Method of forming absorptive hollow fiber membranes using solvent of dimethyl isosorbide

Abstract

Described is a method of producing adsorptive hollow fiber membranes, comprising the steps of dissolving the polymer in a solvent to provide a polymer solution comprising dimethyl isosorbide, suspending an adsorbant material in the polymer solution to provide a polymer suspension, and spinning the polymer suspension to provide hollow fibers.

Description

[0001] METHOD OF FORMING ABSORPTIVE HOLLOW FIBER MEMBRANES USING SOLVENT OF DIMETHYL ISOSORBIDE

[0002] Field of the Invention

[0003] The present invention relates to a method of forming hollow fiber membranes. In particular, the present invention relates to a method of forming absorptive hollow fiber membranes which utilises an environmentally friendly, ‘green’, solvent: dimethyl isosorbide (DMI).

[0004] Background

[0005] Membrane operations offer valuable solutions to many chemical separation processes, they show potential for replacing conventional energy-intensive separation techniques. Owing to their energy efficiency, selectivity, flexibility, robustness, and ease of scale-up, membrane operations represent a viable solution to the Process Intensification strategy [1 ,2],

[0006] Both polymer and ceramic membranes are frequently used in membrane operations. However from the viewpoint of manufacturing cost, polymeric membranes are often the most economically viable solution. This decreased cost in comparison to alternatives has led polymeric membranes frequently to be used in microfiltration, ultrafiltration, nanofiltration, reverse osmosis, dialysis, fuel cells and other such processes.

[0007] While the use of such membranes often results in a highly energy efficient and green process, the membrane production process itself cannot generally be classified as a green or sustainable process as it produces a lot of waste. This waste is in the form of contaminated wastewaters containing residual solvents which are used in the formation of the membranes themselves. Typically these include dangerous solvents such as dimethyl formamide (DMF), dimethyl acetamide (DMA), N-methyl pyrrolidone (NMP), dimethyl sulfoxide (DMSO) and chlorinated solvents. These wastewaters account for more than 95% of the total waste generated during the membrane fabrication process on an industrial level.

[0008] Currently, the management and recycling of wastewater is a crucial aspect when manufacturing membranes. Recycling must be taken into consideration when adopting solvent mitigation strategies, such as adsorption (utilising graphene, zeolites, metal organic frameworks and the like) or through the use of membrane filtration technology itself [3],

[0009] However, another strategy for improving the sustainability of polymeric membrane fabrication is by replacing traditional solvents with greener alternatives, thereby reducing the requirement for costly wastewater processing. Solvent replacement is an ongoing challenge attracting growing attention by the scientific community [4],

[0010] Green solvents are those which have lower impacts on the environment and those which are less toxic to both humans and wildlife. Ideally these green solvents are biodegradable and do not persist in the environment. Renewable solvents are those sourced from biomass, common examples include shortchain alcohols, ketones and esters (for example, ethanol, methylethylketone and ethyl acetate) as well as cyclic ethers and ketones such as 2-methyltetrahydrofuran (2-MeTHF) and dihydrolevoglucosenone (Cyrene™).

[0011] Dimethyl isosorbide (DMI), formula (I), is a green polar solvent. It is also a renewable solvent as it can be synthesized from d-glucose via catalytic processes [5], In addition to a high boiling point (235 °C), low level of volatility, good stability, and low viscosity, DMI has negligible bioaccumulation. It also has a very low acute toxicity due to its greater affinity for water than for organic compounds in the body. It has been used as a replacement for toxic solvents such as dimethyl formamide (DMF), N-methyl pyrrolidone (NMP), and tetrahydrofuran (THF) in membrane production [6], pharmaceutical additives [7] and cosmetics.

[0012] [Formula (I)]

[0013] According to the Hildebrand solubility parameter, it can be ascertained which polymers would be expected to dissolve in DMI. This is done by matching solubility parameters between DMI and the polymer of interest. DMI has a Hildebrand parameter (6) of 20.4 MPa1 / 2; this closely matches the 6 values of both PVDF (24.2 MPa1 / 2) and PES (23.2 MPa1 / 2). Therefore it would be expected that both polymers would be soluble or solubilised by DMI.

[0014] Hildebrand solubility parameters can also indicate which solvents can be replaced with DMI. For instance traditional solvents (such as NMP, DMF, DMA, chloroform), and green solvents (such as ethyl lactate, Cyrene™, Polarclean®) have very similar 6 values, so they would be expected to be interchangeable and compatible with DMI in forming homogenous polymer solutions.

[0015] DMI has been demonstrated to fully solubilise both polyvinylidene difluoride (PVDF) and polyethersulfone (PES), and has been used in the formation of flat sheet membranes [6],

[0016] In [6], the flat sheet membranes are formed by first dissolving PVDF or PES in DMI at a 10:90 polymer to solvent ration to provide a viscous solution (>400 cP for 60 kg / mol PES solutions) that is then cast on a glass plate to form a flat sheet. This cast is then subjected to a humid environment to effect vapour induced phase separation (VIPS) in order to increase porosity before the cast is placed into a water coagulation bath to effect nonsolvent induced phase separation (NIPS). The flat sheet membrane is then washed and dried.

[0017] Flat sheet membranes formed from this VIPS-NIPS method show high porosity (between 78-90%), high water permeability (between 2500-6300 L m-2hbar1for PES membranes), with average pore sizes of 0.035-0.43 pm for PES membranes.

[0018] These properties are well-suited for their intended use in ultrafiltration or microfiltration; however the pore size is too small and the high water permeability is undesirable for use with certain other processes. Some such processes include those utilising adsorptive membranes. These processes require the membrane to contain a material that can selectively adsorb species in a feedstock. These adsorbent materials are incorporated into the membrane as, for example, fine particulates. They extract a target component from the feedstock as the feedstock flows through or over the membrane; there is negligible flow through the membrane in such processes. The extraction is, in effect, chemical rather than physical as in ultrafiltration, microfiltration, nanofiltration and the like.

[0019] For example, adsorptive membranes are used in direct mineral extraction, such as direct lithium extraction (DLE). DLE is a process where lithium is selectively extracted from impure solutions containing small amounts of lithium among large amounts of multiple other ionic species, wherein the majority of other components are left in solution. There are a few DLE techniques known in the art, including electrodialysis, nanofiltration, adsorption and ion-exchange. The latter two approaches hold the most promise in terms of lithium selectivity, energy consumption and cost. DLE represents an alternative to conventional lithium brine extraction processes which utilise successive crystallisation stages (often with the use of evaporation ponds) which remove impurity compounds such as sodium chloride and leave the lithium in solution.

[0020] General DLE methods are well known in the art and will not be discussed in detail here.

[0021] In these adsorptive processes, pore sizes must be larger than those in membranes used for ultrafiltration and microfiltration in order to facilitate holding and effective utilization of the particulate adsorbent material.

[0022] Furthermore, flat sheet membranes are less preferable in comparison to other membrane morphologies for adsorptive processes. Other forms such as fibers, hollow fibers and beads are more difficult to form than flat sheets but have desirable properties such as being self supporting, having higher surface area per unit mass, and also allowing for higher packing density of membranes per unit volume. This allows for higher effective areas and therefore more potential for separation via absorption

[0023] The known method of forming membranes of PES and PVDF using DMI as solvent is therefore unsuitable for producing, for example, hollow fiber membranes with larger pores and lower water permeability suitable for adsorptive processes.

[0024] The present invention has been devised in the light of the above considerations.

[0025] Summary of the Invention

[0026] The present inventors sought to provide a method of producing hollow fiber membranes using DMI as a solvent. While flat polymer membranes have been formed using DMI as solvent in [6], the methodology cannot easily be transferred to a method of forming hollow fibers. The methods are incompatible due to the conditions of spinning being so different from those of flat casting - factors like the air drop, coagulation bath, spinneret release conditions and so on are entirely missing from flat membrane formation and must be accounted for. At its broadest, the present invention relates to a method of producing hollow fiber membranes using DMI as solvent.

[0027] An object of the invention is to provide a method of producing hollow fiber membranes using DMI as solvent, wherein the hollow fiber membranes comprise an adsorbent material.

[0028] The methodology known in [6] is further incompatible with forming hollow fiber membranes comprising an adsorbent material. This is because the adsorbent material itself can affect the structure of the membranes, for instance by increasing or decreasing the hydrophilicity of the polymer solution during spinning.

[0029] Accordingly, in a first aspect, the invention may provide a method of producing adsorptive hollow fiber membranes comprising the steps of:

[0030] (i) dissolving a polymer in a solvent comprising dimethyl isosorbide to provide a polymer solution;

[0031] (ii) suspending an adsorbent material in the polymer solution to provide a polymer suspension; and

[0032] (iii) spinning the polymer suspension to provide hollow fibers.

[0033] The inventors have found that this method can be effective in producing hollow fibers with the following advantages: (1) the hollow fibers formed from this process can have pore sizes of 0.4-1 pm, more suitable for adsorptive processes. This range of pore sizes can also improve the anti-fouling properties of the membranes; (2) the hollow fiber can have a thin active adsorbent layer on each of the bore side and shell side (that is, the internal face of the hollow fiber and the external face of the hollow fiber respectively); this improves the rate of adsorption by reducing the distance that the feedstock and its solutes must diffuse and permeate.

[0034] The method of the invention can be effective in producing membranes with a pore size of about 0.4-1 pm, and a porosity of >60%. A porosity of <85% can also be obtained.

[0035] The method of the invention uses DMI as solvent. DMI is commercially available. It can also be produced from D-glucose, first by hydrogenation to D-sorbitol, then by dehydration to isosorbide through the loss of two equivalents of water. Isosorbide can then be methylated with dimethyl carbonate and a base to provide DMI. This base can be any suitably strong non-nucleophilic base, for example potassium tert- butoxide, diazabicycloundecene, triazabicyclodecane and the like.

[0036] The method of the invention can also include the optional step of adding a pore forming agent to either the polymer solution or polymer suspension. A pore-forming agent is not required in order to form pores in the membrane, as pores naturally form when the fiber enters the coagulation bath during the spinning process; however, the addition of a pore-forming agent allows for more control over the membrane pore size. Suitably the pore forming agent is hydrophilic. Suitably, the pore forming agent is one selected from a hydrophilic polymer, a silica aerogel, graphene oxide and derivatives thereof. Furthermore, hydrophilic pore forming agents have surprisingly been found to improve the fouling characteristics of the membrane product. Improved fouling characteristics prolong the lifespan of the membranes. Fouling occurs when particulate matter blocks the porous structure of the membrane. This can be caused by colloidal particles, dust, microorganisms and precipitated solute components.

[0037] Increased hydrophilicity reduces the propensity for particulates to adhere to the surfaces of the membranes; in particular it lowers the propensity for microorganisms to adhere to the surfaces of the membranes.

[0038] In some embodiments the pore forming agent comprises a hydrophilic polymer, for example polyvinylpyrrolidone (PVP), pluronic, tetronic, polyethylene glycol (PEG), polyethylene glycol) or a derivative, poly(vinylpyrrolidone), cellulose nanocrystals, poly(vinyl alcohol), poly(acrylic acid), polydopamine, a zwitterion or amphiphilic zwitterion, an amphiphilic copolymer, a nature-derived biopolymer such as vanillin, chitosan, chitin, lignin, lignocellulose, caramel, capsaicin, or acacia gum, or an isocyanate organic acid such as p-aramid. These operate by increasing the affinity of the polymer-LIS suspension to water, which in turn increases the rate of demixing and precipitation of the polymer from solution. By modulating the hydrophilicity of the polymer-LIS suspension, the pore size can be changed.

[0039] In some embodiments, the pore forming agent comprises a silica gel such as silica aerogel. The addition of silica aerogel can cause a decrease the kinetic instability of the polymer system, increasing the rate of demixing and giving rise to higher porosity.

[0040] The adsorbent material is not particularly limited as it varies depending on the desired use of the finished membrane. Any suitable adsorbent material can be used such as zeolites, metal-organic frameworks, alumina, zirconia, mixed metal oxides and the like.

[0041] The adsorbent material is suitably an insoluble particulate material. The particles may suitably have an average (Dso) particle size, as measured by SEM, of < 50 pm, suitably < 25 pm, or < 10 pm. Most suitable is a particle size of < 5 pm, for example about 2-3 pm. In some embodiments a smaller particle size may be preferred in order to increase the available surface area for adsorption.

[0042] It is preferable that the adsorbent material is insoluble in water. This is advantageous as water is the most commonly used solvent for the feedstock solution to be processed using the membranes and an insoluble adsorbent material will limit the leeching of the adsorbent material from the membrane. This prolongs the lifespan of the membranes.

[0043] In one embodiment, the adsorbent material is a lithium-ion sieve (LIS). These materials selectively adsorb lithium over other ionic species such as sodium, potassium and other metallic species. These allow for the membranes produced from the method of the invention to be used in lithium extraction.

[0044] In order to conserve charge, clearly, in order to desorb (release) the lithium ions they must be replaced with a suitable cation; this is often a proton as suitable protic acidic solutions are readily available. Similarly, using basic solutions encourages the adsorption (take up) of lithium from solution, by removing protons from the lithium-ion sieve material. By varying pH, the selective ‘adsorption’ (take up) and ‘desorption’ (release) of lithium can be carefully controlled. A more basic feedstock solution provides the fastest lithium extraction rate; a more acidic release solution provides the fastest lithium release rate. However, rate must also be balanced against other factors such as acid / base safety and toxicity, additional cost and so on.

[0045] In one embodiment, the polymer used is polyethersulfone (PES). PES has desirable solubility in DMI and has excellent fiber forming properties, chemical and heat resistance and dimensional stability.

[0046] In one embodiment the polymer is PES and the adsorbent material is a lithium-ion sieve (LIS). This has the unexpected advantage that the leeching rate of the lithium-ion sieve from the resultant membrane reduced in comparison to the pure lithium-ion sieve. This is likely due to chemical coordination between the polymer and the LIS.

[0047] Suitably, the spinning step is performed using wet spinning. Wet spinning involves extruding the polymer suspension through a spinneret into a coagulation bath. This coagulation bath contains a non-solvent for the polymer. The coagulation bath thus causes the polymer (incorporating the adsorbent material) to precipitate from the polymer suspension producing a fiber as the non-solvent reduces the solubility of the polymer in the polymer suspension. This is advantageous as the rate of polymer precipitation can be controlled to alter the characteristics of the pore structure of the membrane.

[0048] In one embodiment, the method of spinning is dry-jet wet spinning. Dry-jet wet spinning is a modification of the wet spinning process. This involves extruding the polymer suspension into air before the fiber enters into the coagulation bath. This contrasts with wet spinning wherein the suspension is extruded directly into the coagulation bath. This is advantageous as extruding into air allows for more parameters to be controlled in order to influence the properties of the membranes. For example, the humidity of the air and air gap distance (that is, the distance between the spinneret and the coagulation bath) can be varied to control pore size.

[0049] General wet spinning and dry-jet wet spinning methods are well known in the art and will not be discussed in great detail here.

[0050] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0051] Detailed Description of the Invention

[0052] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0053] Components

[0054] Solvent “Solvent” is used to refer to the combination of one or more solvents in which the polymer is dissolved to form the polymer solution and hence the polymer suspension suitable for spinning.

[0055] The solvent used in the method of the invention comprises dimethyl isosorbide (DMI). Suitably, the solvent is mostly (that is, >50 vol%) DMI. It may suitably be >60 vol% DMI; >70 vol% DMI; >80 vol% DMI; >90 vol% DMI; >95 vol% DMI or >98 vol% DMI. In some embodiments the solvent is DMI; that is, the solvent comprises substantially only DMI.

[0056] DMI is commercially available and can often be used from commercial sources without further purification.

[0057] Polymer

[0058] “Polymer” is used to refer to the polymeric components of the fiber other than any adsorbent materials and pore-forming agents, if present. The polymer provides structure to the finished membrane.

[0059] The polymer used in the method of the invention preferably comprises polyethersulfone (PES). In some embodiments, the polymer is polyethersulfone; that is, the polymer only comprises PES.

[0060] PES is available in many molecular weights; the average molecular weight of the polyethersulfone used in the present invention is not particularly limited. In some embodiments, the average molecular weight of the PES is between 10 kDa and 500 kDa, preferably between 25 kDa and 100 kDa, more preferably around 50 kDa.

[0061] In some embodiments, the PES is one with a glass transition temperature Tg of 210-250°C, for example 225-235°C, suitably around 230°C. Glass transition temperature may be measured by, for example, differential scanning colorimetry according to ASTM E1356 (2008, R14 edition).

[0062] The PES may suitably comprise SO2 functional groups, for example as terminal groups.

[0063] If necessary, the polymer may be dried to remove any humidity (moisture content) before mixing to form a polymer solution (discussed in more detail below).

[0064] Pore forming agents

[0065] “Pore-forming agent” is used to refer to a material or species added to the solvent, polymer solution or polymer suspension that modulates the formation of pores at the surface and in the structure of the fiber as it soldifies.

[0066] Optionally a pore-forming agent is added to the polymer solution or the polymer suspension.

[0067] The pore forming agent can be one or more hydrophilic materials. Examples include hydrophilic polymers such as polyvinylpyrrolidone (PVP) or a hydrophilic solid such as silica aerogel.

[0068] In one embodiment, the pore forming agent comprises PVP. PVP is commercially available in many molecular weights, in the present invention PVP having a molecular weight of 10kDa to 40 kDa is preferred. More preferably, the molecular weight is 20 kDa to 30 kDa. In one embodiment, the pore forming agent comprises a polyethylene oxide or polypropylene oxide based polymer, including polyethylene glycol (PEG), polypropylene glycol (PPG) and copolymers of polyethylene glycol and polypropylene glycol. Such polymers preferably have a molecular weight of 10 kDa to 40 kDa. More preferably, the molecular weight s 20 kDa to 30 kDa. Preferred examples include Tetronic T904 and Pluronic P31 R1 .

[0069] In one embodiment, the pore forming agent comprises silica aerogel. Preferably, the silica aerogel has an average particle size (Dso) of 1-20 nm. More preferably, the average particle size is 5-15 nm.

[0070] Suitable choice of pore forming agent, or indeed other additives, can lead to control of final membrane properties. For example, addition of additives may cause changes in the thermodynamics and kinetics of the membrane formation steps. In general, non-solvent additives like PEG can reduce thermodynamic miscibility of the spinning dope and eventually faster precipitation of the spinning dope tends to form macrovoids with finger-like membrane structure (thermodynamic effect). On the other hand, slow phaseinversion results in sponge-like structure membranes. Additives may also increase spinning dope viscosity which results in a decrease of mutual diffusion between solvent in the spinning dope and nonsolvent in the coagulation bath (kinetic effect).

[0071] Adsorbent material

[0072] “Adsorbent material” is used to refer to a material that can selectively adsorb species from solution. This can occur through physical adsorption of species onto surfaces and into pores of the adsorbent material. It can also occur through chemical adsorption through the incorporation of species into a crystal structure or by ion-exchange.

[0073] The adsorbent material may suitably be provided in the form of particles. In some embodiments, the particle size of the selective material is < 50 pm, suitably < 25 pm, or < 10 pm. Most suitable is a particle size of < 5 pm, or < 3 pm. On the other hand, the particles may suitably have a size of > 200 nm, for example > 400 nm, > 1 pm, or > 2 pm.

[0074] In one embodiment, the adsorbent material is a lithium-ion sieve (LIS). It may alternatively be a potassium ion sieve or a calcium ion sieve.

[0075] Suitable lithium-ion sieves include, but are not limited to, zeolites, metal organic frameworks, layered double hydroxides, lithium metal oxides such as lithium manganese oxide and lithium titanium oxide, and the like.

[0076] In the present invention, the selective material is optionally a titanium-based compound. In particular, it is lithium titanate or a precursor compound which can be converted to lithium titanate in situ. An example of such a precursor is hydrogen titanium oxide, which is converted to lithium titanate through exposure to a suitably basic (pH>7) solution containing lithium. The hydrogen titanium oxide may suitably itself be derived from lithium titanium oxide (by exposure of it to a protic acidic solution, with initiates an ion exchange whereby protons enter the structure to replace lithium ions. The ‘gaps’ left by the lithium ions are sized such that hydrogen can enter them; then, when the hydrogen titanium oxide is expose to a solution containing lithium, lithium ions can re-enter the structure, the gaps being suitably sized to receive them.)

[0077] Lithium titanate (lithium titanium oxide, LTO) is available in many forms including lithium titanate spinel (LixTisOi2, wherein 4 < x < 7), lithium metatitanate (Li2TiOs), lithium orthotitanate (Li4TiC>4), and ramsdellite lithium titanate (Li2Ti3O?). In the present invention, Li2TiO3 is preferred.

[0078] Hydrogen titanium oxide (titanic acid, HTO) is available in many forms including H2TiO3.

[0079] Intermediate titanium-containing compounds can also be used containing both lithium and hydrogen. It will be appreciated that a given ‘molecule’ will transitions through several such compound states, as well as potentially ‘pure’ LTO and ‘pure’ HTO states, during a cycle (extraction + release) of the DLE process.

[0080] Polymer solution - step (i)

[0081] Typically, the first step of the present method involves dissolving the polymer in the solvent to afford a solution of the polymer in the solvent (the so-called polymer solution). Suitably this involves dissolving PES in DMI.

[0082] The skilled person would be aware of methods of dissolving the polymer in the solvent, which can be readily applied in this invention. Mixing methods can be employed to accelerate dissolution. Suitable methods include mechanical mixing and ultrasonic mixing. In a preferred embodiment, the mixing is continued until the polymer is entirely dissolved in the solvent.

[0083] In one embodiment, this dissolution of the polymer is performed at room temperature; that is, without heating. In some embodiments this is performed at elevated temperature. In a preferred embodiment, this is done at room temperature.

[0084] The polymer is preferably added to the solvent in a ratio of 10-16 wt%.

[0085] Preferably, the viscosity of this solution is sufficient for it to be durable in the spinning process.

[0086] In one embodiment, a pore forming agent is added to the solvent before dissolving the polymer in it. In one embodiment, a pore forming agent is added to the solvent after dissolving the polymer in it; that is, the pore forming agent is added to the polymer solution.

[0087] Wherein the pore forming agent is soluble in the solvent, the pore forming agent is dissolved in the solvent or polymer solution. Wherein the pore forming agent is insoluble in the solvent, the pore forming agent is suspended in the solvent or polymer solution.

[0088] Wherein pore forming agents are used, they may be included at a ratio of up to 50 wt% of the polymer in some embodiments. However, 0.1 to 5 wt% of the polymer is a more suitable content. In some embodiments, they are preferably added in a ratio of up to 1 wt% of the polymer where an aerogel poreforming agent is used, or up to 3 wt% of the polymer where a hydrophilic polymer pore-forming agent is used.

[0089] Polymer suspension - step (ii) Typically, the second step involves suspending the adsorbent material in the polymer solution of step (i).

[0090] The skilled person would be aware of methods of suspending the adsorbent material in the polymer solution, which can be readily applied in this invention. Suitable methods include mechanical mixing and ultrasonic mixing.

[0091] Suitably, the adsorbent material is present in the polymer suspension in an amount of up to 100 wt% of the polymer in the polymer suspension; for example 20 to 70 wt% or 40 to 60 wt%. A high ratio / content amount of the adsorbent material provides a greater level of ‘activity per unit volume’ in the produced membrane and hence has efficiency and cost benefits.

[0092] In some embodiments, the ratio of polymer to adsorbent material added to the solvent is between 1 :0.1 to 1 :2

[0093] In one embodiment, a pore forming agent is added to the polymer suspension.

[0094] Where pore forming agents are added to the polymer suspension, they may be included at a ratio of up to 50 wt% of the polymer in some embodiments. However, 0.1 to 5 wt% of the polymer is a more suitable content. In some embodiments, they are preferably added in a ratio of up to 1 wt% of the polymer where an aerogel pore-forming agent is used, or up to 3 wt% of the polymer where a hydrophilic polymer poreforming agent is used.

[0095] Spinning - step (iii)

[0096] Spinning is a term of the art which refers to a method of forming fibers, often polymer fibers. Spinning is a specialised form of extrusion that forms continuous fine filaments. The skilled person would be aware of various methods of spinning, including solution spinning.

[0097] Solution spinning is a method of forming fibers from a solution of a polymer (also known as the ‘dope’). This is in contrast to melt spinning, wherein the polymer is heated to a liquid state before spinning. Solution spinning allows the use of lower temperatures, and allows for the spinning of polymers which degrade at or before their melting points.

[0098] The fiber is then formed from the solution through a phase inversion in which the solvent is removed form the spun solution. This can occur in air (dry spinning methods) or in liquids (wet spinning methods)

[0099] Suitably, the method of spinning in step (iii) is wet spinning, including dry-jet wet spinning.

[0100] Coagulation baths

[0101] Common to all methods of wet spinning polymer solutions is the use of a coagulation bath; this allows for a solid fiber to be formed from the extruded solution.

[0102] The coagulation bath suitably contains a non-solvent to the solvent used to dissolve the polymer. The non-solvent should be miscible with the solvent, yet the polymer should have poor solubility in the nonsolvent in comparison to the solvent. In the present invention, the solvent is suitably DMI, therefore it is preferred that the non-solvent is miscible with DMI and it is preferred if the polymer is poorly soluble in the non-solvent.

[0103] In some embodiments, the non-solvent comprises water or is an aqueous solution. In one embodiment, the non-solvent is water.

[0104] In one embodiment, this coagulation bath is held at room temperature; that is, the coagulation bath is not heated. In some embodiments, the coagulation bath is held at an elevated temperature. In some embodiments, the coagulation bath is held at a reduced temperature. In a preferred embodiment, this coagulation bath is held at room temperature.

[0105] Spinneret

[0106] During spinning, the polymer solution is extruded through an orifice called a spinneret.

[0107] Suitably, the spinneret allows for an extrusion of a hollow fiber and thus has an inner diameter and an outer diameter.

[0108] Bore solution

[0109] As the method of the present invention suitably uses a hollow fiber spinneret, a bore solution is required to form the internal bore of the fiber. The bore solution is not particularly limited. Examples of bore solutions include water, aqueous alcohol solutions, aqueous salt solutions

[0110] In some embodiments, the bore solution is an aqueous ethanol solution. Preferably, the bore solution is an aqueous ethanol solution between 10-90% ethanol. More preferably, the aqueous ethanol solution is 50 wt% ethanol. In other embodiments the bore solution is deionized water; in still other embodiments the bore solution is a mixture of deionized water with either isopropyl alcohol or acetone.

[0111] Orifice size(s)

[0112] The orifice size of the spinneret is not particularly limited as the diameter of the finished fiber will depend on the intended use; these will be apparent to those skilled in the art. Generally, the diameter of the orifice is greater than the diameter of the fiber it produces.

[0113] In some embodiments, the orifice outer diameter (OD) is about 2 mm.

[0114] In some embodiments, the orifice internal diameter (ID) is about 0.1 mm.

[0115] Air gap distance

[0116] Wherein the method of spinning is dry-jet wet spinning, the air gap distance (that is, the distance between the spinneret and the coagulation bath) can be varied to modulate the properties of the hollow fiber. In some embodiments, the air gap distance is 0-10 cm.

[0117] Humidity

[0118] Wherein the method of spinning is dry-jet wet spinning, the humidity of the air can be varied to modulate the properties of the hollow fiber.

[0119] In some embodiments, the humidity is 30-50%.

[0120] Washing step

[0121] Optionally, after spinning the fibers are washed. This is done to remove residual solvent from the fibers. In one embodiment, this is done by washing the fibers with water. In some embodiments, this is done once, optionally twice, optionally three times, optionally more than three times.

[0122] Drying step

[0123] Optionally, after spinning the fibers, or after the fibers are washed, the fibers are then dried. This is to remove residual solvent or water from the fibers.

[0124] In one embodiment, the drying step is performed at room temperature; that is, without heating. In some embodiments this is performed at elevated temperature. In a preferred embodiment, this is done at room temperature.

[0125] Membranes

[0126] Membranes formed using the method of the present invention can have favourable properties for adsorptive processes.

[0127] Membranes formed using the method of the invention preferably have a porosity >60%. More preferably, the membranes have a porosity >70%. A porosity of about 80% may be particularly suitable.

[0128] The pores of the membranes may have a size (Dso) of < 2 pm, for example < 1 pm. A pore size of 10-40 nm may be particularly suitable.

[0129] Membrane porosity is measured by the known gravimetric method. In detail, the dried membranes were immersed in deionised water for 24 h at room temperature. Thereafter, samples were carefully taken out, wiped using tissue paper to remove excess water from the surface, and weighed. Afterwards, samples were placed in a vacuum oven at 60 °C for 8 h then weighed. The porosity, s, was calculated using the following equation:

[0130] Ww~ Wd

[0131] E =Ww- Wdwdx 100

[0132] Pw Pp where Wwis the weight of the wet membrane, Wd is the weight of the dry membrane, pwand pPare the density of water and of the polymer, respectively.

[0133] Pore size (that is, mean pore radius, rm) is measured using the Guerout-Elford-Ferry equation:

[0134] (2.9 - 1.75s) X 8t lQ rm~ J sAAP

[0135] Where s is porosity, Q is the volume of the permeated pure water per unit time (m3 / s), q is the water viscosity (8.9 x10-4Pa s at 25 °C) and AP is the applied pressure (0.4 MPa), A is the membrane area, I is the membrane thickness.

[0136] Suitable hollow fiber dimensions will be apparent to those skilled in the art. For example, a hollow fiber length of 0.2-2 m (suitably 0.5-1 .5 m), an outer diameter of 0.4-5 mm (suitably 0.6-2 mm), and a wall thickness of 10-200 pm (suitably 20-100 pm) may be obtained from the method of the invention.

[0137] The hollow fibers themselves may have, for example, a length of about 1 m, an outer diameter of about 1 mm, and an inner (bore) diameter of about 0.9 mm.

[0138] ***

[0139] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0140] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0141] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0142] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0143] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0144] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

[0145] References

[0146] A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

[0147] [1] Drioli, E.; Fontananova, E. Membrane Materials for Addressing Energy and Environmental Challenges. Annu. Rev. Chem. Biomol. Eng. 2012, 3 (1), 395-420.

[0148] [2] Macedonio, F.; Drioli, E. Membrane Engineering for Green Process Engineering. Engineering 2017, 3

[0149] [3], 290-298.

[0150] [3] Razali, M.; Kim, J. F.; Attfield, M.; Budd, P. M.; Drioli, E.; Lee, Y. M.; Szekely, G. Sustainable Wastewater Treatment and Recycling in Membrane Manufacturing. Green Chem. 2015, 17 (12), 5196- 5205.

[0151] [4] Figoli, A.; Marino, T.; Simone, S.; Di Nicolo, E.; Li, X.-M.; He,' T.; Tornaghi, S.; Drioli, E. Towards NonToxic Solvents for Membrane Preparation: A Review. Green Chem. 2014, 16 (9), 4034-4059.

[0152] [5] Green Synthesis of Dimethyl Isosorbide - Tundo - 2010 - ChemSusChem - Wiley Online Library

[0153] [6] Dimethyl Isosorbide As a Green Solvent for Sustainable Ultrafiltration and Microfiltration Membrane Preparation | ACS Sustainable Chemistry & Engineering

[0154] [7] Efficient pretreatment using dimethyl isosorbide as a biobased solvent for potential complete biomass valorization - Green Chemistry (RSC Publishing)

Claims

Claims:1 . A method of producing adsorptive hollow fiber membranes comprising the steps of:(i) dissolving a polymer in a solvent comprising dimethyl isosorbide to provide a polymer solution;(ii) suspending an adsorbent material in the polymer solution to provide a polymer suspension; and(iii) spinning the polymer suspension to provide hollow fibers.

2. The method according to claim 1 , wherein the polymer comprises polyethersulfone.

3. The method according to claim 1 or claim 2, wherein the solvent consists of dimethyl isosorbide.

4. The method according to any one of claims 1-3, wherein the adsorbent material is a lithium-ion sieve material.

5. The method according to claim 4, wherein the lithium-ion sieve material is lithium manganese oxide or lithium titanium oxide.

6. The method according to any one of the preceding claims, wherein a pore forming agent is added to the polymer solution or the polymer suspension before step (iii) is conducted.

7. The method according to any one of the preceding claims, wherein the content of polymer added to the solvent in step (i) is 10-16 wt%.

8. The method according to any one of the preceding claims, wherein the spinning conducted in step (iii) is wet spinning, preferably dry-jet wet spinning.

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