Ultra‐Tuning of the Aperture Size in Stiffened ZIF‐8_Cm Frameworks with Mixed‐Linker Strategy for Enhanced CO2/CH4 Separation

ZIF‐8 membrane has the potential for CO₂/CH₄ separation based on size exclusion. But if traditionally prepared by solvothermal methods, it shows only negligible selectivity due to the linker mobility. Here, ≈500 nm‐thin hybrid ZIF‐7_x‐8 membranes with suppressed linker mobility and narrowed window aperture are prepared by a fast current‐driven synthesis (FCDS) within 20 min. The in situ electric field during FCDS allows the formation of stiffened ZIF‐8_Cm as parent skeleton and the mixed‐linker strategy is applied to narrow the aperture size simultaneously. The ZIF‐7₂₂‐8 membrane shows significantly sharpened molecular sieving for CO₂/CH₄ with a separation factor above 25, which soared tenfold compared with other unmodified ZIF‐8 membranes. Additionally, the membrane shows exceptional separation performance for H₂/CH₄ and CO₂/N₂, with separation factors of 71 and 20, respectively. After 180 h temperature swing operation, it still maintains the excellent separation performance.     

MIL‐53(Al) as a Versatile Platform for Ionic‐Liquid/MOF Composites to Enhance CO2 Selectivity over CH4 and N2

Five different imidazolium‐based ionic liquids (ILs) were incorporated into a metal–organic framework (MOF), MIL‐53(Al), to investigate the effect of IL incorporation on the CO₂ separation performance of MIL‐53(Al). CO₂, CH₄, and N₂ adsorption isotherms of the IL/MIL‐53(Al) composites and pristine MIL‐53(Al) were measured to evaluate the effect of the ILs on the CO₂/CH₄ and CO₂/N₂ selectivities of the MOF. Of the composite materials that were tested, [BMIM][PF₆]/MIL‐53(Al) exhibited the largest increase in CO₂/CH₄ selectivity, 2.8‐times higher than that of pristine MIL‐53(Al), whilst [BMIM][MeSO₄]/MIL‐53(Al) exhibited the largest increase in CO₂/N₂ selectivity, 3.3‐times higher than that of pristine MIL‐53(Al). A comparison of the CO₂ separation potentials of the IL/MOF composites showed that the [BMIM][BF₄]‐ and [BMIM][PF₆]‐incorporated MIL‐53(Al) composites both showed enhanced CO₂/N₂ and CO₂/CH₄ selectivities at pressures of 1–5 bar compared to composites of CuBTC and ZIF‐8 with the same ILs. These results demonstrate that MIL‐53(Al) is a versatile platform for IL/MOF composites and could help to guide the rational design of new composites for target gas‐separation applications.     

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 Co²⁺ substitution mainly occurs at the outer surface of ZIF‐108 and results in a core–shell structure. On the contrary, the Cu²⁺‐substituted ZIF has a RHO topology with a homogeneous distribution of Cu²⁺ ions in the structure. Substitution with Ni²⁺ resulted in a remarkable enhancement in adsorption selectivity toward CO₂ over N₂ by a factor of up to 227. With Co²⁺‐substituted nanoparticles as inorganic filler, a mixed matrix membrane based on polysulfone displayed greatly improved performance in the separation of H₂/CH₄, CO₂/N₂ and CO₂/CH₄.     

ZIF‐8 Membrane Separation Performance Tuning by Vapor Phase Ligand Treatment

Vapor phase ligand treatment (VPLT) of 2‐aminobenzimidazole (2abIm) for 2‐methylimidazole (2mIm) in ZIF‐8 membranes prepared by two different methods (LIPS: ligand induced permselectivation and RTD: rapid thermal deposition) results in a notable shift of the molecular level cut‐off to smaller molecules establishing selectivity improvements from ca. 1.8 to 5 for O₂/N₂; 2.2 to 32 for CO₂/CH₄; 2.4 to 24 for CO₂/N₂; 4.8 to 140 for H₂/CH₄ and 5.2 to 126 for H₂/N₂. Stable (based on a one‐week test) oxygen‐selective air separation performance at ambient temperature, 7 bar(a) feed, and 1 bar(a) sweep‐free permeate with a mixture separation factor of 4.5 and oxygen flux of 2.6×10^?3 mol m^?2 s^?1 is established. LIPS and RTD membranes exhibit fast and gradual evolution upon a 2abIm‐VPLT, respectively, reflecting differences in their thickness and microstructure. Functional reversibility is demonstrated by showing that the original permeation properties of the VPLT‐LIPS membranes can be recovered upon 2mIm‐VPLT.     

Azide‐based Cross‐Linking of Polymers of Intrinsic Microporosity (PIMs) for Condensable Gas Separation

Cross‐linked polymers of intrinsic microporosity (PIM)s for gas separation membranes, were prepared by a nitrene reaction from a representative PIM in the presence of two different diazide cross‐linkers. The reaction temperature was optimized using TGA. The homogenous membranes were cast from THF solutions of different ratios of PIM to azides. The resulting cross‐linked structures of the PIMs membranes were formed at 175 °C after 7.5 h and confirmed by TGA, XPS, FT‐IR spectroscopy and gel content analysis. These resulting cross‐linked polymeric membranes showed excellent gas separation performance and can be used for O₂/N₂ and CO₂/N₂ gas pairs, as well as for condensable gases, such as CO₂/CH₄, propylene/propane separation. Most importantly, and differently from typical gas separation membranes derived from glassy polymers, the crosslinked PIMs showed no obvious CO₂ plasticization up to 20 atm pressure of pure CO₂ and CO₂/CH₄ mixtures.

     

A MOF Glass Membrane for Gas Separation

Metal–organic framework (MOF) glasses are promising candidates for membrane fabrication due to their significant porosity, the ease of processing, and most notably, the potential to eliminate the grain boundary that is unavoidable for polycrystalline MOF membranes. Herein, we developed a ZIF‐62 MOF glass membrane and exploited its intrinsic gas‐separation properties. The MOF glass membrane was fabricated by melt‐quenching treatment of an in situ solvothermally synthesized polycrystalline ZIF‐62 MOF membrane on a porous ceramic alumina support. The molten ZIF‐62 phase penetrated into the nanopores of the support and eliminated the formation of intercrystalline defects in the resultant glass membrane. The molecular sieving ability of the MOF membrane is remarkably enhanced via vitrification. The separation factors of the MOF glass membrane for H₂/CH₄, CO₂/N₂ and CO₂/CH₄ mixtures are 50.7, 34.5, and 36.6, respectively, far exceeding the Robeson upper bounds.     

Electropolymerization of Molecular‐Sieving Polythiophene Membranes for H2 Separation

Membrane technologies that do not rely on heat for industrial gas separation would lower global energy cost. While polymeric, inorganic, and mixed‐matrix separation membranes have been rapidly developed, the bottleneck is balancing the processability, selectivity, and permeability. Reported here is a softness adjustment of rigid networks (SARs) strategy to produce flexible, stand‐alone, and molecular‐sieving membranes by electropolymerization. Here, 14 membranes were rationally designed and synthesized and their gas separation ability and mechanical performance were studied. The separation performance of the membranes for H₂/CO₂, H₂/N₂, and H₂/CH₄ can exceed the Robeson upper bound, among which, H₂/CO₂ separation selectivity reaches 50 with 626 Barrer of H₂ permeability. The long‐term and chemical stability tests demonstrate their potential for industrial applications. This simple, scalable, and cost‐effective strategy holds promise for the design other polymers for key energy‐intensive separations.     

Zeolitic Imidazolate Framework/Graphene Oxide Hybrid Nanosheets as Seeds for the Growth of Ultrathin Molecular Sieving Membranes

A defect‐free zeolitic imidazolate framework‐8 (ZIF‐8)/graphene oxide (GO) membrane with a thickness of 100 nm was prepared using two‐dimensional (2D) ZIF‐8/GO hybrid nanosheets as seeds. Hybrid nanosheets with a suitable amount of ZIF‐8 nanocrystals were essential for producing a uniform seeding layer that facilitates fast crystal intergrowth during membrane formation. Moreover, the seeding layer acts as a barrier between two different synthesis solutions, and self‐limits crystal growth and effectively eliminates defects during the contra‐diffusion process. The resulting ultrathin membranes show excellent molecular sieving gas separation properties, such as with a high CO₂/N₂ selectivity of 7.0. This 2D nano‐hybrid seeding strategy can be readily extended to the fabrication of other defect‐free and ultrathin MOF or zeolite molecular sieving membranes for a wide range of separation applications.     

Polymer Composite Membrane with Penetrating ZIF‐7 Sheets Displays High Hydrogen Permselectivity

Oriented and penetrating molecular sieving membranes display enhanced separation performance. A polyimide (PI) solution containing highly dispersed ZIF‐7(III) sheets in CHCl₃ was deposited on a glass side and subjected to flat‐scraping with a membrane fabricator. In this way we developed a novel oriented and penetrating ZIF‐7@PI mixed matrix membrane (MMM) with 50 wt. % ZIF‐7 loading. Because the height of the ZIF‐7 sheets (5 μm) is higher than the film thickness, every ZIF‐7 sheet penetrates both surfaces of the polyimide film. Since the ZIF‐7 channels are the dominant pathway for gas permeation, the ZIF‐7@PI MMM displays a high molecular sieve performance for the separation of H₂ (0.29 nm) from larger gas molecules. At 100 °C and 2 bar, the mixture separation factors of H₂/CO₂ and H₂/CH₄ are 91.5 and 128.4, with a high H₂ permeance of about 3.0×10^?7 mol m^?2 s^?1 Pa^?1, which is promising for hydrogen separation by molecular sieving.     

Formation of Ultrathin,Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

Metal–organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF‐8) membranes on titania‐functionalized porous polymeric supports. The coherent ZIF‐8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF‐8, with hydrogen permeance up to 201×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and an ideal H₂/CO₂ selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.     

Formation of Ultrathin,Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

Metal–organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF‐8) membranes on titania‐functionalized porous polymeric supports. The coherent ZIF‐8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF‐8, with hydrogen permeance up to 201×10^?7 mol m^?2 s^?1 Pa^?1 and an ideal H₂/CO₂ selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.     

A novel acrylonitrile–butadiene–styrene/poly(ethylene glycol) membrane: preparation,characterization, and gas permeation study

Polymeric membranes were prepared by blending different grades of poly(ethylene glycol) (PEG) as the added polymer with acrylonitrile–butadiene–styrene as the backbone structure. The membranes were characterized by Fourier transform infrared, X‐ray diffractometry, dynamic mechanical thermal analysis, differential scanning calorimetry, and scanning electron microscopy. Furthermore, the gas permeation and separation properties of CO₂/CH₄ were studied. In addition, the effect of pressure (1–8 bar) and the effect of PEG content (0–40 wt%) on CO₂ and CH₄ permeability/selectivity were investigated. The results showed that, in more cases, with the introduction of PEG molecules, CO₂/CH₄ selectivity increases without significant changes in CH₄ permeability, indicating that the incorporation of intermolecular interaction is suitable for the separation of gas pairs with no molecular size domination but the solution–diffusion. From the viewpoint of gas separation applications, the resultant data are in the commercially attractive region. Copyright © 2011 John Wiley & Sons, Ltd.     

Syntheses and gas transport properties of polyarylates based on isophthalic and terephthalic acids and mixtures of bisphenol A and 1, 1 bi‐2‐naphthol

Polyarylates based on isophthalic (IA) and terephthalic (TA) acids and an equimolar mixture of the diols Bisphenol A (BPA) and 1,1 bi‐2‐naphthol (BN) were synthesized to produce BPA‐BN/IA and BPA‐BN/TA polymers and to measure their gas permeability coefficients, P(i), at several pressures and 35 °C, to the gases O₂, N₂, CH₄, and CO₂. For the BPA‐BN/IA membranes, at a 2 atm up‐stream pressure, the P(O₂) and P(CO₂) are 0.93 and 4.0 Barrers with O₂/N₂ and CO₂/CH₄ ideal separation factors of 6.7 and 27. For the BPA‐BN/TA, at a 2 atm up‐stream pressure, the P(O₂) and P(CO₂) are 2.0 and 9.9 Barrers with O₂/N₂ and CO₂/CH₄ ideal separation factors of 5.6 and 21. Comparing the selectivity–permeability balance of properties shown by the BPA/TA membranes with that shown by the copolymer BPA‐BN/TA, the balance moves in the direction of higher selectivity and lower permeability because of the incorporation of BN, which is a more rigid monomer than BPA. However, when the balance of properties for the pair O₂/N₂ shown by BPA‐BN/TA is compared with the one shown by other membranes such as those based on mixtures of diols and diacids, that is the bisphenol A‐naphthalene/I‐T polymers reported in the literature, the balance moves up and to the right of the typical selectivity–permeability trade‐off observed in the BPA‐polyarylate family. Thus, simultaneous incorporations of flexible and rigid monomers in both the diols and the diacids lead to more productive and more selective membranes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 256–263, 2006     

An Hetero‐Epitaxially Grown Zeolite Membrane

The secondary growth methodology to form zeolite membranes has stringent requirements for homogeneous epitaxial intergrowth of the seed layer and limits the number of accessible high‐quality zeolite membranes. Despite previous reports on hetero‐epitaxial growth, high‐performance zeolite membranes have yet to be reported using this approach. Here, the successful hetero‐epitaxial growth of highly siliceous ZSM‐58 (DDR‐type zeolite) films from a SSZ‐13 (CHA‐type zeolite) seed layer is reported. The resulting membranes show excellent CO₂ perm‐selectivities, having maximum CO₂ /N₂ and CO₂ /CH₄ separation factors (SFs) as high as about 17 and 279, respectively, at 30 °C. Furthermore, the hybrid membrane maintains the CO₂ perm‐selectivity in the presence of water vapor (the third main component in both cases), that is, CO₂ /N₂ SF of about 14 and CO₂ /CH₄ SF of about 78, respectively, at 50 °C (a representative temperature of both CO₂‐containing streams).     

High‐Flux Zeolitic Imidazolate Framework Membranes for Propylene/Propane Separation by Postsynthetic Linker Exchange

While zeolitic imidazolate framework, ZIF‐8, membranes show impressive propylene/propane separation, their throughput needs to be greatly improved for practical applications. A method is described that drastically reduces the effective thickness of ZIF‐8 membranes, thereby substantially improving their propylene permeance (that is, flux). The new strategy is based on a controlled single‐crystal to single‐crystal linker exchange of 2‐methylimidazole in ZIF‐8 membrane grains with 2‐imidazolecarboxaldehyde (ZIF‐90 linker), thereby enlarging the effective aperture size of ZIF‐8. The linker‐exchanged ZIF‐8 membranes showed a drastic increase in propylene permeance by about four times, with a negligible loss in propylene/propane separation factor when compared to as‐prepared membranes. The linker‐exchange effect depends on the membrane synthesis method.     

Tuning Pore Size in Square‐Lattice Coordination Networks for Size‐Selective Sieving of CO2

Porous materials capable of selectively capturing CO₂ from flue‐gases or natural gas are of interest in terms of rising atmospheric CO₂ levels and methane purification. Size‐exclusive sieving of CO₂ over CH₄ and N₂ has rarely been achieved. Herein we show that a crystal engineering approach to tuning of pore‐size in a coordination network, [Cu(quinoline‐5‐carboxyate)₂]_n ( Qc‐5‐Cu ) ena+bles ultra‐high selectivity for CO₂ over N₂ (S_CN≈40 000) and CH₄ (S_CM≈3300). Qc‐5‐Cu‐sql‐β , a narrow pore polymorph of the square lattice ( sql ) coordination network Qc‐5‐Cu‐sql‐α, adsorbs CO₂ while excluding both CH₄ and N₂. Experimental measurements and molecular modeling validate and explain the performance. Qc‐5‐Cu‐sql‐β is stable to moisture and its separation performance is unaffected by humidity.     

Molecularly Engineered 6FDA‐Based Polyimide Membranes for Sour Natural Gas Separation

Glassy polyimide membranes are attractive for industrial applications in sour natural gas purification. Unfortunately, the lack of fundamental understanding of relationships between polyimide chemical structures and their gas transport properties in the presence of H₂S constrains the design and engineering of advanced membranes for such challenging applications. Herein, 6FDA‐based polyimide membranes with engineered structures were synthesized to tune their CO₂/CH₄ and H₂S/CH₄ separation performances and plasticization properties. Under ternary mixed sour gas feeds, controlling polymer chain packing and plasticization tendency of such polyimide membranes via tuning the chemical structures were found to offer better combined H₂S and CO₂ removal efficiency compared to conventional polymers. Fundamental insights into structure–property relationships of 6FDA‐based polyimide membranes observed in this study offer guidance for next generation membranes for sour natural gas separation.     

Rapid magnetic solid‐phase extraction of Congo Red and Basic Red 2 from aqueous solution by ZIF‐8@CoFe2O4 hybrid composites

Core–shell metal–organic framework materials have attracted considerable attention mainly due to their enhanced or new physicochemical properties compared with their single‐component counterparts. In this work, a core–shell heterostructure of CoFe₂O₄‐Zeolitic Imidazolate Framework‐8 (ZIF‐8@CoFe₂O₄) is successfully fabricated and used as an solid‐phase extraction adsorbent to efficiently extract Congo Red and Basic Red 2 dyes from contaminated aqueous solution. Vibrating sample magnetometry indicates that the saturated magnetization of ZIF‐8@CoFe₂O₄ is 3.3 emu/g, which is large enough for magnetic separation. The obtained hybrid magnetic metal‐organic framework based material ZIF‐8@CoFe₂O₄ can remove the investigated dyes very fast within 1 min of the contact time. The adsorbent ZIF‐8@CoFe₂O₄ also shows a good reusability. After regeneration, the adsorbent can still exhibit high removal efficiency (～97%) toward Congo Red for five cycles of desorption–adsorption. This work reveals the great potential of core–shell ZIF‐8@CoFe₂O₄ sorbents for the fast separation and preconcentration of organic pollutants in aqueous solution before high‐performance liquid chromatography analysis.     

The effect of synthesis procedure on the catalytic performance of isostructural ZIF‐8

We demonstrate the synthesis of isostructural zeolitic imidazole framework (ZIF‐8) using four distinct synthetic methods. Subsequently, the variations in physicochemical properties were analyzed through the catalytic reaction of CO₂ cycloaddition of epoxide. It was thus demonstrated that simply by changing the type of synthetic method for the preparation of ZIF‐8, the physicochemical properties were changed significantly which in turn influenced the catalytic activity of ZIF‐8. It was found that the synthetic method affected the crystal growth and consequently influenced the physicochemical properties which are crucial aspects in metal–organic framework applications. There is an almost exponential relationship between the reactivity of various ZIF‐8 samples in CO₂ cycloaddition of epoxide and the surface area, CO₂ adsorption and pore volume.     

Remarkably Enhanced Gas Separation by Partial Self‐Conversion of a Laminated Membrane to Metal–Organic Frameworks

Separation methods based on 2D interlayer galleries are currently gaining widespread attention. The potential of such galleries as high‐performance gas‐separation membranes is however still rarely explored. Besides, it is well recognized that gas permeance and separation factor are often inversely correlated in membrane‐based gas separation. Therefore, breaking this trade‐off becomes highly desirable. Here, the gas‐separation performance of a 2D laminated membrane was improved by its partial self‐conversion to metal–organic frameworks. A ZIF‐8‐ZnAl‐NO₃ layered double hydroxide (LDH) composite membrane was thus successfully prepared in one step by partial conversion of the ZnAl‐NO₃ LDH membrane, ultimately leading to a remarkably enhanced H₂/CH₄ separation factor and H₂ permeance.