WO2022016232A1

WO2022016232A1 - Macrocycle-metal organic frameworks

Info

Publication number: WO2022016232A1
Application number: PCT/AU2021/050799
Authority: WO
Inventors: Huanting Wang; Simeng Zhang; Jun Lu; Mark Banaszak HOLL; Ranwen OU
Original assignee: Monash University
Priority date: 2020-07-24
Filing date: 2021-07-23
Publication date: 2022-01-27

Abstract

Disclosed herein is a metal organic framework composition comprising a porous metal organic framework material; and a stimuli responsive polymer confined within pores of the porous metal organic framework material, the stimuli responsive polymer comprising a stimuli responsive backbone with a macrocyclic structure grafted thereto.

Description

MACROCYCLE-METAL ORGANIC FRAMEWORKS

Field

[0001] The invention relates to macrocycle modified metal organic frameworks, such as for use as an adsorbent, such as in ion adsorption columns and/or membranes.

Background

[0002] Adsorbents 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 separating ions of the same valence and/or similar sizes using these adsorbents. Furthermore, subsequent regeneration of the adsorbent can be energetically expensive and time consuming.

[0003] Adsorbent systems 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 MOF membranes for pressure driven gas separation technologies, there has been little reported research into the use of MOFs whether incorporated into adsorption systems for selective adsorption, and/or separation of ions in liquids.

[0004] It is an object of the invention to address at least one of the shortcomings of the prior art and/or provide a useful alternative.

Summary of Invention

[0005] In a first aspect of the invention, there is provided a metal organic framework composition comprising: a porous metal organic framework material; and a stimuli responsive polymer confined within pores of the porous metal organic framework material, the stimuli responsive polymer comprising a stimuli responsive backbone with a macrocyclic structure grafted thereto. [0006] Also disclosed herein is a metal organic framework composition comprising: a porous metal organic framework material having first and second surfaces with pore windows therein, channels extending between respective pore windows in the first and second surfaces, and a stimuli responsive polymer confined within the channels, the stimuli responsive polymer comprising a stimuli responsive backbone with a macrocyclic structure grafted thereto.

[0007] In an embodiment, the stimuli responsive polymer has a first state and a second state, and the stimuli responsive polymer undergoes a conformal change from the first state to the second state in response to a chemical stimulus (e.g. pH), or an exogenous stimulus selected from the group consisting of: temperature, light (including visible, IR, or UV light), electric field, magnetic field.

[0008] In a form of the above embodiment, the macrocyclic structure is arranged to adsorb ions in the first state, and desorb ions in the second state.

[0009] In a form of the above embodiment, the first state is a hydrophilic state and the second state is a hydrophobic state.

[0010] In an embodiment, the stimuli responsive polymer is a thermo-responsive polymer.

[0011] In an embodiment, the stimuli responsive polymer exhibits a nanostructure due to confinement within the channel.

[0012] In a form of the above embodiment, the thermo-responsive polymer has a lower solution critical temperature of 90 °C or less.

[0013] In an embodiment, a loading of the stimuli responsive copolymer in the metal organic framework composition is from about 5 wt% up to about 60 wt%. Preferably, the loading is at least 7 wt%. More preferably, the loading is at least 8 wt%. Most preferably, the loading is at least 10 wt%. Additionally, or alternatively, it is preferred that the loading is up to about 50 wt%. More preferably, the loading is up to about 40 wt%. Most preferably, the loading is up to about 30 wt%. [0014] In an embodiment the stimuli responsive backbone is a homopolymer or a copolymer comprising at least one repeating unit selected from the group consisting of: N- isopropylacrylamide, N-[2-(diethylamino)ethyl acrylamide], N,N-dimethylaminoethyl methacrylate, N,N-diethylaminoethyl methacrylate, 2-(N-morpholine)ethyl methacrylate, oligo(ethylene glycol)methacrylate, and N,N-diethylacrylamide, N-

(isobutoxymethyl)acrylamide, 2-carboxyisopropylacrylamide, 2-ethoxyethyl vinyl ether, 2- methoxyethylvinyl ether, 2-(2-methoxyethoxy)ethyl methacrylate, oligo(ethylene glycol) methacrylate, N- (hydroxymethyl) acrylamide.

[0015] In an embodiment, the stimuli responsive backbone is formed from poly(N- isopropylacrylamide), and may be a poly(N-isopropylacrylamide) homopolymer or a poly(N- isopropylacrylamide) copolymer. The poly(N-isopropylacrylamide) copolymer may be a block copolymer or a random copolymer.

[0016] In an embodiment, the stimuli responsive polymer is cross-linked.

[0017] In an embodiment, a stoichiometric ratio of the at least one repeating unit to the macrocyclic structure is greater than or equal to 2.5:1.

[0018] In one or more embodiments, the porous metal organic framework material comprises pore windows, and the pore windows have a diameter of from about 5 A up to about 35 A. Preferably, the pore windows have a diameter of from about 8 A. More preferably, the pore windows have a diameter of from about 10 A. Most preferably, the pore windows have a diameter of from about 15 A. Additionally or alternatively, the pore windows have a diameter of up to about 30 A. More preferably, the pore windows have a diameter of up to about 28 A. Most preferably, the pore windows have a diameter of up to about 25 A.

[0019] In an embodiment, the pore windows have a diameter that is greater than the hydrated diameter of an ion for which the macrocyclic structure is selective.

[0020] In an embodiment, the metal organic framework structure is selected from the group consisting of: MOF-808, MIL-53, UiO-66, UiO-66-IPA, UiO-66-COOH, UiO-66-(COOH)₂, UiO-66-NH₂, Ui0-66-N0₂, ZIF- 8, MIL- 121, ZJU-24, NU-125-IPA, and NU-125-HBTC.

[0021] In an embodiment, the MOF is a water stable MOF. [0022] In an embodiment, the macrocyclic structure comprises an organic macrocycle having at least one N or O atom.

[0023] In an embodiment, the macrocyclic structure is selected from the group consisting of: an organic crown structure, a cryptand structure, a porphyrin structure, a calixarene structure, or a cyclodextrin structure.

[0024] In an embodiment, the macrocyclic structure is a crown ether structure.

[0025] In one form of the above embodiment, the crown ether structure comprises a crown ether 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.

[0026] In one form of the above embodiment, the crown ether structure comprises a crown ether selected from the group consisting of: aminobenzo-, aminomethyl-, or benzo- X-crown-Y; wherein X is an integer selected from 12, 15, or 18 and Y is an integer selected from 4, 5, or 6.

[0027] In one form of the above embodiment, the crown ether structure comprises a crown ether 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.

[0028] In an embodiment, the macrocyclic structure is adapted to adsorb sodium ions (Na⁺) and potassium ions (K⁺) preferentially with respect to lithium ions (Li⁺).

[0029] In an alternative embodiment, the macrocyclic structure is adapted to adsorb lithium ions (Li⁺), such as in preference to non-Li⁺ cations (particularly Na⁺ and K⁺).

[0030] In an embodiment, the metal organic framework structure further comprises a stimuli responsive compound, different to the stimuli responsive polymer, and wherein the stimuli responsive compound is contained within the channel.

[0031] In a form of the above embodiment, the stimuli responsive compound is adsorbed to a surface of the channel or confined within the channel. The stimuli response compound may be responsive to a chemical stimulus (e.g. pH), or an exogenous stimulus selected from the group consisting of: temperature, light (including visible, IR, or UV light), electric field, magnetic field.

[0032] In a form of the above embodiment, the stimuli responsive compound adsorbs ions in the first state, and desorb ions in the second state. It is preferred that the first state is a hydrophilic state and the second state is a hydrophobic state.

[0033] The stimuli responsive-polymer may be chemically bonded to the metal organic framework. However, in a preferred embodiment, the stimuli responsive polymer is not chemically bonded to the metal organic framework.

[0034] In a second aspect of the invention, there is provided an ion adsorbent comprising the metal organic framework composition of the first aspect, and/or embodiments thereof, and/or forms thereof, and/or as disclosed above.

[0035] In an embodiment, the ion adsorbent is, or is a component of, an adsorption membrane, an adsorption membrane module, adsorption media, or an adsorption column.

[0036] In an embodiment, the metal organic framework composition is disposed on, in, and/or around a substrate layer. Preferably, the substrate layer is a porous substrate selected from the group consisting of a porous metal, a porous ceramic, and a porous polymer.

[0037] In an embodiment, the ion adsorbent provides the selective adsorption of certain ions such as, but not limited to Na and K ions in solution.

[0038] Preferably, the ion adsorbent has a selective adsorption ratio of K⁺:Li⁺ is at least 2:1 when compared using 0.5 M aqueous solutions at standard temperature and pressure. More preferably, the selective adsorption ratio is at least 5:1. Even more preferably, the selective adsorption ratio is at least 10:1. Most preferably, the selective adsorption ratio is at least 12:1.

[0039] Preferably, the ion adsorbent has a adsorption ratio of Na⁺:Li⁺ is at least 2: 1 when compared using 0.5 M aqueous solutions at standard temperature and pressure. More preferably, the selective adsorption ratio is at least 5:1. Most preferably, the selective adsorption ratio is at least 10:1. [0040] In a third aspect of the invention, there is provided a membrane module comprising the ion adsorbent the second aspect, and/or embodiments thereof, and/or forms thereof.

[0041] In a fourth aspect of the invention, there is provided an ion adsorption column, ion adsorption media, or ion adsorption membrane comprising the metal organic framework composition of the first aspect, and/or embodiments thereof, and/or forms thereof, and/or as disclosed above.

[0042] The ion adsorption media may be in the form of a powder or monolith, e.g. which can be packed in a column or dispersed in a solvent (such as water) for adsorption.

[0043] Preferably, the ion adsorption column, ion adsorption media, or ion adsorption membrane has a selective adsorption ratio of K⁺:Li⁺ is at least 2:1 when compared using 0.5 M aqueous solutions at standard temperature and pressure. More preferably, the selective adsorption ratio is at least 5:1. Even more preferably, the selective adsorption ratio is at least 10:1. Most preferably, the selective adsorption ratio is at least 12:1.

[0044] Preferably, the ion adsorption column, ion adsorption media, or ion adsorption membrane has a adsorption ratio of Na⁺:Li⁺ is at least 2:1 when compared using 0.5 M aqueous solutions at standard temperature and pressure. More preferably, the selective adsorption ratio is at least 5:1. Most preferably, the selective adsorption ratio is at least 10:1.

[0045] In a fifth aspect of the invention, there is provided a method of preparing a metal organic framework composition comprising: providing a porous metal organic framework material, and initiating a polymerisation reaction within pores of the porous metal organic framework material between at least one monomer suitable for being polymerised to form a stimuli responsive polymer and a macrocyclic structure to form a stimuli responsive polymer comprising a stimuli responsive backbone with a macrocyclic structure grafter thereto confined within the pores.

[0046] In a sixth aspect of the invention, there is provided a method of treating a liquid media to selectively remove ions from the liquid media, the method comprising: contacting a liquid media containing one or more target ions with a surface of the ion adsorbent of the second aspect and/or embodiments thereof, and/or forms thereof, or with a membrane module of the third aspect and/or embodiments thereof, and/or forms thereof, or with an ion adsorption column, ion adsorption media, or ion adsorption membrane of the fourth aspect of the invention and/or embodiments thereof, and/or forms thereof under conditions in which the stimuli responsive polymer is in a first state in which the macrocyclic structure adsorbs the one or more target ions; adsorbing the one or more target ions to the macrocyclic structure; subsequently subjecting the stimuli responsive polymer to a stimulus sufficient that the stimuli responsive polymer undergoes a conformal change from the first state to a second state in which the one or more target ions desorb from the macrocyclic structure; and flushing the ion adsorption membrane with a liquid media to remove desorbed ions from the ion adsorption membrane or the ion adsorption column media.

[0047] In an embodiment, the liquid media is an aqueous solution comprising at least Li⁺, and one or both of Na⁺ or K⁺, and the one or more target ions are one or both of Na⁺ or K⁺; and during the step of contacting at least the portion of the liquid media with the ion adsorption membrane or the ion adsorption column media, one or both of Na⁺ or K⁺ are selectively adsorbed by the macrocyclic structure with respect to Li⁺.

[0048] In another disclosure , there is provided a method of treating a liquid media to selectively remove ions from the liquid media, the method comprising: contacting a liquid media containing one or more target ions with a metal organic framework composition comprising: a porous metal organic framework material having first and second surfaces with pore windows therein, channels extending between respective pore windows in the first and second surfaces, and a stimuli responsive polymer confined within the channels, the stimuli responsive polymer comprising a stimuli responsive backbone with a macrocyclic structure grafted thereto; transporting at least a portion of the liquid media through the channels from a first surface of the membrane to a second surface of the membrane, under conditions in which the stimuli responsive polymer is in a first state in which the macrocyclic structure adsorbs the one or more target ions; adsorbing the one or more target ions to the macrocyclic structure; subsequently subjecting the stimuli responsive polymer to a stimulus sufficient that the stimuli responsive polymer undergoes a conformal change from the first state to a second state in which the one or more target ions desorbs from the macrocyclic structure; and flushing the ion adsorption membrane with a liquid media to remove desorbed ions from the channels.

[0049] In an embodiment of the above disclosure, the liquid media is an aqueous solution comprising at least Li⁺, and one or both of Na⁺ or K⁺, and the one or more target ions one or both of Na⁺ or K⁺; and during the step of transporting at least the portion of the liquid media through the channels, Li⁺ is selectively transported through the membrane preferentially with respect to one or both of Na⁺ or K⁺.

[0050] In an embodiment, there is provided a use of a metal organic framework composition of the first aspect, and/or embodiments thereof, and/or forms thereof, and/or as disclosed above in an ion adsorption process.

[0051] In an embodiment, there is provided a use of a metal organic framework composition of the first aspect, and/or embodiments thereof, and/or forms thereof, in a method of forming one or more of an adsorption membrane, an adsorbent component of an adsorption membrane, an adsorption membrane module, an adsorption media, or an adsorption column.

[0052] In an embodiment, there is provided a method comprising: forming an adsorption membrane, an adsorbent component of an adsorption membrane, an adsorption membrane module, an adsorption media, or an adsorption unit comprising the metal organic framework composition of the first aspect, and/or embodiments thereof, and/or forms thereof.

[0053] Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.

[0054] As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps. [0055] Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief Description of Drawings

[0056] Figure 1: Photographs of temperature responsive swelling/shrinkage of pNIPAM-co-AA hydrogel with different monomer reactant molar ratio in response to 20, 32 and 45°C. The pNIPAM-co-AA hydrogel was prepared by dissolving NIP AM, AA, MBA and APS in DI water with different monomer reactant molar ratio (NIP AM: AA=5:1, 4:1, 3:1, 2.5:1 and 2:1, monomer concentration of 20wt%). The solution was stirred at 70°C for 2h.

[0057] Figure 2: Schematic representation of adsorption and desorption process. (a,b) Selective adsorption of monovalent metal ions and temperature-triggered desorption in thermally reversible pNCE/MOF-808. (c,d) Reversible change of pNCE chains within MOF-808 in selective metal ion (M⁺) adsorption and desorption processes. The pNCE chains are hydrophilic and loose below the LCST, then become hydrophobic and entangled above the LCST, resulting in adverse M⁺ binding environment and two adjacent B18C6 groups to release the adsorbed M⁺.

[0058] Figure 3: Schematic showing the thermally driven change of hydrogen bonds within the pNCE networks.

[0059] Figure 4: SEM images of (a) pristine MOF-808 and (b) pNCE/MOF-808 crystals (c) Elemental mapping as measured by EDS for pNCE/MOF-808. (d) N2 adsorption-desorption isotherms of MOF-808 and pNCE/MOF-808 at 77 K (solid and open symbols depict adsorption and desorption, respectively). Inset: Pore size distribution, y-axis: dV/dlog(W) pore volume (cm³/g), x-axis pore width (A), curve with peak at 18.6 A: MOF-808, and curve with peak at 10.3 A: pNCE/MOF-808. (e) XRD patterns of MOF-808 and pNCE/MOF-808. (f) FTIR spectra of MOF-808, pNIPAM/MOF-808 and pNCE/MOF-808.

[0060] Figure 5: SEM images of (a) pristine MOF-808 crystals and (b) pNCE/MOF-808 crystals.

[0061] Figure 6: FTIR spectra of MOF-808, pNIPAM/MOF-808 and pNCE/MOF-808 (wavenumber range from 1300 cm ¹ to 1000 cm ¹). [0062] Figure 7: (a) Graph showing Li⁺, Na⁺ and K⁺ adsorption capacity of MOF-808, pNIPAM/MOF-808 and pNCE/MOF-808 in 0.5M salt solution (Cl as the anion) (b) Graph showing effects of different polymer loadings on K⁺ adsorption capacity (c) Graph showing adsorption capacity of samples with different polymer loadings at varying adsorption time (d) Graph showing effects of desorption temperature on the desorption performance of pNCE/MOF- 808. (e) Graph showing cycling performance of pNCE/MOF-808. The adsorption tests were conducted in 0.5M KC1 solution and regenerated at 45 °C in Milli Q water (f) Photographs showing the cyclic hydrophilicity-hydrophobicity of pNCE/MOF-808 by water contact angle tests at 20°, 32, 45 and 50°C.

[0063] Figure 8: Thermogravimetric analysis (TGA) curves and derivative curves of (a) NIPAM, (b) AB18C6, (c) pNCE, (d) MOF-808, (e) pNCE/MOF-808 and (f) pNCE/MOF-808- K⁺. The TGA was conducted under atmospheric airflow.

[0064] Figure 9: FTIR spectra of pNCE/MOF-808 before adsorption, after K⁺ adsorption and after thermal-regeneration (wavenumber range from 1300 cm ¹ to 1000 cm ¹).

[0065] Figure 10: Thermogravimetric analysis curves of ID, 3D, 6D and 10D. The TGA was conducted under air flow.

[0066] Figure 11: SEM images of pNCE/MOF-808 crystals prepared with different impregnation time: (a) 1 day, (b) 3 days, (c) 6 days and (d) 10 days.

[0067] Figure 12: XRD patterns of pNCE/MOF-808 before salt adsorption, after salt adsorption and after five cycles of thermal regeneration.

[0068] Figure 13: SEM images of pNCE/MOF-808 crystals: (a) before salt adsorption, (b) after salt adsorption, (c) after five cycles of thermal regeneration.

[0069] Figure 14: Graphs showing ion adsorption capacity and selectivity of pNCE/MOF-808 in the 0.5M, 0.1M and 0.02M mixed salt solution with an equal molar ratio of LiCl, NaCl and KC1. (a) adsorption capacity of pNCE/MOF-808, and (b) mixed ion selectivity.

[0070] Figure 15: Elemental mapping images pNCE/MOF-808. EDS images (scale bar, 500 nm) and the corresponding elemental mappings for pNCE/MOF-808 (a) after adsorption (adsorbed in 0.5M mixed ion solution) and after thermal-desorption in water at b) 25 °C, c)

32°C, d) 45°C and e) 50°C.

Description of Embodiments

[0071] The present disclosure relates to a metal organic framework (MOF) material that includes a stimuli responsive polymer confined within channels of the MOF, the stimuli responsive polymer comprises a stimuli responsive backbone with a macrocyclic structure grafted thereto. The invention also relates to adsorbents formed from this MOF, methods of forming this MOF, and methods of using this MOF, for example in an adsorption column or as an adsorption membrane or component thereof.

[0072] The invention finds applications in a range of fields, but particularly in applications where molecular or ion adsorption is useful, for example in an adsorption process. However, the skilled person will appreciate that the invention is not limited to a particular application.

[0073] The inventors have found that the presence of a macrocycle within the MOF channels allows the selective adsorption of ions within the MOF membrane, and that the presence of the stimuli responsive backbone provides a means for desorbing adsorbed ions from the MOF on application of a stimulus.

[0074] The invention is not particularly limited to a particular MOF and may be practiced with a wide variety of different MOFs. It will be appreciated that it is important that the MOF is stable in the environment in which it is used, e.g. for water treatment or water-based separation processes, the MOF must be stable in water. The MOF also requires pores that are suitably sized to house and confine the stimuli responsive polymer. Given these criteria, the skilled person is able to select appropriate MOFs for a given application and stimuli responsive polymer.

[0075] Without wishing to be bound by theory, the inventors are of the view that confinement of the stimuli responsive polymer within the channels of the MOF is important for achieving a high adsorption efficiency (as compared with bulk stimuli responsive polymer). It is thought that channel confinement causes the stimuli responsive polymer to adopt a nano-structured state in which the macrocyclic structures are able to selectively adsorb molecules and/or ions with a high adsorption efficiency. This arrangement is not present in the bulk stimuli responsive polymer. [0076] The stimuli responsive polymer comprises a stimuli responsive backbone with a macrocyclic structure grafted thereto. The macrocyclic component is important for imparting selectivity, e.g. to allow selective passage of ions through the MOF channels and/or selective adsorption of ions within the MOF channels. This is discussed in greater detail herein, particularly in relation to alkali metal ions.

[0077] Particularly preferred macrocycles are crown ethers. Crown ethers are a class of synthetic macrocyclic compounds with repeating units of ethyleneoxy functional groups (i.e. -CH2CH2O- of polyethers) that are capable of binding cations because of the central hydrophilic cavities featuring electronegative binding oxygen atoms. Generally, crown ether tends to form 1:1 cation-crown ether complex with a strong electrostatic bond with a cation of similar size to the crown ether cavity.

[0078] Different crown ethers have different binding affinity to alkali cations with different stability and formation dynamics as function of the size matching of ion and cavity, ion dehydration effect, and the steric substituents simultaneously. The diameters of alkali cations and cavities of crown ethers are listed in Table 1 below.

Table 1: Diameters of alkali cations and cavities of crown ethers

Cation Ionic radius (A) Crown ether Cavity radius (A)

Li⁺ 0.6 All 14-crown-4 0.6-0.75

Na⁺ 0.95 All 15-crown-5 0.85-1.1

K÷ T33 All 18 -crown -6 1.3- 1.6

Rb⁺ T42 All 21 -crown-7 1.7-2.15

[0079] In aqueous solution, the cation is not necessarily wrapped in the crown ether cavity because of the size-matching relationship between the hydrated ion and cavity and the dehydration of the hydrated cation. The hydrated ion radius, ion hydration enthalpy, and stability constants of 12-crown-4 (12C4), cyclohexyl- 15-crown-5 (cyclohexyl- 15C5) and 18- crown-6 (18C6) complexes with cations (Li⁺, Na⁺ and K⁺) in aqueous solution are shown in Table 2 below. Table 2: The hydrated ion radius, hydration enthalpy, and stability constants (Log K) of cation- crown ether complexes with different substituents (in water at 25 °C, anion is CT in aqueous solution).

Hydration Log K

Hydrated ion enthalpy

Cation - radius(A) Cyclohexyl-

12C4 18C6 B18C6 B2I8C6

(kj mol ¹) 15C5

Li⁺ 3.82 -519 ~0 <1.0 ~0 ~0 0

Na⁺ 3.58 -406 0 <0.3 0.82 1.38 1.16

K⁺ 3.31 -322 0 0.6 2.034 1.79 1.67

[0080] From Table 2 it can be seen that the highest stability constant is obtained between K⁺ and 18C6, suggesting it forms the most stable alkali ion-crown ether complex as compared to the Li⁺ and Na⁺ counterparts. The synergistic effects caused by ionic hydration and ion-ligand complexing compete with each other to generating the difference in binding constants. During the complexation between the cation and crown ether, water molecules present in the aqueous hydration state are mostly removed (dehydration of the cation) to accommodate by the cavity of crown ether. The high charge density of small cations (such as Li⁺) result in strong binding by water molecules and a greater energetic cost associated with dehydration (i.e. a high hydration enthalpy) when competing with complexation by crown ether. By way of contrast, the relatively low charge density of the larger cations (such as K⁺ with low hydration enthalpy) can more easily undergo the dehydration process to bind within crown ether.

[0081] Given the above, 18C6 was selected as the macrocycle for use in the experiments discussed below. However, the skilled person will appreciate that other macrocyclic compounds may be used depending on the intended application.

[0082] Turning to the stimuli responsive backbone, this permits the conformation state of the stimuli responsive polymer to be switched, on application of an appropriate stimulus, between a first state in which the macrocycle is able to adsorb molecules / ions and a second state in which adsorbed molecules / ions are desorbed from the macrocycle. In this way, the stimuli responsive polymer exhibits molecular / ion selectivity (e.g. via the crown ether) and can be readily regenerated after adsorption (e.g. through stimulation of the stimuli responsive polymer to desorb adsorbed ions).

[0083] The skilled person will appreciate that a range of different stimuli responsive polymers may be used, for example chemical responsive polymers (and in particular pH responsive polymers), or polymers responsive to an exogenous stimulus such as thermo-responsive polymers, photo-responsive polymers, UV responsive polymers, magnetic or electric field responsive polymers etc. Particularly preferred polymers are those that exhibit a hydrophilic / hydrophobic transition on exposure to a stimulus, e.g. thermo-responsive polymers such as poly(N-isopropylacrylamide) which exhibits a hydrophilicity at temperatures below its lower critical solution temperature (LCST) and hydrophobicity at temperatures above its LCST.

[0084] Without wishing to be bound by theory, the inventors are of the view that at temperatures below the LCST the polymer chains are soluble in water and thus the crown ether structures are able to selectively adsorb ions whereas at temperatures above the LCST, the polymer chains become insoluble and adopt a condensed state which causes the steric forces to dislodge adsorbed ions from the macrocycle structures.

[0085] The invention also contemplates inclusion or confinement of one or more additional stimuli responsive compounds within the channels of the MOF which may take the form of molecules, oligomers, surfactants, or polymers to provide additional functionality. For example, in one or more embodiments, channel walls are functionalised with the one or more stimuli responsive compounds (e.g. the one or more stimuli responsive compounds are physisorbed or chemisorbed to the channel walls); in other embodiments, the one or more stimuli responsive compounds are confined within the channels.

[0086] This arrangement may advantageously allow additional control over the adsorption and desorption process, e.g. selective adsorption and/or desorption. By way of example, two different stimuli responsive polymers are confined within the channel in a water purification process, a first thermo-responsive polymer with a stimuli responsive backbone and a crown ether structure for targeting K⁺ ions and a second photo-responsive polymer for targeting Na⁺ ions, after use in a water purification process, K⁺ ions can be selectively recovered by heating the MOF above its LCST and then Na⁺ ions can be subsequently recovered by exposing the MOF to light.

[0087] In an alternative arrangement, in a Li⁺ recovery process, a MOF includes a thermo- responsive polymer with a stimuli responsive backbone and a crown ether structure for targeting Na⁺ and/or K⁺ ions and the channel walls are functionalised with a further thermo-responsive molecule for targeting Na⁺ and/or K⁺. This may advantageously enhance the overall Na⁺ and/or K⁺ loading capacity of the MOF. Desorption from both the thermo-responsive polymer and the thermo-responsive molecule can then be triggered by heating the MOF to an appropriate temperature.

[0088] The invention will now be described below in relation to one embodiment thereof. Having described currently preferred embodiments of compounds and ion selective adsorption 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.

Examples

Example 1

[0089] This example reports preparation of thermally regenerable water-stable poly(N- isopropylacrylamide)-crown ether (pNCE) with MOF-808 (pNCE/MOF-808) with selective adsorption of Na⁺ and K⁺ over Li⁺ due to the incorporation of crown ether benzo-18-crown-6 (B18C6) as the ion selective affinity group.

[0090] The pNCE/MOF-808 showed excellent Na⁺/Li⁺ and K⁺/Li⁺ selectivity in mixed salt solution, which was much greater than that in single salt solution. This indicated that Na⁺ and K⁺ preferentially occupied the adsorption sites, effectively lowering Li⁺ adsorption. Release of Na⁺ and K⁺ ions in 45 °C warm water was facilitated by temperature-responsive poly(N- isopropylacrylamide) (pNIPAM) segments. Cycling adsorption-desorption experiments showed that pNCE/MOF-808 exhibited good reusability and stability. This strategy can be extended to develop regenerable MOF adsorbents with different MOF material and stimuli-responsive crown ether (CE) polymers for separation of other ions and their chemical-free regeneration.

Synthesis of MOF-808:

[0091] MOF-808 was synthesised using a solvothermal process trimesic acid (H3BTC) (lOmmol) and Zirconyl chloride octahydrate (ZrOCk· 8H2O) (30mmol) were dissolved in a mixture of dimethylformamide (DMF)/formic acid (30mL/30mL) and thoroughly stirred for lh. The solution was transferred into a 100 mL Teflon liner and the sealed autoclave was put into an oven at 130 °C. After 48 h, the autoclave was cooled to room temperature. A white product was collected by centrifugal separation and washed 5 times with 30 mL portions of DMF. In order to achieve solvent exchange, material was immersed in 30mL of anhydrous acetone for 3 days with three solvent changes per day. The acetone exchanged sample was dried at room temperature for 24 h and at 150 °C for another 24 h. The resulting white powder was dried, ground, and stored in a sealed container.

Synthesis of pNCE/MOF-808:

[0092] 4’-aminobenzo-18-crown-6 (AB18C6) (50mg, 0.15mmol), acrylic acid (AA) (9.8mg, 0.14mmol), l-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) (28.75mg, 0.15mmol) and N-hydroxysuccinimide (NHS) (17.26mg, 0.15mmol) were dissolved in lmL 2-(N- morpholino)ethanesulfonic acid (MES) buffer solution (0.1M) and allowed to react for 12h at 4°C. MOF-808 (50mg), N-isopropylacrylamide (NIP AM) (47.5mg, 0.42mmol), N,N- methylenebis(acrylamide) (MBA) (0.85mg, cross-linker) and ammonium persulfate (APS) (0.58mg, free -radical initiator) were added into the solution. The solution was stirred for 1-10 days in a sealed container. The mixture was heated at 70 °C under stirring for 12h. After cooling down to room temperature, the solid product was isolated by centrifugation, washed using several portions of Milli Q water until the conductivity of supernatant was lower than 5 m cm ¹, and then dried at 60 °C for 48 h.

[0093] In order to determine the useful ratio of acrylic acid for use in the copolymer, a series of initial experiments were conducted to ascertain the effect of AA on the thermo-responsiveness of a resultant poly(N-isopropylacrylamide)-co-acrylic acid (pNIPAM-co-AA) hydrogel. [0094] As shown in Figure 1, all the pNIPAM-co-AA hydrogels were swollen at 20°C. This is because of the strong hydrogen bonds generated between amide groups in poly(N- isopropylacrylamide) (pNIPAM) and thO. On heating to 32 and 45 °C the hydrogels with monomer molar ratio of 5: 1, 4: 1 and 3:1 shrunk since the hydrogen bonds are replaced by the intramolecular/intermolecular hydrogen bonding of amide groups from NIP AM. However, the hydrogels with monomer molar ratio of 2.5:1 and 2:1 did not exhibit any polymer shrinkage (e.g. no water was squeezed out). This is thought to be because the formation of intramolecular/intermolecular hydrogen bonding is limited by AA loading. Specifically, the weight loss percentage of hydrogel with monomer molar ratio of 5:1, 4:1 and 3:1 were 18.78+0.51%, 16.18+0.48% and 11.57+0.22% in respond to 32°C, and no obvious increase to 45°C (see results in Table 3). However, minimal weight loss was observed when the monomer molar ratio decreased to 2.5:1 and no weight loss was observed at 2:1, even when the temperature was elevated to 45°C. Given this, the monomer molar ratio of NIP AM: AA is 3: 1 was selected to form the pNCE.

Table 3: The weight loss percentage of pNIPAM-co-AA hydrogel with different NIPAM/AA reactant molar ratio in respond to heating to temperatures of 32°C and 45°C

NIPAM/AA 5:1 4:1 3:1 2.5:1 2:1

32°C 18.78+0.51% 16.18+0.48% 11.57+0.22% 0.22+0.02% —

45°C 19.13+0.58% 15.98+0.75 12.26+0.15% 0.11+0.01% —

Characterization

[0095] Cation concentrations were determined by inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin Elmer Optima 7000 DV). Standard curve method was used with a linear correlation of 0.99989+0.00005 (concentration gradient was set as 0, 2, 5, 10, 15, 20 ppm of each cation) throughout the experiment, which were determined by ICP-OES. The powder XRD (PXRD) patterns of MOF materials were measured using an X-ray diffractometer (Rigaku Miniflex 600, Tokyo, Japan) employing a 2Q range of 2-50° at room temperature with a Cu Ka radiation source (scanning rate of 2° min^_1at 40 kV of voltage and 15 mA of current). The morphologies of MOF crystals were measured by field emission scanning electronic microscope (Nova NanoSEM 450, FEG SEM, FEI, USA). Elemental mappings by energy dispersive X-ray spectrum (EDS) were obtained using a Magellan 400, FEG SEM, FEI, USA. Fourier transform infrared (FT-IR) spectra were measured using a PerkinElmer Spectrometer (Spectrum 100, Shelton, CT). The Nitrogen (N2) adsorption-desorption isotherm curves, BET surface areas and pore width distributions were acquired using a Micromeritics 3Flex (N2porosimetry/ surface characterization at 77 K, USA). Thermogravimetric analysis (TGA) was collected using a PerkinElmer Thermal Gravimetric Infra-Red hyphenated system, with a temperature range of 50-800 °C, a temperature scanning rate of 50 °C min^-1 in 20 mL min^_1air flow. The wettability of the pNCE/MOF-808 was evaluated by a contact angle goniometer (OCA15, Dataphysics, Filderstadt, Germany). The pNCE/MOF-808 tablet was prepared by pressing pNCE/MOF-808 powder with a pressing machine (Quick Press KBr Pellet Kit, International Crystal Laboratories) for the contact angle measurements. The water contact angle was determined by the average of at least three measurements.

Adsorption performance measurement

[0096] All adsorption experiments were conducted at 20°C. The pNCE/MOF-808 (50mg) was uniformly dispersed in 5mL salt solution with varied concentration, stirred at 500 rpm for 12h. Then the supernatants were collected by centrifuging for the determination of cation concentration by using ICP-OES. The adsorption capacity was calculated by the Equation 1 as follows:

Adsorption capacity (mmol g ') =

M_w - w

[0097] where Ci (ppm) is the initial concentration of cation, Ce (ppm) is the equilibrium concentration after adsorption, V(L) is the volume usage of salt solution, MW (g mol^-1) is the relative atomic mass of cation, and w (g) is the amount of the pNCE/MOF-808 adsorbent used.

[0098] The adsorption capacity variations within 30h were tested. 50 mg of pNCE/MOF-808 was added to 5 mL of 0.5M KC1 solution stirred at 500 rpm. The supernatants were collected for the determination of K+ concentration with different adsorption time of 0-30h by ICP-OES, then the adsorption capacity was calculated.

Desorption of pNCE/MOF-808 with adsorbed ions

[0099] The pNCE/MOF-808 samples with adsorbed ions were uniformly dispersed in 15mL Milli Q water, heated at 45 °C for 12h. The solids were centrifuged, followed by washing with Milli Q water. The adsorbents after washing were dried in an oven and were kept for reuse.

Results and discussion

[0100] The pNCE/MOF-808 was synthesized by impregnating NIP AM, acryloylamidobenzo- 18-crown-6 (AmB18C6) and N,N-methylenebis(acrylamide) (MBA) as a crosslinker into solvothermally synthesized MOF-808, and subsequently inducing polymerization at 70 °C. As shown in Figure 2, pNCE/MOF-808 achieves both selective ion adsorption and temperature- triggered desorption in a cycling adsorption-regeneration process. Specifically, at ambient temperature (e.g., 20°C), the pNCE/MOF-808 shows selective adsorption of K⁺ and Na⁺due to their stronger affinity with the B18C6, leaving purified Fi⁺in aqueous solution (see Figure 2a). Then the adsorbed metal ions (M⁺) are desorbed from the adsorbent in warm water at elevated temperature (e.g. 45 °C) to realize high-efficient thermal regeneration (see Figure 2b). Figure 2c and Figure 2d illustrate the thermal-regeneration mechanism. The pNCE chains within pNCE/MOF-808 display a hydrophilic and swollen coil structure arising from the strong hydrogen bonds between amide groups in poly(N-isopropylacrylamide) (pNIPAM) and H2O at ambient temperature. In this porous structure, B18C6 groups are exposed to solution within the loose polymer networks and are highly accessible to metal ions (see Figure 2c). On the contrary, the pNCE chains shrink to a hydrophobic state above the lower critical solution temperature (FCST) (such as 45°C), resulting in an adverse M⁺ binding environment and two adjacent B18C6 groups closely packed within an entangled hydrophobic polymer, and thus squeezing out the adsorbed ions (see Figure 2d). This is because the intramolecular hydrogen bonding of amide groups in pNCE networks becomes stronger than the amide groups-tbO hydrogen bonding (see Figure 3)

[0101] The pristine MOF-808 and pNCE/MOF-808 were characterized using various techniques. As shown in Figure 4a and Figure 5a, pristine MOF-808 shows a typical octahedral morphology with smooth surfaces and sharp facets. After functionalization, pNCE/MOF-808 exhibits rough crystal surfaces, although its octahedral morphology remains (see Figure 4b and Figure 5b). This suggests that the crystal surfaces of pNCE/MOF-808 are covered by pNCE. N₂ adsorption-desorption isotherms and pore size distribution curves are shown in Figure 4d. A large decrease in N₂ uptake was observed after polymer functionalization. The Brunauer- Emmett-Teller (BET) surface decreased from 1588 to 111 m²/g and the peak pore size reduced from 18.6A to 10.3 A, suggesting the successful incorporation of pNCE into MOF and the high accessibility of the pores of pNCE/MOF-808. The powder X-ray diffraction (PXRD) pattern of pNCE/MOF-808 (see Figure 4e) displays typical peaks associated with (111), (113) and (222) planes that are well matched to those of pristine MOF-8O8, confirming retention of MOF-8O8 framework after polymer functionalization.

[0102] The chemical composition of incorporated polymer was verified by Fourier transform infrared spectroscopy (FTIR). Figure 4e shows the differences in the characteristic peaks of pristine MOF-8O8, pNIPAM functionalized MOF-8O8 and pNCE functionalized MOF-8O8. These three samples all have the characteristic peak at 658 cm^-1, which arises from the Zr-0 framework in MOF-8O8. pNIPAM and pNCE functionalized MOFs show the following characteristic peaks of NIP AM: C-N stretching vibration peak at 1215 cm ¹ (see the spectra of wavenumber range from 1300 cm ¹ to 1000 cm ¹ in Figure 6), N-H stretching vibration (3440 cm ¹), C=0 stretching vibration (1670 cm ¹) from amide, and the corresponding C-H stretching vibration peaks (-CH₃ and -CH) at 2975 and 2880 cm ¹. Furthermore, the C-0 stretching peaks (1263 cm ¹ from aromatic ester, 1151 cm ¹ and 1085 cm ¹ from aliphatic ether), C-O-C stretching peaks from alkyl aryl ether (1251 cm ¹) and Ar-O-R stretching vibration (1010 cm ¹) are the characteristic peaks of ester groups, suggesting successful incorporation of the B18C6. Moreover, the uniform distribution of N (amide groups from NIP AM and acryloylamidobenzo- 18-crown-6 (AmB18C6)) within pNCE/MOF-808 was revealed by energy dispersive X-ray spectroscopy (EDS), as shown in Figure 4c, further confirming the even dispersion and incorporation of pNCE in MOF-8O8.

[0103] The absorption properties of pNCE/MOF-808 were studied using 0.5 M aqueous solutions of LiCl, NaCl and KC1. As shown in Figure 7a, pNCE/MOF-808 exhibits an adsorption capacity of 0.054, 0.64 and 0.85mmol/g for Li⁺, Na⁺ and K⁺, respectively. The single ion selectivity was 15.7 for K⁺/Li⁺ and 11.9 for Na⁺/Li⁺. As discussed previously B18C6 exhibits a binding affinity to alkali cations, and especially to K⁺, whose ionic size well matches with the crown cavity size. [0104] To understand the salt adsorption contributions from each functional unit of the pNCE/MOF-808, the difference in capacity between pristine MOF-808, pNIPAM/MOF-808, and pNCE/MOF-808 was compared. Pristine MOF-808 and pNIPAM/MOF-808 show little adsorption of Fi⁺, Na⁺ and K⁺. By way of contrast, pNCE/MOF-808 exhibits a small adsorption capacity of Fi⁺ but dramatically increased adsorption capacity of K⁺ and Na⁺, indicating that B18C6 plays an essential role in binding of these two alkali metal cations. pNCE/MOF-808-K⁺ was used to be an example for characterization after ion adsorption.

[0105] Thermogravimetric analysis (TGA) was conducted in order to further understand the chemical composition changes of NIP AM, AB18C6, pNCE, MOF-808, pNCE/MOF-808 and pNCE/MOF-808-K⁺. Figure 8 shows the respective TGA curves. To be specific, TGA curves of the functional units NIP AM and AB18C6 are illustrated by Figure 8a and Figure 8b. The pyrolysis mainly occurred from 100-220 °C for NIP AM and from 260-380 °C for AB18C6, respectively. However, as shown in Figure 8c after polymerization to pNCE, the main pyrolysis range was changed to 215-481°C, and ended at 515 °C with weight loss of 92.9 %, consistent with the presence of a polymerised chemical structure. The TGA curves of pNCE/MOF-808 (see Figure 8e), except for the pyrolysis range of 200-378°C and 510-680 °C, are similar to the pyrolysis stages of MOF-808 (see Figure 8d). The new pyrolysis stage of 378-515 °C was generated corresponding to the pyrolysis range of pNCE. From 215-515°C, the weight loss of MOF-808 and the pNCE/MOF-808 were 18.0% and 33.2%, respectively, indicating the increased weight loss arises from the incorporated pNCE. Therefore, the approximate loading of pNCE in pNCE/MOF-808 was 16.4%. Similarly, as shown in Figure 8f, the weight loss of pNCE/MOF-808 after adsorption was around 32.1%, suggesting a pNCE loading of 15.2 wt% (similar to the fresh material). However, the main pyrolysis stages of pNCE/MOF-808 were different from the fresh pNCE/MOF-808, which may be explained by the presence the K⁺- B18C6 complex. The difference in FTIR spectra between pNCE/MOF-808 before and after K⁺ adsorption demonstrates weaker stretching vibration peaks of the characteristic ester groups, further confirming K⁺ binding to the crown ether (Figure 9).

[0106] pNCE/MOF-808 formed with different impregnation times (1 day, 3 days, 6 days and 10 days) of functional units was prepared. TGA curves shown in Figure 10 show the different weight loss of the 1-day, 3-day, 6-day, and 10-day samples. The TGA curves show that all the samples generated weight loss at temperatures lower than 515 °C, which is due to the incorporation of pNCE. This weight loss difference suggests different amount of pNCE present in 1, 3, 6 and 10 day impregnated materials. To be specific, the weight loss from 215-515 °C was 27.3 wt%, 33.2 wt%, 38.0 wt% and 42.4% for 1, 3, 6 and 10 day material, respectively. Combining the weight loss of pNCE (92.9 wt%) and MOF-808 (18.0 wt%) from 215-515 °C, the calculated incorporation of 1, 3, 6, and 10 days materials were around 10.0, 16.4, 21.5, and 26.2 wt%. The results further quantified the incremental incorporated pNCE amount with the impregnation time of functional units increasing, suggesting the adsorption capacity tendency was affected by the impregnated loadings of functional units, and thus pNCE loading.

[0107] SEM images show that with increasing the impregnation time, the original octahedral shape of MOF-808 changes due to polymer wrapping, especially after 6 days and 10 days of impregnation (Figure 11). A comparison of pNCE loading on the adsorption capacity of pNCE/MOF-808 is provided in Figure 7b. pNCE/MOF-808 with 16.4 wt% pNCE shows the highest equilibrium adsorption capacity (0.85 mmol/g), followed by 10.0 wt% pNCE loading (0.73mmol/g), 21.5 wt% (0.72mmol/g) and 26.2 wt% (0.62mmol/g). The adsorption kinetics curves of samples with different pNCE loadings in 0.5M K⁺ salt solutions are shown in Figure 7c. Adsorption reaches equilibrium around 8 to 10 hours for all four samples. The equilibrium time increased with increasing pNCE loading, which can be explained by the fact that samples with lower polymer loadings have higher pore volume and faster ion diffusion. By way of contrast, the samples with higher polymer loadings exhibit lower pore volume, resulting in slower ion diffusion and lower adsorption capacity. Hence, pNCE/MOF-808 with 16.4% pNCE loading was used in the following experiments.

[0108] To investigate thermal regeneration and recyclability, pNCE/MOF-808 with absorbed K⁺ was washed in pure water at 25, 32, 45 and 50 °C. The desorption percentage of K⁺ was calculated by taking the ratio of the secondary-cycle adsorption capacity to the initial adsorption capacity of the sample (Figure 7d). With increasing the water temperature, the desorption percentage increases and reaches to a plateau (98%) at around 45 °C. This indicates that the LCST of pNCE is around 45 °C, which is higher than that of pNIPAM (32 °C). The increase of LCST is attributed to the pNCE copolymer networks and the change of polymer conformation induced by the hydrated ion-crown ether complex. This change is further supported by the increased hydrophilicity of pNCE at elevated temperature. The water contact angle of pNCE increases from 40.8+2° to 63.9+2° to 96.5+1° when the temperature rises from 25 to 32 to 45 °C; however, it does not further increase at 50 °C. After one cycle, the water contact angle changes back to the original value when pNCE cools to 20 °C. This confirms the hydrophilicity- hydrophobicity transition is at mild temperature and fully reversible (se Figure 7f).

[0109] The reusability of pNCE/MOF-808 was examined by measuring its salt adsorption and desorption capacity for five cycles. As shown in Figure 7e, pNCE/MOF-808 retains around 90% of its maximum adsorption capacity after five cycles. The characteristic peaks of B18C6 from FTIR spectra of pNCE/MOF-808 after thermal regeneration proved the adsorption sites were recovered (see Figure 8). The PXRD patterns of pNCE/MOF-808 indicated that its crystal structure remained the same before salt adsorption, after salt adsorption, and after regeneration (see Figure 12). The SEM images also showed the PNCE/MOF-808 crystal morphology did not change before adsorption, after adsorption, and after thermal-regeneration (see Figure 13).

These confirm that the pNCE/MOF-808 has excellent stability.

[0110] pNCE/MOF-808 was further tested in a mixed solution of LiCl, NaCl and KC1 (with a molar ratio of 1:1:1) to determine the ion selectivity. The adsorption capacity of Li⁺, Na⁺ and K⁺ in 0.5M salt solution was 0.016, 0.47 and 0.55 mmol/g (see Figure 14a), respectively, and the mixed ion selectivity was calculated to be 29.4 for Na⁺/Li⁺ and 34.4 for K⁺/Li⁺ (see Figure 14b). Interestingly, the mixed ion selectivity of pNCE/MOF-808 is much greater than the single ion selectivity. During complexation between the cation and crown ether, hydrated ions become partially dehydrated upon crown ether binding. Li⁺ has smallest ionic radius, larger hydration enthalpy, and lower complexation stability constant with B18C6, suggesting it is less favorable to form a stable Li⁺-B18C6 complex. By way of contrast, Na⁺ and K⁺ preferentially occupy the adsorption sites due to stronger complexation forces and size-matching effect, resulting in much lower Li⁺ adsorption capacity in the mixed ion solution.

[0111] To study the effects of ion concentration on mixed ion selectivity, adsorption tests were conducted in 0.1M and 0.02M solutions. The adsorption capacity of Li⁺, Na⁺ and K⁺ in 0.1M solution was 0.007, 0.14 and 0.24 mmol/g. Adsorption further decreased to 0.001, 0.016 and 0.037 mmol/g, respectively, in 0.02M solution (see Figure 14a). The mixed ion selectivity of Na⁺/Li⁺ and K⁺/Li⁺was 20.0 and 34.3 in 0.1M solution, and 16 and 37 in 0.02M solution, respectively. The above results show that the adsorption capacity for all three ions and Na⁺/Li⁺ selectivity generally decline with decreasing the ion concentration; however, the K⁺/Li⁺ ion selectivity is less affected by ion concentration (see Figure 14b). The preferential adsorption of K⁺ is consistent with relative binding force to B18C6 (K⁺> Na⁺). Elemental mapping images of pNCE/MOF-808 regenerated at 25, 32, 45 and 50°C are shown in Figure 15, indicating that pNCE/MOF-808 can be effectively regenerated in water at 45°C (please see the detailed EDS analysis in Note S4). As a result, thermally regenerable pNCE/MOF-808 with selective adsorption ability shows great potential for separation of Na⁺ and K⁺ from Li⁺, and thus for environmentally friendly lithium purification.

Example 2

[0112] This example is intended to illustrate that further stimuli responsive polymers, such as photo-responsive polymer poly(spiropyran acrylate) (PSP), may be incorporated into the channel in combination with a stimuli responsive polymer comprising a stimuli responsive backbone with a crown ether structure grafted thereto.

[0113] It should be noted that this example only reports the confinement of PSP within an A1 based MOF having a MIL-53 framework as proof of concept that this additional photo- responsive polymer exhibits good adsorption and desorption characteristics. This example does not report a stimuli responsive polymer comprising a stimuli responsive backbone with a crown ether structure grafted thereto in accordance with the present invention. However, the skilled person will appreciate, based on the teachings herein, that a further stimuli-responsive compound, such as the PSP reported in this example, can be incorporated into the channels of a MOF along with a stimuli responsive polymer comprising a stimuli responsive backbone with a crown ether structure grafted thereto in accordance with the present invention.

[0114] In more detail, this example reports the confinement of poly(spiropyran acrylate) (PSP) molecules within an MIL-53 (Al) frameworks to develop a sunlight-regenerable salt adsorbent (PSP-MIL-53) for sustainable desalination. MIL-53 was selected as support due to its high surface area, suitable pore size, smart breathing effect, and high water-stability. PSP-MIL-53 quickly adsorbs monovalent and divalent ions from salt water under dark condition, up to 2.88 mmol-g^-1 of NaCl, and undergoes fast regeneration under sunlight irradiation.

Synthesis of MIL-53

[0115] MIL-53 was synthesized under hydrothermal conditions. 2.60 g of A1(Nq₃)₃·9H₂q and 0.576 g of terephthalic acid (fhBDC) were added into 10 mL water in a 50 mL Teflon-lined autoclave. The autoclave was sealed and placed in oven at 150 °C for 24 hours. The resultant white powder was centrifuged and washed several times with pure water until the pH of supernatant stable at 4-5. The as-prepared MIL-53 was then dried in 60 °C oven overnight. To empty the pores, MIL-53 powder was calcined in a temperature-programmed box furnace that heated to 330 °C at a heating rate of 5 °C-min^-1, then stayed at 330 °C for 5 hours.

Synthesis of PSP-MIL-53

[0116] 0.04 g of calcined MIL-53, 0.025 g of spiropyran acrylate, and 0.01 g of azobisisobutyronitrile (AIBN) were added into 1 mL ethanol. The suspension was stirred for 3 days before allowing evaporation of ethanol in fume hood until visibly dry. The sample was polymerized in an oven at 70 °C for 2 days. The sample was then washed with ethanol and water until the conductivity of supernatant less than 5 pS cm^-1. The obtained PSP-MIL-53 was dried in 60 °C oven.

Characterization

[0117] A Rigaku Miniflex 600 X-ray diffractometer was applied to determine the powder X-ray diffraction (XRD) patterns at 40 kV and 25 mA at a scanning rate of 10° min^-1 and 2Q range of 5-60°. The UV-Vis spectra were determined by a UV-Vis spectrophotometer (UV mini 1240). The conductivity of salt solutions was characterized by a laboratory conductivity meter (Cond 730, inoLab). The thermogravimetric data was measured by a Thermal Gravimetric Infra-Red hyphenated system (TGIR, PerkinElmer) performed over a temperature range of 50 - 800 °C, at a temperature scanning rate of 40 °C min^-1, in a 20 mL min^-1 air flow. Scanning electron microscope images were taken by Nova NanoSEM 450 and Magellan 400 microscope (FEG, SEM, FEI, USA). The NaCl content of PSP-MI-53 was determined by Scanning Electron Microscopy/Energy Dispersive X-Ray Spectroscopy (SEM-EDS) analysis. The original, salt adsorbed, and regenerated PSP-MIL-53 was collected and washed with water to remove the salt on the surface of PSP-MIL-53 for characterization. The NaCl content was calculated from the sodium amount determined by EDS analysis. N2 adsorption-desorption isotherm was collected by a surface characterization analyzer (3 Flex, micromeritics, USA). The water contact angles were determined by a contact angle goniometer (Dataphysis OCA15, Dataphysics, Germany). The MIL-53 and PSP-MIL-53 powders were pressed into tablets (Quick Press KBr Pellet Kit, International Crystal Laboratories), and then characterized with ~1 pL water droplet.

Ion adsorption loading of PSP-MIL-53 [0118] Salt solutions, including LiCl, NaCl, KC1, MgCL, and CaCF, were used for the measurement of ion adsorption loading. 0.05 g of PSP-MIL-53 and 15.00 mL of salt solution (pH 8) were put in a 50 mL centrifuge tube. The pH of salt solutions were adjusted by 0.01 M NaOH solution. The adsorption occurred under UV light (254 nm, p/n 90-0001-05, UVP) or dark. The UV light intensity irradiated on the adsorbent and solution was determined to be -270 pW cm ² by a radiometer (FZ-A, photoelectric instrument factory of Beijing Normal University, China). After the adsorption of salt, the adsorbent suspension was centrifuged, and the supernatant was collected in another tube for measurement of conductivity. The ion adsorption loading (mmol-g^-1) was calculated as follows:

[0119] where n is the number of cations and anions of a salt. C (mg-L^_1) is the concentration difference of the salt solution before and after adsorption determined by the conductivity meter. V (L) is the volume of salt solution, Ms (g-moL¹) is the molecular weight of the salt, and w (g) is the weight of PSP-MIL-53 sample.

Desorption of salts adsorbed PSP-MIL-53

[0120] Visible light source with three different intensities were used for the regeneration of PSP-MIL-53, including room lighting (0.13 mW-cm ²), 0.1 sun (~10 mW-cm ²), and one sun (-100 mW-cm ²). A sunlight simulator (CHF-XM-500W, TrusTech, China) was used to generate sunlight with intensity of 0.1 sun and one sun. 15.00 mL of water (pH 8) was added to the salt adsorbed PSP-MIL-53 with visible light irradiation. Then, the sample was centrifuged after a certain of time, and washed 2 to 3 times until the conductivity of supernatant <5 pS cm^-1. The desorbed sample was dried in 60 °C oven for further uses. As for the regeneration of adsorbent in single-column setup, the valve of the column was turned off, then, water was added into column with visible light illumination.

Adsorption performance testing of PSP-MIL-53 in a single-column setup

[0121] The single-column has an inner diameter of 0.6 cm. A certain amount of PSP-MIL-53 was put into the single-column setup with an adsorbent packing density of 607 g-L^_1. 1,000, 10,000, and 35,000 ppm NaCl solutions were individually added into the column for the breakthrough curves measurement. The water drained at the bottom of the column at 1, 2, 3, 4, 5, 10, 15, 20, 30, 45, 60, 90, 120, 180, 240, and 300 min(s) were collected for analysis. The salt concentration of the collected water was determined by the conductivity meter. The filtration flux was controlled by the valve at the bottom of the column to be -100 or -1,000 L-m^-2-h^-1 (LMH).

Desalination of synthetic brackish water and synthetic seawater by PSP-MIL-53

[0122] Synthetic brackish water with 2,233 ppm salts, including 1,083 ppm Cl-, 215 ppm SO₄ ^2-. 25 ppm NO₃-, 171 ppm HCO₃-, 448 ppm Na⁺, 127 ppm Ca²⁺, and 136 ppm Mg²⁺, was prepared according to an underground brackish water in the region of Doukkala (Centre of Morocco). The pH of synthetic brackish water was 7.8. 35,000 ppm synthetic seawater was prepared by dissolving 1.75 g of sea salts (s9883, Sigma Aldrich) in pure water to form a 50 g solution. The synthetic seawater showed a pH of 8.3. The synthetic salt waters were desalted by passing through PSP-MIL-53-packed single-column setup. The ions concentration of synthetic brackish water (HCO3-, SO4^2-, Cl-, NO3-, Ca²⁺, Mg²⁺, and Na⁺), seawater (HCO3-, SO4^2-, Cl-, Ca²⁺, Mg²⁺, Na⁺, and K⁺), and PSP-MIL-53 treated solutions were sent to ALS Environmental (Melbourne, Australia) for analysis.

Results and discussion

[0123] PSP-MIL-53 was synthesized by introducing spiropyran acrylate into the pores of MIL- 53, followed by in situ polymerization. MIL-53 exhibits a breathing effect, which comprises two consecutive breathing transitions upon adsorption of some molecules (e.g., CO2, H2O). The empty MIL-53 framework is expanded with a pore size of 8.5 A. The pores become contracted (2.6x13.6 A) while partially filled, and further adsorption can reopen the pores to the expanded state (8.5x8.5 A). This highly flexible framework and proper pore size allow the entrance of SP (-5.4x5.8x11.6 A) into the MOP channels. MIL-53 with expanded pores and contracted pores show distinct X-ray powder diffraction (XRD) patterns, providing a simple way to determine the framework structure. The XRD patterns of calcined MIL-53 suggested that its pores were partially filled and contracted. The introduction of PSP expanded the contracted pores, and PSP- MIL-53 showed a pore size of -8.5 A. Therefore, PSP was successfully introduced into the framework of MIL-53, and the -8.5 A pore size effectively confined the polymer chains. In addition, this highly porous MIL-53 with a Brunauer-Emmett-Teller surface area of 875 m²-g^-1. Photo reversible capture and release of salts in water

[0124] PSP-MIL-53 shows the ability to reversibly capture and release of salts (e.g., NaCl) in water. Under dark condition or UV light irradiation, PSP transforms to the zwitterionic PMC. The positively charged indolium group and the negatively charged phenolate group simultaneously adsorb anions and cations in water, respectively. The UV-Vis spectra of merocyanine aqueous solution (pink) showed a strong peak at 512 nm, indicating the formation of extended p-conjugation between indolium and chromene moieties, and the zwitterionic structure. Then, the ion adsorbed PMC transforms to PSP under visible light, releasing the salts. The spiropyran solution became transparent after visible light illumination, exhibiting distinctively different UV-Vis spectra compared to that of merocyanine solution. Most notably, there was no peak at 512 nm. This reversible process imparts PSP-MIL-53 with the ability of photocontrol capture and release of salts from water. Whilst MIL-53 showed no adsorption of salt in 1,000 ppm NaCl solution, PSP-MIL-53 exhibited high ion (cation and anion) adsorption capacities of 0.68 mmol-g^-1 in dark, and 0.96 mmol-g^-1 under UV light (254 nm) irradiation. Importantly, PSP-MIL-53 rapidly reached adsorption equilibrium within 30 min in dark or under UV light stimulation, and underwent ultrafast regeneration in 4 min under one sun illumination, which demonstrated efficient desalination. The photo-reversibility of PSP-MIL-53 was verified repeatedly by switchable wettability, color, and composition. Concerning water wettability, PSP-MIL-53 powders were hydrophobic (119+3°) after visible light irradiation. The irradiated PMC-MIL-53 with zwitterionic structure showed relatively hydrophilic water contact angles of 84+1° (dark) and 73+1° (UV), which greatly benefits the adsorption process in water. Secondly, PSP-MIL-53 aqueous suspension showed visible and reversible color change between reddish brown (dark) and orange brown (Vis). To further confirm the efficacy of salt adsorption and regeneration, EDS analysis demonstrated that the original PSP-MIL-53 did not contain any NaCl and dried PSP-MIL-53 had 2.5 wt.% NaCl after salt adsorption in 1,000 ppm NaCl solution. The NaCl content of dried PSP-MIL-53 decreased to <0.1 wt.% after visible light regeneration. Importantly, after 10 adsorption-desorption cycles, the XRD pattern and morphology of PSP-MIL-53 demonstrated its good water- stability.

NaCl adsorption loading of PSP-MIL-53

[0125] The PSP loading of PSP-MIL-53 was varied to optimize the ion adsorption loading. The ion adsorption loading of PSP-MIL-53 in 1,000 ppm NaCl solution were 0.37, 0.96, 0.57, and 0.23 mmol-g^-1 (under UV light) with an increasing PSP loading of 15.6, 22.5, 28.7, and 36.3 wt.%, respectively. Theoretically, larger amounts of PSP could provide the composites with more adsorption sites and higher ion adsorption loading. However, the accessibility of adsorption sites may become restricted by the PSP blocked pores, causing lower adsorption. Therefore, PSP-MIL-53 with a PSP loading of 22.5 wt.% exhibited an optimized adsorption performance, and was then used for further study.

[0126] The pH of the electrolyte solution also affected the adsorption performance of PSP-MIL- 53. At a pH range of 4-9, the ion adsorption loading of PSP-MIL-53 firstly increased with increasing solution pH, then decreased, and reached the maximum at a pH of 8. The spiropyran solution was kept in dark condition at varied pH for UV-Vis characterization. The UV-Vis spectra of pink solution excited at the pH of 8 showed a strong peak at 512 nm due to the existence of extended p-conjugation between indolium and chromene moieties, which is a strong indicator of the formation of merocyanine (MC). However, at a pH of 6, the weak peak at 512 nm indicated the formation of protonated merocyanine (MCH+) and MC mixture. At pH 9, this peak could be barely seen. Therefore, the zwitterionic MC formed at pH of 8 is crucial for the high adsorption of cations and anions simultaneously, resulting in an efficient removal of salts from water. At higher or lower pH conditions, the surplus H⁺ or OH- ions would bind with the adsorption sites, thereby hindering ion adsorption.

[0127] The ion adsorption loading of PSP-MIL-53 increased with increasing NaCl concentrations of 500-35,000 ppm, and reached adsorption equilibrium in >10,000 ppm solutions. Further, the ion adsorption loading of polymerocyanine (PMQ-MIL-53 activated under UV light is always higher that under dark condition, because PMC-MIL-53 is more hydrophilic and it undergoes a more complete transformation of SP to MC under UV light irradiation. The equilibrium ion adsorption loadings were 1.94 and 2.66 mmol-g^-1 under dark and UV light, respectively. These are comparable with the best published adsorbents/ion- exchangers for desalination. PSP-MIL-53 adsorbed 0.48 (dark) and 0.72 (UV) mmol-g^-1 cations and anions in 500 ppm NaCl solution.

Fast adsorption and desorption kinetics

[0128] PSP-MIL-53 exhibited rapid adsorption and ultrafast desorption processes. When the NaCl concentration was 5,000 ppm or higher, PSP-MIL-53 reached adsorption equilibrium within 30 min under Dark/UV light illumination. The desorption kinetics of PSP-MIL-53 depended on the intensity of visible light. Desorption of PSP-MIL-53 under one sun (100 mW-cm ²) illumination only took 4 min to complete. When the intensity was decreased to 0.1 sun (~ 10 mW-cm ²), the regeneration time was increased to 30 min. PSP-MIL-53 could also be desorbed with room lighting irradiation (0.13 mW-cm ²), and it took 120 min to regenerate the adsorbent. Moreover, 98% of adsorbed ions and ion adsorption loading were recovered in slightly alkaline water (pH 8). Commercial or reported adsorbents/ion-exchangers that involved in double-bed, mono-bed, and Sirotherm processes generally take 20-900 min for the adsorption process and 20-960 mins for the desorption process. Their regeneration relied on strong acid/base solutions and/or thermal energy (>70 °C), which is chemically and/or energy intensive. By contrast, PSP-MIL-53 showed fast ion adsorption (30 min) and ultrafast desorption (4 min) processes for efficient desalination. Importantly, the abundant and renewable visible light can be used for regeneration, which can dramatically reduce the energy cost and environmental impact of the desalination process.

Desalination of NaCl solution in a single-column setup

[0129] To demonstrate its potential for practical application, PSP-MIL-53 was further tested to produce fresh water by desalting NaCl solutions in dark in a single-column setup. NaCl solutions with initial concentrations of 1,000, 10,000, and 35,000 ppm were subjected to a desalination process by passing them individually through a single-column setup with an adsorbent packing density of 607 g-L^_1. With a feed solution flux of -100 L-m^_2-h^_1 (LMH) (flow velocity of 0.1 m-h ¹), the ion adsorption loading of PSP-MIL-53 reached 1.69, 2.73, and 2.88 mmol-g^-1 after desalting 1,000, 10,000, and 35,000 ppm NaCl solutions, respectively. Accordingly, 82.5, 7.1, and 2.5 mL of fresh water (<600 ppm TDS) per gram of PSP-MIL-53 (mL-g^_1) were produced in one adsorption-desorption cycle. For desalting with 2000, 4000, 7500, and 20,000 ppm NaCl solutions, 28.4, 14.6, 8.9, and 4.1 mL-g^_1 of fresh water could be produced in each cycle, respectively. If the flux of 1,000 ppm NaCl solution was increased to -1,000 LMH (1.0 m-h ¹), the fresh water amount obtained would be reduced to 75.8 mL-g^_1 per cycle. PSP-MIL-53 exhibited excellent cations and anions adsorption performance in single column setup, which was way better than those of measured in batch adsorption experiments performed in dark condition. The amount of fresh water needed for regenerating PSP-MIL-53 was determined to be 1.5 mL-g^_1 due to the good mobility of suspension. In addition, PSP-MIL- 53 showed remarkable cycling performance in dark condition and room lighting stimulation. In ten cycles of desalting 1,000 ppm NaCl solution with a feed flux of -100 LMH, the amount of fresh water obtained slightly decreased from 82.5 to 79.5 mL-g^-1 per cycle, while reached a plateau from the third cycle.

Desalination of synthetic salt water

[0130] Apart from NaCl, PSP-MIL-53 was capable of adsorbing various monovalent and multivalent salts with excellent cycling performance. In batch adsorption experiments, PSP- MIL-53 adsorbed up to 2.30, 1.94, 1.71, 0.68, and 0.75 mmol-g^-1 of ions in the 10,000 ppm salt solutions of LiCl, NaCl, KC1, MgCh, and CaCh in dark, respectively. When the salt concentration was decreased to 1,000 ppm, the relevant ion adsorption capacities dropped to 0.76, 0.68, 0.45, 0.17, and 0.30 mmol-g^-1. PSP-MIL-53 showed lower adsorption of divalent salts than that of monovalent salts for two reasons. Firstly, a divalent cation occupies two adsorption sites. Secondly, the respective hydrated diameter of Mg²⁺ and Ca²⁺ is 8.8 and 8.4 A, which are larger than hydrated Na⁺ ion diameter (7.16 A). These large hydrated ions could experience greater resistance to diffuse into the adsorbent with a pore size of <8.5 A. Moreover, PSP-MIL-53 showed excellent cycling performance for reversible adsorption of both monovalent and divalent salts, with ion adsorption capacities only slightly decreasing from 100 % to > 90 % in 10 cycles.

[0131] PSP-MIL-53-packed single-column setup was further used to desalt 2,233 ppm synthetic brackish water and 35,000 ppm synthetic seawater with a feed solution flux of -100 LMH. The synthetic brackish water had a pH of 7.8, while the synthetic seawater showed a pH of 8.3. With regards to the desalination of synthetic brackish water, the ion adsorption loading of PSP-MIL- 53 was 1.06 mmol-g^-1 and 1.33 meq-g^-1 (milliequivalent-g^-1). Accordingly, 23.4 mL-g^-1 of fresh water (< 600 ppm TDS) could be produced in an adsorption-desorption cycle. There was 21.4 wt.% of divalent ions in synthetic brackish water, in which explains the lower adsorption loading than that in adsorption of NaCl alone. In terms of desalting synthetic seawater, it exhibited an ion adsorption loading of 2.47 mmol-g^-1 and 2.66 meq-g^-1. A low fresh water productivity of 2.3 mL-g^-1 per cycle was produced because of the high salinity of seawater.

[0132] To confirm the feasibility of PSP-MOF for water desalination, a simple and yet conservative analysis of its water desalination capacity and energy consumption was conducted on a classical two-bed desalination system. According to the results of desalting 2,233 ppm synthetic brackish water, estimated net water capacity of the system would reach 63 m³-d^_1 (139.5 L-kg^_1-d^_1), with water recovery of 88%. In this calculation, sunlight is used for regeneration at daytime (12 hours), while room lighting is used during the night. The energy consumption of the additional energy input, including pumping and lighting, is estimated to be 0.11 kWh-m^-3. With room lighting using for regeneration only, this value increases to 0.21 kWh-m^-3. It is estimated that the energy consumption of a brackish water reverse osmosis (BWRO) desalination process by desalting 2000 ppm solution. With a water recovery of 70%, the estimated SEC was 0.38 kWh-m³. Thus, this sunlight-regenerable adsorbent-based desalination process with sunlight as regeneration source is as energy-efficient as that of BWRO process for fresh water production. Therefore, by adsorbing multiple salts from water in dark and regeneration under sunlight, PSP-MIL-53 is an energy-efficient and sustainable adsorbent for desalting saline water, especially for brackish water.

[0133] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

1. A metal organic framework composition comprising: a porous metal organic framework material; and a stimuli responsive polymer confined within pores of the porous metal organic framework material, the stimuli responsive polymer comprising a stimuli responsive backbone with a macrocyclic structure grafted thereto.

2. The metal organic framework composition of claim 1, wherein the stimuli responsive polymer has a first state and a second state, and the stimuli responsive polymer undergoes a conformal change from the first state to the second state in response to a chemical stimulus, or an exogenous stimulus selected from the group consisting of: temperature, light, electric field, magnetic field.

3. The metal organic framework composition of claim 2, wherein the macrocyclic structure is arranged to adsorb ions in the first state, and desorb ions in the second state.

4. The metal organic framework composition of claim 2 or 3, wherein the first state is a hydrophilic state and the second state is a hydrophobic state.

5. The metal organic framework composition of any one of the preceding claims, wherein the stimuli responsive polymer is a thermo-responsive polymer.

6. The metal organic framework composition of claim 5, wherein the thermo-responsive polymer has a lower solution critical temperature of 90 °C or less.

7. The metal organic framework composition of any one of the preceding claims, wherein a loading of the stimuli responsive copolymer in the metal organic framework composition is from about 5 wt% up to about 60 wt%.

8. The metal organic framework composition of any one of the preceding claims, wherein the stimuli responsive backbone is a homopolymer or a copolymer comprising at least one repeating unit selected from the group consisting of: N-isopropylacrylamide, N-[2- (diethylamino)ethyl acrylamide], N,N-dimethylaminoethyl methacrylate, N,N-diethylaminoethyl methacrylate, 2-(N-morpholine)ethyl methacrylate, oligo(ethylene glycol)methacrylate, and N,N-diethylacrylamide, N-(isobutoxymethyl)acrylamide, 2-carboxyisopropylacrylamide, 2- ethoxyethyl vinyl ether, 2-methoxyethylvinyl ether, 2-(2-methoxyethoxy)ethyl methacrylate, oligo(ethylene glycol) methacrylate, N-(hydroxymethyl) acrylamide.

9. The metal organic framework composition of any one of the preceding claims, wherein a stoichiometric ratio of the at least one repeating unit to the macrocyclic structure is greater than or equal to 2.5:1.

10. The metal organic framework composition of any one of the preceding claims, wherein the pore windows have a diameter of from about 10 A up to about 30 A.

11. The metal organic framework composition of any one of the preceding claims, wherein the metal organic framework structure is selected from the group consisting of: MOF-808, MIL- 53, UiO-66, UiO-66-IPA, UiO-66-COOH, UiO-66-(COOH)₂, UiO-66-NH₂, Ui0-66-N0₂, ZIF- 8, MIL- 121, ZJU-24, NU-125-IPA, and NU-125-HBTC.

12. The metal organic framework composition of any one of the preceding claims, wherein the macrocyclic structure is selected from the group consisting of: an organic crown structure, a cryptand structure, a porphyrin structure, a calixarene structure, or a cyclodextrin structure.

13. The metal organic framework composition of any one of the preceding claims, wherein the macrocyclic structure is a crown ether structure.

14. The metal organic framework composition of claim 13, wherein the crown ether structure comprises: a crown ether 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; or a crown ether selected from the group consisting of: aminobenzo- or aminomethyl-X- crown-Y, wherein X is an integer selected from 12, 15, or 18 and Y is an integer selected from 4, 5, or 6; or a crown ether 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.

15. The metal organic framework composition of any one of the preceding claims, wherein the crown ether structure is adapted to adsorb sodium ions (Na⁺) and potassium ions (K⁺).

16. The metal organic framework composition of any one of the preceding claims, wherein the metal organic framework structure further comprises a stimuli responsive compound, different to the stimuli responsive polymer, and wherein the stimuli responsive compound is contained within the channel.

17. An ion adsorbent comprising the metal organic framework composition of any one of the preceding claims.

18. The ion adsorbent of claim 17, wherein the metal organic framework composition is disposed on, in, and/or around a substrate layer.

19. The ion adsorbent of claim 18, wherein the substrate layer is a porous substrate selected from the group consisting of a porous metal, a porous ceramic, and a porous polymer.

20. A membrane module comprising the ion adsorbent of any one of claims 17 to 19.

21. An ion adsorption unit, ion adsorption media, or ion adsorption membrane comprising the metal organic framework composition of any one of claims 1 to 16, or the ion adsorbent of any one of claims 17 to 19.

22. A method of preparing a metal organic framework composition comprising: providing a porous metal organic framework material, and initiating a polymerisation reaction within pores of the porous metal organic framework material between at least one monomer suitable for being polymerised to form a stimuli responsive polymer and a macrocyclic structure to form a stimuli responsive polymer comprising a stimuli responsive backbone with a macrocyclic structure grafter thereto confined within the pores.

23. A method of treating a liquid media to selectively adsorb ions from the liquid media, the method comprising: contacting a liquid media containing one or more target ions with a surface of the ion adsorbent of any one of claims 17 to 19 or the ion adsorption unit or ion adsorption media or ion adsorption membrane according to claim 21 under conditions in which the stimuli responsive polymer is in a first state in which the macrocyclic structure adsorbs the one or more target ions; adsorbing the one or more target ions to the macrocyclic structure; subsequently subjecting the stimuli responsive polymer to a stimulus sufficient that the stimuli responsive polymer undergoes a conformal change from the first state to a second state in which the one or more target ions desorb from the macrocyclic structure; and flushing the ion adsorption membrane with a liquid media to remove desorbed ions from the ion adsorption membrane or the ion adsorption column media.

24. The method of claim 22, wherein the liquid media is an aqueous solution comprising at least Li⁺, and one or both of Na⁺ or K⁺, and the one or more target ions are one or both of Na⁺ or K⁺; and during the step of contacting at least the portion of the liquid media with the ion adsorption membrane or the ion adsorption column media, one or both of Na⁺ or K⁺ are selectively adsorbed by the macrocyclic structure with respect to Li⁺.

25. Use of a metal organic framework composition of any one of claims 1 to 16 in an ion adsorption process.

26. Use of the metal organic framework composition of any one of claims 1 to 16 in a method of forming one or more of an adsorption membrane, an adsorbent component of an adsorption membrane, an adsorption membrane module, an adsorption media, or an adsorption column.

27. A method comprising: forming an adsorption membrane, an adsorbent component of an adsoprtion membrane, an adsorption membrane module, an adsorption media, or an adsorption unit comprising the metal organic framework composition of any one of claims 1 to 16.

Monash University Patent Attorneys for the Applicant SPRUSON & FERGUSON