Porous carbon production method and carbon dioxide separation method

Abstract

The present disclosure provides a porous carbon production method by which porous carbon containing a large amount of ultra-micropores can be easily and inexpensively produced. A porous carbon production method according to an aspect of the present disclosure comprises: a first pulverization step for pulverizing an easily-graphitizable carbon raw material; a step for infusibilizing the easily-graphitizable carbon raw material pulverized in the first pulverization step; and a step for solid-phase-carbonizing the easily-graphitizable carbon raw material infusibilized in the infusibilization step.

Description

Porous carbon manufacturing method and carbon dioxide separation method

[0001] The present disclosure relates to a method for producing porous carbon and a method for separating carbon dioxide.

[0002] In recent years, carbon dioxide is considered to have a large impact on global warming. Carbon dioxide separation technology has attracted attention as an effective measure against this global warming problem.

[0003] Porous carbon, which has many pores, has attracted attention as a material for separating gases such as carbon dioxide. This porous carbon, also known as molecular sieve carbon, can separate not only carbon dioxide but also other gases depending on the size of the pores.

[0004] JP 2016-41656 A

[0005] Porous carbon capable of separating carbon dioxide is thought to be useful if it contains a large number of ultramicropores with a pore size of 0.7 nm or less.

[0006] Patent Document 1 describes a method for producing porous carbon, in which a mixture of a polyamic acid resin and an oxide is carbonized, the oxide is removed, and amorphous porous carbon is obtained, and the amorphous porous carbon is then heat-treated at a temperature equal to or higher than the crystallization temperature. However, such a method using a non-graphitizable carbon raw material involves a complicated production process, which tends to increase the cost of producing porous carbon.

[0007] On the other hand, one possible method for producing porous carbon is to carbonize a carbon source such as coal or a plant and activate it with steam. However, this method has problems such as difficulty in controlling the pore size, high activation costs, and low yield.

[0008] The present disclosure has been made in light of the above circumstances, and aims to provide a method for producing porous carbon that can easily and inexpensively produce porous carbon containing a large amount of ultramicropores.

[0009] A method for producing porous carbon according to one embodiment of the present disclosure includes a first pulverization step of pulverizing a graphitizable carbon raw material, a step of stabilizing the graphitizable carbon raw material pulverized in the first pulverization step, and a step of solid-phase carbonization of the graphitizable carbon raw material stabilized in the stabilization step.

[0010] A method for producing porous carbon according to one embodiment of the present disclosure can easily and inexpensively produce porous carbon containing a large amount of ultramicropores.

[0011] FIG. 1 is a flow diagram showing a method for producing porous carbon according to an embodiment of the present disclosure. FIG. 2 is a flow diagram showing a carbon dioxide separation method using the method for producing porous carbon shown in FIG. 1. FIG. 3 is a flow diagram showing a method for producing porous carbon according to an embodiment different from FIG. 1. FIG. 4 is a flow diagram showing a carbon dioxide separation method using the method for producing porous carbon shown in FIG. 3. FIG. 5 is a graph showing a cumulative curve of particle size distribution of petroleum pitch after the first crushing step of No. 1. FIG. 6 is a graph showing the relationship between the treatment time of porous carbon No. 1 and the amount of carbon dioxide adsorbed. FIG. 7 is a graph showing the heat generation process due to carbon dioxide adsorption in porous carbon No. 1. FIG. 8 is a schematic cross-sectional view estimated for porous carbon No. 1. FIG. 9 is a graph showing the relationship between the particle size of porous carbon No. 1 and the proportion of the inner layer. FIG. 10 is a graph showing the relationship between the treatment time of porous carbon No. 2 and the amount of carbon dioxide adsorbed. FIG. 11 is a graph showing the heat generation process due to carbon dioxide adsorption in porous carbon No. 2. FIG. 12 is a schematic cross-sectional view estimated for porous carbon No. 1. 1 is a graph showing the relationship between the treatment time of porous carbon in Example 5 and the amount of carbon dioxide adsorbed.

[0012] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0013] (1) A method for producing porous carbon according to one aspect of the present disclosure includes a first pulverization step of pulverizing a graphitizable carbon raw material, a step of stabilizing the graphitizable carbon raw material pulverized in the first pulverization step, and a step of solid-state carbonization of the graphitizable carbon raw material stabilized in the stabilization step.

[0014] Since the method for producing porous carbon uses the graphitizable carbon raw material, it is possible to produce the porous carbon more inexpensively than conventional methods using non-graphitizable carbon raw materials. Furthermore, the method for producing porous carbon can spontaneously form ultramicropores in the resulting porous carbon by subjecting the graphitizable carbon raw material pulverized in the first pulverization step to the infusibilization step and the solid-phase carbonization step. Therefore, the method for producing porous carbon can easily and inexpensively produce porous carbon containing a large amount of ultramicropores.

[0015] (2) In the method (1), it is preferable to further include a second pulverization step of pulverizing the graphitizable carbon raw material after the infusibilizing step. With this configuration, porous carbon having ultramicropores can be easily produced.

[0016] (3) In the above (2), the second pulverization step may be carried out before the solid-phase carbonization step, which makes it easier to increase the gas adsorption amount and adsorption rate of the resulting porous carbon.

[0017] (4) In any one of (1) to (3) above, the graphitizable carbon raw material may have an average particle size of 20 μm or less during the solid-state carbonization step. This configuration makes it easier to increase the gas adsorption amount, adsorption rate, etc. of the resulting porous carbon.

[0018] (5) In any one of (1) to (3) above, the graphitizable carbon raw material may be pulverized to an average particle size of 20 μm or less in the first pulverization step. This configuration allows the gas adsorption amount, adsorption rate, etc. of the resulting porous carbon to be increased.

[0019] (6) In any of (1) to (5) above, the amount of carbon dioxide adsorbed by the porous carbon may be 4 parts by mass or more per 100 parts by mass of the porous carbon at 30° C. and 100 kPa in a 100% carbon dioxide atmosphere. By using this method for producing porous carbon, the amount of carbon dioxide adsorbed can be made equal to or greater than the lower limit.

[0020] (7) In any one of the above (1) to (6), the porous carbon may be a carbon dioxide separating material. The porous carbon is suitable as a carbon dioxide separating material.

[0021] (8) A carbon dioxide separation method according to another aspect of the present disclosure includes a step of contacting a gas containing carbon dioxide with porous carbon produced by the method for producing porous carbon described in any one of (1) to (6) above.

[0022] According to this carbon dioxide separation method, carbon dioxide can be separated easily and inexpensively.

[0023] (9) In the above (8), the gas may be a combustion exhaust gas generated when a carbon-containing material is burned. Recently, as a measure against global warming, it has been desired to efficiently separate carbon dioxide from combustion exhaust gas generated by the use of fossil fuels, etc. In this regard, the carbon dioxide separation method can efficiently separate carbon dioxide and can effectively function as a measure against global warming.

[0024] In the present disclosure, the term "graphitizable carbon raw material" refers to a group of organic raw material compounds for producing carbon that graphitizes when heat-treated at a high temperature of 2500°C or higher under atmospheric pressure. The "graphitizable carbon raw material" preferably has the property of liquid-phase carbonization. The term "non-graphitizable carbon raw material" refers to a group of organic raw material compounds for producing carbon that maintains a random structure and does not graphitize even when heat-treated at 2500°C under atmospheric pressure.

[0025] The term "pulverization" in the first pulverization step refers to pulverizing the graphitizable carbon raw material to an average particle size of 150 μm or less. The term "average particle size" of the graphitizable carbon raw material refers to the 50% value (50% particle size, D50) of a volume-based cumulative particle size distribution curve measured by a laser diffraction method.

[0026] The term "infusible" refers to a treatment for preventing the graphitizable carbon raw material from melting in the solid-phase carbonization step. The term "solid-phase carbonization" refers to carbonization without softening (melting).

[0027] [Details of the Embodiments of the Present Disclosure] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the drawings as appropriate. It should be noted that with respect to the numerical values ​​described in this specification, it is possible to adopt only one of the upper and lower limit values ​​described, or to arbitrarily combine the upper and lower limit values. In this specification, all combinable numerical ranges are described as suitable ranges. In this disclosure, the terms "first" and "second" are used to distinguish the components to which they are attached, and do not limit the number, order, priority, etc.

[0028] [First embodiment] <Method for producing porous carbon> As shown in FIG. 1 , the method for producing porous carbon includes a first pulverization step S1 of pulverizing a graphitizable carbon raw material, a step (infusibility step S2) of stabilizing the graphitizable carbon raw material pulverized in the first pulverization step S1, and a step (solid-phase carbonization step S3) of solid-phase carbonizing the graphitizable carbon raw material stabilized in the stabilization step S2.

[0029] Since the method for producing porous carbon uses the graphitizable carbon raw material, it is possible to produce porous carbon more inexpensively than conventional methods using non-graphitizable carbon raw materials. Furthermore, the method for producing porous carbon allows the graphitizable carbon raw material pulverized in the first pulverization step S1 to undergo a stabilization step S2 and a solid-phase carbonization step S3, thereby spontaneously forming ultramicropores in the resulting porous carbon. Therefore, the method for producing porous carbon can easily and inexpensively produce porous carbon containing a large number of ultramicropores. Note that "ultramicropores" refers to pores with a pore diameter of 0.7 nm or less.

[0030] [Graphitizable Carbon Raw Material] Examples of the graphitizable carbon raw material include pitches, ashless coal, etc. Examples of the pitches include coal pitch and petroleum pitch. These raw materials may be used alone or in combination of two or more.

[0031] Pitches are preferred as the graphitizable carbon raw material. Pitches are graphitizable because they are used as binders for producing coke in steelmaking and as raw materials for producing graphite. Porous carbon has a large number of uniformly sized ultramicropores, which allows it to appropriately separate gas molecules based on their size. From this perspective, pitches are considered to be advantageous for forming uniformly sized ultramicropores because their microstructure does not contain heterogeneous structures found in other coals and plants. Furthermore, pitches are relatively inexpensive, which allows for reduced production costs of porous carbon.

[0032] Ashless coal is also preferred as the graphitizable carbon raw material. Ashless coal is a thermoplastic liquid at around 300°C, and is graphitizable so that a liquid-phase carbonization reaction proceeds. Ashless coal is obtained by extraction from coal at 400°C or less, and therefore contains many heteroatoms such as oxygen, which makes it easy to increase the specific surface area per unit volume. Furthermore, ashless coal has a large heteroatom content and a large molecular weight, which makes it easy to improve the yield in the solid-phase carbonization step S3.

[0033] In the present disclosure, "ashless coal" (Hypercoal, HPC) is a type of modified coal obtained by modifying coal, and is modified coal obtained by removing as much ash and insoluble components as possible from coal using a solvent. On the other hand, ashless coal may contain ash to the extent that it does not significantly impair the fluidity or expandability of the ashless coal. Generally, coal contains 7% by mass or more and 20% by mass or less of ash, but ashless coal may contain about 2% by mass, and in some cases about 5% by mass of ash. Note that "ash content" refers to a value measured in accordance with JIS-M8812:2004.

[0034] (First Pulverization Step) In the first pulverization step S1, the graphitizable carbon raw material is pulverized to an average particle size of 150 μm or less. In the first pulverization step S1, a plurality of the graphitizable carbon raw materials are mechanically pulverized.

[0035] In the first pulverization step S1, it is preferable to pulverize the graphitizable carbon raw material to an average particle size of 20 μm or less. According to the findings of the present inventors, by pulverizing the graphitizable carbon raw material to an average particle size of 20 μm or less in the first pulverization step S1, it is easy to increase the gas adsorption amount of the resulting porous carbon. Furthermore, this configuration makes it easy to increase the gas adsorption rate of the resulting porous carbon. The reason why pulverizing the graphitizable carbon raw material in the first pulverization step S1 can increase the gas adsorption amount and gas adsorption rate of the porous carbon is not clear, but the following reason is presumed. The mechanism by which the graphitizable carbon raw material becomes porous is not necessarily clear, but it is thought that the low-molecular-weight gases (CO, CO) generated during the carbonization process increase the gas adsorption amount and gas adsorption rate of the porous carbon. ２ , C.H. ４ It is believed that traces of water escaping from the particles and traces of water escaping from the particles contribute to the formation of pores. Therefore, by pulverizing the carbon before the stabilization step S2 and then proceeding with the stabilization all the way to the center of the particles in the stabilization step S2, oxygen, which is a source of low-molecular-weight gases, can be uniformly supplied to the inside of the particles, and by promoting the detachment of the low-molecular-weight gases, etc. in the solid-phase carbonization step S3, it is believed that the porosity can be improved, and in turn, the amount and rate of gas adsorption by the porous carbon can be increased.

[0036] As a means for finely pulverizing the graphitizable carbon raw material in the first pulverization step S1, for example, freeze pulverization is preferred. The upper limit of the average particle size of the graphitizable carbon raw material pulverized in the first pulverization step S1 may be 15 μm, 10 μm, or 5 μm. The lower limit of the average particle size of the graphitizable carbon raw material pulverized in the first pulverization step S1 is not particularly limited, but may be 1 μm or 1.2 μm from the viewpoint of preventing the labor required for pulverization from becoming unnecessarily large and from the viewpoint of making it easy to control the particle size of the resulting porous carbon. In the present disclosure, the term "adsorption amount" simply refers to the adsorption amount per unit volume.

[0037] (Infusibilizing Step) In the infusibilizing step S2, the graphitizable carbon raw material is prevented from melting during the carbonization process. By including the infusibilizing step S2, the method for producing porous carbon can produce porous carbon having desired ultramicropores.

[0038] In the infusibilization step S2, the graphitizable carbon raw material is infusibilized by heating in an oxygen-containing atmosphere using, for example, a known heating furnace. The oxygen-containing atmosphere is generally an air atmosphere.

[0039] In the stabilizing step S2, the graphitizable carbon raw material is maintained within a predetermined temperature range. The lower limit of the stabilizing treatment temperature in the stabilizing step S2 is preferably 150°C, more preferably 180°C, from the viewpoint of achieving sufficient stabilization and preventing the stabilization time from becoming too long. On the other hand, the upper limit of the stabilizing treatment temperature is preferably 350°C, more preferably 320°C, from the viewpoint of preventing the graphitizable carbon raw material from melting before stabilization. The temperature rise rate in the stabilizing step S2 can be, for example, 0.01°C / min or more and 10°C / min or less.

[0040] The lower limit of the holding time at the infusibilization treatment temperature in the infusibilization step S2 is preferably 10 minutes, more preferably 20 minutes, from the viewpoint of achieving sufficient infusibilization, while the upper limit of the holding time is preferably 120 minutes, more preferably 90 minutes, from the viewpoint of improving the production efficiency of the porous carbon.

[0041] (Solid-state carbonization step) In the solid-state carbonization step S3, the graphitizable carbon raw material after the infusibility step S2 is solid-state carbonized by heat treatment. That is, in this method for producing porous carbon, the graphitizable carbon raw material is not melted in the carbonization process. In this method for producing porous carbon, porous carbon is obtained by the solid-state carbonization step S3. The solid-state carbonization step S3 can be performed, for example, by a known heating unit.

[0042] The heating section carbonizes the graphitizable carbon raw material by heating while substantially maintaining its aggregated state. A known electric furnace, for example, can be used as the heating section. In the solid-phase carbonization step S3, the low-crystalline graphitizable carbon raw material is inserted into the heating section, the atmosphere in the heating section is replaced with an inert gas, and then heating is performed while blowing the inert gas into the heating section, thereby solid-phase carbonizing the graphitizable carbon raw material. The inert gas is not particularly limited, but examples include nitrogen and argon, with inexpensive nitrogen being preferred.

[0043] In the solid-phase carbonization step S3, the graphitizable carbon raw material is maintained within a predetermined temperature range. The lower limit of the solid-phase carbonization temperature in the solid-phase carbonization step S3 is preferably 600°C, more preferably 900°C, from the viewpoint of sufficiently promoting carbonization and the development of pores (ultramicropores). On the other hand, the upper limit of the solid-phase carbonization temperature is preferably 1300°C, more preferably 1100°C, from the viewpoint of maintaining porosity and preventing unnecessary increases in production costs. The temperature rise rate in the solid-phase carbonization step S3 can be, for example, 0.01°C / min or more and 10°C / min or less.

[0044] The lower limit of the holding time at the solid-state carbonization treatment temperature in the solid-state carbonization step S3 is preferably 10 minutes, more preferably 20 minutes, from the viewpoint of achieving sufficient solid-state carbonization, while the upper limit of the holding time is preferably 10 hours, more preferably 8 hours, from the viewpoint of improving the production efficiency of the porous carbon.

[0045] The upper limit of the average particle size of the graphitizable carbon raw material during the solid-phase carbonization step S3 (at the start of the solid-phase carbonization step S3) is preferably 20 μm, more preferably 15 μm, and even more preferably 10 μm. In other words, the upper limit of the average particle size of the porous carbon obtained by the porous carbon production method is preferably 20 μm, more preferably 15 μm, and even more preferably 10 μm. According to the findings of the present inventors, when the average particle size of the obtained porous carbon is equal to or less than the above upper limit, the gas adsorption capacity of the porous carbon can be increased. More specifically, the porous carbon can sufficiently increase the gas adsorption capacity while maintaining a high adsorption rate. Note that the lower limit of the average particle size of the graphitizable carbon raw material during the solid-phase carbonization step S3 may be 1 μm or 1.2 μm, from the viewpoint of preventing unnecessary increase in the labor required for pulverization.

[0046] (Other Steps) The method for producing porous carbon only needs to include the first pulverization step S1, the infusibilization step S2, and the solid-phase carbonization step S3, and may not include other steps. In other words, the method for producing porous carbon does not need to include an activation step or the like, as has been conventionally performed. However, the method for producing porous carbon does not intend to exclude the activation step. In addition, the method for producing porous carbon may include a molding step to improve the bulk density of the porous carbon or to make it easier to use the porous carbon as a molecular sieve carbon. The activation step and the molding step will be described in detail below.

[0047] (Activation Step) The activation step can be carried out, for example, after the solid-phase carbonization step S3 or in parallel with the solid-phase carbonization step S3. The activation step can be carried out by a known method, for example, by partially gasifying the surface of the graphitizable carbon raw material (C + CO ２ →2CO↑) ２ This can be done using activation.

[0048] (Molding step) The molding step can be carried out, for example, before the infusibilizing step S2, between the infusibilizing step S2 and the solid-phase carbonization step S3, or after the solid-phase carbonization step S3. In the molding step, pressure molding is carried out so that the bulk density of the porous carbon after the solid-phase carbonization step S3 is increased. In the molding step, the porous carbon may be molded into pellets. By molding the porous carbon into pellets, for example, when the molded article obtained by the molding step is used as a gas separation material, the packing density, gas contact efficiency, handleability, etc. of the molded article can be improved. Examples of molding methods in the molding step include known extrusion molding and granulation methods.

[0049] The molding step can be carried out using a known molding machine. The size and shape of the cavity of the mold used in the molding machine can be set appropriately depending on the intended use of the porous carbon, etc. For example, when the porous carbon is molded into a pellet, the porous carbon can be cylindrical with a diameter of 1 mm to 3 mm and a length of 3 mm to 10 mm. Note that the molding step may be carried out by adding a binder to the porous carbon or the graphitizable carbon raw material in order to improve the moldability of the porous carbon or the graphitizable carbon raw material.

[0050] When molding is carried out by adding a binder, any known binder that can suppress deterioration of the porosity of the porous carbon can be used as the binder, and examples thereof include pitches, resins, molasses, and starch.

[0051] When a binder is added for molding, the lower limit of the amount of the binder added to the obtained molded body is preferably 2% by mass, more preferably 4% by mass, from the viewpoint of preventing insufficient molding, while the upper limit of the amount of the binder added is preferably 10% by mass, more preferably 8% by mass, from the viewpoint of preventing an increase in production costs.

[0052] In the molding step, a heat treatment may be performed to remove or stabilize the binder in the molded body. For example, when the molding step is performed after the solid-phase carbonization step S3, it is preferable to perform the heat treatment.

[0053] From the viewpoint of production costs, the molding step is preferably carried out at room temperature (for example, 25° C.). The molding step may also be carried out at elevated temperatures in order to improve moldability.

[0054] In the molding step, it is preferable to form a molded body having voids of about 2 μm or more so as not to impair the gas diffusion performance of the resulting molded body.

[0055] [Porous Carbon] The porous carbon produced by the porous carbon production method is configured as, for example, a gas separation material. The porous carbon may be used, for example, to separate nitrogen and oxygen in air, or to separate nitrogen and methane. In particular, the porous carbon is preferably used as a carbon dioxide separation material. The porous carbon produced by the porous carbon production method contains a large number of ultramicropores, and these ultramicropores have a relatively uniform pore size, so that carbon dioxide can be easily separated from a gas containing carbon dioxide. As described above, the porous carbon can be formed into a molded body through the molding process. Even in this case, the performance of the molded body is determined by the substrate. Therefore, when the porous carbon contains a large number of ultramicropores, the gas adsorption amount, adsorption rate, etc. of the molded body can be easily increased.

[0056] The amount of carbon dioxide adsorbed by the porous carbon at 30°C and 100 kPa in a 100% carbon dioxide atmosphere is preferably 3 parts by mass or more, more preferably 3.5 parts by mass or more, even more preferably 4 parts by mass or more, and particularly preferably 4.5 parts by mass or more, per 100 parts by mass of the porous carbon. According to this method for producing porous carbon, it is possible to make the amount of carbon dioxide adsorbed equal to or more than the lower limit.

[0057] As described above, the porous carbon is suitable for use as a gas separation material. It is known that in porous carbon made from organic compounds, contraction of the carbon matrix occurs at temperatures above 700°C, resulting in a decrease in pore size (Yuzo Sanada et al., Activated Carbon - Fundamentals and Applications, Kodansha (1992)). This property can be utilized to control the pore size and, ultimately, the gas separation performance of the porous carbon depending on the gas type. However, the present disclosure does not intend to limit the use of the porous carbon to gas separation materials. The porous carbon can also be used, for example, as an adsorbent for water treatment, electronic components, and electrode materials for electric double layer capacitors.

[0058] <Carbon dioxide separation method> The porous carbon obtained by this porous carbon manufacturing method can be used in a gas separation method, and in particular in a carbon dioxide separation method. Furthermore, this porous carbon manufacturing method can be configured as part of a gas separation method. Generally, when porous carbon is used as a gas separation material, it is desirable that the porous carbon have excellent adsorption capacity, adsorption rate, and the like. In this regard, the porous carbon manufactured by this porous carbon manufacturing method is sufficiently pulverized in the first pulverization step S1 and manufactured through the infusibilization step S2 and solid-phase carbonization step S3. Therefore, the porous carbon contains a large number of ultramicropores, and has excellent adsorption capacity and adsorption rate. In particular, this porous carbon manufacturing method can be suitably configured as part of a carbon dioxide separation method. Hereinafter, a carbon dioxide separation method using porous carbon obtained by the porous carbon manufacturing method of FIG. 1 will be described with reference to FIG. 2 .

[0059] As shown in Figure 2, the carbon dioxide separation method includes a step of contacting a gas containing carbon dioxide with porous carbon produced by the method for producing porous carbon (contact step S5). The carbon dioxide separation method also includes a step of shaping the porous carbon produced by the method for producing porous carbon (shaping step S4). The porous carbon used in the carbon dioxide separation method is obtained through a first pulverization step S1, a stabilization step S2, and a solid-phase carbonization step S3. The first pulverization step S1, the stabilization step S2, and the solid-phase carbonization step S3 are the same as the first pulverization step S1, the stabilization step S2, and the solid-phase carbonization step S3 in Figure 1. Furthermore, the porous carbon used in the carbon dioxide separation method may be obtained through the activation step described above in the method for producing porous carbon in Figure 1.

[0060] The carbon dioxide separation method uses porous carbon produced by the porous carbon production method, and therefore can separate carbon dioxide easily and inexpensively.

[0061] (Shaping Step) The shaping step S4 can be carried out, for example, before the infusibilizing step S2, between the infusibilizing step S2 and the solid-phase carbonization step S3, or after the solid-phase carbonization step S3. The shaping step S4 can be carried out in the same procedure as the shaping step described in the method for producing the porous carbon.

[0062] In the contacting step S5, a gas containing carbon dioxide is brought into contact with the compact formed in the forming step S4. The concentration of carbon dioxide in the gas, the contact temperature between the gas and the porous carbon, and the like can be set based on the implementation environment of the carbon dioxide separation method, the carbon dioxide adsorption efficiency, and the like.

[0063] The gas is not particularly limited as long as it contains carbon dioxide, and may be, for example, composed solely of carbon dioxide. Furthermore, the gas may be a combustion exhaust gas generated when a carbon-containing material is burned. Recently, in order to combat global warming, it has become desirable to efficiently separate carbon dioxide from combustion exhaust gas generated by the use of fossil fuels, etc. In this regard, the carbon dioxide separation method can efficiently separate carbon dioxide and effectively function as a global warming countermeasure.

[0064] [Second embodiment] <Method for producing porous carbon> As shown in Fig. 3, the method for producing porous carbon includes a first pulverization step S11 of pulverizing a graphitizable carbon raw material, a step (infusibility step S12) of stabilizing the graphitizable carbon raw material pulverized in the first pulverization step S11, and a step (solid-phase carbonization step S14) of solid-phase carbonizing the graphitizable carbon raw material stabilized in the stabilization step S12. The method for producing porous carbon also includes a second pulverization step S13 of pulverizing the graphitizable carbon raw material after the stabilization step S12. The method for producing porous carbon can be performed in the same manner as the method for producing porous carbon shown in Fig. 1, except that the second pulverization step S13 is included in addition to the first pulverization step S11 when pulverizing the graphitizable carbon raw material.

[0065] The method for producing porous carbon uses the graphitizable carbon raw material, and therefore can produce the porous carbon more inexpensively than conventional methods using non-graphitizable carbon raw materials. The method for producing porous carbon can easily reduce the particle size of the graphitizable carbon raw material by performing the second pulverization step S13 after the infusibilization step S12. Therefore, the method for producing porous carbon can more easily produce porous carbon containing a large amount of ultramicropores.

[0066] The graphitizable carbon raw material used in this method for producing porous carbon can be the same as the graphitizable carbon raw material in the method for producing porous carbon of Fig. 1. The infusible step S12 in this method for producing porous carbon can be carried out in the same procedure as the infusible step S2 in the method for producing porous carbon of Fig. 1. Furthermore, the solid-phase carbonization step S14 in this method for producing porous carbon can be carried out in the same procedure as the solid-phase carbonization step S3 in the method for producing porous carbon of Fig. 1. Therefore, only the first pulverization step S11 and the second pulverization step S13 will be described below. Note that, like the method for producing porous carbon of Fig. 1, this method for producing porous carbon may include one or both of the activation step and the shaping step as other steps.

[0067] (First Pulverization Step) In the first pulverization step S11, the graphitizable carbon raw material is pulverized to an average particle size of 150 μm or less. In the first pulverization step S11, a plurality of the graphitizable carbon raw materials are mechanically pulverized. In the first pulverization step S11, the graphitizable carbon raw material may be pulverized to an average particle size of 20 μm or less. On the other hand, the average particle size of the graphitizable carbon raw material pulverized in the first pulverization step S11 may be more than 20 μm. For example, the lower limit of the average particle size of the graphitizable carbon raw material pulverized in the first pulverization step S11 may be 50 μm or 100 μm. In the first pulverization step S11, the smaller the particle size of the graphitizable carbon raw material, the more easily the graphitizable carbon raw material generates heat and a plurality of graphitizable carbon raw materials bond together. In contrast, by controlling the average particle size of the graphitizable carbon raw material within the above range in the first pulverization step S11, it is possible to easily control the particle size of the graphitizable carbon raw material and form a large amount of ultramicropores in the resulting porous carbon.

[0068] (Second Pulverization Step) In the present embodiment, the second pulverization step S13 is performed between the infusibilization step S12 and the solid-phase carbonization step S14. That is, in the method for producing porous carbon, the second pulverization step S13 is performed before the solid-phase carbonization step S14. This configuration makes it easy to increase the gas adsorption amount, adsorption rate, etc. of the obtained porous carbon. It is not clear why performing the second pulverization step S13 between the infusibilization step S12 and the solid-phase carbonization step S14 makes it easy to increase the gas adsorption amount, adsorption rate, etc. of the porous carbon, but it is thought that by performing pulverization before the solid-phase carbonization step S14, it is possible to promote the desorption of low-molecular-weight gases, etc. in the solid-phase carbonization step S14, improve the porosity of the porous carbon, and ultimately increase the gas adsorption amount, adsorption rate, etc. of the porous carbon.

[0069] In the second pulverization step S13, a plurality of the graphitizable carbon raw materials are mechanically pulverized. In the second pulverization step S13, it is preferable to pulverize the graphitizable carbon raw materials to an average particle size of 20 μm or less. By pulverizing the graphitizable carbon raw material to an average particle size of 20 μm or less in the second pulverization step S13, the gas adsorption amount, adsorption rate, etc. of the resulting porous carbon are likely to be increased. The upper limit of the average particle size of the graphitizable carbon raw material pulverized in the second pulverization step S13 may be 15 μm, 10 μm, or 5 μm. Note that the lower limit of the average particle size of the graphitizable carbon raw material pulverized in the second pulverization step S13 is not particularly limited, but may be 1 μm or 1.2 μm from the viewpoint of preventing unnecessary increase in the labor required for pulverization and from the viewpoint of easily controlling the particle size of the resulting porous carbon.

[0070] <Carbon dioxide separation method> The porous carbon obtained by this method for producing porous carbon can be used in a gas separation method, and in particular in a carbon dioxide separation method, similar to the porous carbon obtained by the method for producing porous carbon in Fig. 1. Furthermore, this method for producing porous carbon can be configured as part of a gas separation method, and in particular can be suitably configured as part of a carbon dioxide separation method, similar to the method for producing porous carbon in Fig. 1.

[0071] As shown in Figure 4, the carbon dioxide separation method includes a step of contacting a gas containing carbon dioxide with porous carbon produced by the porous carbon production method (contact step S16). The carbon dioxide separation method also includes a step of shaping the porous carbon produced by the porous carbon production method (shaping step S15). The porous carbon used in the carbon dioxide separation method is obtained through a first pulverization step S11, a stabilization step S12, a second pulverization step S13, and a solid-phase carbonization step S14. The first pulverization step S11, the stabilization step S12, the second pulverization step S13, and the solid-phase carbonization step S14 are the same as the first pulverization step S11, the stabilization step S12, the second pulverization step S13, and the solid-phase carbonization step S14 in Figure 3. Furthermore, the porous carbon used in the carbon dioxide separation method may be obtained through the activation step described in the porous carbon production method of Figure 3.

[0072] The forming step S15 in this carbon dioxide separation method is the same as the forming step S4 in the carbon dioxide separation method of Fig. 2. Furthermore, the contacting step S16 in this carbon dioxide separation method is the same as the contacting step S5 in the carbon dioxide separation method of Fig. 2.

[0073] The carbon dioxide separation method uses porous carbon obtained by the porous carbon production method, and therefore can separate carbon dioxide easily and inexpensively.

[0074] [Other Embodiments] The above-described embodiments do not limit the configuration of the present invention. Therefore, the above-described embodiments may include omissions, substitutions, or additions of components based on the description in this specification and common general technical knowledge, and all of these should be construed as falling within the scope of the present invention.

[0075] In the second embodiment, a configuration has been described in which the second pulverization step is performed before the solid-phase carbonization step. However, when the method for producing porous carbon includes the second pulverization step, the order of the second pulverization step is not particularly limited. For example, the method for producing porous carbon can also be configured so that the second pulverization step is performed after the solid-phase carbonization step. By performing the second pulverization step after the solid-phase carbonization step, the gas adsorption rate of the resulting porous carbon can be increased. Note that the method for producing porous carbon can also be configured to include another pulverization step in addition to the first and second pulverization steps.

[0076] The present disclosure will be described in detail below based on examples, but the present disclosure should not be construed as being limited based on the description of these examples.

[0077] [Example] (No. 1) Porous carbon was produced using commercially available petroleum pitch (softening point 200°C) as the graphitizable carbon raw material. First, the petroleum pitch was pulverized in an agate mortar to an average particle size of 150 μm or less (first pulverization step). The average particle size (D50) of the petroleum pitch after the first pulverization step was measured by laser diffraction and found to be 22.2 μm. Figure 5 shows a cumulative curve of the particle size distribution of the petroleum pitch. Next, the petroleum pitch pulverized in the first pulverization step was heated to 300°C at a heating rate of 1°C / min in air and held at 300°C for 1 hour to be infusibilized (infusibilization step). Subsequently, the petroleum pitch infusibilized in the infusibilization step was heated to 900°C at a heating rate of 3°C / min in a nitrogen atmosphere and held at 900°C for 0.5 hours to be solid-phase carbonized (solid-phase carbonization step).

[0078] The carbon dioxide adsorption process of the porous carbon obtained by the solid-phase carbonization step was measured by gravimetric method at 30°C under a 100% carbon dioxide atmosphere at 100 kPa. The amount of carbon dioxide adsorbed was 3 parts by mass or more (3.7 parts by mass) per 100 parts by mass of porous carbon. The relationship between the treatment time of the porous carbon and the amount of carbon dioxide adsorbed by this measurement is shown in Figure 6. Note that Figure 6 shows values ​​measured using 20 mg of porous carbon. In Figure 6, the vertical axis represents the weight change with the initial weight being 100%, and the difference from 100 represents the CO２ The graph shows the amount of adsorption. No correction was made for the change in buoyancy due to gas switching. In addition, in Figure 6, the end point of the arch-shaped curve where the mass of the porous carbon increases beyond 100% indicates the start time of carbon dioxide adsorption (0 seconds).

[0079] On the other hand, the amount of nitrogen adsorption of the same porous carbon was measured in the same manner as above at 30°C under a 100% nitrogen atmosphere at 100 kPa by gravimetric method, and the result was that the amount of nitrogen adsorption was 0.5 parts by mass per 100 parts by mass of porous carbon.

[0080] The porous carbon produced in No. 1 exhibits a significantly larger carbon dioxide adsorption capacity than nitrogen gas. From this, it can be said that the porous carbon produced by the porous carbon production method of the present disclosure has abundant ultramicropores and is suitable for use in carbon dioxide separation methods. In this example, the porous carbon was produced through a stabilization step and a solid-phase carbonization step. Although shrinkage and partial aggregation due to carbonization may occur in the solid-phase carbonization step, the primary particle size of the porous carbon can be approximately equal to the particle size of the petroleum pitch after the first pulverization step.

[0081] (Study Based on No. 1) The present inventors conducted extensive studies on the porous carbon of No. 1 in order to further improve the carbon dioxide adsorption efficiency. First, in order to investigate the carbon dioxide adsorption process in more detail, the exothermic process due to carbon dioxide adsorption was measured using a differential scanning calorimeter (DSC) using 10 mg of the porous carbon of No. 1. The measurement results are shown in Figure 7.

[0082] As shown in Figure 7, the exothermic peak of No. 1 porous carbon has two components: a fast adsorption component that ends within one minute after the start of adsorption, and a slow adsorption component that continues to tail even after 10 minutes from the start of adsorption. From these results, it is inferred that No. 1 porous carbon has an outer layer 1 in which adsorption is completed relatively early, and an inner layer 2 that exists inside this outer layer 1 and has a slow adsorption rate, as shown in Figure 8. Furthermore, in Figure 7, the ratio of the fast adsorption component to the slow adsorption component was calculated from the integrated values ​​of the graph, and was found to be fast component:slow component = 60.1:39.9.

[0083] Based on the above speculation, it is believed that in order to produce porous carbon with higher adsorption efficiency, it is effective to increase the proportion (volume proportion) of the outer layer 1 in the porous carbon. Theoretically, it can be speculated that the carbon dioxide adsorption efficiency can be effectively increased by controlling the particle diameter of the porous carbon to be no more than twice the thickness T of the outer layer 1.

[0084] Based on this assumption, and assuming that (1) the thickness of the outer layer 1 is constant regardless of the particle size of the porous carbon, and (2) the porous carbon is spherical, the thickness T of the outer layer 1 at which the proportion of the outer layer 1 is 60.1% was determined by trial and error using the cumulative curve of the particle size distribution in Figure 5. As a result, T was calculated to be approximately 5.0 µm. The relationship between the proportion (volume proportion) of the inner layer 2 and the particle size of the porous carbon based on this calculation result is shown in Figure 9.

[0085] [Study Results] As a result of the above study, it is estimated that the particle size of the porous carbon is preferably 10 μm or less from the viewpoint of carbon dioxide adsorption efficiency. Furthermore, based on Fig. 9, even when the particle size of the porous carbon is 20 μm, the proportion of the inner layer 2 is estimated to be about 20%, so it is estimated that a configuration with a particle size of 20 μm or less will provide sufficiently high carbon dioxide adsorption efficiency.

[0086] (No. 2) Based on the above study results, porous carbon was produced using the same petroleum pitch as in No. 1 as the graphitizable carbon raw material. First, the petroleum pitch was freeze-pulverized. The average particle size of the petroleum pitch after this pulverization was 1.7 μm (first pulverization step). Note that, after pulverization in the first pulverization step, the particle fraction of the petroleum pitch having a particle size of 10 μm or less was 90% or more. The petroleum pitch after pulverization in the first pulverization step was subjected to the infusibilization step and solid-phase carbonization step in the same procedures as in No. 1 to produce porous carbon.

[0087] For the porous carbon No. 2, the carbon dioxide adsorption process was measured by a gravimetric method using a thermogravimetric analyzer under a 100% carbon dioxide atmosphere at 30°C and 100 kPa. The relationship between the treatment time of the porous carbon and the amount of carbon dioxide adsorbed by this measurement is shown in Figure 10. Note that Figure 10 shows values ​​measured using 20 mg of porous carbon. In Figure 10, the vertical axis represents the weight change with the initial weight taken as 100%, and the difference from 100 represents the CO ２ The graph shows the amount of carbon dioxide adsorption. No correction was made for changes in buoyancy due to gas switching. In addition, in FIG. 10, the end point of the arched curve where the mass of the porous carbon increases beyond 100% represents the start point of carbon dioxide adsorption (0 seconds). FIG. 10 also shows the relationship between the treatment time of the porous carbon in No. 1 and the amount of carbon dioxide adsorption.

[0088] As shown in Fig. 10, the porous carbon of No. 2 has improved carbon dioxide adsorption capacity and adsorption rate compared to the porous carbon of No. 1. For example, the porous carbon of No. 2 has an adsorption capacity of 4 parts by mass or more per 100 parts by mass of porous carbon in a 100% carbon dioxide atmosphere at 30°C and 100 kPa.

[0089] The exothermic process due to carbon dioxide adsorption was measured by a differential scanning calorimeter (DSC) using 10 mg of porous carbon No. 2, and the results are shown in Figure 11. Figure 11 also shows the exothermic process due to carbon dioxide adsorption using porous carbon No. 1. As shown in Figure 11, in No. 2, the tailing component with a slow adsorption rate disappeared even after 10 minutes from the start of adsorption, and the proportion of the component with a fast adsorption rate became extremely large.

[0090] (No. 3) Porous carbon was produced using ashless coal prepared from bituminous coal as the graphitizable carbon raw material. First, the ashless coal was freeze-pulverized. The average particle size of the ashless coal after this pulverization was 2.1 μm (first pulverization step). The ashless coal after pulverization in the first pulverization step was subjected to a stabilization step and a solid-phase carbonization step in the same procedures as in No. 1, thereby producing porous carbon.

[0091] The carbon dioxide adsorption process of porous carbon No. 3 was measured by gravimetric method at 30°C under a 100% carbon dioxide atmosphere at 100 kPa, and the amount of carbon dioxide adsorbed was 9.2 parts by mass per 100 parts by mass of porous carbon. The carbon dioxide adsorption rate was also comparable to that of porous carbon No. 2.

[0092] (No. 4) Porous carbon was produced using ashless coal prepared from bituminous coal as the graphitizable carbon raw material. First, the ashless coal was freeze-pulverized. The average particle size of the ashless coal after this pulverization was not measured, but since the ashless coal was freeze-pulverized in the same manner as in No. 3, it is estimated to be around 2 μm (first pulverization step). The ashless coal after pulverization in the first pulverization step was subjected to a stabilization step and a solid-phase carbonization step in the same procedures as in No. 1, to produce porous carbon.

[0093] The carbon dioxide adsorption process of porous carbon No. 4 was measured by gravimetric method at 30°C under a 100% carbon dioxide atmosphere at 100 kPa, and the amount of carbon dioxide adsorbed was 7.6 parts by mass per 100 parts by mass of porous carbon. The carbon dioxide adsorption rate was also comparable to that of porous carbon No. 2.

[0094] Furthermore, for porous carbon No. 4, a 0.01 g sample was vacuum heated at 250°C for 3 hours, and then a nitrogen adsorption isotherm was measured up to 100 kPa at 25°C using a 0.01 g sample amount using a "Belsorp-mini" manufactured by Microtrac-Bell, but no adsorption isotherm was obtained. From this, it is considered that porous carbon No. 4 has excellent carbon dioxide / nitrogen adsorption selectivity and is suitable for separating carbon dioxide from combustion exhaust gas.

[0095] (No. 5) The porous carbon obtained after the solid-phase carbonization step in No. 1 was further pulverized (second pulverization step), and the porous carbon obtained after the second pulverization step was obtained as porous carbon No. 5. The average particle diameter (D50) of porous carbon No. 5 was measured by laser diffraction, and was found to be 1.8 μm.

[0096] For No. 5 porous carbon, the carbon dioxide adsorption process was measured by gravimetric method under a 100% carbon dioxide atmosphere at 30°C and 100 kPa. The relationship between the treatment time of the porous carbon and the amount of carbon dioxide adsorbed by this measurement is shown in Figure 12. Note that Figure 12 shows values ​​measured using 20 mg of porous carbon. In Figure 12, the vertical axis represents the weight change with the initial weight being 100%, and the difference from 100 represents the CO ２ The graph shows the amount of carbon dioxide adsorption. No correction was made for changes in buoyancy due to gas switching. In addition, in FIG. 12, the end point of the arched curve where the mass of the porous carbon increases beyond 100% represents the start point of carbon dioxide adsorption (0 seconds). FIG. 12 also shows the relationship between the treatment time of the porous carbon in No. 1 and the amount of carbon dioxide adsorption.

[0097] As shown in Fig. 12, the porous carbon No. 5 has an improved carbon dioxide adsorption rate compared to the porous carbon No. 1 by undergoing the second pulverization step. Furthermore, the porous carbon No. 5 has a slightly improved carbon dioxide adsorption amount compared to the porous carbon No. 1 by undergoing the second pulverization step. Note that the porous carbon No. 5 is inferior to the porous carbon No. 2 in both the carbon dioxide adsorption amount and adsorption rate. Therefore, in the present disclosure, from the viewpoint of the carbon dioxide adsorption amount and adsorption rate, it is considered more preferable to promote the microparticulation of the graphitizable carbon raw material in the first pulverization step prior to the stabilization step.

[0098] Comparative Example (No. 6) Porous carbon was produced using coal pitch as the graphitizable carbon raw material, and the infusibilization step and solid-phase carbonization step were carried out in the same manner as in No. 1. No. 6 differs from No. 1 and No. 5 in that neither the first pulverization step nor the second pulverization step was carried out. The carbon obtained by No. 6 had a particle size in the range of 200 μm to 800 μm. The amount of carbon dioxide adsorption of this carbon was measured at 30° C. in a 100% carbon dioxide atmosphere at 100 kPa, and was below the measurement limit.

[0099] [Evaluation Results] Pitch and other graphitizable carbonaceous materials are graphitizable carbonaceous materials. When they are carbonized as is, liquid-phase carbonization proceeds, leading to the development of a layered structure of polycyclic aromatic compounds, making it difficult to form ultramicropores. On the other hand, when the graphitizable carbonaceous material is stabilized and then subjected to solid-phase carbonization, a large number of ultramicropores are formed. While the reason for the formation of a large number of ultramicropores is unclear, several factors are speculated, including the isotropic molecular orientation of the raw material, which results in the formation and maintenance of intermolecular voids; the formation of oxygen bridges during stabilization inhibits layer formation, creating and maintaining intermolecular voids; and the generation and desorption of low-molecular-weight gases during carbonization, which results in the formation of voids. Furthermore, it has been found that by appropriately controlling the particle size of such porous carbon before solid-phase carbonization, the proportion of the outer layer in the porous carbon can be increased, thereby improving the carbon dioxide adsorption amount and adsorption rate. Furthermore, according to the present disclosure, it is possible to control the pore size of ultramicropores, thereby achieving good carbon dioxide / nitrogen separation properties and enabling efficient separation of carbon dioxide from combustion exhaust gas. Therefore, the method for producing porous carbon according to the present disclosure can be said to be effective as a measure against global warming.

[0100] 1 Outer layer 2 Inner layer T Thickness of outer layer

Claims

1. A method for producing porous carbon, comprising: a first pulverization step of pulverizing a graphitizable carbon raw material; a step of stabilizing the graphitizable carbon raw material pulverized in the first pulverization step; and a step of solid-phase carbonization of the graphitizable carbon raw material stabilized in the stabilization step.

2. The method for producing porous carbon according to claim 1, further comprising a second pulverization step of pulverizing the graphitizable carbon raw material after the infusibilization step.

3. The method for producing porous carbon according to claim 2, wherein the second crushing step is carried out before the solid-phase carbonization step.

4. A method for producing porous carbon according to any one of claims 1 to 3, wherein the graphitizable carbon raw material has an average particle size of 20 μm or less during the solid-phase carbonization step.

5. A method for producing porous carbon according to any one of claims 1 to 3, wherein the graphitizable carbon raw material is pulverized to an average particle size of 20 μm or less in the first pulverization step.

6. A method for producing porous carbon described in any one of claims 1 to 3, wherein the amount of carbon dioxide adsorbed by the porous carbon at 30°C and 100 kPa in a 100% carbon dioxide atmosphere is 4 parts by mass or more per 100 parts by mass of the porous carbon.

7. A method for producing porous carbon according to any one of claims 1 to 3, wherein the porous carbon is a carbon dioxide separation material.

8. A method for separating carbon dioxide, comprising a step of contacting a gas containing carbon dioxide with porous carbon produced by the method for producing porous carbon according to any one of claims 1 to 3.

9. The carbon dioxide separation method according to claim 8, wherein the gas is a combustion exhaust gas generated when a carbon-containing material is burned.

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