaction-condition perspective, making it difficult to scale-up. We now reveal a highly dispersible and scalable additive based on α,α'-dichloro-p-xylene (p-DCX), that inhibits aging more effectively, and crucially almost doubles gas-transport selectivity. These synergistic effects are related to the intimately mixed nanocomposite that is formed though the high dispersibility of p-DCX in the gas-separation polymer. This reduces particle-size effects and the internal free volume is almost unchanged over time. This study shows this inexpensive and scalable polymer additive delivers exceptional gas-transport performance and selectivity.

Gas-separation membranes loaded with porous aromatic frameworks that improve with age.

Porosity loss, also known as physical aging, in glassy polymers hampers their long term use in gas separations. Unprecedented interactions of porous aromatic frameworks (PAFs) with these polymers offer the potential to control and exploit physical aging for drastically enhanced separation efficiency. PAF-1 is used in the archetypal polymer of intrinsic microporosity (PIM), PIM-1, to achieve three significant outcomes. 1) hydrogen permeability is drastically enhanced by 375% to 5500 Barrer. 2) Physical aging is controlled causing the selectivity for H2 over N2 to increase from 4.5 to 13 over 400 days of aging. 3) The improvement with age of the membrane is exploited to recover up to 98% of H2 from gas mixtures with N2 . This process is critical for the use of ammonia as a H2 storage medium. The tethering of polymer side chains within PAF-1 pores is responsible for maintaining H2 transport pathways, whilst the larger N2 pathways gradually collapse.

Porous aromatic frameworks impregnated with fullerenes for enhanced methanol/water separation.

Molecular simulation techniques have revealed that the incorporation of fullerenes within porous aromatic frameworks (PAFs) remarkably enhances methanol uptake while inhibiting water uptake. The highest selectivity of methanol over water is found to be 1540 at low pressure (1 kPa) and decreases gradually with increasing pressure. The adsorption of water is very small compared to methanol, a useful material property for membrane and adsorbent-based separations. Grand canonical Monte Carlo (GCMC) simulations are utilized to calculate the pure component and mixture adsorption isotherms. The water and methanol mixture simulations show that water uptake is further inhibited above the pure component results because of the dominant methanol adsorption. Molecular dynamics (MD) simulations confirm that water diffusivity is also inhibited by strong methanol adsorption in the mixture. Overall, this study reveals profound hydrophobicity in C60@PAF materials and recommends C60@PAFs as suitable applicants for adsorbent and membrane-based separations of methanol/water mixtures and other alcohol/water separation applications.