WO2020152442A1

WO2020152442A1 - Filtration membranes

Info

Publication number: WO2020152442A1
Application number: PCT/GB2020/050109
Authority: WO
Inventors: Rahul RAVEENDRAN NAIR; Vishnu SREEPAL
Original assignee: The University Of Manchester
Priority date: 2019-01-21
Filing date: 2020-01-20
Publication date: 2020-07-30
Also published as: GB201900824D0

Abstract

This invention relates to membranes comprising 2D phyllosilicate coatings, e.g. 2D vermiculite coatings. The 2D phyllosilicateflakes are associated with cations. The invention also relates to methods of using said membranes, e.g. in filtration methods.

Description

Filtration Membranes

[0001] This invention relates to membranes comprising 2D phyllosilicate coatings. The invention also relates to methods of using said membranes.

BACKGROUND

[0002] Membranes find numerous uses in modern industry. Membranes can be used to separate oil and water, to remove solutes or solvents from water, e.g. to generate drinking water or to selectively separate one solute from another.

[0003] Shao et al (‘Self assembled two-dimensional nanofluidic proton channels with high thermal stability’; Nat. Commun. 6:7602 doi10.1038/ncomms8602 (2015)) have shown that laminate films formed from exfoliated vermiculite can conduct protons. The 2D vermiculite flakes were associated with lithium ions and the laminate films were micrometers thick.

[0004] There remains a need however to find and characterise new membranes that might provide performance improvements in any number of applications.

BRIEF SUMMARY OF THE DISCLOSURE

[0005] In a first aspect of the present invention there is provided a membrane, said membrane comprising a plurality of 2D phyllosilicate flakes and a plurality of cations associated with said 2D phyllosilicate flakes, wherein the plurality of cations does not comprise lithium.

[0006] The membrane may be a composite membrane, wherein said composite membrane comprises a porous support membrane, the first face of which is coated with a coating, said coating comprising the plurality of 2D phyllosilicate flakes and the plurality of cations associated with said 2D phyllosilicate flakes.

[0007] It may be that the coating is no more than 1 pm thick.

[0008] In use, said plurality of 2D phyllosilicate flakes (e.g. said coating) will typically be hydrated. However, it could be that the membrane is supplied in a dry state and is hydrated before use.

[0009] In a second aspect of the invention is provided a separation method, the method comprising:

A) contacting a first face of a membrane with a mixture of a first product or group of products and a second product or group of products to provide the separated first product or group of products at a first face of the membrane and the separated second product or group of products at a second face of the membrane; and

B) recovering the second product or group of products from or downstream from the second face of the membrane; and/or recovering the first product or group of products from or downstream from the first face of the membrane;

said membrane comprising a plurality of 2D phyllosilicate flakes and a plurality of cations associated with said 2D phyllosilicate flakes. It may be that the membrane is a membrane of the first aspect.

[0010] In a third aspect of the present invention there is provided a use of a membrane in the separation of a first product or group of products and a second product or group of products from a mixture of the first product or group of products and the second product or group of products, said membrane comprising a plurality of 2D phyllosilicate flakes and a plurality of cations associated with said 2D phyllosilicate flakes. It may be that the membrane is a membrane of the first aspect.

[0011] The inventors have found that the properties of membranes comprising 2D phyllosilicate flakes can be tuned by varying the nature of the cations that are associated with the flakes.

[0012] The inventors have also found that the membranes of the invention exhibit selective ion diffusion.

[0013] In a fourth aspect of the invention is provided a method of making a composite membrane, said method comprising the steps:

A) forming a membrane comprising a plurality of 2D phyllosilicate flakes and a plurality of lithium ions associated with said 2D phyllosilicate flakes; and

B) displacing said lithium ions with a plurality of cations to form a membrane

comprising a plurality of 2D phyllosilicate flakes and a plurality of cations associated with said 2D phyllosilicate flakes, said wherein the plurality of cations does not comprise lithium.

[0014] The inventors have found that novel membranes can be formed by forming membranes comprising a plurality of 2D phyllosilicate flakes and a plurality of lithium ions and subjecting said membranes to cation exchange, displacing the lithium ions with a cation of choice. The inventors have also found that the properties of 2D phyllosilicate can be tuned by exchanging lithium cations for other cations. Membranes

[0015] It may be that the plurality of cations comprises metal ions. It may be that the plurality of cations are all metal ions.

[0016] It may be that the plurality of cations comprises organic ions. It may be that the plurality of cations are all organic ions. Organic ions include those having a nitrogen atom that is directly attached to four other atoms, e.g. four carbon or hydrogen atoms. Organic ions include tetraalkyl ammonium ions and N-alkylated nitrogen heteroaryl groups (e.g. pyridines, imidazoles, pyrazoles, indoles, pyridazine, pyrazines, pyrimidines).

[0017] It may be that the plurality of cations comprises H⁺ ions.

[0018] It may be that the plurality of cations comprises alkali metal ions (but not lithium).

It may be that the plurality of cations are all alkali metal ions (but not lithium). It may be that the plurality of cations comprises alkali earth metal ions. It may be that the plurality of cations are all alkali earth metal ions. It may be that the plurality of cations comprises transition metal ions. It may be that the plurality of cations are all transition metal ions. It may be that the plurality of cations comprises lanthanide or actinide metal ions. It may be that the plurality of cations are all lanthanide or actinide metal ions.

[0019] Potassium has a particularly strong association with phyllosilicates. The plurality of cations may all be potassium ions. It may be that the plurality of cations comprises a plurality of potassium ions and a plurality of at least one other cation (but not lithium). It may be that the plurality of cations comprises a plurality of potassium ions and a plurality of alkali metal ions (but not lithium). It may be that the plurality of cations comprises a plurality of potassium ions and a plurality of alkali earth metal ions. It may be that the plurality of cations comprises a plurality of potassium ions and a plurality of transition metal ions. It may be that the plurality of cations comprises a plurality of potassium ions and a plurality of lanthanide or actinide metal ions.

[0020] It may be that the plurality of cations comprises ions having a single positive charge (but not lithium). It may be that the plurality of cations are all ions having a single positive charge (but not lithium). It may be that the plurality of cations comprises ions having a 2+ charge. It may be that the plurality of cations are all ions having 2+ charge. It may be that the plurality of cations comprises ions having a 3+ charge. It may be that the plurality of cations are all ions having 3+ charge. It may be that the plurality of cations comprises ions having a 4+ charge. It may be that the plurality of cations are all ions having 4+ charge.

[0021] It may be that the plurality of cations comprises a plurality of potassium ions and a plurality of ions having a single positive charge (but not lithium). It may be that the plurality of cations comprises a plurality of potassium ions and a plurality of ions having a 2+ charge. It may be that the plurality of cations comprises a plurality of potassium ions and a plurality of ions having a 3+ charge. It may be that the plurality of cations comprises a plurality of potassium ions and a plurality of ions having a 4+ charge.

[0022] In certain specific embodiments, the plurality of cations comprise a metal selected from potassium, calcium, lanthanum, tin and silver. In certain specific embodiments, the plurality of cations comprise a metal selected from potassium, calcium, lanthanum, tin, silver and sodium. The plurality of cations may all be potassium ions. The plurality of cations may comprise a plurality of potassium ions and a plurality of ions of a metal selected from calcium, lanthanum, tin and silver. The plurality of cations may comprise a plurality of potassium ions and a plurality of ions of a metal selected from calcium, lanthanum, tin, silver and sodium.

[0023] Throughout the specification, the term‘all’ when applied to the plurality of cations may mean greater than 80 molar%. It may mean greater than 90 molar%. It may mean greater than 95 molar% or greater than 99 molar%.

[0024] It may be that the flakes and the cations associated with them together have no charge. It may be that the flakes and the cations associated with them together have a negative charge. 2D phyllosilicate flakes are typically negatively charged. The cations that are associated with the flakes reduce the negative charge and, if enough cations are present, can completely neutralise the negative charge. If fewer cations are present, however, the flakes and the cations associated with them will together have a negative charge. Where the cation is potassium, for example, it does not typically associate in sufficient amounts with the flakes in order to neutralise the flake and the flakes together with the potassium ions have a negative charge. The net charges of the flakes and cations associated with them can be measured by measuring the zeta potential. It may be that the flakes and the cations associated with them together have a zeta potential that is more negative than -0.001. It may be that the flakes and the cations associated with them together have a zeta potential that is more negative than -0.01. Alternatively, it may be that the flakes and the cations associated with them together have a zeta potential that is negative but is less negative than -0.04. It may be that the flakes and the cations associated with them together have a zeta potential that is negative but is less negative than -0.02. It may be that the flakes and the cations associated with them together have a zeta potential that is negative but is less negative than -0.01.

[0025] It may be that the 2D phyllosilicate flakes and the associated cations are selected such that if the flakes and associated cations are formed into a hydrated 5 pm thick laminate membrane, the surface of that membrane would have a water contact angle that is less than 100°. It may be that the 2D phyllosilicate flakes and the associated cations are selected such that if the flakes and associated cations are formed into a hydrated 5 pm thick laminate membrane, the surface of that membrane would have a water contact angle that is less than 80°. It may be that the 2D phyllosilicate flakes and the associated cations are selected such that if the flakes and associated cations are formed into a hydrated 5 pm thick laminate membrane, the surface of that membrane would have a water contact angle that is greater than 0°. It may be that the 2D phyllosilicate flakes and the associated cations are selected such that if the flakes and associated cations are formed into a hydrated 5 pm thick laminate membrane, the surface of that membrane would have a water contact angle that is greater than 20°. It may be that the 2D phyllosilicate flakes and the associated cations are selected such that if the flakes and associated cations are formed into a hydrated 5 pm thick laminate membrane, the surface of that membrane would have a water contact angle that is greater than 40°.

[0026] The 2D phyllosilicate flakes may be 2D vermiculite flakes. The membrane (e.g. the coating) may comprise a mixture of 2D flakes of two or more different 2D

phy 11 os i I i cates , e.g. a mixture of 2D vermiculite flakes and 2D flakes of at least one other phyllosilicate.

[0027] It may be that membrane (e.g. the coating) comprises a plurality of 2D

phyllosilicate flakes having a size distribution such that greater than 50 wt% (e.g. greater than 85% or greater than 95%) of the flakes have a flake size that is less than 20 pm. It may be that the membrane (e.g. the coating) comprises a plurality of 2D phyllosilicate flakes having a size distribution such that greater than 50 wt% (e.g. greater than 85% or greater than 95%) of the flakes have a flake size that is less than 10 pm. It may be that the membrane (e.g. the coating) comprises a plurality of 2D phyllosilicate flakes having a size distribution such that greater than 50 wt% (e.g. greater than 85% or greater than 95%) of the flakes have flake size that is less than 2 pm.

[0028] It may be that the membrane (e.g. the coating) comprises a plurality of 2D phyllosilicate flakes having a size distribution such that greater than 50 wt% (e.g. greater than 85% or greater than 95%) of the flakes have a flake size that is greater than 100 nm.

It may be that the membrane (e.g. the coating) comprises a plurality of 2D phyllosilicate flakes having a size distribution such that greater than 50 wt% (e.g. greater than 85% or greater than 95%) of the flakes have a flake size that is greater than 200 nm. It may be that the membrane (e.g. the coating) comprises a plurality of 2D phyllosilicate flakes having a size distribution such that greater than 50 wt% (e.g. greater than 85% or greater than 95%) of the flakes have a flake size that is greater than 500 nm. [0029] It may be that greater than 50% by weight (e.g. greater than 85% or greater than 95%) of the 2D phyllosilicate has a thickness of from 1 to 5 phyllosilicate layers. It may be that greater than 50% by weight (e.g. greater than 85% or greater than 95%) of the 2D phyllosilicate has a thickness of from 1 to 3 phyllosilicate layers. It may be that greater than 50% by weight (e.g. greater than 85% or greater than 95%) of the 2D phyllosilicate has a thickness of a single phyllosilicate layer.

[0030] It may be that the membrane does not comprise a porous support membrane. It may be that the plurality of 2D phyllosilicate flakes forms a free-standing laminate membrane. It may be that the membrane comprises only the plurality of 2D phyllosilicate flakes, the plurality of cations and optionally a binder. It may be that the membrane is no more than 15 pm thick. It may be that the membrane is no more than 10 pm thick. It may be that the coating is no less than 100 nm thick. It may be that the coating is no less than 500 nm thick. It may be that the coating is no less than 1 pm thick.

[0031] Where the membrane does comprise a porous support membrane, it may be that the coating is no more than 1 pm thick. It may be that the coating is no more than 500 nm thick. It may be that the coating is no more than 100 nm thick. It may be that the coating is no more than 60 nm thick. In certain porous membranes, 60 nm is the thickness above which the coating forms a laminate structure. Where the coating forms a laminate structure the water flux is typically lowered. The thickness at which the coating becomes a laminate varies depending on the material, surface roughness and pore size of the porous membrane. It may be that the coating is no more than 20 nm thick. It may be that the coating is no less than 10 nm thick. The inventors have found that membranes having a coating thickness below 10 nm are still effective at separating oil and water but are less resistant to fouling than membranes having a coating that is thicker than 10 nm.

[0032] It may be that the coating thickness is such that the water flux through the composite membrane is reduced by no more than 50% relative to the water flux through the uncoated porous membrane. It may be that the coating thickness is such that the water flux through the composite membrane is reduced by no more than 75% relative to the water flux through the uncoated porous support membrane.

[0033] The porous support membrane may have an average pore size that is no more than 20 pm. The porous support membrane may have an average pore size that is no more than 5 pm. The porous support membrane may have an average pore size that is no more than 2 pm. The porous support membrane may have an average pore size that is no less than 100 nm. The porous support membrane may have an average pore size that is no less than 300 nm. The porous support membrane may have an average pore size that is no more than 500 nm. [0034] The porous support membrane may be a polymeric membrane. The porous support may comprise a polymer selected from: polytetrafluoroethylene (PTFE), polyvinylidenedifluoride (PVDF), Poly(ether sulfone) (PES) and Polyamide (e.g. Nylon). The porous membrane may comprise nylon.

[0035] The porous support membrane may be inorganic, e.g. it may comprise alumina.

[0036] The porous support may comprise a woven fabric, e.g. cotton.

[0037] The porous support membrane may comprise a metal mesh, e.g. steel.

[0038] The porous support membrane may, when uncoated, have a water contact angle that is less than 90 °. The porous support membrane may, when uncoated, have a water contact angle that is less than 60 °. The porous support membrane may, when uncoated, have a water contact angle that is less than 40

[0039] There may be an adhesion promoter between the coating and the porous support membrane. The adhesion promoter may be a positively charged polymer coating, e.g a coating of Polydiallyldimethylammonium chloride (PDADMAC).

[0040] The surface of the porous support membrane may have been treated to increase adhesion between the support and the coating, e.g. plasma treatment or chrono treatment.

[0041] The porous support membrane make take the form of a hollow porous tube or fiber.

[0042]

[0043] The membrane (e.g. the coating) may further comprise a binder, e.g. Poly vinyl acetate.

[0044] It may be that the plurality of 2D phyllosilicate flakes and the plurality of cations associated with said 2D flakes together make up no less than 65% by weight of the membrane (e.g. the coating). It may be that the plurality of 2D phyllosilicate flakes and the plurality of cations associated with said 2D flakes together make up no less than 85% by weight of the membrane (e.g. the coating).

[0045] The membrane (e.g. the composite membrane) may take the form of a flat sheet. The composite membrane may take the form of a hollow fibre.

[0046] It may be that the second face of the porous support membrane is coated with the same coating as the first face. This can facilitate manufacture and/or be of use in multidirectional separation systems. [0047] The membrane may form part of a filtration device. Said filtration device may comprise a plurality of said membranes. Said plurality may be arranged in parallel or in series.

Methods of separating mixtures using said membranes and uses of said

membranes in separating mixtures

The following fall backs are expressed as applying to a method of the invention but they apply equally to both the first or third aspects of the invention.

[0048] The method may be continuous. Thus, steps A) and B) may be carried out simultaneously or substantially simultaneously.

[0049] It may be that step B) comprises recovering the second product or group of products from or downstream from the second face of the membrane. It may be that step B) comprises recovering the first product or group of products from or downstream from the first face of the membrane. It may be that step B) comprises recovering the second product or group of products from or downstream from the second face of the membrane and recovering the first product or group of products from or downstream from the first face of the membrane.

[0050] It may be that the mixture is agitated as it is contacted with the membrane.

[0051] Contacting the first face of a composite membrane with the mixture will typically comprise causing the second product or group of products in the mixture to pass through the composite membrane. It may be that the second group of products passes through the composite membrane by diffusion. It may be that a force is applied to the mixture as it is in contact with the first face of the composite membrane, said force being directed such as to cause the product or group of products to pass through the composite membrane. The force may be gravity. It may be that the force is pressure. The pressure may be less than 5 bar. The pressure may be less than 3 bar. The pressure will typically be greater than 1 bar.

[0052] The membrane used in step A) may be a hydrated membrane. The method may comprise the step of hydrating a dry membrane to produce the hydrated composite membrane used in step A). The step of hydrating the dry composite membrane may comprise soaking the dry membrane in water.

[0053] The method may involve contacting a plurality of said membranes with the mixture. Thus, the filtration device of the third aspect may comprise a plurality of said membranes. These may be arranged in parallel (to increase the flux capacity of the process/device) or in series (where the separation achieved by a single membrane is less than desired). [0054] The separation may be complete or it may be partial.

[0055] The first product or set of products may be an organic solvent. The second product or set of products may be an aqueous solution. The aqueous solution may be water.

[0056] The first product or set of products may be a first ion or group of ions, e.g. a first cation or group of cations. The first product or set of products may be a first metal cation or group of metal cations. The second product or set of products may be a second ion or group of ions, e.g. a second cation or group of cations. The second product or set of products may be a second metal cation or group of metal cations. Said ions will typically be in an aqueous solution.

[0057] The inventors have found that potassium ions diffuse selectively through a membrane in which the cations associated with the 2D phyllosilicate flakes are potassium. In particular, potassium ions diffuse selectively over lanthanum ions.

[0058] The inventors have also found that silver ions diffuse selectively through a membrane in which the cations associated with the 2D phyllosilicate flakes are silver. In particular, silver ions diffuse selectively over magnesium ions.

Methods of making the composite membrane

[0059] The method may further comprise exfoliating a bulk phyllosilicate suspended in an aqueous solution comprising lithium ions to provide a suspension of 2D phyllosilicate flakes that are associated with lithium ions. The solution may thus comprise lithium ions (e.g. lithium chloride). It may be that some bulk phyllosilicate remains in the suspension after the exfoliation. If this is the case, the bulk phyllosilicate can be removed by centrifugation to provide a suspension of 2D phyllosilicate flakes that are associated with said cations. It may be that the solid components of the suspension, i.e. the 2D

phyllosilicate flakes and the bulk phyllosilicate, may be removed from the suspension, e.g. by filtration. The solid components may be washed, e.g. with water. Once removed from the suspension and/or washed, the solid components may then be re-suspended in water and then subjected to centrifugation.

[0060] The exfoliation step may be achieved by applying energy to the suspension. Said energy may be sonic energy. The sonic energy may be ultrasonic energy. It may be delivered in using a bath sonicator or a tip sonicator. Alternatively the energy may be a mechanical energy, e.g. shear force energy or grinding. Preferably, however, the energy is heat energy. Thus, the step of exfoliating may comprise heating the suspension of the bulk phyllosilicate in the aqueous solution of the cations, e.g. heating to reflux. [0061] The suspension may be subjected to energy (e.g. heat) for a length of time from 15 min to 1 week, depending on the properties and proportions (flake size and thickness) desired. The particles may be subjected to energy (e.g. heat) for a length of time from 12 hours to 3 days.

[0062] Prior to being exfoliated in the solution aqueous solution comprising the cations, the bulk phyllosilicate may be heated in a saturated NaCI solution. The resulting solid may be isolated, e.g. by filtration, and optionally washed, e.g. with water, before being suspended in the aqueous solution comprising the cations.

[0063] The bulk phyllosilicate may be vermiculite, e.g. thermally expanded vermiculite.

[0064] A method of forming 2D vermiculite flakes associated with lithium ions is described in Shao et al (‘Self assembled two-dimensional nanofluidic proton channels with high thermal stability’; Nat Commun. 6:7602 doi10.1038/ncomms8602 (2015)), incorporated herein by reference.

[0065] The suspension can then be used to coat a porous membrane. This may be done by passing the suspension through the porous membrane for a predetermined period of time, said period of time being sufficient to achieve the desired thickness of coating.

[0066] The method may further comprise, before coating the membrane with the 2D phyllosilicate, treating the surface of the porous support membrane to increase adhesion between the membrane and the coating. Said treatment may be plasma treatment or chrono treatment. Alternatively, said treatment may comprise coating the membrane with an adhesion promoter. The adhesion promoter may be a positively charged polymer coating, e.g a coating of Polydiallyldimethylammonium chloride (PDADMAC).

[0067] Alternatively, the suspension can then be used to provide free-standing laminate membrane. This may be done by passing the suspension through a porous support membrane for a predetermined period of time, said period of time being sufficient to achieve the desired thickness of laminate membrane to provide a laminate membrane supported on a porous support membrane; and then separating the laminate membrane from the porous support membrane to provide the freestanding laminate membrane.

[0068] The free-standing laminate or the composite membrane comprising a coating will at this stage comprise lithium ions. The free-standing laminate or the composite membrane comprising a coating can then be subjected to step B) of the fourth aspect of the invention. Step B) may be carried out by contacting the membrane (e.g. the free standing laminate or the composite membrane comprising the coating) with a solution of the desired cations (e.g. metal cations). The solution may be a solution of the chloride of the desired cation. Exemplary solutions include potassium chloride, calcium chloride lanthanum chloride, tin chloride and silver chloride.

[0069] The membrane (e.g. the free-standing laminate or the composite membrane comprising the coating) comprising a coating prepared from the suspension will typically comprise lithium ions and potassium ions. If a potassium solution is used to displace the lithium ion, the plurality of cations in the resultant membrane (e.g. the free-standing laminate or the composite membrane comprising the coating) will all be potassium ions. If a solution of a different metal (i.e. a metal other than potassium or lithium, e.g. Ca, La, Sn, Ag) is used to displace the lithium ion, the plurality of cations in the resultant membrane (e.g. the free standing laminate or the composite membrane comprising the coating) will comprise a plurality of potassium ions and a plurality of ions of that other metal (e.g. Ca, La, Sn, Ag).

[0070] The step of contacting the membrane (e.g. the free-standing laminate or the composite membrane comprising the coating) with a solution of the desired cations (e.g. metal cations) may comprise immersing the membrane (e.g. the free standing laminate or the composite membrane comprising the coating) in a solution of the desired cations (e.g. metal cations).

[0071] The membrane may be immersed in the solution for a period of time greater than 10 minutes, e.g. a period of time greater than 30 minutes. It may be that the membrane is immersed in the solution for a period of time less than 24 hours, e.g. less than 6 hours or less than 3 hours.

[0072] The immersion may take place at a temperature between 0 °C and 60 °C. The immersion may take place at a temperature between 10 °C and 30 °C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1A shows a transmission electron microscope (TEM) image of exfoliated lithium vermiculite flakes. Scale bar, 10 pm. Figure 1 B shows the XRD patterns for

LiV, KV, CaV, LaV and SnV-laminates that were vacuum dried for 12 h, then exposed to air (-40% relative humidity) and subsequently soaked in water.

Figure 2 shows an AFM image of vermiculite flakes drop-casted on a silicon wafer. Inset: Height profile along the dotted line showing vermiculite flakes has an average thickness of 1.5 nm. Scale bar, 750 nm. Figure 3 shows wetting properties of vermiculite laminates. (A-E) Water contact angle of LiV, KV, CaV, LaV, and SnV-laminates in dry and wet states. Scale bar, 750 mm. (F) Evaluation of water pinning property of free-standing wet LiV laminate measured by capturing XRD from wet LiV laminate before and after immersing in kerosene for a week. As a reference, a hydrophilic GO laminate was also tested in the same experimental conditions, it was found that water molecules were released from the membrane after 12 hours of kerosene exposure.

Figure 4 shows the effect of ionic strength on contact angle and zeta potential. (A) The contact angle as a function of ionic strength of the solution for various V-laminates. (B) Zeta potential obtained for various V-laminates. Inset: variation of zeta potential as a function of ionic strength of the solution for KV- and LiV-laminates.

Figure 5 shows the surface free energy for different V-laminates. Dispersion ( Os^d ) and polar (s ) components of the surface free energy for various vermiculite laminates estimated from the contact angle data.

Figure 6 shows a) permeation rate of K and La through a membrane of the invention taken at an interval of 24hrs showing selective permeation of potassium b) Concentration dependence of the ratio of ions in permeate side plotted. Concentration of LaCh was fixed to be 0.5M

DETAILED DESCRIPTION

[0074] Throughout this specification, the term‘oil’ is used to describe any organic species that is poorly miscible or immiscible with water. Examples may include fuel oils, cooking oils, greases and fats, solvents and essential oils. The oil will typically be a liquid at room temperature, but it may be a solid at room temperature, in which case the separation process of the first aspect may be conducted at a temperature that is higher than the melting point of the oil.

[0075] The term‘coating’ is intended to mean that a layer of 2D phyllosilicate flakes lies on the support membrane. The coating may be on both sides of the substrate. It is possible, depending on the materials involved, the size of the pores and the size of the flakes, that the 2D phyllosilicate flakes coat the surfaces of the pores inside the support membrane, e.g. to a depth of up to 50 nm inside the support membrane. It may be however that substantially no (e.g. no more than 1 % by weight) 2D phyllosilicate flakes are found inside the pores of the support membrane.

[0076] The term‘aqueous medium’ refers to a mixture of substances which comprises at least 50% water by weight. It may comprise at least 75% water by weight, e.g. at least 90% water by weight. The term‘aqueous medium’ preferably refers to either water or an aqueous solution. However, it is not intended to exclude the possibility that there might be particulate matter suspended in the aqueous medium. Of course, it is expected that any particulate matter that is larger than the pores of the composite membrane will not pass through the membrane.

[0077] Phyllosilicates are materials that comprise parallel sheets of silicate. The silicate within the sheets is typically in the form of a layer of interconnected tetrahedra and the ratio of silicon to oxygen in the sheets is typically 2:5. Exemplary phyllosilicates include Antigorite (Mg₃Si₂05(0H)4), Chrysotile (Mg₃Si₂05(0H)4), Lizardite (Mg₃Si₂0₅(0H)₄), Halloysite (AI₂Si₂0₅(0H)₄), Kaolinite (AI₂Si₂0₅(0H)₄), lllite

((K,H₃0)(AI,Mg,Fe)₂(Si,AI)₄Oio[(OH)₂,(H₂0)]), Montmorillonite

((Na,Ca)o_.33(AI,Mg)₂Si₄Oio(OH)₂ nH₂0), Vermiculite ((MgFe,AI)₃(AI,Si)₄O₁₀(OH)₂-4H₂O),

Talc (Mg₃SUOio(OH)₂), Sepiolite (Mg₄Si₆0i₅(0H)₂-6H₂0), Attapulgite

((Mg,AI)₂Si₄Oio(OH)-4(H₂0)), Pyrophyllite (AI₂Si₄O₁₀(OH)₂), Biotite

(K(Mg,Fe)₃(AISi₃)Oio(OH)₂), Fuchsite (K(AI,Cr)₂(AISi₃O₁₀)(OH)₂), Muscovite

(KAI₂(AISi₃)Oio(OH)₂), Phlogopite (KMg₃(AISi₃)O₁₀(OH)₂), Lepidolite (K(Li,AI)₂_

3(AISi₃)Oio(OH)₂), Margarite (CaAI₂(AI₂Si₂)O₁₀(OH)₂), Glauconite

((K,Na)(AI,Mg,Fe)₂(Si,AI)₄Oio(OH)₂) and Chlorite ((Mg,Fe)₃(Si,AI)₄O₁₀(OH)₂ (Mg,Fe)₃(OH)₆). Further information on phyllosilicates can be found in W.A. Deer, R.A. Howie, and J.

Zussman. (2013). 3rd ed. An Introduction to the Rock-forming Minerals. London: Mineralogical Society. ISBN 9780903056274. 498pp.

[0078] In certain embodiments, the phyllosilicate is a clay-type phyllosilicate (e.g.

Halloysite, Kaolinite, lllite, Montmorillonite, Vermiculite, Talc, Sepiolite, Attapulgite, Pyrophyllite). In certain preferred embodiments, the phyllosilicate is vermiculite. In other embodiments, the phyllosilicate is a mica-type phyllosilicate (e.g. Biotite, Fuchsite, Muscovite, Phlogopite, Lepidolite, Margarite or Glauconite).

[0079] The term‘2D phyllosilicate flake’ refers to a flake of phyllosilicate that is from 1 to 5 phyllosilicate layers thick. The precise identity of a phyllosilicate layer varies depending on the identity of the bulk phyllosilicate from which the 2D material was obtained. A phyllosilicate layer is typically two sheets of silicate with a further mineral sheet (typically comprising aluminium, magnesium, iron and mixture thereof) sandwiched between them. The silicate sheets are typically comprised of interlinked tetrahedra and the further mineral sheet is typically comprised of interlinked octahedra, with the three layers being linked through oxygen atoms. In the bulk phyllosilicate material (indeed in 2D phyllosilicate flakes that are not a single phyllosilicate layer thick), the phyllosilicate layers are held together by interlayer hydrated cations. In vermiculite, for example, each phyllosilicate layer is comprised of one magnesium based octahedral sheet sandwiched between two tetrahedral silicate sheets. Substitutional Al³⁺ impurities in the silicate sheets give the phyllosilicate sheets a net negative charge that is balanced by the cations situated between the phyllosilicate sheets. 2D vermiculite flakes therefore comprise from 1 to 5 phyllosilicate layers, with each phyllosilicate layer comprising two silicate tetrahedral sheets and a magnesium based octahedral sheet sandwiched between. A‘2D

phyllosilicate X flake’ (where‘phyllosilicate X is a particular phyllosilicate) may be considered to mean‘a flake that is from 1 to 5 phyllosilicate layers thick and formed by exfoliation of said phyllosilicate X’.

[0080] For the absence of doubt, the cations that are associated with the phyllosilicate flakes in the membranes of the invention are not necessarily the cations that are present in between the phyllosilicate layers in the bulk phyllosilicate. Ion exchange can occur during the exfoliation process, for example.

[0081] The membranes of the invention are formed from a plurality of 2D phyllosilicate flakes and a plurality of cations associated with said 2D phyllosilicate flakes. The term ‘associated’ is intended to mean that the cations are attached to the flake by ionic interactions. The cations may be situated within the phyllosilicate layers and/or the cations may be situated in between the phyllosilicate layers (where the flakes have more than 1 phyllosilicate layer) and/or the cations may be situated on the external surfaces of the 2D phyllosilicate flakes.

[0082] The term‘hydrated’ means that the coating or membrane comprises water. The water will typically be associated with the cations, e.g. datively bonded to the cations.

[0083] Throughout the description and claims of this specification, the words“comprise” and“contain” and variations of them mean“including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0084] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments.

The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0085] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

EXAMPLES

[0086] Stable aqueous dispersions of lithium vermiculite (LiV) (inset of Fig. 1A) flakes were prepared from thermally expanded vermiculite crystals using a reflux ion exchange methodology. The transmission electron microscopy (TEM) (Fig. 1A) and atomic force microscopy (AFM) image (Fig. 2) confirms that the individual flakes are defect free and have a thickness of -1.5 nm. Micrometre-thick (5 pm) V-laminates (Fig. 1 B) were prepared from the above dispersion using vacuum filtration as. We used a simple ion exchange method to replace Li ions in the interlayers of the vermiculite membranes to other desired cations such as sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), lanthanum (La³⁺), and tin (Sn⁴⁺) . Inductively coupled plasma mass spectrometry (ICP-MS) analysis confirms that Li has been completely interchanged with the desired cations (Table 1 below).

[0087] Cations in interlayer galleries of the bulk vermiculite crystals are hydrated such that the extent of hydration could depend on the competition of cation hydration energy and the electrostatic interaction between the cation and the layer charge. Consistent with this, the authors have found that vermiculite laminates with different interlayer cations show different Interlayer spacing (d-spacing) and different hydration behaviour. Fig. 1 B shows X-ray diffraction (XRD) of V-laminates vacuum dried for 12 h, then exposed to air (-40% relative humidity) and subsequently soaked in water (20). d-spacing of vacuum dried samples are in the range of 12.2 A to 11.5 A, typical of bulk vermiculite with a single hydration layer. On the other hand, when exposed to ambient air the d-spacings increased to 12.6 A, 13.6 A, 13.9 A, 14.7 A and 15.2 A for K-vermiculite (KV), Li-vermiculite (LiV), Sn- vermiculite (SnV), Ca-vermiculite (CaV), and La-vermiculite (LaV) laminates, respectively. Further exposure to liquid water only increased the d-spacing of LiV (13.6 to 15.2 A), and no change was observed for CaV, LaV, SnV and KV laminates. The observed swelling behaviour of V-laminates is consistent with swelling of bulk vermiculite and further confirms the exchange of Li cations to the other cations used.

[0088] Compared to the bulk vermiculite crystals one of the unique features of the exfoliated vermiculite is its potential to use as a membrane or coating. Hence a thorough understanding of the wetting properties of such films is essential. We have used contact angle measurements, an experimentally accessible parameter, to characterize the surface wetting properties of V-laminates. Fig. 3A-E show the water contact angle measurements on cation exchanged V-laminates in dry and wet states. We found that cation-exchange modifies the wetting properties of V-laminate in a tunable manner. The water contact angle in the air for LiV-laminate is 15°±1 ° whereas 52°± 1 °, 56°±2°, 63°±3°, 75°±2°, and 101 °±2° for Na⁺, K⁺, Ca²⁺, La³⁺, and Sn⁴⁺ exchanged laminates respectively. It is surprising to note that LiV-laminate is more hydrophilic compared to all other cation exchanged V-laminates, and in the wet state, LiV-laminate is superhydrophilic with zero contact angle while all other V-laminates retained the same wetting behaviour.

[0089] T o test whether the unique superhydrophilicity of LiV is associated with surface charge density, we probed the ionic strength dependence of the contact angle and found negligible change with ionic strength, suggesting surface charge is not the crucial factor (Fig. 4). To confirm this further, we have also measured the zeta potential of V-laminates, which is directly related to the surface charge density, and its dependence on the ionic strength. Our results show that the zeta-potential varies with ionic strength unlike that of the contact angle behavior (Fig. 4).

[0090] Further, we estimated the surface free energy from contact angle data using the primary equation of Fowkes' surface energy theory. The dispersive component of surface energy is roughly same for all vermiculite membranes irrespective of the intercalated cation whereas the polar component decreases in the order LiV> KV> CaV> LaV> SnV (Fig. 5).

[0091] In summary, we have revealed that the properties of V-laminates can be tuned by exchanging lithium cations for other cations.

Materials and Methods

Fabrication of vermiculite laminates [0092] Vermiculite dispersion was obtained from the thermally expanded vermiculite (Sigma Aldrich, UK) via two-step ion exchange method as reported previously ( G. F.

Walker, W. G. Garrett, Chemical Exfoliation of Vermiculite and the Production of Colloidal Dispersions. Science 156, 385 (1967); J. J. Shao, K. Raidongia, A. R. Koltonow, J. Huang, Self-assembled two-dimensional nanofluidic proton channels with high thermal stability.

Nat Commun 6, 7602 (2015)). 100 mg of vermiculite granules were added to 100 ml_ saturated NaCI (36 wt. %) solution and stirred under refluxing at 100 °C for 24 h to replace the interlayer cations with Na. The solution was then filtered out, and the collected vermiculite flakes were repeatedly washed with water and ethanol to remove any residual salt. Sodium exchanged bulk vermiculite was then dispersed in 2M LiCI solution and refluxed for another 24 h followed by filtration and extensive wash with water and ethanol. Lithium vermiculite (LiV) flakes so-obtained were sonicated in water for 20 minutes in order to exfoliate into monolayer LiV flakes and subsequently centrifuged at 6000 rpm to remove any multilayers and bulk residues left in the solution. The thickness of the exfoliated vermiculite flakes (Fig. 2) was measured by using Veeco Dimension 3100 atomic force microscopy (AFM). Electron microscopy characterization of the flakes was carried out by using the Titan G2 80-200 S/TEM analytical scanning transmission electron microscope.

[0093] The LiV laminates of thickness ^« 5 pm were prepared by vacuum filtration of the LiV dispersion through a Whatman Anodise alumina membrane filter (0.02 pm pore size and a diameter of 47 mm). The resulting vermiculite films on alumina filters were peeled off from the substrate to obtain free-standing LiV-laminates (Fig. 1).

[0094] The other cation exchanged vermiculite laminates (V-laminates) were prepared by immersing the LiV-laminate in 1 M aqueous chloride solution of the desired cation for an hour.

Characterization of vermiculite laminates

We have used inductively coupled plasma atomic emission spectrometry (ICP-AES), and X- ray diffraction (XRD) to characterize the V-laminates. The ICP-AES analysis provided the concentration of the interlayer cations in the V-laminates and confirmed the efficient cation exchange. The samples for ICP-AES analysis were prepared by digesting the V-laminate in a mixture of 1 ml of 38% concentrated HCI, and 1 ml of 70% concentrated HNO₃ for overnight. The samples were heated at 70 °C on a hot block in the tubes before being made up to 10 ml with deionized water. The concentration of cations present in the V-laminates was evaluated in moles per mg of the dry V-laminate and is shown in Table 1.

Table 1 : ICP Analysis. Concentration of cations in various vermiculite laminates. BDL denotes‘below detection limit’.

Cation exchange method used here efficiently replaces the initial cations in the laminates with cations of interest. For example, the concentration of Li-ions in KV-laminate obtained by replacing Li-ions with K-ions is below the detection limit of the instrument (Table 1). It should be noted that the K⁺-ion is present in all the laminates and the starting bulk vermiculite since its strong binding to the vermiculite surface. Nevertheless, in KV-laminates the concentration of K-ions is significantly higher than that of other laminates.

XRD experiments were performed to study the laminar structure and swelling properties of the V-laminates with 5 pm in thickness. We used Rigaku smart lab thin film XRD system (Cu-Ka radiation) operated at 1.8 kW. For acquiring XRD from dry V-laminates, initially, all the samples were vacuum dried and stored in the glove box for 48 h. The dehydrated samples were sealed in air-tight X-ray sample holder inside the glove box and taken out for further XRD measurements. Then the same laminates were exposed to ambient air (~ 40 % RH) for 24 hours, and the measurements were repeated. For wet XRD, the laminates are soaked in the corresponding liquid for a minimum of 30 minutes and then immediately acquired the diffraction data.

Contact angle measurements

Sessile drop method (KRLISS drop shape analyzer, DSA100S) was used to measure the water contact angle on the laminates. Free-standing laminates were placed on a holey flat stage such a way that the central part of the laminate would be on top of the hole. A micro syringe needle was used to precisely control the drop volume (2 pi). The needle was lowered slowly until the drop touched the vermiculite laminates and then gently raised. The contact angle measurement module was operated in video mode at a capture speed of 60 frames per second.

Vermiculite coated nylon membranes

The vermiculite coated nylon membranes used for studying the antifouling oil- water/emulsion separation were prepared by filtering the lithium vermiculite dispersion through a porous nylon support (170 pm thick EMD Millipore™ nylon hydrophilic membrane filters with 47 mm diameter) using a dead-end pressure filtration system (Sterlitech HP4750, 0.5 bar of overpressure), followed by overnight vacuum drying. The vermiculite coating thicknesses of 15, 30, 45, 60, 75, and 90 nm were fabricated. The coating thicknesses were estimated from the equation,

where t is the thickness of the coating, C is the concentration of the vermiculite dispersion, V is the volume of the dispersion deposited, A is the coating area, and D is the density of vermiculite film. The density of the vermiculite film was obtained by measuring the weight and volume of a thick (~15 pm) freestanding V-laminate.

Influence of ionic strength on contact angle

To study the impact of surface charge density on the contact angle of V-laminates, we measured the contact angle at different concentrations (10 ⁴ M to 1 M) of salt solutions. At high ionic strength, the surface charge density decreases due to the compression of the electric double layer (EDL)and hence studying the contact angle variation with ionic strength directly probe its dependence on surface charge density. These measurements were carried out by investigating the contact angle of different V-laminates using a salt solution of varying concentrations. To avoid any ion exchange during the contact angle measurements, we used the salt solution corresponds to the interlayer cations of the V- laminates (e.g., LiCI solution for LiV-laminate and KCI for KV-Laminates). Fig. 4 shows the contact angle as a function of the concentration of salt solution for five different V- laminates. For all five types of V-laminates, contact angle did not show any significant change with the ionic strength. It should also be noted that, for LaV and SnV laminates, the contact angle slightly changes at higher concentration of the salt solution. This is due to the change in pH (salt solution become more acidic) of the salt solution at these

concentrations.

Zeta potential measurements

To further confirm the absence of correlation between surface charge density and the contact angle, we have measured the zeta potential of different V-laminates and its ionic strength dependence via the streaming potential technique (Anton Paar SurPASS3). These measurements were carried out by placing two V-laminates inside the measuring cell forming a capillary with 100 pm height. Then the test liquid (a mixture of LiCI and KCL) with known ionic strength was injected through the capillary at a specific pressure (200-600 mbar), and the potential difference was measured between the two ends of the streaming channel as the streaming potential.

For samples with a planar surface, its zeta potential can be related with the streaming potential by Helmholtz-Smoluchowski equation:

where U_str is the measured streaming potential at a specific cross-capillary pressure D p , K \ S the conductivity of the capillary, and h and e e₀are the viscosity and dielectric coefficient of the electrolyte solution.

Fig. 4 shows the zeta potential for different V-laminates. We found that LiV has the maximum zeta potential and it decreases to nearly zero for LaV and SnV with multivalent ions as the interlayer cations. This could be due to the charge neutralisation of the negatively charged silicate layer with positively charged cations. Even though the variation of zeta potential qualitatively correlates with the change in contact angle (Laminates with higher zeta potential shows minimum contact angle), the large change in the contact angle for LiV-laminates compared to other laminates was not correlating with the zeta potential measurements. This absence of correlation between the zeta potential and the contact angle is further confirmed by measuring the ionic strength dependence of zeta potential. As shown in Fig. 4, the zeta potential decreases with ionic strength as expected, whereas the contact angle is largely independent of ionic strength.

[0095] Surface free energy estimation

Fowkes' surface energy theory describes the contact angle ( Q ) of a liquid on a solid surface as

where y₍ is surface tension of the liquid, y , yf are the dispersive and polar component of the surface tension of the wetting liquid, respectively, and y , yf are the dispersive and polar component of the solid surface energy, respectively. The surface energy component of the laminates can be determined from eq. 4 by using the contact angle measurements with liquids of known polar and dispersive components. We used the contact angle data of a nonpolar aprotic liquid (diiodomethane, y_l = 50.8 mN /m, yf = 0, and yf = 50.8 miV/m ) and a polar protic liquid (water, y_l = 72.8 mN /m, yf = 51 miV/m, and yf = 21.8 mN /m ) on the V-laminates (Table 2) and calculated the yf and yf (Fig. 5). The Total surface energy (y_s) of the V-laminate can be calculated as in eq. 5. Ys = Ys^d + Ys (5)

Table 2: Contact angles. Water and diiodomethane Contact angle of different V-laminates

As can be seen from the above analyses, the properties of vermiculite membranes can be tuned by varying the cations associated with the vermiculite.

Ion diffusion through potassium vermiculite membranes (K-V)

Ion selective membranes are of immense interest in terms of biological and industrial applications. To explore the selective permeation of ions through K-V, we glued the K-V membranes onto a PET disc with a hole of 1.33 cm² area. The glued membranes were then sealed between two Teflon compartments labelled feed and permeate. The feed compartment was filled with a mixture of LaCh and KCI. The concentration of LaCL was kept constant at 0.5M while we varied the concentration of KCI between 0.01 M and 0.5M. The permeate solution was collected at an interval of 24 hrs after which both compartments are loaded with fresh solution. We observed that the permeation is faster for potassium ions for all the concentrations of KCI tested. The permeation rate is plotted against number of cycles of repeating the experiment which showed that the membrane performance is stable after repeated use. It is interesting to note that the concentration of K in permeate is higher even after 8 days.

Selectivity of the membrane was found to be dependent on the concentration of the feed mixture. To understand the dependence of potassium concentration in the feed on the permeation properties, we plotted the ratio of the ion concentration in permeate side after 8 cycles with the concentration of KCI in feed (Concentration of La was kept at 0.5M). The ratio increased more than one order when going from 0.01 M to 0.1M KCI in feed and thereafter seems to have an exponential dependence. We also made a vermiculite membrane with silver as the associated cation (using the methodology described above under Fabrication of Vermiculite Membranes) and tested the selectivity of silver permeation over magnesium (0.5M MgNC>3 and 0.5M AgNC>3 in the feed side). Permeation of Ag was found to be more than 2 orders higher than Mg. This shows that the interlayer ion has an influence in the permeation rate in vermiculite membranes. So far, we observed selective permeation of potassium ion through vermiculite membranes. The selectivity towards potassium in K-V remains even when the concentration of K in the feed is less than that of the counter ion. Similar observations were made in the permeation of Ag through Ag intercalated vermiculite membranes.

Claims

1. A membrane, said membrane comprising a plurality of 2D phyllosilicate flakes and a plurality of cations associated with said 2D phyllosilicate flakes, wherein the plurality of cations does not comprise lithium.

2. A membrane of claim 1 , wherein the plurality of 2D phyllosilicate flakes forms a free-standing laminate membrane.

3. A membrane of claim 2, wherein the membrane is no more than 15 pm thick.

4. A membrane of claim 2 or claim 3, wherein the membrane is no less than 100 nm thick.

5. A membrane of claim 2 or claim 3, wherein the membrane comprises a binder.

6. A membrane of claim 1 , wherein the membrane is a composite membrane, wherein said composite membrane comprises a porous support membrane, the first face of which is coated with a coating, said coating comprising the plurality of 2D phyllosilicate flakes and the plurality of cations associated with said 2D phyllosilicate flakes.

7. A membrane of claim 6, wherein the coating is no more than 1 pm thick.

8. A membrane of claim 6 or claim 7, wherein the coating is no less than 10 nm thick.

9. A membrane of any one of claims 6 to 8, wherein the porous membrane is a polymeric membrane, optionally where the membrane comprises nylon.

10. A membrane of any one of claims 1 to 9, wherein the plurality of cations comprises metal ions.

11. A membrane of claim 10, wherein the plurality of cations comprises potassium ions.

12. A membrane of claim 11 , wherein the plurality of cations are all potassium ions.

13. A membrane of claim 12, wherein the plurality of cations comprises a plurality of potassium ions and a plurality of at least one other cation.

14. A method of claim 13, wherein the at least one other cation is selected from sodium, lanthanum, calcium, magnesium, tin and silver.

15. A method of any one of claims 1 to 14, wherein the 2D phyllosilicate flakes are 2D vermiculite flakes.

16. A separation device comprising a composite membrane of any one of claims 1 to 15.

18. A separation method, the method comprising:

A) contacting a first face of a membrane of any one of claims 1 to 15 with a mixture of a first product or group of products and a second product or group of products to provide the separated first product or group of products at a first face of the membrane and the separated second product or group of products at a second face of the membrane; and

B) recovering the second product or group of products from or downstream from the second face of the membrane; and/or recovering the first product or group of products from or downstream from the first face of the membrane.

19. A use of a membrane of any one of claims 1 to 15 in the separation of a first product or group of products and a second product or group of products from a mixture of the first product or group of products and the second product or group of products.

20. A method of claim 18 or a use of claim 19, wherein the first product or set of products is a first ion or group of ions and the second product or set of products is a second ion or group of ions.

21. A method of making a membrane of any one of claims 1 to 15, said method comprising the step:

B) displacing said lithium ions with a plurality of cations to form a membrane