EP4259641A1

EP4259641A1 - Polycrystalline iron-containing metal-organic framework membranes for organic solvent nanofiltration

Info

Publication number: EP4259641A1
Application number: EP21907265.9A
Authority: EP
Inventors: Dan Zhao; Weidong Fan; Xin Yu
Original assignee: National University of Singapore
Current assignee: National University of Singapore
Priority date: 2020-12-14
Filing date: 2021-12-10
Publication date: 2023-10-18
Also published as: WO2022132045A1; JP2023553187A; CN116888134A; EP4259641A4

Abstract

Disclosed is a polycrystalline metal-organic framework (MOF) membrane PCN-250 formed on a porous substrate such as alumina and said polycrystalline MOF membrane material used as a filter material in organic solvent nanofiltration (OSN). Also disclosed is a method of preparing the MOF membrane by rub seeding.

Description

POLYCRYSTALLINE IRON-CONTAINING METAL-ORGANIC FRAMEWORK MEMBRANES FOR ORGANIC SOLVENT NANOFILTRATION

Field of Invention

The current invention relates to polycrystalline iron-containing metal-organic framework membranes, and their use as a filter material in organic solvent nanofiltration.

Background

The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

The petrochemical and pharmaceutical industries involve intensive separation processes in organic solvents to purify and concentrate the products, recover solvents, and recycle catalysts. At present, distillation and crystallization are the most ubiquitous separation techniques for separating liquid-liquid and solid-liquid mixtures. However, substantial costs are incurred through distillation processes because of the massive energy penalty for successive heating and cooling, drying, and evaporation (D. S. Sholl & R. P. Lively, Nature 2016, 532, 435-437). For instance, separation processes account for 45-55% of the total energy consumption in the United States, whereby distillation and crystallization account for 49% and 31%, respectively (D. S. Sholl & R. P. Lively, Nature 2016, 532, 435-437). Besides the high energy consumption, these separation approaches are unsuitable for separating azeotropic mixtures and thermally sensitive chemicals. Therefore, efficient methods to separate chemicals from organic solvents under mild conditions are urgently needed.

Membrane-based separation technology is attractive in terms of its economic and environmental impact. Organic solvent nanofiltration (OSN), an emerging membrane-based technique for liquid-phase separation, is low-cost and environmentally friendly (W. R. Bowen & J. S. Welfoot, Chem. Eng. Sci. 2002, 57, 1121-1137; A. W. Mohammad et al., Desalination 2015, 356, 226-254; and X. Wang et al., ACSAppl. Mater. Interfaces 2017, 9, 37848-37855). Currently, the efficient separation of large molecules in the biochemical industry requires the membranes to be robust in organic solvents and have regular pore sizes. However, such requirements are not met by conventional polymer (F. M. Sukma & P. Z. Qulfaz-Emecen, J. Membr. Sci. 2018, 545, 329-336), ceramic (A. Buekenhoudt et al., J. Membr. Sci. 2013, 439, 36-47), and zeolite membranes due to their swelling, leaching, and small and irregular pores (Y. Cai et a!., J. Membr. Sci. 2020, 615, 118551).

Metal-organic frameworks (MOFs), consisting of metal ions coordinated to organic linkers, have great potential for catalysis, drug delivery, gas storage, sensing, and separation, owing to their uniform and controllable pore sizes, and various functionalities. Furthermore, MOFs with high-valent metal ions, such as Zr⁴*, Al³⁺, Cr³⁺, Fe³⁺, along with the Co²⁺/Zn²⁺ based UiO- 66, MIL-101 , PCN, and ZIF series, are chemically stable in the presence of water and organic solvents (M. Kandiah etal., Chem. Mater. 2010, 22, 6632-6640; G. Ferey etal., Science 2QQ5, 309, 2040-2042; D. Feng et al., Angew. Chem. Int. Ed. 2012, 51, 10307-10310; K. Wang et al., J. Am. Chem. Soc. 2014, 136, 13983-13986; and M. Ding, X. Cai & H.-L. Jiang, Chem. Sci. 2019, 10, 10209-10230). In particular, recent advances in preparing thin and chemically stable MOF membranes are promising for engineering advanced OSN membranes. For example, Cai etal. reported a UiO-66-NH₂(Zr) membrane synthesized on carboxylated carbon cloth for OSN (Y. Cai et al., J. Membr. Sci. 2020, 615, 118551), achieving very high rejection of Nile red (> 99.8%) with methanol permeance of 0.30 LMH/bar. Similarly, Lin and co-workers reported ZIF-8 membranes grown on plant polyphenol tannic acid modified polyethersulfone film for separating hydrated ions from water (Y. Xu et al., J. Membr. Sci. 2021 , 618, 118726). Indeed, these continuous MOF membranes have demonstrated excellent molecular sieving characteristics coupled with superior chemical resistance. However, the permeance of the existing MOF membranes is still too low for industrial OSN applications. For example, general industrial organic solvents, such as ethanol (EtOH, dinette = 0.45 nm) and strong polar solvents like N-methyl-2-pyrrolidone (NMP, dinette = 0.55 nm), can hardly permeate through the UiO-66 and ZIF-8 membranes due to their nanometer-sized pore windows (ZIF-8 of 0.314 nm (A. W. Thornton et al., Energy Environ. Sci. 2012, 5, 7637), UiO-66 of 0.6 nm (X. Li et al., Nat. Commun. 2019, 10, 2490)).

Therefore, there exists a need to develop MOF membranes for high solvent permeance for industrial OSN applications.

Summary of Invention

Aspects and embodiments of the invention will now be discussed by reference to the following numbered clauses.

1. A composite material comprising: a porous substrate having a first surface and a second surface; and a polycrystalline membrane material formed on the first surface, wherein the polycrystalline membrane material is a metal organic framework (MOF) that is PCN-250 (iron azobenzene tetracarboxylic, also known as MIL-127(Fe), soc-MOF(Fe)).

2. The composite material according to Clause 1 , wherein the polycrystalline membrane material is PCN-250.

3. The composite material according to Clause 1 or Clause 2, wherein the MOF has a pore size of from 5 to 10 A.

4. The composite material according to Clause 3, wherein the MOF has a pore size of from 5.8 to 9.2 A.

5. The composite material according to any one of the preceding clauses, wherein the MOF has a BET surface area of from 1000 to 2000 m².g^-1, such as about 1305 m².g-¹.

6. The composite material according to any one of the preceding clauses, wherein the thickness of the polycrystalline membrane material on the first surface of the porous substrate is from 0.1 pm to 20 pm.

7. The composite material according to Clause 6, wherein the thickness of the polycrystalline membrane material on the first surface of the porous substrate is from 0.5 pm to 15 pm.

8. The composite material according to Clause 7, wherein the thickness of the polycrystalline membrane material on the first surface of the porous substrate is from 1 pm to 10 pm, such as from 2 to 5.4 pm, such as from 2.6 to 5 pm, such as about 3.7 pm.

10. The composite material according to any one of the preceding clauses, wherein the composite material is suitable for use as a filter material.

11. The composite material according to Clause 10, wherein the composite material is suitable for use as a filter material in organic solvent nanofiltration.

12. The composite material according to any one of the preceding clauses, wherein the composite material displays a permeance of greater than 20 L rm² h'¹ bar¹ for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A, optionally wherein the composite material displays a permeance of from 25 to 140 L rrr² IT¹ bar¹ for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A.

13. The composite material according to any one of the preceding clauses, wherein the composite material displays a dye rejection of 93.7% for methyl blue using N-methyl-2- pyrrolidone (NMP) with a flux of 2.6 L rm² IT¹ bar¹.

14. The composite material according to any one of the preceding clauses, wherein the substrate is selected from one or more of a polymer, a ceramic (e.g. alumina), a carbon cloth, a metal, and a metal oxide.

15. The composite material according to Clause 14, wherein the substrate is a porous AI2O3 substrate, optionally wherein the substrate is a porous a-AhOs substrate.

16. The composite material according to any one of the preceding clauses, wherein the substrate is provided in the form of a tube or, more particularly, a mesh, a sheet or in the form of hollow fibers (e.g. porous alumina ceramic hollow fibers) and other arrangements that are obtainable by the folding of a tube or, more particularly, a mesh, a sheet and hollow fibers.

17. The composite material according to any one of the preceding clauses, wherein the polycrystalline membrane material formed on the first surface is defect-free.

18. The composite material according to any one of the preceding clauses, wherein a surface of the polycrystalline membrane material is substantially free of metal organic framework crystals deposited on top of said surface.

19. The composite material according to any one of Clauses 1 to 11 and 14 to 18, wherein the composite material displays a permeance of greater than 10 L rm² h'¹ bar¹ for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A, optionally wherein the composite material displays a permeance of from 15 to 350 L rm² IT¹ bar¹ , such as from 12 to 337 L rm² h'¹ bar¹ , for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A.

20. The composite material according to any one of the preceding clauses, wherein the composite material is stable for at least two weeks in an aqueous solution with a pH of 2.

21 . The composite material according to any one of the preceding clauses, wherein: (a) the composite material is hydrophilic with water contact angle of around 7.6°; and/or

(b) the composite material is hydrophilic with a roughness of around 543 nm.

22. A method of forming a composite material according to any one of Clauses 1 to 21 , comprising the steps of:

(a) providing a porous substrate that has been seeded by a rub-seeding operation with a plurality of crystals and or powdered particulates of a metal organic framework (MOF) that is PCN-250 (iron azobenzene tetracarboxylic, also known as MIL-127(Fe), soc-MOF(Fe)); and

(b) placing the seeded substrate in a vessel and immersing it in a mother solution comprising a solvent, an organic ligand, a metal precursor compound, and a modulator compound, the seeded substrate should be fully covered by mother solution and then heating the resulting mixture for a period of time to provide the composite material, wherein the porous substrate is substantially perpendicularly orientated with respect to the base of the vessel.

23. A method of forming a composite material according to any one of Clauses 1 to 21 , comprising the steps of:

(a) providing a porous substrate and a mother solution comprising a solvent, an organic ligand, a metal precursor compound, and a modulator compound;

(b) immersing the porous substrate in the mother solution and then heating the resulting mixture for a period of time to provide the composite material.

24. A method of using a polycrystalline metal-organic framework membrane as described in any one of Clauses 1 to 21 in a process of separating a fluid into a filtrate fluid and a retentate fluid, the process comprising the steps of:

(a) providing a fluid in need of separation to a composite material as described in any one of Clauses 1 to 21 ;

(b) allowing or enabling a portion of the fluid to pass through the composite material to provide a filtrate fluid and thereby providing a filtrate fluid; and

(c) collecting the filtrate fluid and retentate fluids.

25. The method according to Clause 24, wherein the fluid to be separated is selected from: a mixture of gases; an aqueous solution comprising one or more inorganic materials; an aqueous solution comprising one or more organic materials; an aqueous solution comprising one or more inorganic materials and one or more organic materials; a mixture of organic liquids; a mixture of one or more organic liquids and water; a mixture of one or more organic liquids and one or more organic materials; a mixture of one or more organic liquids and one or more inorganic materials; a mixture of one or more organic liquids, one or more organic materials and one or more inorganic materials; a mixture of water, one or more organic liquids and one or more organic materials; a mixture of water, one or more organic liquids and one or more inorganic materials; and a mixture of water, one or more organic liquids, one or more organic materials and one or more inorganic materials.

Drawings

FIG. 1 depicts the schematic diagram of the separation apparatus used for OSN. The transmembrane pressure of the upper membrane was controlled within 1-5 bar.

FIG. 2 depicts the schematic illustration of the PCN-250 membrane synthesis and its application in OSN.

FIG. 3 depicts the optical images of (a) the blank substrates; (b) the seed layers after rub seeding; and (c) the rub seed layers after 3, 4, and 6 h of secondary growth.

FIG. 4 depicts the scanning electron microscopy (SEM) images of PCN-250 MOF seeds under a synthesis time of 2 h.

FIG. 5 depicts the field emission scanning electron microscopy (FESEM) images of (a) the bare AI2O3 support; and (b) the PCN-250 seed layer by rub seeding; (c, d) Energy-dispersive X-ray spectroscopy (EDX) mapping of the rub seed layer; (e) Cross-section SEM image of the rub seed layer; and (f) SEM image of the PCN-250 seed layer prepared by the solvothermal method at 150 °C for 2 h.

FIG. 6 depicts the SEM images of the polycrystalline PCN-250 membrane by solvothermal seeding followed by secondary growth: (a) surface view; and (b) cross-section view.

FIG. 7 depicts the synthesis of the PCN-250 membrane by placing the seeded support horizontally (a1) and vertically (b1), and the SEM images of the resultant membranes (a2 and b2).

FIG. 8 depicts the FESEM images of the alumina supported PCN-250 seed layer (a) and the PCN-250 membrane (b-d); (e-f) EDX mapping of the alumina supported PCN-250 membrane: Fe; and Al; and (g) X-ray diffraction (XRD) patterns of the as-prepared PCN-250 powder, PCN- 250 membrane, and PCN-250 membranes after immersing in H2O and various solvents for two weeks. FIG. 9 depicts the water contact angle of PCN-250 membrane.

FIG. 10 depicts the atomic force microscope (AFM) image of PCN-250 membrane on AI2O3 substrate. The membrane was obtained after 3 h growth, washing, and drying. The roughness was estimated as 543 nm.

FIG. 11 depicts the thickness of the PCN-250 membrane after rub seeding and secondary growth of (a) 2 h, (b) 4 h, and (c) 8 h, respectively.

FIG. 12 depicts the H2O permeance and methyl blue (MB) rejection of the PCN-250 membrane after 2 h, 3 h, 4 h, and 8 h of secondary growth. Note that the low rejection of the membrane synthesized under 2 h may be due to the presence of defects.

FIG. 13 depicts (a) the permeance of H₂O and several organic solvents through the PCN-250 membrane measured at 25 °C; (b) the relationship of solvent permeance against the combined solvent property (viscosity: i , total solubility parameter: 8_t, and molecular diameter: dm) for the PCN-250 membrane; (c) the H2O permeance and salt rejections of PCN-250 membrane; (d) the H2O permeance and dye rejections of PCN-250 membrane; and the ultraviolet-visible (UV- Vis) spectra of MB in (e) NMP; and (f) ethanol.

FIG. 14 depicts the UV-Vis spectra of aqueous dye feed (50 ppm) and filtrate solutions after separation by the PCN-250 membrane under 3 bar at room temperature (25 °C).

FIG. 15 depicts the H2O permeance and polyethylene glycol (PEG) rejection of the PCN-250 membrane under 5 bar at room temperature (25 °C). Based on the 90% rejection, the molecular weight cut-off (MWCO) of the PCN-250 membrane was approximately 1300 g/mol.

FIG. 16 depicts the N2 sorption isotherms (closed, adsorption; open, desorption). Inset: pore size distributions (density functional theory (DFT) method) of the PCN-250 crystal powders.

FIG. 17 depicts the UV-Vis spectra of the feed and filtrated solutions (aqueous methyl blue solutions, 50 ppm) of the reproduced 10 membranes (see Table 5 for details).

FIG. 18 depicts the long-term stability test of the PCN-250 membrane for H₂O permeance and NaCI rejection (1000 ppm) under 3 bar at 25 °C. FIG. 19 depicts the UV-Vis spectra of aqueous MB solution separated by PCN-250 membrane treated under different conditions (feed; filtrate): (a) neutral solution; (b) HCI solution (pH = 2) for 2 weeks; and (c) NaOH solution (pH = 12) for 2 h.

FIG. 20 depicts the SEM images of the PCN-250 membrane after soaking in various solvents for 2 weeks: (a) dimethylformamide (DMF); (b) ethanol; (c) NMP; (d) H2O; (e) hydrochloric acid (HCI) aqueous solution (pH = 2); and (f) sodium hydroxide (NaOH) aqueous solution (pH = 12).

FIG. 21 depicts the H2O permeance and MB rejection of the original PCN-250 membrane, and that of the membrane after treatment with pH = 2 (HCI aqueous) for 2 weeks and subsequent treatment under pH = 12 (NaOH aqueous) for 2 h. Both treatments were carried out under 4 bar at room temperature (25 °C). The aim of choosing a dense and thick membrane (reaction time of 8 h) for the pH resistance test is to reflect the intrinsic quality of the PCN-250 membrane.

FIG. 22 depicts the long-term continuous test of a dense (reaction time of 4 h) PCN-250 membrane for the permeance of (a) NMP; and then (b) ethanol (ethanol/methyl blue rejection) at room temperature under 3 bar.

FIG. 23 depicts the UV-Vis spectra of aqueous dye solutions (50 ppm, 200 mL each) before and after adding PCN-250 powders (0.3 mg each).

FIG. 24 depicts the dye rejections of PCN-250 membrane compared with other OSN membranes (see Table 7 for details). The solvents investigated were (a) NMP; and (b) ethanol.

Description

In a first aspect of the invention, there is provided a composite material comprising: a porous substrate having a first surface and a second surface; and a polycrystalline membrane material formed on the first surface, wherein the polycrystalline membrane material is a metal organic framework (MOF) that is PCN-250 (iron azobenzene tetracarboxylic, also known as MIL-127(Fe), soc-MOF(Fe)).

In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components/features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of’ or synonyms thereof and vice versa.

The phrase, “consists essentially of” and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greater than 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure.

The composite materials disclosed herein have a high mechanical strength. Considering the stimuli-responsive nature of the N=N bond and the flexibility of the Fe-0 bond, PCN-250 exhibits dynamic behavior. Under a mechanical pressure of 150 MPa, the structure of PCN- 250 barely changes. (S. Yuan et al., Joule, 2017, 1 , 806-815.) Therefore, it is believed that the fabricated polycrystalline PCN-250 membranes possess high mechanical strength.

The composite materials disclosed here have a uniform and large pore size. The pore size of PCN-250 is 5-10 A, which is larger than that of water (3.8 A) and many organic solvents, such as methanol (5.1 A), acetone (6.2 A), toluene (7.0 A), and hexane (7.5 A). It is believed that this provides favorable conditions for the preferential permeation of water and common organic solvents.

The composite materials disclosed herein have high chemical stability. The PCN-250 membrane disclosed herein is stable in aqueous environments with a wide pH range (from pH = 1 to pH = 12) and almost all kinds of organic solvents. This will allow PCN-250 membranes to be used in practical applications involving organics, such as organic solvent nanofiltration.

It is believed that a number of additional benefits may be obtained by the production of a polycrystalline MOF that is defect-free (as reported in embodiments herein), and this is expanded upon in the examples section below.

Taking these advantages together, the high separation performance and outstanding stability of the composite materials disclosed herein suggest that the composite materials can separate large molecules in aggressive organic solvents, while at the same time being easy to manufacture. As will be appreciated, each of the metal-organic frameworks (MOFs) mentioned herein are materials that comprise bonds (i.e. coordination bonds) between metal cations and multidentate organic linkers, and they form a porous structure with a plurality of cavities within each MOF. As noted above, the polycrystalline membrane material is PCN-250.

As noted hereinbefore, the MOF may be described as a porous structure. As such, the MOF may have a pore size of from 5 to 10 A. In particular embodiments that may be mentioned herein, the MOF may have a pore size of from 5.8 to 9.2 A. The MOF may have any suitable surface area that will allow it to perform the desired function. For example, the MOF may have a BET surface area of from 1000 to 2000 m².g^-1, such as about 1305 m².g-¹.

As will be appreciated, the polycrystalline metal-organic framework attached to the surface of the substrate material will result in a layer on top of the substrate material that will have a thickness. This membrane material may have any suitable thickness, for example, the thickness of the polycrystalline membrane material on the first surface of the substrate may be from 0.1 pm to 20 pm. For example, the thickness of the polycrystalline membrane material on the first surface of the porous substrate may be from 0.5 pm to 15 pm. Alternatively or additionally, as it has been found that a PCN-250 membrane with a thickness of 5.4 pm shows a water permeability of 2.6 L rm² IT¹ bar¹, particular embodiments of the invention may refer to a polycrystalline membrane material that has a thickness of less than or equal to 5 pm on the first surface of the porous substrate. As such, embodiments of the invention that may be discussed herein may be ones where the thickness of the polycrystalline membrane material on the first surface of the porous substrate is from 1 pm to 10 pm, such as from 2 to 5.4 pm, such as from 2.6 to 5 pm, such as about 3.7 pm. It is noted that a thickness of 3.7 pm of PCN- 250 in the composite material may provide a water permeability of 18.9 L rm² IT¹ bar¹.

For the avoidance of doubt, it is explicitly contemplated that where a number of numerical ranges related to the same feature are cited herein, that the end points for each range are intended to be combined in any order to provide further contemplated (and implicitly disclosed) ranges. Thus, below, there is disclosed a polycrystalline membrane having a thickness of: from 0.1 to 0.5 pm, from 0.1 to 1 pm, from 0.1 to 2 pm, 0.1 to 2.6 pm, 0.1 to 3.7 pm, from 0.1 to 5 pm, 0.1 to 5.4 pm, from 0.1 to 10 pm, from 0.1 to 15 pm, from 0.1 to 20 pm; from 0.5 to 1 pm, from 0.5 to 2 pm, 0.5 to 2.6 pm, 0.5 to 3.7 pm, from 0.5 to 5 pm, 0.5 to 5.4 pm, from 0.5 to 10 pm, from 0.5 to 15 pm, from 0.5 to 20 pm; from 1 to 2 pm, 1 to 2.6 pm, 1 to 3.7 pm, from 1 to 5 pm, 1 to 5.4 pm, from 1 to 10 pm, from 1 to 15 pm, from 1 to 20 pm;

2 to 2.6 pm, 2 to 3.7 pm, from 2 to 5 pm, 2 to 5.4 pm, from 2 to 10 pm, from 2 to 15 pm, from 2 to 20 pm; 2.6 to 3.7 pm, from 2.6 to 5 pm, 2.6 to 5.4 pm, from 2.6 to 10 pm, from 2.6 to 15 pm, from 2.6 to 20 pm; from 3.7 to 5 pm, 3.7 to 5.4 pm, from 3.7 to 10 pm, from 3.7 to 15 pm, from 3.7 to 20 pm; from 5 to 5.4 pm, from 5 to 10 pm, from 5 to 15 pm, from 5 to 20 pm; from 5.4 to 10 pm, from 5.4 to 15 pm, from 5.4 to 20 pm; from 10 to 15 pm, from 10 to 20 pm; and from 15 to 20 pm.

Any of the ranges mentioned above may be applied to any combination of embodiments listed herein unless otherwise specified.

The composite material may be used in any suitable use. However, the composite material disclosed herein may be particularly suited for use as a filter material. Yet more particularly the composite material may be suitable for use as a filter material in organic solvent nanofiltration.

In certain embodiments, the composite material may display a permeance of greater than 20 L rrr² IT¹ bar¹ for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A. More particularly, the composite material may display a permeance of from 25 to 140 L rm² IT¹ bar¹ for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A. In alternative embodiments, the composite material may display a permeance of greater than 10 L rm² h'¹ bar¹ for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A. More particularly, the composite material may display a permeance of from 15 to 350 L rrr² IT¹ bar¹, such as from 12 to 337 L rrr² IT¹ bar¹, for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A.

The composite material disclosed herein may display a dye rejection of greater than 78% for methylene blue using dichloromethane with a flux of 79.2 L rm² h'¹ bar¹. Additionally or alternatively, the composite material may display a dye rejection of 93.7% for methyl blue using N-methyl-2-pyrrolidone (NMP) with a flux of 2.6 L rm² h'¹ bar¹.

The porous substrate used herein may be any suitable porous material. For example, the substrate may be selected from one or more of a polymer, a ceramic (e.g. alumina), a carbon cloth, a metal, and a metal oxide. The substrate can be in any suitable form, which include, but is not limited to, meshes, sheets and hollow fibers (e.g. porous alumina ceramic hollow fibers), plus other forms that can be obtained by the folding of these primary forms.

Examples of substrates in the form of sheets include, but are not limited to, polymer film and carbon film/cloth. Meshes may include, but are not limited to, metal meshes and metal oxide meshes. Hollow fiber structures that may be mentioned herein include, but are not limited to ceramics (e.g. alumina) and polymer films.

It is noted that certain substrates (e.g. carbon films/cloths or stainless steel meshes) can be functionalized by carboxylation or amination, which can facilitate the growth of crystal seeds. In addition, the flexibility of carbon films/cloths as substrates can offer good mechanical properties to the resultant membranes.

Examples of polymers that may be used as substrates include, but are not limited to, polyethyleneimine (PEI), polyethersulfone (PES), polyvinylidene fluoride (PVDF), polyetherimide (llltem™ 1000), poly(ether-block-amide) (PEBA), polydimethylsiloxane (PDMS), poly(amic acid), polybenzimidazole (PBI), Pebax, Matrimid™, 6FDA-DAM, 6FDA/BPDA-DAM, poly(amide-imide), polydopamine (PDA), poly tetra fluoroethylene (PTFE), and combinations thereof.

In particular embodiments that may be mentioned herein, the the substrate may be a porous AI2O3 substrate, optionally wherein the substrate is a porous a-ALOs substrate. An example of a suitable porous a-AfeOs substrate are porous asymmetric a-ALOs supports with a maximum pore size of 70 nm, a diameter (for the substrate) of 18 mm, and a thickness of 1 mm, which may be purchased from the Fraunhofer Institut fur Keramische Technologien and Systeme (IKTS), Germany. As will be appreciated, materials with different pore sizes may also be used, for example the maximum pore size may be from 30 to 200 nm.

The substrate may be provided in any suitable form. For example, the substrate may be provided in the form of a of a tube or, more particularly, a mesh, a sheet or in the form of hollow fibers (e.g. porous alumina ceramic hollow fibers) and other arrangements that are obtainable by the folding of a tube or, more particularly, a mesh, a sheet and hollow fibers.

The composite material disclosed herein may be one in which the polycrystalline membrane material formed on the first surface is defect-free. A defect-free membrane may be determined herein by a membrane where the rejection of methyl blue is equal to or greater than 90% in H2O (e.g. see examples section below for details on how to perform this experiment). Alternatively or additionally, a defect-free membrane may be determined herein by a membrane where the rejection of methyl blue is equal to or greater than 84% in ethanol (e.g. see examples section below for details on how to perform this experiment).

The composite material disclosed herein may be one in which a surface of the polycrystalline membrane material is substantially free of metal organic framework crystals deposited on top of said surface. That is, the surface of the polycrystalline membrane material may be completely devoid of metal organic framework crystals deposited on the plane of the surface or it may contain a minor amount of surface coverage when a sample area is viewed using FESEM (e.g. the coverage may be less the 5%, such as less than 1 %, such as less than 0.1 %, such as less than 0.01 %).

The composite materials herein may be stable in acidic conditions for an extended period of time. For example, the composite materials disclosed herein may be stable for at least two weeks in an aqueous solution with a pH of 2.

The composite materials disclosed herein may be hydrophilic in nature. As such, the composite materials disclosed herein may have a water contact angle of around 7.6°. The water contact angle may be measured using a sessile water droplet test.

In a further aspect of the invention, there is provided a method of forming a composite material as described hereinbefore, comprising the steps of:

(b) placing the porous seeded substrate that has been seeded in a vessel and immersing it in a mother solution comprising a solvent, an organic ligand, a metal precursor compound, and a modulator compound, and then heating the resulting mixture for a period of time to provide the composite material, wherein the porous substrate is substantially perpendicularly orientated with respect to the base of the vessel.

In the method, the porous seeded substrate may be provided by taking seed crystals of the MOF PCN-250 and rubbing these onto the substrate’s surface using a soft rubber material in one rubbing direction. This rubbing step may be repeated from 1 to 10 times, such as three times. As will be appreciated, the immersion discussed in the method above refers to the entire immersion of the porous substrate that has been seeded in the mother solution, so as to ensure that the entire substrate can be used to grow the MOF.

The substrate is placed perpendicularly with respect to the base of the vessel, as it has been surprisingly found that this orientation allows for the formation of a defect-free polycrystalline membrane material formed on the first surface of the substrate. As will be appreciated, only one surface of the membrane material is intended to be exposed to the MOF growth solution and so any suitable way to mask the other surface(s) may be used. For example, two substrates may be prepared at the same time and the back surface (i.e. the surfaces that have not been prepared by rub seeding) may be placed to face the back surface of a membrane holder’s card slot, thereby preventing them being exposed to the solution. This way, only the surface prepared by rub seeding is exposed to the MOF growth solution. As will be appreciated, “perpendicular” is not intended to refer to placement that is only at 90° relative to the orientation of the base of the vessel, as it may refer to any orientation that may achieve the desired result. As such, the variation from 90° may be from ±0.1 ° to ±5°, such as from ±0.5° to ±2°, such as from ±1 ° to ±1.5°.

After the reaction has been completed, the reaction vessel is allowed to cool down (e.g. to ambient temperature) and the resulting composite material may be washed with a suitable solvent before use. Examples of suitable solvents this washing step include, but are not limited to, DMF and/or acetone.

Alternatively, the method of forming a composite material as described hereinbefore, may comprising the steps of:

The methods described herein us a simple solvothermal reaction, but nonetheless provide high-quality polycrystalline PCN-250 membranes (e.g. which are defect-free). This provides the possibility for the scaled-up production of polycrystalline MOF membranes of this type.

Any suitable solvent may be used in the mother liquor. Examples of suitable solvents include, but are not limited to DMF. Any suitable organic ligand may be used in the mother liquor. Examples of suitable organic ligands include, but are not limited to 3, 3’, 5,5’- azobenzenetetracarboxylic acid (F ABTC). Any suitable metal precursor compound may be used in the mother liquor. Examples of suitable metal precursor compounds include, but are not limited to ferric chloride. Any suitable modulator compound may be used in the mother liquor. Examples of suitable modulator compounds include, but are not limited to acetic acid.

The membranes described above may have a broad utility in the separation of fluids and materials within said fluids. Thus there is also disclosed a method of using a polycrystalline metal-organic framework membrane as described hereinbefore in a process of separating a fluid into a filtrate fluid and a retentate fluid, the process comprising the steps of:

(a) providing a fluid in need of separation to a composite material as described hereinbefore;

(c) collecting the filtrate fluid and retentate fluids.

There are multiple possible separations where the current invention may be beneficial. For example, the fluid to be separated may be selected from: a mixture of gases; an aqueous solution comprising one or more inorganic materials; an aqueous solution comprising one or more organic materials; an aqueous solution comprising one or more inorganic materials and one or more organic materials; a mixture of organic liquids; a mixture of one or more organic liquids and water; a mixture of one or more organic liquids and one or more organic materials; a mixture of one or more organic liquids and one or more inorganic materials; a mixture of one or more organic liquids, one or more organic materials and one or more inorganic materials; a mixture of water, one or more organic liquids and one or more organic materials; a mixture of water, one or more organic liquids and one or more inorganic materials; and a mixture of water, one or more organic liquids, one or more organic materials and one or more inorganic materials.

Particular applications that may be mentioned herein for the membranes include, but are not limited to: separation of organic/water mixtures or organic systems; desalination and wastewater purification (i.e. the removal of ions or dyes from wastewater, known as organic solvent nanofiltration); and gas separation. The examples below provide detailed descriptions and results for various membranes of the current invention applied to these technologies. As will be appreciated, application to the other separation methods can be extrapolated from the methods disclosed herein and would be readily achieved by a skilled person based upon the instruction provided in this document and their common knowledge. Further aspects and embodiments of the invention are provided in the following non-limiting examples.

Examples

Materials

All the reagents were obtained from commercial suppliers and used without further purification. Iron (III) Chloride anhydrous (FeC , 98%) was purchased from FISHER UK. HCI (37%), acetic acid (HAc, 99.5%), N,N-dimethylacetamide (DMA, >99.0%), xylene isomers (p-, m-, o-xylene, AR, > 98%), rose bengal (RB), methyl orange (MO, > 98%), PEG (200, 400, 600, 1000, 2000, 4000 g/mol, ACS reagent) were purchased from TCI. Sodium chloride (NaCI, > 99.0%), NaOH (> 98%), 5-nitroisophthalic acid (98%) and D-glucose (99.5%) were purchased from Sigma. Methyl blue (MB, 62%), acid fuchsin (AF), acid blue 25 (45%) and chloranilic acid (> 98%) were purchased from Sigma-Aldrich. ACS reagent solvents (99.5%), such as methanol (MeOH), ethanol (EtOH), acetone, NMP, DMF, hexane, toluene, tetrahydrofuran (THF), dichloromethane (DCM), 1-propanol (NPA), and 2-propanol (IPA) were purchased from Fisher Chemical. The deionized water (H₂O) produced by Millipore-Q System (Millipore, Billerica, MA, USA) was used in all experiments. Porous asymmetric a-AhOs supports with a top pore size of 70 nm, a diameter of 18 mm, and a thickness of 1 mm were purchased from Fraunhofer Institut fur Keramische Technologien and Systeme (IKTS).

PXRD

PXRD patterns were obtained on a Rigaku Miniflex 600 X-ray powder diffractometer equipped with a Cu sealed tube (A = 1.540598 A) at a scan rate of 2 deg/min. The pH values were measured by a pH meter (VWR pH 1100L).

Energy-dispersive X-ray spectroscopy (EDX) and FESEM

The composition and morphology of the polycrystalline PCN-250 membrane were examined with EDX and FESEM (JSM-7610F, JEOL). Before observation, all samples were sputtered with Pt by a sputter coater (Cressington 208 HR) under a current of 20 mA for 60 s.

Atomic force microscopy (AFM)

The roughness of the membrane was characterized by AFM (Bruker Dimension Icon). Pore size distribution

N2 sorption isotherms were collected at 77 K (Micromeritics, ASAP 2020), and pore-size distributions were obtained from the DFT method.

Determination of salt concentrations

The salt concentrations were measured by a conductivity meter (SI analytics, Lab 955).

UV-vis spectroscopy

UV-Vis absorption spectra of dye solutions were measured by a UV-Vis spectrophotometer (Cary 60, Agilent).

Determination of total organic carbon (TOC)

The organic carbon concentrations were measured by TOC (Shimadzu, TOC-L CSH).

Example 1. OSN performance tests

Single-solvent permeation test

A separation apparatus used for OSN is depicted in FIG. 1. As shown, there is a separation apparatus 100 that includes a retentate compartment 110 comprising, solvent, dyes and large molecules 111 , a feed compartment 112 comprising solvent, dyes and large molecules 113, and a magnetic stirrer 114 comprising a stir bar 115, a membrane module 116 and a membrane 117, where the retentate compartment, feed compartment, and magnetic stirrer are fluidly connected to one another by a fluid pathway 118 comprising a back-pressure regulator 119 and a pressure gauge 120. As will be appreciated, in use, the fluid may be circulated through the fluid pathway by any suitable means, such as by use of a suitable pumping system (e.g. plunger pump 121). The magnetic stirrer 114 also includes a permeate compartment 122 comprising solvent and small molecules 123, that is connected by a suitable fluid pathway 124 to the magnetic stirrer 114. This is a straightforward approach to evaluate the stability of the membranes.

OSN measurements

The organic solvent nanofiltration performance (solvent (analytical grade) permeance) of the PCN-250 membranes prepared in the next example was evaluated in a crossflow system (FIG. 1) at room temperature under a transmembrane pressure of 1~5 bar. Water or common single organic solvent (MeOH, EtOH, IPA, NPA, hexane, DMF, DMA, toluene, p-xylene, m-xylene, o-xylene, THF, acetone, DCM or NMP) was poured into the setup with the prepared PCN-250 membranes to test their flux. The molecular weight cut-off (MWCO) of the membranes was studied by filtrating PEG with different molecular weights of 200, 400, 600, 1000, 2000, and 4000 g/mol. The separation of dye (MB, AF, MO, RB, acid blue 25, chloranilic acid) or PEG was evaluated by feeding raw aqueous solution of dye (50 ppm) or PEG (1000 ppm) into the crossflow stirred cell membrane module with the PCN-250 membrane to test the dye rejection. The solution was fed at room temperature by a plunger pump, and the pressure of the feed side was maintained at about 2.0 bar. Before collecting the samples, 6 h was given to the system for stabilization. The pure organic solvents were collected to determine the filtrate volume of permeation. The dye rejection was determined by analyzing the concentrations of dye in the filtrate using a UV-Vis spectrometer. The performance of the membrane was evaluated by calculating the rejection and organic solvent flux.

The permeation flux, f, was calculated according to Equation 1 : f = 77 Equation 1 where V (L) is the total volume of solution collected from the permeate side, A (m²) is the effective separation area of the membrane, t (h) is the testing time, and P (bar) is the transmembrane pressure (1~5 bar). The effective area of the membrane was accurately measured. The permeate samples were collected three times, and the average value was obtained.

The dye or PEG rejection (F?i) performance of PCN-250 membrane was calculated using Equation 2:

R = l - — x 100% Equation 2 where C_p and Cr are the concentrations of dye (or PEG) permeate and feed solutions measured by the UV-Vis spectrometer and TOC, respectively.

The salt rejection (R2) performance of the PCN-250 membrane was calculated using Equation 3: Equation 3 where e_p and er are the concentrations of salt permeate and feed solutions measured by a conductivity meter.

Example 2. Preparation of high-quality polycrystalline PCN-250 membrane

Synthesis of3,3’,5,5’-azobenzenetetracarboxylic acid (H4ABTC) The H4ABTC ligand was prepared according to the reported procedure (Y. Feng etal., J. Mater.

Chem. A 2020, 8, 13132-13141).

Synthesis of PCN-250 seeds

Anhydrous FeCL (175 mg, 1.08 mmol) and H4ABTC (150 mg, 0.419 mmol) were dissolved in anhydrous DMF (20 mL, 99.5%) and HAc (10 mL, 99.5%) by ultrasonic sonication. The obtained suspension was sealed in an autoclave (Teflon-lined with a stainless-steel cover) and heated in a 150 °C oven for 2 h. After the cooling and resting period, PCN-250 powder seeds were obtained, and subsequently washed with DMF and soaked in acetone to replace DMF. Finally, the obtained seeds were centrifuged and then dried at 80 °C under vacuum for 1 day to remove the solvent. The as-synthesized powder crystals were used for preparing the seed layer on the substrates by rub seeding.

Synthesis of PCN-250 membrane by rub seeding

FIG. 2 depicts the PCN-250 membrane synthesis. Briefly, PCN-250 seeds were applied to the a-AhOs membrane surface by rubbing along one direction several times with a soft rubber. Subsequently, PCN-250 membranes were prepared by secondary growth through solvothermal reaction (same mother solution as that of seed preparation) for 3 h at 150 °C followed by cooling to room temperature and washed by DMF and acetone.

Synthesis of PCN-250 membrane by general, simple in-situ solvothermal method

The polycrystalline PCN-250 membranes were fabricated on the surface of porous AI2O3 substrates by simple solvothermal reactions. The substrates were horizontally placed into the Teflon-lined stainless steel autoclave with the smooth side facing up, and immersed into the mother solution with a molar composition of H₄ABTC (20 mg), FeCh (30 mg), DMF (660 pL) and HAc (330 pL). The mixed solution and substrates were sealed and heated at 150 °C for 12 h. After cooling to room temperature, the PCN-250 membranes were intensively washed with DMF, ethanol, and then dried at room temperature overnight.

Results and discussion

The solution used for seed preparation and secondary growth of the PCN-250 membrane was the same. Here, the MOF seed layers were prepared by rub coating, which is different from the conventional solvothermal growth of MOF seeds. Rub seeding is a common approach for zeolite membrane synthesis because of the uniform control over membrane quality and thickness (X. Wang et al., J. Membr. Sci. 2014, 455, 294-304; S. Li, J. L. Falconer & R. D. Noble, Adv. Mater. 2006, 18, 2601-2603). Therefore, PCN-250 seeds were applied to the a- AI2O3 membrane surface by rubbing along one direction several times with soft rubber. For the simple in-situ solvothermal reaction fabrication process, it only took 12 h to form a well- intergrown polycrystalline PCN-250 membrane at 150 °C.

Example 3. Characterization of seed layers

The materials prepared in Example 2 were taken for characterization studies using various analytical methods.

Results and discussion

The PCN-250 structure based on H4ABTC as the ligand and acetic acid as the modulator and structure directing agent is presented in FIG. 2. FIG. 3 shows the blank substrates, seed layers, and dense MOF layers. A similar seed layer (FIG. 3b) was easily formed by rubbing MOF seeds (FIG. 4). FIG. 3c shows the color of the membrane surface over different lengths of reaction time. Briefly, by keeping the reaction time constant, membranes with similar surfaces were synthesized, indicating the high reproducibility of the membrane fabrication owing to the uniform seed layers (FIG. 5c-d).

For comparison, the seed layer prepared by general solvothermal reaction at 150 °C for 2 h (FIG. 5f) resulted in large but discontinuous crystal particles, which are detrimental to forming continuous and thin polycrystalline MOF layers. After 8 h of solvothermal reaction, the MOF layer was 15.4 pm thick (FIG. 6) with H₂O permeance below 0.1 L rm² IT¹ bar¹. It is thus important to decrease the MOF layer’s thickness for efficient permeation. It is also remarkable that the rub seeding method shortened the seeding time to 1 min. For example, the MOF seed layers prepared by solvothermal or dip-coating methods, such as UiO-66(Zr) and MOF-5, may require 12 to 24 h with solvents (Y. Sun et al., ACS Appl. Mater. Interfaces 2019, 12, 4494- 4500; F. Wu et al., J. Membr. Sci. 2017, 544, 342-350; R. Ranjan & M. Tsapatsis, Chem. Mater. 2009, 21, 4920-4924; and Z. Zhao et a!., Ind. Eng. Chem. Res. 2013, 52, 1102-1108). Therefore, rub seeding is effective, rapid, energy-saving, and does not require additional chemicals.

FIG. 7 shows that particle deposition on the substrate during the membrane growth could be avoided by placing the substrate vertically in the autoclave. Hence, after 3 h in the autoclave, the vertically placed membrane substrate had a continuous, defect-free polycrystalline PCN- 250 layer with a thickness of around 3.7 pm (FIG. 8d), a water contact angle of around 7.6° (FIG. 9) and surface roughness of around 543 nm (FIG. 10). Notably, this membrane is hydrophilic (FIG. 9) and has a small thickness comparable to the MOF membranes prepared by other approaches (e.g. thermal synthesis, Y. Cai et al., J. Membr. Sci. 2020, 615, 118551 ; X. Wang et al., ACS Appt. Mater. Interfaces 2017, 9, 37848-37855; and X. Wu et al., Angew.

Chem. Int. Ed. Engl. 2018, 57, 15354-15358).

To further study the formation of MOFs on a substrate, membranes prepared after 2, 4, and 8 h of growth were observed by SEM. The secondary growth duration strongly influenced the MOF layer’s thickness (from 2.6 to 20 pm, FIG. 11). By considering the permeance and rejection following different durations of secondary growth (FIG. 12), the optimized reaction time was determined to be 3 h. EDX mapping confirmed the formation of the alumina- supported PCN-250 membrane (FIG. 8e-f). The sharp transition of the MOF layer (Fe) from the support (Al) showed that no detectable MOF crystals have nucleated within the ceramic support. As shown in FIG. 8g, the XRD patterns of the PCN-250 membrane and powder samples agree well with the simulated one (Y. Feng et al., J. Mater. Chem. A 2020, 8, 13132- 13141), indicating the successful construction of the PCN-250 membrane. The PXRD patterns showed well-defined diffraction peaks, and the structure remained stable after heating at 240 °C for 10 h.

Example 4. Permeance and stability of PCN-250 membrane

The PCN-250 membrane prepared by rub seeding in Example 2 was taken for permeance and stability studies by following the protocols in Example 1. Before OSN measurements, single-solvent permeation tests were conducted at room temperature to evaluate the stability of the membranes.

Results and discussion

As shown in FIG. 13a, the membrane had excellent permeance for MeOH (337.9 L rm² IT¹ bar¹), while the permeance for EtOH, DMF, and NMP were 69.2, 31.7, and 17.2 L rm ² IT¹ bar¹, respectively. The difference in solvent permeance is consistent with the combined property in terms of the molecular diameter, dielectric constant, and the total Hansen solubility (FIG. 13b, Table 1 , S. Karan, Z. Jiang & A. G. Livingston, Science 2015, 348, 1347-1351). Moreover, the PCN-250 membrane’s ability to separate different dyes and salts in H2O was investigated (FIG. 13c-d). The membrane showed high rejection of 98.8% and 92.9% for RB (Mw = 1017.7 g/mol) and MB (Mw = 799.8 g/mol) with water permeance of around 20 L rm² IT ¹ bar¹ (FIG. 14, Table 2). Dyes with lower molecular weights, such as MO (327.33 g/mol), were only partly rejected because of their comparatively smaller molecular sizes (rejection of -10%, Table 3). Besides organic dyes, PEG with various molecular weights was also used as the probe to determine the MWCO of the PCN-250 membrane (FIG. 15). On the basis of 90% PEG rejection, the PCN-250 membrane’s MWCO was determined to be around 1300 g/mol. In addition, the membrane exhibited very high water permeance for aqueous salt solutions. For example, the water flux of NaCI solution was about 83.3 L rm² h'¹ bar¹ (Table 4). However, the NaCI rejection was only 4.4%. Other salts also had low rejections (< 13%) due to the large pore size of the membrane (0.6-1.0 nm, FIG. 16). After washing with DMF and acetone, the activated PCN-250 membrane exhibited Type I N2 sorption isotherms at 77 K, with a Brunauer- Emmett-Teller (BET) surface area of 1305 m² g-¹ (FIG. 16). In addition, the pore size of the crystal calculated via gas sorption data was 5.8-9.2 A (FIG. 16 inset), which is larger than water (3.8 A) and many organic solvents including methanol (5.1 A), acetone (6.2 A), toluene (7.0 A), and hexane (7.5 A), indicating the possibility of using the prepared PCN-250 membrane for preferential transportation of water and common organic solvents.

Table 1. The Hansen solubility parameter (5), viscosity (77) , and molecular diameter (d_m) of the organic solvents used in this study.

Viscosity, Hansen solubility parameter (Mpa^1/2) Molar diameter, mPa s nm

H₂O 0.916 47.8 15.6 16 42.3 0.265

Methanol 0.49 29.7 15.1 12.3 22.3 0.51

Acetonitrile 0.342 24.6 15.3 18 6.1 0.55

Ethanol 1.17 26.6 15.8 8.8 19.4 0.57

Acetone 0.29 20.1 15.5 10.4 7 0.62

1-Propanol 1.959 24.6 16 6.8 17.4 0.62

2-Propanol 2.058 23.5 15.8 6.1 16.4 0.623

DMF 0.77 24.8 17.4 13.7 11.3 0.63

1-Butanol 2.9 23.1 16 5.7 15.8 0.66 p-Xylene 0.6475 17.9 17.6 1 3.1 0.67

Toluene 0.555 18.2 18 1.4 2 0.7

NMP 1.89 23 18 12.3 7.2 0.54 o-Xylene 0.8102 18 17.8 1 3.1 0.74 m-Xylene 0.62 18.7 18.4 2.6 2.3 0.74

Note: Viscosity, Hansen solubility parameter, and molar diameter data were taken from literature (S. Karan, Z. Jiang & A. G. Livingston, Science 2015, 348, 1347-1351 ; and A. Buekenhoudt et al., J. Membr. Sci. 2013, 439, 36-47). 8_d = solubility parameter due to dispersion forces, 8_p = solubility parameter due to dipole forces, and 8_h = solubility parameter due to hydrogen bonding (or in general due to donor acceptor interactions). 8_Totai was calculated Lara et al., Int. J. Curr. Res. 2017, 9, 47860- 47867; and C. Hansen, Hansen Solubility Parameters: A User's Handbook, 2nd Edition, CRC

Press, Boca Raton, FL, 2007).

Table 2. The H2O permeance and dye rejection of the PCN-250 membrane under 4 bar at room temperature (25 °C).

Dyes Molecular weight H2O permeance Rejection

(g/mol) (L h ¹ nr² bar¹) (%)

Rose bengal 1017.7 23.5 ± 0.47 98.8 ± 0.1

Methyl blue 799.8 18.9 ± 0.76 92.9 ± 2.2

Acid fuchsin 585.5 23.1 ± 0.58 64.4 ± 2.5

Acid blue 25 416.4 19.8 ± 0.28 50.4 ± 2.3

Methyl orange 327.3 26.2 ± 0.15 29.9 ± 1.8

Chloranilic acid 208.9 43.3 ± 0.74 0 ± 2.2

Table 3. The molecular size of various organic solutes. Table 4. The H2O permeance and salt rejection of PCN-250 membrane under 3 bar at room temperature (25 °C).

Salts Permeance (L rrr² h’¹ bar¹) Rejection (%)

NaCI 83.3 ± 7.5 4.4 ± 0.3

KCI 104.6 ± 9.0 6.6 ± 1.1

MgCI₂ 76.7 ± 9.3 12.5 ± 1.5

MgSO₄ 34.9 ± 7.0 6.5 ± 0.9

Mg(OAc)₂ 61.7 ± 10.0 8.0 ± 0.7

To evaluate the reproducibility of PCN-250 membranes, 10 pieces of membranes were prepared by secondary growth (3 h) with the rub seeding method. FIG. 17 and Table 5 shows the permeance, and MB/H2O rejection. Notably, 7 out of the 10 reproduced membranes achieved MB rejections > 90%, indicating good reproducibility.

Table 5. The methyl blue rejection and water permeance of the reproduced ten PCN-250 membranes.

Membrane # Methyl blue rejection Water permeance (L rm² IT¹ bar¹)

1 58.7% 340.3

2 48.4% 312.8

3 80.3% 40.8

4 92.6% 26.5

5 94.2% 102.2

6 96.2% 131.2

7 94.5% 76.4

8 94.8% 39.7

9 90.0% 58.6 Membrane # Methyl blue rejection Water permeance (L rm² IT¹ bar¹)

10 95.9% 8.7

To confirm the membrane’s stability in solvents, the permeance and rejection of MB in NMP and EtOH were measured periodically (FIG. 13e-f). Herein, the rejection of MB was approximately 93.7% in NMP with NMP permeance of 2.6 L rm² IT¹ bar¹, and provides the first report to confirm OSN through microporous crystalline PCN-250 MOF membranes. In ethanol, the MB rejection was 84.7% with ethanol permeance of 27.5 L rm² h'¹ bar¹. It also highlights the membrane stability due to the exceptional chemical stability of the MOF materials. Notably, unlike the charge repulsion separation mechanism for MB in H2O, MB in ethanol was separated based on size exclusion (F. M. Sukma & P. Z. Qulfaz-Emecen, J. Membr. Sc/. 2018, 545, 329-336; and C. Wang et a/., J. Mater. Chem. A 2020, 8, 15891-15899).

The stability of the PCN-250 membrane was further investigated under different conditions, and the microstructural properties of the resulting PCN-250 membrane and powder were characterized using several methods. The membrane showed stable water permeance over 50 h for NaCI rejection (FIG. 18), confirming its high stability in water. In addition, HCI (pH = 2) and NaOH (pH = 12) aqueous solutions were fed into the permeation cell containing PCN- 250 membrane. After a continuous permeance test using aqueous HCI solution under 3 bar for 2 weeks, the membrane showed similar permeance. It maintained 99% rejection of MB (FIG. 19, Table 6), confirming its high acid stability. Since the XRD peaks (FIG. 8g) and morphology (FIG. 20) barely changed even after 2 weeks of continuous stability testing using H₂O, acid (pH = 2, aqueous HCI solution), and organic solvents (e.g. DMF, ethanol, and NMP), the excellent stability of PCN-250 membranes is ascertained. However, the permeance increased drastically after immersion in an aqueous NaOH solution for 2 h (FIG. 21 , Table 6), and the MB rejection decreased to 76.3% (Table 6). This result can be explained by the damage to the MOF’s crystalline structure under alkali conditions, forming larger pores with increased permeance and reduced rejection. In addition, the membrane performed well after a continuous operation of more than 70 h (FIG. 22), showing its excellent solvent resistance and antifouling ability despite a membrane roughness of 543 nm (FIG. 10). The weak XRD peaks and change in the membrane morphology at pH = 12 (aqueous NaOH solution) also suggest that the crystallinity was lost after exposure to basic conditions (J.-B. Tan & G.-R. Li, J. Mater. Chem. A 2020, 8, 14326-14355). Table 6. The H2O permeance and MB rejection of the original PCN-250 membrane, and that of the membrane after treatment with pH = 2 (HCI aqueous) for 2 weeks and subsequent treatment with pH = 12 (NaOH aqueous) for 2 h. Both treatments were carried out under 4 bar at room temperature (25 °C). The aim of choosing a dense and thick membrane for the pH resistance test is to reflect the intrinsic quality of the PCN-250 membrane.

Rejection Permeance (L nr² h ¹ bar¹) (%)

Original 98.0 0.8

After treating in HCI solution (pH = 2) 99.0 0.5

After treating in NaOH solution (pH = 12) 76.3 39.3

Further, the membrane performed well after being immersed in solvents for more than 100 h (FIG. 22), showing its excellent solvent resistance. Based on these tests, the PCN-250 membrane has low rejection toward small dyes such as MO, and is suitable for removing small molecules or salts from large molecules in aggressive organic solutions, such as the recovery of salts and enzymes during the purification of chemical and pharmaceutical products (J. F. Jenck, F. Agterberg & M. J. Droescher, Green Chem. 2004, 6, 544-556; and P. A. Marrone et al., J. Supercrit. Fluids 2004, 29, 289-312).

Example 5. Physisorption of dyes

Physisorption test

To quantitatively determine the possible physisorption of dyes inside the membrane matrix, the UV-Vis spectra of aqueous dye solutions (with a volume of 200 mL for each dye solution) were collected before and after adding PCN-250 powders for certain periods. Accordingly, PCN-250 powders with a mass (0.3 mg) close to the amount of PCN-250 in the membrane layer (0.278 mg, based on effective membrane diameter of 10 mm, membrane thickness of 3.7 pm, and PCN-250 crystal density of 1.011 g cm'³) were used for each physisorption test.

Results and discussion

The UV-Vis peaks remained almost unchanged for all the tested dye solutions (FIG. 23), indicating that the amounts of physisorbed dyes, if there are any, are almost negligible, and the dye rejection demonstrated in PCN-250 membranes is mainly achieved by size exclusion under equilibrium conditions. Comparative Example 1

Most notably, the PCN-250 membrane exhibited high permeance and dye rejection compared with other lab-fabricated MOF-, inorganic-, polymer-, thin-film composite-, and mixed matrix membranes (FIG. 24, see below for the detailed description of the legend). Its MB rejection is comparable to and even higher than that of polymer and thin-film composite membranes. Specifically, its NMP and ethanol permeance is remarkably higher than that of previous membranes. This performance can be attributed to the transport pathways through the PCN- 250 membrane’s large and regular pores.

Detailed description of the legend in FIG. 24

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Table 7. Organic solvent nanofiltration performance of the PCN-250 membrane and other state-of-the-art membranes.

MMM: Mixed matrix membrane; TFC: Thin-film composite; RB: Rose bengal; MB: Methyl blue;

LA: Linoleic acid; VB: Victoria blue; SO: Styrene oligomers; CBT : Chrome black T ; BB: Brilliant blue; NR: Nile red; AF: Acid fuchsin; RBB: Remazol brilliant blue R; RDB: Rhodamine B; SB35: Solvent blue 35; BBR2: Brilliant blue R250; CR: Congo red; VB12: Vitamin B12; CV: Crystal violet. *: pure ethanol

Claims

2. The composite material according to Claim 1 , wherein the polycrystalline membrane material is PCN-250.

3. The composite material according to Claim 1 or Claim 2, wherein the MOF has a pore size of from 5 to 10 A.

4. The composite material according to Claim 3, wherein the MOF has a pore size of from 5.8 to 9.2 A.

5. The composite material according to any one of the preceding claims, wherein the MOF has a BET surface area of from 1000 to 2000 m².g^-1, such as about 1305 m².g-¹.

6. The composite material according to any one of the preceding claims, wherein the thickness of the polycrystalline membrane material on the first surface of the porous substrate is from 0.1 pm to 20 pm.

7. The composite material according to Claim 6, wherein the thickness of the polycrystalline membrane material on the first surface of the porous substrate is from 0.5 pm to 15 pm.

8. The composite material according to Claim 7, wherein the thickness of the polycrystalline membrane material on the first surface of the porous substrate is from 1 pm to 10 pm, such as from 2 to 5.4 pm, such as from 2.6 to 5 pm, such as about 3.7 pm.

10. The composite material according to any one of the preceding claims, wherein the composite material is suitable for use as a filter material.

11. The composite material according to Claim 10, wherein the composite material is suitable for use as a filter material in organic solvent nanofiltration.

12. The composite material according to any one of the preceding claims, wherein the composite material displays a permeance of greater than 20 L rm² h'¹ bar¹ for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A, optionally wherein the composite material displays a permeance of from 25 to 140 L rrr² IT¹ bar¹ for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A.

13. The composite material according to any one of the preceding claims, wherein the composite material displays a dye rejection of 93.7% for methyl blue using N-methyl-2- pyrrolidone (NMP) with a flux of 2.6 L rm² IT¹ bar¹.

14. The composite material according to any one of the preceding claims, wherein the substrate is selected from one or more of a polymer, a ceramic (e.g. alumina), a carbon cloth, a metal, and a metal oxide.

15. The composite material according to Claim 14, wherein the substrate is a porous AI2O3 substrate, optionally wherein the substrate is a porous a-AhOs substrate.

16. The composite material according to any one of the preceding claims, wherein the substrate is provided in the form of a tube, a mesh, a sheet or in the form of hollow fibers (e.g. porous alumina ceramic hollow fibers) and other arrangements that are obtainable by the folding of a tube, a mesh, a sheet and hollow fibers.

17. The composite material according to any one of the preceding claims, wherein the polycrystalline membrane material formed on the first surface is defect-free.

18. The composite material according to any one of the preceding claims, wherein a surface of the polycrystalline membrane material is substantially free of metal organic framework crystals deposited on top of said surface.

19. The composite material according to any one of Claims 1 to 11 and 14 to 18, wherein the composite material displays a permeance of greater than 10 L rm² h'¹ bar¹ for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A, optionally wherein the composite material displays a permeance of from 15 to 350 L rm² IT¹ bar¹ , such as from 12 to 337 L rm² h'¹ bar¹ , for water and organic solvents having a molecular diameter less than or equal to 9A, such as less than or equal to 8A.

20. The composite material according to any one of the preceding claims, wherein the composite material is stable for at least two weeks in an aqueous solution with a pH of 2.

21 . The composite material according to any one of the preceding claims, wherein:

(a) the composite material is hydrophilic with water contact angle of around 7.6°; and/or

(b) the composite material is hydrophilic with a roughness of around 543 nm.

22. A method of forming a composite material according to any one of Claims 1 to 21 , comprising the steps of:

(a) providing a porous substrate that has been seeded by a rub-seeding operation with a plurality of crystals and or powdered particulates of a metal organic framework (MOF) that is PCN-250 (iron azobenzene tetracarboxylic, also known as MIL-127(Fe), soc-MOF(Fe));

(b) placing the porous substrate that has been seeded in a vessel and immersing it in a mother solution comprising a solvent, an organic ligand, a metal precursor compound, and a modulator compound and then heating the resulting mixture for a period of time to provide the composite material, wherein the porous substrate is substantially perpendicularly orientated with respect to the base of the vessel.

23. A method of forming a composite material according to any one of Claims 1 to 21 , comprising the steps of:

24. A method of using a polycrystalline metal-organic framework membrane as described in any one of Claims 1 to 21 in a process of separating a fluid into a filtrate fluid and a retentate fluid, the process comprising the steps of:

(a) providing a fluid in need of separation to a composite material as described in any one of Claims 1 to 21 ;

(c) collecting the filtrate fluid and retentate fluids.

25. The method according to Claim 24, wherein the fluid to be separated is selected from: a mixture of gases; an aqueous solution comprising one or more inorganic materials; an aqueous solution comprising one or more organic materials; an aqueous solution comprising one or more inorganic materials and one or more organic materials; a mixture of organic liquids; a mixture of one or more organic liquids and water; a mixture of one or more organic liquids and one or more organic materials; a mixture of one or more organic liquids and one or more inorganic materials; a mixture of one or more organic liquids, one or more organic materials and one or more inorganic materials; a mixture of water, one or more organic liquids and one or more organic materials; a mixture of water, one or more organic liquids and one or more inorganic materials; and a mixture of water, one or more organic liquids, one or more organic materials and one or more inorganic materials.