In situ electrochemical potential-induced synthesis of metal organic framework membrane on polymer support for H2/CO2 separation.

Metal-Organic Framework (MOF) membranes act as selective layers have offered unprecedented opportunities for energy-efficient and cost-effective gas separation. Searching for the green and sustainable synthesis method of dense MOF membrane has received huge attention in both academia and industry. In this work, we demonstrate an in situ electrochemical potential-induced synthesis strategy to aqueously fabricate Metal Azolate Framework-4 (MAF-4) membranes on polypropylene (PP) support. The constant potential can induce the heterogeneous nucleation and growth of MAF-4, resulting an ultrathin membrane with the thickness of only 390 nm. This high-quality membrane exhibits a high H2/CO2 separation performance with the H2 permeance as high as 1565.75 GPU and selectivity of 11.6. The deployment of this environment friendly one-step fabrication method under mild reaction conditions, such as low-cost polymer substrate, water instead of organic solvent, room temperature and ambient pressure shows great promise for the scale-up of MOF membranes.

Energy-efficient extraction of linear alkanes from various isomers using structured metal-organic framework membrane.

Extraction of low concentration linear alkanes (C5-C7) from various isomers is critical for the petrochemical industry. At present, the separation of alkane isomers is mainly accomplished by distillation, which results in substantial energy expenditure. Metal-organic frameworks (MOFs) with well-tailored nanopores have been demonstrated to be capable of realizing molecule-level separation. In this study, oriented HKUST-1 membranes are formulated according to the morphology-biased principle and finally realized with a low dose synthesis method for terminating undesired crystal nucleation and growth. The fully exposed triangular sieving pore array of the membrane induces configuration entropic diffusion to split linear alkanes from mono-branched and di-branched isomers as well as their cyclical counterparts. Typically, the current separation technique consumes 91% less energy than vacuum distillation. Furthermore, our membranes can realize one-step extraction of normal-pentane, normal-hexane and normal-heptane from a ten-component alkane isomer solution that mimics light naphtha.

Heat-driven molecule gatekeepers in MOF membrane for record-high H2 selectivity.

Hydrogen/carbon dioxide (H2/CO2) separation for sustainable energy is in desperate need of reliable membranes at high temperatures. Molecular sieve membranes take their nanopores to differentiate sizes between H2 and CO2 but have compromised at a marked loss of selectivity at high temperatures owing to diffusion activation of CO2. We used molecule gatekeepers that were locked in the cavities of the metal-organic framework membrane to meet this challenge. Ab initio calculations and in situ characterizations demonstrate that the molecule gatekeepers make a notable move at high temperatures to dynamically reshape the sieving apertures as being extremely tight for CO2 and restitute with cool conditions. The H2/CO2 selectivity was improved by an order of magnitude at 513 kelvin (K) relative to that at the ambient temperature.

A LDH Template Triggers the Formation of a Highly Compact MIL-53 Metal-Organic Framework Membrane for Acid Upgrading.

Highly compact metal-organic framework (MOF) membranes offer hope for the ambition to cope with challenging separation scenarios with industrial implications. A continuous layer of layered double hydroxide (LDH) nanoflakes on an alumina support as a template triggered a chemical self-conversion to a MIL-53 membrane, with approximately 8 hexagonal lattices (LDH) traded for 1 orthorhombic lattice (MIL-53). With the sacrifice of the template, the availability of Al nutrients from the alumina support was dynamically regulated, which resulted in synergy for producing membranes with highly compact architecture. The membrane can realize nearly complete dewatering from formic acid and acetic acid solutions, respectively, and maintain stability in a continuous pervaporation over 200 h. This is the first success in directly applying a pure MOF membrane to such a corrosive chemical environment (lowest pH value of 0.81). The energy consumption is saved by up to 77 % when compared with the traditional distillation.

Reversibly Phase-Transformative Zeolitic Imidazolate Framework-108 and the Membrane Separation Utility.

Reversible phase transformations (RPTs) of metal-organic frameworks not only create material diversity but also promise a self-restoration of crystals in a controllable manner. However, there are only limited examples because seeking for a convenient and effective trigger for RPTs, especially for RPTs with respect to spatiotemporal harmony in cleavage and reconstruction of metal-linker chemical bonds, is challenging. In this work, we found that zeolitic imidazolate framework (ZIF)-108 with Zn-N coordination bonds showing moderate strength was an ideal platform. We reported three crystal phases of ZIF-108, namely, sodalite (SOD), diamondoid (DIA), and large pore_sodalite (lp_SOD) topologies, and identified RPTs between phases: (1) when exposed to water or water vapor, the SOD structure could transform to a compact DIA version as a result of the decomposition of four-membered rings and synchronous reorganization of six-membered rings. Then, the DIA structure could also return back to SOD when soaked in dimethylformamide (DMF) or DMF vapor. (2) High-temperature treatment of SOD gives rise to lp_SOD, which then reverts to SOD by DMF. (3) lp_SOD could also be compressed into the DIA phase by water or water vapor and can then be restored via a two-step treatment, namely, soaking in DMF (DIA → SOD) right before a high-temperature therapy (SOD → lp_SOD). From the perspective of the separation utility, we found that the lp_SOD version of ZIF-108, relative to SOD-structured ZIF-108, can produce mixed matrix membranes having an interesting interfacial structure with the polymer chains, though both share the same chemical composition. We verified that the large pore of lp_SOD can allow being penetrated by polymer chains, which contributed to not only reinforcing the bi-phase interface but also sharpening the molecule sieve properties of fillers toward CO2 and CH4.

Multidimensional Building Blocks for Molecular Sieve Membranes.

Chemical separations aiming for high-purity commodities are critical to modern society. Compared to distillation, chemical absorption, and adsorption, membrane separation is attractive for its energy efficiency, ease of operation, and compact footprint. Molecular sieve membranes (MSMs) are broadly defined as membranes that are constructed from intrinsically and artificially porous materials. On the basis of our recent studies, this Account will first summarize the evolution of MSMs from the viewpoint of dimensionality of building blocks, which fundamentally determines the stacking architectures, intercrystalline gaps, and mass transfer channels of MSMs. Intergrowth of three-dimensional (3D) crystals as primary building blocks gives rise to classical MSMs. However, the poor connection between crystals inherent to those membranes results in intercrystalline gaps that are catastrophic for separation selectivity. We adopted a variety of strategies to close the crystal boundary gaps, including microwave synthesis, electrochemical-ionothermal synthesis, and modular integration. These efforts make us better understand the structure-performance relationship in membranes and create solutions for industrial processes. Excitingly, we first scaled-up the microwave synthesis of a Linde type A (LTA) zeolite membrane and built the world's largest ethanol dehydration membrane unit with an annual capacity of 100,000 tons. MSMs can also be made of two-dimensional (2D) nanosheets as primary building blocks. Those strike a balance between permeation rate and selectivity because the nanometer thickness ensures the minimization of the mass-transfer resistance of the membrane and the layer-by-layer stacking mode can significantly reduce the intercrystalline gaps. By publishing our first report on metal-organic framework (MOF) nanosheet membranes in Science, we committed to establishing top-down and bottom-up methods for assembly of laminae. Once the stacking, orientation, and connection between the layers are meticulously controlled, nanosheet building blocks with diversity open the door for ultrapermeable and selective MSMs. We recently proposed a supramolecule array membrane (SAM) with zero-dimensional (0D) molecules as primary building blocks, which has great potential to absolutely eliminate intercrystalline gaps in membranes. In contrast to the classical transport through nanopores of membranes, selective transport through the intermolecular spacing of supramolecules is creatively realized within the SAM, which marks a new breakthrough in ultraprecise sieving of molecules with tiny differences in size and revolutionizes MSMs in regard to stacking modes, intercrystalline gaps, and transport channels. MSMs have proven to be successful in diverse applications and have triggered wide interest. A unique perspective on the dimensionality evolution of building blocks will accelerate the progress of MSMs. The synergy of multidimensional MSMs will be a positive response to fundamental bottlenecks and industrial questions of membranes and will unlock the potential of membranes to displace the existing separation technologies in the future.

MIL-53 and its OH-bonded variants for bio-polyol adsorption from aqueous solution.

The adsorption of bio-polyols from dilute aqueous solution is important but faces challenges in the sustainable bio-refinery process. One solution to increase adsorption efficiency is to leverage host-guest interactions between the polyols and materials to grant a preference for polyols. In this study, we synthesized MIL-53 and diverse OH-bonded variants, and studied their adsorption properties towards ethanediol, 1,3-propanediol and glycerol in water. Among the four materials, OH-MIL-53 exhibited fast adsorption kinetics and high capacity, and could be completely regenerated through ethanol elution. Hydrophobic interactions between the alkyl chains of the polyols and the organic linkers of OH-MIL-53 and hydrogen bonding interactions between their OH groups were identified. The synergistic effect of the host-guest interactions is responsible for the unique adsorption performances of OH-MIL-53 towards polyols, and particularly for 1,3-propanediol.

Hetero-Lattice Intergrown and Robust MOF Membranes for Polyol Upgrading.

Metal-organic framework membranes are frequently used in gas separations, but rare in pervaporation for liquid chemical upgrading, especially for separating water from polyols, due to lack of highly compact and robust micro-architecture. Here, we report hetero-lattice intergrown membranes in which amino-MIL-101 (Cr) particles embedded into the micro-gaps of MIL-53 (Al) rod arrays after secondary growth. By means of high-resolution TEM and two-dimensional topologic simulation, the connection between these two distinct MOF lattices at the molecular-level and their crystallographic geometry harmony is identified, which leads to a close-knit structure at the crystal boundaries of membranes. Typically, the membrane shows a separation factor as high as 13 000 for a 90/10 ethanediol/water solution in pervaporation, yields polymer-grade ethanediol, and saves ca. 32 % of energy consumption vs. vacuum distillation. It has a highly robust micro-architecture, with great tolerance to high pressure, durability against ultrasonic therapy and long-term separation stability over 600 h.

A Highly Selective Supramolecule Array Membrane Made of Zero-Dimensional Molecules for Gas Separation.

We orderly assembled zero-dimensional 2-methylimidazole (mim) molecules into unprecedented supramolecule array membranes (SAMs) through solvent-free vapor processing, realizing the intermolecular spacing of mim at ca. 0.30 nm available as size-sieving channels for distinguishing the tiny difference between H2 (kinetic diameter: 0.289 nm) and CO2 (kinetic diameter: 0.33 nm). The highly oriented and dense membranes yield a separation factor above 3600 for equimolar H2 /CO2 mixtures, which is one order of magnitude higher than those of the state-of-the-art membranes defining 2017's upper bound for H2 /CO2 separation. These SAMs define a new benchmark for molecular sieve membranes and are of paramount importance to precombustion carbon capture. Given the range of supramolecules, we anticipate SAMs with variable intermolecular channels could be applied in diversified separations that are prevalent in chemical processes.

ZIF-L membrane with a membrane-interlocked-support composite architecture for H2/CO2 separation.

Metal-organic framework (MOF) membranes hold great promise in energy-efficient chemical separations. The outstanding challenges of the microstructural design stem from (1) thinning of membranes to immensely reduce the mass-transfer resistance (for high permeances); (2) tuning of orientation to optimize the selective transport of gas molecules, and (3) reinforcement of intercrystalline structure to subside leakage through defective gaps (for high selectivity). Here, we propose the ZIF-L membrane that is completely confined into the voids of the alumina support through an interfacial assembly process, producing an appealing membrane-interlocked-support (MIS) composite architecture that meets the requirements of the microstructural design of MOF membranes. Consequently, the membranes show average H2 permeances of above 4000 GPU and H2/CO2 separation factor (SF) of above 200, representing record-high separation performances of ZIF-L membranes and falling into the industrial target zone (H2 permeance > 1000 GPU and H2/CO2 SF > 60). Furthermore, the ZIF-L membrane possessing the MIS composite architecture that is established with alumina particles as scaffolds shows mechanical stability, scraped repeatedly by a piece of silicon rubber causing no selectivity loss.

Tuning of Delicate Host-Guest Interactions in Hydrated MIL-53 and Functional Variants for Furfural Capture from Aqueous Solution.

Capture of high-boiling-point furfural from diluted aqueous solution is a critical but challenging step in sustainable bio-refinery processes, but conventional separation methods such as distillation and liquid-liquid extraction requires prohibitive energy consumption. We report control over the microenvironment of hydrated MIL-53 and isoreticular variants with diversified functional terephthalic acid linkers for the purpose of preferential binding of furfural through delicate host-guest interactions. Methyl-bounded MIL-53 with improved binding energy in the hydrated form results in highly efficient capture ratio (ca. 98 %) in the extremely low concentration of furfural solution (0.5-3 wt %) and 100 % furfural specificity over xylose. The distinct hydrogen bonding sites and multiple Van de Wall interactions for furfural adsorption was testified by computational modeling. Furthermore, the recovery ratio of furfural reaches ca. 93 % in desorption.

Metal-Organic Framework Membranes and Membrane Reactors: Versatile Separations and Intensified Processes.

Metal-organic frameworks are an emerging and fascinating category of porous solids that can be self-assembled with metal-based cations linked by organic molecules. The unique features of MOFs in porosity (or surface areas), together with their diversity for chemical components and architectures, make MOFs attractive candidates in many applications. MOF membranes represent a long-term endeavor to convert MOF crystals in the lab to potentially industry-available commodities, which, as a promising alternative to distillation, provide a bright future for energy-efficient separation technologies closely related with chemicals, the environment, and energy. The membrane reactor shows a typical intensified process strategy by combining the catalytic reaction with the membrane separation in one unit. This review highlights the recent process of MOF-based membranes and the importance of MOF-based membrane reactors in relative intensified chemical processes.

Chemically Cross-Linked MOF Membrane Generated from Imidazolium-Based Ionic Liquid-Decorated UiO-66 Type NMOF and Its Application toward CO2 Separation and Conversion.

Carbon dioxide capture and transformation are of great importance to make cuts in greenhouse gas emissions. Nanometal-organic frameworks (NMOFs) could serve as ideal fillers for polymer membranes owing to their structural diversity and regulable microenvironment of the nanocage. Herein, a bifunctional, robust, and chemically cross-linked NMOF-based membrane was successfully constructed by the postsynthetic polymerization of imidazolium-based ionic liquid (IL)-decorated UiO-66 type nanoparticles (NPs) and the isocyanate-terminated polyurethane oligomer under mild conditions. The IL-modified MOF-polymer membranes exhibit a highly selective adsorption for CO2 over N2 and CH4. In addition, the obtained membrane can also be a highly active heterogeneous catalyst for CO2 transformation by cycloaddition with epoxide under an ambient pressure.

Two-Dimensional Metal-Organic Framework Nanosheets for Membrane-Based Gas Separation.

Metal-organic framework (MOF) nanosheets could serve as ideal building blocks of molecular sieve membranes owing to their structural diversity and minimized mass-transfer barrier. To date, discovery of appropriate MOF nanosheets and facile fabrication of high performance MOF nanosheet-based membranes remain as great challenges. A modified soft-physical exfoliation method was used to disintegrate a lamellar amphiprotic MOF into nanosheets with a high aspect ratio. Consequently sub-10 nm-thick ultrathin membranes were successfully prepared, and these demonstrated a remarkable H2 /CO2 separation performance, with a separation factor of up to 166 and H2 permeance of up to 8×10-7  mol m-2 s-1 Pa-1 at elevated testing temperatures owing to a well-defined size-exclusion effect. This nanosheet-based membrane holds great promise as the next generation of ultrapermeable gas separation membrane.

Microstructural Engineering and Architectural Design of Metal-Organic Framework Membranes.

In the past decade, a huge development in rational design, synthesis, and application of molecular sieve membranes, which typically included zeolites, metal-organic frameworks (MOFs), and graphene oxides, has been witnessed. Owing to high flexibility in both pore apertures and functionality, MOFs in the form of membranes have offered unprecedented opportunities for energy-efficient gas separations. Reports on the fabrication of well-intergrown MOF membranes first appeared in 2009. Since then there has been tremendous growth in this area along with an exponential increase of MOF-membrane-related publications. In order to compete with other separation and purification technologies, like cryogenic distillation, pressure swing adsorption, and chemical absorption, separation performance (including permeability, selectivity, and long-term stability) of molecular sieve membranes must be further improved in an attempt to reach an economically attractive region. Therefore, microstructural engineering and architectural design of MOF membranes at mesoscopic and microscopic levels become indispensable. This review summarizes some intriguing research that may potentially contribute to large-scale applications of MOF membranes in the future.

High performance carbon molecular sieving membranes derived from pyrolysis of metal-organic framework ZIF-108 doped polyimide matrices.

Carbon molecular sieve membranes (CMSMs) were fabricated by pyrolysis of MOF-doped polyimide mixed matrix membranes. ZIF-108 (Zn(2-nitroimidazolate)2) was used as a dopant to tailor the micropores of the as-prepared CMSMs into narrow ultramicropores, providing a remarkable combination of permeability and selectivity of membranes in CO2/CH4, O2/N2 and N2/CH4 separation.

Confinement of Ionic Liquids in Nanocages: Tailoring the Molecular Sieving Properties of ZIF-8 for Membrane-Based CO2 Capture.

Fine-tuning of effective pore size of microporous materials is necessary to achieve precise molecular sieving properties. Herein, we demonstrate that room temperature ionic liquids can be used as cavity occupants for modification of the microenvironment of MOF nanocages. Targeting CO2 capture applications, we tailored the effective cage size of ZIF-8 to be between CO2 and N2 by confining an imidazolium-based ionic liquid [bmim][Tf2 N] into ZIF-8's SOD cages by in-situ ionothermal synthesis. Mixed matrix membranes derived from ionic liquid-modified ZIF-8 exhibited remarkable combinations of permeability and selectivity that transcend the upper bound of polymer membranes for CO2 /N2 and CO2 /CH4 separation. We observed an unusual response of the membranes to varying pressure, that is, an increase in the CO2 /CH4 separation factor with pressure, which is highly desirable for practical applications in natural gas upgrading.

Mixed-matrix membranes containing functionalized porous metal-organic polyhedrons for the effective separation of CO2-CH4 mixture.

To enhance dispersion and adhesion, functionalized porous metal-organic polyhedrons were incorporated into polysulfone as a filler to obtain mixed-matrix membranes, which exhibit largely improved gas permeability and separation factor simultaneously for CO2-CH4 separation.

Membranes. Metal-organic framework nanosheets as building blocks for molecular sieving membranes.

Layered metal-organic frameworks would be a diverse source of crystalline sheets with nanometer thickness for molecular sieving if they could be exfoliated, but there is a challenge in retaining the morphological and structural integrity. We report the preparation of 1-nanometer-thick sheets with large lateral area and high crystallinity from layered MOFs. They are used as building blocks for ultrathin molecular sieve membranes, which achieve hydrogen gas (H2) permeance of up to several thousand gas permeation units (GPUs) with H2/CO2 selectivity greater than 200. We found an unusual proportional relationship between H2 permeance and H2 selectivity for the membranes, and achieved a simultaneous increase in both permeance and selectivity by suppressing lamellar stacking of the nanosheets.

Metal-substituted zeolitic imidazolate framework ZIF-108: gas-sorption and membrane-separation properties.

A series of dual-metal zeolitic imidazolate framework (ZIF) crystals with SOD and RHO topologies was synthesised by metal substitution from ZIF-108 (Zn(2-nitroimidazolate)2 , SOD topology) as the parent material. This was based on the concept that metal substitution of ZIF-108 requires a much lower activation energy than homogenous nucleation owing to the metastability of ZIF-108. In-depth investigations of the formation processes of the daughter ZIFs indicated that the transformation of ZIF-108 is a dissolution/heterogeneous nucleation process. Typical isostructural Co(2+) substitution mainly occurs at the outer surface of ZIF-108 and results in a core-shell structure. On the contrary, the Cu(2+) -substituted ZIF has a RHO topology with a homogeneous distribution of Cu(2+) ions in the structure. Substitution with Ni(2+) resulted in a remarkable enhancement in adsorption selectivity toward CO(2) over N(2) by a factor of up to 227. With Co(2+) -substituted nanoparticles as inorganic filler, a mixed matrix membrane based on polysulfone displayed greatly improved performance in the separation of H(2)/CH(4), CO(2)/N(2) and CO(2)/CH(4).