ZIF-62 glass foam self-supported membranes to address CH4/N2 separations.

Membranes with ultrahigh permeance and practical selectivity could greatly decrease the cost of difficult industrial gas separations, such as CH4/N2 separation. Advanced membranes made from porous materials, such as metal-organic frameworks, can achieve a good gas separation performance, although they are typically formed on support layers or mixed with polymeric matrices, placing limitations on gas permeance. Here an amorphous glass foam, agfZIF-62, wherein a, g and f denote amorphous, glass and foam, respectively, was synthesized by a polymer-thermal-decomposition-assisted melting strategy, starting from a crystalline zeolitic imidazolate framework, ZIF-62. The thermal decomposition of incorporated low-molecular-weight polyethyleneimine evolves CO2, NH3 and H2O gases, creating a large number and variety of pores. This greatly increases pore interconnectivity but maintains the crystalline ZIF-62 ultramicropores, allowing ultrahigh gas permeance and good selectivity. A self-supported circular agfZIF-62 with a thickness of 200-330 µm and area of 8.55 cm2 was used for membrane separation. The membranes perform well, showing a CH4 permeance of 30,000-50,000 gas permeance units, approximately two orders of magnitude higher than that of other reported membranes, with good CH4/N2 selectivity (4-6).

Porous Organic Polymers for Efficient and Selective SO2 Capture from CO2-rich Flue Gas.

The quest for effective technologies to reduce SO2 pollution is crucial due to its adverse effects on the environment and human health. Markedly, removing a ppm level of SO2 from CO2-containing waste gas is a persistent challenge, and current technologies suffer from low SO2/CO2 selectivity and energy-intensive regeneration processes. Here using the molecular building blocks approach and theoretical calculation, we constructed two porous organic polymers (POPs) encompassing pocket-like structures with exposed imidazole groups, promoting preferential interactions with SO2 from CO2-containing streams. Markedly, the evaluated POPs offer outstanding SO2/CO2 selectivity, high SO2 capacity, and an easy regeneration process, making it one of the best materials for SO2 capture. To gain better structural insights into the notable SO2 selectivity of the POPs, we used dynamic nuclear polarization NMR spectroscopy (DNP) and molecular modelling to probe the interactions between SO2 and POP adsorbents. The newly developed materials are poised to offer an energy-efficient and environment-friendly SO2 separation process while we are obliged to use fossil fuels for our energy needs.

A Zeolite-Like Metal-Organic Framework Based Membrane for Reverse Selective Hydrogen Separation and Butane Isomer Sieving.

Here, the fabrication of a defect-free membrane that is based on a zeolite-like metal-organic framework (ZMOF) with the underlying ana topology is reported. The unique ana-ZMOF structure provides high degree of pore connectivity, which is reflected by the fast transport of gases. Prominently, it offers an optimum pore-aperture size, affording notable sieving selectivity for butane/isobutane, and optimal pore energetics for reverse CO2 /H2 separation. This emphasize the potential for the application of pure MOF membranes, paving the way to more sustainability of energy resources.

Reticular Chemistry for the Construction of Highly Porous Aluminum-Based nia-Metal-Organic Frameworks.

Edge-transitive nets are regarded as appropriate blueprints for the practice of reticular chemistry, and in particular, for the rational design and synthesis of functional metal-organic frameworks (MOFs). Among edge-transitive nets, type I edge-transitive nets have unique coordination figures, offering only one edge-transitive target for their associated expressed net-cBUs. Here, we report the reticulation of the binodal edge-transitive (6, 6)-c nia net in MOF chemistry, namely, the deliberate assembly of trinuclear aluminum clusters and 6-connected hexacarboxylate ligands into highly porous nia-MOFs. Further studies reveal that Al-nia-MOF-1 shows promising attributes as a storage media for oxygen (O2) at high-pressure adsorption studies.

Quest for an Optimal Methane Hydrate Formation in the Pores of Hydrolytically Stable Metal-Organic Frameworks.

Porous metal-organic frameworks (MOFs) capable of storing a relatively high amount of dry methane (CH4) in the adsorbed phase are largely explored; however, solid CH4 storage in confined pores of MOFs in the form of hydrates is yet to be discovered. Here we report a rational approach to form CH4 hydrates by taking advantage of the optimal pore confinement in relatively narrow cavities of hydrolytically stable MOFs. Unprecedentedly, we were able to isolate methane hydrate (MH) nanocrystals with an sI structure encapsulated inside MOF pores with an optimal cavity dimension. It was found that confined nanocrystals require cavities slightly larger than the unit cell crystal size of MHs (1.2 nm), as exemplified in the experimental case study performed on Cr-soc-MOF-1 vs smaller cavities of Y-shp-MOF-5. Under these conditions, the excess amount of methane stored in the pores of Cr-soc-MOF-1 in the form of MH was found to be ≈50% larger than the corresponding dry adsorbed amount at 10 MPa. More importantly, the pressure gradient driving the CH4 storage/delivery process could be drastically reduced compared to the conventional CH4-adsorbed phase storage on the dry Cr-soc-MOF-1 (≤3 MPa vs 10 MPa).

Publisher Correction: Mixed matrix formulations with MOF molecular sieving for key energy-intensive separations.

In the version of this Article originally published, the units of the y axis of Fig. 3b were incorrectly given as '106 cm2 s-1'; they should have been '10-8 cm2 s-1'. This has been corrected in the online versions of the Article.

Polyoxometalate-Cyclodextrin Metal-Organic Frameworks: From Tunable Structure to Customized Storage Functionality.

Self-assembly allows structures to organize themselves into regular patterns by using local forces to find the lowest-energy configuration. However, assembling organic and inorganic building blocks in an ordered framework remains challenging due to  difficulties in rationally interfacing two dissimilar materials. Herein, the ensemble of polyoxometalates (POMs) and cyclodextrins (CDs) as molecular building blocks (MBBs) has yielded two unprecedented POM-CD-MOFs, namely [PW12O40]3- and α-CD MOF (POT-CD) as well as [P10Pd15.5O50]19- and γ-CD MOF (POP-CD), with distinct properties not shared by their isolated parent MBBs. Markedly, the POT-CD features a nontraditional enhanced Li storage behavior by virtue of a unique "amorphization and pulverization" process. This opens the door to a new generation of hybrid materials with tuned structures and customized functionalities.

Mixed matrix formulations with MOF molecular sieving for key energy-intensive separations.

Membrane-based separations can improve energy efficiency and reduce the environmental impacts associated with traditional approaches. Nevertheless, many challenges must be overcome to design membranes that can replace conventional gas separation processes. Here, we report on the incorporation of engineered submicrometre-sized metal-organic framework (MOF) crystals into polymers to form hybrid materials that successfully translate the excellent molecular sieving properties of face-centred cubic (fcu)-MOFs into the resultant membranes. We demonstrate, simultaneously, exceptionally enhanced separation performance in hybrid membranes for two challenging and economically important applications: the removal of CO2 and H2S from natural gas and the separation of butane isomers. Notably, the membrane molecular sieving properties demonstrate that the deliberately regulated and contracted MOF pore-aperture size can discriminate between molecular pairs. The improved performance results from precise control of the linkers delimiting the triangular window, which is the sole entrance to the fcu-MOF pore. This rational-design hybrid approach provides a general toolbox for enhancing the transport properties of advanced membranes bearing molecular sieve fillers with sub-nanometre-sized pore-apertures.

Porous materials with optimal adsorption thermodynamics and kinetics for CO2 separation.

The energy costs associated with the separation and purification of industrial commodities, such as gases, fine chemicals and fresh water, currently represent around 15 per cent of global energy production, and the demand for such commodities is projected to triple by 2050 (ref. 1). The challenge of developing effective separation and purification technologies that have much smaller energy footprints is greater for carbon dioxide (CO2) than for other gases; in addition to its involvement in climate change, CO2 is an impurity in natural gas, biogas (natural gas produced from biomass), syngas (CO/H2, the main source of hydrogen in refineries) and many other gas streams. In the context of porous crystalline materials that can exploit both equilibrium and kinetic selectivity, size selectivity and targeted molecular recognition are attractive characteristics for CO2 separation and capture, as exemplified by zeolites 5A and 13X (ref. 2), as well as metal-organic materials (MOMs). Here we report that a crystal engineering or reticular chemistry strategy that controls pore functionality and size in a series of MOMs with coordinately saturated metal centres and periodically arrayed hexafluorosilicate (SiF(2-)(6)) anions enables a 'sweet spot' of kinetics and thermodynamics that offers high volumetric uptake at low CO2 partial pressure (less than 0.15 bar). Most importantly, such MOMs offer an unprecedented CO2 sorption selectivity over N2, H2 and CH4, even in the presence of moisture. These MOMs are therefore relevant to CO2 separation in the context of post-combustion (flue gas, CO2/N2), pre-combustion (shifted synthesis gas stream, CO2/H2) and natural gas upgrading (natural gas clean-up, CO2/CH4).

Achieving Superprotonic Conduction with a 2D Fluorinated Metal-Organic Framework.

A hydrolytically stable metal-organic framework (MOF) material, named KAUST-7', was derived from a structural phase change of KAUST-7 upon exposure to conditions akin to protonic conduction (363 K/95% relative humidity). KAUST 7' exhibited a superprotonic conductivity as evidenced by the impedance spectroscopic measurement revealing an exceptional conductivity up to 2.0 × 10-2 S cm-1 at 363 K and under 95% RH, a performance maintained over 7 days. Ab initio molecular dynamics simulations suggested that the water-mediated proton transport mechanism is governed by water assisted reorganization of the H-bond network involving the fluorine moieties in KAUST-7' and the guest water molecules. The notable level of performances combined with a very good hydrolytic stability positions KAUST-7' as a prospective proton-exchange membrane alternative to the commercial benchmark Nafion. Furthermore, the remarkable RH sensitivity of KAUST-7' conductivity, substantially higher than previously reported MOFs, offers great opportunities for deployment as a humidity sensor.

Enriching the Reticular Chemistry Repertoire: Merged Nets Approach for the Rational Design of Intricate Mixed-Linker Metal-Organic Framework Platforms.

Rational design and construction of metal-organic frameworks (MOFs) with intricate structural complexity are of prime importance in reticular chemistry. We report our latest addition to the design toolbox in reticular chemistry, namely the concept of merged nets based on merging two edge-transitive nets into a minimal edge-transitive net for the rational construction of intricate mixed-linker MOFs. In essence, a valuable net for design enclosing two edges (not related by symmetry) is rationally generated by merging two edge-transitive nets, namely (3,6)-coordinated spn and 6-coordinated hxg. The resultant merged-net, a (3,6,12)-coordinated sph net with net transitivity [32] enclosing three nodes and two distinct edges, offers potential for deliberate design of intricate mixed-linker MOFs. We report implementation of the merged-net approach for the construction of isoreticular rare-earth mixed-linker MOFs, sph-MOF-1 to -4, based on the assembly of 12-c hexanuclear carboxylate-based molecular building blocks (MBBs), displaying cuboctahedral building units, 3-c tritopic ligands, and 6-c hexatopic ligands. The resultant sph-MOFs represent the first examples of MOFs where the underlying net is merged from two 3-periodic edge-transitive nets, spn and hxg. Distinctively, the sph-MOF-3 represents the first example of a mixed-linker MOF to enclose both trigonal and hexagonal linkers. The merged-nets approach allows the logical practice of isoreticular chemistry by taking into account the mathematically correlated dimensions of the two ligands to afford the deliberate construction of a mixed-linker mesoporous MOF, sph-MOF-4. The merged-net equation and two key parameters, ratio constant and MBB constant, are disclosed. A merged-net strategy for the design of mixed-linker MOFs by strictly controlling the size ratio between edges is introduced.

Enhanced Separation of Butane Isomers via Defect Control in a Fumarate/Zirconium-Based Metal Organic Framework.

The discovery of appropriate synthetic reaction conditions for fabricating a stable zirconium-based molecular sieve (Zr-fum-fcu-MOF) with minimal defects and its utilization in the challenging separation of linear paraffins from branched paraffins is reported. The crystallinity and structural defects were modulated and adjusted at the molecular level by controlling the synthetic reaction conditions (i.e., amounts of modulators and ligands). The impact of molecular defects on the separation of n-butane from iso-butane was studied through the preparation, fine characterization, and performance evaluation of Zr-fum-fcu-MOFs with varying degrees of defects. Defect-rich Zr-fum-fcu-MOFs were found to have poor n-butane/iso-butane separation, mainly driven by thermodynamics, while Zr-fum-fcu-MOFs with fewer or minimal defects showed efficient separation, driven mainly by kinetics and full molecular exclusion mechanisms. The impact of intrinsic defects (i.e., missing organic or inorganic blocks) on the associated mechanisms involved in the separation of n-butane/iso-butane was evidenced through single-gas adsorption, mixed-gas column breakthrough experiments, and calorimetric studies. This investigation demonstrates, for the first time, the importance of controlling intrinsic defects to maintain the selective exclusion behavior of hydrocarbon isomers when using molecular sieves.

Gas/vapour separation using ultra-microporous metal-organic frameworks: insights into the structure/separation relationship.

The separation of related molecules with similar physical/chemical properties is of prime industrial importance and practically entails a substantial energy penalty, typically necessitating the operation of energy-demanding low temperature fractional distillation techniques. Certainly research efforts, in academia and industry alike, are ongoing with the main aim to develop advanced functional porous materials to be adopted as adsorbents for the effective and energy-efficient separation of various important commodities. Of special interest is the subclass of metal-organic frameworks (MOFs) with pore aperture sizes below 5-7 Å, namely ultra-microporous MOFs, which in contrast to conventional zeolites and activated carbons show great prospects for addressing key challenges in separations pertaining to energy and environmental sustainability, specifically materials for carbon capture and separation of olefin/paraffin, acetylene/ethylene, linear/branched alkanes, xenon/krypton, etc. In this tutorial review we discuss the latest developments in ultra-microporous MOF adsorbents and their use as separating agents via thermodynamics and/or kinetics and molecular sieving. Appreciably, we provide insights into the distinct microscopic mechanisms governing the resultant separation performances, and suggest a plausible correlation between the inherent structural features/topology of MOFs and the associated gas/vapour separation performance.

Enabling Fluorinated MOF-Based Membranes for Simultaneous Removal of H2 S and CO2 from Natural Gas.

Membrane-based gas separations are energy efficient processes; however, major challenges remain to develop high-performance membranes enabling the replacement of conventional separation processes. Herein, a new fluorinated MOF-based mixed-matrix membrane is reported, which is formed by incorporating the MOF crystals into selected polymers via a facile mixed-matrix approach. By finely controlling the molecular transport in the channels through the MOF apertures tuned by metal pillars and at the MOF-polymer interfaces, the resulting fluorinated MOF-based membranes exhibit excellent molecular sieving properties. These materials significantly outperform state-of-the-art membranes for simultaneous removal of H2 S and CO2 from natural gas-a challenging and economically important application. The robust fluorinated MOFs (NbOFFIVE-1-Ni, AlFFIVE-1-Ni), pave a way to efficient membrane separation processes that require precise discrimination of closely sized molecules.

A Fine-Tuned Metal-Organic Framework for Autonomous Indoor Moisture Control.

Conventional adsorbents, namely zeolites and silica gel, are often used to control humidity by adsorbing water; however, adsorbents capable of the dual functionality of humidification and dehumidification, offering the desired control of the moisture level at room temperature, have yet to be explored. Here we report Y-shp-MOF-5, a hybrid microporous highly connected rare-earth-based metal-organic framework (MOF), with dual functionality for moisture control within the recommended range of relative humidity (45%-65% RH) set by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). Y-shp-MOF-5 exhibits exceptional structural integrity, robustness, and unique humidity-control performance, as confirmed by the large number (thousand) of conducted water vapor adsorption-desorption cycles. The retained structural integrity and the mechanism of water sorption were corroborated using in situ single-crystal X-ray diffraction (SCXRD) studies. The resultant working water uptake of 0.45 g·g-1 is solely regulated by a simple adjustment of the relative humidity, positioning this hydrolytically stable MOF as a prospective adsorbent for humidity control in confined spaces, such as space shuttles, aircraft cabins, and air-conditioned buildings.

Applying the Power of Reticular Chemistry to Finding the Missing alb-MOF Platform Based on the (6,12)-Coordinated Edge-Transitive Net.

Highly connected and edge-transitive nets are of prime importance in crystal chemistry and are regarded as ideal blueprints for the rational design and construction of metal-organic frameworks (MOFs). We report the design and synthesis of highly connected MOFs based on reticulation of the sole two edge-transitive nets with a vertex figure as double six-membered-ring (d6R) building unit, namely the (4,12)-coordinated shp net (square and hexagonal-prism) and the (6,12)-coordinated alb net (aluminum diboride, hexagonal-prism and trigonal-prism). Decidedly, the combination of our recently isolated 12-connected (12-c) rare-earth (RE) nonanuclear [RE9(µ3-OH)12(µ3-O)2(O2C-)12] carboxylate-based cluster, points of extension matching the 12 vertices of hexagonal-prism d6R, with 4-connected (4-c) square porphyrinic tetracarboxylate ligand led to the formation of the targeted RE-shp-MOF. This is the first time that RE-MOFs based on 12-c molecular building blocks (MBBs), d6R building units, have been deliberately targeted and successfully isolated, paving the way for the long-awaited (6,12)-c MOF with alb topology. Indeed, combination of a custom-designed hexacarboxylate ligand with RE salts led to the formation of the first related alb-MOF, RE-alb-MOF. Intuitively, we successfully transplanted the alb topology to another chemical system and constructed the first indium-based alb-MOF, In-alb-MOF, by employing trinuclear [In3(µ3-O)(O2C-)6] as the requisite 6-connected trigonal-prism and purposely made a dodecacarboxylate ligand as a compatible 12-c MBB. Prominently, the dodecacarboxylate ligand was employed to transplant shp topology into copper-based MOFs by employing the copper paddlewheel [Cu2(O2C-)4] as the complementary square building unit, affording the first Cu-shp-MOF. We revealed that highly connected edge-transitive nets such shp and alb are ideal for topological transplantation and deliberate construction of related MOFs based on minimal edge-transitive nets.

A Fine-Tuned Fluorinated MOF Addresses the Needs for Trace CO2 Removal and Air Capture Using Physisorption.

The development of functional solid-state materials for carbon capture at low carbon dioxide (CO2) concentrations, namely, from confined spaces (<0.5%) and in particular from air (400 ppm), is of prime importance with respect to energy and environment sustainability. Herein, we report the deliberate construction of a hydrolytically stable fluorinated metal-organic framework (MOF), NbOFFIVE-1-Ni, with the appropriate pore system (size, shape, and functionality), ideal for the effective and energy-efficient removal of trace carbon dioxide. Markedly, the CO2-selective NbOFFIVE-1-Ni exhibits the highest CO2 gravimetric and volumetric uptake (ca. 1.3 mmol/g and 51.4 cm(3) (STP) cm(-3)) for a physical adsorbent at 400 ppm of CO2 and 298 K. Practically, NbOFFIVE-1-Ni offers the complete CO2 desorption at 328 K under vacuum with an associated moderate energy input of 54 kJ/mol, typical for the full CO2 desorption in conventional physical adsorbents but considerably lower than chemical sorbents. Noticeably, the contracted square-like channels, affording the close proximity of the fluorine centers, permitted the enhancement of the CO2-framework interactions and subsequently the attainment of an unprecedented CO2 selectivity at very low CO2 concentrations. The precise localization of the adsorbed CO2 at the vicinity of the periodically aligned fluorine centers, promoting the selective adsorption of CO2, is evidenced by the single-crystal X-ray diffraction study on NbOFFIVE-1-Ni hosting CO2 molecules. Cyclic CO2/N2 mixed-gas column breakthrough experiments under dry and humid conditions corroborate the excellent CO2 selectivity under practical carbon capture conditions. Pertinently, the notable hydrolytic stability positions NbOFFIVE-1-Ni as the new benchmark adsorbent for direct air capture and CO2 removal from confined spaces.

Reticular Synthesis of HKUST-like tbo-MOFs with Enhanced CH4 Storage.

Successful implementation of reticular chemistry using a judiciously designed rigid octatopic carboxylate organic linker allowed the construction of expanded HKUST-1-like tbo-MOF series with intrinsic strong CH4 adsorption sites. The Cu-analogue displayed a concomitant enhancement of the gravimetric and volumetric surface area with the highest reported CH4 uptake among the tbo family, comparable to the best performing metal organic frameworks (MOFs) for CH4 storage. The corresponding gravimetric (BET) and volumetric surface areas of 3971 m(2) g(-1) and 2363 m(2) cm(-3) represent an increase of 115% and 47%, respectively, in comparison to the corresponding values for the prototypical HKUST-1 (tbo-MOF-1), and are 42% and 20% higher than that of tbo-MOF-2. High-pressure methane adsorption isotherms revealed a high total gravimetric and volumetric CH4 uptakes, reaching 372 cm(3) (STP) g(-1) and 221 cm(3) (STP) cm(-3), respectively, at 85 bar and 298 K. The corresponding working capacities between 5 and 80 bar were found to be 294 cm(3) (STP) g(-1) and 175 cm(3) (STP) cm(-3) and are placed among the best performing MOFs for CH4 storage particularly at relatively low temperature. To gain insight on the mechanism accounting for the resultant enhanced CH4 storage capacity, molecular simulation study was performed and revealed the presence of very strong CH4 adsorption sites near the organic linker with similar adsorption energetics as the open metal sites. The present findings support the potential of tbo-MOFs based on the supermolecular building layer (SBL) approach as an ideal platform to further enhance the CH4 storage capacity via expansion and functionalization of the quadrangular pillars.

Reticular Chemistry at Its Best: Directed Assembly of Hexagonal Building Units into the Awaited Metal-Organic Framework with the Intricate Polybenzene Topology, pbz-MOF.

The ability to direct the assembly of hexagonal building units offers great prospective to construct the awaited and looked-for hypothetical polybenzene (pbz) or "cubic graphite" structure, described 70 years ago. Here, we demonstrate the successful use of reticular chemistry as an appropriate strategy for the design and deliberate construction of a zirconium-based metal-organic framework (MOF) with the intricate pbz underlying net topology. The judicious selection of the perquisite hexagonal building units, six connected organic and inorganic building blocks, allowed the formation of the pbz-MOF-1, the first example of a Zr(IV)-based MOF with pbz topology. Prominently, pbz-MOF-1 is highly porous, with associated pore size and pore volume of 13 Å and 0.99 cm3 g-1, respectively, and offers high gravimetric and volumetric methane storage capacities (0.23 g g-1 and 210.4 cm3 (STP) cm-3 at 80 bar). Notably, the pbz-MOF-1 pore system permits the attainment of one of the highest CH4 adsorbed phase density enhancements at high pressures (0.15 and 0.21 g cm-3 at 35 and 65 bar, respectively) as compared to benchmark microporous MOFs.