Understanding the Anomalous Alkane Selectivity of ZIF-7 in the Separation of Light Alkane/Alkene Mixtures

C2 and C3 alkanes are selectively adsorbed from mixtures over the corresponding alkenes on the zeolite imidazolate framework ZIF-7 through a gate-opening mechanism. As a result, the direct production of the pure alkene upon adsorption and the pure alkane upon desorption in packed columns is possible. Herein, a detailed investigation of the step-wise adsorption and separation of alkanes and alkenes is presented, together with a rigorous performance assessment. A molecular picture of the gate-opening mechanism underlying the unprecedented selectivity towards alkane adsorption is proposed based on DFT calculations and a thermodynamic analysis of the adsorption-desorption isotherms.     

Structural Optimization of Interpenetrated Pillared‐Layer Coordination Polymers for Ethylene/Ethane Separation

With the goal of achieving effective ethylene/ethane separation, we evaluated the gas sorption properties of four pillared‐layer‐type porous coordination polymers with double interpenetration, [Zn₂(tp)₂(bpy)]_n ( 1 ), [Zn₂(fm)₂(bpe)]_n ( 2 ), [Zn₂(fm)₂(bpa)]_n ( 3 ), and [Zn₂(fm)₂(bpy)]_n ( 4 ) (tp=terephthalate, bpy=4,4′‐bipyridyl, fm=fumarate, bpe=1,2‐di(4‐pyridyl)ethylene and bpa=1,2‐di(4‐pyridyl)ethane). It was found that 4 , which contains the narrowest pores of all of these compounds, exhibited ethylene‐selective sorption profiles. The ethylene selectivity of 4 was estimated to be 4.6 at 298 K based on breakthrough experiments using ethylene/ethane gas mixtures. In addition, 4 exhibited a good regeneration ability compared with a conventional porous material.     

Ionic Liquids Supported on Metal‐Organic Frameworks: Remarkable Adsorbents for Adsorptive Desulfurization

Acidic ionic‐liquids (IL) supported on metal–organic frameworks (MOFs) have been shown to be beneficial for adsorptive desulfurization. A remarkable improvement in the adsorption capacity (ca. 71%) was observed in for ILs supported on MIL‐101 compared with virgin MIL‐101. The improved adsorptive performance might be explained by the acid–base interactions between the acidic ionic liquid and basic benzothiophene (BT). Moreover, from this study, it can be suggested that porous MOFs, supported with ionic liquids, may introduce a new class of highly porous adsorbents for the efficient adsorption of various compounds.     

Molecular Sieving of C2‐C3 Alkene from Alkyne with Tuned Threshold Pressure in Robust Layered Metal–Organic Frameworks

C₂‐C₃ alkyne/alkene separation is of great importance; however, designing materials for an efficient molecular sieving of alkenes from alkynes remains challenging. Now, two hydrolytically stable layered MOFs, [Cu(dps)₂(GeF₆)] (GeFSIX‐dps‐Cu, dps=4,4′‐dipyridylsulfide) and [Zn(dps)₂(GeF₆)] (GeFSIX‐dps‐Zn), can achieve almost complete exclusion of both C₃H₆ and C₂H₄ from their alkyne analogues. GeFSIX‐dps‐Cu displays a notable advanced threshold pressure for alkynes adsorption and thus substantial uptakes at lower pressures, providing record C₃H₄/C₃H₆ uptake ratios and capacity‐enhanced C₂H₂/C₂H₄ sieving for a wide composition range. Metal substitution (Zn to Cu) affords fine tuning of linker rotation and layer stacking, creating slightly expanded pore aperture and interlayer space coupled with multiple hydrogen‐bonding sites, allowing easier entrance of alkyne while excluding alkene. Breakthrough experiments confirmed tunable sieving by these MOFs for C₃H₄/C₃H₆ and C₂H₂/C₂H₄ mixtures.     

Adsorptive Separation of Acetylene from Light Hydrocarbons by Mesoporous Iron Trimesate MIL‐100(Fe)

A reducible metal–organic framework (MOF), iron(III) trimesate, denoted as MIL‐100(Fe), was investigated for the separation and purification of methane/ethane/ethylene/acetylene and an acetylene/CO₂ mixtures by using sorption isotherms, breakthrough experiments, ideal adsorbed solution theory (IAST) calculations, and IR spectroscopic analysis. The MIL‐100(Fe) showed high adsorption selectivity not only for acetylene and ethylene over methane and ethane, but also for acetylene over CO₂. The separation and purification of acetylene over ethylene was also possible for MIL‐100(Fe) activated at 423 K. According to the data obtained from operando IR spectroscopy, the unsaturated Fe^III sites and surface OH groups are mainly responsible for the successful separation of the acetylene/ethylene mixture, whereas the unsaturated Fe^II sites have a detrimental effect on both separation and purification. The potential of MIL‐100(Fe) for the separation of a mixture of C₂H₂/CO₂ was also examined by using the IAST calculations and transient breakthrough simulations. Comparing the IAST selectivity calculations of C₂H₂/CO₂ for four MOFs selected from the literature, the selectivity with MIL‐100(Fe) was higher than those of CuBTC, ZJU‐60a, and PCP‐33, but lower than that of HOF‐3.     

Separation of Amyl Alcohol Isomers in ZIF‐77

The separation of pentanol isomer mixtures is shown to be very efficient using the nanoporous adsorbent zeolitic imidazolate framework ZIF‐77. Through molecular simulations, we demonstrate that this material achieves a complete separation of linear from monobranched—and these from dibranched—isomers. Remarkably, the adsorption and diffusion behaviors follow the same decreasing trend, produced by the channel size of ZIF‐77 and the guest shape. This separation based on molecular branching applies to alkanes and alcohols and promises to encompass numerous other functional groups.     

Efficient Separation of n‐Butene and iso‐Butene by Flexible Ultramicroporous Metal‐Organic Frameworks with Pocket‐like Cavities

Efficient separation of n‐butene (n‐C₄H₈) and iso‐butene (iso‐C₄H₈) is of significance for the upgrading of C4 olefins to high‐value end products but remains one of the major challenges in hydrocarbon purifications owing to their similar structures. Herein, we report a flexible metal‐organic framework, MnINA (INA=isonicotinate), featuring one‐dimensional pore channels with periodically large pocket‐like cavities connected by narrow bottlenecks, for the first time for efficient n‐/iso‐C₄H₈ separation. MnINA with smaller pore size (4.62 Å) compared with CuINA (4.84 Å), exhibits steep adsorption isotherms and high capacity of 1.79 mmol g^?1 for n‐C₄H₈ (4.46 Å) through strong host‐guest interactions via C?H???π bonding. The narrow bottlenecks exert barriers for the large molecules of iso‐C₄H₈ (4.84 Å) within the gate‐opening pressure range of 0–0.1 bar. This gives rise to MnINA with excellent separation selectivity of 327.7 for n‐/iso‐C₄H₈ mixture. The adsorption mechanism for n‐C₄H₈ and the gate‐opening effect were investigated by dispersion‐corrected density functional (DFT‐D) theory, verifying the strong interactions between n‐C₄H₈ and the frameworks as well as the gate‐opening effect derived from the rotation of organic linkers. The breakthrough tests confirmed MnINA and CuINA can be promising candidates for n‐/iso‐C₄H₈ separation.     

Mixed Metal–Organic Framework with Multiple Binding Sites for Efficient C2H2/CO2 Separation

The separation of C₂H₂/CO₂ is particularly challenging owing to their similarities in physical properties and molecular sizes. Reported here is a mixed metal–organic framework (M′MOF), [Fe(pyz)Ni(CN)₄] ( FeNi‐M′MOF , pyz=pyrazine), with multiple functional sites and compact one‐dimensional channels of about 4.0 Å for C₂H₂/CO₂ separation. This MOF shows not only a remarkable volumetric C₂H₂ uptake of 133 cm³ cm^?3, but also an excellent C₂H₂/CO₂ selectivity of 24 under ambient conditions, resulting in the second highest C₂H₂‐capture amount of 4.54 mol L^?1, thus outperforming most previous benchmark materials. The separation performance of this material is driven by π–π stacking and multiple intermolecular interactions between C₂H₂ molecules and the binding sites of FeNi‐M′MOF . This material can be facilely synthesized at room temperature and is water stable, highlighting FeNi‐M′MOF as a promising material for C₂H₂/CO₂ separation.     

Post‐Synthetic Polymerization of UiO‐66‐NH2 Nanoparticles and Polyurethane Oligomer toward Stand‐Alone Membranes for Dye Removal and Separation

Metal–organic frameworks (MOFs) are widely used as porous materials in the fields of adsorption and separation. However, their practical application is largely hindered by limitations to their processability. Herein, new UiO‐66‐Urea‐based flexible membranes with MOF loadings of 50 ( 1 ), 60 ( 2 ), and 70 wt % ( 3 ) were designed and prepared by post‐synthetic polymerization of UiO‐66‐NH₂ nanoparticles and a polyurethane oligomer under mild conditions. The adsorption behavior of membrane 3 towards four hydrophilic dyes, namely, eosin Y (EY), rhodamine B (RB), malachite green (MG), and methylene blue (MB), in aqueous solution was studied in detail. It exhibits strong adsorption of EY and RB but weak adsorption of MG and MB in aqueous solution. Owing to the selective adsorption of these hydrophilic dyes, membrane 3 can remove EY and RB from aqueous solution and completely separate EY/MB, RB/MG, and RB/MB mixtures in aqueous solution. In addition, the membrane is uniformly textured, easily handled, and can be reused for dye adsorption and separation.     

Exploiting Large‐Pore Metal–Organic Frameworks for Separations through Entropic Molecular Mechanisms

We review the molecular mechanisms behind adsorption and the separations of mixtures in metal–organic frameworks and zeolites. Separation mechanisms can be based on differences in the affinity of the adsorbate with the framework and on entropic effects. To develop next‐generation adsorbents, the separation efficiency of the materials needs to be improved. The performance under industrially relevant conditions largely depends on two factors: 1) the separation selectivity and 2) the pore volume capacity of the material. Enthalpic mechanisms can lead to increased selectivities, but these are mostly restricted to the low loading regime, and hence these mechanisms are unable to make use of all of the large‐pore volume that a metal–organic framework can provide. Industrial processes routinely operate in the pore saturation regime. In this Review, we focus on entropic molecular separation mechanisms that are effective under these conditions and, in particular, on a recent methodology to obtain high selectivities at high pore loading.     

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.     

Fluorocarbon Separation in a Thermally Robust Zirconium Carboxylate Metal–Organic Framework

Fluorocarbons have important applications in industry, but are environmentally unfriendly, and can cause ozone depletion and contribute to the global warming with long atmospheric lifetimes and high global warming potential. In this work, the metal–organic framework UiO‐66(Zr) is demonstrated to have excellent performance characteristics to separate fluorocarbon mixtures at room temperature. Adsorption isotherm measurements of UiO‐66(Zr) display high fluorocarbon sorption uptakes of 5.0 mmol g^?1 for R22 (CHClF₂), 4.6 mmol g^?1 for R125 (CHF₂CF₃), and 2.9 mmol g^?1 for R32 (CH₂F₂) at 298 K and 1 bar. Breakthrough data obtained for binary (R22/R32 and R32/R125) and ternary (R32/R125/R134a) mixtures reveal high selectivities and capacities of UiO‐66(Zr) for the separation and recycling of these fluorocarbon mixtures. Furthermore, the UiO‐66(Zr) saturated with R22 and R125 can be regenerated at temperatures as low as 120 °C with excellent desorption–adsorption cycling stabilities.     

Ultrathin Chiral Metal–Organic‐Framework Nanosheets for Efficient Enantioselective Separation

Chiral metal–organic framework (CMOF) nanosheets only a few layers thick remain a virgin land waiting for exploration. Herein, the first examples of ultrathin CMOF nanosheets are prepared by the confinement growth of two‐dimensional (2D) chiral layers, which are assembled by helical metal–organic chains within microemulsion. This convenient and easily scaled up inverse microemulsion method gives a series of 2D CMOF nanosheets composed of variable metal nodes or chiral ligands. More significantly, thanks to the exceptionally large number of chiral sites exposed on surfaces, the as‐obtained CMOF nanosheets exhibit much higher enantioselectivity in chiral separation compared with their bulk counterparts.     

An Allosteric Metal–Organic Framework That Exhibits Multiple Pore Configurations for the Optimization of Hydrocarbon Separation

The function of allosteric enzymes can be activated or inhibited through binding of specific effector molecules. Herein, we describe how the skeletal deformation, pore configuration, and ultimately adsorptive behavior of a dynamic metal–organic framework (MOF), (Me₂NH₂)[In(atp)]₂ (in which atp=2‐aminoterephthalate), are controlled by the allocation and orientation of its counter ions triggered by the inclusion/removal of different guest molecules. The power of such allosteric control in MOFs is highlighted through the optimization of the hydrocarbon separation performance by achieving multiple pore configurations but without altering the chemical composition.     

Chiral Metal–Organic Framework Hollow Nanospheres for High‐Efficiency Enantiomer Separation

Chiral ZIF‐8 hollow nanospheres with d ‐histidine as part of chiral ligands (denoted as H‐d ‐his‐ZIF‐8) were prepared for separation of (±)‐amine acids. Compared to bulk d ‐his‐ZIF‐8 without a hollow cavity, the prepared H‐d ‐his‐ZIF‐8 showed 15 times higher separation capacity and higher ee values of 90.5 % for alanine, 95.2 % for glutamic acid and 92.6 % for lysine, respectively.