WO2022018697A1 - Crown ether metal organic framework compositions and methods - Google Patents

Abstract

Crown ether (CE) functionalized metal organic framework (MOF) compositions (CEMOFs) and membranes comprising CEMOFs for lithium or other ion transport and/or ion separation, and related components, systems and processes utilizing CEMOFs for controlling ion separation or transport in liquid media. In some embodiments, ion selectivity or transport efficiency in CEMOFs is voltage tunable and the ions are Li, K and/or Mg in solution in a polar solvent.

Description

CROWN ETHER METAL ORGANIC FRAMEWORK COMPOSITIONS AND

METHODS

Cross-reference to related applications

[0001] This application claims the benefit of priority to U.S. Provisional Application No. 63/056,198 filed July 24, 2020, which is hereby incorporated herein by reference in its entirety.

Field

[0002] The invention relates to crown ether (CE) modified metal organic framework (MOF) compositions (CEMOFs) and related membranes for use in, among other things, ion transport and/or separation systems and processes.

Background

[0003] Membranes with ultrafast ion permeation and high ion selectivity are highly desirable for efficient mineral separation, water purification, and energy conversion. However, challenges arise in efficiently transporting and/or separating atomic ions of the same valence and/or similar sizes using synthetic membranes.

[0004] Membranes which incorporate metal organic frameworks (MOFs) with a narrow distribution of pore sizes, especially in the angstrom range, are of interest for use in gas separation technologies. While there has been interest in the use of MOFs for pressure driven gas separation technologies, there has been little reported research into the use of MOFs for selective transport and/or separation of ions in liquids.

[0005] It is an object of the invention to address at least one shortcoming of the prior art and/or to provide alternative MOF compounds, membranes, systems, and methods to that of the prior art.

Summary of Invention

[0006] The present disclosure provides, among other things, crown ether metal organic framework compositions, comprising a porous MOF having a first surface with a first pore window and a second surface with a second pore window and a channel therebetween, in which the MOF is functionalized by one or more CE structures contained within or on the channel of the MOF. Such crown ether metal organic framework compositions are referred to generally herein as “CEMOF” compositions and in preferred aspects as “CE-20X” compositions. The MOF structure of CE-20X may include a material selected from the group consisting of UiO-66, ZIF-7, and ZIF- 8; the CE structure may include a compound selected from the group consisting of 12-crown-4, 15-crown-5, 18-crown-6, 21-crown-7, 24-crown-8, 27-crown-9, and 30-crown- 10. In a preferred embodiment, the crown ether metal organic framework (CE-20X) composition is stable in aqueous media and has an average channel diameter of about 1 angstrom to about 10 angstroms.

[0007] Preferably, the MOF structure of the crown ether metal organic framework (CE-20X) composition comprises a plurality of carboxylic groups. In one aspect of the present disclosure, the crown ether metal organic framework (CE-20X) composition includes a MOF structure selected from the group consisting of UiO-66-IPA, UiO-66-COOH, UiO-66-(COOH)2, UiO-66- NFh, U1O-66-NO2, MIF-121, ZJU-24, NU-125-IPA, and NU-125-HBTC. In another aspect, the CE-20X structure has one or more missing-linkers selected from the group consisting of uncoordinated linkers and dangling linkers. In a particularly preferred aspect, the CE-20X structure includes uncoordinated BDC (benzene-dicarboxylate) linkers. Preferably, the CE-20X MOF structures are selected from UiO-66-COOH and UiO-66-(COOH)2.

[0008] In one aspect of the present disclosure, the crown ether metal organic framework and/or CE-20X composition includes a crown ether selected from the group of consisting 2- aminomethyl- 15 -crown-5, 2-aminomethyl-18-crown-6, 4'-aminobenzo-15-crown-5 and 4'- aminobenzo- 18 -crown-6. In another aspect, the crown ether structure includes a structure selected from the group consisting of aminobenzo- or aminomethyl-X-crown-Y wherein X is one of 12, 15, or 18 and Y is one of 4, 5, or 6. In another case, the crown ether includes a structure selected from the group consisting of 4'-Aminobenzo-12-crown-4, 4'-Aminobenzo-15- crown-5, 4'-Aminobenzo-18-crown-6, 4'-Amino-5'-nitrobenzo-15-crown-5, 4'-Aminodibenzo- 18-crown-6, 2-Aminomethyl- 15 -crown-5, and 2-Aminomethyl-18-crown-6.

[0009] The present disclosure also provides crown ether metal organic framework (CE-20X) compositions wherein the MOF channel comprises an ion forming moiety capable of providing ion transport charge selectivity and/or ion binding affinity selectivity in polar solution. In certain embodiments, the ion forming moiety is selected from the group consisting of -COOH and -SO3H. [00010] Further, the present disclosure provides crown ether metal organic framework (CE-20X) compositions wherein the compositions are capable of selectively transporting one or more ions though the pore channel in a polar solvent upon application of a bias voltage from about 10 mV up to about 2 V. In one case, the crown ether metal organic framework composition exhibits, upon application of a voltage bias in a polar solvent, a higher selectivity for K+ than for Li+ in the range of a K+/Li+ selectivity from 10: 1 to 1000: 1. In another aspect, the voltage bias is in the range of from about 0.05 V to about 0.5 V. The present disclosure also provides crown ether metal organic framework compositions which exhibit, upon application of a voltage bias in a polar solvent, a higher selectivity for Na+ than for Li+ in the range of a Na+/Li+ selectivity from 10: 1 to 1000: 1. In such cases the voltage bias may be, for example, in the range of from about 0.5 V to about 2 V.

[00011] The present disclosure also provides ion separation or transport membranes including crown ether modified metal organic framework CE-20X structures, comprising first and second surfaces, and one or more ion transport channels between respective pore windows in the first and second surfaces, wherein the ion transport channels contain one or more crown ethers. In some embodiments, the ion separation or transfer membranes are ion selective separation membranes specific for certain ions such as, but not limited to Li ions in solution.

[00012] In one aspect of the ion separation membranes, the respective pore windows have a diameter that is less than the hydrated diameter of an ion for which the ion separation membrane transports and/or is selective. In another aspect, the crown ether has a crown portion that has a size that is less than the hydrated diameter of an ion for which the ion separation membrane transports and/or is selective. The pore windows typically have a pore size of less than 1 nm, but any suitable pore size may be selected given the intended end-use application and the teachings herein.

[00013] In another aspect, the ion separation or transport membrane is based on a ZIF-8 or UiO- 66 type MOF. In yet another aspect, the crown ether includes a structure selected from the group consisting of 12-crown-4, 15-crown-5, 18-crown-6, 21-crown-7, 24-crown-8, 27-crown-9, 30- crown- 10, and mixtures thereof.

[00014] The ion separation or transport membrane also may comprise a substrate layer, wherein the crown ether modified metal organic framework structure is disposed on, in, and/or around the substrate layer. The substrate layer may be, for example, a porous substrate having one or more channels, wherein the crown ether modified metal organic framework structure is disposed within said one or more of channels. The channels may have an average cross-sectional diameter of, for example, from about 5 nm to about 200 nm. In another aspect, the channels taper from a first diameter at the first surface to a second diameter at the second surface, wherein the first diameter and the second diameter are different. The channels may have, for example, a bullet like shape. The porous substrate layer may comprise, for example, a material selected from the group consisting of a porous metal layer, a porous ceramic layer, and a porous polymer layer.

[00015] In another aspect, the ion separation or transport membrane may comprise a nonporous substrate having a crown ether modified metal organic framework (CEMOF) structure disposed on or within said substrate. For example, the substrate layer may comprise a nonporous substrate and the crown ether modified metal organic framework (CEMOF) structure being in the form of CEMOF particles. The particles may have a size in the range of, for example, 1 nm to 500 nm. The crown ether modified MOF particles are typically present in the membrane in a range of about 20 to about 60 percent by weight.

[00016] The substrate layer may comprise, for example, a polymer selected from the group consisting of polyethylene terephthalate (PET), polycarbonates, and polyimides. The substrate layer may also be an essentially 2D layered material, for example, graphene and/or graphene oxide, zeolite, M0S2, WS2, and BN.

[00017] The ion separation or transport membrane may further include a support layer. The crown ether modified metal organic framework may comprise a layer or a component of a layer formed on at least one surface of the support layer. The support layer may include, for example, a porous ceramic, porous metal, porous paper, or porous polymer.

[00018] In another aspect, the ion separation or transport membrane is a voltage tunable membrane, wherein the ion separation or transport efficiency and/or ion selectivity is controllable with applied trans-membrane voltage. The membrane may further include first and second electrodes to apply a trans-membrane voltage bias.

[00019] In yet another aspect, the present disclosure provides an ion separation module for an ion separation membrane system, the membrane module including an ion separation membrane comprising a crown ether modified metal organic framework structure comprising first and second surfaces, and ion transport channels between respective pore windows in the first and second surfaces; wherein the ion transport pore channels include one or more crown ethers. [00020] In yet a further aspect, the present disclosure provides a method for transporting, conducting, or separating ions in a liquid media, the method including: exposing a liquid media containing ions to a first surface of a membrane; and applying a trans-membrane bias voltage to transport ions into a pore window in the first surface, through the ion transport channel, and out of a pore window in a second surface; wherein the membrane is an ion separation or transport membrane according to any of the aspects of the present disclosure. The method incudes, for example, a method for selectively separating ions; wherein the liquid media contains at least first and second ions, and the step of applying a trans-membrane bias voltage selectively transports the first ions preferentially with respect to the second ions. The trans-membrane bias voltage may be, for example, from about 10 mV up to about 2 V.

[00021] A still further aspect provides methods of forming ion separation or transport membranes according to any of the aspects disclosed herein, the methods comprising: coupling crown ethers to at least internal surfaces of an ion transport channel between respective pore windows in first and second surfaces of a metal organic framework structure.

[00022] These and other embodiments are illustrated herein below and described in the appended claims.

Brief Description of Figures

[00023] Figure 1 is a schematic illustrating a fabrication process for UiO-66-(COOH)₂-15CE5 by facilitated interfacial growth in sub-nanochannels (SNC).

[00024] Figure 2(a), Figure 2(c), and Figure 2(d) are SEM images of tip, cross-section, and base side of a bullet-shaped PET nanochannel (NC) respectively.

[00025] Figure 2(b) is a schematic of the cross section of the bullet-shaped nanochannel.

[00026] Figures 2(e) and Figure 2(f) are size distribution graphs for the tip diameter and base diameter of the nanochannel respectively.

[00027] Figure 3(a) is a pH responsive I-V curves of PETNC for 0.1 M KC1 at pH values of 2.0, 5.7, and 8.0.

[00028] Figure 3(b) is a pH responsive I-V curves of PET@EDA NC for 0.1 M KC1 at pH values of 2.0, 5.7, and 8.0. [00029] Figure 4(a), Figure 4(b), and Figure 4(c) are SEM images of (a) tip side, (b) base side and (c) whole cross section of a MOFSNC.

[00030] Figure 5 is a graph showing PXRD patterns of a multi-PET NC membrane before and after interfacial growth of UiO-66-(COOH)2.

[00031] Figure 6(a) is a graph showing FTIR results for (i) MOF (UiO-66-(COOH)2 crystals), (ii) crown ether (CE) (raw 15-crown-5 ether powder), and (iii) MOF-CE (UiO-66-(COOH)2 - 15CE5 crystals).

[00032] Figure 6(b) is a graph showing C¹³-NMR results for (i) MOF (UiO-66-(COOH)2 crystals), (ii) CE (raw 15-crown-5 ether powder), and (iii) MOF-CE (UiO-66-(COOH)2 -15CE5 crystals).

[00033] Figure 7(a), Figure 7(b), Figure 7(c), and Figure 7(d) are I-V curves of: (a) PETNC, (b) PET@EDANC, (c) PET@EDA-UiO-66-(COOH)₂ SNC, and (d) UiO-66-(COOH)₂-15CE5 SNC for 0.1 M LiCl, NaCl and KC1.

[00034] Figure 8(a) is an I-V curve of UiO-66-(COOH)₂-15CE5 SNC for (i) 0.5 M KC1, (ii) 1.0 M KC1, (iii) 0.5 M NaCl, (iv) 1.0 M NaCl, and (v) 0.5 M KC1 + 0.5 M NaCl.

[00035] Figure 8(b) is a schematic illustrating different facilitated transport mechanisms for K⁺ and Na⁺ in single component systems.

[00036] Figure 8(c) is a graph showing K⁺ selectivity of UiO-66-(COOH)₂-15CE5 at low trans membrane voltage of 0.1 V over Na⁺ and Li⁺.

[00037] Figure 8(d) is a graph showing Na⁺ selectivity of UiO-66-(COOH)2-15CE5 at higher trans-membrane voltage bias of 1 V over K⁺ and Li⁺.

[00038] Figure 9 is a schematic illustration of competitive facilitated transport mechanisms between Na⁺ and K⁺ of UiO-66-(COOH)2-15CE-5 in a mixed ion system.

[00039] Figure 10 is a graph showing the ideal ion selectivity of UiO-66-(COOH)2-15CE5 based on single component ion permeation experiments.

[00040] Figure 11A - Figure HE are schematics illustrating a fabrication process for a ZIF- 8/GO/AAO membrane: (A) Schematic of AAO support. (B) Spin-coating of hybrid ZIF-8/GO nanosheets onto the AAO support to form a uniform and ultrathin seeding layer. (C) Air plasma treatment of the ZIF-8/GO nanosheets to obtain the nanoporous seeding layer. (D) Secondary growth of the nanoporous seeding layer by contra-diffusion method. (E) Schematic of the ZIF- 8/GO membrane of the AAO support.

[00041] Figure 12(A) is a schematic illustration of ion transport through a ZIF-8/GO/AAO membrane with ~3.4 A pore windows for ion selectivity and ~11.6 A pore cavities for fast ion transport (drawing not to scale). The inset indicates the crystal structure of ZIF- 8.

[00042] Figure 12(B) is a scanning electron microscopy (SEM) image of the hybrid ZIF-8/GO nanosheet seeds coated on the AAO support.

[00043] Figure 12(C) is an SEM image of the plasma-treated nanoporous ZIF-8/GO seeds.

[00044] Figure 12(D) is an SEM image of the ZIF-8/GO/AAO membrane surface.

[00045] Figure 12(E) is an SEM image of the membrane cross section reveal that a ~446-nm- thick ZIF-8/GO layer is densely grown on the top of the AAO support.

[00046] Figure 12(F) illustrates XRD patterns of the AAO support, the seeding layer, the plasma-treated seeding layer, the ZIF-8/GO/AAO membrane, and simulated ZIF-8 structure.

[00047] Figure 13 is a schematic illustrating the fabrication of aZIF-8/PET single- nanochannel membrane by an interfacial growth method.

[00048] Figure 14 is a schematic illustrating the fabrication of a ZIF-7/PET single-nanochannel membrane by an interfacial growth method with ~2.9 A pore windows.

[00049] Figure 15 is a schematic illustrating the fabrication of a UiO-66/PET single-nanochannel membrane with ~6.0 A pore windows.

[00050] Figure 16 is a schematic of linker defects in an octahedral pore of UiO-66 showing, from left to right, a defect-free structure; missing linker; a dangling linker with coordinatively unsaturated transversal CUS; and a dangling linker with coordinated modulator on transversal cus.

Detailed Description of Preferred Embodiments

[00051] The present disclosure provides metal organic framework (MOF) compounds, structures and membranes including ion transport pore channels, wherein the ion transport pore channels comprise one or more crown ethers disposed therein. The inventors have found that the presence of a crown ether within the transport pore channels may enhance the ion transport, conduction, or selectivity of the MOF or membrane in comparison with a similar MOF or membrane absent the crown ether.

[00052] This enhanced ion transport efficiency and/or selectivity is a surprising technical effect since, to the inventors’ knowledge, crown ethers have not previously been used in the context of ion selective or transport MOF membranes, and while crown ethers have been used to functionalize polymers, this has ostensibly been to form ion adsorbing polymer resins (which must then be treated with a stripping solution to release adsorbed ions) and not to form ion selective or transport membranes comprising CEMOFs which permit the control or passage (conduction) of certain ions or ions for which they are selective. In certain embodiments, the ability to control or enhance the conduction or transport of ions is desirable apart from any specific or potential ion selectivity. In other embodiments, ion selectivity may be enhanced by CEMOF structures capturing or adsorbing certain ions within the CEMOF, as discussed herein below.

[00053] The present disclosure thus provides CE functionalized MOFs, CE-20X compounds, ion selective membranes, systems, modules, methods of fabrication, ion separation methods, and other features wherein the membrane is or comprises a crown ether functionalized MOF structure which is capable of providing ion selectivity or transport across the membrane when contacted with a liquid media, such as an aqueous or other liquid or solution containing one or more ions. By “functionalized” herein we mean that the crown ether component, for example, the “crown” or “ring” portion of the crown ether structure, is directly or indirectly coupled such as chemically bound (e.g., covalently or by hydrogen bonds, etc.) to the internal surface of the MOF’s ion transport channel or pore window in a configuration that restricts the size or binding activity of the MOF’s ion transport channel or pore window compared to the MOF’s channel or pore window without the crown ether. In certain embodiments, multiple (such as three or more crown ethers) are stacked axially (such as vertically, see, e.g., Figure 9) within the MOF’s pore or ion transport channel, thus modifying the capture or transfer of one or more ions in or through the channel and/or effectively restricting the MOF pore size. In preferred aspects the ion separation or transfer membrane comprises a metal organic framework structure comprising first and second surfaces, and ion transport pore channels formed between respective pore windows in the first and second surfaces, wherein the ion transport pore channels include and/or are functionalized with one or more crown ethers. [00054] Ion selective and/or transport membrane modules also are provided herein for use in ion separation membrane systems, the membrane modules comprising an ion separation membrane comprising a metal organic framework structure comprising first and second surfaces, and ion transport channels formed between respective pore windows in the first and second surfaces, wherein the ion transport pore channels contain and/or are functionalized with one or more crown ethers.

[00055] By “ion selective” it is meant that the separation membrane is operable to more freely permit the passage of the ion(s) for which the ion selective membrane is/are selective. By way of example, certain ion selective membranes herein are selective for Na+, in which case the ion selective membrane is operable to provide a higher transport ratio of Na+ relative to one or more other ions (e.g. such as K+ or Li+). In preferred forms of the present disclosure, the ion selective separation membrane is selective for an ion selected from the group consisting of: Na+, K+, and Li+, for example, over other ions of the afore mentioned group. Thus, the present disclosure provides significant advantages in efficient resource recovery methods, specifically including lithium extraction, from raw materials or waste streams for use in batteries and other industrial and consumer products.

[00056] The transport pore channels of the membranes herein typically have a diameter that is less than the fully hydrated diameter of one or more ions for which the membrane is not selective. By way of example, the fully hydrated diameter for Li+ (7.64 A) > Na+ (7.16 A) >

K+ (6.62 A), as such a pore channel or size 7 A provides size selectivity for fully hydrated K+ over Li+ and Na+. In various aspects of the present disclosure, the respective pore windows have a diameter that is less than the fully hydrated diameter for which the ion selective separation membrane transports and/or is selective. In this way, the pore windows provide ion selectivity for a partially or fully dehydrated form of the ion for which the membrane is selective. In some embodiments, ions that are not transported may be captured within the MOF structures. The present disclosure is not limited to any specific mode of CEMOF activity or mechanism of action as long as ion selectivity and/or transport is achieved.

[00057] The crown ether structure may have a crown portion that has a size that is less than the fully hydrated diameter for which the ion separation membrane transports and/or is selective. In this way, the crown ether provides size selective transport of these ions in situations where they have been partially or fully dehydrated to a diameter less than their fully hydrated diameter. Selectivity may thus be affected by controlling dehydration of ions, such as by applying bias voltage across the membrane. In such cases, ions that have been fully or partially dehydrated can pass either through the crown portion of the crown either (if partially or fully dehydrated to a size that is less than the size of the crown portion) or can be transported via exchange between adjacent crown ethers, for example, under an applied voltage.

[00058] In one or more aspects, the ion pore channels may further include an ion forming moiety to further provide charge selectivity and/or binding affinity selectivity. For example, a non limiting list of ion forming moieties, includes, such as, -COOH and -SO₃H. The ion forming moiety may enhance ion selectivity through electrostatic charge (e.g., negatively charged nanochannels generally have metal ion selectivity due to charge selectivity, for example electron attraction of a negatively charged nanochannel with positively charged metal ions). The ion forming moiety may in one or more forms enhance ion selectivity through binding affinity. For example, for -COOH the binding affinity for alkali metals is Li > Na > K, which provides a transport selectivity of K > Na > Li; for -SO₃H the binding affinity for alkali metals is K > Na > Li, which provides a transport selectivity of Li > Na > K. The combination of size selectivity, charge selectivity, and/or binding affinity selectivity of the MOF structure with the size selectivity of the crown portion can provide an ion separation structure or membrane with enhanced ion selectivity.

[00059] A wide range of metal organic framework materials may be used. Any suitable metal organic framework can be modified via CE functionalization given the teachings herein. The MOFs are generally compounds comprising metal nodes connected to organic ligands forming one-, two-, or three- dimensional structures, typically having voids or porous structures. However, the choice of MOF is dependent on the intended application, for example, the MOF should be stable in the liquid media or solvent in which it is to be used. Furthermore, for ion separation processes it is preferred that the MOF have pore windows of less than 1 nm in size. Thus, in certain embodiments of the present disclosure, the MOF is a solvent stable MOF with sub-nanometer pore windows (e.g., pore windows having a pore size of less than 1 nm). Preferably, the MOF is a ZIF-8 or UiO-66 type MOF or a derivative thereof.

[00060] In a specific embodiment the pore size of the pore windows is from about 2.8 A and less than 1 nm. In one example of this embodiment, the pore size is up to about 6.5 A, more preferably up to about 6.4 A, and most preferably up to about 6.3 A. In another example of this embodiment, the pore size is up to about 4 A, preferably about 3.8 A, and more preferably about 3.6 A. In other terms, the average pore diameter is, for example, from 1 to 10, 2.8 to 6.5, 3.8 to 6.4, 4 to 6.3, or 3.6 to 6.3 angstroms in diameter, or any numerical range subsumed within these ranges.

[00061] A wide range of different crown ether compounds and/or structures may be used. Thus, the crown ether component includes, but is not limited to, for example, a crown structure selected from the group consisting of: 12-crown-4, 15-crown-5, 18-crown-6, 21-crown-7, 24- crown-8, 27-crown-9, 30-crown-10, and mixtures thereof. Preferred crown ethers include 2- hydroxymethyl-12-crown-4, 2-hydroxymethyl-15-crown-5, 2-hydroxymethyl- 18-crown-6, 2- aminomethyl- 15 -crown-5, 2-aminomethyl-18-crown-6, 4'-aminobenzo-15-crown-5 and 4'- aminobenzo-18-crown-6. Most preferably, the crown ethers include 2-aminomethyl-15-crown-5, 2-aminomethyl-18-crown-6, 4'-aminobenzo-15-crown-5, and 4'-aminobenzo-18-crown-6.

[00062] A variety of different CEMOF compounds, membranes, membrane systems, and/or membrane modules are contemplated by the present disclosure. By way of example, the membranes, membrane systems, and/or membrane modules may be plate -and-frame membranes, tubular membranes, spiral-wound membranes, or hollow-fiber membranes. The metal organic framework structure may be disposed or formed on, in, and/or around a substrate layer, or may be self-supporting thus providing a CEMOF layer or membrane structure with or without any additional substrate or support. In other embodiments, the CEMOFs are formed or otherwise disposed in pores or nanochannels in a substrate, such as a polymer matrix.

[00063] For example, the substrate layer may be a porous substrate having a plurality of channels, and the metal organic framework component or layer may comprise a plug of metal organic framework material within the plurality of channels. Such channels may be nanochannels having a size, for example, of from about 5 nm up to about 200 nm (e.g., from 5 nm to 100 nm, from 100 nm to 200 nm, from 5 nm to 50 nm, from 50 nm to 100 nm, from 100 nm to 150 nm, from 150 nm to 200 nm, from 5 nm to 190 nm, from 10 nm to 200 nm, or from 10 nm to 190 nm). For example, the channels can have a size of 5 nm or more (e.g., 10 nm or more, 15 nm or more, 20 nm or more, 25 nm or more, 30 nm or more, 35 nm or more, 40 nm or more, 45 nm or more, 50 nm or more, 60 nm or more, 70 nm or more, 80 nm or more, 90 nm or more, 100 nm or more, 110 nm or more, 120 nm or more, 130 nm or more, 140 nm or more, 150 nm or more, 160 nm or more, 170 nm or more, 180 nm or more, or 190 nm or more). In some examples, the channel can have a size of 200 nm or less (e.g., 190 nm or less, 180 nm or less,

170 nm or less, 160 nm or less, 150 nm or less, 140 nm or less, 130 nm or less, 120 nm or less,

110 nm or less, 100 nm or less, 90 nm or less, 80 nm or less, 70 nm or less, 60 nm or less, 50 nm or less, 45 nm or less, 40 nm or less, 35 nm or less, 30 nm or less, 25 nm or less, 20 nm or less,

15 nm or less, or 10 nm or less). Such channels also may taper from a first diameter at the first surface to a second diameter at the second surface, wherein the first diameter and the second diameter are different. Preferably, the second diameter is less than the first diameter. More preferably, the second diameter is the narrow-most diameter of the channel. By way of example, the channel may exhibit a ‘bullet-like’ shape.

[00064] Suitable porous substrate layers include, for example, a porous metal layer (non limiting examples of which includes porous stainless steel), a porous ceramic layer (non-limiting examples of which includes porous alumina, titania, etc.), and a porous polymer layer (non limiting examples of which includes porous polyethylene terephthalate (PET), polycarbonate, polyimide (PI), etc.). It will be appreciated that the porous substrate layer may include other additives (e.g., porous or non-porous metals, ceramics, polymers, organic compounds, or inorganic compounds which additives may be in the form of nanoparticles).

[00065] An alternative substrate layer is an essentially 2D layered material. Preferably, the essentially 2D layered material is selected from the group consisting of: graphene and/or graphene oxide, zeolite, M0S2, WS2, and BN. Most preferably, the essentially 2D layered material is graphene oxide.

[00066] The ion separation or transport membrane may further include a support layer. In one form of this embodiment, the metal organic framework is a layer on at least one surface of the support layer. In another form of this embodiment in which the ion separation or transport membrane comprises a porous substrate layer, the porous substrate layer is on at least one surface of the support layer. Preferably, the support layer is a porous ceramic, porous metal, porous paper, or porous polymer. An example of a suitable support layer is an anodic aluminum oxide layer.

[00067] In an embodiment, the membrane is a voltage tunable membrane, e.g., the ion separation efficiency and/or selectivity of the ion separation membrane is capable of being adjusted and controlled by applying, or varying an applied, trans-membrane voltage. In one form of this embodiment, the ion separation membrane has a selectivity for one or more of K+, Na+, or Li+ and the selectivity is tunable with applied trans-membrane voltage. By way of example, at a first voltage the ion selective membrane is operable as a K+/Na+ selective membrane, whereas at a second voltage the ion selective is operable as a Na+/K+ selective membrane. In another example, at a first voltage the ion selective membrane is operable as a K+/Na+ selective membrane, whereas at a second voltage the ion selective membrane is operable as a Li+/Na+ selective membrane. In an embodiment, the ion selective separation membrane further includes first and second electrodes to apply a trans-membrane voltage bias.

[00068] In an embodiment, the ion selective membrane provides, or is operable under an applied trans-membrane voltage to provide, a higher selectivity for K+ than for Li+. Preferably, the K+/Li+ selectivity is at least 10: 1 (e.g., at least 25:1, at least 50: 1, at least 75:1, at least 100: 1, at least 125:1, at least 150:1, at least 175:1, at least 200:1, at least 250:1, at least 300:1, at least 350:1, at least 400:1, at least 450:1, at least 500:1, at least 600:1, at least 700:1, at least 800:1, at least 900:1, at least 1000:1, at least 1100:1, at least 1200:1, at least 1300:1, at least 1400:1, or at least 1500: 1). More preferably, the K+/Li+ selectivity is at least 100:1. Even more preferably, the K+/Li+ selectivity is at least 1000: 1. In a particular form of this embodiment, the trans membrane voltage bias is in the range of from about 0.05 V to about 0.5 V. Preferably, the trans membrane voltage bias is about 0.1 V. In one embodiment the ion selective membrane provides or is operable under an applied trans-membrane voltage to provide a higher selectivity for Na+ than for Li+. Preferably, the Na+/Li+ selectivity is at least 10:1 (e.g., at least 25: 1, at least 50:1, at least 75:1, at least 100:1, at least 125:1, at least 150:1, at least 175:1, at least 200:1, at least 250:1, at least 300:1, at least 350:1, at least 400:1, at least 450:1, at least 500:1, at least 600:1, at least 700: 1, at least 800: 1, at least 900: 1, at least 1000: 1, at least 1100: 1, at least 1200: 1, at least 1300: 1, at least 1400: 1, or at least 1500: 1). More preferably, the Na+/Li+ selectivity is at least 100: 1. Even more preferably, the Na+/Li+ selectivity is at least 1000: 1. In a particular form of this embodiment, the trans-membrane voltage bias is in the range of from about 0.5 V to about 2 V. Preferably, the trans-membrane voltage bias is about 1.0 V.

[00069] The ion separation membranes according to the present disclosure may be used in methods for separating ions in a liquid media, such as a polar solution, the methods including: providing an ion separation membrane according to the present disclosure; exposing a liquid media containing ions to the first surface; and optionally applying a trans-membrane bias voltage to transport ions into a pore window in the first surface, through the ion transport channel, and out of a pore window in the second surface. Where the method is a method for selectively separating ions, the step of applying a trans-membrane bias voltage selectively transports the selected ions. The inventors have found that in embodiments application of a voltage potential difference across the ion separation membrane can enhance the passage of ions through the ion separation membrane from the first surface to the second surface, and can provide for the selective transport of ions. Furthermore, the inventors have also found that the presence of crown ethers allows ion selectivity to be tunable with voltage. Thus, in some embodiments the ion separation membrane is a voltage tunable ion separation membrane. The trans-membrane bias voltage may be, for example, about 10 mV up to about 10 V. Preferably, the trans-membrane bias voltage is from about 30 mV. More preferably, the trans-membrane bias voltage is from about 50 mV. Most preferably, the trans-membrane bias voltage is from about 80 mV. Alternatively, or additionally, the trans-membrane bias voltage is up to 8 V. Preferably, the trans-membrane bias voltage is up to 6 V. More preferably, the trans-membrane bias voltage is up to 4 V. Most preferably, the trans-membrane bias voltage is up to 2 V. By way of example, the trans-membrane bias voltage may be a value of from about 0.1 V up to about 1 V. However, applied voltage or bias is not necessary in all transport or separation embodiments. The CEMOFs and membranes herein can function under various passive or active mechanisms of ion conduction control, for example, but not limited to, diffusion, pressure, and other modes of ion or liquid transport or capture.

[00070] The liquid media will typically include a polar solvent or solution, for example, but not limited to, water, methanol, ethanol, isopropyl alcohol, n-butanol, formic acid, acetic acid, dichloromethane, tetrahydrofuran, ethyl acetate, acetonitrile, dimethyl sulfoxide, acetone, hexamethylphosphoric triamide, dimethylformamide, nitromethane, propylene carbonate, or mixtures thereof. However, it is preferred that the liquid media is aqueous and that the polar component is water.

[00071] The ion separation and/or transport membrane may be formed by coupling crown ethers to at least internal surfaces of an ion separation or transport channel formed between respective pore windows in first and second surfaces of a metal organic framework structure. Crown ether (CE) functionalization of MOFs may be carried out to make CE-20X compounds by any suitable method, for example, by grafting crown ethers into MOF channels via a condensation reaction of an amino group of the CE and a carboxylic group of the MOF, mediated by EDC/NHSS, to anchor the crown ether molecules into the MOF nanochannels. CE functionalization may be carried out on MOFs prior to incorporation into the membrane substrate or the MOFs may be incorporated or applied to the substrate and subsequently functionalized.

[00072] The metal organic framework can comprise any suitable metal organic framework. Examples of metal organic frameworks include, but are not limited to, UiO-66, ZIF, HKUST-1, derivatives thereof, and combinations thereof. In some examples, the metal organic framework (CE-20X) comprises ZIF-8, ZIF-7, derivatives thereof, or combinations thereof. In some examples, the metal organic framework comprises UiO-66, derivatives thereof, or combinations thereof. The metal organic-framework can, for example, be selected from the group consisting of UiO-66, UiO-66-(COOH)2, UiO-66-NH2, Ui0-66-S03H, UiO-66-Br, and combinations thereof.

[00073] Preferred CE-20X MOFs are relatively stable in water, typically have microporous pore size below 10A, and may have free carboxylic groups throughout the MOF channel. The MOFs most preferably have free carboxylic groups in every one or two linker units: such as UiO-66- IPA, UiO-66-COOH, UiO-66-(COOH)₂, MIF-121, ZJU-24, NU-125-IPA, and NU-125-HBTC.

[00074] The CE-20X MOFs can include missing-linker defects. In one aspect, one or more BDC (benzene -dicarboxy late) linkers present in the MOF only have one end coordinated with a metal-oxide, while the other end is uncoordinated, but having free carboxyl groups (see Figure 16). These MOFs include, for example, missing-linker UiO-66, UiO-66-NH2 and U1O-66-NO2. Of these types, UiO-66-COOH and UiO-66-(COOH)2 are most preferable for some applications.

[00075] While any suitable crown ether may be used given the teachings herein, currently preferred crown ethers for CE-20X compositions are rich in amino groups, such as aminobenzo- or aminomethyl-X-crown-Y, (wherein X=12, 15, 18; Y=4, 5, 6). Suitable crown ethers also include, but are not limited to, for example, 12-crown-4, 15-crown-5, 18-crown-6, 4'- Aminobenzo-12-crown-4, 4'-Aminobenzo-15-crown-5, 4'-Aminobenzo-18-crown-6, 4'-Amino- 5 '-nitrobenzo- 15 -crown-5, 4'-Aminodibenzo-18-crown-6, 2-Aminomethyl- 15 -crown-5, and 2- Aminomethyl- 18-crown-6.

[00076] The substrate, if any, may comprise any suitable material, and may be porous or non- porous (in which event permeability is provided essentially only via the CEMOFs incorporated in or on the substrate), and may be selected from a metal layer (non-limiting examples of which include porous stainless steel), a ceramic layer (non-limiting examples of which include alumina, titania, etc.), and a polymer layer (non-limiting examples of which include polyethylene terephthalate (PET), polycarbonate, polyimide (PI), polysulfone, poly(amideimide), polybenzimidazole, polyethersulfone, polyphenylsulfone, polyacrylonitile, poly (ethylene oxide), poly(ether ether ketone), poly(vinylidene fluoride), polyethylene chlorotrifluoroethylene), polycarbonate, polystyrene, poly(ether-block-amide), acrylonitrile butadiene styrene, and derivatives and combinations of the aforementioned). The polymer can similarly comprise bisphonenolsulfone (BPS) polymers and derivatives thereof. The polymer can also include sulfonated polymer derivates, for example, sulfonated poly(ether ether ketone), sulfonated polysulfone, and sulfonated poly (ether sulfone). It will be appreciated that the substrate layer may include other additives (e.g., porous or non-porous metals, ceramics, polymers, organic compounds, or inorganic compounds which additives may be in the form of nanoparticles) and may also be an essentially 2D layered material, such as graphene and/or graphene oxide, zeolite, M0S2, WS2, and BN.

[00077] Examples of suitable substrates further include, but are not limited to, other polymers (e.g., porous polymers), glass fibers, glass, quartz, silicon, non-woven fibers, ceramic, metal and combinations thereof.

[00078] The MOF or CEMOF materials may be incorporated into the membrane substrate, if any, in any suitable amount for the desired application. For most applications, the CEMOF mass loading is high enough to provide one or more CEMOF ion transport channels from one surface or side of the membrane, for example, an ion separation membrane layer or module, to the opposite side or surface of the membrane or module. Suitable loadings include, but are not limited to, 10% or more by weight of the CEMOF material or particles relative to the membrane substrate (e.g., 15% or more, 20% or more, 25% or more, 30% or more, 35% or more, 40% or more, 45% or more, 50% or more, 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, or 80%, or 90% or more). In some examples, the ion separation membrane can comprise 95% or less by weight of the CEMOF material relative to the membrane substrate (e.g., 85% or less, 80% or less, 75% or less, 70% or less, 65% or less, 60% or less, 55% or less, 50% or less, 45% or less, 40% or less, 35% or less, 30% or less, 25% or less, or 20% or less). The average weight loading of the CEMOFs in the membrane can range from any of the minimum values described above to any of the maximum values described above. For example, the membrane can comprise from 10% to 90% by weight of the CEMOF component relative to the membrane substrate (e.g., from 10% to 55%, from 55% to 90%, from 10% to 40%, from 40% to 60%, from 60% to 90%, from 10% to 80%, from 20% to 90%, from 30% to 90%, from 50% to 90%, from 60% to 90%, from 20% to 60%, or from 20% to 40%). In some examples, the CEMOF component can be distributed substantially homogeneously throughout the membrane substrate and/or distributed in percent mass loadings sufficient to form a percolating network of CEMOF material to form ion transport channels across the membrane. [00079] The weight loading of the CEMOF component, such as in the form of a plurality of metal organic framework particles in the membrane, can be selected in view of a variety of factors. For example, an average weight loading for the CEMOF components can be selected in view of the desired mechanical and transport properties of the membrane. For example, the loading of the plurality of metal organic framework particles in the membrane can be inversely related to the mechanical properties and directly related to the transport properties. As the weight loading of the metal organic framework particles in the membrane is increased, the mechanical properties of the membrane may be reduced, i.e., the membranes can become more brittle and likely to crack under stress. Conversely, the transport properties of the membrane can improve as the MOF weight loading in the membrane increases. The average weight loading of the CEMOF component in the membrane can be selected in view of this tradeoff between decreasing mechanical properties (e.g., increasing brittleness) and increasing transport properties as the weight loading increases.

[00080] In some examples, the CEMOF component is in the form of a plurality of particles disposed or dispersed within or coated on a membrane or substrate, such as a nonporous polymer substrate, wherein the CEMOF component average particle size is of 1 micrometer (micron, pm) or less (e.g., 900 nm or less, 800 nm or less, 700 nm or less, 600 nm or less, 500 nm or less, 450 nm or less, 400 nm or less, 350 nm or less, 300 nm or less, 275 nm or less, 250 nm or less, 225 nm or less, 200 nm or less, 175 nm or less, 150 nm or less, 125 nm or less, 100 nm or less, 95 nm or less, 90 nm or less, 85 nm or less, 80 nm or less, 75 nm or less, 70 nm or less, 65 nm or less, 60 nm or less, 55 nm or less, 50 nm or less, 45 nm or less, 40 nm or less, 35 nm or less, 30 nm or less, 25 nm or less, 20 nm or less, 15 nm or less, 10 nm or less, 9 nm or less, 8 nm or less, 7 nm or less, 6 nm or less, or 5 nm or less).

[00081] The average particle size of the plurality of CEMOF particles can range from any of the minimum values described above to any of the maximum values described above. For example, the plurality of CEMOF particles can have an average particle size of from 1 nm to 1 pm (e.g., from 1 nm to 900 nm, from 1 nm to 800 nm, from 1 nm to 700 nm, from 1 nm to 600 nm, from 1 nm to 500 nm, from 1 nm to 400 nm, from 1 nm to 300 nm, from 1 nm to 200 nm, from 1 nm to 100 nm, from 5 nm to 100 nm, from 10 nm to 100 nm, from 25 nm to 100 nm, or from 50 nm to 100 nm).

[00082] The average particle size of the plurality of CEMOF particles can be selected in view of a variety of factors. For example, the average particle size can be selected based on the average thickness of the membrane, such that the average particle size of the particles is less than the average thickness of the membrane. In some examples, the average particle size of the plurality of metal organic framework particles can be less than the average thickness of the membrane by an order of magnitude. If the average particle size of the metal organic framework particle is on the same size order as the resulting ion separation membrane thickness, defects can sometimes be formed during casting of the fdms for example due to interactions with the casting substrate or due to the casting blade/technique. For example, the metal organic particles can interact either more favorably or less favorably with the substrate, causing the metal organic framework particles to separate from the polymer or agglomerate away from the casting substrate, respectively. For example, if a casting blade is used to deposit the polymer/metal organic framework/solvent system onto a substrate, then the average particle size of the metal organic framework particles needs to be less than the height at which the casting blade is set. Otherwise, the metal organic framework particles can contact the blade during casting and streak across the surface, causing macro-sized defects in the fdm. Furthermore, the average particle size of the metal organic framework particles can be selected in view of the desired mechanical properties of the ion transport or ion separation mixed matric membrane. Alternatively, the MOF component may be grown in SNCs formed in a substrate material, essentially forming MOF “plugs” in an otherwise porous or nonporous polymer. The MOF plugs are then CE functionalized, in some embodiments, to provide ion selectivity across the membrane.

[00083] The average thickness of the ion separation and/or transport membrane can be any suitable thickness for the desired application. For example, the membrane can have an average thickness of from 50 nm to 50 pm (e.g., from 100 nm to 50 pm, from 500 nm to 50 pm, from 500 nm to 20 pm, 1 pm to 30 pm, from 1 pm to 10 pm, from 500 nm to 10 pm, or from 500 nm to 5 pm). The average thickness of the membrane can be selected in view of a variety of factors. For example, the average thickness of the membrane can be selected in view of the average particle size of the plurality of metal organic framework properties, the average diameter of the SNCs/ plugs, the desired mechanical properties of the ion separation membrane, the desired ion transport or capture properties of the membrane, or combinations thereof.

[00084] Also provided herein are methods for use of any of the MOF compounds and membranes described herein. For example, the materials described herein can be used to separate a target ion from a non-target ion in a liquid medium (e.g., in an aqueous solution). In some examples, the materials described herein can be used for mineral separation, ion separations, water purification, energy conversion, or a combination thereof. In some examples, the materials described herein can be used for the transfer or selective removal of Li from a high salinity aqueous or other solution in a batch or continuous process.

[00085] Also provided herein are methods comprising separating a target ion from a non-target ion in a liquid medium using an ion separation membrane, wherein the membrane comprises a plurality of crown ether modified metal organic framework structures or particles formed or dispersed in a substrate, for example a porous or nonporous/continuous polymer phase.

[00086] Also disclosed herein are systems comprising any of the membranes disclosed herein and liquid medium comprising the target ion and the non-target ion, such that the target ion and the non-target ion are solvated. Also disclosed herein are systems comprising any of the membranes disclosed herein and an aqueous solution comprising the target ion and the non target ion, such that the target ion and the non-target ion are hydrated. In some examples, the systems can further comprise an electrode and a voltage source, wherein the voltage source and electrode are configured to apply a potential bias to generate an electric field gradient that influences the flow and/or transport of the target ion through the CEMOF membrane, or capture of the target or other ions within the CEMOFs. If desired, a solvent or stripping solution may be applied to facilitate release of the captured ions, in either batch or continuous ion separation processes.

[00087] Aspects of the invention are described below in relation to exemplary embodiments which include examples that both illustrate the fabrication of currently preferred CEMOF compounds, ion selective separation membranes and provide corresponding characterization data and testing data. Having described currently preferred embodiments of compounds and ion selective and/or transport membranes, and having shown illustrative details of particular embodiments, it will be understood that the specific examples given below are employed in a descriptive sense only and are not for the purpose of limitation. Various modifications to the embodiments may be made without departing from the spirit and scope of the present invention which is limited only by the appended claims.

Example 1

[00088] This example reports methods for forming MOF membranes.

Fabrication of ultrathin ZIF-8 membrane on an anodic aluminum oxide (AAO) support [00089] Figure 11A-Figure HE provides a general illustration for the fabrication of ultrathin ZIF-8 membrane on an AAO support.

[00090] Hybrid two-dimensional graphene oxide (GO) nanosheets with ZIF-8 crystals were fabricated as seeds and assembled onto the AAO support by spin-coating to produce an ultrathin seeding layer (Figure 11A and Figure 11B).

[00091] Hybrid ZIF-8/GO nanosheets were prepared by mixing 6 ml methanol solution of 0.183 g Zn (Nq3)₂ 6H₂O, 10 ml methanol solution with 0.405 g 2-methylimidazole (Hmim), and 4 ml 1 mg ml-1 GO suspension in mixture of methanol -water (4: 1, v/v), and stirring the mixture for 3 hr. This led to the formation of ZIF-8/GO nanosheets precipitate. The precipitate was collected via centrifugation (8000 rpm for 5 min) and washed with methanol three times.

[00092] The hybrid ZIF-8/GO nanosheets were re-dispersed in methanol to form a stable colloid suspension with a concentration of 20 mg ml ¹. Then the suspension was spin-coated onto the AAO support to form ultrathin and uniform ZIF-8/GO seeding layer. The spin-coating process was performed for 30 s at 1000 rpm. After coating, the support with seeding layer was dried at 50 °C for 2 h and then coated again. Twice-coating was carried out to ensure formation of a uniform seeding layer on the AAO support. The coated support was dried overnight at 50 °C.

[00093] The ZIF-8/GO nanosheets were then subjected to plasma treatment with air plasma to form nanopores with the GO nanosheets to facilitate fast crystal intergrowth during membrane formation (Figure 11C). Plasma treatment was carried out using Harrick Plasma PDC-32G-2 with 18 watts of power (max) at 1 mbar pressure for 30 s.

[00094] Subsequently, ZIF-8/GO/AAO membranes were synthesized via a counter-diffusion method at room temperature (Figure 11D and Figure 11E). The ultrathin ZIF-8/GO membrane was prepared by secondarily growing the plasma-treated seeding layer on the AAO support via counter-diffusion method. Zn²⁺ and Hmim solution were prepared by dissolving Zn(N0₃)₂ 6H₂O (0.183 g) and Hmim (0.405 g) in 10 ml methanol. The coated AAO support with nanoporous seeding layer was mounted on a custom-made setup, where the Zn²⁺ and Hmim solutions were separated by the coated support, the seeding layer faced the Zn²⁺ side, and the AAO support was vertically aligned. During secondary growth, the nanoporous GO nanosheets act as a barrier between two different synthesis solutions, which self-limits crystal growth and to reduce defects during the counter-diffusion process. After reaction at room temperature for 3 h, the ZIF- 8/GO/AAO membrane was removed, rinsed with fresh methanol, and dried overnight at 50 °C. [00095] Figure 12A is a schematic illustration of ion transport through a ZIF-8/GO/AAO membrane with ~3.4 A pore windows for ion selectivity and ~11.6 A pore cavities for fast ion transport. The inset indicates the crystal structure of the ZIF-8. Figure 12B shows scanning electron microscopy (SEM) images of ZIF-8/GO hybrid nanosheets uniformly coated on the AAO support, a cross section of the seeding layer (see Figure 3C), nanoporous ZIF-8/GO nanosheets obtained by air plasma treatment (see Figure 12C), a ZIF-8/GO/AAO membrane after secondary growth (see Figure 12D), and the membrane cross section (see Figure 12E).

The average thickness of the ZIF-8/GO membrane on the AAO support was 446 ± 74 nm. XRD patterns confirmed that a highly crystalline ZIF-8 structure was formed in the ZIF-8/GO membrane after secondary growth (see Figure 12F).

[00096] Once formed, the pore channels of the ZIF-8/GO/AAO membrane is functionalized to include a crown ether moiety. Example 2 reports one method for post-functionalization of MOF membranes with a crown ether.

Fabrication of ZIF-7/PET, ZIF-8/PET, and UiO-X/PET membranes

[00097] This series of examples report the fabrication ofZIF-7, ZIF-8, and UiO-X within bullet shaped nanochannels etched into a layer of PET.

Preparation of bullet-shaped nanochannels in PET

[00098] Single bullet-shaped nanochannels were fabricated in 12 pm thick polyethylene terephthalate (PET) membranes (diameter of 30 mm) by adopting the method of surfactant- protected ion-track-etching method. One side of the membrane was etched by 6 M NaOH + 0.025% sodium dodecyl diphenyloxide disulfonate, while the other side was etched by 6 M NaOH at 60 °C. During the etching process, a constant voltage of 1.0 V was applied across the film. After etching for about 3 min, a 1 M KC1 + 1 M HCOOH solution that is able to neutralize the etchant was added into the containers on both sides of the membrane, thus slowing down and finally stopping the etching process, and single bullet-shaped nanochannels were produced in the PET membranes. The nanochannel membranes were then soaked in MilliQ water to remove residual salts. Average tip diameter of the bullet-shaped nanochannel is 33 ± 6 nm, while average base diameter is 239 ± 20 nm.

ZIF-8/PET membrane [00099] ZIF-8/PET membranes were fabricated by interfacially growing ZIF-8 crystals into the base regions of the bullet-shaped single -nanochannel PET membranes formed according to the method discussed above. The ZIF-8/PET membrane was prepared using an interfacial growth method in which the base side of the single nanochannel was fully fdled with the ZIF-8 material. A schematic illustrating this is provided in Figure 13.

[000100] The ZIF-8/PET membrane was formed using a counter-diffusion method. To form the membrane, the single-nanochannel PET membrane was mounted with the base side of the nanochannel membrane exposed to a Zn²⁺ solution (prepared by dissolving Zn(NC>3)2 6H2O (0.055 g) in 10 ml octanol) and the tip side of the nanochannel membrane exposed to a 2- methylimidazole (Hmim) solution (prepared by dissolving Hmim (1.125 g) in 10 ml water). The Zn²⁺ and Hmim solutions were separated by the membrane, and the single-nanochannel support was vertically aligned. After reacting at room temperature for 48 h, the ZIF-8/PET membrane was taken out and rinsed with fresh methanol, before being dried overnight at 25 °C.

[000101] Once formed, the pore channels of these membranes are functionalized to include a crown ether moiety. Example 2 reports one method for post-functionalization of MOF membranes with a crown ether.

ZIF-7/PET

[000102] ZIF-7/PET membranes were fabricated by interfacially growing ZIF-7 crystals into the base regions of the bullet-shaped single -nanochannel PET membranes formed according to the method discussed above.

[000103] The ZIF-7/PET membrane was prepared using an interfacial growth method in which the base side of the single nanochannel was fully filled with the ZIF-8 material. A schematic illustrating this is provided in Figure 14.

[000104] The ZIF-7/PET membrane was formed using a counter-diffusion method. To form the membrane, the single-nanochannel PET membrane was mounted with the base side of the nanochannel membrane exposed to a Zn²⁺ solution (prepared by dissolving Zn(NC>3)2 6H2O (0.1 g) in 10 ml DMF) and the tip side of the nanochannel membrane exposed to a benzimidazole (Bim) solution (prepared by dissolving Bim (0.256 g) in 10 ml DMF). The Zn²⁺ and Bim solutions were separated by the membrane, and the single-nanochannel support was vertically aligned. After reacting at room temperature for 24 h, the ZIF-7/PET membrane was taken out and rinsed with fresh methanol, before being dried overnight at 25 °C.

[000105] Once formed, the pore channels of these membranes are functionalized to include a crown ether moiety.

UiO-66-X/PET

[000106] UiO-66-X/PET membranes were fabricated by interfacially growing UiO-66-X crystals into the base regions of the bullet-shaped single -nanochannel PET. While the below methods relates to X being H, NEh, N⁺(CH3)3, or COOH, the skilled person will appreciate that X may be other chemical functional groups. For example, X may include a moiety that allows post treatment crown ether functionalization with a crown ether compound (such as through standard coupling reactions). Alternatively, X may include or be modified to include a crown ether moiety prior to the interfacial growth of the UiO-66-X MOF, in which case post treatment crown ether functionalization step is not required.

[000107] A UiO-66/PET membrane was prepared using an in-situ solvothermal synthesis method. A schematic illustrating this is provided in Figure 15. ZrCU and BDC were dissolved in 40 ml DMF under stirring to give a molar composition: Zr⁴⁺/BDC/DMF=1: 1 : 500. This clear solution was transferred into a Teflon-lined stainless steel autoclave in which a single nanochannel PET membrane was placed vertically. Afterwards the autoclave was placed in a convective oven and heated at 100 °C for 24 h. After cooling, each membrane was washed with methanol and dried overnight at 25 °C.

[000108] Once formed, the pore channels of the UiO-66/PET membrane are functionalized to include a crown ether moiety.

[000109] A uίO-όό-NEE/RET membrane was fabricated via the in-situ growth of UiO-66-NH2 crystals into 12-pm-thick single-nanochannel PET membranes using a similar method to that described above. ZrCU (150 mg) and BDC-NFh (120 mg) in DMF (25 ml) were ultrasonically dissolved in a glass bottle. The obtained clear solution was transferred into a Teflon-lined stainless-steel autoclave, in which the PET membrane with a single nanochannel was placed vertically with a holder. Subsequently, the autoclave was placed in an oven and heated at 100 °C for 24 h. After cooling down to room temperature, the as-prepared nanochannel membrane was washed with ethanol three times, followed by drying in a vacuum oven overnight at 25 °C. [000110] A UiO-66-N⁺(CH₃)₃ membrane was synthesized by quartemization of UiO-66-NEh with CH3I. To form the UiO-66-N⁺(CH₃)₃ membrane, the UiO-66-NEh membrane was immersed into a CEEI-methanol solution for 48 h for the quatemization process, followed by washing with methanol three times and drying in a vacuum oven overnight at 25 °C.

[000111] Once formed, the pore channels of the UiO-66-NEh/PET membrane or UiO-66- N⁺(CH3)3 membrane are functionalized to include a crown ether moiety. Example 2 reports one method for post-functionalization of MOF membranes with a crown ether.

[000112] A UiO-66-COOH/PET membrane was formed by first forming UiO-66(COOH)2 seeds via a hydrothermal method, and then using a facilitated interfacial growth strategy to assemble the UiO-66(COOH)₂ MOF.

[000113] 1.4 g ofZrCU was dissolved in 5 ml of MilliQ water and sonicated for 10 mins. 1.5 g of H48TEC was dispersed into 15 ml of MilliQ water and stirred at 600 RPM at room temperature for 20 mins. Afterwards, the above two solutions were mixed and stirred for another 20 mins. This mixture was sealed into a PTFE-lined autoclave and then transferred into a preheated oven at 100 °C for 48 h under static conditions. After cooling down to room temperature, the synthesized product was centrifuged and washed with water and methanol for 3 times respectively and finally dried at 80 °C under vacuum for 16 h. The obtained white product was ground into fine powder to be used as the UiO-66-(COOH)2 seeds.

[000114] The facilitated interfacial growth strategy was then used to assemble UiO-66-(COOH)2 into the confined nanochannel of PET film. 0.1 g of UiO-66-(COOH)2 seeds were dispersed into 10 ml of MilliQ water and sonicated for 1 h before being put into two cells separated by the PET film. Driven under -2 V for 20 mins, the UiO-66-(COOH)2 seed particles migrated and then deposited into the nanochannel. After washing with distillated water to remove the seed attached on the film surface, the seeded PET NC was clamped by home-made interfacial synthesis equipment consisting of two cells, one of which was filled with 5 ml of ZrCU (0.35 g) solution and the other side with 5 ml of EEBTEC (0.38 g) solution. The interfacial synthesis equipment was then sealed into a PTFE-lined autoclave and transferred into a preheated oven at 100 °C and maintained for 48 h under static conditions. When the interfacial synthesis ended, the MOF modified PET film was taken out, washed with distilled water, and dried at room temperature.

[000115] Once formed, the pore channels of the UiO-66(COOH)2 membrane are functionalized to include a crown ether moiety. Example 2

[000116] This example reports the preparation of an ion selective membrane based on the MOF UiO-66-(COOH)2 which is then post-functionalized with a crown ether.

[000117] Figure 1 is a schematic that provides an overview of the method of preparing an ion selective metal organic framework (MOF) membrane, based on the MOF UiO-66-(COOH)2, according to an embodiment of the invention.

[000118] In this embodiment, the membrane includes a polyethylene terephthalate fdm that includes bullet shaped nanochannels that pass therethrough. The bullet shaped channels are loaded with a metal organic framework component. The metal organic framework component includes pore channels that have been functionalized with a crown ether such that the crown component of the crown ether is disposed within the pore channels. The inventors have found that functionalizing the pore channels of the metal organic framework in this way provides improved ion selectivity when the membrane is used in ion separation process.

[000119] Turning to the synthetic procedure, polyethylene terephthalate (PET) fdm was first etched to introduce bullet shaped nanochannels therethrough as illustrated in Figure l(i).

[000120] PET membranes (12 pm thick with single or multiple ion tracks in the center) were subjected to an asymmetrical etching process to produce single or multiple bullet-shaped nanochannels. One side of the PET membrane was exposed to an etching solution of 6 M NaOH + 0.025% sodium dodecyl diphenyloxide disulfonate at 60 °C whilst the other side of the PET membrane was exposed to an etching solution of 6 M NaOH at 60 °C. During the etching process, a constant voltage was applied across the PET film. A pico-ammeter was used to observe the current change of a single-nanochannel membrane during the etching process.

[000121] The etching process was terminated by adding a mixture of 1 M KC1 and 1 M HCOOH aqueous solution to neutralize the alkaline etching solution when the current reached a value of about 5*10⁸A. The resultant PET nanochannel containing film (PET NC) was then cooled to room temperature before being thoroughly washed with distilled water.

[000122] The morphologies and diameters of the nanochannels were observed with SEM using multichannel membranes prepared according to the same etching method as per the single channel membranes. Figure 2(a) - Figure 2(f) provides characterization data for the structure of single bullet-shaped PETNC embedded within PET membranes. Figure 2(a) and Figure 2(d) are SEM images of tip and base side of the bullet-shaped PET NC. Figure 2(b) is a schematic of the cross section of the bullet-shaped nanochannel. Figure 2(c) is an SEM image of the cross section of the bullet-shaped nanochannel (scale bar 1 pm). Figure 2(e) is a graph showing the size distribution of the tip diameter with an average value of 52.0 ± 14.4 nm. Figure 2(f) is a size distribution of the base diameter with an average value of 360.2 ± 52.6 nm. The PET fdm with nanochannel is generally referred to herein as PET NC.

[000123] In the next step, and as illustrated in Figure l(ii), the surfaces of the bullet-shaped nanochannel are functionalized to provide pendant amine groups (e.g. to alter the inner wall from being rich with -COOH as shown in Figure l(i) to -NEh groups as shown in Figure l(ii)). The purpose of this is to assist the growth of the UiO-66-(COOH)2 within the bullet-shaped nanochannels since the pendant amine groups assist with the deprotonation of pyromellitic acids close to the wall surface and thus promotes the primary nucleation of UiO-66-(COOH)2.

[000124] To achieve this functionalization the following methodology was employed: (i) 60 mg EDC |/V-(3-Dimethylami nopropyl )-/V'-ethylcarbodiimide hydrochloride] and 12 mg NHSS [N- Hydroxysulfosuccinimide sodium salt] were dissolved in 4 mL of 10 mM MES [2-(N- Morpholino)ethanesulfonic acid] solution (pH =5.5) and was fdled in the two PTFE cells which clamped the base-etched PET NC film; (ii) after 1 hour of activation period, the above solution was taken out and 4 mL of 0.1 M ethanediamine (EDA) solution was added; (iii) after 18 hours, the obtained EDA modified PET NC film was washed thoroughly with MilliQ water. The EDA modified PET NC is generally referred to herein as PET@EDA NC.

[000125] Figure 3(a) is a graph showing I-V curves of PET NC for 0.1 M KC1 at pH values of 2.0, 5.7, and 8.0, respectively. Figure 3(b) is a graph showing I-V curves of EDA modified PET NC for 0.1 M KC1 at pH values of 2.0, 5.7, and 8.0, respectively.

[000126] Returning to Figure 1, Figure l(iii) illustrates the assembly of the UiO-66-(COOH)2 within the bullet-shaped nanochannel of the PET. A facilitated interfacial growth strategy was used to assemble UiO-66-(COOH)2 into the EDA modified PET NC. One EDA modified PET NC was clamped using an interfacial synthesis apparatus consisting of two cells, one of which was filled with 5 mL of ZrCU (0.35 g) solution with the other side being filled with 5 mL of H4BTEC (0.38 g) solution. Both solutions were preheated to a temperature preheated to 90 °C.

[000127] The interfacial synthesis equipment was then sealed into a PTFE-lined autoclave preheated to 100 °C and maintained in the autoclave for 48 h under static conditions to form the UiO-66-(COOH)2-SNC. The UiO-66-(COOH)2-SNC was removed from the autoclave and subsequently washed with distilled water before being dried at room temperature.

[000128] Figure 4(a), Figure 4(b), and Figure 4(c) are SEM images illustrating the presence of the UiO-66-(COOH)2 is shown in the SEM image of tip side, base side and cross section of the PET NC.

[000129] Figure 5 is an XRD pattern of PET@EDA NC before and after interfacial synthesis verifying the presence of UiO-66-(COOH)2 in the PETNC.

[000130] The PET@EDA NC modified to include UiO-66-(COOH)2 is generally referred to herein as PET@EDA-UiO-66-(COOH)₂ SNC.

[000131] The next step, as illustrated in Figure l(iv), is the grafting of a crown ether onto the MOF structure. In this case, 4'-Aminobenzo-15-crown-5 was grafted onto the UiO-66-(COOH)2 via a condensation reaction between an -NEC group on the crown ether and the -COOH on the MOF. It will be appreciated that the nature of the grafting reaction may be different for different crown ether compounds and MOFs.

[000132] The following procedure was used to graft the 4 '-Aminobenzo- 15 -crown-5 was grafted onto the UiO-66-(COOH)2. 60 mg EDC, 12 mg NHSS and 20 mg 15-crown-5 were dissolved in 4 mL of MilliQ water and was filled in the two PTFE cells which clamped UiO-66-(COOH)2- SNC film. pH value of crown ether was ~ 6 and the amino groups in the crown ether ring were protonated, becoming positively charged.

[000133] 1 V of trans-membrane forward voltage was applied to the two cells with Pt electrode for 30 mins. Driven by the constant voltage applied across the UiO-66-(COOH)2-SNC, crown ethers entered into MOF channels and were grafted into the triangle windows of the surface MOF SNC. This is generally referred to herein as UiO-66-(COOH)₂-15CE5 SNC. Subsequently, the equipment was placed in a dark environment overnight and then the prepared UiO-66- (COOH)₂-15CE5 SNC was taken out, washed with MilliQ water, and finally dried at room temperature.

[000134] Figure 6(a) and Figure 6(b) provide FTIR and C¹³-NMR results respectively for (i) MOF (UiO-66-(COOH)2 crystals), (ii) CE (raw 15-crown-5 ether powder), and (iii) MOF-CE (UiO-66-(COOH)2 -15CE5 crystals). The denoted N-H, C-H, C=C and C-O-C stretch in Figure 6(a) and -CH2-CH2-O in Figure 6(b) verify the 15-crown-5 ether has been successfully grafted into MOF, indicating the successful crown ether modification of MOF SNC.

Example 3

[000135] This example reports testing of the ion selectivity of the membrane of Example 2.

[000136] To study the variation of ion conduction in nanochannels with precisely-controlled reductions in pore size, the current-voltage (I-V) curves for three monovalent chloride aqueous solutions (LiCl, NaCl and KC1) were tested in PET NC, PET@EDA NC, UiO-66-(COOH)₂ SNC and UiO-66-(COOH)2-15CE5 SNC, respectively.

[000137] Figure 7(a)-Figure 7(d) provides I-V curves of (a) PET NC, (b) PET@EDA NC, (c) PET@EDA-UiO-66-(COOH)₂ SNC, and (d) UiO-66-(COOH)₂-15CE5 SNC for 0.1 M LiCl, NaCl and KC1. After CE modification, the UiO-66-(COOH)₂-15CE5 SNC show facilitated transport for Na⁺ and K⁺, while excluding Li⁺ conduction.

[000138] The change in current values in Figure 7(a)-Figure 7(d) illustrates the ion conduction properties at different stages. For the pristine bullet-shaped PET NC, asymmetric I-V curves were observed in all electrolyte solutions (see Figure 7(a)), showing that the negatively charged NC could preferentially transport cations from the tip side to the base side. Without wishing to be bound by theory, the inventors currently are of the view that this is due to the asymmetric negative charge of the PET NC wall resulting from deprotonation of carboxyl groups at pH 5.7. The current values were at 10⁹A and followed the order: KC1 > NaCl > LiCl.

[000139] With reference to Figure 7(b), it can be seen that for the PET@EDA NC the current still follows the same order as observed in Figure 7(a). However, the shape of the I-V curves are reversed due to the polarity of the terminal groups of the inner surface (e.g. amino groups instead of the -COOH groups). When the solution pH was 5.7, smaller than 9.2, i.e. pKa of-MU, the inner surface of PET@EDA NC was positively charged owing to the protonation of amino groups; and the preferential cation conduction reversed to be from the base side to the tip side.

[000140] With reference to Figure 7(c), less asymmetric I-V curves in UiO-66-(COOH)2 SNC were observed because MOF was assembled within the channel reducing structure and chemistry asymmetry. The current values in the UiO-66-(COOH)2 SNC reduced to 10 ¹⁰ A and followed the decreasing order in terms of hydrated diameters of cations: KC1 > NaCl > LiCl. Until this stage, the UiO-66-(COOH)2 SNC showed no specific monovalent cation selectivity over its rivals. However, with reference to Figure 7(d), after crown ether modification, the current of LiCl in the UiO-66-(COOH)₂-15CE5 SNC was significantly decreased to 10 ¹² A, because the incorporation of crown ether molecules into the tip side of MOF SNC narrowed down the channel size greatly, resulting in large transport resistance for Li⁺. In contrast, the current of KC1 and NaCl still maintained at the same magnitude as that in UiO-66-(COOH)2 SNC, which implied a facilitated transport pathway for K⁺ and Na⁺ in the UiO-66-(COOH)2-15CE5 SNC.

[000141] In order to clarify this, I-V curves of pure solution (0.5 M/1.0 KC1 and 0.5 M/1.0 M NaCl) and mixture (0.5 M KC1 + 0.5 NaCl) were tested. The results of this test are shown in Figure 8(a). The current of 0.5 M KC1 + 0.5 M NaCl was between that of 0.5 M KC1 and 0.5 NaCl. There is no additionality in current for mixture solution (0.5 M KC1 + 0.5 M NaCl) in comparison with that of 0.5 M KC1 and 0.5 M NaCl, which illustrated the facilitated transport of Na⁺ and K⁺ is competitive. Without wishing to be bound by theory, the inventors have proposed two different mechanisms for K⁺ and Na⁺ transport (as shown in Figure 8(b)). In MOF-15CE5 SNC, the two adjacent 15-crown-5 molecules will anchor one partially dehydrated K⁺ in a sandwich manner and then permit fast transport of this partially dehydrated K⁺ under applied voltages; whereas for Na⁺, the Na⁺ ion must fully dehydrate in order to fit into the - 1.9 A-sized cavity of 15-crown-5 ether and therefore the facilitated transport of Na⁺ occurs is a single-file manner.

[000142] In order to study the ion selective property of UiO-66-(COOH)2-15CE5 SNC, ion permeation experiments were conducted. Based on the cation concentrations in the permeate side determined by ICP-OES, the permeability ratio was calculated to evaluate the cation selectivity. Ideal selectivity based on a single component permeation experiment is shown in Figure 10. The ideal K⁺/Na⁺ and K⁺/Li⁺ selectivity was 4.6 and 2800, respectively, qualitatively consistent with the I-V curves, which as well confirmed the UiO-66-(COOH)2-15CE5-SNC can selectively transport K⁺ and then Na⁺ over Li⁺ in single component systems. But for a mixed ion system, the proposed facilitated mechanism for Na⁺ and K⁺ are competitive because these two ions share the same transport pathway. Mixed ion permeation experiments were first conducted at relative low voltage, i.e. 0.1 V for 24 h. It was found that in this system, the K⁺/Na⁺ selectivity was 45.9 and the K⁺/Li⁺ selectivity was 1650 (see Figure 8(c)). Therefore, the UiO-66-(COOH)2-15CE5 SNC exhibited excellent K⁺ selectivity over Na⁺ at lower trans-membrane voltage bias.

[000143] Ion permeation experiments were then conducted at a higher voltage of 1.0 V. Interestingly, the UiO-66-(COOH)₂-15CE5 SNC reversed to be Na⁺ selective. As shown in Figure 8(d) the Na⁺/K⁺ selectivity of UiO-66-(COOH)2-15CE5-SNC was 62.8. Furthermore, the Na⁺/Li⁺ selectivity was 3700. These results indicate that highly selective ion transport channels can be prepared by incorporation of crown ethers into MOF structures.

[000144] The ultrahigh and voltage-tunable mixture ion selectivity of UiO-66-(COOH)₂-15CE5 SNC can be explained by the trade-off between the facilitated transport mechanism for K⁺ and Na⁺ respectively in the mixture and system driven under trans-membrane voltage.

[000145] Before crown ether modification, the UiO-66-(COOH)2 SNC has aperture channel size of ~ 6 A and high density of free carboxylic groups are dangling along the channel. In this system, the monovalent ion selectivity (K⁺ vs Na⁺) of the UiO-66-(COOH)2 SNC is relatively weak due to several reasons. First, the sole size-sieving effect relying on the sub-nanometer sized channel does not strongly discriminate between these three monovalent metal ions (i.e., Na⁺, K⁺, Li⁺), considering their hydrated diameters are close to each other: 6.62 A for K⁺, 7.16 A for Na⁺ and 7.64 A for Li⁺. Further, the binding affinity for these monovalent ions with -COOH is not sufficiently high for the MOFSNC to discriminate one ion from its rivals. Still further, defects (e.g. originating from the missing of ligand units in the framework of MOFs) will enlarge the local pore size and may then undermine the size exclusion effect.

[000146] The incorporation of 4'-Aminobenzo-15-crown-5 endowed MOF-SNC with improved K⁺ or Na⁺ selective transport, as well as the extremely low conduction for Li⁺. This is thought to be because the chemical grafted crown ether can address the above discussed adverse factors. Due to the less diffusion resistance, defect sites tend to be modified with crown ethers first compared with non-defect areas. The presence of crown ether narrows down the channel size at angstrom-sized scale and then offsets the unfavorable effect from the defects. The size exclusion effect of UiO-66-(COOH)₂- 15CE5 is enhanced and as such the conduction of Li⁺ is depressed, as shown the current ratio of 10 ¹² A at 2 V. More importantly, the 15-crown-5 ether has high and exclusively binding affinity for Na⁺. Consequently, the grafted 15-crown-5 moieties serve as an independent binding site for Na⁺, and the conduction of Na⁺ in UiO-66-(COOH)₂-15CE5 SNC is facilitated. The triangle shaped windows of UiO-66-(COOH)2 possess three -COOH for attachment of 15 -crown-5 molecules by a condensation reaction so the grafted three 15 -crown-5 in the triangle window will arrange in a cascade form along the direction vertical to the triangle window plane (e.g. see Figure 1 (iv)). In this case, the two adjacent crown ethers are close enough to sandwich one hydrated K⁺, which facilitates the transport of K⁺ in UiO-66-(COOH)2- 15CE5. [000147] In a single component system, these two types of facilitated mechanisms mean different free energy barriers for K⁺ and Na⁺. For the sandwich manner, K⁺ only needs to lose part of its hydration number to fit between two neighboring crown ethers. In contrast, Na⁺ must lose its whole hydration shell to fit into the cavity of 15-crown-5. Therefore, the free energy barrier for Na⁺ passing through the 15-crown-5 grafted triangle window is larger than that of K⁺. These can justify why the UiO-66-(COOH)₂-15CE5 showed K⁺ selective property form both I-V curves and ideal selectivity. However, these two facilitated mechanisms will co-exist, in a competitive relationship with each other in a mixed ion system containing K⁺ and Na⁺. On one hand, K⁺ can readily enter the gap of two crown ether due to the reasonable distance of two crown ethers, which reduce the local negative charge of the ether ring. As a result, the binding of Na⁺ to the cavity of crown ether is restrained. On the other hand, once the cavity capturing bared (dehydrated) Na⁺, the grafted crown ether becomes positively charged and then shift along the trans-membrane potential gradient. At low voltage, the free energy barrier is the dominant factor so that the K⁺ is preferentially conveyed across the UiO-66-(COOH)2-15CE5. Under relative high voltage, such as 1 V, the distance between two adjacent crown ethers will be enlarged and then reduce the possibility of sandwich manner for K⁺. What is more, the disadvantage for Na⁺ in energy barrier can be compensated to a certain extent by relatively high trans-membrane voltage bias because the Na⁺ can be favorably dehydrated to fit into the cavity and single-file transport is finally enhanced over sandwich manner.

[000148] Figure 9 is a schematic illustrating the competitive facilitated transport mechanism between Na⁺ and K⁺ of the UiO-66-(COOH)₂-15CE-5 in a mixed ion system. In the mixed ion system, the co-existed facilitated transport mechanism for K⁺ and Na⁺ compete with each other and was voltage-tunable: (i) The sandwich manner for K⁺ conduction was predominant at low voltage (ii) High applied voltage enlarged distance between two adjacent crown ether molecules in triangle windows, depressing the sandwich manner for K⁺ conduction; energy cost for full dehydration for Na⁺ was compensated, which enhanced single-file transport for Na⁺. The red and yellow arrows are used to illustrate the translocation ways of K⁺ and Na⁺ throughout the narrowest part of UiO-66-(COOH)₂-15CE-5.

Claims

1. A crown ether metal organic framework composition, comprising: a porous metal organic framework (MOF) structure having a first surface with a first pore window and a second surface with a second pore window and a channel therebetween, the MOF structure being functionalized by one or more crown ether (CE) structures contained within or on the channel; the MOF structure comprising a material selected from the group consisting of UiO-66, ZIF-7, ZIF- 8, MIL-121, ZJU-24, NU-125-IPA, and NU-125-HBTC; and the CE structure comprising a compound selected from the group consisting of 12- crown-4, 15-crown-5, 18-crown-6, 21-crown-7, 24-crown-8, 27-crown-9, and 30-crown-10.

2. The crown ether metal organic framework composition according to claim 1, wherein the composition is stable in aqueous media and has an average channel diameter of 1 angstrom to 10 angstroms.

3. The crown ether metal organic framework composition according to claim 1, wherein the MOF structure comprises a plurality of carboxylic groups.

4. The crown ether metal organic framework composition according to claim 1, wherein the MOF structure comprises a material selected from the group consisting of UiO-66-IPA, UiO- 66-COOH, UiO-66-(COOH)₂, U1O-66-NH2, U1O-66-NO2, MIL- 121, ZJU-24, NU-125-IPA, and NU-125-HBTC.

5. The crown ether metal organic framework composition according to claim 1, wherein the MOF structure has one or more missing-linker defects selected from the group consisting of uncoordinated linkers and dangling linkers.

6. The crown ether metal organic framework composition according to claim 1, wherein the MOF structure includes an uncoordinated BDC (benzene-dicarboxylate) missing-linker.

7. The crown ether metal organic framework composition according to claim 1, wherein the MOF structure is selected from UiO-66-COOH and UiO-66-(COOH)2.

8. The crown ether metal organic framework composition according to claim 1, wherein the crown ether structure includes a compound selected from the group of consisting 2- aminomethyl- 15 -crown-5, 2-aminomethyl-18-crown-6, 4'-aminobenzo-15-crown-5 and 4'- aminobenzo- 18 -crown-6.

9. The crown ether metal organic framework composition according to claim 1, wherein the crown ether structure includes a compound selected from the group consisting of aminobenzo- or aminomethyl-X-crown-Y wherein X is 12, 15, or 18 and Y is 4, 5, or 6.

10. The crown ether metal organic framework composition according to claim 1, wherein the crown ether structure includes a compound selected from the group consisting of 4'- Aminobenzo-12-crown-4, 4'-Aminobenzo-15-crown-5, 4'-Aminobenzo-18-crown-6, 4'-Amino- 5 '-nitrobenzo- 15 -crown-5, 4'-Aminodibenzo-18-crown-6, 2-Aminomethyl- 15 -crown-5, and 2- Aminomethyl- 18-crown-6.

11. The crown ether metal organic framework composition according to claim 1, wherein the channel comprises an ion forming moiety capable of providing ion transport charge selectivity and/or binding affinity selectivity in polar solution.

12. The crown ether metal organic framework composition according to claim 11, wherein the ion forming moiety is selected from the group consisting of -COOH and -SO₃H.

13. The crown ether metal organic framework composition according to claim 1, wherein the composition is capable of selectively transporting one or more ions though said channel in a polar solvent upon application of a bias voltage from about 10 mV up to about 2 V.

14. The crown ether metal organic framework composition according to claim 1, wherein upon application of a voltage bias in a polar solvent the composition exhibits a higher selectivity for K+ than for Li+ in the range of a K+/Li+ selectivity from 10: 1 to 1000: 1.

15. The crown ether metal organic framework composition according to claim 14, wherein the voltage bias is in the range of from about 0.05 V to about 0.5 V.

16. The crown ether metal organic framework composition according to claim 1, wherein upon application of a voltage bias in a polar solvent the composition exhibits a higher selectivity for Na+ than for Li+ in the range of a Na+/Li+ selectivity from 10: 1 to 1000: 1.

17. The crown ether metal organic framework composition according to claim 16, wherein the voltage bias is in the range of from about 0.5 V to about 2 V.

18. An ion separation or transport membrane comprising : a crown ether (CE) functionalized metal organic framework (MOF) structure comprising: first and second surfaces, and one or more ion transport channels between respective pore windows in the first and second surfaces, wherein said ion transport channels contain one or more crown ethers.

19. The membrane of claim 18, wherein the membrane is an ion selective separation membrane.

20. The membrane of claim 18, wherein the respective pore windows have a diameter that is less than the hydrated diameter of an ion for which the membrane transports and/or is selective.

21. The membrane of claim 18, wherein the crown ether has a crown portion that has a size that is less than the hydrated diameter of an ion for which the membrane transports and/or is selective.

22. The membrane of claim 18, wherein the pore windows have a pore size in the range of 1 angstrom to 6.5 angstroms.

23. The membrane of claim 18, wherein the MOF is a ZIF-8 or UiO-66 type MOF.

24. The membrane of claim 18, wherein the crown ether includes a structure selected from the group consisting of: 12-crown-4, 15-crown-5, 18-crown-6, 21-crown-7, 24-crown-8, 27- crown-9, 30-crown- 10, and mixtures thereof.

25. The membrane of claim 18, wherein the crown ether modified metal organic framework structure is disposed on, in, and/or around a substrate layer.

26. The membrane of claim 25 wherein the substrate layer is a substrate having a one or more channels, and the crown ether modified metal organic framework structure is disposed within said one or more of channels.

27. The membrane of claim 26 wherein the channels have an average cross-sectional diameter of from about 5 nm to about 200 nm.

28. The membrane of claim 27, wherein the channels taper from a first diameter at the first surface to a second diameter at the second surface, wherein the first diameter and the second diameter are different.

29. The membrane of claim 28, wherein the channels have a bullet-like shape.

30. The membrane of claim 26, wherein the substrate layer comprises a material selected from the group consisting of a porous metal layer, a porous ceramic layer, and a porous polymer layer.

31. The membrane of claim 25, wherein the substrate layer is a nonporous substrate having crown ether modified metal organic framework (CE MOF) structure disposed on or within said substrate.

32. The membrane of claim 25, wherein the substrate layer is a nonporous substrate and the crown ether modified metal organic framework (CE MOF) structure is in the form of CE MOF particles.

33. The membrane of claim 25, wherein the substrate layer comprises a polymer selected from the group consisting of polyethylene terephthalate (PET), polycarbonates, and polyimides.

34. The membrane of claim 32, wherein the particles have a size in the range of 1 nm to 500 nm.

35. The membrane of claim 32 wherein the crown ether modified MOFs are present in the membrane in a range of about 20 to about 60 percent by weight.

36. The membrane of claim 25, wherein the substrate layer is a 2D layered material.

37. The membrane of claim 36, wherein the 2D layered material is selected from the group consisting of: graphene and/or graphene oxide, zeolite, M0S2, WS2, and BN.

38. The membrane of claim 18, further comprising a support layer.

39. The membrane of claim 38, wherein the crown ether modified metal organic framework is a layer or a component of a layer formed on at least one surface of the support layer.

40. The membrane of claim 38, wherein the support layer is a porous ceramic, porous metal, or porous polymer.

41. The membrane of claim 18, wherein the membrane is a voltage tunable ion separation membrane in which ion separation efficiency and/or ion selectivity is controllable with an applied trans-membrane voltage.

42. The membrane of claim 18, wherein the membrane further includes first and second electrodes configured to apply a trans-membrane voltage bias.

43. A membrane module for an ion separation or transport membrane system, the membrane module comprising: an ion separation or transport membrane comprising: a crown ether modified metal organic framework structure comprising: first and second surfaces, and ion transport channels between respective pore windows in the first and second surfaces; wherein the ion transport pore channels include one or more crown ethers.

44. A method for separating ions in a liquid media, the method including: exposing a liquid media containing ions to a first surface of an ion separation membrane; and applying a trans-membrane bias voltage to transport ions into a pore window in the first surface, through the ion transport channel, and out of a pore window in a second surface; wherein the ion separation membrane is an ion separation membrane according to claims

18 or 25.

45. The method of claim 44, wherein the method is a method for selectively separating ions; wherein the liquid media contains at least first and second ions, and the step of applying a trans membrane bias voltage selectively transports the first ions preferentially with respect to the second ions.

46. The method of claim 44 or 45, wherein the trans-membrane bias voltage is from about 10 mV up to about 2 V.

47. A method of forming an ion separation membrane according to any one of claims 18 or 25, or the membrane module of claim 43, the method comprising: coupling crown ethers to at least internal surfaces of an ion transport channel between respective pore windows in first and second surfaces of a metal organic framework structure.