Chiral Recognition and Separation by Chirality‐Enriched Metal–Organic Frameworks

Endowed with chiral channels and pores, chiral metal–organic frameworks (MOFs) are highly useful; however, their synthesis remains a challenge given that most chiral building blocks are expensive. Although MOFs with induced chirality have been reported to avoid this shortcoming, no study providing evidence for the ee value of such MOFs has yet been reported. We herein describe the first study on the efficiency of chiral induction in MOFs using inexpensive achiral building blocks and fully recoverable chiral dopants to control the handedness of racemic MOFs. This method yielded chirality‐enriched MOFs with accessible pores. The ability of the materials to form host–guest complexes was probed with enantiomers of varying size and coordination and in solvents with varying polarity. Furthermore, mixed‐matrix membranes (MMMs) composed of chirality‐enriched MOF particles dispersed in a polymer matrix demonstrated a new route for chiral separation.     

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.     

Computational Methods for MOF/Polymer Membranes

Metal–organic framework (MOF)/polymer mixed matrix membranes (MMMs) have received significant interest in the last decade. MOFs are incorporated into polymers to make MMMs that exhibit improved gas permeability and selectivity compared with pure polymer membranes. The fundamental challenge in this area is to choose the appropriate MOF/polymer combinations for a gas separation of interest. Even if a single polymer is considered, there are thousands of MOFs that could potentially be used as fillers in MMMs. As a result, there has been a large demand for computational studies that can accurately predict the gas separation performance of MOF/polymer MMMs prior to experiments. We have developed computational approaches to assess gas separation potentials of MOF/polymer MMMs and used them to identify the most promising MOF/polymer pairs. In this Personal Account, we aim to provide a critical overview of current computational methods for modeling MOF/polymer MMMs. We give our perspective on the background, successes, and failures that led to developments in this area and discuss the opportunities and challenges of using computational methods for MOF/polymer MMMs.     

Molecular Level Characterization of the Structure and Interactions in Peptide‐Functionalized Metal–Organic Frameworks

We use density functional theory, newly parameterized molecular dynamics simulations, and last generation ¹⁵N dynamic nuclear polarization surface enhanced solid‐state NMR spectroscopy (DNP SENS) to understand graft–host interactions and effects imposed by the metal–organic framework (MOF) host on peptide conformations in a peptide‐functionalized MOF. Focusing on two grafts typified by MIL‐68‐proline ( ‐Pro ) and MIL‐68‐glycine‐proline ( ‐Gly‐Pro ), we identified the most likely peptide conformations adopted in the functionalized hybrid frameworks. We found that hydrogen bond interactions between the graft and the surface hydroxyl groups of the MOF are essential in determining the peptides conformation(s). DNP SENS methodology shows unprecedented signal enhancements when applied to these peptide‐functionalized MOFs. The calculated chemical shifts of selected MIL‐68‐NH‐ Pro and MIL‐68‐NH‐ Gly‐Pro conformations are in a good agreement with the experimentally obtained ¹⁵N NMR signals. The study shows that the conformations of peptides when grafted in a MOF host are unlikely to be freely distributed, and conformational selection is directed by strong host–guest interactions.     

In Situ Modification of Metal–Organic Frameworks in Mixed‐Matrix Membranes

Processable films of metal–organic frameworks (MOFs) have been long sought to advance the application of MOFs in various technologies from separations to catalysis. Herein, MOF–polymer mixed‐matrix membranes (MMMs) are described, formed on several substrates using a wide variety of MOF materials. These MMMs can be delaminated from their substrates to create free‐standing MMMs that are mechanically stable and pliable. The MOFs in these MMMs remain highly crystalline, porous, and accessible for further chemical modification through postsynthetic modification (PSM) and postsynthetic exchange (PSE) processes. Overall, the findings here demonstrate a versatile approach to preparing stable functional MMMs that should contribute significantly to the advancement of these materials.     

Microporous 3D aluminum MOF doped into chitosan‐based mixed matrix membranes for ethanol/water separation

An aluminum metal–organic framework (Al‐MOF), [Al(OH)(BPDC)] (DUT‐5; BPDC = Biphenyl‐4,4′‐dicarboxylate), was synthesized using solvothermal reactions. The high surface area and micropores (approximately 1.2 nm) of DUT‐5 were characterized using N₂ gas sorption measurements. The thermal stability of DUT‐5 and its phase purity were also investigated. The different amounts of DUT‐5 (0.1, 0.15, and 0.2 wt%) were successfully incorporated into the chitosan (CS) polymer to prepare a mixed matrix membrane (MMM) for the pervaporation of water/ethanol at 25°C. In particular, when 0.15 wt% of DUT‐5 was loaded, the DUT‐5@CS MMMs displayed excellent permeability and selectivity in ethanol/water separation. The results indicated that compared with pristine chitosan membranes, the flux of DUT‐5@CS membranes with 0.15 wt% loading significantly increased from 315 to 378 (g/m² h^?1) and the separation factor from 347 to 3,429. These promising results of the microporous Al‐MOF doped into chitosan MMMs reveal its good application potential for the bio‐ethanol separation processes.     

Homochiral metal–organic framework used as a stationary phase for high‐performance liquid chromatography

Metal–organic frameworks are promising porous materials. Chiral metal–organic frameworks have attracted considerable attention in controlling enantioselectivity. In this study, a homochiral metal–organic framework [Co₂(D‐cam)₂(TMDPy)] (D‐cam = d ‐camphorates, TMDPy = 4,4′‐trimethylenedipyridine) with a non‐interpenetrating primitive cubic net has been used as a chiral stationary phase in high‐performance liquid chromatography. It has allowed the successful separation of six positional isomers and six chiral compounds. The good selectivity and baseline separation, or at least 60% valley separation, confirmed its excellent molecular recognition characteristics. The relative standard deviations for the retention time of run‐to‐run and column‐to‐column were less than 1.8 and 3.1%, respectively. These results demonstrate that [Co₂(D‐cam)₂(TMDPy)] may represent a promising chiral stationary phase for use in high‐performance liquid chromatography.     

Gel-to-crystal route towards MOF-mixed MOF-matrix membranes

Metal-organic framework (MOF) membranes attract intensive attention for precisely molecular sieving. It is of greatly scientific interest to adjust transport property of MOF membranes. Inspired by formation and separation mechanisms of mixed matrix membranes (MMMs), in this study, we report a kind of MOF-mixed MOF-matrix membranes (M₅), which consist of dispersed MOF fillers and continuous MOF matrixes and combine different features of two MOFs. A simple, controllable, and versatile gel-to-crystal methodology, similar to solvent-evaporation induced phase-inversion of polymer-based membranes and performed by precursor coating and thermally treating crystallization, is proposed to construct M₅. The incorporated MOF fillers can improve membrane crystallization and have good compatibility toward MOF matrixes. As an example, because of UiO-66-NH₂ filler has larger aperture than ZIF-8 matrix, the prepared M₅ shows enhanced permeability while invariable selectivity compared with the pure one.     

Chiral Amplification of Oligopeptides in the Polymerization of α‐Amino Acid N‐Carboxyanhydrides in Water

This article is concerned with the chiral amplification of oligopeptides formed in the polymerization of chiral, nonracemic mixtures of the N‐carboxyanhydride (NCA) of Leu and Glu in aqueous solution. Labeling (deuteration) of one enantiomer and reversed‐phase and normal‐phase high‐performance liquid chromatography mass spectrometry (RP‐ and NP‐HPLC/MS, respectively) were used to determine the product distribution, both with respect to oligopeptide chain length and stereoisomer distribution. Starting the polymerization with an enantiomeric excess (ee) of 20% of the L ‐enantiomer (L ‐amino acid/D ‐amino acid 6 : 4) leads to an ee of 73% at the level of the homochiral enantiomeric (Leu)₅, and of 71% at the level of the homochiral enantiomeric (Glu)₇. For the Leu system and in the presence of a solid support (quartz), the ee reached values of up to 100%. We argue that such amplification processes could be relevant for the chemical evolution towards single‐handedness.     

Amino Acid Ionic Liquids as Chiral Ligands in Ligand‐Exchange Chiral Separations

Recently, amino acid ionic liquids (AAILs) have attracted much research interest. In this paper, we present the first application of AAILs in chiral separation based on the chiral ligand exchange principle. By using 1‐alkyl‐3‐methylimidazolium L ‐proline (L ‐Pro) as a chiral ligand coordinated with copper(II), four pairs of underivatized amino acid enantiomers—dl ‐phenylalanine (dl ‐Phe), dl ‐histidine (dl ‐His), dl ‐tryptophane (dl ‐Trp), and dl ‐tyrosine (dl ‐Tyr)—were successfully separated in two major chiral separation techniques, HPLC and capillary electrophoresis (CE), with higher enantioselectivity than conventionally used amino acid ligands (resolution (R_s)=3.26–10.81 for HPLC; R_s=1.34–4.27 for CE). Interestingly, increasing the alkyl chain length of the AAIL cation remarkably enhanced the enantioselectivity. It was inferred that the alkylmethylimidazolium cations and L ‐Pro form ion pairs on the surface of the stationary phase or on the inner surface of the capillary. The ternary copper complexes with L ‐Pro are consequently attached to the support surface, thus inducing an ion‐exchange type of retention for the dl ‐enantiomers. Therefore, the AAIL cation plays an essential role in the separation. This work demonstrates that AAILs are good alternatives to conventional amino acid ligands for ligand‐exchange‐based chiral separation. It also reveals the tremendous application potential of this new type of task‐specific ILs.     

Preparation and Gas Separation Properties of Metal‐Organic Framework Membranes

Continuous and intergrown metal‐organic framework (MOF) membranes, MIL‐100(In) (MIL represents Materials Institute Lavoisier), were prepared directly on porous anodic alumina oxide (AAO) membranes using an in situ crystallization method. The pore surface of MIL‐100(In) is conferred with polarity due to the presence of the 1, 3,5‐benzenetricarboxylic acid. The thickness of MIL‐100(In) membranes was tuned by varying the reactant concentration of indium chloride and 1, 3,5‐benzenetricarboxylic acid. Single gas permeation measurements on this MOF membrane indicate the large permeances of 0.90 × 10^–6 and 0.81 × 10^–6 mol · m^–2·s^–1·Pa^–1 for CO₂ and CH₄, and relatively high ideal selective factors of 3.75 and 3.38 for CO₂/N₂ and CH₄/N₂, respectively.     

Mixed‐Matrix Membranes

Research into extended porous materials such as metal‐organic frameworks (MOFs) and porous organic frameworks (POFs), as well as the analogous metal‐organic polyhedra (MOPs) and porous organic cages (POCs), has blossomed over the last decade. Given their chemical and structural variability and notable porosity, MOFs have been proposed as adsorbents for industrial gas separations and also as promising filler components for high‐performance mixed‐matrix membranes (MMMs). Research in this area has focused on enhancing the chemical compatibility of the MOF and polymer phases by judiciously functionalizing the organic linkers of the MOF, modifying the MOF surface chemistry, and, more recently, exploring how particle size, morphology, and distribution enhance separation performance. Other filler materials, including POFs, MOPs, and POCs, are also being explored as additives for MMMs and have shown remarkable anti‐aging performance and excellent chemical compatibility with commercially available polymers. This Review briefly outlines the state‐of‐the‐art in MOF‐MMM fabrication, and the more recent use of POFs and molecular additives.     

l‐Lysine‐derived ionic liquids as chiral ligands of Zn(II) complexes used in ligand‐exchange CE

Amino acid ionic liquids (AAILs) with l ‐lysine (l ‐Lys) as anion were synthesized and applied as new chiral ligands in Zn(II) complexes for chiral ligand‐exchange CE. After effective optimization, baseline enantioseparation of seven pairs of dansylated amino acids was achieved with a buffer of 100.0 mM boric acid, 5.0 mM ammonium acetate, 3.0 mM ZnSO₄, and 6.0 mM [C₆mim][l ‐Lys] at pH 8.2. To validate the unique behavior of AAILs, a comparative study between the performance of Zn(II)‐l ‐Lys and Zn(II)‐[C₆mim][l ‐Lys] systems was conducted. In Zn(II)‐[C₆mim][l ‐Lys] system, it has been found that the improved chiral resolution could be obtained and the migration times of the three test samples were markedly prolonged. Then the separation mechanism was further discussed. The role of [C₆mim][l ‐Lys] indicated clearly that the synthesized AAILs could be used as chiral ligands and would have potential utilization in separation science in future.     

Engineering of the Filler/Polymer Interface in Metal–Organic Framework‐Based Mixed‐Matrix Membranes to Enhance Gas Separation

Traditional films cannot fully adapt to industrial applications and to intensified processes. Advanced mixed‐matrix membranes comprising metal–organic frameworks (MOF) embedded in a polymer matrix have been developed with the goal of breaking the trade‐off effect of traditional polymer membranes and achieving separation performance beyond Robeson's upper limit. The key challenges in the fabrication of MOF‐based mixed‐matrix membranes are an enhancement in compatibility between the inorganic filler and the polymer matrix, elimination of the irregular morphology and non‐selective interfacial defects, and further improvement in the gas‐separation performance. This review summarizes the recent advances in protocols and strategies in terms of designing interfacial interactions to enhance the MOF/polymer interface compatibility. This review aims at providing some meaningful insights into preparing MOF‐based mixed‐matrix membranes targeting ideal interfacial morphology and leading to excellent gas‐separation performance.     

Chiral Isocamphoric Acid: Founding a Large Family of Homochiral Porous Materials

Homochiral metal–organic frameworks (MOFs) are an important class of chiral solids with potential applications in chiral recognition; however, relatively few are available. Of great importance is the availability of low‐cost, racemization‐resistant, and versatile enantiopure building blocks. Among chiral building blocks, d ‐camphoric acid is highly prolific, yet, its trans‐isomer, l ‐isocamphoric acid, has remained unknown in the entire field of solid‐state materials. Its rich yet totally untapped synthetic and structural chemistry has now been investigated through the synthesis of a large family of homochiral metal isocamphorates. The first observation of diastereoisomerism in isostructural MOFs is presented. Isocamphorate has a powerful ability to create framework topologies unexpected from common inorganic building blocks, and isocamphoric acid should allow access to hundreds of new homochiral materials.     

The Cooperative Effect of Both Molecular and Supramolecular Chirality on Cell Adhesion

Although helical nanofibrous structures have great influence on cell adhesion, the role played by chiral molecules in these structures on cells behavior has usually been ignored. The chirality of helical nanofibers is inverted by the odd–even effect of methylene units from homochiral l ‐phenylalanine derivative during assembly. An increase in cell adhesion on left‐handed nanofibers and weak influence of cell behaviors on right‐handed nanofibers are observed, even though both were derived from l ‐phenylalanine derivatives. Weak and negative influences on cell behavior was also observed for left‐ and right‐handed nanofibers derived from d ‐phenylalanine, respectively. The effect on cell adhesion of single chiral molecules and helical nanofibers may be mutually offset.     

A three‐dimensional homochiral metal–organic framework constructed from manganese(II) with S‐carboxymethyl‐N‐(p‐tosyl)‐l‐cysteine and 4,4′‐bipyridine

In the chiral polymeric title compound, poly[aqua(4,4′‐bipyridine)[μ₃‐S‐carboxylatomethyl‐N‐(p‐tosyl)‐l ‐cysteinato]manganese(II)], [Mn(C₁₂H₁₃NO₆S₂)(C₁₀H₈N₂)(H₂O)]_n, the Mn^II ion is coordinated in a distorted octahedral geometry by one water molecule, three carboxylate O atoms from three S‐carboxyatomethyl‐N‐(p‐tosyl)‐l ‐cysteinate (Ts‐cmc) ligands and two N atoms from two 4,4′‐bipyridine molecules. Each Ts‐cmc ligand behaves as a chiral μ₃‐linker connecting three Mn^II ions. The two‐dimensional frameworks thus formed are further connected by 4,4′‐bipyridine ligands into a three‐dimensional homochiral metal–organic framework. This is a rare case of a homochiral metal–organic framework with a flexible chiral ligand as linker, and this result demonstrates the important role of noncovalent interactions in stabilizing such assemblies.     

Pseudophomins A and B,a class of cyclic lipodepsipeptides isolated from a Pseudomonas species

The crystal structures of pseudophomins A and B, with primary structures β‐hydroxy­decanoyl–l ‐Leu–d ‐Glu–d ‐allo‐Thr–d ‐Ile–d ‐Leu–d ‐Ser–l ‐Leu–d ‐Ser–l ‐Ile monohydrate, C₅₅H₉₇N₉O₁₆·H₂O, and β‐hydroxy­dodecanoyl–l ‐Leu–d ‐Glu–d ‐allo‐Thr–d ‐Ile–d ‐Leu–d ‐Ser–l ‐Leu–d ‐Ser–l ‐Ile monohydrate, C₅₇H₁₀₁N₉O₁₆·H₂O, new cyclic lipodepsipeptides isolated from Pseudomonas fluorescens strain BRG100, have been solved. The absolute configuration of pseudophomin A has been determined from anomalous dispersion and the stereochemistry of the β‐hydroxy acid group is R.     

Extra‐Framework Aluminum Species of Zeolite that Surrogate the Growth of Metal Organic Framework from Zeolite Matrix

Constructing a robust hybrid material with a porous inorganic and a porous organic framework is highly intriguing owing to its diverse functionality and porosity. However, the line of synthesis is not straightforward, since their nucleation and crystal growth processes are incompatible. Here, a simple method for the fabrication of hybrid zeolite/metal–organic framework of different framework structures is developed wherein the less‐useful extra‐framework aluminum species present in the zeolite surrogate the growth of metal organic framework (MOF) from the zeolite matrix in the presence of organic linkers of the corresponding MOF. An NMR study confirms that all the octahedral Al species are converted to Al‐MOF. TGA analysis shows that 32 % Al of H‐Beta is converted to Al‐MOF. Furthermore, NH₃ TPD analysis shows that most of the weak acid sites disappear but strong acid sites are preserved suggesting the utilization of weakly bound Al species of H‐Beta in the growth of Al‐MOF. The synthesis strategy is successfully demonstrated using H‐Beta, H‐ZSM‐5, and H‐Y zeolites for the growth of MIL‐53 and MIL‐96 MOFs from the zeolite matrix. This synthesis strategy enables application‐based engineering of the framework structures, functionality, and porosity of the materials.     

MOF Scaffold for a High‐Performance Mixed‐Matrix Membrane

A novel composite membrane consisting of an interconnected MOF scaffold coated with cross‐linked poly(ethylene glycol) (PEG) has been developed. As a result of its unique structure, the membrane shows an exceptional 18‐fold permeability enhancement as compared to pristine PEG membranes, without compromising the selectivity. This performance is unattainable with current mixed‐matrix membranes (MMMs). Our optimized membrane has a permeability of 2700 Barrer with a CO₂/N₂ selectivity of 35, which surpasses the latest Robeson upper bound.