IONIC LIQUID ENCAPSULATED METAL ORGANIC FRAMEWORK BASED THIN FILM NANOCOMPOSITE MEMBRANE

FOR METAL IONS SEPARATION

FIELD OF THE INVENTION

The present invention relates to filtration membranes useful in metal ion separation, gas separation and water purification/desalination. In particular, the present invention relates to a new type of membrane including metal organic frameworks (MOFs), thin- film nanocomposite (TFN), and ionic liquids (ILs).

BACKGROUND OF THE INVENTION

Metal ion separation and recovery from e-waste is of sustainable importance due to the rapid depletion of geographical mineral ores, limited natural resources of critical metals and deteriorating environment owing to the improper disposal of e-waste. Among all the separation techniques, membrane separation has received increasing attention for its ecologically and economically friendly traits. On the other hand, there are growing appeals for using metal organic frameworks (MOFs) in metal ion separation field for their designable structures, ordered porosities and versatile organic linkers. Hybridization of porous MOFs with membrane can provide an efficient alternative for metal ion separation.

The main problems of the membranes used for metal ion separation application, include:

(i) the current development of membrane used in metal ion separation area is always hindered by its poor selectivity;

(ii) nanofiller encapsulated thin-film nanocomposite (TFN) membranes usually exhibit low permeability;

(iii) MOFs encapsulated TFN membranes usually show weak mechanical strength and poor adherence between MOFs and membrane substrates.

The current solutions to the aforementioned existing problems of membrane are summarized below.

(i) The common practice for improving the selectivity of the TFN membrane is hybridizing various nanofillers such as boron nitride, nano-clays and graphene oxide with the TFN membrane layers. However, the structures of these nanofillers are always lack variability and are difficult to modulate or change. Therefore, the function of the nanofiller hybridized with the TFN membrane is not adjustable. Some work has been reported on the possibility of using MOFs as the nanofiller for tailorable structures using the large variety of available such frameworks. However, it also leads to the low permeability of the TFN membrane.

(ii) To date, the most used MOFs as nanofillers are UiO-66 (Zr), ZIF-8 and CuBDC. The low permeability of these nanofiller encapsulated TFN membrane is mainly caused by the “ pore-blocking ” effect of the MOFs (with small pore size and high microporosities) additives. On the other hand, if MOFs with large pore size and low microporosities are hybridized with the TFN membrane, poor selectivity is observed.

(iii) Many approaches are available in the literature to combine MOFs with membranes, such as low temperature reaction (Liu, S. et al., Journal of Materials Chemistry A, 2017, 5(44), p. 22988-22996), introduction of an intermediate layer (Karan, S. et al., Science, 2015, 348(6241), p. 1347-1351), macromolecule incorporation (Tan, Z. et al., Science, 2018, 360(6388), p. 518-521), electro spraying (Chowdhury, M.R. et al., Science, 2018, 361(6403), p. 682-686) and nano-foaming (Ma, X.-H. et al., Environmental Science & Technology Letters, 2018, 5(2), p. 123-130). However, these methods are always complicated, with multi-steps involved and high associated costs.

For example, Chen et al. (Chen, X. et al., ACS Applied Materials & Interfaces, 2020, 12(35), p. 39227-39235) reported the MOF-808-loaded polyacrylonitrile membrane for heavy metal ion removal. In this work, the MOF-808 is loaded onto the membrane by the electro spraying method, where the electrospinning apparatus is required. In addition, they only reported the single metal ion removal performance and no metal ion pairs separation results are provided. Furthermore, neither the cyclic stability nor the permeability performance is reported. In another study, Li et al. (Li, X. et al. Advanced Materials Technologies, 2021, 6(10), p. 2000790) implanted several MOEs (with smaller pore size) onto an Anodic aluminum oxide (AAO) type of porous substrate via various techniques such as in situ growth strategy, contra-diffusion growth and solid confinement conversion. However, the operation process of these methods is much more complicated when compared with that of, for example, Interfacial Polymerization (IP) reaction. Other drawbacks such as poor control of MOP growth onto the substrate and possible uneven MOP distribution or agglomeration of MOP on the substrate surface make it less competitive. In addition, AAO substrate membrane is extremely fragile, presents very small flexibility and cannot be produced for large surface area. The lack of mechanical strength and flexibility makes this type of MOF based TFN membrane not suitable for cyclic metal ion separation application or even gas separation.

A chromium terephthalate-based MOF namely MIL-lOl(Cr) has been considered as a potential candidate for Thin Film Nanocomposite (TFN) membrane technology (Ferey G. et al., Science, 2005, 309(5743): p. 2040-2042). Structurally, MIL-lOl(Cr) is a micro-mesoporous material with giant pore size (~ 30 to 34 A), high pore volume (~ 702,000 A ³ ) and large Langmuir surface area (~ 5900 + 300 m ² /g). Furthermore, MIL- 10 l(Cr) is chemically stable in aqueous environment due to the strong coordination bonds between carboxylate-type linkers and high-valence metal ions Cr ³⁺ (Wang, C., et al., Chemical Society Reviews, 2016, 45(18), p. 5107-5134). One of the most prominent merits of MIL-lOl(Cr) is that it possesses two meso-porous cages with internal free o o diameters of ~ 29 A and 34 A, respectively. These large openings always lead to high solution flux. In one study, Sorribas et al. used MIL- 101 (Cr) based TFN membrane for organic solvent nanofiltration and an exceptional increase in permeance from 1.7 up to 11.1 L-m ^h^-bar ¹ for tetrahydrofuran/styrene oligomer solution was observed (Sorribas, S., etal., Journal of the American Chemical Society, 2013, 135(40), p. 15201- 15208). To date and to inventors’ knowledge, although several works have reported the noteworthy permeability of MIL-lOl(Cr) based TFN membranes in organic solvent nanofiltration application, study on MIL-lOl(Cr) based TFN membranes used for metal ion separation has not been previously investigated. This is mainly caused by the two foremost drawbacks of the original MIL-lOl(Cr): oversize cages and low stability in aqueous environment. Moreover, the lack of mechanical strength and flexibility makes this type of MOF based TFN membrane not suitable for cyclic metal ion separation application or gas separation.

There is thus a need to develop new MOF based TFN membranes suitable for cyclic ion separation applications such as metal ion separation, metal ion pairs separation, gas separation, water purification/desalination, with improved mechanical strength and flexibility, and enhanced adherence between MOFs and TFN membranes.

Furthermore, there is a real need for new MOF based TFN membranes having improved stability in aqueous environment, high permeability and enhanced selectivity for ion separation in applications such as metal ion separation, metal ion pairs separation, gas separation, water purification/desalination.

SUMMARY OF THE INVENTION The present invention addresses the above-identified needs among others by providing a MOF based thin-layer nanocomposite membrane (TFN), characterized in that it comprises:

- a first substrate layer, the substrate including anodic aluminum oxide (AAO), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polysulfone (PSF),

- a second polyamide (PA) layer on top of the first substrate layer, and

- a third metal-organic framework (MOF) layer on top of the second polyamide layer, the MOF being MIL-lOl(Cr), and in that an ionic liquid (IL) is encapsulated onto the MOF particles of the MOF layer, the IL being selected among

[BMIM] type ILs including but not limited to l-butyl-3-methylimidazolium acetate, 3-methylimidazolium chloride, 3-methylimidazolium bromide, 3- methylimidazolium methanesulfonate, 3-methylimidazolium tetrafluoroborate, 3-methylimidazolium tetrachloro-aluminate, 3-methylimidazolium hexafluorophosphate, and

[EMIM] type ILs including but not limited to l-ethyl-3-methylimidazolium chloride, 1 -ethyl- 3-methylimidazolium bromide.

The metal-organic framework (MOF) layer on top of the first substrate layer is a continuous layer containing MOF nanoparticles which are octahedral with their sizes ranging between 20-100 nm, preferably between 20-80 nm. The “size” of an octahedral may be defined as a straight line segment that passes through the center of the octahedral structure and whose endpoints lie on the sides of the octahedral structure. It is of note that the size change of the original MIL-lOl(Cr) when implanted by an IL is negligeable. The expression “IL encapsulated onto or into the MOF” or “IL implanted onto or into the MOF”, also denoted as IL@MOF, designates IL functionalized MOFs or MOF- supported IL where IL is encapsulated within the MOF structure via chemical reaction (which is stable) instead of just physically penetrating inside the MOF pores (which is unstable).

For sake of clarity, the encapsulation onto a MOF (MIL-lOl(Cr)) by an IL ([BMIM] ⁺ [A1C14]’) is shown in [Fig. 3]. The IL is encapsulated onto the MOF structure via chemical reaction (which is stable) instead of penetrating inside the MOF pores via physical reaction (which is instable). Fig.3 shows the IL encapsulation process. As shown in Fig.3(a), the black atom represents the Cr atom of the original MIL-lOl(Cr) MOF and each Cr is connected with one water (H2O) molecule (also shown in Fig.3(b)). First, dehydration process is applied to the original MIL- 101 (Cr) and the water molecule is removed permanently. Hence, coordinatively unsaturated chromium sites (CUS) are created (Fig.3(b)). Next, ionic liquid (IL) is encapsulated to the CUS as shown in Fig.3(c)).

Expressions such as “hybridization of MOFs with a membrane” or “MOFs hybridized with a membrane” are to be understood as MOFs bonded to and/or embedded in a membrane resulting in a composite membrane that exhibits improved filtration performance and specific properties.

In the context of the present invention, the term “original MOF” designates a MOF that has not been modified/functionalized by an IL, and the term “pure substrate” designates a substrate that has not been modified by a MOF.

In the present invention the “polyamide” is the product of the IP reaction and its composition depends on the reactants. The possible reactants for IP reaction includes but not limited to:

• reactants in aqueous phase (solution A) such as piperazine (PIP), m- phenylenediamine (MPD), benzylamine;

• reactants in organic phase (solution B) such as 1,3,5-benzenetricarbonyl trichloride (trimesoyl chloride or TMC), benzene- 1,4-dicarbonyl chloride;

• organic solvent in organic phase (solution B) such as pentane, hexane, cyclohexane, benzene, toluene.

The polyamide layer is thus formed via the step-growth polymerization (IP reaction) process, at the interface between two immiscible phases namely “aqueous phase” and “organic phase” as described above. Hence, the polyamide layer is constrained to the interface.

In the present invention, “substrate” refers to AAO, PVDF, PTFE or PSF membrane, which is the bottom layer of the MOF based thin-layer nanocomposite membrane (TFN). It is of note that AAO, PVDF, PTFE or PSF substrate could be used as “membrane” per se. To differentiate pure AAO/PVDF/PTFE/ PSF membrane from the MOF based thin-layer nanocomposite membrane, the term “substrate” is used to show that AAO/PVDF/PTFE/ PSF is part of the invention’s MOF based TFN membrane and that it is located at the bottom. The invention also concerns a process for manufacturing a MOF based thin-layer nanocomposite membrane (TFN) according to the present invention, characterized in that it comprises the steps of:

(i) Synthesis of a MOF, the MOF being MIL-lOl(Cr), by hydrothermal method, by mixing a metal salt and at least one acid in deionized water to form a solution, placing the solution in an autoclave reactor and heating at a temperature ranging from 200-400°C to form a reaction mixture containing metal organic framework (MOF) crystals, cooling the reaction mixture and filtering the MOF crystals therefrom, drying the MOF crystals at a temperature ranging from 40 to 100°C, preferably from 60 to 90°C, optionally submitting the as-synthesized MOF crystals to a thermal treatment at a temperature between 120 and 300°C, preferably between 150 and 250°C;

(ii) Modifying the resulting MOF with an IL as defined above by a wet impregnation method, by mixing the dehydrated optionally activated MIL- 101 (Cr) obtained in step (i) with ionic liquid in an aliphatic alcohol of formula R-OH where R is methyl, ethyl, n-propyl, iso-propyl, n-butyl, s-butyl, tert-butyl, iso-butyl liquor at a weight ratio IL:MOF from 0.5:1 to 3:1. The mixture is stirred for 6 to 48 hours, preferably for 12 to 24 hours, at ambient temperature (25 ± 5 °C) and pressure (1 atmosphere = 1.01325 bars = 101325 Pa); and filtrating the resulting IL@MOFs sample, optionally purifying with an aliphatic alcohol of formula R-OH where R is methyl, ethyl, n-propyl, iso-propyl, n-butyl, s-butyl, tert-butyl, iso-butyl, and drying at a temperature of 60 to 120°C, preferably 80 to 100°C, for 6 to 15 hours, activating the resulting IL@MOFs at a temperature of 140 to 230°C, preferably 160 to 200°C, for 1 to 10 hours, preferably 3 to 5 hours, and

(iii) Manufacturing MOFs based TFN membrane by interfacial polymerization (IP), by the following steps

(a) solution preparation Solution A (aqueous phase): an aqueous solution containing the modified MIL- 101 (Cr) obtained in step (ii), a monomer (A) and deionized water is prepared for example via sonication or any other known mechanical stirring method, Solution B (organic phase): an organic solution is prepared by dissolving a monomer (B) in an organic solvent;

(b) vacuum filtration

Solution A is poured onto the vacuum filtration system and filtered through the substrate;

(c) interfacial polymerization (IP)

Solution B is added onto the monomer (A)-MOF embedded substrate obtained in step (b) leading to the formation of a polyamide layer;

(d) thermal treatment after removal of excess organic liquids by draining for example, the resulting membrane is heated, at a temperature of 40 to 80°C, preferably between 50 and 70°C, for 1 to 10 min., preferably 1 to 5 min.

Another object of the invention is the use of a MOF based thin-layer nanocomposite membrane (TFN) according to the invention, or obtained by a process according to the invention, for

- selective separation of ions,

- water desalination,

- water purification, and

- gas separation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by the following drawings and examples which illustrate embodiments thereof. These examples and drawings should not in any way be interpreted as limiting the scope of the present invention.

[Fig.l] is a comparison study on the metal ion separation (Na^/Ni ² ^ performance in terms of rejection rate for pure PVDF substrate, original and ionic liquids encapsulated MOF-based TFN membranes.

[Fig.2] represents MOF-based TFN membrane. (a)&(b): without IP reaction; (c): with IP reaction and (d)&(e): cyclic stability test with IP reaction. The MOF-based TFN membrane structure with IP reaction shows great mechanical strength and does not delaminate when folded, an important feature toward industrialization.

[Fig.3] represents the ionic liquid encapsulation process onto the original MIL-lOl(Cr). [Fig.4] is a SEM picture of MOF encapsulated PVDF based TFN membrane, where the polyamide layer resulting from IP reaction is shown for comprehensive understanding. The SEM image is taken by using the FE-SEM apparatus (JEOL JSM-7600F). Pre-gold- coating work on membrane is conducted for the electrical conductivity enhancement. The investigation voltage/mode/magnification = 15.0 kV/EM/x 500.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a MOF based thin-layer nanocomposite membrane (TFN) comprising: a MOF based thin-layer nanocomposite membrane (TFN), characterized in that it comprises:

- a first substrate layer, the substrate including anodic aluminum oxide (AAO), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polysulfone (PSF),

- a second polyamide (PA) layer on top of the first substrate layer, and

- a third metal-organic framework (MOF) layer on top of the second polyamide layer, the MOF being MIE-lOl(Cr), and in that an ionic liquid (IE) is encapsulated onto the MOF particles of the MOF layer, the IE being selected among

[BMIM] type IFs including but not limited to l-butyl-3-methylimidazolium acetate, 3-methylimidazolium chloride, 3-methylimidazolium bromide, 3- methylimidazolium methanesulfonate, 3-methylimidazolium tetrafluoroborate, 3-methylimidazolium tetrachloro-aluminate, 3-methylimidazolium hexafluorophosphate, and

[EMIM] type IFs including but not limited to l-ethyl-3-methylimidazolium chloride, 1 -ethyl- 3-methylimidazolium bromide.

In a preferred embodiment, the substrate is polyvinylidene fluoride (PVDF).

In another preferred embodiment, the substrate is poly vinylidene fluoride (PVDF) and the MOF is MIE-lOl(Cr).

In this invention, the MOF is used as nanofiller for TFN membrane modification. The MOF is hybridized with the membrane substrate by IP reaction' As a result, the surface physicochemical properties are improved, additional molecular pathways are created and the metal ion separation, gas separation or water purification/desalinisation performance are improved.

When the MOF is MIE-lOl(Cr), structurally, MIE-101 (Cr) possesses relatively giant pore size (~ 30 to 34 A) and pore volume (~ 702,000 A ³ ). Therefore, the permeability of the MIE- 101 (Cr) based TFN membrane is maintained. Ionic liquid (IE) implantation onto the MOF, improves the selectivity of the membrane, pore size reduction work. By encapsulating certain ionic liquid onto the MOF framework, the pore size of the MIL- 101 (Cr) can be precisely tailored and controlled. The optimal ILs-MIL-101 (Cr) based TFN membrane can exhibit both relatively high permeability and promising selectivity for metal ion or gas separation applications as well as water purification/desalinisation. In the present invention, the weak mechanical strength and poor adherence problems are overcome by implanting MOFs on the surface of a mechanically robust and flexible membrane substrate in general, and poly-vinylidene fluoride (PVDF) membrane in particular, via interfacial polymerization (IP) reaction method (Raaijmakers, M.J. et al., Progress in Polymer Science, 2016, 63, p. 86-142. IP reaction is a diffusion-reaction process, where the poly-condensation occurs at the interface of organic monomers and aqueous diamine monomers. As compared with other methods discussed above, IP reaction is easier to control, is timesaving, and is therefore a cost efficient process.

In the MOF based thin-layer nanocomposite membrane (TFN) of the invention, the MOF/TFN ratio (in terms of weight) is between 0.35 and 1.07, preferably between 0.525 and 0.875.

The polyamide/TFN ratio (in terms of weight) is between 1:10 and 1:6, preferably between 1:9 and 1:7.

In the MOF based thin-layer nanocomposite membrane (TFN) the invention, the IL/MOF ratio (in terms of weight) is between 0.5:1 and 4:1, preferably between 1:1 and 3:1.

The thickness of the first layer substrate layer is around 55pm, no matter how much amount of MOF are encapsulated. By “around 55pm”, it is meant “55 + 5pm.

The thickness of the second polyamide (PA) layer which is on top of the first substrate layer, is between 10pm and 30pm, preferably between 15pm and 25pm.

The thickness of the third metal-organic-framework (MOF) layer is between 15 pm and 45 pm, preferably between 22.5 pm and 37.5 pm.

The overall thickness of a MOF based thin-layer nanocomposite membrane (TFN) according to the invention is between 85pm and 115pm, preferably between 92.5pm and 107.5pm.

The invention also concerns a process for manufacturing a MOF based thin-layer nanocomposite membrane (TFN) according to the present invention, characterized in that it comprises the steps of: (i) Synthesis of a MOF, the MOF being MIL-lOl(Cr), by hydrothermal method, by mixing a metal salt and at least one acid in deionized water to form a solution, placing the solution in an autoclave reactor and heating at a temperature ranging from 200-400°C to form a reaction mixture containing metal organic framework (MOF) crystals, cooling the reaction mixture and filtering the MOF crystals therefrom, drying the MOF crystals at a temperature ranging from 40 to 100°C, preferably from 60 to 90°C, optionally submitting the as-synthesized MOF crystals to a thermal treatment at a temperature between 120 and 300°C, preferably between 150 and 250°C;

(ii) Modifying the resulting MOF with an IL as defined above by a wet impregnation method, by mixing the dehydrated optionally activated MIL- 101 (Cr) obtained in step (i) with ionic liquid in an aliphatic alcohol of formula R-OH where R is methyl, ethyl, n-propyl, iso-propyl, n-butyl, s-butyl, tert-butyl, iso-butyl liquor at a weight ratio IL:MOF from 0.5:1 to 3:1. The mixture is stirred for 6 to 48 hours, preferably for 12 to 24 hours, at ambient temperature (25 ± 5 °C) and pressure (1 atmosphere = 1.01325 bars = 101325 Pa); and filtrating the resulting IL@MOFs sample, optionally purifying with an aliphatic alcohol of formula R-OH where R is methyl, ethyl, n-propyl, iso-propyl, n-butyl, s-butyl, tert-butyl, iso-butyl, and drying at a temperature of 60 to 120°C, preferably 80 to 100°C, for 6 to 15 hours, activating the resulting IL@MOFs at a temperature of 140 to 230°C, preferably 160 to 200°C, for 1 to 10 hours, preferably 3 to 5 hours, and

(iii) Manufacturing MOFs based TFN membrane by interfacial polymerization (IP), by the following steps

(a) solution preparation

Solution A (aqueous phase): an aqueous solution containing the modified MIL- 101 (Cr) obtained in step (ii), a monomer (A) and deionized water is prepared for example via sonication or any other known mechanical stirring method, Solution B (organic phase): an organic solution is prepared by dissolving a monomer (B) in an organic solvent;

(b) vacuum filtration Solution A is poured onto the vacuum filtration system and filtered through the substrate;

(c) interfacial polymerization (IP)

Solution B is added onto the monomer (A)-MOF embedded substrate obtained in step (b) leading to the formation of a polyamide layer;

(d) thermal treatment after removal of excess organic liquids by draining for example, the resulting membrane is heated, at a temperature of 40 to 80°C, preferably between 50 and 70°C, for 1 to 10 min., preferably 1 to 5 min.

The heat treatment in step (d) can be done by an annealing treatment or any other suitable thermal treatment known to the skilled person in the art.

The thickness of the MOF layer in step (ii) is controlled by the amount of MOF containing solution that is filtered.

The amount of modified MIL-101 (Cr) in Solution A can be comprised between 0.01% to 0.50% (w/v), preferably between 0.15% to 0.25% (w/v).

The amount of monomer (A) in Solution A can be comprised between 1 to 50 mmol/L, preferably between 15 to 25 mmol/L.

The amount of monomer (B) in Solution B can be comprised between 0.1 to 50 mol/L, preferably between 1 to 10 mol/L.

The monomer (A) can be selected among but not limited to piperazine (PIP), m- phenylenediamine (MPD), benzylamine.

The monomer (B) can be selected among but not limited to 1,3,5-benzenetricarbonyl trichloride (trimesoyl chloride or TMC), benzene- 1,4-dicarbonyl chloride.

The vacuum filtration system can be by batch in a container for example, or a roll-based continuous filtration system.

The organic solvent can be selected among but not limited to pentane, cyclohexane, hexane, benzene, toluene.

As shown in [Fig.4], the IP reaction leads to the formation of a continuous MOF layer on top of the substrate (PVDF). In addition, the polyamide layer resulted from the IP reaction acts like glue, to enhance the adhesion between the MOF layer and the PVDF layer.

Non-limiting examples of metal salts include metal salts formed from Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, La, W, Os, Ir, Pt, Au, Al, Ga, In, Ge, Sn, Pb, alkali metals selected among Li, Na, K, Rb, Cs, alkaline earth metals chosen among Mg, Ca, Sr, Ba.

The acid used in step (i) may be selected among HC1, HNO3, H2SO4, terephtalic acid, or a mixture thereof.

In an embodiment according to the invention, the synthesis of MIL- 101 (Cr) based TFN membranes includes the following main steps:

(i) Synthesis of a modified MIL-lOl(Cr) by hydrothermal method as described in the examples;

(ii) Modifying the resulting MOF with an IL as defined above by a wet impregnation method, by mixing the dehydrated activated MIL- 101 (Cr) obtained in step (i) with ionic liquid in 100 ml of ethanol liquor at a given weight ratio (from 0.5:1 to 3:1 for IL:MOF). The mixture is stirred for 24h at ambient temperature (25 ± 5 °C) and pressure (1 atmosphere = 1.01325 bars = 101325 Pa), the IL@MOFs sample is filtered, purified with ethanol (2 x 50 ml), dried at 80°C overnight, and activated at 160 °C for 5 h; and

(iii) Manufacturing MOFs based TFN membrane fabrication by interfacial polymerization (IP), by

(a) solution preparation

Solution A: 100 ml of aqueous solution containing 0.1% (w/v) of original or modified MIL-101 (Cr), 0.2% (w/v) of piperazine (PIP) and deionized water is prepared via sonication. Solution B: 100 ml of organic solution is prepared by dissolving 1,3,5-benzenetricarbonyl trichloride (0.15% (w/v)) in hexane;

(b) vacuum filtration

5 to 15 ml of Solution A is poured into the vacuum filtration container and filtered through the nanoporous anodic aluminum oxide (AAO) or Polyvinylidene fluoride (PVDF) substrates;

(c) interfacial polymerization (IP)

6 ml of Solution B is added onto the PIP-MOFs embedded AAO, or PVDF, substrates for 1 min to perform IP (interfacial polymerization) reaction, from the reaction of 1,3,5-benzenetricarbonyl trichloride with the piperazine; and

(d) thermal treatment the excess organic liquids are removed, the membrane is heated at 70 °C for 5 min. Hence, one MIL-lOl(Cr) based TFN membrane via IP reaction is obtained. The interfacial polymerization (IP) is a type of step-growth polymerization process, in which the polymerization occurs at the interface between two immiscible phases. The IP reaction enhances the adhesion between MOF and the substrate and thus, the overall adhesion strength of the TFN membrane.

The ILs are used to tailor the porosities (such as the BET surface area, micropore volume and the total pore volume) of the original MOF. By tailoring the MOF porosities, the metal ion separation performance will be regulated and improved.

Another object of the invention is the use of a MOF based thin-layer nanocomposite membrane (TFN) according to the invention, or obtained by a process according to the invention, for

- selective separation of ions,

- water desalination,

- water purification, and

- gas separation.

The MOF based thin-layer nanocomposite membranes (TFN) according to the invention are useful for metal ion separation related applications, such as heavy metal removal, water desalination, lithium ion battery and water purification.

The invention will be further described and illustrated by the following examples.

EXAMPLES

Materials and chemicals

Commercially available hydrophilic polyvinylidene fluoride (PVDF) substrate is purchased from Merck. Chromic nitrate nonahydrate (Cr(NO3)3-9H ₂ O, 99%), terephthalic acid (H ₂ BDC, C ₆ H ₄ -1,4-(CO ₂ H) ₂ , 98%, H ₂ BDC), ammonium fluoride (NH ₄ E, > 98%), piperazine (C ₄ HION ₂ , 99%, PIP), 1,3,5- Benzenetricarbonyl Trichloride (C6H3(COC1)3, 98%, TMC), dimethylformamide (DMF, 99.8%), ethanol (C2H5OH, > 99.8%), concentrated hydrochloric acid (HC1, 37%), hexane (CH3(CH ₂ ) ₄ CH3, > 97%) and all the ionic liquids (ILs) are purchased from Merck. Collectively, the imidazolium-based ILs used in this work contain the identical cation of 1- Butyl-3-methylimidazolium [BMIM] ⁺ , but seven different anions namely chloride [Cl]’, bromide [Br]“, hexafluorophosphate [PEe]’, tetrafluoroborate [BF4]", tetrachloroaluminate [AICI4]’, methanesulfonate [CH3O3S ]’ and acetate [CH3COO] .

All chemicals are of analytical grade and used without further purification.

1. Fabrication of ionic liquid encapsulated MIL-101 (Cr) based TFN membrane

The fabrication of ionic liquid encapsulated MIL-lOl(Cr) based TFN membrane involves three main steps, namely, (i) MIL-lOl(Cr) synthesis, (ii) Modification/functionalization on MIL-lOl(Cr) and (iii) MOFs based TFN membrane fabrication, which are shown below.

(i) MIL-lOl(Cr) synthesis

MOF namely MIL-lOl(Cr) is synthesized via hydrothermal reaction method. Firstly, equimolar (0.01 mol) of chromic nitrate nonahydrate (Cr(NO3)3-9H2O), terephthalic acid (H2BDC, CeFL- 1 HCChFfh) and 1 ml of concentrated HC1 (37%) are mixed in 48 ml of deionized water. The mixture is stirred for 30min. under ambient environment. Secondly, the mixture is transferred into the liner, tightly sealed in the Teflon-lined stainless-steel autoclave reactor and heated under 220 °C for 16h. Thirdly, the autoclave is cooled down naturally. The mixture is filtrated from the solution and crystalline green solid is obtained. It is noted that certain amount of recrystallized or unreacted H2BDC is trapped both inside/outside the pores of MIL- 101 (Cr). These impurities always lead to a low yield and poor quality of the final product, therefore, rigorous purification procedures are needed. Initially, the mixture is filtered through the large pore fritted glass filter (no.2) and small pore (no.5) paper filter consecutively. Next, the green solid is eluted with hot dimethylformamide (DMF) and ethanol (three times) at 80°C. Finally, the as-synthesized MIL-lOl(Cr) is dried at 80°C overnight and activated at 160°C for 5h under vacuum condition. The activation process aims to eliminate thoroughly the water vapor, residual reagent or any other impurities.

(ii) Modification on MIL-lOl(Cr)

The wet-impregnation method is used for the ionic liquids encapsulation onto MIL-lOl(Cr). Firstly, 1 g of activated MIL-lOl(Cr) is immersed in 200 ml of the ammonium fluoride (NH4F, 1 M) and stirred at 80°C for 24h. This dehydration process aims to create the coordinatively unsaturated chromium sites (CUS) at the secondary building unit (SBU) of the MIL-lOl(Cr). The dehydrated MIL-lOl(Cr) is directly washed with hot ethanol (3 x 50 ml) without cooling, dried at 80°C overnight and vacuum-heated at 160°C for 5h. Secondly, 1 g of activated dehydrated MIL-lOl(Cr) and 1 g of l-Butyl-3- Methylmidazolium methanesulfonate ([BMIM] ⁺ [MeSO3]’) are dispersed in 100 ml of ethanol liquor. The mixture is stirred for 24h at ambient pressure (1 atmosphere = 1.01325 bars = 101325 Pa) and temperature (20 + 5°C) and the ILs are implanted onto the CUS of the dehydrated MIL-101 (Cr). Next, the ILs@MOFs, sample is filtrated, washed with ethanol (2 x 50 ml), dried at 80 °C overnight and activated at 160 °C for 5 h. Hence, [BMIM] ⁺ [MeSO3]’ type ILs encapsulated MIL-lOl(Cr) with an impregnation weight ratio of 1:1 is obtained and nominalized as “1:1 [BMIM] ⁺ [ MeSCh]’© MIL-lOl(Cr)”. It is noted that the ILs encapsulation procedures remain the same for all the ILs@MOFs materials, except for the change of type as well as the amount of the ILs. By performing ionic liquid implantation onto the parent MOF based TFN membrane, the ionic conductivity will be improved, additional transfer pathways are created, the ion exchange property could be revised. In addition, the ionic liquid additives would enhance the electrostatic repulsion of the membrane surface, which would bring favorable effect on the rejection of multi-valence metal ions. Furthermore, improved metal ion separation performance will be obtained as the size sieving effect is enhanced by the embedment of ionic liquids.

(Hi) MOFs based TFN membrane fabrication

Four main steps are included for the synthesis of MIL-lOl(Cr) based TFN membranes:

1) Solutions preparation

Solution A: 100 ml of aqueous solution containing 0.1% (w/v) of modified MIL-lOl(Cr), 0.2% (w/v) of piperazine (PIP) and deionized water is prepared via sonication.

Solution B: 100 ml of organic solution is prepared by dissolving 1,3,5- Benzenetricarbonyl Trichloride (0.15% (w/v)) in hexane;

2) Vacuum filtration 10 ml of Solution A is poured onto the vacuum filtration container and filtered through the nanoporous poly-vinylidene fluoride (PVDF) substrates;

3) Interfacial polymerization (IP)

6 ml of Solution B is added onto the PIP-MOFs embedded PVDF substrates for Imin to perform IP (interfacial polymerization) reaction, from the reaction of 1,3,5-Benzenetricarbonyl Trichloride with the piperazine; and

4) Thermal treatment

The excess organic liquids are removed, the membrane is heated at 70°C for 5 min. Hence, one MIL-lOl(Cr) based TFN membrane via IP reaction is obtained.

2. Metal ion separation experiments

An embodiment of the present invention is the ionic liquid encapsulated MIL- 10 l(Cr) based PVDF type TFN membrane, where the implantation of MOF onto the substrate membrane is achieved by the IP reaction. Therefore, three main characteristics are included in this embodiment, i.e. (i) MOF implanted PVDF membrane; (ii) ionic liquid encapsulated MOF and (iii) IP reaction. Other embodiments which are not fully covering these three characteristics have also been fabricated and their performances are compared with this embodiment.

For example, a comparison study on the metal ion separation NcC/Ni ² *') performance in terms of rejection rate for pure PVDF substrate, original and ionic liquids encapsulated MOFs based TFN membranes are shown in [Fig. 1], The experiments are conducted in the H-cell apparatus. Typically, two half-cell chambers are connected tightly and MOFs based TFN membranes are installed in the middle. The feed side solution on the left side of the H-cell contains metal ions and deionized water is used as the permeate (right side of the H-cell). The concentration of metal ion on the permeate side is tested every one hour (up to 8 hours) and the rejection rate for Na ⁺ /Ni ²⁺ is calculated accordingly. Here, the rejection rate is defined as:

R (rejection rate) = 1 — where C _p is the metal concentration of permeate cf solution and Cf is the metal concentration of feed solution. In the present study, the initial Na ⁺ /Ni ²⁺ concentration fot the feed and permeate solution is 0.0 IM mg/L and 0 (pure deionized or DI water), respectively.

From Fig.1(a), no significant difference of rejection rate is observed between Na ⁺ /Ni ²⁺ pair when the pure PVDF substrate is used as the membrane, especially for the first 8 hours. After 24 hours, the rejection rate for Na ⁺ and Ni ²⁺ is 71.3% and 75.0%, respectively.

Improvement is obtained when MIL-lOl(Cr) implanted PVDF type TFN membrane is applied (Fig.1(b)). Here, larger rejection rate difference between Na ⁺ (75.4%) and Ni ²⁺ (82.1%) is found after 24 h.

The best Na ⁺ /Ni ²⁺ separation performance is achieved by the ionic liquid encapsulated MIL-lOl(Cr) based PVDF membrane, as shown in Fig.1(c). Evident separation effect is observed from the first hour and the rejection rate difference becomes more important as time increases. After 24 hours, a 21.1% of rejection rate difference is observed for Na ⁺ (30.7%) and Ni ²⁺ (51.8%) pair. Such a larger rejection rate difference indicates the improved separation performance of Na ⁺ /Ni ²⁺ pair by using ionic liquid encapsulated MIL-lOl(Cr) based TFN membrane.

Besides the selectivity, the mechanical strength, flexibility as well as the cyclic stability are also important for membrane evaluation. For example, Fig.2(a)&(b) show the MOF based TFN membrane without IP reaction. Either weak adherence between MOF and PVDF substrate (Fig.2(a)) or poor mechanical stability of MOF layer in the aqueous environment (Fig.2(b)), is found. On the other hand, enhanced mechanical strength and adherence between MOF and PVDF substrate is obtained when IP reaction is involved, as shown in Fig.2(c). Additionally, cyclic metal ion separation experiments are performed using IP reaction involved MOF based TFN membrane and promising stability is observed (Fig.2(d)). After a cyclic metal ion separation experiment over five days, the MOF layer is robustly embedded on the PVDF substrate as it was first fabricated. Here, Fig.2(e) shows that the adherence and the flexibility of the overall structure is maintained after cyclic metal ion separation test.

These results show that the metal ion separation performance of the TFN membrane can be tailored and improved by using ionic liquids implanted MOFs. The mechanical properties of these membranes are likely to be compatible with industrial manufacturing processing and implementation.

More generally, the membranes of the invention are useful for metal ion separation, heavy metal removal, gas separation, water desalination, lithium ion battery and water purification.