Modified polysaccharide, material for forming gas separation membrane, film, gas separation membrane, gas separation membrane module, and gas separation method

Abstract

To provide a material that makes it possible to produce a gas separation membrane having gas permeability and gas selectivity (gas separability) in a good balance.SOLUTION: A modified polysaccharide includes a polysaccharide skeleton, and a structure bonded to the polysaccharide skeleton, which is derived from at least one selected from an aromatic ring, and a monocarboxylic acid including a carbon-carbon unsaturated double bond, directly bonded to the aromatic ring, an ester thereof, and an acid halide thereof.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to modified polysaccharides, materials for forming gas separation membranes, films, gas separation membranes, gas separation membrane modules, and gas separation methods. [Background technology]

[0002] Global warming, an environmental problem progressing on a global scale, is known as a problem that must be solved. Global warming is thought to be mainly caused by the increase in carbon dioxide emitted into the atmosphere from thermal power plants and other sources.

[0003] For this reason, technologies for separating and capturing carbon dioxide are currently being investigated, and membrane separation is known as one such technology. Since each polymer material has its own unique gas permeability, a gas separation membrane made of a polymer material can be used to selectively permeate and separate the desired gas. Cellulose compounds are known as materials for forming gas separation membranes (see, for example, Patent Document 1).

[0004] Membrane separation is a highly economical, clean separation technology that uses the pressure difference (concentration difference) across a membrane as the driving force and does not involve chemical reactions or phase changes. Therefore, membrane separation is expected to be a highly economical separation technology in the separation and recovery process of target gases such as carbon dioxide. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2020-73249 Summary of the Invention [Problem to be solved by the invention]

[0006] Gas separation membranes with excellent gas permeability and gas selectivity (separability) are useful because they can efficiently separate a specific gas from a mixed gas containing two or more gases. However, gas permeability and gas selectivity generally have a trade-off relationship. In conventional gas separation membranes, for example, increasing gas permeability sometimes results in a decrease in gas selectivity.

[0007] An object of the present disclosure is to provide a material that enables the production of a gas separation membrane that has an excellent balance between gas permeability and gas selectivity (gas separation performance). [Means for solving the problem]

[0008] The modified polysaccharide according to one embodiment of the present disclosure comprises a polysaccharide backbone and a structure attached to the polysaccharide backbone, the structure being derived from at least one selected from a monocarboxylic acid containing an aromatic ring and a carbon-carbon unsaturated double bond directly bonded to the aromatic ring, an ester thereof, and an acid halide thereof. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide modified polysaccharides, which are materials that enable the production of gas separation membranes that have an excellent balance between gas permeability and gas selectivity (gas separation properties). [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 shows the 1H-NMR spectrum of the polymer obtained in the Examples section. [Figure 2] FIG. 2 shows the TGA curve of the film obtained in the Examples section. [Figure 3] FIG. 3 shows the second curve of the DSC of the film obtained in the Examples section. [Figure 4] FIG. 4 shows the gas sorption isotherm of the membrane obtained in the Examples section. [Figure 5] FIG. 5 shows the pressure dependence of the gas permeability coefficient of the membrane obtained in the Examples section. DETAILED DESCRIPTION OF THE INVENTION

[0011] In this specification, "A to B" indicating a range of values ​​includes the values ​​A and B written before and after "to" as the lower and upper limits, respectively. For example, "1 to 10" means a range of values ​​from 1 to 10.

[0012] As used herein, "gas separation" means that the concentration of a specific gas (e.g., carbon dioxide) becomes higher after permeation through a gas separation membrane than before permeation, but does not mean that only the specific gas permeates the gas separation membrane. Furthermore, a gas that permeates a gas separation membrane is also called a "permeating gas," and a gas that does not permeate is also called a "non-permeating gas."

[0013] [Modified polysaccharide] The modified polysaccharide of the present disclosure (hereinafter also referred to as "modified polysaccharide (S)") is a polysaccharide backbone; a structure bound to the polysaccharide skeleton, which is derived from at least one selected from a monocarboxylic acid containing an aromatic ring and a carbon-carbon unsaturated double bond directly bound to the aromatic ring, an ester thereof, and an acid halide thereof (hereinafter also referred to as "monocarboxylic acids (m)"); Includes:

[0014] In one embodiment of the modified polysaccharide (S), the structure derived from the monocarboxylic acid (m) is introduced into the polysaccharide by an ester bond (esterification reaction) between a hydroxy group contained in the polysaccharide and a carboxy group, an ester group thereof, or an acid halide group thereof contained in the monocarboxylic acid (m).

[0015] In one embodiment, a membrane containing the modified polysaccharide (S) exhibits an excellent balance of gas permeability and gas selectivity. This is presumably because the introduction of a structure derived from monocarboxylic acids (m) into the polysaccharide moderately reduces the number of hydroxyl groups in the polysaccharide, and simultaneously introduces aromatic rings, thereby reducing the hydrogen bonding strength (cohesion) of the polysaccharide and thereby reducing the membrane density (i.e., increasing the free volume). Furthermore, in one embodiment, a membrane containing the modified polysaccharide (S) also exhibits excellent resistance to plasticization under high-pressure conditions. This is presumably due to the effective use of pseudo-crosslinking based on stacking between the aromatic rings introduced into the polysaccharide. Note that the above explanation is merely presumed and does not limit the modified polysaccharide (S) of the present disclosure in any way. Therefore, the modified polysaccharide (S) is useful as a material for forming a gas separation membrane.

[0016] <Polysaccharide> Examples of polysaccharides that constitute the polysaccharide backbone include cellulose, starch, glycogen, dextrin, amylose, amylopectin, xanthan gum, alginic acid, chitin, chitosan, hyaluronic acid, agarose, and carrageenan.

[0017] The polysaccharide may be a derivative of the above specific examples, such as a derivative in which some of the hydroxy groups contained in the above specific examples of polysaccharides have been acylated, etherified, or subjected to other known substitution treatments, or an alkali metal salt such as a sodium salt or potassium salt of the above specific examples of polysaccharides.

[0018] Examples of the derivatives include cellulose derivatives and starch derivatives. Examples of cellulose derivatives include etherified celluloses such as acylated celluloses such as cellulose acetate, cellulose nitrate, cellulose sulfate, alkyl-etherified celluloses such as methyl cellulose and ethyl cellulose, hydroxyalkyl-etherified celluloses such as hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethyl cellulose, and hydroxypropylethyl cellulose, and carboxyalkyl-etherified celluloses such as carboxymethyl cellulose and carboxyethyl cellulose. Examples of starch derivatives include starch acetate.

[0019] Among polysaccharides, from the viewpoints of gas permeability, gas selectivity, and chemical durability, cellulose and cellulose derivatives are preferred, cellulose derivatives are more preferred, acylated cellulose is even more preferred, and cellulose acetate is particularly preferred. Acylated cellulose is a polysaccharide in which some of the hydroxy groups contained in cellulose are acylated. The acylated cellulose may contain one type of acyl group or two or more types of acyl groups.

[0020] The acyl group is preferably an aliphatic acyl group (R—CO—; R is an aliphatic group, preferably an alkyl group having 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms), more preferably an acetyl group, a propanoyl group, or a butanoyl group, and particularly preferably an acetyl group.

[0021] The degree of substitution in cellulose derivatives, particularly in the case of acylated cellulose, with acyl groups is preferably 0.1 or more and less than 3.0, more preferably 0.5 to 2.8, even more preferably 1.0 to 2.6, and particularly preferably 1.5 to 2.4. The degree of substitution indicates the average number of hydrogen atoms constituting hydroxy groups that have been replaced with substituents (e.g., acyl groups) among all hydroxy groups contained in glucose units, which are repeating structural units in cellulose. Therefore, the upper limit of the degree of substitution in general cellulose derivatives is 3.

[0022] The above-mentioned degree of substitution is the degree of substitution of the cellulose derivative into which the monocarboxylic acid (m) is introduced, and therefore does not include substitution with the monocarboxylic acid (m).

[0023] The degree of substitution is 1 It can be determined by H-NMR etc. Specific examples of the method include the method described in Cellulose Communication, 1999, Vol. 6, pp. 73-79.

[0024] Generally, the lower the degree of substitution of a cellulose derivative, the smaller the free volume of the membrane tends to be due to the effect of the hydroxyl group, and the denser the membrane tends to be. Therefore, the lower the degree of substitution of a cellulose derivative, the lower the membrane's permeability to gases other than water vapor tends to be. Furthermore, such membranes are prone to plasticization under high-pressure conditions, which can reduce gas selectivity. In contrast, in one embodiment, the modified polysaccharide (S) of the present disclosure contains a structure derived from monocarboxylic acids (m), and therefore the resulting gas separation membrane has excellent properties, such as high gas permeability and high resistance to plasticization under high-pressure conditions.

[0025] <Monocarboxylic acids> The monocarboxylic acid (m) is at least one selected from the group consisting of a monocarboxylic acid containing an aromatic ring and a carbon-carbon unsaturated double bond directly bonded to the aromatic ring, an ester of the monocarboxylic acid, and an acid halide of the monocarboxylic acid. The monocarboxylic acid (m) contains one carboxy group, an ester group thereof, or an acid halide group thereof, an aromatic ring, and a carbon-carbon unsaturated double bond directly bonded to the aromatic ring. The modified polysaccharide (S) may contain one or more structures derived from the monocarboxylic acid (m).

[0026] Examples of the ester group include alkoxycarbonyl groups such as methoxycarbonyl and ethoxycarbonyl groups. Examples of the acid halide group include acid chloride groups.

[0027] Examples of aromatic rings contained in the monocarboxylic acids (m) include a benzene ring, a naphthalene ring, and an anthracene ring. The aromatic ring may contain one or more substituents (m1). Examples of the substituent (m1) include hydrocarbon groups such as alkyl groups such as methyl and ethyl groups, aryl groups such as phenyl groups, and aralkyl groups such as benzyl groups; hydroxy groups; alkoxy groups such as methoxy and ethoxy groups; aryloxy groups such as phenoxy groups; acyl groups such as acetyl, propanoyl, and butanoyl groups; acyloxy groups such as acetoxy, propanoyloxy, and butanoyloxy groups; and halogen atoms such as fluorine and chlorine. The substituent (m1) may be of one type or of two or more types.

[0028] The number of carbon atoms in the alkyl group and alkoxy group is preferably 1 to 10, more preferably 1 to 5. The number of carbon atoms in the aryl group and aryloxy group is preferably 6 to 15, more preferably 6 to 10. The number of carbon atoms in the aralkyl group is preferably 7 to 16, more preferably 7 to 11. The number of carbon atoms in the acyl group and acyloxy group is preferably 2 to 10, more preferably 2 to 5.

[0029] The carbon-carbon unsaturated double bond contained in the monocarboxylic acid (m) may be, for example, a monovalent group (-CH=CH), a divalent group (-CH=CH-), or a trivalent group (-CH=C<), but is preferably a divalent group. Note that the bond (-) in these formulas is bonded to a group other than a hydrogen atom. The carbon-carbon unsaturated double bond is directly bonded to an aromatic ring, and preferably also directly bonded to a carboxy group, an ester group thereof, or an acid halide group thereof.

[0030] The monocarboxylic acids (m) preferably have 9 to 50 carbon atoms, more preferably 9 to 30 carbon atoms, and even more preferably 9 to 20 carbon atoms.

[0031] The monocarboxylic acid in the monocarboxylic acid (m) is preferably cinnamic acid (CHCH=CHCOOH) and cinnamic acid derivatives. Examples of cinnamic acid derivatives include compounds containing one or more substituents (m1) on the aromatic ring in cinnamic acid. Specific examples of the substituents (m1) are as described above.

[0032] Examples of cinnamic acid derivatives include ferulic acid and acylated ferulic acid in which the phenolic hydroxy group is acylated. The number of carbon atoms in the acyl group in the acylated ferulic acid is preferably 2 to 10, more preferably 2 to 5, and even more preferably 2 to 4. The acylated ferulic acid is preferably acetylated ferulic acid.

[0033] In particular, the introduction of a structure derived from at least one compound selected from ferulic acid and its derivatives, their esters, and their acid halides into cellulose acetate, which is one of the gas separation membrane materials in practical use, is preferred because it improves the gas permeability and gas selectivity of the cellulose acetate membrane and also improves its resistance to plasticization under high-pressure conditions.

[0034] Ferulic acid, a type of cinnamic acid derivative, can be obtained from rice bran, the outer layer and germ of brown rice removed during the rice milling process. Currently, most rice bran is underutilized and disposed of as industrial waste. However, in recent years, processes for extracting compounds such as ferulic acid and γ-oryzanol from rice bran have been established. Ferulic acid can also be extracted from corn. Therefore, ferulic acid is preferably derived from rice bran or corn, and more preferably from rice bran. In this case, ferulic acid and ferulic acid derivatives obtained from rice bran, a type of unused biomass (renewable resource), can be effectively utilized. The effective use of renewable resources reduces the consumption of fossil resources and, due to its carbon-neutral properties, contributes to reducing carbon dioxide emissions into the atmosphere.

[0035] One embodiment of the modified polysaccharide (S) is a bio-based polymer made from renewable resources such as plants. In particular, by introducing a structure derived from at least one compound selected from ferulic acid and its derivatives, esters thereof, and acid halides thereof into cellulose acetate, a bio-based polymer with an increased biomass ratio can be obtained.

[0036] In the modified polysaccharide (S), the introduction rate of the structure derived from the monocarboxylic acid (m) into the polysaccharide backbone, particularly the acylated cellulose backbone, is, for example, 0.1 to 100%, preferably 1 to 80%, and in one embodiment, 5 to 60%, 10 to 55%, or 15 to 50%. An introduction rate that further improves the gas permeability and gas selectivity of the target gas separation membrane can be selected. The introduction rate (%; number basis) is based on the number of hydroxy groups in the polysaccharide backbone of acylated cellulose or the like (for example, the number of unacetylated hydroxy groups in cellulose acetate) before the structure derived from the monocarboxylic acid (m) is introduced. For example, when the structure derived from the monocarboxylic acid (m) is introduced into all hydroxy groups in the acylated cellulose, the introduction rate is 100%. The introduction rate is 1 It can be determined by H-NMR or the like.

[0037] <Suitable examples of modified polysaccharides> The modified polysaccharide (S) is preferably a modified acylated cellulose containing the repeating structural unit shown in formula (S-1). [ka]

[0038] In formula (S-1), R 1 ~R 3 are each independently a hydrogen atom, an acyl group, or -CO-CH=CH-CR 4The acyl group is preferably an aliphatic acyl group (R-CO-; R is an aliphatic group, preferably an alkyl group having 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms), more preferably an acetyl group, a propanoyl group, or a butanoyl group, and particularly preferably an acetyl group. 4 are each independently a hydrogen atom or a substituent (m1). Specific examples of the substituent (m1) are as described above.

[0039] However, R is selected so that the degree of substitution with acyl groups is 0.1 or more and less than 3.0, preferably 0.5 to 2.8, more preferably 1.0 to 2.6, and particularly preferably 1.5 to 2.4, and the introduction rate of the structure derived from the monocarboxylic acids (m) into the acylated cellulose skeleton is 0.1 to 100%. 1 ~R 3 is selected from the group above.

[0040] The weight-average molecular weight (Mw) of the modified polysaccharide (S) is preferably 5,000 to 500,000 g / mol, more preferably 10,000 to 300,000 g / mol, and even more preferably 10,000 to 150,000 g / mol, from the viewpoints of the membrane strength and formability of the gas separation membrane. The weight-average molecular weight is measured by gel permeation chromatography (GPC) and is a converted value calculated using a calibration curve based on standard polystyrene.

[0041] In one embodiment, the modified polysaccharide (S) has excellent solubility in various solvents. For example, a solution of the modified polysaccharide (S) can be easily prepared, and therefore, the solution can be used to form a layer containing the modified polysaccharide (S).

[0042] Examples of solvents that constitute the solution include aprotic solvents such as halogenated hydrocarbon solvents such as chloroform and methylene chloride; ester solvents such as methyl acetate, ethyl acetate, and butyl acetate; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone, and cyclohexanone; ether solvents such as dibutyl ether, tetrahydrofuran, and dioxane; nitrogen-containing solvents such as N-methylpyrrolidone, dimethylformamide, dimethylimidazolidinone, and dimethylacetamide; and sulfur-containing solvents such as dimethyl sulfoxide; and protic solvents such as mono-substituted glycol ether solvents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether, ethylene glycol monophenyl ether, propylene glycol monophenyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, and triethylene glycol monoethyl ether.

[0043] Among these, the modified polysaccharide (S) tends to have high solubility in aprotic solvents, and among aprotic solvents, it tends to have higher solubility in halogenated hydrocarbon solvents, ketone solvents, ether solvents, nitrogen-containing solvents, and sulfur-containing solvents.

[0044] In one embodiment, the modified polysaccharide (S) has excellent film-forming properties, and therefore can be easily formed into a film, and the obtained film can be used as a free-standing film. On the other hand, for example, a radical copolymer of a monocarboxylic acid (m) and styrene tends to have poor film-forming properties.

[0045] <Method for producing modified polysaccharide (S)> The method for producing the modified polysaccharide (S) is not particularly limited. The modified polysaccharide (S) can be obtained by introducing the target structure into the polysaccharide through a conventionally known esterification reaction between the polysaccharide and a monocarboxylic acid (m). In the reaction for introducing the structure derived from the monocarboxylic acid (m) into the polysaccharide, for example, a monocarboxylic acid, an ester of a monocarboxylic acid, or an acid halide may be used depending on the reactivity. For example, the target product can be obtained by reacting a hydroxy group in the polysaccharide with an acid chloride of a monocarboxylic acid.

[0046] [Films and gas separation membranes] The films of the present disclosure contain a modified polysaccharide (S). The gas separation membrane of the present disclosure includes a layer containing a modified polysaccharide (S) (hereinafter also referred to as "gas separation layer (S1)"). The gas separation membrane may be a single-layer or multi-layer membrane consisting of only the gas separation layer (S1), or may be a composite multi-layer membrane containing the gas separation layer (S1) and other layers. The other layers include, for example, a gas-permeable support layer and a protective layer.

[0047] Examples of composite multilayer membranes include a multilayer membrane comprising a gas-permeable support layer, a gas separation layer (S1) formed on the support layer, and, if necessary, a protective layer formed on the gas separation layer (S1) on the opposite side of the gas-permeable support layer. Such composite multilayer membranes tend to have excellent gas permeability and gas selectivity as well as excellent mechanical strength. The composite multilayer membrane may also include an additional layer between the gas-permeable support layer and the gas separation layer (S1).

[0048] The gas-permeable support layer is, for example, a porous layer, a woven fabric, or a nonwoven fabric. The gas-permeable support layer may be one layer or two or more layers, and when two or more layers are used, the layers may be the same or different. The thickness of the gas-permeable support layer is preferably 1 to 3,000 μm, more preferably 5 to 1,000 μm, and even more preferably 10 to 500 μm.

[0049] Examples of materials for forming the gas-permeable support layer include polyolefin resins such as polyethylene, polypropylene, and polymethylpentene; fluorine-containing resins such as polytetrafluoroethylene, polyvinyl fluoride, polyvinylidene fluoride, and amorphous fluoropolymer; polystyrene, polyacrylonitrile, polyester, polyurethane, polyphenylene oxide, polysulfone, polyethersulfone, polyimide, polyamide, cellulose acetate, and poly(1-trimethylsilyl-1-propyne). One or more of the above-mentioned materials can be used.

[0050] The content of the modified polysaccharide (S) in the film and gas separation layer (S1) is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more, from the viewpoints of gas permeability and gas selectivity. The content of the modified polysaccharide (S) in the film and gas separation layer (S1) may be 100% by mass or less, or may be 99% by mass or less.

[0051] The film and gas separation layer (S1) may further contain additives to adjust the membrane properties. Examples of additives include plasticizers, organic lubricants, crystal nucleating agents, organic particles, inorganic particles, ultraviolet absorbers, infrared absorbers, color inhibitors, matting agents, antibacterial agents, antistatic agents, deodorizers, flame retardants, weather resistance agents, antistatic agents, antioxidants, ion exchange agents, antifoaming agents, color pigments, and dyes. One or more additives may be used.

[0052] The thickness of the film and the gas separation layer (S1) is preferably 0.01 to 500 μm, more preferably 0.1 to 300 μm, and even more preferably 0.1 to 100 μm.

[0053] The above-mentioned film and gas separation layer (S1) can be produced, for example, by applying a coating solution containing the modified polysaccharide (S) to a support (e.g., a glass plate or a plastic sheet) and drying the coating. The composite multilayer membrane can be produced, for example, by applying a coating solution containing the modified polysaccharide (S) to a gas-permeable support layer and drying the coating to form the gas separation layer (S1).

[0054] The content of the modified polysaccharide (S) in the coating solution is preferably 0.1 to 70% by mass, more preferably 0.5 to 50% by mass, from the viewpoint of film-forming properties. One or more types of modified polysaccharide (S) can be used.

[0055] The solvent constituting the coating solution may be the aprotic solvent or protic solvent described above, and aprotic solvents are preferred because they have excellent solubility for the modified polysaccharide (S). One or more kinds of solvents may be used.

[0056] Examples of methods for applying the coating solution include casting, dipping, screen printing, spray coating, and spin coating. The amount of coating is appropriately determined depending on the thickness of the film and the gas separation layer (S1). Drying conditions are appropriately determined depending on the type of solvent, etc.

[0057] The gas separation membrane of the present disclosure may be a so-called asymmetric membrane containing a modified polysaccharide (S). The asymmetric membrane can be formed by a phase inversion method using a solution containing the modified polysaccharide (S). The phase inversion method, proposed by Rob Srirajan et al., is a known method for forming a membrane by contacting a polymer solution with a coagulation liquid to cause phase inversion. For example, the solution on the surface of a membrane-shaped polymer solution is evaporated to form a thin dense layer. The resulting membrane is then immersed in a coagulation liquid that does not substantially dissolve the polymer but is compatible with the solvent of the polymer solution. The resulting phase separation phenomenon is utilized to form micropores, forming a porous layer. Therefore, the asymmetric membrane comprises a dense layer and a porous layer.

[0058] From the viewpoint of gas permeability and gas selectivity, the content of the modified polysaccharide (S) in the asymmetric membrane is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more. The content of the modified polysaccharide (S) in the asymmetric membrane may be 100% by mass or less, or may be 99% by mass or less. The asymmetric membrane may further contain the above-mentioned additives.

[0059] The thickness of the asymmetric membrane is preferably 0.01 to 500 μm, more preferably 0.1 to 300 μm, and even more preferably 1 to 200 μm.

[0060] Examples of the shape of the gas separation membrane include a hollow fiber membrane (hereinafter also referred to as a "hollow fiber membrane") and a flat membrane (hereinafter also referred to as a "flat membrane"). Among these, a hollow fiber membrane is preferred because the module can be easily manufactured and the effective membrane area per unit volume of the module is large.

[0061] In one embodiment, the gas separation membrane of the present disclosure exhibits excellent gas permeability without compromising gas selectivity, as compared to, for example, conventional gas separation membranes made of cellulose acetate, and also exhibits stable gas selectivity regardless of the pressure of the mixed gas supplied to the gas separation membrane.

[0062] The gas separation membrane of the present disclosure can selectively separate a specific gas from a mixed gas containing two or more gases, such as helium (He), oxygen (O2), nitrogen (N2), saturated hydrocarbons such as methane (CH4), and carbon dioxide (CO2). In one embodiment, the gas separation membrane of the present disclosure has excellent carbon dioxide permeability and selectivity, and is therefore useful as a gas separation membrane for selectively allowing carbon dioxide to permeate from a mixed gas containing carbon dioxide and other gases (e.g., saturated hydrocarbons such as methane).

[0063] Furthermore, in actual plants, when the mixed gas supplied is at high pressure, the gas separation membrane may plasticize, increasing the mobility of the polymer segments and reducing the gas selectivity of the gas separation membrane. This problem is likely to occur when a glassy polymer is used as the material for forming the gas separation membrane. In contrast, in one embodiment, the gas separation membrane of the present disclosure has excellent resistance to plasticization under high-pressure conditions. Therefore, the gas separation membrane of the present disclosure has high gas permeability and gas selectivity, and can separate the target gas even under high-pressure mixed gas supply conditions, thereby enabling high-speed and highly selective gas separation.

[0064] [Gas separation module] The gas separation membrane module of the present disclosure comprises the gas separation membrane of the present disclosure. Examples of module configurations include hollow fiber, flat membrane, tubular, spiral, pleated, and plate-and-frame types, with hollow fiber separation membrane modules being preferred.

[0065] A gas separation membrane module includes, for example, a container having a mixed gas inlet, a permeate gas outlet, and a non-permeate gas outlet, and a gas separation membrane fixed in the container.

[0066] A hollow fiber gas separation membrane module, for example, comprises an assembly of multiple hollow fiber membranes aligned in the longitudinal direction. The assembly may contain, for example, 100 to 1,000,000 hollow fiber membranes. The assembly may take various shapes. Examples include bundles, sheets, and sheet laminates containing multiple hollow fiber membranes aligned approximately parallel in the longitudinal direction, as well as cylinders or cylindrical bodies formed by winding the sheets or laminates once or twice or more times. The hollow fiber membranes preferably have an inner diameter of 10 to 3,000 μm and an outer diameter of 20 to 6,000 μm.

[0067] In one embodiment of a hollow fiber gas separation membrane module, an assembly of hollow fiber membranes is housed and fixed in a container so as to define an inner space and an outer space of the hollow fiber membranes. A mixed gas inlet is connected to one end of the assembly of hollow fiber membranes, and a non-permeate gas outlet is connected to the other end. Both ends of the assembly are fixed to the container by any method (for example, a method using an adhesive).

[0068] [Gas separation method] The gas separation method of the present disclosure includes, specifically, a step of obtaining a gas having an increased concentration of a specific gas from a mixed gas containing two or more gases using the gas separation membrane or gas separation membrane module of the present disclosure. In one embodiment, the gas separation membrane of the present disclosure has particularly excellent carbon dioxide permeability and selectivity, and therefore, it is preferable that the mixed gas contains carbon dioxide and the specific gas is carbon dioxide.

[0069] The mixed gas preferably contains carbon dioxide and saturated hydrocarbons such as methane. In the mixed gas containing carbon dioxide and saturated hydrocarbons, the volume ratio of carbon dioxide to saturated hydrocarbons (carbon dioxide:saturated hydrocarbons) is preferably 1:99 to 99:1, more preferably 10:90 to 90:10.

[0070] The supply pressure of the mixed gas during gas separation is preferably 1 to 100 bar, more preferably 1 to 50 bar, even more preferably 2 to 30 bar, and particularly preferably 2 to 10 bar. It is also preferable to provide a pressure difference between the gas supply side and the gas permeation side of the gas separation membrane. This pressure difference is provided, for example, by reducing the pressure on the gas permeation side.

[0071] The ambient temperature during gas separation is not particularly limited, but is preferably -20 to 120°C, more preferably 10 to 80°C.

[0072] In one embodiment of the gas separation method of the present disclosure, a mixed gas containing two or more gases is brought into contact with one side of a gas separation membrane, and a specific gas in the mixed gas is selectively allowed to permeate to the other side of the gas separation membrane.

[0073] For example, in the case of a hollow fiber gas separation membrane module, a mixed gas is supplied from a mixed gas inlet into the inner space of a hollow fiber membrane, and as the mixed gas flows through the inner space of the hollow fiber membrane, a specific gas in the mixed gas selectively permeates the hollow fiber membrane, which is a gas separation membrane. The permeated gas that has permeated the hollow fiber membrane and has a high concentration of the specific gas is discharged from a permeated gas outlet, and the non-permeated gas that has not permeated the hollow fiber membrane is discharged from a non-permeated gas outlet, thereby selectively separating the specific gas.

[0074] The gas separation method of the present disclosure can be used, for example, to separate carbon dioxide from combustion exhaust gas generated in thermal power plants, steel plants, cement factories, etc. Furthermore, since natural gas and biogas are mixed gases containing methane and carbon dioxide, the gas separation method of the present disclosure can also be used to refine light hydrocarbons in natural gas and refine methane in biogas.

[0075] The embodiments disclosed in this specification, as well as specific examples and preferred examples of each element, are merely illustrative and do not limit the present disclosure in any way.

[0076] The present disclosure relates to, for example, the following [1] to

[14] . [1] A modified polysaccharide comprising a polysaccharide backbone and a structure attached to the polysaccharide backbone, the structure being derived from at least one selected from a monocarboxylic acid containing an aromatic ring and a carbon-carbon unsaturated double bond directly bonded to the aromatic ring, an ester thereof, and an acid halide thereof. [2] The modified polysaccharide according to [1] above, wherein the monocarboxylic acid is at least one selected from cinnamic acid and cinnamic acid derivatives. [3] The modified polysaccharide according to [1] or [2] above, wherein the monocarboxylic acid is at least one selected from ferulic acid and ferulic acid derivatives. [4] The modified polysaccharide described in [3] above, wherein ferulic acid is a component derived from rice bran. [5] The modified polysaccharide according to any one of [1] to [4] above, wherein the polysaccharide backbone is at least one selected from a cellulose backbone and a cellulose derivative backbone. [6] The modified polysaccharide according to any one of [1] to [5] above, wherein the polysaccharide backbone is an acylated cellulose backbone, and the degree of substitution by acyl groups in the acylated cellulose backbone is 0.1 or more and less than 3.0. [7] The modified polysaccharide according to [6] above, wherein the introduction rate of a structure derived from at least one selected from monocarboxylic acids, their esters, and their acid halides into the acylated cellulose skeleton is 0.1 to 100%. [8] A material for forming a gas separation membrane, comprising the modified polysaccharide according to any one of [1] to [7] above. [9] A film containing the modified polysaccharide according to any one of the above [1] to [7].

[10] A gas separation membrane comprising a layer containing the modified polysaccharide according to any one of the above [1] to [7].

[11] The gas separation membrane according to

[10] above, which is used to selectively allow carbon dioxide to permeate from a mixed gas containing carbon dioxide.

[12] A gas separation membrane module comprising the gas separation membrane according to

[10] or

[11] above.

[13] A gas separation method comprising a step of obtaining a gas in which the concentration of a specific gas has been increased from a mixed gas containing two or more gases, using the gas separation membrane according to

[10] or

[11] above or the gas separation membrane module according to

[12] above.

[14] The gas separation method according to

[13] above, comprising a step of obtaining a gas having an increased concentration of carbon dioxide from a mixed gas containing carbon dioxide. [Example]

[0077] The modified polysaccharide (S) etc. of the present disclosure will be explained in more detail below based on the following test examples, but the modified polysaccharide (S) etc. of the present disclosure is not limited to these test examples.

[0078] [reagent] Cellulose acetate (product name "LM-80", acetylation degree: 51.6%, substitution degree: 2.2; Daicel Chemical Industries, Ltd.) was dissolved, reprecipitated, and purified before use in the following synthesis. Ferulic acid (purity 99% or higher; Tsuno Foods Industry) was used as is in the following synthesis. Reagents and solvents other than cellulose acetate and ferulic acid were purchased commercially and used in the following synthesis.

[0079] [Synthesis] The synthetic route for the modified polysaccharide is shown in Scheme 1. First, ferulic acid was acetylated, then converted to acid chloride. The resulting acetylated ferulic acid chloride was esterified with cellulose acetate to introduce a structure derived from acetylated ferulic acid into the cellulose acetate. Cellulose acetate is also called "CA," and the modified cellulose acetate obtained by this synthesis is also called "FA-CA." FT-IR and 1 Structural analysis of FA-CA by H-NMR confirmed the reduction of hydroxyl groups in cellulose acetate and the introduction of structures derived from acetylated ferulic acid into cellulose acetate. Figure 1 shows the structure of FA-CA. 1 The H-NMR spectrum is shown.

[0080] [ka] In Scheme 1, * indicates a bond, and n indicates that the structure in [ ] is a repeating structural unit. Other symbols are as explained in the text.

[0081] <Acetylated ferulic acid> A 1 L flask equipped with a stirrer and calcium chloride tube was charged with 38.84 g (0.20 mol) of ferulic acid (FA), 37.8 mL (0.40 mol) of acetic anhydride, 56 mL (0.40 mol) of triethylamine (TEA), and 400 mL of tetrahydrofuran (THF). The contents were stirred with a magnetic stirrer at room temperature overnight. After completion of the reaction, the reaction solution was concentrated using an evaporator and poured into a large amount of hot water, and the precipitate was collected. The precipitate was washed once with hot water to obtain acetylated ferulic acid (AcFA) as a pale yellow powder.

[0082] <Synthesis of acetylated ferulic acid chloride> A 200 mL flask equipped with a stirrer and a three-way stopcock was charged with 4.72 g (20 mmol) of AcFA. After purging with nitrogen, 80 mL of anhydrous THF was added and the contents were stirred. After cooling the contents in an ice bath, 1.7 mL (20 mmol) of oxalyl dichloride was added dropwise via syringe, followed by several drops of N,N-dimethylformamide (DMF). After confirming the generation of bubbles upon addition of DMF, the mixture was returned to room temperature and allowed to react overnight. After the reaction, the disappearance of the raw material spots was confirmed by thin-layer chromatography (TLC) (methyl esterified), marking the reaction complete. The solvent was then distilled off to yield acetylated ferulic acid chloride (AcFA-Cl) as a pale yellow solid.

[0083] <Modification of cellulose acetate (esterification reaction)> A 300 mL flask equipped with a stirrer and a three-way stopcock was charged with 4.92 g (19 mmol) of cellulose acetate (CA). After purging with nitrogen, 100 mL of anhydrous THF was added to dissolve the cellulose acetate in THF. Next, while stirring, 10 mL of pyridine was added, and a solution of 5.1 g of AcFA-Cl in 50 mL of anhydrous THF was added dropwise via syringe. The reaction was allowed to proceed overnight. After completion of the reaction, the resulting gray reaction mixture was filtered and poured into a large amount of methanol. The precipitate was collected by filtration and dried to yield a milky-white solid: modified cellulose acetate (FA-CA), in which the structure derived from acetylated ferulic acid (AcFA structure) had been introduced into the cellulose acetate. 1 H-NMR analysis revealed that the OH conversion (introduction rate of AcFA structure) was approximately 30%. FA-CA, like CA, was soluble in organic solvents such as tetrahydrofuran, but insoluble in water and methanol.

[0084] <Film forming> The membrane was formed by solvent casting using tetrahydrofuran. The filtered polymer solution with an FA-CA concentration of 3% by mass was poured into a glass petri dish, and the solvent was slowly evaporated over 24 to 48 hours to obtain a film. The obtained film was heated and dried in a vacuum dryer at 100°C for 24 hours to remove the solvent and obtain an FA-CA membrane. The thickness of the dried FA-CA membrane was measured with a micrometer (Mitsutoyo) and was found to be 60 μm. A CA membrane was also obtained using cellulose acetate. Both were transparent, free-standing membranes.

[0085] [Characterization] <Nuclear Magnetic Resonance (NMR) Spectrum> Proton NMR ( 1 H-NMR spectroscopy was performed using an ECX300 (manufactured by JEOL Ltd.) in deuterated chloroform as a solvent.

[0086] <Fourier transform infrared (FT-IR) spectrum> FT-IR spectrum measurement was carried out using FT / IR-4100 (manufactured by JASCO Corporation). The film was measured by attenuated total reflection spectroscopy (ATR-FTIR).

[0087] <Density measurement> Film density ρ polymer was calculated from Archimedes' principle. The dry weight w of the membrane at 25°C was measured using a precision electronic balance AUX220 (Shimadzu Corporation) equipped with a specific gravity measurement kit. air and weight in hexane w Hexane The density of the film was calculated by the following formula: Hexane is the density of hexane.

[0088]

number

[0089] <Moisture content> The moisture content of the membrane is the dry weight w of the membrane at 35°C. dryand the weight when equilibrium is reached after immersion in pure water, w wet was measured and calculated using the following formula:

[0090]

number

[0091] <Thermal analysis> Differential scanning calorimetry (DSC) measurements were performed using a Thermo plus DSC-8230 (Rigaku Corporation) in a temperature range of 35 to 250°C under a nitrogen atmosphere at a heating rate of 10°C / min. The glass transition temperature (Tg) was calculated from the midpoint of the heat change in the second scan.

[0092] Thermogravimetric analysis (TGA) was performed using a Thermo plus EVO 8120 (Rigaku Corporation) in the temperature range of 35 to 900°C under a nitrogen atmosphere at a heating rate of 10°C / min. The thermal decomposition temperature (Td) was calculated from the onset of the largest thermal weight loss.

[0093] <Gas permeability measurement> The permeabilities of oxygen (O2), nitrogen (N2), methane (CH4), and carbon dioxide (CO2) were measured for CA and FA-CA membranes using a prototype gas permeation apparatus (constant pressure volume change method) at a temperature of 35°C and a supply pressure of 2 bar. The pressure dependence of carbon dioxide was observed at a temperature of 35°C and a supply pressure range of 2 to 30 bar. To standardize the effect of membrane plasticization, the permeability coefficient was calculated from the value 1 hour after gas supply. The reproducibility of the measurements was confirmed using two or more independent membranes.

[0094] Gas permeability coefficient P (barrer unit: 10 -10 cm 3 (STP)·cm / (cm 2 The ideal separation factor (α(A / B)) is calculated from the slope of the pressure change per unit time at steady state. STP means the standard state of 0°C and 1 atmosphere. The ideal separation factor (α(A / B)) is the permeability coefficient P of gas A and gas B. A and P BThe gas permeability coefficient P is expressed as the product of the solubility coefficient S and the diffusion coefficient D, so the ideal separation factor is the ratio of the solubility coefficients S of gas A and gas B. A and S B and its diffusion coefficient D A and D B It can be expressed as a product of the ratio of

[0095]

number

[0096] <Gas sorption measurement> The gas sorption amount of the membrane was measured by a gravimetric method using a Belsorp HP (manufactured by Japan Bell Co., Ltd.) to measure the sorption amounts of carbon dioxide, methane, and water at a temperature of 35°C. From the gas sorption amount measurement C, the solubility coefficient S of each membrane was calculated using the following formula, with the measurement pressure p.

number

[0097] [result] Table 1 summarizes the physical properties of the CA membrane and the FA-CA membrane.

[0098] [Table 1]

[0099] The density of the FA-CA membrane is lower than that of the CA membrane. This is presumably due to the introduction of AcFA structures into the hydroxyl groups in the CA, which reduces the number of hydroxyl groups in the FA-CA membrane and weakens the hydrogen bonding strength (reducing the cohesive strength between cellulose acetate chains). The water content of the FA-CA membrane is lower than that of the CA membrane. This is presumably due to the reduction in hydroxyl groups in the FA-CA membrane and the introduction of benzene rings derived from acetylated ferulic acid, which increases the hydrophobicity of the FA-CA membrane.

[0100] Figure 2 shows the TGA curves for the CA and FA-CA membranes. The thermal decomposition of the CA and FA-CA membranes was a single stage, with the thermal decomposition temperatures (Td) of 337°C and 341°C, respectively. This indicates that the reduction of hydroxyl groups and the introduction of AcFA structures did not significantly affect heat resistance. The residual amounts of the CA and FA-CA membranes at 900°C were 10% and 15% by mass, respectively.

[0101] Figure 3 shows the second curves of the DSC for the CA and FA-CA films. Like the CA film, the FA-CA film was in a rigid glassy state at room temperature. Clear glass transitions were observed at 184°C and 162°C for both the CA and FA-CA films. The decrease in glass transition temperature (Tg) due to the introduction of the AcFA structure is presumably due to the weakening of the influence of hydrogen bonds caused by the reduction in hydroxyl groups in the cellulose acetate, resulting in a decrease in cohesion (an increase in free volume).

[0102] Tables 2 and 3 show the permeability coefficients P (35°C, 2 bar) and ideal separation coefficients α of each gas for the CA membrane and the FA-CA membrane.

[0103] [Table 2]

[0104] [Table 3]

[0105] The gas permeability coefficient of the FA-CA membrane is larger than that of the CA membrane for any gas. This is presumably due to an increase in free volume caused by the introduction of the AcFA structure and a decrease in hydroxyl groups. The gas permeability coefficients of each membrane are in the order of CO2>O2>N2>CH4, which is the reverse order of the kinetic diameters of the gases (CO2 (3.30 Å) < O2 (3.46 Å) < N2 (3.64 Å) < CH4 (3.80 Å)). The values of the kinetic diameters are described, for example, in S. Kanehashi and K. Nagai, Gas and Vapor Transport in Membranes, Membrane Characterization, Elsevier, 2017.

[0106] The ideal separation coefficients of the FA-CA membranes were all equal to or higher than those of the CA membrane. Generally, there is a trade-off relationship between the gas permeability coefficient and the ideal separation coefficient in polymer membranes. However, it has also been clarified that by introducing the AcFA structure into cellulose acetate, the free volume increases, the gas permeability coefficient improves, and furthermore, the ideal separation coefficient also improves. To clarify the increase in this gas permeability coefficient P and ideal separation coefficient α, the solubility coefficient S of each membrane was calculated by gas sorption measurement, and the diffusion coefficient D was calculated.

[0107] Figures 4(a) and (b) show the sorption isotherms of CO2 and CH4 for the CA membrane and the FA-CA membrane. The gas sorption amount in each membrane followed a behavior conforming to a convex dual sorption model with respect to the pressure axis regardless of the type of gas. These behaviors are consistent with those observed in glassy polymer membranes including cellulose acetate. The CO2 sorption amount of the FA-CA membrane is smaller than that of the CA membrane, and the CH4 sorption amount of the FA-CA membrane is larger than that of the CA membrane. From this, it is inferred that by introducing the AcFA structure into cellulose acetate, the interaction between the membrane and methane increases and its adsorption sites increase.

[0108] Furthermore, the water sorption isotherms for both membranes showed a binary sorption pattern in the low activity range, but above a certain activity range, they showed a Flory-Huggins sorption pattern, where the sorption amount increased exponentially with increasing activity. This is typical behavior observed for glassy polymer membranes. As shown in Figure 4(c), the water sorption amount for the FA-CA membrane was approximately 70% lower than that for the CA membrane at the maximum measured activity (α = 0.8). As predicted from the results of the membrane water content, the introduction of the AcFA structure made the membrane more hydrophobic, which is presumably why the water sorption amount decreased significantly.

[0109] Table 4 shows the solubility coefficient S of each membrane calculated from gas sorption measurements, and also summarizes the gas permeability coefficient P and the diffusion coefficient D calculated from the relationship between the solubility coefficient and the diffusion coefficient (P = S × D).

[0110] [Table 4]

[0111] The solubility coefficient of the FA-CA membrane was roughly the same for CO2 as that of the CA membrane, but approximately 60% higher for CH4. Meanwhile, the diffusion coefficients of each gas in the FA-CA membrane were all higher than those in the CA membrane, with the increase in the diffusion coefficient of CO2 being particularly large. As predicted from the membrane density, the introduction of the AcFA structure suppressed packing between polymer chains, increasing the free volume of the membrane, presumably resulting in a higher diffusion coefficient for CO2, which has a smaller molecular size, than for CH4. From these results, it is believed that the increase in CO2 permeability is due to increased diffusivity associated with the increased free volume, while the increase in CH4 permeability is due to increased solubility rather than diffusivity.

[0112] Figure 5 shows the CO2 pressure dependence of the CO2 permeability coefficient of the CA membrane and FA-CA membrane. For both membranes, the CO2 permeability coefficient initially decreased as the CO2 pressure increased. This is thought to be due to a decrease in the solubility coefficient of CO2 according to the two-dimensional sorption model. Once the plasticization onset pressure was exceeded, the CO2 permeability coefficient began to increase. The plasticization onset pressure Pplasticization is about 5 bar for the CA membrane, while P for the FA-CA membrane is plasticization The pressure was approximately 10 bar, and plasticization was significantly suppressed in the FA-CA membrane compared to the CA membrane. It is presumed that the introduction of the AcFA structure into cellulose acetate effectively created pseudo-crosslinks based on the stacking of benzene rings contained in the AcFA structure, which suppressed plasticization in the FA-CA membrane compared to the CA membrane. This demonstrates that the introduction of the AcFA structure into cellulose acetate not only improves the gas permeability coefficient and ideal separation factor, but also has the effect of suppressing membrane plasticization caused by high-pressure CO2.

Claims

1. A material for forming a gas separation membrane contains a modified polysaccharide, which comprises a polysaccharide skeleton and a structure bound to the polysaccharide skeleton, the structure being derived from at least one selected from the group consisting of a monocarboxylic acid containing an aromatic ring and a carbon-carbon unsaturated double bond directly bonded to the aromatic ring, an ester thereof, and an acid halide thereof, wherein the polysaccharide skeleton is at least one selected from the group consisting of a cellulose skeleton and a cellulose acetate skeleton.

2. 2. The material for forming a gas separation membrane according to claim 1, wherein the polysaccharide skeleton is a cellulose acetate skeleton, and the degree of substitution of the cellulose acetate skeleton with acetyl groups is 0.1 or more and less than 3.

0.

3. 3. The material for forming a gas separation membrane according to claim 1, wherein the introduction rate of the structure derived from at least one selected from the group consisting of monocarboxylic acids, esters thereof, and acid halides thereof into the cellulose acetate skeleton is 0.1 to 100%.

4. A film for forming a gas separation membrane contains a modified polysaccharide, which comprises a polysaccharide skeleton and a structure bound to the polysaccharide skeleton, the structure being derived from at least one selected from a monocarboxylic acid containing an aromatic ring and a carbon-carbon unsaturated double bond directly bonded to the aromatic ring, an ester thereof, and an acid halide thereof, wherein the polysaccharide skeleton is at least one selected from a cellulose skeleton and a cellulose acetate skeleton.

5. 5. The film for forming a gas separation membrane according to claim 4, wherein the polysaccharide skeleton is a cellulose acetate skeleton, and the degree of substitution by acetyl groups in the cellulose acetate skeleton is 0.1 or more and less than 3.

0.

6. 6. The film for forming a gas separation membrane according to claim 4 or 5, wherein an introduction rate of the structure derived from at least one selected from the group consisting of monocarboxylic acids, esters thereof, and acid halides thereof into the cellulose acetate skeleton is 0.1 to 100%.

7. A gas separation membrane comprising a layer containing a modified polysaccharide, the modified polysaccharide comprising a polysaccharide skeleton and a structure bound to the polysaccharide skeleton, the structure being derived from at least one selected from the group consisting of a monocarboxylic acid containing an aromatic ring and a carbon-carbon unsaturated double bond directly bound to the aromatic ring, an ester thereof, and an acid halide thereof, wherein the polysaccharide skeleton is at least one selected from the group consisting of a cellulose skeleton and a cellulose acetate skeleton.

8. 8. The gas separation membrane according to claim 7, wherein the polysaccharide skeleton is a cellulose acetate skeleton, and the degree of substitution of the cellulose acetate skeleton with acetyl groups is 0.1 or more and less than 3.

0.

9. 9. The gas separation membrane according to claim 7, wherein the introduction rate of the structure derived from at least one selected from the group consisting of monocarboxylic acids, esters thereof, and acid halides thereof into the cellulose acetate skeleton is 0.1 to 100%.

10. The gas separation membrane according to any one of claims 7 to 9, which is used to selectively permeate carbon dioxide from a mixed gas containing carbon dioxide.

11. A gas separation membrane module comprising the gas separation membrane according to any one of claims 7 to 10.

12. A gas separation method comprising a step of obtaining a gas in which the concentration of a specific gas is increased from a mixed gas containing two or more gases, using the gas separation membrane according to any one of claims 7 to 10 or the gas separation membrane module according to claim 11.

13. The gas separation method according to claim 12, comprising a step of obtaining a gas having an increased concentration of carbon dioxide from a mixed gas containing carbon dioxide.

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