Separation membrane and method for producing same

Abstract

This separation membrane has a separation function layer composed of etherified cellulose obtained by substituting at least some hydrogens in hydroxy groups of cellulose with substituents including an aromatic ring, wherein the degree of substitution of the hydroxy groups in the etherified cellulose is 0.3-2.4.

Description

Separation membrane and method for manufacturing the same

[0001] This disclosure relates to a separation membrane and a method for producing the same.

[0002] Membrane separation, compared to distillation, has the advantage of significantly reducing the energy required for separation because it does not involve phase transition. Furthermore, since it is a process that does not require heating, it is suitable for purifying substances that are susceptible to thermal degradation. Therefore, membrane technology is applied in a wide range of industrial fields, including seawater desalination, drinking water treatment, sewage treatment, and blood purification.

[0003] In recent years, there has been a growing need for energy-saving technologies to recover valuable substances from wastewater, and membrane concentration methods for inorganic salts or organic solvents using separation membranes have attracted attention. One example of a membrane concentration method involves placing solutions containing ions or solutes of adjusted concentration on both the feed liquid side and the permeate side of a membrane, and then performing reverse osmosis filtration while maintaining a constant osmotic pressure difference between the membranes to highly concentrate the ions or solvents.

[0004] For example, when recovering valuable substances such as metal ions or organic solvents contained in wastewater using a separation membrane, reverse osmosis (RO) membranes or nanofiltration (NF) membranes capable of highly blocking ions or solvent molecules can be used.

[0005] For example, RO membranes made of cellulose acetate have excellent solvent resistance and can be operated under high pressure of 50 atmospheres or more in aqueous solutions containing aprotic polar solvents such as N-methyl-2-pyrrolidone (NMP) or dimethylformamide (DMF). They also exhibit excellent inhibition of NMP or DMF, making them suitable for membrane concentration treatment of wastewater containing these organic solvents.

[0006] Furthermore, RO membranes made of cellulose acetate have extremely excellent ion blocking properties, as demonstrated in water production processes such as seawater desalination, and are therefore suitably used for membrane concentration of brine (NaCl concentrated water) in salt production processes and for membrane concentration of lithium-containing salt wastewater in lithium-ion battery manufacturing processes.

[0007] However, wastewater containing lithium ions, for example, can be found to have a strongly alkaline pH of 13 or higher. While direct membrane concentration is preferable considering the cost of neutralization, cellulose acetate membranes may not withstand alkaline conditions. Specifically, it is known that the acetyl groups in the side chains of cellulose acetate are hydrolyzed and converted to hydroxyl groups under alkaline conditions. Therefore, improving the durability of the side chain substituents of separation membranes made of cellulose acetate under alkaline conditions may lead to the creation of superior RO membranes.

[0008] On the other hand, compared to cellulose acetate, cellulose is generally known to be insoluble in organic solvents and is a polymer with excellent resistance to alkaline hydrolysis. However, due to its high crystallinity and hydrophilicity, it is not easy to obtain high separation performance of ions and low molecular weight compounds as a reverse osmosis membrane, which is a challenge.

[0009] Therefore, in order to achieve both excellent ion blocking performance as an RO membrane and improved hydrolysis resistance to alkalis, modifying the hydroxyl groups of cellulose with something other than acetyl groups is considered a promising strategy.

[0010] Conventional approaches to this kind include, for example, Patent Document 1 (International Publication No. 2017 / 175600), which discloses a technology for obtaining a separation membrane with improved chlorine resistance and alkali resistance compared to cellulose triacetate membranes by benzoylating the hydroxyl groups of cellulose and introducing aromatic ester groups.

[0011] International Publication No. 2017 / 175600

[0012] The modified cellulose membrane substituted with aromatic esters disclosed in Patent Document 1 has ester bonds, and therefore may not have sufficient hydrolysis resistance under strongly alkaline conditions.

[0013] The purpose of this disclosure is to provide a separation membrane that has excellent resistance, particularly under alkaline conditions, and excellent separation performance, in view of the above circumstances.

[0014] (1) A separation membrane having a separation functional layer made of etherified cellulose in which at least some of the hydrogens of the hydroxyl groups of cellulose are substituted by substituents containing aromatic rings, wherein the degree of substitution of the hydroxyl groups in the etherified cellulose is 0.3 or more and 2.4 or less.

[0015] (2) The separation membrane according to (1), wherein the substituent containing the aromatic ring is a benzyl group which may have a substituent represented by chemical formula (1).

[0016]

[0017] (In the formula, R １ ~R ５ Each of these independently represents either hydrogen or a substituent other than hydrogen.

[0018] (3) The separation membrane described in (1) or (2), wherein, after an alkaline immersion test in a 0.1 M sodium hydroxide aqueous solution at 40°C for one week, a reverse osmosis test is performed using an aqueous solution at 25°C, 4.8 MPa, and a NaCl concentration of 1500 mg / L as the feed solution, and the NaCl rejection rate is 50% or more.

[0019] (4) A method for producing a separation membrane, comprising: a cellulose dissolution step of dissolving cellulose or a cellulose derivative in an aqueous solution of a tetraalkylammonium salt or tetraalkylphosphonium salt whose counterion is a hydroxide ion to obtain an aqueous cellulose solution; a cellulose modification step of contacting the aqueous cellulose solution with an electrophile containing optionally substituted benzyl groups to replace at least some of the hydrogens of the hydroxyl groups of the cellulose with the optionally substituted benzyl groups to obtain etherified cellulose; and a film formation step of obtaining a separation functional layer by solution film formation using a film-forming stock solution containing the etherified cellulose.

[0020] (5) The manufacturing method according to (4), wherein the electrophile is benzyl halide.

[0021] According to this disclosure, it is possible to provide a separation membrane that has excellent resistance, particularly under alkaline conditions, and excellent separation performance.

[0022] Figure 1 shows an example of an FT-IR spectrum showing deacetylation of cellulose acetate. Figure 2 shows etherification of the hydroxyl group of the cellulose side chain. １ This figure shows an example of an H-NMR spectrum.

[0023] The embodiments of this disclosure will be described below, but this disclosure is not limited to these embodiments.

[0024] <Separation Membrane> The separation membrane (semipermeable membrane) of this disclosure mainly comprises etherified cellulose, in which at least some of the hydrogen atoms of the hydroxyl groups of cellulose are substituted by substituents containing aromatic rings. That is, the separation membrane of this disclosure is a separation membrane constructed using said etherified cellulose as the main component.

[0025] The etherified cellulose constituting the separation membrane of this disclosure is a modified cellulose in which at least a portion of the hydroxyl groups of the side chains are protected via ether bonds by specific substituents (protecting groups) containing aromatic rings. Because it forms chemically stable ether bonds, it has excellent resistance, especially under alkaline conditions. For this reason, it remains stable for a long period of time even when used for membrane separation under strongly alkaline conditions. Furthermore, since the specific substituents (protecting groups) containing aromatic rings are hydrophobic and bulky, the introduction of these substituents into the side chains of cellulose moderately suppresses the high hydrophilicity and crystallinity of cellulose, resulting in excellent separation performance equivalent to or better than that of cellulose acetate. Examples of aromatic rings include benzene rings.

[0026] Therefore, the separation membrane composed of the modified cellulose of this disclosure has excellent resistance, particularly under alkaline conditions, and excellent separation performance. For this reason, the separation membrane of this disclosure can be suitably used, for example, in membrane concentration (membrane separation treatment) under high alkaline conditions, such as membrane concentration treatment of wastewater under strongly alkaline conditions or lithium ion recovery processes.

[0027] The separation membranes of this disclosure are so-called semipermeable membranes, and examples of separation membranes include reverse osmosis (RO) membranes, osmosis-assisted reverse osmosis (OARO) membranes, forward osmosis (FO) membranes, nanofiltration (NF) membranes, ultrafiltration (UF) membranes, and the like.

[0028] The shape of the separation membrane is not particularly limited. The semipermeable membrane may be, for example, a flat membrane such as a spiral membrane (spiral-type semipermeable membrane) or a hollow fiber membrane (hollow fiber-type semipermeable membrane), and is preferably a hollow fiber membrane. Compared with flat membranes, hollow fiber membranes are advantageous in that they have a smaller membrane thickness, can increase the membrane area per module, and can improve volumetric efficiency.

[0029] In the present disclosure, the separation membrane (semipermeable membrane) may be an asymmetric membrane or a composite membrane. In addition, in the present disclosure, the separation functional layer refers to a layer formed on the surface of the separation membrane that is responsible for the intended separation function, and refers to a thin film, a film, or a skin layer. In the present disclosure, the separation functional layer may also be referred to as a dense layer.

[0030] In the present disclosure, an asymmetric membrane has a cross-sectional structure including a porous support membrane and a separation functional layer (skin layer) whose pores are adjusted to be denser than that of the support membrane. In addition, the support membrane and the separation functional layer are composed mainly of the same material. Components other than the main component constituting the asymmetric membrane may include materials different from the above-mentioned specific material, fillers, additives, etc., and the concentration distribution thereof may differ within the separation active layer, within the support layer, and between the separation active layer and the support layer. An asymmetric membrane is typically formed by a phase separation method using a specific material as a raw material as the main component (generally 50% by mass or more). In the present disclosure, the above-mentioned specific material is the etherified cellulose (modified cellulose) of the present disclosure.

[0031] In the present disclosure, a composite membrane refers to a separation membrane obtained by forming a separation functional layer (thin film or film) on the surface and / or surface layer portion of a porous support membrane to form a composite. In a composite membrane, the support membrane and the separation functional layer are generally made of different materials, but there is no problem even if they are formed of the same material. As a method for forming the separation functional layer, known methods can be used, such as bonding a film onto a porous support membrane, coating the support membrane with a solution of the material by a coating drying method, and forming a thin film on the support membrane by an interfacial polymerization method. In the present disclosure, the main component (generally 50% by mass or more) of the separation functional layer is the etherified cellulose (modified cellulose) of the present disclosure.

[0032] Note that the main component is the most abundant component on a mass basis in the entire separation membrane in a dry state, and is preferably a component that accounts for 50% by mass or more of the whole. The content of the main component is more preferably 80% by mass or more, still more preferably 90% by mass or more, and the entire separation membrane may be composed only of the main component. By setting the content within the above range, the separation membrane can exhibit more excellent alkali resistance.

[0033] When the separation membrane of the present disclosure is an asymmetric flat membrane, it may have a dense layer on one surface, the dense layer substantially serves as a separation active layer that defines the pore diameter of the hollow fiber membrane, and the other surface may be a membrane having a lower density than the dense layer. Further, when the separation membrane of the present disclosure is a composite flat membrane, it may be a membrane having a two-layer structure including a dense layer that exhibits separation performance on one surface of a porous support membrane.

[0034] When the separation membrane of the present disclosure is an asymmetric hollow fiber membrane, it may have a dense layer (separation functional layer) on the outer peripheral surface, the dense layer substantially serves as a separation active layer that defines the pore diameter of the hollow fiber membrane, and the inner peripheral surface may be a membrane having a lower density than the dense layer. Alternatively, it may have a dense layer (separation functional layer) on the inner peripheral surface, the dense layer substantially serves as a separation active layer that defines the pore diameter of the hollow fiber membrane, and the outer peripheral surface may be a membrane having a lower density than the dense layer. On the other hand, when the separation membrane of the present disclosure is a composite hollow fiber membrane, it may be a membrane having a two-layer structure including a dense layer (separation functional layer) that exhibits separation performance on the outer peripheral surface of a porous support layer, or may be a membrane having a two-layer structure including a dense layer (separation functional layer) that exhibits separation performance on the inner peripheral surface of a porous support membrane.

[0035] From the viewpoint of obtaining separation performance such as desalination performance, the substituent containing an aromatic ring is preferably a substituent represented by chemical formula (1). Note that the substituent represented by chemical formula (1) is a benzyl group which may have a substituent.

[0036]

[0037] (wherein R １ to R ５ each independently represent hydrogen or a substituent other than hydrogen.)

[0038] Examples of substituents other than hydrogen include halogens such as fluorine, alkyl groups such as trifluoromethyl group, nitrile group, nitro group, carboxyl group, sulfonic acid group, phenolic hydroxy group, methyl group, and tert-butyl group, alkoxy groups such as methoxy group, alkylcarbonyl group (R ７ -COO-), alkylsulfonyl group (R ８ -SOO-) and the like. The alkyl moieties (R ７ - and R ８ -) in the alkylcarbonyl group and the alkylsulfonyl group may be alkyl groups having 1 to 6 carbon atoms, and the alkyl groups may have a substituent.

[0039] The separation membrane of the present disclosure preferably has a NaCl rejection of 50% or more when a reverse osmosis test is performed after a predetermined alkali immersion test using an aqueous salt solution having a pressure of 4.8 MPa and a NaCl concentration of 1500 mg / L as a feed solution. The NaCl rejection is more preferably 70% or more, and still more preferably 80% or more. In the alkali immersion test described above, 500 mL of 0.1 M aqueous sodium hydroxide solution is placed in a 2 L container, the separation membrane (thin film sample) is immersed in the aqueous sodium hydroxide solution, and the container is placed in an oven at 40° C. and held for one week. The size of the thin film sample is a 10 cm square. The predetermined alkali immersion test refers to, for a membrane surface area of 200 cm ２ , a test in which 500 mL of 0.1 M aqueous sodium hydroxide solution is placed in a 2 L container, the separation membrane (thin film sample) is immersed in the aqueous sodium hydroxide solution, and the container is placed in an oven at 40° C. and held for one week.

[0040] <Method for Producing Separation Membrane> An example of the method for producing the separation membrane of the present disclosure will be described below. An example of the method for producing the separation membrane of the present disclosure includes a cellulose dissolving step, a cellulose modifying step, and a membrane forming step. Modified cellulose (etherified cellulose) is prepared by the cellulose dissolving step and the cellulose modifying step.

[0041] (Cellulose dissolution step) In the cellulose dissolution step, cellulose or a cellulose derivative is dissolved in an aqueous solution of a tetraalkylphosphonium salt or tetraalkylammonium salt whose counterion is a hydroxide ion to obtain an aqueous cellulose solution.

[0042] The modified cellulose (etherified cellulose) constituting the separation membrane of this disclosure can be prepared, for example, from cellulose or a cellulose derivative as a raw material. The chemical structure of cellulose or a cellulose derivative is represented by the following chemical formula (2).

[0043]

[0044] (In the formula, n represents the repeating unit of the polymer. R represents either hydrogen or a non-hydrogen substituent, independently.)

[0045] In formula (2), an acetyl group can be considered a substituent other than hydrogen for R.

[0046] As the raw material cellulose, for example, regenerated cellulose may be used, or cellulose obtained by deacetylating cellulose acetate with alkali treatment may be used. Examples of cellulose derivatives that can be used as raw materials include cellulose acetates such as cellulose triacetate and cellulose diacetate.

[0047] The number-average molecular weight of cellulose and cellulose derivatives used as raw materials is preferably 1,000 to 200,000, from the viewpoint of imparting sufficient viscosity to the film-forming solution to improve coating properties and stringability, and ensuring the strength of the film.

[0048] Generally, cellulose has very low solubility in organic solvents, and the cellulose hydroxyl groups have low acidity and poor reactivity, so the etherification reaction is not always easy. Therefore, when modifying the hydroxyl groups of cellulose, it is preferable to modify the cellulose in a dissolved state after alkoxideization in a specific strong alkaline solvent. In this disclosure, it is preferable to dissolve the cellulose in a concentrated solution of 40% by mass or more of a tetraalkylammonium or tetraalkylphosphonium hydroxide salt, which is a phase transfer catalyst (PTC). In the above PTC solution, the hydroxyl groups of cellulose are ionized and dissolved, and the alkoxide groups act as highly reactive nucleophiles.

[0049] It is preferable that the PTC is water-soluble, has a certain degree of solubility in the organic phase, and can move between the two phases. Based on this requirement, it is preferable that the PTC is a quaternary ammonium salt or quaternary phosphonium salt having an alkyl group with 2 to less than 10 carbon atoms, and, as described above, the counterion is preferably a hydroxide ion in order to alkoxideize the hydroxyl groups of cellulose under strongly basic conditions.

[0050] Therefore, in this disclosure, an aqueous solution of a tetraalkylphosphonium salt or tetraalkylammonium salt whose counterion is a hydroxide ion is used as the PTC. An example of a tetraalkylphosphonium salt whose counterion is a hydroxide ion is tetrabutylphosphonium hydroxide (TBPOH). An example of a tetraalkylammonium salt whose counterion is a hydroxide ion is tetraalkylammonium hydroxide. Commercially available PTCs can be used. Alternatively, for example, tetrabutylphosphonium bromide (or iodide) or tetrabutylammonium bromide (or iodide) may be converted to the hydroxide form by ion exchange and used.

[0051] The concentration of PTC in the PTC aqueous solution is preferably 40% by mass or higher. More preferably 45% by mass or higher. Cellulose dissolves well within this concentration range.

[0052] (Cellulose modification process) In the cellulose modification process, an aqueous solution of cellulose is brought into contact with an electrophile containing an aromatic ring, thereby substituting at least some of the hydrogen atoms of the hydroxyl groups of cellulose with benzyl groups to obtain etherified cellulose.

[0053] In the cellulose modification step following the cellulose dissolution step described above, it is preferable to carry out a phase transfer reaction between the two phases by stirring and mixing an aqueous cellulose solution (an aqueous phase containing cellulose and PTC) with an organic phase containing an electrophile that is immiscible with the aqueous phase. By carrying out a phase transfer reaction between the two phases, the deactivation of the electrophile in the organic phase by the nucleophilicity of hydroxide ions in the aqueous phase can be suppressed, thereby enabling etherification with a high degree of substitution.

[0054] As an electrophile for modifying the hydroxyl groups of cellulose, it is preferable to select a hydrophobic and bulky structure from the viewpoint of readily reacting with alkoxides and modifying the polymer structure to one that is suitable as a material for RO membranes, etc., by suppressing the hydrophilicity and crystallinity of cellulose. A polymer structure suitable for RO membranes, etc., refers to a polymer chemical structure that has an appropriate free volume that allows only water molecules to pass through, while blocking hydrated ions and low molecules, in order to block solutes of angstrom size such as ions. From the viewpoint of increasing the free volume, it is well known that selecting a bulky main chain or side chain is effective.

[0055] As electrophiles, for example, benzyl halides (benzyl halogenated compounds) can be used. The halogen group of these electrophiles can be selected from chlorine, bromine, and iodine, with bromine being preferred due to its simplicity and high reactivity. However, from the viewpoint of increasing reactivity, iodine may be selected, or methods such as adding potassium iodide or the like to an electrophile having a chlorine or bromine halogen group to perform halogen exchange and promote the substitution reaction may be employed. Furthermore, from the viewpoint of avoiding elimination reactions, it is preferable that the number of carbon atoms bonded to the α-carbon of the halogen group is 1, i.e., that it is a primary halide.

[0056] When the substituent of the etherified cellulose that forms the constituent material of the separation membrane is a substituent represented by chemical formula (1), that is, a benzyl group which may have substituents, a benzyl halide compound can be suitably used as the electrophile.

[0057] As the benzyl halogenated compound (benzyl halide), benzyl bromide is preferred due to its simplicity. Derivatives of benzyl bromide are also preferred. As derivatives of benzyl bromide, the R of chemical formula (1) of the benzyl group is used to control the chemical stability as a protecting group for the cellulose hydroxyl group. １ ~R ５ At least one of the positions is, for example, an electron-withdrawing group such as a halogen group, trifluoromethyl group, nitrile group, nitro group, carboxyl group, sulfonic acid group, alkylcarbonyl group (R ７ -COO-), alkylsulfonyl group (R ８ You may choose a group that is substituted with -SOO-, etc., or a group that is substituted with an electron-donating group such as a phenolic hydroxyl group, an alkyl group such as a methyl group or tert-butyl group, or an alkoxy group such as a methoxy group. Alkyl carbonyl group and the alkyl portion (R) in alkyl sulfonyl group ７ - and R ８ -) may be an alkyl group having 1 to 6 carbon atoms, and the alkyl group may have substituents.

[0058] It is preferable to use liquid benzyl halide. Using the undiluted benzyl halide directly in the above-mentioned phase transfer reaction is preferable because it increases the reactivity. However, solid benzyl halide can also be used in the above-mentioned phase transfer reaction after being dissolved in a nonpolar solvent such as hexane, cyclohexane, or toluene.

[0059] For electrophiles that are liquid in the range of room temperature to 100°C, it is preferable to add the undiluted electrophile directly to the cellulose solution from the viewpoint of increasing reactivity. When the electrophile is in solid form and an organic layer obtained by dissolving the electrophile in a nonpolar solvent is added to the cellulose solution, it is preferable that the concentration of the electrophile in the organic phase be 5% by mass or more and 25% by mass or less.

[0060] In the phase transfer reaction, the volume ratio of the PTC aqueous solution to the organic phase containing the electrophile can be appropriately selected, but for example, a 1:1 ratio is preferable as it promotes the reaction.

[0061] The reaction temperature and reaction time can be selected as appropriate, but for example, it is preferable to carry out the reaction at room temperature for one hour or more under strong stirring conditions.

[0062] The modified cellulose obtained by the above reaction can be recovered as a polymer by reprecipitation in a solvent such as ethanol or water, followed by neutralization and washing, and then drying.

[0063] The degree of substitution of hydroxyl groups in the etherified cellulose described above (degree of etherification substitution) is preferably 0.3 to 2.4, more preferably 0.4 to 2.0, and even more preferably 0.5 to 1.5, with respect to the three hydroxyl groups of the glucose ring constituting the repeating unit of cellulose. Since the side chain of cellulose has three hydroxyl groups, the maximum degree of substitution in etherified cellulose is 3. If the separation membrane of this disclosure contains not only etherified cellulose but also unsubstituted cellulose and / or cellulose derivatives other than etherified cellulose, the average degree of substitution of the three hydroxyl groups of the glucose ring constituting the repeating unit of cellulose, including these derivatives (average degree of etherification substitution), is considered to be the degree of substitution of etherified cellulose.

[0064] Based on the inventors' research, it is believed that when the degree of substitution of etherified cellulose is within the above range, a separation membrane with excellent alkali resistance and salt removal performance can be more reliably obtained. When the degree of substitution is 0.3 or higher, the crystallinity and hydrophilicity of the etherified cellulose can be appropriately suppressed, resulting in excellent separation performance as an RO membrane, which is preferable. When the degree of substitution is 2.4 or lower, the amount of reaction reagent used can be reduced, and processing can be done in a short reaction time, which is preferable from an economic standpoint. Furthermore, because hydrophobicity can be appropriately suppressed, excellent water permeability of the RO membrane can be achieved. The degree of etherification substitution can be adjusted by the amount of electrophile added to the cellulose aqueous solution.

[0065] (Film Forming Process) In the film forming process, a separation membrane can be obtained by solution film formation using the etherified cellulose (modified cellulose) obtained above.

[0066] For example, the modified cellulose obtained can be dissolved in a suitable organic solvent, and a separation membrane can be fabricated by the solution casting method or the non-solvent-induced phase separation method.

[0067] The concentration of modified cellulose (polymer concentration) in the film-forming stock solution, obtained by dissolving modified cellulose in an organic solvent, is not particularly limited. Furthermore, the appropriate polymer concentration may vary depending on the structure and degree of substitution of the substituents on the hydroxyl groups of cellulose, and on the film-forming method. When asymmetric membranes are produced by the non-solvent-induced phase separation method, the concentration is typically 30% by mass or more, and more preferably 35% by mass or more. When forming a separation active layer made of modified cellulose by the solvent casting method, a film-forming stock solution with a polymer concentration of approximately 5% to 15% by mass can typically be used.

[0068] From the viewpoint of solubility of modified cellulose, aprotic polar solvents such as NMP, DMF, dimethylacetamide (DMAc), sulfolane, and dioxolane can be preferably used as the aforementioned organic solvent.

[0069] The film-forming stock solution may contain glycol-based solvents such as ethylene glycol, diethylene glycol, triethylene glycol, glycerin, polyethylene glycol, or polypropylene glycol as non-solvents (sometimes referred to as poor solvents) from the viewpoint of controlling phase separation, or it may contain hydrophilic polymers such as polyvinylpyrrolidone.

[0070] Furthermore, the film-forming stock solution may contain surfactants such as ionic surfactants and nonionic surfactants, as well as inorganic salts such as lithium salts, sodium salts, and potassium salts. In addition, hydrophilicity and pore size can be controlled by other known additives.

[0071] The method for forming the above-mentioned film-forming stock solution into a separation membrane is not limited, and various known methods can be used.

[0072] For example, when preparing a flat film using the solution casting method, a polymer film-forming solution containing a pore size control agent is applied to a heated substrate such as glass, dried to form a thin film, and then bonded to the surface of a porous substrate to obtain a composite film. Alternatively, a composite film can be obtained by applying a dilute polymer solution to the surface of a porous substrate and drying it to form a thin film. Furthermore, a separation film can be obtained by applying a film-forming solution to the surface of a nonwoven fabric with a doctor blade or the like, and then immersing it in a coagulation bath to perform non-solvent-induced phase separation.

[0073] In the case of hollow fiber membranes, for example, the film-forming raw material is extruded from the outer slit of a double cylindrical nozzle, and at the same time, the internal liquid with adjusted coagulation ability is discharged from the central hole and immersed in a coagulation bath to form a hollow fiber shape. It is preferable that the film is thoroughly washed after coagulation. From the viewpoint of controlling the pore size, annealing treatment such as immersion in a hot water bath may be performed on the film.

[0074] The present disclosure will be described in more detail below with reference to examples, but will not be limited thereto.

[0075] (Example 1) (1) Preparation of Cellulose 30 g of cellulose triacetate (CTA, LT-35, manufactured by Daicel Corporation, acetyl substitution degree 2.9, number average molecular weight 75,900) was prepared and dispersed in 600 mL of 1 M NaOH / ethanol solution. Deacetylation treatment was performed by stirring at 25°C for 24 hours. The polymer (cellulose) after deacetylation treatment was filtered, thoroughly washed with ethanol, and dried. Next, the degree of acetylation of the obtained polymer was evaluated using fast Fourier transform infrared spectroscopy (FT-IR) by the following method. Specifically, 1025 cm －１ In the spectrum normalized by the peak corresponding to the C-O-C stretching vibration of the cellulose skeleton, the peak corresponding to the C=O stretching vibration of the acetyl group is 1740 cm⁻¹. －１ The degree of acetylation was evaluated from the intensity ratio of the peaks. From the obtained FT-IR spectrum, it was confirmed that the acetyl groups of CTA were completely converted to hydroxyl groups (Figure 1). In other words, it was confirmed that cellulose had been obtained.

[0076] (Evaluation of acetylation degree by FT-IR) FT-IR measurements were performed using the following method. Sample preparation method: 5 mg of polymer powder was mixed with 1500 mg of KBr powder, formed into tablets using a hydraulic press, and the IR spectrum was measured. For membrane samples, the film was cut into 1 cm squares and the IR spectrum was measured. Measurement device: Carry 670 (manufactured by Agilent Technologies, Inc.) Measurement conditions: Transmission method, wavenumber resolution 4 cm －１ , cumulative count 64 times

[0077] (2) Etherification of Cellulose (Cellulose Dissolution Process) As an aqueous solution of PTC (phase transfer catalyst), an aqueous solution containing 40% by mass of tetrabutylphosphonium hydroxide (TBPOH) (manufactured by Tokyo Chemical Industry Co., Ltd.) was prepared and concentrated to 47% by mass using an evaporator. 7.0 g of the cellulose prepared in (1) above was added to 216 mL of this aqueous solution and stirred at 25°C for 12 hours to dissolve it. (Cellulose Modification Process) After confirming that the cellulose was dissolved, 15 mL of benzyl bromide (BnBr) was added to the solution and stirred to form an O / W emulsion, and the phase transfer reaction was carried out at 25°C for 3 hours. After the reaction was completed, the solution was added dropwise to ethanol to precipitate the polymer. The polymer was filtered off, thoroughly washed with ethanol, and then dried. In this way, etherified cellulose (benzyl cellulose) was obtained in which some of the hydroxyl groups of the side chains of cellulose were etherified.

[0078] Furthermore, the obtained benzylated cellulose was subjected to proton nuclear magnetic resonance ( １ The degree of etherification substitution was evaluated by performing the following measurements using 1H-NMR. Specifically, the obtained １ Regarding the 1H-NMR spectrum, the area of ​​the peak corresponding to the phenyl group CH at 7.0–7.5 ppm and the area of ​​the cellulose skeleton CH and benzyl group CH at 3.0–5.5 ppm ２ The degree of substitution was determined from the area of ​​the corresponding peaks (Figure 2). More specifically, the degree of substitution was determined from the following formula: (Degree of substitution) = 7A / (5B - 2A) (where A is the area of ​​the peak corresponding to the phenyl group CH at 7.0 to 7.5 ppm, and B is the area of ​​the cellulose skeleton (glucose ring) CH and benzyl group CH at 3.0 to 5.5 ppm) ２ (This is the area of ​​the peak corresponding to ) As a result, the degree of substitution of the obtained benzylated cellulose was 0.5. １(Measurement by H-NMR) Sample preparation method: 30 mg of polymer was dissolved in 1 mL of deuterated dimethyl sulfoxide (manufactured by Tokyo Chemical Industry Co., Ltd.). Measurement device: AVANCE NEO 500 (manufactured by Bruker Japan Co., Ltd.) Measurement conditions: Number of accumulations: 64, waiting time: 1 s, pulse angle: 30°, pulse interval: 4.28 s, measurement temperature: 30°C

[0079] (3) Preparation of separation membrane A separation membrane was prepared using the benzylated cellulose obtained in (2) above by the solution casting method. Specifically, the following procedure was carried out. The benzylated cellulose obtained in (2) above was dissolved in NMP to prepare a film-forming stock solution containing 10% by mass of benzylated cellulose. The film-forming stock solution was applied to a glass plate with an applicator, the glass plate was quickly heated to 150°C, and the NMP was evaporated for 3 hours to obtain a thin-film cast film, which was then thoroughly washed with pure water to obtain the separation membrane of Example 1.

[0080] (4) Preparation of a laminate for separation performance evaluation The obtained separation membrane was cut into a circular shape with a diameter of 75 mm to obtain a separation membrane sample (thin film sample). Then, the separation membrane sample was placed on the front side of an ultrafiltration porous membrane of the same size (Toyo Roshi Co., Ltd., ADVANTEC Ultrafilter Q100 150E, made of polysulfone) to obtain a laminate for separation performance evaluation.

[0081] (Example 2) Using benzylated cellulose with a degree of substitution of 0.5 obtained by the same method as in (1) and (2) of Example 1, a separation membrane was obtained by a non-solvent-induced phase separation method as an example of another film-forming method for "(3) Preparation of separation membrane". Specifically, a benzylated cellulose polymer was dissolved in NMP, and a predetermined amount of ethylene glycol was added as a poor solvent to prepare a film-forming stock solution. The film-forming stock solution was applied with an applicator to a polyphenylene sulfide (PPS) nonwoven fabric attached to a glass plate heated on a hot plate, and the fabric was quickly immersed in a coagulation bath, which was an NMP aqueous solution at 25°C, for 5 minutes to allow phase separation to proceed. In this way, the separation membrane of Example 2 was obtained, and a laminate for evaluating the separation performance of Example 2 was obtained by the same method as in Example 1.

[0082] (Example 3) In "(2) Etherification of Cellulose," the amount of BnBr added was changed to 19 mL. Otherwise, etherified cellulose with a degree of substitution of 0.8 was obtained using the same method as in Example 1, and a separation membrane and a laminate for evaluating separation performance were prepared.

[0083] (Example 4) In "(2) Etherification of Cellulose," the amount of BnBr added was changed to 22 mL. Otherwise, the same method as in Example 1 was used to obtain etherified cellulose with a degree of substitution of 1.3, and a separation membrane and a laminate for evaluating separation performance were prepared.

[0084] (Example 5) In "(2) Etherification of Cellulose," the amount of BnBr added was changed to 33 mL. Otherwise, the same method as in Example 1 was used to obtain etherified cellulose with a degree of substitution of 2.0, and a separation membrane and a laminate for evaluating separation performance were prepared.

[0085] (Example 6) In "(2) Etherification of Cellulose," 4-fluorobenzyl bromide (F-BnBr) was used instead of BnBr. Otherwise, etherified cellulose with a degree of substitution of 1.2 was obtained by the same method as in Example 1, and a separation membrane and a laminate for evaluating separation performance were prepared.

[0086] Furthermore, the obtained benzylated cellulose was subjected to nuclear magnetic resonance ( １３ The degree of etherification substitution was determined by evaluation using ¹³C NMR. Specifically, the following １３ Obtained by 13C-NMR １３ The degree of substitution was determined from the C-NMR spectrum by comparing the area of ​​the peak corresponding to the cellulose skeleton C at 100–105 ppm and the area of ​​the peak corresponding to the phenyl group C at 114–116 ppm. More specifically, the degree of substitution was calculated using the following formula: (Degree of substitution) = B / 2A (where A is the area of ​​the peak corresponding to the cellulose skeleton C at 100–105 ppm, and B is the area of ​​the peak corresponding to the phenyl group C at 114–116 ppm). As a result, the degree of substitution of the obtained benzylated cellulose was 1.2. １３(Measurement by 13C-NMR) Sample preparation: 30 mg of the sample was dissolved in 0.6 ml of deuterated dimethyl sulfoxide and centrifuged. The supernatant was then collected and decoupled using a reverse-gate decoupling method. １３ C-NMR measurements were performed. Measurement equipment: Fourier transform nuclear magnetic resonance spectrometer (BRUKER, AVANCE NEO600) Resonance frequency: 150 MHz Measurement conditions: 3500 integration cycles, 10 s waiting time, 30° pulse angle, 12 s pulse interval, 30°C measurement temperature

[0087] (Comparative Example 1) For comparison with the etherified cellulose membrane of the Example, a separation membrane was prepared using cellulose triacetate. Specifically, cellulose triacetate (CTA, LT-35 manufactured by Daicel Corporation, acetyl substitution degree 2.9, number average molecular weight 75,900) was dissolved in NMP to prepare a 10% by mass film-forming stock solution. The film-forming stock solution was applied to a glass plate with an applicator, and the glass plate was quickly heated to 150°C. The NMP was evaporated for 3 hours to obtain the separation membrane of Comparative Example 1 as a thin-film cast film. Using the obtained separation membrane, a laminate for evaluating separation performance was obtained by the same method as in Example 1.

[0088] (Comparative Example 2) A CTA film (separation membrane) obtained by the same method as in Comparative Example 1 was immersed in a 1.0 M NaOH aqueous solution at 40°C for one week to completely remove the acetyl groups and obtain a cellulose film (separation membrane) with a degree of substitution of 0. Using this cellulose film, a laminate for evaluating separation performance was obtained by the same method as in Example 1.

[0089] (Comparative Example 3) In "(2) Etherification of Cellulose," the amount of BnBr added was changed to 6 mL. Otherwise, the same method as in Example 1 was used to obtain etherified cellulose with a degree of substitution of 0.2, and a separation membrane and a laminate for evaluating separation performance were obtained.

[0090] (Comparative Example 4) In "(2) Etherification of Cellulose," the amount of BnBr added was changed to 50 mL. Otherwise, the same method as in Example 1 was used to obtain etherified cellulose with a degree of substitution of 2.5, and a separation membrane and a laminate for evaluating separation performance were obtained.

[0091] <Alkali Resistance Evaluation> To evaluate the alkali resistance of the separation membranes prepared in the examples and comparative examples, the following alkali immersion test (accelerated alkali degradation test) was performed on the separation membranes (thin film samples). Using the separation membranes prepared before and after the test, the following measurements of water permeability (pure water transmission rate) and desalination performance (NaCl rejection rate) were performed on the laminates prepared for evaluation of separation performance. The measurement results of pure water transmission rate and NaCl rejection rate are shown in Table 1. However, the separation membrane obtained in Comparative Example 3 was made of etherified cellulose with an extremely low degree of substitution, and undissolved material was found in the thin film due to poor solubility, making it impossible to measure the desalination performance (NaCl rejection rate). Also, the separation membrane obtained in Comparative Example 4 was made of etherified cellulose with excessive etherification substitution, and its water permeability was extremely low, making it impossible to measure the pure water transmission rate and desalination performance (NaCl rejection rate).

[0092] (Alkali Immersion Test) 500 mL of 0.1 M sodium hydroxide solution was placed in a 2 L container, and the separation membrane (thin film sample) was immersed in the sodium hydroxide solution. The container was then placed in a 40°C oven and kept there for one week. The size of the thin film sample was 10 cm square.

[0093] <Measurement of Pure Water Permeability and NaCl Rejection Rate> The permeability (pure water permeability) and desalination performance (NaCl rejection rate) were measured using the following method. A spin flow cell (manufactured by Iwai Pharmatec) was used as the housing for membrane evaluation. The evaluation apparatus was configured by connecting this housing to a liquid transfer pump (NP-FX-100 manufactured by Nippon Precision Science Co., Ltd.).

[0094] (NaCl rejection rate and permeate volume) The laminate for separation performance evaluation was installed in the evaluation apparatus described above, and the apparatus was operated for a predetermined time at 25°C and a pressure of 4.8 MPa using an aqueous NaCl solution prepared to a concentration of 1500 mg / L as the feedwater. After that, permeate was collected from the laminate for separation performance evaluation, and the mass of the permeate was measured using an electronic balance. The mass of the permeate was converted to the permeate volume per unit time using the following formula. The density was 1.0 g / cm³. ３The following was done: Permeate volume (mL / min) = Permeate mass (g) / Elapsed time (min) The NaCl rejection rate was evaluated as follows: The conductivity of the membrane permeate collected in the above permeate volume measurement and the feed water was measured using an electrical conductivity meter ("CM-25R" Toa DKK Co., Ltd.). The NaCl rejection rate was calculated using the following formula: NaCl rejection rate [%] = (1 - Membrane permeate conductivity [μS / cm] / Feed water conductivity [μS / m]) × 100 However, if the permeate volume was extremely low and it was difficult to measure the conductivity with an electrical conductivity meter, the ion concentration of the feed water and permeate was measured by ion chromatography. The NaCl rejection rate was calculated using the following formula: NaCl rejection rate [%] = (1 - Permeate concentration [mg / L] / Feed water concentration [mg / L]) × 100

[0095] (Pure water permeability) The pure water permeability A (Permeance) was calculated using the evaluation data of the NaCl rejection rate and permeate volume described above. A [L / (m 2 [h bar] = Permeate volume of pure water [L] / Membrane area [m²] 2 ] / Sampling time [h] / Measured pressure [bar]

[0096]

[0097] The results shown in Table 1 indicate that the separation membranes of Examples 1 to 6, which were made from etherified cellulose in which at least some of the hydrogen atoms of the hydroxyl groups of cellulose are substituted with substituents containing aromatic rings, showed little change in salt removal performance (NaCl rejection rate) before and after the alkaline immersion test, demonstrating excellent resistance to alkaline conditions and excellent separation performance. Regarding the separation membranes of Comparative Examples 3 and 4, which had similar compositions to the examples, the separation membrane of Comparative Example 3 had insufficient etherification substitution, resulting in extremely poor solubility and the presence of undissolved material in the solution, making it impossible to measure film formation and desalting performance (NaCl rejection rate). Furthermore, the separation membrane of Comparative Example 4 was composed of etherified cellulose with excessive etherification substitution, resulting in extremely low water permeability, making it impossible to measure desalting performance (NaCl rejection rate). Therefore, it is preferable that the degree of substitution of etherified cellulose be between 0.3 and 2.4.

[0098] <Measurement of Molecular Weight> (1) Measurement of Molecular Weight of Cellulose Triacetate Separation Membrane The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of the cellulose triacetate separation membrane used as the raw material for Comparative Example 1 (before the above-mentioned alkaline immersion test) and the cellulose triacetate separation membrane of Comparative Example 1 after the above-mentioned alkaline immersion test were measured using gel permeation chromatography (GPC). Specifically, 8 mg of the sample was weighed, and 8 mL of an eluent consisting of N,N-dimethylacetamide (DMAc) containing 30 mM lithium bromide (LiBr) was added to the sample to prepare the sample solution. The sample solution was filtered through a 0.2 μm membrane filter. GPC measurement was performed on the obtained sample solution under the following conditions. The molecular weight was calculated on a standard polyethylene glycol basis. (GPC analysis conditions) Measurement device: HLC-8220GPC (manufactured by Tosoh Corporation) Column: TSKgel SuperAWM-H + TSKgel SuperAW2500 (manufactured by Tosoh Corporation) Detector: Differential refractometer (RI) Eluent: DMAc / 30mM LiBr Membrane filter: PTFE, 0.2μm (manufactured by GL Sciences Co., Ltd.) Measurement conditions: Flow rate 0.35 mL / min, injection volume 10 μL, temperature 40℃

[0099] (2) Measurement of the molecular weight of the cellulose separation membrane The cellulose separation membrane obtained by deacetylating cellulose triacetate in Comparative Example 2 (before the above-mentioned alkali immersion test), and the cellulose separation membrane of Comparative Example 2 after the above-mentioned alkali immersion test, were subjected to GPC analysis by Tosoh Analysis Center Co., Ltd. under the following conditions, and the number-average molecular weight Mn and weight-average molecular weight Mw of cellulose were measured. The molecular weight was calculated in terms of standard pullulan. (GPC analysis conditions) Measuring device: HLC-8420GPC (manufactured by Tosoh Corporation) Column: TSKgel guardcolumn SuperH-H (4.6 mm.D. × 3.5 cm) + TSKgel SuperHM-H (6.0 mm.D. × 15 cm) × 2 (manufactured by Tosoh Corporation) Detector: Differential refractometer (RI) polarity = (+) Eluent: DMAc-based solution Filter: PTFE filter

[0100] (3) Measurement of Molecular Weight of Etherified Cellulose Separation Membrane The etherified cellulose obtained in Example 4 (before the above-mentioned alkali immersion test) and the etherified cellulose separation membrane of Example 4 after the above-mentioned alkali immersion test were subjected to GPC analysis in the same manner as in (1) above under the following conditions, and the number-average molecular weight (Mn) and weight-average molecular weight (Mw) were measured. Molecular weight was calculated on a standard polystyrene basis. (GPC Analysis Conditions) Measuring device: HLC-8420GPC (manufactured by Tosoh Corporation) Column: TSKgel SuperAWM-H x 2 (manufactured by Tosoh Corporation) Detector: Differential refractometer (RI) Eluent: 30 mM LiBr / DMAc Filter: PTFE 0.2 μm (manufactured by GL Sciences Co., Ltd.) Measurement conditions: Flow rate 0.3 mL / min, injection volume 10 μL, temperature 40°C

[0101] Furthermore, the degree of substitution after the alkali immersion test was measured in the same manner as above for the cellulose triacetate separation membrane of Comparative Example 1, the cellulose separation membrane of Comparative Example 2, and the etherified cellulose separation membrane of Example 4.

[0102] Table 2 summarizes the molecular weight and degree of substitution measurements before and after the alkali immersion test for the cellulose triacetate separation membrane of Comparative Example 1, the cellulose separation membrane of Comparative Example 2, and the etherified cellulose separation membrane of Example 4.

[0103]

[0104] Table 2 shows that the cellulose triacetate membrane (Comparative Example 1) lacks alkali resistance because the acetyl groups in its side chains are completely removed after the alkali immersion test, meaning it is converted into a cellulose membrane. Furthermore, the membrane after alkali immersion became insoluble in the solvent system used to measure the molecular weight of the cellulose triacetate separation membrane, making it impossible to perform GPC analysis under the same conditions. On the other hand, the cellulose membrane (Comparative Example 2) showed no change in molecular weight before and after the alkali immersion test, indicating that the structure of the cellulose main chain was maintained even under alkaline conditions, demonstrating excellent alkali resistance. However, as shown in Comparative Example 2 of Table 1, the cellulose membrane is undesirable as an RO membrane, NF membrane, etc., because its high hydrophilicity and crystallinity result in no (extremely low) NaCl rejection rate. In comparison to these results, the etherified cellulose membrane (Example 4) showed no change in substitution degree or molecular weight before and after the alkali immersion test, demonstrating excellent alkali resistance. In addition, as shown in Examples 1 to 6 of Table 1, the etherified cellulose membrane exhibits excellent NaCl rejection rates and maintains this performance even after alkali treatment. Therefore, the etherified cellulose membrane of this disclosure has high alkali resistance in terms of both chemical structure and separation membrane performance.

Claims

1. A separation membrane having a separation functional layer made of etherified cellulose in which at least some of the hydrogens of the hydroxyl groups of cellulose are substituted by substituents containing aromatic rings, wherein the degree of substitution of the hydroxyl groups in the etherified cellulose is 0.3 or more and 2.4 or less.

2. The separation membrane according to claim 1, wherein the substituent containing the aromatic ring is a benzyl group which may have substituents represented by chemical formula (1). (In the formula, R １ ~R ５ Each of these independently represents either hydrogen or a substituent other than hydrogen.

3. The separation membrane according to claim 1 or claim 2, wherein, after an alkaline immersion test in a 0.1 M sodium hydroxide aqueous solution at 40°C for one week, a reverse osmosis test is performed using an aqueous solution at 25°C, 4.8 MPa, and a NaCl concentration of 1500 mg / L as the feed solution, and the NaCl rejection rate is 50% or more.

4. A method for producing a separation membrane, comprising: a cellulose dissolution step of dissolving cellulose or a cellulose derivative in an aqueous solution of a tetraalkylammonium salt or tetraalkylphosphonium salt whose counterion is a hydroxide ion to obtain an aqueous cellulose solution; a cellulose modification step of contacting the aqueous cellulose solution with an electrophile containing optionally substituted benzyl groups to replace at least some of the hydrogens of the hydroxyl groups of the cellulose with the optionally substituted benzyl groups to obtain etherified cellulose; and a film formation step of obtaining a separation functional layer by solution film formation using a film-forming stock solution containing the etherified cellulose.

5. The manufacturing method according to claim 4, wherein the electrophile is benzyl halide.

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