Composite particles, method for producing composite particles, gas separation membrane, method for producing gas separation membrane, gas separation module, and gas separation device

Abstract

One aspect of the present disclosure provides composite particles comprising: a plurality of coated particles that contain an inorganic crystal and a reduced graphene oxide and have a coating layer provided on the surface of the inorganic crystal; and a sheet containing a reduced graphene oxide, wherein the sheet includes secondary particles in which the plurality of coated particles are aggregated.

Description

Composite particles, method for manufacturing composite particles, gas separation membrane, method for manufacturing a gas separation membrane, gas separation module, and gas separation apparatus.

[0001] This disclosure relates to composite particles, a method for producing composite particles, a gas separation membrane, a method for producing a gas separation membrane, a gas separation module, and a gas separation apparatus.

[0002] Gas separation technology using gas separation membranes is attracting attention as one of the technologies expected to be energy-saving. Graphene oxide can be considered as a single-molecule sheet with a two-dimensional extent, and its use as a separation membrane, taking advantage of the presence of defects within the sheet, is being investigated.

[0003] Zeolite microcrystals possess pores inherent to their crystal structure, and while ideal separation performance can be expected, when a membrane is formed from zeolite microcrystals, voids are formed between the microcrystals, creating gas pathways that bypass the zeolite microcrystals. As a result, the membrane does not exhibit the performance expected from zeolite microcrystals.

[0004] Therefore, efforts are being made to improve the performance of separation membranes by reducing the voids in the separation membrane made of zeolite microcrystals. For example, techniques have been disclosed for preparing coated particles in which a graphene oxide coating layer is formed on zeolite microcrystals using graphene oxide, which is smaller than zeolite microcrystals, and for preparing gas separation membranes using said coated particles (for example, Patent Documents 1-4, Non-Patent Document 1).

[0005] Japanese Patent Publication No. 2018-192378, International Publication No. 2021 / 153515, Japanese Patent Publication No. 2024-006292, Japanese Patent Publication No. 2024-075946

[0006] Radovan Kukobat et. al., “Ultrapermeable 2D-channeled graphene-wrappedzeolite molecular sieving membranes for hydrogen separation”, Science Advances,2022, 8, 3521, p. 1-11

[0007] It has been confirmed that membranes obtained by compression molding an aggregate of coated particles as described above have improved separation performance compared to membranes made of zeolite microcrystals. Even with membranes obtained as described above, voids remain between the coated particles. Although gas separation is expected to occur by passing through the coated particles, the presence of these voids can cause separated gas molecules to remix, and in some cases, gases may pass through the gas separation membrane without passing through the coated particles at all. Depending on the application of the gas separation membrane, higher performance may be required.

[0008] This disclosure aims to provide novel composite particles and methods for producing them. This disclosure also aims to provide gas separation membranes with fewer defects.

[0009] This disclosure provides, for example, the following [1] to

[13] .

[0010] [1] A composite particle comprising an inorganic crystal and a plurality of coated particles having a coating layer provided on the surface of the inorganic crystal, and a sheet containing reduced graphene oxide, wherein the sheet encloses secondary particles formed by the aggregation of the plurality of coated particles. [2] The composite particle according to [1], wherein the mass loss rate at 250 to 950°C when heated to 1000°C under air flow is 6 to 15% by mass. [3] The composite particle according to [1] or [2], wherein the inorganic crystal is a zeolite. [4] A gas separation membrane composed of the composite particle according to any one of [1] to [3]. [5] A method for producing composite particles, comprising: a first step of settling aggregates from a dispersion containing inorganic crystals, graphene oxide, and an ammonium salt; and a second step of reducing the aggregates, wherein the content of graphene oxide in the dispersion is 11% by mass or more, based on the total amount of the inorganic crystals and the graphene oxide; the concentration of the ammonium salt in the dispersion is greater than 0.15 mol / L and less than 0.30 mol / L; and the average particle size of the graphene oxide is larger than the average particle size of the inorganic crystals. [6] A further mixing step of mixing a dispersion A containing graphene oxide and a dispersion B containing inorganic crystals and an ammonium salt to obtain the dispersion, wherein the π-π observed when the ultraviolet-visible absorption spectrum of the dispersion A is measured is ＊[5] The manufacturing method according to [5], wherein the wavelength of the excitation light corresponding to the maximum value of the absorption peak associated with the transition is 229.8 to 231.2 nm. [7] The manufacturing method according to [5] or [6], wherein the average particle size of the graphene oxide is 10 μm or more. [8] The manufacturing method according to any one of [5] to [7], wherein the reduction treatment in the second step is carried out by heating the aggregate to 300°C or higher. [9] The manufacturing method according to any one of [5] to [8], wherein the second step is carried out in an atmosphere containing at least one selected from the group consisting of noble gas, nitrogen gas, and hydrogen gas.

[10] The manufacturing method according to any one of [5] to [9], wherein the ammonium salt contains at least one selected from the group consisting of ammonium chloride, ammonium acetate, ammonium nitrate, ammonium bicarbonate, ammonium carbonate, ammonium oxalate, and ammonium sulfate.

[11] A method for manufacturing a gas separation membrane, comprising: a first step of settling aggregates from a dispersion containing an inorganic crystal, graphene oxide, and an ammonium salt; a second step of reducing the aggregates to obtain a reduced product; and a third step of compression molding the reduced product.

[12] A gas separation module comprising the gas separation membrane described in [4].

[13] A gas separation module comprising a housing and a gas separation membrane housed in the housing and dividing the internal space of the housing into a first space and a second space, wherein the housing is provided with a gas supply port communicating with the first space, a first gas outlet communicating with the first space, and a second gas outlet communicating with the second space, and the gas separation membrane includes the gas separation membrane described in [4].

[14] A gas separation apparatus comprising the gas separation module described in

[12] or

[13] .

[0011] This disclosure provides novel composite particles and methods for producing them. Furthermore, this disclosure provides gas separation membranes with fewer defects.

[0012] Figure 1 is a schematic diagram showing an example of composite particles. Figure 2 is a schematic diagram illustrating a method for manufacturing composite particles. Figure 3 is a scanning electron microscope image showing the appearance of graphene oxide used in (a) Example 1 and (b) Comparative Example 1. Figure 4 is a scanning electron microscope image showing the appearance of composite particles prepared in (a) Example 1, (b) Comparative Example 1, and (c) Comparative Example 2, and the appearance of (d) Reference Example (MFI zeolite). Figure 5 is a graph showing the evaluation results in the examples. Figure 6 is a schematic diagram showing a gas separation membrane composed of conventional coated particles. Figure 7 is a schematic cross-sectional view showing an example of a gas separation module. Figure 8 is a schematic cross-sectional view showing an example of a gas separation module. Figure 9 is a schematic diagram showing an example of a gas separation apparatus.

[0013] Embodiments of this disclosure will be described below, with reference to the drawings as appropriate. However, the following embodiments are illustrative examples for illustrating this disclosure and are not intended to limit this disclosure to the following. In the description, the same reference numerals will be used for elements that are the same or have the same function, and redundant explanations will be omitted as appropriate. Furthermore, positional relationships such as up, down, left, and right will be based on the positional relationships shown in the drawings unless otherwise specified. In addition, the dimensional ratios of each element are not limited to the ratios shown. Also, in this specification, numerical ranges indicated by the symbol "~" include lower and upper limits. That is, numerical ranges indicated by "x~y" mean x or greater and y or less.

[0014] Unless otherwise specified, the materials exemplified herein may be used individually or in combination of two or more. The content of each component in a composition means the total amount of any multiple substances present in the composition, unless otherwise specified, if there are multiple substances corresponding to each component in the composition.

[0015] In this specification, reduced graphene oxide refers to a sheet-like compound obtained by reducing graphene oxide. Reduced graphene oxide can also be considered as a monolayer. Graphene oxide generally has defects in its structure, which are sometimes referred to as windows. The average aperture diameter (arithmetic mean of aperture diameters) of the windows may be, for example, 0.3 to 1.5 nm. Reduced graphene oxide, like graphene oxide, has defects in its structure, which gives it the ability to allow gas molecules and the like to pass through.

[0016] One embodiment of a composite particle comprises a plurality of coated particles and a sheet containing reduced graphene oxide. The coated particles contain an inorganic crystal and reduced graphene oxide, and have a coating layer provided on the surface of the inorganic crystal. In the composite particle, the sheet encloses secondary particles formed by the aggregation of the plurality of coated particles.

[0017] Figure 1 is a schematic diagram showing an example of a composite particle. The composite particle 10 is composed of a plurality of coated particles 6 and a sheet 8 containing reduced graphene oxide. Each coated particle 6 contains an inorganic crystal 2 and a coating layer 4. The coated particle 6 has a void 3 between the inorganic crystal 2 and the coating layer 4. In Figure 1, an example is shown in which the composite particle 10 contains three coated particles 6, but the number of coated particles 6 in the composite particle 10 is not limited to this, and there may be two or more, for example, 2 to 10, 2 to 8, or 2 to 6.

[0018] The coating layer 4 has a laminated structure in which multiple reduced graphene oxides are stacked. In this laminated structure, the reduced graphene oxides are stacked on top of each other with gaps at the molecular level between them, and gas molecules can pass through these gaps. Gas can then pass through the coating layer 4 via the defects in each reduced graphene oxide and these gaps. In other words, the coating layer 4 has paths through which gas molecules can pass. These paths do not need to be through holes penetrating in the thickness direction of the coating layer 4; the positions of the defects in each reduced graphene oxide constituting the coating layer 4 can be offset from each other, as long as gas molecules can pass from the inorganic crystal 2 side of the coating layer 4 to the side opposite to the inorganic crystal 2 side of the coating layer 4.

[0019] The sheet 8 may also be composed of reduced graphene oxide, and may have a laminated structure in which multiple reduced graphene oxide sheets are stacked. In the composite particle 10, the sheet 8 may be enclosed on its outer surface by a single reduced graphene oxide sheet, or it may be enclosed by multiple reduced graphene oxide sheets. Since the sheet 8 is composed of reduced graphene oxide, it has a path through which gas molecules can pass. This allows gas molecules to pass inside and outside the composite particle 10. In the composite particle 10, gaps 9 are formed between multiple coated particles 6, but these gaps 9 are distinguished from the space outside the composite particle 10 by the sheet. The composite particle 10 has a form in which secondary particles formed by the aggregation of multiple coated particles 6 are enclosed by the sheet 8, and the gaps that will occur between the coated particles are covered in advance by the sheet 8. This configuration allows for a more significant reduction in the occurrence of pathways through which gas molecules pass through the separation membrane without passing through reduced graphene oxide and the coating particles (hereinafter also referred to as leak pathways) when preparing a gas separation membrane, compared to the conventional method where the coating particles exist individually. In other words, it is possible to prepare a gas separation membrane with fewer defects.

[0020] The inorganic crystal 2 may or may not have pores. The inorganic crystal 2 having pores may have a porous structure. When the inorganic crystal 2 has a porous structure, gas molecule movement paths are provided inside the inorganic crystal 2, which can increase the gas permeability of the composite particles 10 and the gas separation membrane prepared using them. An example of the inorganic crystal 2 having a porous structure is a zeolite. An example of the inorganic crystal 2 not having pores is apatite.

[0021] When the inorganic crystal 2 is a zeolite, the crystal surface of the zeolite is relatively smooth, making it easier to adjust the void 3 formed between it and the coating layer 4, and thus improving the gas selectivity (e.g., nitrogen selectivity) of the resulting composite particles 10.

[0022] The zeolite may be either natural or synthetic. Synthetic zeolites may be, for example, high-silica zeolites and molecular sieves. More specifically, examples of synthetic zeolites include MFI zeolite, BEA zeolite, FAU zeolite, AEI zeolite, and CHA zeolite.

[0023] The upper limit of the average particle diameter of the inorganic crystal 2 may be, for example, 10 μm or less, 7 μm or less, or 5 μm or less. By having the upper limit of the average particle diameter of the inorganic crystal 2 within the above range, composite particles can be obtained in which the proportion of inorganic crystal portions not encapsulated by reduced graphene oxide is further reduced. By forming a gas separation membrane using such composite particles, the leakage path of gas generated in the gas separation membrane can be further reduced, and the separation performance of the resulting gas separation membrane can be further improved. The lower limit of the average particle diameter of the inorganic crystal 2 may be, for example, 0.01 μm or more, 0.10 μm or more, or 0.25 μm or more. By having the upper limit of the average particle diameter of the inorganic crystal 2 within the above range, the proportion of inorganic crystals in the composite particles can be increased, and the gas permeability performance of the composite particles themselves can be further improved. By forming a gas separation membrane using such composite particles, high gas permeability performance in the gas separation membrane can be expected.

[0024] In this specification, the average particle size of inorganic crystals refers to the value measured by the following method. First, the composite particles to be measured are heated to 1000°C using an airflow heating system at a heating rate of 3°C / min or less, and the resulting inorganic crystals are used as the measurement sample. The average particle size can also be determined by measuring the dispersion sample obtained by dispersing the obtained measurement sample in water using a dynamic light scattering method. The dispersion sample may be adjusted to a dilute concentration such that incident light reaches the dispersed particles and the light scattered by the dispersed particles reaches the detector sufficiently. A light scattering device can be used for dynamic light scattering measurement. For example, a "Zetasizer nano ZS" manufactured by Malvern Instruments can be used as a light scattering device. When preparing composite particles oneself, the inorganic crystals used as raw materials can be used as the measurement sample, and the above-mentioned dispersion sample can be prepared and measured.

[0025] The composite particles 10 may be adjusted so that the mass loss rate between 250 and 950°C when heated to 1000°C under air flow is within a predetermined range. The above mass loss rate is mainly due to the decomposition of reduced graphene oxide and can be considered an indicator of the reduced graphene oxide content. The lower limit of the above mass loss rate may be, for example, 6% by mass or more, 7% by mass or more, 8% by mass or more, or 9% by mass or more. The fact that the lower limit of the above mass loss rate is within the above range means that the proportion of the coating layer in the coated particles and the proportion of the sheet in the composite particles are large, and by using composite particles in which the proportion of reduced graphene oxide is adjusted in this way, the occurrence of gas leakage paths in the resulting gas separation membrane can be further reduced. The upper limit of the above mass loss rate in the composite particles 10 may be, for example, 15% by mass or less, 13% by mass or less, 12% by mass or less, or 11% by mass or less. The fact that the upper limit of the above-mentioned mass loss rate is within the above range corresponds to ensuring that the proportion of the coating layer in the coated particles and the proportion of the sheet in the composite particles are not too large. By using composite particles in which the proportion of reduced graphene oxide is adjusted in this way, the decrease in the gas permeability of the resulting gas separation membrane can be further suppressed, and the high-speed separation of gases can be improved. When the composite particles 10 are heated to 1000°C under air flow, the mass loss rate at 250 to 950°C can be adjusted within the above range, for example, 6 to 15% by mass, or 7 to 15% by mass.

[0026] In this specification, the mass loss rate refers to the value measured by thermogravimetric analysis (TG). Specifically, first, the composite particles to be measured are heated under an argon flow (flux: 0.8 m / min) at a rate of 1°C / min to 350°C, and then heated at 350°C for 30 minutes to obtain the powder to be measured. The mass loss rate between 250 and 950°C is determined by thermogravimetric analysis of this powder, which is heated under an air flow at a rate of 3°C / min to 1000°C.

[0027] The composite particles described above are useful as constituent materials for gas separation membranes. One embodiment of a gas separation membrane is composed of the composite particles described above. The gas separation membrane may be a compressed molded body of an aggregate of the composite particles described above.

[0028] The above gas separation membrane can be used as a separation membrane having nitrogen selectivity. In this specification, having nitrogen selectivity means that the ratio of the nitrogen permeation coefficient (unit: mol / (m ２ ·s·Pa)) to the oxygen permeation coefficient (unit: mol / (m ２ ·s·Pa)) exceeds 1.1.

[0029] The above gas separation membrane has an improved gas permeation rate by about 1000 times compared to conventional polymer gas separation membranes. The reason for such a difference is not necessarily clear. In the case of polymer gas separation membranes, separation is mainly achieved by utilizing gas dissolution and diffusion, so it is necessary to dissolve the raw material gas in the gas separation membrane. Therefore, a considerably high pressure needs to be applied. In contrast, the gas separation membrane according to the present disclosure basically does not need to dissolve the raw material gas in other substances, and separates the gas by utilizing the molecular sieve effect based on the difference in the size of gas molecules and the difference in adsorption interaction on the solid surface of the separation membrane material in the state of the gas. Therefore, it is not always necessary to supply the raw material gas to the gas separation membrane at a high pressure. For this reason, it can be used at a low pressure of about several tens of kPa (for example, a pressure of 20 kPa or less). Since it can exhibit the above-mentioned nitrogen selectivity even at this multiple, it can be used safely and is useful. Being able to be used at a low pressure enables miniaturization of gas separation modules and gas separation devices incorporating the gas separation membrane, and is presumed to have excellent versatility.

[0030] One embodiment of the method for producing composite particles includes a first step of precipitating aggregates from a dispersion liquid containing inorganic crystals, graphene oxide, and an ammonium salt, and a second step of subjecting the above aggregates to a reduction treatment to obtain a reduction-treated product.

[0031] The inorganic crystals used in the above production method can be those exemplified in the description of the above composite particles.

[0032] The graphene oxide used in the above manufacturing method may be separately prepared from graphite. That is, the above manufacturing method may further include a step of preparing the graphene oxide by oxidizing graphite. In this step, as graphite is oxidized, the graphene constituting graphite becomes graphene oxide, thereby exfoliating graphene oxide from graphite and obtaining graphene oxide. The oxidation of graphite may be carried out in an aqueous solution. In this case, the above graphene oxide can be obtained in a state dispersed in the aqueous solution. Note that an oxidizing agent may be used for the oxidation of graphite. Examples of the oxidizing agent include potassium permanganate, sodium permanganate, potassium ferrate, potassium chlorate, and sodium chlorate. Further, instead of the preparation method using an oxidizing agent, the obtained graphene oxide may be used by electrochemical oxidation and exfoliation.

[0033] In this case, as the graphite used as the raw material, those having an average particle size of 0.2 mm or more are used. The lower limit value of the average particle size of graphite may be, for example, 0.5 mm or more, 1.0 mm or more, or 2.0 mm or more. By using graphite having the lower limit value of the average particle size within the above range, it becomes easier to prepare graphene oxide having a relatively large average particle size. The upper limit value of the average particle size of graphite may be, for example, 10.0 mm or less, 5.0 mm or less, or 3.0 mm or less. By using graphite having the upper limit value of the average particle size within the above range, the oxidation reaction can proceed more sufficiently, and thereby the exfoliation of graphene oxide can proceed more sufficiently. Further, by preparing graphene oxide in this way, it becomes easier to prepare graphene oxide.

[0034] The average particle size of graphite in this specification means a value measured in accordance with the method described in dry sieving described in JIS Z 8815-1994 "General Rules for Sieve Analysis Test Methods".

[0035] The graphite may be of excellent crystallinity and may have a flaky shape. For example, graphite from Madagascar can be used.

[0036] The step of preparing graphene oxide by oxidizing graphite may be carried out, for example, by a modified Hammers process as described later. The step of preparing graphene oxide may be a method of preparing a dispersion of graphene oxide by heating an aqueous solution containing graphite, an inorganic acid, and potassium permanganate at a temperature above 35°C to oxidize the graphite.

[0037] The inorganic acid may include, for example, at least one acid selected from the group consisting of phosphoric acid, sulfuric acid, hydrochloric acid, and nitric acid. From the viewpoint of reducing the inhibition of the oxidation reaction of graphite by the oxidizing agent, the inorganic acid preferably includes at least one acid selected from the group consisting of phosphoric acid and sulfuric acid, and more preferably includes phosphoric acid and sulfuric acid. The inorganic acid may be, for example, a mixture of 98% by mass sulfuric acid and phosphoric acid in a volume ratio of 90:10.

[0038] The heating temperature of the aqueous solution is 35°C or higher, but may be, for example, 35-50°C, 35-40°C, or 35-38°C. The heating time of the aqueous solution may be, for example, 2-10 hours, 2-5 hours, or 4-5 hours.

[0039] The heating treatment of the aqueous solution for oxidizing graphite may be carried out, for example, under stirring. By controlling the temperature of the aqueous solution and stirring it, the graphene constituting the graphite is oxidized, and functional groups having a predetermined amount of oxygen are introduced to the surface of the graphene, making it easier to disperse graphene oxide in the aqueous solution. More specifically, the introduction of the functional groups increases the distance between the layers of graphene constituting the graphite, and water molecules and other dispersion media penetrate between the layers, thereby further promoting the detachment and dispersion of graphene oxide in the solution.

[0040] The above process for preparing graphene oxide may further include adding hydrogen peroxide to the solution after oxidizing graphite to stop the oxidation reaction, and acid washing. The hydrogen peroxide concentration may be, for example, 15% by mass or more. Acid washing can reduce the manganese content in the dispersion.

[0041] The above manufacturing method may further include a mixing step of mixing dispersion A containing graphene oxide with dispersion B containing inorganic crystals and an ammonium salt to obtain the above-mentioned dispersion. In this case, when the ultraviolet-visible absorption spectrum of dispersion A is measured, the π-π observed is... ＊ The excitation light wavelength corresponding to the maximum value of the absorption peak associated with the transition may be 229.8–231.2 nm. As described above, graphite with a relatively large average particle size is used as the raw material, π–π ＊ By using dispersion A, in which the wavelength of excitation light corresponding to the maximum value of the absorption peak associated with the transition falls within the above-mentioned range, the graphene oxide in dispersion A can be given a more sufficient surface charge, and the flexibility of the graphene oxide can be further improved. Therefore, when mixed with dispersion B, the inclusion of inorganic crystals can be made easier. Furthermore, in dispersion A as described above, the graphene oxide becomes in a state where it has sufficient defects on the graphene sheet (for example, nanowindows) due to gas permeation, making it more suitable for the formation of coated particles and composite particles and for improving the separation performance of gas molecules. The π-π observed when the ultraviolet-visible absorption spectrum of dispersion A is measured ＊ The wavelength of the excitation light corresponding to the maximum value of the absorption peak associated with the transition is, for example, preferably 230.2 to 230.8 nm.

[0042] In this specification, the above π-π ＊The wavelength of excitation light corresponding to the maximum absorption peak associated with the transition is the value measured as follows: First, dispersion A is diluted to prepare a graphene oxide dispersion with a concentration of 0.1 mass%, and then a measurement sample is obtained by further diluting it 30 times with distilled water. The absorbance of this measurement sample is measured in 0.1 nm steps in the range of 200 to 600 nm using an ultraviolet-visible near-infrared spectrophotometer, thereby obtaining the π-π value. ＊ The wavelength of the excitation light corresponding to the maximum value of the absorption peak associated with the transition is determined. The concentration of the measurement sample should be adjusted so that the absorption intensity of the peak top is approximately 1 when measured with an ultraviolet-visible-near-infrared spectrophotometer. For example, a spectrophotometer such as the "V670" manufactured by JASCO Corporation can be used.

[0043] In the above manufacturing method, the average particle size of the graphene oxide is larger than the average particle diameter of the inorganic crystal. The average particle size of the graphene oxide may be, for example, 1.1 times or more, 2 times or more, 3 times or more, 4 times or more, 5 times or more, 6 times or more, 8 times or more, or 10 times or more, relative to the average particle diameter of the inorganic crystal. By using graphene oxide with an average particle size within the above range, it becomes easier to coat the inorganic crystal and to enclose secondary particles composed of multiple coated particles. The average particle size of the graphene oxide may be, for example, 10,000 times or less, 700 times or less, 500 times or less, 200 times or less, 100 times or less, 80 times or less, 60 times or less, 30 times or less, 15 times or less, or 12 times or less, relative to the average particle diameter of the inorganic crystal. By using graphene oxide with an average particle size within the above range, it is possible to prevent the structure from simply having inorganic crystals resting on the surface of the graphene oxide, making it easier to prepare the inorganic crystals encapsulated by the graphene oxide. In other words, it becomes easier to encapsulate primary or secondary particles, or coating particles, of the inorganic crystal with one or more graphene oxide sheets, and it is possible to obtain composite particles with an encapsulation structure that further suppresses the generation of leakage pathways.

[0044] The lower limit of the average particle size of the graphene oxide described above may be, for example, 10 μm or more, 14 μm or more, or 20 μm or more. By using graphene oxide such that the lower limit of the average particle size falls within the above range, coating of inorganic crystals and encapsulation of secondary particles composed of multiple coated particles becomes easier. The upper limit of the average particle size of the graphene oxide described above may be, for example, 100 μm or less, 70 μm or less, or 50 μm or less.

[0045] In this specification, the average particle size of graphene oxide refers to the value measured by the following method. Specifically, a dispersion of graphene oxide is prepared using water as a medium to a concentration of 0.01% by mass. 0.05 mL of this dispersion is dropped onto a silicon substrate, and a sample is prepared by drying at 25°C for 1 hour. Scanning electron microscope images are acquired using this sample as the measurement target. The maximum diameter (long axis) and minimum diameter (short axis) of each graphene oxide in the acquired image are determined, and their arithmetic mean is taken as the particle size of the graphene oxide. The particle sizes are similarly determined for 50 or more graphene oxides, and their arithmetic mean is taken as the average particle size of the graphene oxide.

[0046] The graphene oxide content in the above dispersion is 11% by mass or more, based on the total amount of the inorganic crystal and the graphene oxide. By adjusting the graphene oxide content in this way, it becomes possible to coat single particles of the inorganic crystal and to enclose secondary particles of the coated particles.

[0047] The amount of graphene oxide in the dispersion can be further adjusted according to the amount of coating layer desired for the coated particles or the sheet (such as the thickness of the coating layer and sheet, and the number of layers). The lower limit of the graphene oxide content may be, for example, 12% by mass or more, 13% by mass or more, or 14% by mass or more, based on the total amount of the inorganic crystal and the graphene oxide. By setting the lower limit of the graphene oxide content within the above range, the thickness of the coating layer in the coated particles or the sheet that forms the outer layer of the composite particles can be increased. In addition, the amount of graphene oxide that plays a role in binding multiple inorganic particles and coated particles together can be increased. Since the layer containing reduced graphene oxide is more flexible than the inorganic crystal, in the above case, when the aggregate of composite particles obtained is compressed and molded to prepare a gas separation membrane, the contact area between the composite particles can be increased, and defects in the membrane can be further reduced. A larger thickness of the coating layer in the coated particles or the sheet that forms the outer layer of the composite particles also corresponds to increasing the number of layers of reduced graphene oxide in the coating layer of the obtained coated particles or in the sheet. Increasing the number of layers in this way corresponds to lengthening the pathway for gas molecules to penetrate the interior of the coated particles or composite particles. Even if there are somewhat large defects in each layer of reduced graphene oxide, as the number of layers increases and the pathway for gas molecules to penetrate lengthens, gas molecules must pass through multiple layers of reduced graphene oxide to enter the interior of the coated particles. This makes the overall pathway for gas molecules to pass through narrower on average, thereby improving the gas separation performance. The upper limit of the graphene oxide content may be, for example, 19% by mass or less, 18% by mass or less, 17% by mass or less, or 16% by mass or less, based on the total amount of the inorganic crystal and the graphene oxide. By setting the upper limit of the graphene oxide content within the above range, excessive inclusion by graphene oxide can be prevented, and a gas separation membrane with high gas permeability can be prepared.

[0048] The content of the graphene oxide described above may be determined using the following formula for the apparent number of inclusion layers. The surface area of the graphene oxide used in the formula is a value calculated using the amount of graphene oxide compounded (g), the density of graphene (2.05 g / cm ３ ), and the thickness of the graphene monolayer (0.35 nm). Also, the external surface area of the inorganic crystal is a value calculated from the adsorption isotherm of nitrogen gas adsorption. [Apparent number of inclusion layers] = [Surface area of graphene oxide] / [External surface area of inorganic crystal] [Surface area of graphene oxide] = [Amount of graphene oxide compounded (g)] / {[Density of graphene (g / cm ３ )] × [Thickness of graphene monolayer (cm)]}

[0049] The ammonium salt used in the above production method plays an important role in forming a coating layer on the inorganic crystal. Taking FIG. 2 as an example, the process of forming coated particles in the above production method will be described. Since graphene oxide 4a has polar groups 4b (for example, hydroxyl groups, ether groups, carboxyl groups, etc.) on its surface, it becomes negatively charged when dispersed in water. When the inorganic crystal 2 is also negatively charged in an aqueous solution, it is difficult to coat it due to electrostatic repulsion with the negatively charged and dispersed graphene oxide 4a. Here, by interposing an ammonium salt, among the anions 30 and cations 40 generated by the ionization of the ammonium salt, the negative charge on the surface of the graphene oxide 4a can be weakened by the cations 40, and the thickness of the electric double layer can be reduced (it can also be said that the distance (Debye length) affected by the electric field when the graphene oxide 4a is regarded as a charged particle is shortened). By such an action, the graphene oxide 4a can be brought close to and coated on the surface of the inorganic crystal 2, and a precursor of the coated particle can be formed.

[0050] The ammonium salt that acts as a modifier for the electrical double layer as described above may be removable by heating in a later step (for example, heating at 300°C or higher). By sufficiently removing the ammonium salt, the remaining cations or anions in the voids formed between the inorganic crystal and the coating layer can be sufficiently suppressed, and a gas passage path can be more reliably established, thereby further improving the gas permeability and gas separation performance of the resulting composite particles. Since the ammonium salt is easily removed by heating, it may contain at least one selected from the group consisting of ammonium chloride, ammonium acetate, ammonium nitrate, ammonium bicarbonate, ammonium carbonate, ammonium oxalate, and ammonium sulfate. From the viewpoint of suppressing the deterioration of the composite particles obtained by making the dispersion neutral or basic, it is more preferable that it be at least one of ammonium chloride and ammonium oxalate.

[0051] In the above dispersion, the concentration of ammonium salt is greater than 0.15 mol / L and less than 0.30 mol / L, but it may be adjusted from the viewpoint of improving the productivity of composite particles. The lower limit of the concentration of ammonium salt in the above dispersion may be, for example, 0.16 mol / L or more, 0.18 mol / L or more, or 0.20 mol / L or more. By setting the lower limit of the ammonium salt concentration within the above range, the distance between graphene oxide and inorganic crystal (Debye length) can be shortened, making it easier to form a coating layer on the surface of the inorganic crystal. Also, by setting the lower limit of the ammonium salt concentration within the above range, electrostatic repulsion between graphene oxides is suppressed, making it easier to form a multilayer structure of graphene oxide on the surface of the inorganic crystal, and further improving the gas separation performance of the resulting composite particles. Furthermore, by setting the lower limit of the ammonium salt concentration within the above range, the gap formed between the inorganic crystal and graphene oxide in the coated particles can be made smaller, enabling the preparation of composite particles that can separate even when the size difference of gas molecules in the gas to be separated is small (for example, oxygen and nitrogen). The upper limit of the ammonium chloride concentration in the dispersion may be, for example, 0.29 mol / L or less, 0.27 mol / L or less, or 0.25 mol / L or less. When the aggregates are later allowed to settle, if the aggregation of the coated particles progresses too much, the density of the settled portion of the aggregates becomes high, and when separated from the dispersion medium and dried, the resulting powder deposition structure tends to be dense. If the powder deposition structure is dense, the molding pressure during pressure molding cannot be fully utilized, and additional measures may be required to create a dense film structure and suppress gas leakage paths. In contrast, by setting the upper limit of the ammonium chloride concentration in the dispersion within the above range, the aggregation of the coated particles can be made more appropriate. Furthermore, by setting the upper limit of the ammonium chloride concentration in the dispersion within the above range, coated particles with an even more uniform inclusion structure can be obtained. In addition, by setting the upper limit of the ammonium salt concentration within the above range, it is possible to more effectively suppress the excessive narrowing of the gap formed between the inorganic crystal and graphene oxide in the coated particles.The concentration of the ammonium salt in the above dispersion may be adjusted within the range described above, for example, 0.16 to 0.29 mol / L.

[0052] The first step is to allow the aggregates to settle from the dispersion, but the settling of the aggregates may be done by letting the dispersion stand. The time for letting the dispersion stand may be, for example, 24 hours or more, 48 hours or more, 72 hours or more, 96 hours or more, or 120 hours or more. By setting the standing time within the above range, the coating of the inorganic crystals with graphene oxide and the inclusion of the coated particles can be made more sufficient. The time for letting the dispersion stand is not particularly limited, but for example, it may be 240 hours or less, 216 hours or less, 192 hours or less, 168 hours or less, or 144 hours or less.

[0053] In the first step, the aggregates may be separated from the dispersion medium. The dispersion medium can be removed, for example, by decantation. Furthermore, to ensure more complete removal of the dispersion medium from the aggregates, the dispersion medium can also be removed by methods such as heat drying and freeze-drying.

[0054] The second step is a reduction process in which the aggregates obtained in the first step are subjected to a reduction treatment. In this step, the polar groups of graphene oxide are removed and reduced, thereby reducing the electrostatic repulsion between the inorganic crystals and graphene oxide, and between the graphene oxides themselves, and making the voids between the inorganic crystals and reduced graphene oxide in the resulting composite particles smaller. Furthermore, by reducing the ammonium salt content in the aggregates in this step, the decrease in the gas permeability of the gas separation membrane due to the residual anions and cations derived from the ammonium salt in the aforementioned voids can be suppressed. The voids between the inorganic crystals and reduced graphene oxide formed in this way can be, for example, 0.4 nm or less. The reduction treatment may be either a heat treatment (dry treatment) or a wet treatment, but a heat treatment is preferred. In the case of a heat reduction treatment, the aggregates obtained in the first step may be dried before the heat treatment, and the resulting dried powder may be heat-treated to perform the reduction.

[0055] When the reduction treatment in the second step is carried out by heat treatment, the second step may be carried out in an atmosphere containing at least one of a noble gas, nitrogen gas, and hydrogen gas, or in a noble gas atmosphere, from the viewpoint of further promoting the reduction of graphene oxide. Examples of noble gases include helium (He) and argon (Ar).

[0056] When the reduction treatment in the second step is performed by heat treatment, the lower limit of the heat treatment temperature may be, for example, 300°C or higher, or 350°C or higher. By setting the lower limit of the heat treatment temperature within the above range, the removal of ammonium salts and the reduction of graphene oxide can be made more sufficient, and the gas separation performance of the gas separation membrane using the resulting composite particles can be further improved. The upper limit of the heat treatment temperature in the second step may be, for example, 800°C or lower, or 700°C or lower, under an inert atmosphere. By setting the upper limit of the heat treatment temperature within the above range, the collapse of the inclusion structure of the composite particles can be more sufficiently prevented, and the content of reduced graphene oxide on the coated particles can be sufficiently maintained.

[0057] When the second step is carried out by a wet process, the aggregates obtained in the first step may be reduced by adding a reducing agent to the dispersion containing the aggregates. Either an organic or inorganic reducing agent may be used. Examples of organic reducing agents include hydrazine, formic acid, oxalic acid, and 3,4,5-trihydroxybenzoic acid (gallic acid). Examples of inorganic reducing agents include sodium borohydride and diisobutylaluminum hydride.

[0058] The above-described method for producing composite particles may further include other steps, for example, a third step of compression molding the reduced product.

[0059] One embodiment of a method for manufacturing a gas separation membrane includes a first step of settling aggregates from a dispersion containing an inorganic crystal, graphene oxide, and an ammonium salt; a second step of reducing the aggregates to obtain a reduced product; and a third step of compression molding the reduced product.

[0060] The lower limit of the molding pressure in compression molding of the above-mentioned heat-treated material may be, for example, 100 MPa or more, 300 MPa or more, 500 MPa, 700 MPa or more, or 900 MPa or more. The upper limit of the molding pressure may be 1500 MPa or less, 1200 MPa or less, or 1000 MPa or less.

[0061] Figure 6 shows a schematic diagram of a conventional gas separation membrane composed of coated particles. Figure 6(a) is a schematic cross-sectional view of the gas separation membrane, and Figure 6(b) is an enlarged view of region R in (a). The gas separation membrane is formed by compression molding an aggregate of multiple coated particles 6', and has a structure in which adjacent coated particles 6' are bound together by thin layers of reduced graphene oxide on the surface of the coated particles 6'. The coated particles 6' consist of an inorganic crystal 2' and a layer 4' made of reduced graphene oxide provided on the surface of the inorganic crystal 2', and gas molecules can pass between the inside and outside of the coated particles 6' due to defects in the reduced graphene oxide. Furthermore, when gas molecules pass through the gap between the inorganic crystal 2' and the layer 4' made of reduced graphene oxide, if there is a large difference in the size of the gas molecules, separation between gas molecules is possible via a path through the inside of the coated particles 6'. Figure 6(b) shows that gas molecule A (e.g., nitrogen) and gas molecule B (e.g., methane) are separated. However, if the entry pathway for gas molecules from layer 4' made of reduced graphene oxide to the coated particles 6' is wide, or if the uniformity of the gap between the inorganic crystal 2' and layer 4' made of reduced graphene oxide is low, the expected separation performance may not be obtained. Furthermore, when the sizes of gas molecules A and B are close (e.g., nitrogen and oxygen), it tends to be difficult to obtain the expected separation performance. Furthermore, while gas molecules A and B are expected to separate when they pass through the gap between the inorganic crystal 2' and the layer 4' made of reduced graphene oxide, as shown in Figure 6(b), even when compression molding is performed to press the coated particles 6' together, a certain amount of void 20 remains. As a result, gas molecules separated by passing through the gap between the inorganic crystal 2' and the layer 4' made of reduced graphene oxide may mix again, or they may pass through the gas separation membrane without passing through the gap at all and remain unseparated.

[0062] In contrast, the composite particles according to this disclosure have a novel structure comprising a sheet containing reduced graphene oxide, which encloses secondary particles formed by the aggregation of multiple coated particles. By having such a structure, the occurrence of defects (for example, the void 20 in Figure 6(a) that does not contribute to gas separation) in the gas separation membrane prepared using the composite particles can be suppressed, thereby further improving the gas separation performance.

[0063] The gas separation membrane described above is useful as a component of a gas separation module or the like. One embodiment of a gas separation module includes the gas separation membrane described above. The gas separation module includes, for example, a housing and a gas separation membrane housed in the housing that divides the internal space of the housing into a first space and a second space. The housing is provided with a gas supply port communicating with the first space, a first gas outlet communicating with the first space, and a second gas outlet communicating with the second space. The gas separation membrane includes the gas separation membrane described above according to this disclosure.

[0064] Figure 7 is a schematic cross-sectional view showing an example of a gas separation module. The gas separation module 62 is an example in which the gas separation membrane is arranged in a planar manner, and is also called a flat membrane module. The gas separation module 62 comprises a housing 50, a gas separation membrane 54, and a support 56 that supports the gas separation membrane 54. In the gas separation module 62, the gas separation membrane 54 and the support 56 are fixed to the housing 50 by a sealing material. The support 56 is a substrate that can pass gas through, and is made of, for example, a porous material. Depending on the mechanical strength of the gas separation membrane 54, the support 56 may be omitted. Also, if the gas separation membrane 54 can be fixed inside the housing 50, the sealing material 58 may also be omitted. In Figure 7, the internal space 52 of the housing 50 is divided into a first space 52a and a second space 52b by the gas separation membrane 54. The housing 50 may be made of, for example, two separable members. In this case, for example, the ends of the gas separation membrane 54 may be sandwiched between the two members and fixed.

[0065] In the example shown in Figure 7, the housing 50 supplies raw material gas G1 (e.g., compressed air) into the first space 52a from a gas supply port d1a provided in the housing 50. The gas separation membrane 54 separates the gas into separated gas G3 (e.g., nitrogen-enriched gas) that passes through the gas separation membrane 54 and is supplied to the second space 52b, and residual gas G2 (e.g., oxygen-enriched gas) that remains in the first space 52a without passing through the gas separation membrane 54. The residual gas G2 remaining in the first space 52a is taken out of the gas separation module 62 from the first gas outlet d1b provided in the housing 50, and the separated gas G3 that has passed through the gas separation membrane 54 is taken out of the gas separation module 62 from the second gas outlet d2 provided in the housing 50.

[0066] Figure 8 is a schematic cross-sectional view showing an example of a gas separation module. The gas separation module 64 is an example of a module that separates gases by forming a gas separation membrane in a cylindrical shape and utilizing the space between the inside and outside of the cylinder. The basic configuration is the same as that of the gas separation module 62 shown in Figure 7. The gas separation module 64 differs from the gas separation module 62 in that the housing 50 has a cylindrical shape, and a gas separation membrane 54, also formed in a cylindrical shape, is arranged within the internal space of the cylindrical housing. Figure 8 shows an example in which there is one molded tube of a cylindrically formed gas separation membrane 54 inside the housing 50, but multiple molded tubes may be arranged inside the housing 50. In this case, multiple gas supply ports d1a and multiple first gas outlet ports d1b may be provided so as to communicate with each of the spaces inside each molded tube (first space 52a).

[0067] Since no anisotropy is specifically provided in the gas permeability performance of the gas separation membrane according to this disclosure, it is sufficient that a first gas outlet d1b communicating with the space on the raw material gas supply side (i.e., the space through which the gas supply port communicates) is provided, and a second gas outlet d2 communicating with the space on the other side (i.e., the space through which the gas passing through the gas separation membrane is rejected) is provided. Furthermore, by making the amount of gas removed from the first gas outlet d1b to the outside of the housing less than the amount of gas supplied from the gas supply port d1a, pressure can be applied to the space on the raw material gas supply side of the gas separation membrane. In addition, the gas separation performance of the gas separation membrane can be adjusted by adjusting this pressure. Depending on the composition of the target separation gas, for example, it is possible to adjust by increasing the pressure to lower the separation coefficient of the separation membrane.Therefore, although it depends on the strength of the gas separation membrane, it is desirable to use the gas separation module under pressurized conditions.

[0068] One embodiment of the gas separation apparatus includes the gas separation module described above.

[0069] Figure 9 shows a schematic diagram illustrating an example of a gas separation apparatus. The gas separation apparatus 100 shown in Figure 7 comprises a gas separation module 60, a pump 70 that supplies gas to the gas separation module 60, and a back pressure control valve 80 for adjusting the pressure inside the gas separation module 60. The back pressure control valve 80 has the function of adjusting the amount of gas taken out of the gas separation module 60 from the first space in order to pressurize the first space side of the gas separation module 60, that is, the space on the side where the raw material gas G1 is supplied to the gas separation membrane (first space). In the gas separation apparatus 100 shown in Figure 9, a pressure gauge P for checking the supply pressure of the raw material gas is placed between the pump 70 and the gas separation module 60.

[0070] In the gas separation device 100, the separated gas G3 that has permeated the gas separation membrane and the residual gas G2 that remains in the first space without permeating the gas separation membrane can be separated and recovered. For example, when air is supplied as the raw material gas G1 to the gas separation device 100 using a nitrogen-selective gas separation membrane, the nitrogen in the air permeates the gas separation membrane and flows into the second space, so nitrogen-enriched gas can be obtained as the separated gas G3, and air with reduced nitrogen content (i.e., oxygen-enriched gas) can be obtained as the residual gas G2. The oxygen concentration and nitrogen concentration in the residual gas G2 and separated gas G3, respectively, can be controlled, for example, by adjusting the pressure in the raw material supply space in the gas separation module.

[0071] Although several embodiments have been described above, this disclosure is not limited in any way to the embodiments described above. Furthermore, the descriptions of the embodiments described above are applicable to each other.

[0072] The present disclosure will be described in more detail below with reference to examples, comparative examples, and reference examples. However, the present disclosure is not limited to the following examples.

[0073] (Example 1) [Preparation of graphene oxide] 2 g of graphite, 89 mL of inorganic acid, and 10 g of potassium permanganate were measured into a container and stirred for 4 hours at a temperature of 38°C and a stirring speed of 250 rpm. Then, 300 mL of water (H) was added to the aqueous solution. ２ O) was added and thoroughly stirred. In the process described above, the graphene layer of the graphite was oxidized to generate surface functional groups, and the oxidized graphene layer was further peeled off from the graphite to prepare a solution containing layered graphene oxide. The graphite used was from Madagascar with an average particle size of 0.3 mm. The graphene constituting the graphite has a flaky shape. The inorganic acid was sulfuric acid (H ２ SO ４ Concentration: 98% by mass, and phosphoric acid (H ３ PO ４ An aqueous solution with a concentration of 85% by mass was used, adjusted to a volume ratio of 90:10.

[0074] Then, 40 mL of hydrogen peroxide solution (H) was added to the above aqueous solution. ２ O ２ The oxidation reaction was stopped by adding 15% by mass. The supernatant was removed from the aqueous solution after the reaction was stopped by centrifugation and decantation. After removing the supernatant, the solution was washed five times with hydrochloric acid (HCl aqueous solution, concentration: 5% by mass), and then five times with pure water to remove sulfuric acid, manganese, etc., and to obtain a dispersion (dispersion B) in which graphene oxide was dispersed. The π-π in the ultraviolet-visible absorption spectrum of the dispersion was measured. ＊ The excitation light wavelength corresponding to the maximum value of the absorption peak associated with the transition was in the range of 229.8–231.2 nm. The average particle size of the obtained graphene oxide was 25 μm. Figure 3(a) shows a scanning electron microscope image of graphene oxide placed on a Si substrate.

[0075] [Preparation of Composite Particles] Composite particles were prepared under the conditions shown in Table 1. Specifically, first, MFI zeolite (manufactured by Tosoh Corporation, aluminosilicate high silica zeolite, ZSM-5 type, "HSZ-822HOA" grade, crystal size: 0.1 × 0.5 μm, average particle diameter: 5 μm, average pore diameter: 0.58 nm, SiO₂) was placed in a plastic vial. ２ / Al ２ O ３ 50 mg of the mixture (molar ratio: 24) was measured out, 4 mL of distilled water was added, and ultrasonic irradiation (ultrasonic amplitude: 34.5 mm) was performed for 1 minute to obtain dispersion C. Dispersion C was transferred to a 100 mL vial, and 50 mL of ammonium chloride aqueous solution (concentration: 0.25 mol / L) was added to obtain dispersion B. Next, 9.5 mL of the graphene oxide dispersion A (concentration 0.1 mass%) prepared as described above was added and shaken well. At this time, the mixture was shaken so that the dispersion inside the vial strongly collided with the lid and bottom of the vial. After visually confirming that dispersions A and B were mixed, the dispersions were left to stand for 5 days in a light-shielded environment at room temperature (25°C). A precipitate was formed during this process. The proportions of dispersions A and B described above were adjusted so that the amount of graphene oxide was 16 mass%, based on the total amount of MFI zeolite and graphene oxide.

[0076] Next, the supernatant was removed from the vial by decantation, and the precipitate was flash-frozen with liquid nitrogen and then freeze-dried to obtain a spongy aggregate. The aggregate was then heat-treated at 350°C for 30 minutes under an argon stream (flow rate: 100 cc / min) to reduce graphene oxide and convert it to reduced graphene oxide, and to remove any remaining ammonium chloride from the aggregate. In this way, a black spongy solid was obtained. A scanning electron microscope image of the black spongy aggregate is shown in Figure 4(a).

[0077] (Example 2) In preparing the composite particles, instead of freeze-drying to obtain a sponge-like aggregate from the dispersion after standing for 5 days, the dispersion was spread onto a glass petri dish and heated and dried in a 100°C oven for 2 hours to obtain an aggregate of flake-like particles. The composite particles were prepared in the same manner as in Example 1.

[0078] (Example 3) In preparing the composite particles, instead of freeze-drying to obtain a sponge-like aggregate from the dispersion after standing for 5 days, the dispersion was spread onto a glass petri dish and heated and dried in a 100°C oven for 2 hours to obtain an aggregate of flake-like particles, and the subsequent heat treatment of the aggregate was changed from under an argon atmosphere to under a nitrogen atmosphere. The composite particles were prepared in the same manner as in Example 1.

[0079] (Example 4) In preparing the composite particles, instead of freeze-drying to obtain a sponge-like aggregate from the dispersion after standing for 5 days, the dispersion was spread onto a glass petri dish and heated and dried in a 100°C dryer for 2 hours to obtain an aggregate of flake-like particles, and the temperature of the subsequent heat treatment of the aggregate was changed from 350°C to 500°C. The composite particles were prepared in the same manner as in Example 1.

[0080] (Example 5) In preparing the composite particles, instead of freeze-drying to obtain a sponge-like aggregate from the dispersion after standing for 5 days, the dispersion was spread onto a glass petri dish and heated and dried in a 100°C dryer for 2 hours to obtain an aggregate of flake-like particles. Furthermore, the subsequent heat treatment of the aggregate was changed from under an argon atmosphere and 350°C to under a hydrogen atmosphere and 500°C. The composite particles were prepared in the same manner as in Example 1.

[0081] (Example 6) Composite particles were prepared in the same manner as in Example 1, except that the temperature of the heat treatment of the aggregate was changed from 350°C to 600°C.

[0082] (Example 7) Composite particles were prepared in the same manner as in Example 1, except that the temperature of the heat treatment of the aggregate was changed from 350°C to 700°C.

[0083] (Example 8) Composite particles were prepared in the same manner as in Example 1, except that instead of dispersion C, a graphene oxide dispersion (manufactured by Sanwa Oil & Chemical Industry Co., Ltd.) with an average particle size of 20 μm, synthesized from Chinese graphite with an average particle size of 0.4 mm, was used, and the concentration of the ammonium chloride aqueous solution was changed to 0.2 mol / L.

[0084] (Example 9) Dispersions A and B were mixed in the same manner as in Example 1, and the dispersions were left to stand for 5 days in a light-shielded environment at room temperature (25°C). Then, 7.5 mL of 0.5 mol / L sodium hydroxide aqueous solution was added to 30 mL of the dispersion, and the pH was confirmed to be 9-10 using pH test paper. 79.4 mg of sodium borohydride was dissolved in 30 mL of distilled water over about 5 minutes to obtain an aqueous sodium borohydride solution. Next, the aqueous sodium borohydride solution was slowly added dropwise to the pH-adjusted dispersion using a Pasteur pipette over 3-5 minutes to obtain a mixed solution. The mixed solution was heated to 65°C and stirred for 3 hours to reduce the graphene oxide and convert it to reduced graphene oxide. Then, the dispersion was filtered by suction to prepare an aggregate of composite particles as a sheet on cellulose filter paper.

[0085] (Example 10) First, the dispersions were prepared in the same manner as in Example 1, except that the concentration of the ammonium chloride aqueous solution added to dispersion C in the preparation of the composite particles was changed from 0.25 mol / L to 0.20 mol / L, and the formulation of dispersions A and B was changed so that the amount of graphene oxide was 13% by mass based on the total amount of MFI zeolite and graphene oxide. The dispersions were then left to stand for 5 days at room temperature (25°C), protected from light. After that, 7.5 mL of 0.5 mol / L sodium hydroxide aqueous solution was added to 30 mL of the dispersion, and the pH was confirmed to be 9-10 using pH test paper. 79.4 mg of sodium borohydride was dissolved in 30 mL of distilled water over about 5 minutes to obtain an aqueous sodium borohydride solution. Next, the aqueous sodium borohydride solution was slowly added dropwise to the pH-adjusted dispersion using a Pasteur pipette over 3-5 minutes to obtain a mixed solution. The above mixed solution was heated to 65°C and stirred for 3 hours to reduce graphene oxide and convert it to reduced graphene oxide. Subsequently, the dispersion was filtered by suction to prepare an aggregate of composite particles as a sheet on cellulose filter paper.

[0086] (Example 11) First, in the preparation of the composite particles, the concentration of the ammonium chloride aqueous solution added to dispersion C was changed from 0.25 mol / L to 0.20 mol / L, and the formulation of dispersion A and dispersion B was changed so that the amount of graphene oxide was 19% by mass based on the total amount of MFI zeolite and graphene oxide. Otherwise, the composite particles were prepared in the same manner as in Example 1.

[0087] (Comparative Example 1) [Preparation of Graphene Oxide] Except for using "CB-100" (product name) (average particle size: 0.08 mm) manufactured by Nippon Graphite Industries Co., Ltd. as the raw material, a dispersion (dispersion D) containing dispersed graphene oxide was obtained in the same manner as in Example 1. The average particle size of the obtained graphene oxide was 5 μm. Figure 3(b) shows a scanning electron microscope image of the graphene oxide of Comparative Example 1 placed on a Si substrate. It can be confirmed from the appearance that the particle size is smaller than that of the graphene oxide used in Example 1 shown in Figure 3(a).

[0088] [Preparation of Coated Particles] Composite particles were prepared under the conditions shown in Table 2. Specifically, a black, sponge-like solid was obtained in the same manner as in Example 1, except that dispersion D of graphene oxide prepared as described above was used instead of dispersion A. Scanning electron microscope images were obtained of the obtained black, sponge-like solid, and it was confirmed that no coated particles were formed, that graphene oxide and zeolite particles existed separately, and that coating of inorganic crystals tended to be difficult due to the small average particle size of graphene oxide. The scanning electron microscope images obtained of the obtained solid are shown in Figure 4(b).

[0089] (Comparative Example 2) In the preparation of composite particles, a black, sponge-like solid was obtained in the same manner as in Example 1, except that the concentration of the ammonium chloride aqueous solution added to dispersion C was changed from 0.25 mol / L to 0.10 mol / L. Scanning electron microscope images were obtained of the obtained black, sponge-like solid and it was confirmed that no coating particles were formed, that graphene oxide and zeolite particles existed separately, and that the small average particle size of graphene oxide tended to make coating of inorganic crystals difficult. The scanning electron microscope image obtained of the obtained solid is shown in Figure 4(c).

[0090] (Comparative Example 3) In the preparation of composite particles, a black sponge-like solid was obtained in the same manner as in Example 1, except that the concentration of the ammonium chloride aqueous solution added to dispersion C was changed from 0.25 mol / L to 0.15 mol / L. Scanning electron microscope images were taken of the obtained black sponge-like solid and it was confirmed that no coating particles were formed, graphene oxide and zeolite particles existed separately, and that the small average particle size of graphene oxide tended to make coating with inorganic crystals difficult.

[0091] (Comparative Example 4) In the preparation of composite particles, a black sponge-like solid was obtained in the same manner as in Example 1, except that the concentration of the ammonium chloride aqueous solution added to dispersion C was changed from 0.25 mol / L to 0.30 mol / L. Scanning electron microscope images were taken of the obtained black sponge-like solid and it was confirmed that no coating particles were formed, graphene oxide and zeolite particles existed separately, and that the small average particle size of graphene oxide tended to make coating with inorganic crystals difficult.

[0092] (Comparative Example 5) In the preparation of composite particles, the concentration of the ammonium chloride aqueous solution added to dispersion C was changed from 0.25 mol / L to 0.15 mol / L, and the formulation of dispersion A and dispersion B was changed so that the amount of graphene oxide was 10% by mass based on the total amount of MFI zeolite and graphene oxide. Otherwise, a black, sponge-like solid was obtained in the same manner as in Example 1.

[0093] (Comparative Example 6) In the preparation of composite particles, a black, sponge-like solid was obtained in the same manner as in Example 1, except that the concentration of the ammonium chloride aqueous solution added to dispersion C was changed from 0.25 mol / L to 0.15 mol / L, and the formulation of dispersion A and dispersion B described above was changed so that the amount of graphene oxide was 19% by mass based on the total amount of MFI zeolite and graphene oxide.

[0094] (Comparative Example 7) In the preparation of composite particles, the concentration of the ammonium chloride aqueous solution added to dispersion C was changed from 0.25 mol / L to 0.30 mol / L, and the formulation of dispersion A and dispersion B was changed so that the amount of graphene oxide was 19% by mass based on the total amount of MFI zeolite and graphene oxide. Otherwise, a black, sponge-like solid was obtained in the same manner as in Example 1.

[0095] (Comparative Example 8) In the preparation of composite particles, a black, sponge-like solid was obtained in the same manner as in Example 1, except that the concentration of the ammonium chloride aqueous solution added to dispersion C was changed from 0.25 mol / L to 0.10 mol / L, and the formulation of dispersion A and dispersion B described above was changed so that the amount of graphene oxide was 23% by mass based on the total amount of MFI zeolite and graphene oxide.

[0096] (Reference example) MFI zeolite not encapsulated with graphene oxide was used as the raw material particle for the gas separation membrane. A scanning electron microscope image showing the appearance of the MFI zeolite is shown in Figure 4(d).

[0097]

[0098]

[0099] <Evaluation of Composite and Coated Particles: Nitrogen Selectivity of Gas Separation Membranes> Gas separation membranes were prepared using the composite particles obtained in each example, the coated particles obtained in each comparative example, or the MFI zeolite of the reference example, and their nitrogen selectivity was evaluated. The results are shown in Table 3 and Figure 5. Note that the nitrogen selectivity shown in Table 3 and Figure 5 is a relative value based on the nitrogen selectivity of the gas separation membrane of Comparative Example 1.

[0100] [Preparation of Gas Separation Membranes] In each example or comparative example, the black solid material obtained, or the MFI zeolite from the reference example, was placed in a tablet molder (IR die compression set) and pressurized at 18 kN to prepare a gas separation membrane with a diameter of 5 mm. In Examples 9 and 10, the sheet-like material recovered on filter paper was pressurized at 18 kN in the thickness direction of the sheet to prepare a gas separation membrane with a diameter of 5 mm.

[0101] [Evaluation of Nitrogen Selectivity] A disc-shaped acrylic plate was prepared, and a 1 mm diameter through-hole was made in the center of it. Next, the gas separation membrane prepared as described above was placed on the acrylic plate so that it was located in the center of the disc, and the outer circumference of the gas separation membrane was fixed to the substrate using adhesive (araldite), and this was used as the measurement sample.

[0102] A chamber was created on the side of the gas separation membrane opposite to the acrylic plate side of the measurement sample to allow pressure to be applied with the gas to be measured. The side of the gas separation membrane facing the acrylic plate (the permeate side) was kept at atmospheric pressure, and the measuring device was assembled so that the gas that permeated through the gas separation membrane flowed to a flow meter through a through-hole in the acrylic plate, and the gas permeation rate was measured using the flow meter. The difference between the pressurized side of the gas separation membrane and atmospheric pressure was defined as ΔP.

[0103] Nitrogen gas and oxygen gas were used as the gases to be measured. The permeation time when a unit amount of gas permeated through the gas separation membrane was measured, and the gas permeability R (unit: mol / m³) for the gases to be measured was determined. ２ The gas permeability R(N) was calculated. The ambient temperature during measurement was 25°C. The area of ​​the gas separation membrane was measured, and the gas permeability R(N) for nitrogen gas was calculated. ２ ) and gas permeability R(O) to oxygen gas ２ The nitrogen selectivity of the gas separation membrane was determined by calculating the ratio of the obtained gas permeability using the following formula. [Nitrogen selectivity of gas separation membrane (N ２ / O ２ )] = [Gas permeability R (N) for nitrogen gas ２ )] / [Gas permeability R (O) to oxygen gas ２ )]

[0104]

[0105] As can be seen from the results shown in Table 3, by using the composite particles prepared in the examples, gas separation performance (e.g., nitrogen selectivity) can be improved even when separating gases with similar molecular sizes, such as oxygen and nitrogen.

[0106] This disclosure provides novel composite particles and methods for producing them. Furthermore, this disclosure provides gas separation membranes with fewer defects.

[0107] 2, 2'... Inorganic crystal, 3, 9, 20... Void, 4... Coating layer, 4a... Graphene oxide, 4b... Polar group, 6, 6'... Coating particle, 8... Sheet, 10... Composite particle, 30... Anion, 40... Cation, 50... Housing, 52... Internal space, 52a... First space, 52b... Second space, 54... Gas separation membrane, 56... Support, 58... Sealing material, 60, 62, 64... Gas separation module, 70... Pump, 80... Back pressure control valve, 100... Gas separation device, d1a... Gas supply port, d1b... First gas outlet, d2... Second gas outlet, G1... Raw material gas, G2... Residual gas, G3... Separated gas.

Claims

1. A composite particle comprising an inorganic crystal and a plurality of coated particles having a coating layer provided on the surface of the inorganic crystal, and a sheet containing reduced graphene oxide, wherein the sheet encloses secondary particles formed by the aggregation of the plurality of coated particles.

2. The composite particle according to claim 1, wherein the mass loss rate at 250 to 950°C when heated to 1000°C under airflow is 6 to 15% by mass.

3. The composite particle according to claim 1 or 2, wherein the inorganic crystal contains a zeolite.

4. A gas separation membrane composed of the composite particles described in claim 1 or 2.

5. A method for producing composite particles, comprising: a first step of settling aggregates from a dispersion containing inorganic crystals, graphene oxide, and an ammonium salt; and a second step of reducing the aggregates, wherein the content of graphene oxide in the dispersion is 11% by mass or more, based on the total amount of the inorganic crystals and the graphene oxide; the concentration of the ammonium salt in the dispersion is greater than 0.15 mol / L and less than 0.30 mol / L; and the average particle size of the graphene oxide is larger than the average particle size of the inorganic crystals.

6. The process further includes a mixing step of mixing dispersion A containing graphene oxide with dispersion B containing inorganic crystals and an ammonium salt to obtain the dispersion, and measuring the ultraviolet-visible absorption spectrum of dispersion A, which is observed when the π-π ＊ The manufacturing method according to claim 5, wherein the wavelength of the excitation light corresponding to the maximum value of the absorption peak associated with the transition is 229.8 to 231.2 nm.

7. The manufacturing method according to claim 5 or 6, wherein the average particle size of the graphene oxide is 10 μm or more.

8. The manufacturing method according to claim 5 or 6, wherein the reduction treatment in the second step is carried out by heating the aggregate to 300°C or higher.

9. The manufacturing method according to claim 5 or 6, wherein the second step is carried out in an atmosphere containing at least one selected from the group consisting of noble gases, nitrogen gas, and hydrogen gas.

10. The manufacturing method according to claim 5 or 6, wherein the ammonium salt comprises at least one selected from the group consisting of ammonium chloride, ammonium acetate, ammonium nitrate, ammonium bicarbonate, ammonium carbonate, ammonium oxalate, and ammonium sulfate.

11. A method for producing a gas separation membrane, comprising: a first step of settling aggregates from a dispersion containing inorganic crystals, graphene oxide, and an ammonium salt; a second step of reducing the aggregates to obtain a reduced product; and a third step of compression molding the reduced product.

12. A gas separation module comprising the gas separation membrane described in claim 4.

13. A gas separation apparatus comprising the gas separation module described in claim 12.

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