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

Abstract

One aspect of the present disclosure includes inorganic crystals and a plurality of coated particles having a coating layer provided on the surface of the inorganic crystals, and a sheet containing reduced graphene oxide, wherein the sheet provides composite particles that encapsulate secondary particles formed by aggregation of the plurality of coated particles.

Description

Technical Field

[0001] The present disclosure relates to composite particles, a method for producing the composite particles, a gas separation membrane, a method for producing the gas separation membrane, a gas separation module, and a gas separation device.

Background Art

[0002] The membrane separation technology of gases by gas separation membranes has attracted attention as one of the technologies expected to be energy-saving technologies. Graphene oxide can be regarded as a single-molecule sheet having a two-dimensional spread, and studies have been made on it as a separation membrane utilizing the presence of defects in the sheet.

[0003] Zeolite microcrystals have pores inherent in their crystal structure and can be expected to have ideal separation performance. However, when a membrane composed of zeolite microcrystals is formed, voids are formed between the microcrystals, creating a gas passage that does not pass through the zeolite microcrystals. As a result, the performance expected from the zeolite microcrystals is not exhibited as a separation membrane.

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

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-Patent Literature

[0006]

Non-Patent Literature 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] It has been confirmed that a membrane obtained by compression molding an aggregate of coated particles as described above has improved separation performance compared to a membrane composed of zeolite microcrystals. Even for a membrane obtained as described above, voids remain between the coated particles. Although gas separation is expected to occur through the coated particles, due to the existence of the above voids, once separated gas molecules may mix again, and in some cases, gas 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 is required.

[0008] An object of the present disclosure is to provide novel composite particles and a method for manufacturing the same. Another object of the present disclosure is to provide a gas separation membrane with few defects.

Means for Solving the Problems

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

[13] .

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

[10] The production 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 first step of precipitating aggregates from a dispersion containing inorganic crystals, graphene oxide, and an ammonium salt; A second step of subjecting the aggregates to a reduction treatment to obtain a reduction-treated product; A method for producing a gas separation membrane, comprising a third step of compression-molding the reduction-treated product.

[12] A gas separation module comprising the gas separation membrane according to [4].

[13] A housing; A gas separation membrane housed in the housing and partitioning the internal space of the housing into a first space and a second space; In the housing, A gas supply port communicating with the first space and a first gas extraction port communicating with the first space; A second gas extraction port communicating with the second space; are provided, The gas separation membrane includes the gas separation membrane according to [4], and the gas separation module.

[14] A gas separation device comprising the gas separation module according to

[12] or

[13] .

Advantages of the Invention

[0011] According to the present disclosure, novel composite particles and a method for producing the same can be provided. According to the present disclosure, a gas separation membrane with few defects can also be provided.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings in some cases. However, the following embodiments are examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents. In the description, the same reference numerals are used for the same elements or elements having the same function, and redundant descriptions may be omitted in some cases. Also, the positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings unless otherwise specified. Furthermore, the dimensional ratios of the respective elements are not limited to the ratios shown in the drawings. In this specification, the numerical range indicated by the symbol "~" includes the lower limit value and the upper limit value. That is, the numerical range indicated by "x~y" means x or more and y or less.

[0014] Unless otherwise specified, the materials exemplified in this specification can be used alone or in combination of two or more. The content of each component in the composition means the total amount of the plurality of substances present in the composition when there are a plurality of substances corresponding to each component in the composition, unless otherwise specified.

[0015] In this specification, reduced graphene oxide is a sheet-like compound obtained by reducing graphene oxide. Reduced graphene oxide can be regarded as a single molecular layer. Further, graphene oxide generally has defects in its structure, and these defects may be referred to as windows. The average aperture diameter (arithmetic mean value of the aperture diameter) of the window may be, for example, 0.3 to 1.5 nm. Reduced graphene oxide has defects in its structure in the same manner as graphene oxide, and thereby has the ability to allow gas molecules and the like to pass through.

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

[0017] FIG. 1 is a schematic diagram showing an example of the composite particles. The composite particles 10 are composed of a plurality of coated particles 6 and a sheet 8 containing reduced graphene oxide. Each coated particle 6 includes 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 FIG. 1, an example in which the composite particles 10 include three coated particles 6 is shown, but the number of coated particles 6 in the composite particles 10 is not limited thereto, and 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 a plurality of reduced graphene oxides are laminated. In the above-mentioned laminated structure, the reduced graphene oxides are laminated on each other with a molecular-level gap therebetween, and gas molecules can pass through this gap. And the gas can pass through the coating layer 4 via the defects of each reduced graphene oxide and the above-mentioned gap. In other words, the coating layer 4 has a path through which gas molecules can pass. The path does not necessarily have to be a through-hole penetrating in the thickness direction of the coating layer 4, as long as gas molecules can pass from the side of the inorganic crystal 2 of the coating layer 4 to the surface on the side opposite to the side of the inorganic crystal 2 of the coating layer 4, the positions of the defects of each reduced graphene oxide constituting the coating layer 4 may be shifted from each other.

[0019] The sheet 8 may also be composed of reduced graphene oxide and may have a laminated structure in which a plurality of reduced graphene oxides are laminated. In the composite particle 10, the sheet 8 may be one that wraps around the outer periphery with a single reduced graphene oxide, or may be one that wraps around with a plurality of reduced graphene oxides. Since the sheet 8 is composed of reduced graphene oxide, it has a path through which gas molecules can pass. As a result, gas molecules can pass through the inside and outside of the composite particle 10. In the composite particle 10, a void 9 is formed between the plurality of coated particles 6, and the void 9 is separated from the space outside the composite particle 10 by the sheet. The composite particle 10 has a form in which secondary particles in which a plurality of coated particles 6 are aggregated are wrapped by the sheet 8, and the gaps generated between the coated particles in advance are covered by the sheet 8. By having such a form, compared with the case of using coated particles that exist alone as in the prior art, when preparing a gas separation membrane, the generation of a path (hereinafter also referred to as a leak path) through which gas molecules pass through the separation membrane without passing through the reduced graphene oxide and the coated particles in the separation membrane can be more sufficiently reduced. In other words, a gas separation membrane with fewer defects can be prepared.

[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, since a gas molecule migration path is also provided inside the inorganic crystal 2, the gas permeation rate of the composite particle 10 and the gas separation membrane prepared using the same can be increased. Examples of the inorganic crystal 2 having a porous structure may include zeolite and the like. Examples of the inorganic crystal 2 having no pores may include apatite and the like.

[0021] When the inorganic crystal 2 is zeolite, the crystal surface of the zeolite is relatively smooth, it is easier to adjust the void 3 formed between the inorganic crystal 2 and the coating layer 4, and the gas selectivity (for example, nitrogen selectivity) of the obtained composite particle 10 can be further improved.

[0022] The zeolite may be either natural zeolite or synthetic zeolite. Examples of the synthetic zeolite may include high-silica zeolite and molecular sieve and the like. More specifically, examples of the synthetic zeolite may include MFI zeolite, BEA zeolite, FAU zeolite, AEI zeolite, and CHA zeolite and the like.

[0023] The upper limit value 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 the upper limit value of the average particle diameter of the inorganic crystal 2 being within the above range, composite particles with a reduced proportion of the inorganic crystal portion not included by the reduced graphene oxide can be obtained. By forming a gas separation membrane using such composite particles, the gas leakage path generated in the gas separation membrane can be further reduced, and the separation performance of the obtained gas separation membrane can be further improved. The lower limit value 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 the upper limit value of the average particle diameter of the inorganic crystal 2 being within the above range, the proportion of the inorganic crystal in the composite particles can be improved, and the gas permeation performance of the composite particles themselves can be further improved. By forming a gas separation membrane using such composite particles, high gas permeation performance in the gas separation membrane can be expected.

[0024] The average particle diameter of the inorganic crystal in this specification means the value measured by the method shown below. First, the composite particles to be measured are heated at a heating rate of 3 °C / min or less up to 1000 °C by air fluidization, and the obtained inorganic crystal is used as the measurement sample. The above average particle diameter can also be determined by performing measurement on the dispersion sample obtained by dispersing the obtained measurement sample in water by the dynamic light scattering method. The dispersion sample may be adjusted to a dilute concentration such that the 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 the dynamic light scattering measurement. As the light scattering device, for example, "Zetasizer nano ZS" manufactured by Malvern Instruments can be used. In addition, when preparing the composite particles by oneself, the inorganic crystal as the raw material can be used as the measurement sample, the above dispersion sample can be prepared, and the measurement can also be performed.

[0025] The composite particles 10 may be adjusted such that the mass reduction rate at 250 to 950 °C when heated to 1000 °C under an air flow is within a predetermined range. The above mass reduction rate is mainly due to the decomposition of reduced graphene oxide and can be regarded as an index of the content of reduced graphene oxide. The lower limit value of the above mass reduction 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 value of the above mass reduction rate is within the above range means that the ratio of the coating layer in the coated particles and the ratio of the sheet of the composite particles are large. By using the composite particles in which the ratio of reduced graphene oxide is adjusted in this way, the generation of gas leakage paths in the obtained gas separation membrane can be further reduced. The upper limit value of the above mass reduction 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 value of the above mass reduction rate is within the above range corresponds to the fact that the ratio of the coating layer in the coated particles and the ratio of the sheet of the composite particles are not too large. By using the composite particles in which the ratio of reduced graphene oxide is adjusted in this way, the decrease in gas permeability of the obtained gas separation membrane can be further suppressed, and the high-speed gas separation performance can be improved. The composite particles 10 can be adjusted such that the mass reduction rate at 250 to 950 °C when heated to 1000 °C under an air flow is within the above range, and may be, for example, 6 to 15% by mass, or 7 to 15% by mass.

[0026] The above mass reduction rate in this specification means the value measured by thermogravimetric analysis (TG). Specifically, first, the composite particles to be measured are heated from room temperature to 350 °C at a rate of 1 °C / min under an argon flow (flux: 0.8 m / min), and then heated at 350 °C for 30 minutes to obtain the powder to be measured. Using the powder as the measurement object, the mass reduction rate at 250 to 950 °C is determined in a thermogravimetric analysis when heated to 1000 °C at a rate of 3 °C / min under an air flow.

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

[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, but 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, it is necessary to apply a considerably high pressure. 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 separation is achieved 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), and even if it is doubled, it can exhibit the above-mentioned nitrogen selectivity, so 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 it 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] As the inorganic crystals used in the above production method, those exemplified in the description of the above composite particles can be used.

[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 that constitutes graphite becomes graphene oxide, thereby exfoliating graphene oxide from graphite to obtain graphene oxide. The oxidation of graphite may be carried out in an aqueous solution. In this case, the 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] Graphite may be preferably one with excellent crystallinity and may have a flaky shape. As the graphite, for example, graphite produced in Madagascar can be used.

[0036] The step of preparing the graphene oxide by oxidizing the graphite may be carried out, for example, by a modified Hummers method as described below. The step of preparing the graphene oxide may be a method of preparing a dispersion of graphene oxide by heat-treating 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 contain, 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 contains at least one acid selected from the group consisting of phosphoric acid and sulfuric acid, and more preferably contains phosphoric acid and sulfuric acid. The inorganic acid may be, for example, a mixture of sulfuric acid with a concentration of 98% by mass and phosphoric acid in a volume ratio of 90:10.

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

[0039] The heat treatment of the aqueous solution for oxidizing the graphite may be carried out, for example, under stirring. By controlling the temperature of the aqueous solution and stirring, graphene constituting the graphite is oxidized, and a functional group having a predetermined oxygen is introduced onto the surface of the graphene, making it easier to disperse graphene oxide in the aqueous solution. More specifically, due to the introduction of the functional group, the interlayer distance of the graphene constituting the graphite is enlarged, and water molecules and the like as the dispersion medium penetrate into the interlayer, which can further promote the exfoliation of graphene oxide and its dispersion in the solution.

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

[0041] The above manufacturing method may further include a mixing step of mixing dispersion liquid A containing graphene oxide, and dispersion liquid B containing inorganic crystals and an ammonium salt to obtain the above dispersion liquid. At this time, when ultraviolet-visible absorption spectrum measurement is performed on the above dispersion liquid A, the wavelength of the excitation light corresponding to the maximum value of the absorption peak accompanying the π-π * transition may be 229.8 to 231.2 nm. As described above, graphite having a relatively large average particle diameter is used as the raw material, and the wavelength of the excitation light corresponding to the maximum value of the absorption peak accompanying the π-π * transition is within the above range, and by using the above dispersion liquid A, the graphene oxide in the dispersion liquid A can be in a state with more sufficient surface charge, and the flexibility of the graphene oxide can be further improved. Therefore, when mixed with the above dispersion liquid B, the inclusion of the inorganic crystals can be made easier. Further, in the dispersion liquid A as described above, the graphene oxide is in a state having sufficient defects (for example, nano-windows may also be possible) on the graphene sheet due to gas permeation, and is suitable for the formation of coated particles and composite particles and the improvement of the separation performance of gas molecules. When ultraviolet-visible absorption spectrum measurement is performed on the above dispersion liquid A, the wavelength of the excitation light corresponding to the maximum value of the absorption peak accompanying the π-π * transition is, for example, preferably 230.2 to 230.8 nm.

[0042] In this specification, the above π-π *The wavelength of the excitation light corresponding to the maximum value of the absorption peak associated with the transition means the value measured as follows. First, Dispersion A is diluted to prepare a graphene oxide dispersion with a concentration of 0.1% by mass, and further, a measurement sample diluted 30 times with distilled water is obtained. For the said measurement sample, using an ultraviolet-visible near-infrared spectrophotometer, the absorbance is measured in steps of 0.1 nm in the range of 200 to 600 nm to determine the wavelength of the excitation light corresponding to the maximum value of the absorption peak associated with the π-π * transition. Note that the concentration of the measurement sample may be adjusted so that the absorption intensity at the peak top is about 1 in the measurement by an ultraviolet-visible near-infrared spectrophotometer. As the ultraviolet-visible near-infrared spectrophotometer, for example, "V670" manufactured by JASCO Corporation can be used.

[0043] In the above production method, the average particle size of the graphene oxide is larger than the average particle size 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 based on the average particle size 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 include the secondary particles composed of a plurality of coated particles. The average particle size of the graphene oxide may be, for example, 10000 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 based on the average particle size of the inorganic crystal. By using graphene oxide with an average particle size within the above range, it is possible to prevent a structure in which the inorganic crystal only lies on the surface of the graphene oxide, and it becomes easier to adjust the inorganic crystal included by the graphene oxide. That is, it becomes easier to include the primary particles or secondary particles of the inorganic crystal, or the coated particles by one or a plurality of graphene oxides, and it is possible to obtain composite particles having an inclusion structure in which the generation of leak paths is more suppressed.

[0044] The lower limit of the average particle size of the graphene oxide may be, for example, 10 μm or more, 14 μm or more, or 20 μm or more. By using graphene oxide with the lower limit of the average particle size within the above range, it becomes easier to coat the inorganic crystal and to encapsulate the secondary particles composed of a plurality of coated particles. The upper limit of the average particle size of the graphene oxide may be, for example, 100 μm or less, 70 μm or less, or 50 μm or less.

[0045] The average particle size of the graphene oxide in this specification means a value measured by the method shown below. That is, a dispersion of graphene oxide prepared so that the concentration is 0.01% by mass with water as the medium is prepared, 0.05 mL is dropped onto a silicon substrate, and a sample dried at 25 °C over 1 hour is prepared. The sample is used as the measurement target and a scanning electron microscope image is acquired. For each individual graphene oxide in the acquired image, the maximum diameter (major axis) and the minimum diameter (minor axis) are determined, and the arithmetic mean value thereof is taken as the particle size of the graphene oxide. The particle sizes are similarly determined for 50 or more graphene oxides, and the arithmetic mean value thereof is taken as the average particle size of the graphene oxide.

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

[0047] The content of the graphene oxide in the above dispersion can be further adjusted according to the amount of the coating layer required for the coated particles and the above sheet (such as the thickness and number of layers of the coating layer and the sheet). The lower limit of the content of the graphene oxide 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 content of the graphene oxide within the above range, the thickness of the coating layer on the coated particles or the sheet corresponding to the outer layer of the composite particles can be increased. Also, the amount of graphene oxide that plays a role in connecting multiple inorganic particles and coated particles to each other can be further increased. Since the layer containing reduced graphene oxide is softer than the inorganic crystal, in the above-mentioned case, when the aggregate of the obtained composite particles is compression-molded to prepare a gas separation membrane, the contact area between the composite particles can be increased, and the defects as a membrane can be further reduced. The large thickness of the coating layer on the coated particles or the sheet corresponding to the outer layer of the composite particles also corresponds to an increase in the number of stacked layers of reduced graphene oxide in the coating layer and sheet of the obtained coated particles. Increasing the number of stacked layers in this way corresponds to an increase in the length of the path for gas molecules to penetrate into the coated particles or composite particles. Even if there are somewhat large defects in each layer of the reduced graphene oxide, when the number of stacked layers increases and the penetration path of gas molecules becomes longer, in order to enter the inside of the coated particles, it is necessary to pass through multiple layers of reduced graphene oxide, and the path through which gas molecules pass as a whole can be made narrow on average, and the gas separation performance can be further improved. The upper limit of the content of the graphene oxide 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 content of the graphene oxide within the above range, excessive inclusion by the graphene oxide can be prevented, and a gas separation membrane having 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 the value calculated using the compounding amount (g) of the graphene oxide, 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 the 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] = [Compounding amount (g) of graphene oxide] / {[Density of graphene (g / cm 3 )] × [Thickness of graphene monolayer (cm)]}

[0049] The ammonium salt used in the above manufacturing method plays an important role in forming a coating layer on the inorganic crystal. Taking FIG. 2 as an example, the process of forming the coated particles in the above manufacturing method will be described. Since the 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 the negatively charged and dispersed graphene oxide 4a due to electrostatic repulsion. Here, by interposing an ammonium salt, among the anions 30 and cations 40 generated by the ionization of the ammonium salt, the cations 40 can weaken the negative charge on the surface of the graphene oxide 4a and reduce the thickness of the electric double layer (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 serving as the electric double layer regulator as described above may be removable by heating in a subsequent process (for example, heating at 300 °C or higher). By sufficiently removing the ammonium salt, it is possible to sufficiently suppress the remaining of cations or anions in the voids formed between the inorganic crystal and the coating layer, and by more surely providing a gas passage path, the gas permeation performance and gas separation performance of the obtained composite particles can be further improved. Since the ammonium salt is easily removable 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 preferably at least one of ammonium chloride and ammonium oxalate.

[0051] In the above dispersion, the concentration of the ammonium salt is more 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 the composite particles. The lower limit of the concentration of the 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 concentration of the ammonium salt within the above range, the distance (Debye length) between graphene oxide and the inorganic crystal can be made shorter, and it becomes easier to form a coating layer on the surface of the inorganic crystal. By setting the lower limit of the concentration of the ammonium salt within the above range, the electrostatic repulsion between graphene oxides is also suppressed, and it becomes easier to form a multilayer structure by graphene oxide on the surface of the inorganic crystal, and the gas separation performance of the obtained composite particles can be further improved. Further, by setting the lower limit of the concentration of the ammonium salt within the above range, the gap formed between the inorganic crystal and graphene oxide in the coated particles can be made smaller, and composite particles capable of separation can be prepared even when the size difference of gas molecules in the gas to be separated is small (for example, oxygen and nitrogen, etc.). The upper limit of the concentration of ammonium chloride in the above 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 sedimented later, if the aggregation of the coated particles progresses too much, the density of the sedimented part of the aggregates becomes high, and they are separated from the dispersion medium, and when dried, etc., the deposited structure of the obtained powder tends to be dense. When the deposited structure of the powder is dense, the molding pressure during pressure molding cannot be fully utilized, and an additional device may be required to make the obtained membrane have a dense structure and suppress the gas leakage path. On the other hand, by setting the upper limit of the concentration of ammonium chloride in the above dispersion within the above range, the aggregation of the coated particles can be made more appropriate. Further, by setting the upper limit of the concentration of ammonium chloride in the above dispersion within the above range, coated particles having a more uniform inclusion structure can be obtained. Furthermore, by setting the upper limit of the concentration of the ammonium salt within the above range, it is possible to more sufficiently suppress the gap formed between the inorganic crystal and graphene oxide in the coated particles from becoming excessively narrow.The concentration of the ammonium salt in the dispersion may be adjusted within the above-mentioned range, for example, it may be 0.16 to 0.29 mol / L.

[0052] The first step is a step of sedimenting aggregates from the dispersion, and the sedimentation of the aggregates may be carried out by allowing the dispersion to stand. The time for allowing the dispersion to 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 crystal by graphene oxide and the inclusion of the coated particles can be made more sufficient. The time for allowing the dispersion to stand is not particularly limited, but may be, for example, 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 above aggregates may be separated from the dispersion medium. The removal of the dispersion medium can be carried out, for example, by decantation. Further, in order to make the removal of the dispersion medium from the above aggregates more sufficient, the removal of the dispersion medium can also be carried out, for example, by heat drying and freeze drying.

[0054] The second step is a step of reducing the aggregate obtained in the first step (reduction step). In this step, by desorbing and reducing the polar groups of graphene oxide, the electrostatic repulsion between the inorganic crystal and graphene oxide and between graphene oxides is reduced, and the voids between the inorganic crystal and reduced graphene oxide in the resulting composite particles can be made small. Furthermore, in this step, by reducing the content of the ammonium salt in the aggregate, it is possible to suppress a decrease in the gas permeability of the gas separation membrane due to the remaining anions and cations derived from the ammonium salt in the above voids. The voids between the inorganic crystal and reduced graphene oxide thus formed can be, for example, 0.4 nm or less. Note that the reduction treatment may be either a heat treatment (dry treatment) or a wet treatment, but a heat treatment is preferred. Also, in the case of reduction treatment by heating, the aggregate obtained in the first step may be dried before the heat treatment, and the obtained dry powder may be heat-treated for 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 may be carried out in a noble gas atmosphere, from the viewpoint of further promoting the reduction of graphene oxide. Examples of the noble gas include helium (He) and argon (Ar).

[0056] When the reduction treatment in the second step is carried out by heat treatment, the lower limit value of the heat treatment temperature may be, for example, 300 °C or higher, or 350 °C or higher. By setting the lower limit value of the heat treatment temperature within the above range, the removal of the ammonium salt 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 value of the heat treatment temperature in the second step may be, for example, 800 °C or lower, or 700 °C or lower in an inert atmosphere. By setting the upper limit value 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 wet treatment, the aggregates obtained in the first step may be reduced by adding a reducing agent to the dispersion containing the aggregates. As the reducing agent, either an organic reducing agent or an inorganic reducing agent may be used. Examples of the organic reducing agent include hydrazine, formic acid, oxalic acid, and 3,4,5-trihydroxybenzoic acid (gallic acid). Examples of the inorganic reducing agent include sodium borohydride and diisobutylaluminum hydride.

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

[0059] One embodiment of the method for producing a gas separation membrane includes a first step of precipitating aggregates from a dispersion containing inorganic crystals, graphene oxide, and an ammonium salt, a second step of subjecting the aggregates to a reduction treatment to obtain a reduction-treated product, and a third step of compression molding the reduction-treated product.

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

[0061] Fig. 6 shows a schematic diagram of a gas separation membrane composed of conventional coated particles. Fig. 6(a) is a schematic cross-sectional view of the gas separation membrane, and Fig. 6(b) is an enlarged view of region R in Fig. 6(a). The gas separation membrane is formed by compression molding an aggregate of a plurality of coated particles 6'. There is a structure in which adjacent coated particles 6' are bound by reduced graphene oxides thinly provided on the surface layer of the coated particles 6'. The coated particle 6' includes an inorganic crystal 2' and a layer 4' composed of reduced graphene oxide provided on the surface of the inorganic crystal 2'. Due to the defects of the reduced graphene oxide, gas molecules can pass between the inside and outside of the coated particle 6'. When gas molecules pass through the gap between the inorganic crystal 2' and the layer 4' composed of reduced graphene oxide, if there is a large difference in the size of the gas molecules, the separation between the gas molecules can be achieved through the path passing through the inside of the coated particle 6'. Fig. 6(b) shows that gas molecule A (for example, nitrogen) and gas molecule B (for example, methane) are separated. However, when the gas molecule intrusion path from the layer 4' composed of reduced graphene oxide to the coated particle 6' is wide, or when the uniformity of the gap between the inorganic crystal 2' and the layer 4' composed of reduced graphene oxide is low, etc., the expected separation performance may not be obtained. Furthermore, when the sizes of gas molecule A and gas molecule B are close (for example, nitrogen and oxygen), it tends to be difficult to obtain the expected separation performance. Also, when gas molecules A and B pass through the gap between the inorganic crystal 2' and the layer 4' composed of reduced graphene oxide as shown in Fig. 6(b), although gas separation seems to be achieved, even when compression molding is performed to achieve crimping of the coated particles 6', since a certain void 20 remains, the gas molecules separated through the gap between the inorganic crystal 2' and the layer 4' composed of reduced graphene oxide may be remixed, or may pass through the gas separation membrane without passing through the above gap and without being separated.

[0062] In contrast, the composite particles according to the present disclosure include a sheet containing reduced graphene oxide, and have a novel structure in which a plurality of coated particles are aggregated by the sheet to enclose secondary particles. By having such a structure, the generation of defects (for example, the void 20 in Fig. 6(a) that does not contribute to gas separation) in the gas separation membrane prepared using the composite particles is suppressed, so that the gas separation performance can be further improved.

[0063] The above-mentioned gas separation membrane is useful as a member such as a gas separation module. One embodiment of the gas separation module includes the above-mentioned gas separation membrane. The gas separation module includes, for example, a housing, and a gas separation membrane housed in the housing and partitioning 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 extraction port communicating with the first space, and a second gas extraction port communicating with the second space. And the above-mentioned gas separation membrane includes the above-mentioned gas separation membrane according to the present disclosure.

[0064] Fig. 7 is a schematic cross-sectional view showing an example of the gas separation module. The gas separation module 62 is an example in which the gas separation membrane is arranged in a planar shape, and is also referred to as a flat membrane module. The gas separation module 62 includes 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 base material through which gas can pass, and is, for example, composed of a porous body. Depending on the mechanical strength of the gas separation membrane 54, the support 56 can be omitted. Also, if the gas separation membrane 54 can be fixed in the housing 50, the sealing material 58 can also be omitted. In Fig. 7, the internal space 52 of the housing 50 is partitioned into a first space 52a and a second space 52b by the gas separation membrane 54. The housing 50 may be composed of, for example, two separable members. In this case, for example, the end portions of the gas separation membrane 54 may be fixed by sandwiching them between the two members.

[0065] In the example shown in FIG. 7, the housing 50 supplies the raw material gas G1 (e.g., compressed air) into the first space 52a from the gas supply port d1a provided in the housing 50. The gas separation membrane 54 separates it into the 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 the 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] FIG. 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 forms a gas separation membrane in a cylindrical shape and performs gas separation using the spaces inside and outside the cylinder. The basic configuration is the same as that of the gas separation module 62 described in FIG. 7. The gas separation module 64 is different from the gas separation module 62 in that the housing 50 has a cylindrical shape and a gas separation membrane 54 formed in a cylindrical shape is disposed in the internal space of the cylinder. FIG. 8 shows an example in which there is one forming tube of the gas separation membrane 54 formed in a cylindrical shape in the housing 50, but a plurality of the forming tubes may be disposed in the housing 50. In this case, a plurality of gas supply ports d1a and a plurality of first gas outlets d1b may be provided so as to communicate with each of the spaces (first space 52a) inside each forming tube.

[0067] In the gas separation membrane according to the present disclosure, anisotropy in the gas permeation performance is not particularly provided. Therefore, a first gas outlet d1b communicating with the space on the raw material gas supply side (i.e., the space communicating with the gas supply port) is provided on the side of the space on the raw material gas supply side, and a second gas outlet d2 communicating with the space on the other side (i.e., the space where the gas passing through the gas separation membrane is rejected) is provided on the side of the other space. Note that by making the amount of gas taken out from the first gas outlet d1b to the outside of the housing smaller 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. Further, by adjusting the pressure, the gas separation performance of the gas separation membrane can be adjusted. Depending on the composition of the target separation gas or the like, for example, adjustment such as lowering the separation coefficient of the separation membrane by increasing the pressure is possible. Therefore, although it depends on the strength of the gas separation membrane, it is desirable to use the gas separation module in a pressurized state.

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

[0069] FIG. 9 shows a schematic diagram illustrating an example of the gas separation device. The gas separation device 100 shown in FIG. 7 includes 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 a function of adjusting the amount of gas taken out from the first space to the outside of the gas separation module 60 in order to pressurize the first space side of the gas separation module 60, that is, the space (first space) on the side where the raw material gas G1 is supplied to the gas separation membrane. In the gas separation device 100 shown in FIG. 9, a pressure gauge P for checking the supply pressure of the raw material gas is disposed between the pump 70 and the gas separation module 60.

[0070] In the gas separation device 100, the separated gas G3 that has permeated through the gas separation membrane and the residual gas G2 that has remained in the first space without permeating through 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, nitrogen in the air permeates through the gas separation membrane and flows into the second space. Therefore, a nitrogen-enriched gas can be obtained as the separated gas G3, and air with a reduced nitrogen content (i.e., an oxygen-enriched gas) can be obtained as the residual gas G2. The oxygen concentration and nitrogen concentration in each of the residual gas G2 and the separated gas G3 can be controlled, for example, by adjusting the pressure in the space on the raw material supply side of the gas separation module.

[0071] As described above, several embodiments have been explained, but the present disclosure is not limited to the above embodiments in any way. Also, the description contents of the above-described embodiments can be applied to each other.

Example

[0072] Hereinafter, the present disclosure will be described in more detail using examples, comparative examples, and reference examples. Note that 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 each weighed into a container and stirred for 4 hours under the conditions of a temperature of 38°C and a stirring speed of 250 rpm. Next, 300 mL of water (H2O) was added to the above aqueous solution and stirred well. In the above process, the graphene layers of graphite were oxidized to generate surface functional groups, and a solution containing layered graphene oxide was prepared by further peeling the oxidized graphene layers from the graphite. The graphite used was that with an average particle size of 0.3 mm produced in Madagascar. The graphene constituting the graphite has a flake shape. The inorganic acid used was adjusted so that sulfuric acid (H2SO4, concentration: 98% by mass) and phosphoric acid (H3PO4 aqueous solution, concentration: 85% by mass) had a volume ratio of 90:10.

[0074] Subsequently, 40 mL of hydrogen peroxide solution (H2O2, concentration: 15% by mass) was added to the above aqueous solution to stop the oxidation reaction. The supernatant was removed from the aqueous solution after the reaction was stopped by centrifugation and decantation. After removing the supernatant, washing with hydrochloric acid (HCl aqueous solution, concentration: 5% by mass) was repeated 5 times, and further washing with pure water was repeated 5 times to remove sulfuric acid, manganese, etc., and a dispersion (dispersion B) in which graphene oxide was dispersed was obtained. The wavelength of the excitation light corresponding to the maximum value of the absorption peak associated with the π-π * transition in the ultraviolet-visible absorption spectrum measurement of the dispersion was in the range of 229.8 to 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 an Si substrate.

[0075] [Preparation of Composite Particles] Composite particles were prepared under the conditions shown in Table 1. Specifically, first, 50 mg of MFI zeolite (manufactured by Tosoh Corporation, high-silica zeolite of aluminosilicate, 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 / Al2O3 ratio (molar ratio): 24) was weighed and placed in a plastic vial, 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 an ammonium chloride aqueous solution (concentration: 0.25 mol / L) was added to obtain dispersion B. Next, 9.5 mL of the dispersion A (concentration 0.1% by mass) of graphene oxide prepared as described above was added and shaken well. At this time, the dispersion inside was shaken so that it strongly collided with the lid and bottom of the vial. After visually confirming that dispersions A and B were mixed, the dispersion was allowed to stand for 5 days in a light-shielded environment at room temperature (25°C). A precipitate was formed during this process. The formulation of the above-mentioned dispersions A and B was adjusted so that the blending amount of graphene oxide was 16% by 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 instantaneously frozen with liquid nitrogen and then freeze-dried to obtain a sponge-like aggregate. The above aggregate was heat-treated at 350 °C for 30 minutes under an argon gas stream (flow rate: 100 cc / min) to reduce graphene oxide and convert it into reduced graphene oxide, and ammonium chloride remaining in the aggregate was removed. In this way, a black sponge-like solid was obtained. The scanning electron microscope image obtained for the black sponge-like aggregate is shown in Fig. 4(a).

[0077] (Example 2) In the preparation of the composite particles, except that when obtaining the sponge-like aggregate from the dispersion after standing for 5 days, instead of freeze-drying, the above dispersion was spread on a glass petri dish and heated and dried with a dryer at 100 °C for 2 hours to obtain an aggregate of flaky particles, the composite particles were prepared in the same manner as in Example 1.

[0078] (Example 3) In the preparation of the composite particles, except that when obtaining the sponge-like aggregate from the dispersion after standing for 5 days, instead of freeze-drying, the above dispersion was spread on a glass petri dish and heated and dried with a dryer at 100 °C for 2 hours to obtain an aggregate of flaky particles, and that the subsequent heat treatment of the aggregate was changed from under an argon atmosphere gas stream to under a nitrogen gas stream, the composite particles were prepared in the same manner as in Example 1.

[0079] (Example 4) In the preparation of the composite particles, except that when obtaining the sponge-like aggregate from the dispersion after standing for 5 days, instead of freeze-drying, the above dispersion was spread on a glass petri dish and heated and dried with a dryer at 100 °C for 2 hours to obtain an aggregate of flaky particles, and that 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 the preparation of the composite particles, instead of freeze-drying to obtain a sponge-like aggregate from the dispersion after standing for 5 days, the above dispersion was spread on a glass petri dish and heat-dried at 100 °C for 2 hours to obtain an aggregate of flaky particles. And, except that the subsequent heat treatment of the aggregate was changed from a condition of 350 °C under an argon atmosphere flow to 500 °C under a hydrogen atmosphere flow, composite particles were prepared in the same manner as in Example 1.

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

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

[0083] (Example 8) Instead of Dispersion C, a graphene oxide (manufactured by Sanwa Yuka Kogyo Co., Ltd.) dispersion with an average particle size of 20 μm, synthesized using Chinese graphite with an average particle size of 0.4 mm as a raw material, was used, and the concentration of the ammonium chloride aqueous solution was changed to 0.2 mol / L. Except for this, composite particles were prepared in the same manner as in Example 1.

[0084] (Example 9) In the same manner as in Example 1, dispersions A and B were mixed, and the dispersion was allowed to stand for 5 days in the dark at room temperature (25°C). Then, 7.5 mL of a 0.5 mol / L aqueous sodium hydroxide solution was added to 30 mL of the dispersion, and it was confirmed with pH test paper that the pH had become 9 - 10. 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 dropped into the dispersion with the adjusted pH over 3 - 5 minutes using a Pasteur pipette to obtain a mixed solution. The mixed solution was heated to 65°C and stirred for 3 hours to reduce graphene oxide and convert it into reduced graphene oxide. Then, the dispersion was suction filtered to prepare an aggregate of composite particles as a sheet-like material on a cellulose filter paper.

[0085] (Example 10) First, in the preparation of the composite particles, while changing the concentration of the aqueous ammonium chloride solution added to dispersion C from 0.25 mol / L to 0.20 mol / L, and changing the formulation of the above-mentioned dispersions A and B such that the blending amount of graphene oxide was 13% by mass based on the total amount of MFI zeolite and graphene oxide, in the same manner as in Example 1, a dispersion was prepared, and the dispersion was allowed to stand for 5 days in the dark at room temperature (25°C). Then, 7.5 mL of a 0.5 mol / L aqueous sodium hydroxide solution was added to 30 mL of the dispersion, and it was confirmed with pH test paper that the pH had become 9 - 10. 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 dropped into the dispersion with the adjusted pH over 3 - 5 minutes using a Pasteur pipette to obtain a mixed solution. The mixed solution was heated to 65°C and stirred for 3 hours to reduce graphene oxide and convert it into reduced graphene oxide. Then, the dispersion was suction filtered to prepare an aggregate of composite particles as a sheet-like material on a cellulose filter paper.

[0086] (Example 11) First, in the preparation of the composite particles, while changing the concentration of the ammonium chloride aqueous solution added to the dispersion C from 0.25 mol / L to 0.20 mol / L and changing the formulation of the above-mentioned dispersion A and dispersion B such that the amount of graphene oxide is 19% by mass based on the total amount of MFI zeolite and graphene oxide, composite particles were prepared in the same manner as in Example 1 except for the above changes.

[0087] (Comparative Example 1) [Preparation of Graphene Oxide] A dispersion (dispersion D) in which graphene oxide was dispersed was obtained in the same manner as in Example 1 except that "CB-100" (trade name) (average particle diameter: 0.08 mm) manufactured by Nippon Carbon Industry Co., Ltd. was used as the graphite raw material. The average particle diameter of the obtained graphene oxide was 5 μm. A scanning electron microscope image showing the appearance of the graphene oxide of Comparative Example 1 placed on an Si substrate is shown in Fig. 3(b). 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 Fig. 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 the dispersion D of graphene oxide prepared as described above was used instead of dispersion A. A scanning electron microscope image was obtained for the obtained black sponge-like solid, and it was confirmed that coated particles were not formed, and graphene oxide and zeolite particles existed separately, and the tendency that it was difficult to coat the inorganic crystals due to the small average particle diameter of graphene oxide. The scanning electron microscope image obtained for the obtained solid is shown in Fig. 4(b).

[0089] (Comparative Example 2) In the preparation of the 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 Liquid C was changed from 0.25 mol / L to 0.10 mol / L. A scanning electron microscope image was acquired for the obtained black sponge-like solid, and it was confirmed that coated particles were not formed, that graphene oxide and zeolite particles existed separately, and that it tended to be difficult to coat the inorganic crystals due to the small average particle size of the graphene oxide. The scanning electron microscope image acquired for the obtained solid is shown in Fig. 4(c).

[0090] (Comparative Example 3) In the preparation of the 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 Liquid C was changed from 0.25 mol / L to 0.15 mol / L. A scanning electron microscope image was acquired for the obtained black sponge-like solid, and it was confirmed that coated particles were not formed, that graphene oxide and zeolite particles existed separately, and that it tended to be difficult to coat the inorganic crystals due to the small average particle size of the graphene oxide.

[0091] (Comparative Example 4) In the preparation of the 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 Liquid C was changed from 0.25 mol / L to 0.30 mol / L. A scanning electron microscope image was acquired for the obtained black sponge-like solid, and it was confirmed that coated particles were not formed, that graphene oxide and zeolite particles existed separately, and that it tended to be difficult to coat the inorganic crystals due to the small average particle size of the graphene oxide.

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

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

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

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

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

[0097]

Table 1

[0098]

Table 2

[0099] <Evaluation of Composite Particles and Coated Particles: Nitrogen Selectivity of Gas Separation Membrane> 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 the nitrogen selectivity was evaluated. The results are shown in Table 3 and Fig. 5. Note that the nitrogen selectivity shown in Table 3 and Fig. 5 is a relative value based on the nitrogen selectivity of the gas separation membrane of Comparative Example 1.

[0100] [Preparation of Gas Separation Membrane] The black solid obtained in each example or each comparative example, or the MFI zeolite of the reference example, was placed in a tablet press (IR die compression set) and pressure-molded at 18 kN to prepare a gas separation membrane with a diameter of 5 mm. For Examples 9 and 10, the sheet-like material collected on the filter paper was directly pressure-molded 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 disk-shaped acrylic plate was prepared, and a through-hole with a diameter of 1 mm was provided at the center. Next, the gas separation membrane prepared as described above was placed on the acrylic plate so as to be located at the center of the disk, and the outer periphery of the gas separation membrane was adhered to the substrate using an adhesive (Araldite) so as to be fixed, and this was used as a measurement sample.

[0102] A chamber was provided on the surface of the gas separation membrane in the measurement sample opposite to the acrylic plate side so that the gas to be measured could be pressurized. Also, the surface (permeation side) of the gas separation membrane on the acrylic plate side was set to atmospheric pressure, and the gas that permeated through the gas separation membrane was made to flow through the through-hole provided in the acrylic plate to the flow meter, and the gas permeation amount was measured with the flow meter. Note that the difference between the pressure on the pressurized side of the gas separation membrane and atmospheric pressure was defined as ΔP.

[0103] As the gas to be measured, nitrogen gas and oxygen gas were used. The permeation time when a unit amount of gas permeated through the gas separation membrane was measured, and the gas permeation rate R (unit: mol / (m 2 ·s·Pa)) was calculated for the gas to be measured. The temperature of the measurement atmosphere was set to 25°C. The area of the gas separation membrane was measured, and the gas permeation rate R(N2) for nitrogen gas and the gas permeation rate R(O2) for oxygen gas were calculated. The nitrogen selectivity of the gas separation membrane was determined by calculating the ratio of the obtained gas permeation rates according to the following formula. [Nitrogen selectivity of gas separation membrane (N2 / O2)] = [Gas permeation rate R(N2) for nitrogen gas] / [Gas permeation rate R(O2) for oxygen gas]

[0104]

Table 3

[0105] As can also be confirmed from the results shown in Table 3, by using the composite particles prepared in the examples, the gas separation performance (for example, nitrogen selectivity) can be improved even in the separation of gases with similar molecular sizes such as oxygen and nitrogen.

Industrial Applicability

[0106] According to the present disclosure, novel composite particles can be provided, and a method for manufacturing the same can be provided. According to the present disclosure, a gas separation membrane with few defects can also be provided.

Description of Reference Numerals

[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 extraction port, d2... second gas extraction port, G1... raw material gas, G2... residual gas, G3... separated gas.

Claims

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

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

3. A gas separation membrane composed of the composite particles according to claim 1 or 2.

4. A first step of precipitating aggregates from a dispersion containing an inorganic crystal, graphene oxide, and an ammonium salt; A second step of subjecting the aggregates to a reduction treatment, and comprising: The content of graphene oxide in the dispersion is 11% by mass or more based on the total amount of the inorganic crystal and graphene oxide; The concentration of the ammonium salt in the dispersion is more than 0.15 mol / L and less than 0.30 mol / L; A method for producing a composite particle, wherein the average particle diameter of the graphene oxide is larger than the average particle diameter of the inorganic crystal, and the inorganic crystal includes zeolite.

5. A method for producing a composite particle, further comprising a mixing step of mixing a dispersion A containing graphene oxide and a dispersion B containing an inorganic crystal and an ammonium salt to obtain the dispersion. When the ultraviolet-visible absorption spectrum of the dispersion liquid A is measured, the wavelength of the excitation light corresponding to the maximum value of the absorption peak associated with the π-π ＊ transition is 229.8 to 231.2 nm. The manufacturing method according to claim 4.

6. The production method according to claim 4 or 5, wherein the average particle diameter of the graphene oxide is 10 μm or more.

7. The production method according to claim 4 or 5, wherein the reduction treatment in the second step is performed by heating the aggregates to 300 ° C. or higher.

8. The production method according to claim 4 or 5, wherein the second step is performed in an atmosphere containing at least one selected from the group consisting of a noble gas, nitrogen gas, and hydrogen gas.

9. The production method according to claim 4 or 5, wherein the ammonium salt includes at least one selected from the group consisting of ammonium chloride, ammonium acetate, ammonium nitrate, ammonium bicarbonate, ammonium carbonate, ammonium oxalate, and ammonium sulfate.

10. A first step of precipitating aggregates from a dispersion containing an inorganic crystal, graphene oxide, and an ammonium salt; A second step of subjecting the aggregates to a reduction treatment to obtain a reduction-treated product; A third step of compression molding the reduction-treated product, and comprising: The content of the graphene oxide in the dispersion liquid is 11% by mass or more based on the total amount of the inorganic crystal and the graphene oxide. The concentration of the ammonium salt in the dispersion liquid is more than 0.15 mol / L and less than 0.30 mol / L. The average particle diameter of the graphene oxide is larger than the average particle diameter of the inorganic crystal. A method for manufacturing a gas separation membrane, wherein the inorganic crystal contains zeolite.

11. A gas separation module comprising the gas separation membrane according to Claim 3.

12. A gas separation device comprising the gas separation module according to Claim 11.

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