JPH07185327A - Adsorbent and filter media and utilization of the same - Google Patents

Abstract

PURPOSE:To collect and recover materials in a gas phase or liquid phase and to purify and classify the gas phase or the liquid phase by using carbon nanotube as an adsorbent or a filter media. CONSTITUTION:The nanotube is physically and chemically stable, has easily wettable property that is important from the viewpoint of adsorption and is high in adsorbability. Furthermore, the carbon nanotube forms for oneself a high density fine three dimensional narrow pores by integrating oneself and has not only absorptivity of molecular level but filtration function of fine particle level. And the nanotube is capable of being regenerated as the adsorbent or filter media. The example of collection and classification of a benzene aq. solution by the nanotube is shown in the followings. A spectrum A is a spectrum of the benzene aq. solution before the adsorption by the nanotube, B is spectrum after adsorption, C is a spectrum at the half way of the saturation of adsorption. The nanotube has high adsorption to benzene and then the materials in the liquid phase are removed and the liquid phase is purified.

Description

Detailed Description of the Invention

[0001]

INDUSTRIAL APPLICABILITY The present invention can be used in industrial fields involving purification, collection, recovery or fractionation of substances in a gas or liquid phase and the gas or liquid phase itself. Industrial applications such as medicine, food, metal, and electronics are included.

[0002]

2. Description of the Related Art Among conventional adsorbents, a carbon material typified by activated carbon has excellent physical and chemical stability and is most widely used. Normally, activated carbon is used in powder or granular form, but the former has relatively high adsorption activity,
It has the drawback that it is easily scattered and requires careful handling (health hazard due to dust explosion and inhalation into the lungs). The latter is easy to handle, but has the drawback of low adsorption capacity.

When used in the liquid phase, powdered activated carbon is usually used in a batch contact method (in which the adsorbent is directly introduced into the liquid phase and stirred), and it is not possible to expect an adsorption amount above the equilibrium adsorption amount. . Granular activated carbon is used by a liquid passing method (flowing a liquid phase with an adsorbent as a fixed bed), but requires a large amount of fixed bed due to its low adsorption capacity.

Further, both of them, like other carbon materials, are hard to be wet with a liquid and require special treatment. In addition, low-molecular weight organic substances are usually inherently present in the carbon material containing activated carbon. When using activated carbon or the like as the adsorbent, organic substances contained in the activated carbon or the like may be mixed in the gas phase or liquid phase after adsorption, especially when the activated carbon or the like is used to purify food or drink such as drinking water, the human body There is concern about the impact on Further, generally, the filtering effect on the fine particles of the carbon material such as activated carbon is not remarkable.

[0005]

The problem to be solved by the present invention is the drawbacks and disadvantages of carbon materials represented by activated carbon widely used as an adsorbent, that is, powdered activated carbon is difficult to handle ( (Dust explosion / health hazard), the fact that granular activated carbon has a low adsorption amount per unit, the disadvantage that it is difficult to wet the liquid common to all activated carbon, and the disadvantage that low molecular weight organic substances are mixed. At the same time, superior physical and chemical stability, high adsorption activity, and higher than carbon materials represented by activated carbon.
The aim is to develop an adsorbent / filter agent using nanotubes that has outstanding filtration properties.

[0006]

The gist of the present invention is the use of carbon nanotubes as adsorbents and filters. As the method of use, the batch contact method or the liquid passing method described in the section of the prior art can be used.

Nanotubes have physical and chemical stability equivalent to or higher than that of activated carbon, and are hard to scatter despite having a very fine structure. Since the nanotube is a perfect crystal composed of 100% carbon, it does not contain any low molecular weight organic matter. Further, as will be described later, it has an excellent adsorption ability and is very easily wet with a liquid. It is also renewable. Since nanotubes are very fine needle-like crystals, they are capable of self-forming high-density and minute three-dimensional pores by themselves accumulating. This indicates that it has not only the adsorption ability at the molecular level, but also the filtration function for fine particles of the order of micrometers and nanometers.

[0008]

[Function] Carbon nanotubes are very fine acicular graphite crystals and are attracting attention as new allotropes of carbon. The diameter of the nanotube is from about 1 nanometer to several tens of nanometers, and the length is several hundred nanometers to several micrometers, which is a very elongated straw-like shape. In addition, the side surface has a cylindrical graphite surface, and the tip portion is covered with a graphite curved surface, and has a closed structure.

In order to use the nanotubes industrially, it is necessary to mass-produce them in high yield, but in 1992, a large-scale synthesis method of nanotubes (Japanese Patent Application No. 4-172242).
Issue, Japanese Patent Application No. 4-311846) was found and the problem has been solved. In addition, a method of obtaining pure nanotubes by removing carbon nanoparticles (fine graphite polyhedrons) and amorphous carbon generated as by-products during the synthesis of nanotubes (Japanese Patent Application No. 0-014387 and Japanese Patent Application No. 5-133048) is proposed. Nanotubes that have been developed and have unique electrical and physical / chemical properties can be applied to a wide range of next-generation industrial fields ranging from chemistry to electronics.

Since the nanotube has a structure in which reticulated carbon is closed without breaks and is a perfect crystal without so-called defects or dislocations, the physical and mechanical strength is very high. According to theoretical predictions, tensile strength is said to exceed that of diamond. Further, since the nanotube is a complete crystal composed of 100% carbon, there is no possibility that a low-molecular weight organic substance is mixed in, even considering the synthetic method. Examples of the chemical stability of nanotubes include high temperature resistance (in the presence of oxygen) and chemical resistance (eg, reactivity with strong acids such as sulfuric acid and nitric acid). In the case of C ₆₀ , which is one of the allotropes of carbon, combustion starts at a temperature of less than 500 ° C in the presence of oxygen, whereas in the case of nanotubes, it does not burn up to around 700 ° C (Nature (Na
true), Vol. 362, No. 6420, pp.
522-525, April 8, 1993). In the case of carbon whiskers, which have high heat resistance (in the presence of air) among carbon materials, the heat resistance limit temperature is 700 ° C (for example, manufactured by Nikkiso,
Product name: Glasker GRASKER, described in the characteristics table). Therefore, the high temperature heat resistance of nanotubes in the presence of oxygen is one of the most excellent carbon materials.

Further, a powder of a conventional carbon material (for example,
When amorphous carbon) is boiled in sulfuric acid and nitric acid at 180 ° C for 6 hours, the recovery rate (weight) of the carbon material as a raw material is 10% or less, whereas the nanotube (powder) is treated under the same conditions. When treated, the nanotube recovery is above 80%. The same results have been obtained with other corrosive reactants. These results are derived from the difference in structure between the carbon skeletons of ordinary carbon materials and nanotubes. That is, since a normal carbon material has a structure in which two-dimensional graphite surfaces are stacked in layers and the peripheral portion is open, the reaction substance penetrates between the layers and the reaction proceeds not only from the surface portion but also from the inside. On the other hand, because the nanotube has a structure in which the graphite surface is closed, the reason is that the reaction occurs only on the surface.

Furthermore, in the case of a nanotube, since it is a perfect crystal with no defects or transitions, there is no danglin bond (the bond is cut and the electron is exposed and the reactivity is very high) which is the starting point of the reaction. Another major reason is that nanotubes are inherently less reactive, in contrast to other carbon materials, which have many defects and dislocations. As described above, nanotubes have physical and chemical stability equivalent to or higher than other carbon materials including activated carbon, and these are advantageous characteristics in utilizing nanotubes.

As mentioned above, nanotubes are very fine, elongated needle-like crystals. Therefore, since the nanotubes in the bulk state are intricately entangled with each other in a three-dimensional manner, they are difficult to scatter. Further, by accumulating by themselves, it is possible to self-form high density and minute three-dimensional fine pores. This indicates not only the adsorption ability at the molecular level, but also the filtration function for fine particles and ultrafine particles of the order of micrometers and nanometers.

The wettability of the adsorbent with respect to the liquid is determined by collecting and separating substances in the liquid phase and purifying and recovering the liquid phase.
It is important in terms of whether the adsorbent works effectively. That is, as the adsorbent gets wet, the liquid phase penetrates into the adsorbent, and the adsorption efficiency is improved. The wettability of nanotubes with liquids can be shown by the following simple experiment. Water is dripped from the upper part where the nanotubes are spread. Then, water permeates the nanotube more and more. When a similar experiment is performed using powdered amorphous carbon, water is repelled to form water droplets and does not soak into the interior. The same results are obtained when the glassy carbon powder is used as when the amorphous carbon powder is used.

As described above, the nanotube is very easily wet by the liquid as compared with other carbon materials. It is considered that this is because the graphite surface forming the ordinary carbon material is flat, whereas the graphite surface forming the nanotube is cylindrical and spherical. That is, while the π orbitals of carbon, which is a constituent element, are parallel to each other in the case of other carbon materials, they are not parallel in the case of nanotubes and open outward at a certain angle. It is considered that the constituent molecules (or atoms) easily interact with the carbon atoms on the surface of the nanotube.

Bulk nanotubes are in powder form,
Since each nanotube is extremely fine, it can be compressed to a high density and processed into various shapes. Therefore,
When used as an adsorbent in the liquid phase, either powder or granules can be used. Batch contact method like powdered activated carbon (directly introducing adsorbent into liquid phase and stirring), liquid passing method like granular activated carbon. Both methods (flowing the liquid phase with the adsorbent as a fixed bed) can be used. Further, when the adsorbate is vaporized, by combining a heating device and an adsorbate gas collector, not only the adsorbate can be collected and separated, but also purified and recovered. Furthermore, it is possible to easily regenerate the nanotubes as an adsorbent and withstand repeated use. From the examples shown in the next section, it can be seen that the nanotubes have high adsorption ability.

A technique for purifying a nanotube and opening both ends of the nanotube (Japanese Patent Application No. 5-133048).
It is also possible to use the inside of the nanotube as an adsorption site by using together. When using open nanotubes, an increase in adsorption of 20-50% is seen.

From the above, it can be seen that nanotubes have physical and chemical stability equivalent to or higher than that of activated carbon and are hard to scatter even though they have a very fine structure. Further, it has an excellent adsorption ability and is very easily wetted by a liquid, and does not mix a low molecular weight organic substance into the gas phase or the liquid phase after adsorption. Furthermore, by accumulating by themselves, they form high-density and minute three-dimensional pores by themselves, and have not only adsorption ability at the molecular level, but also filtration function for fine and ultrafine particles of the order of micrometers and nanometers. I understand.

[0019]

【Example】

(Example 1) An example of purifying and collecting pure water by collecting and fractionating ultrafine particles of polymethylmethacrylate (PMMA) in a suspension aqueous solution using nanotubes as a filtering agent is shown below.

(Experiment) A paper filter was laid on the bottom of the syringe barrel, and 0.4 g of nanotubes was laid on the paper filter to form a nanotube filter. The thickness of the nanotube portion was about 2 mm. PM with an average particle size of 1-2 μm
MA ultrafine particles (trade name: acrylic ultrafine powder, MP-14
00, manufactured by Soken Chemical Industry Co., Ltd.) and having an average particle diameter of 0.15 μm
A suspension of PMMA fine particles (the same as above, MP-1451) in water (concentration: 1.0 * 10 -1 g / l) was prepared, the suspension was poured onto the upper part of the nanotube filter, and the ultraviolet light of the aqueous solution flowing out from the lower part was extracted. The absorption spectrum was measured to examine the filtering effect of the nanotubes on the ultrafine particles.

(Result) Average particle size 1 on the upper part of the nanotube
It was found that the nanotubes were very easily wetted with the aqueous solution, because the suspension was soaked more and more when the suspension solution of PMMA ultrafine particles of ˜2 μm was dropped. Next, FIG. 1 shows changes in the UV-visible absorption spectrum of the aqueous solution before and after the outflow. Spectrum A is a spectrum of a suspension aqueous solution of PMMA ultrafine particles before passing through the nanotube filter, and spectrum B is a spectrum of an aqueous solution after passing through the filter. In general, fine particles contained in a suspension scatter light, and therefore, when an ultraviolet / visible absorption spectrum is measured, an apparent broad absorption is seen from the infrared region to the ultraviolet region. Spectrum A has a wavelength of 200-
Broad absorption is seen over 700 nm, which is
Apparent absorption derived from ultrafine particles of PMMA.
In the spectrum after flowing out, no absorption by PMMA ultrafine particles appears at all, thus showing that the flowing out aqueous solution is pure water.

That is, all the ultrafine particles of PMMA in the suspension aqueous solution are removed by the filtering action of the nanotubes,
Pure water is purified and recovered. Average particle size 0.15
A similar experiment was performed on a suspension of PMMA ultrafine particles of μm, and the UV / visible spectrum was measured as shown in FIG. Similar to FIG. 1, spectrum A is a spectrum of an aqueous suspension of PMMA ultrafine particles before passing through the filter, and spectrum B is a spectrum of the aqueous solution after passing through the filter. As is clear from FIG. 2, the average particle size is 1-2.
It can be seen that, as in the case of the PMMA ultrafine particles of μm, the ultrafine particles of PMMA having an average particle size of 0.15 μm were completely filtered by the nanotubes to obtain pure water. From the above two experimental results, it can be seen that the nanotube has a filtering function for substances on the order of micrometers and nanometers. Further, it was confirmed that the nanotubes after filtering the ultrafine particles of PMMA were taken out from the syringe and washed with water (it was possible to use the nanotubes again as a filter agent (after the experiment of FIG. 1, the nanotubes were washed with water, The experiment of FIG. 2 was performed).

Next, for reference, a similar experiment was conducted with a kind of carbon material, amorphous carbon. However, it has been found that the amorphous carbon filter is difficult to wet with an aqueous solution. Furthermore, it was confirmed with the naked eye that the filtrate contained a large amount of fine particles derived from amorphous carbon.
Although it was not possible to judge the removal of ultrafine particles of PMMA from the visible absorption spectrum, what is important is that at least water cannot be purified and recovered with amorphous carbon.

(Example 2) An example in which nanotubes are used as an adsorbent and benzene in an aqueous benzene solution is collected and fractionated is shown below.

(Experiment) 1 g of nanotubes was spread on the bottom of a glass filter to make a nanotube filter. A 5 * 10 ^-3 mol / l benzene aqueous solution was made to flow from the upper part of the filter while being pressurized with a pump, the ultraviolet absorption spectrum of the aqueous solution flowing out from the lower part was measured, and the benzene adsorption action of the nanotubes was investigated to determine the adsorption amount of benzene. .

(Results) When the aqueous benzene solution was dripped on the upper part of the nanotube, it was permeated more and more, and it was found that the nanotube was very easily wet with the aqueous benzene solution. Next, FIG. 3 shows changes in the ultraviolet absorption spectrum of the aqueous solution before and after the outflow. Spectrum A is the spectrum of the benzene solution before passing through the filter, B is the first 5 ml spectrum after passing through the filter, and C is 50.
It is a spectrum after flowing ml. 24 of spectrum A
Absorption with a vibrational structure centered around 5 nm is due to benzene in the aqueous solution. When the aqueous benzene solution is passed through the filter, as shown in spectrum B, all the absorption derived from benzene has disappeared. This indicates that all the benzene in the aqueous solution was adsorbed and removed by the nanotube. Therefore, it can be seen that the nanotube has a high adsorption ability for benzene. In addition, since absorption derived from other organic substances is not seen,
It is confirmed that there are no organic substances such as low molecular weight aromatic hydrocarbons in the nanotubes. Further, the benzene aqueous solution is kept flowing until the adsorption of benzene reaches saturation (spectrum C corresponds to the spectrum in the middle of saturation), and the amount of benzene adsorbed is calculated to be 2 * 10 ^-4 mol / g of benzene. It was found to have adsorption capacity. This adsorption amount corresponds to 30 m ² / g, and the BET method (N ₂ adsorption capacity) performed on nanotubes
It is in good agreement with the result obtained in the experiment by.

As a reference, a similar experiment was conducted with a kind of carbon material, amorphous carbon. As a result, amorphous carbon hardly adsorbed benzene. (Not shown in FIG. 1) because the background is large due to the inclusion of the fine particles of (3), and it extends from the scale of the graph). Further, the amorphous carbon filter was difficult to wet with an aqueous solution, and the filter was not formed well. Further, it was confirmed with naked eyes that amorphous carbon was mixed in the filtrate. This is not the case with the nanotube filter, and it can be seen that a dense filter is formed. After the experiment, the nanotubes were subjected to heat treatment at 60 ° C. for 1 hour under reduced pressure, and the same experiment was performed. As a result, the same adsorption activity as the above result was exhibited. Therefore, it was found that the nanotube can be recycled as an adsorbent.

(Example 3) An example of collecting and fractionating chlorobenzene in an aqueous chlorobenzene solution using nanotubes as an adsorbent is shown below.

(Experimental) A paper filter was laid on the bottom of the cylinder of the syringe, and 0.5 g of nanotubes was laid on the paper filter to prepare a nanotube filter. 1 * 10 inside the syringe
^-4 mol / l chlorobenzene aqueous solution was added, and the ultraviolet absorption spectrum of the aqueous solution flowing out from the lower part was measured to examine the chlorobenzene adsorption action of the nanotubes.

(Results) It was found that the nanotubes are very easily wet with the aqueous chlorobenzene solution. next,
The change in the ultraviolet absorption spectrum of the aqueous solution before and after the outflow is shown in FIG. Spectrum A is the spectrum of the chlorobenzene aqueous solution before passing through the filter, and B is the spectrum of the aqueous solution after passing through the filter. 26 of spectrum A
The absorption having a maximum around 5 nm is due to chlorobenzene in the aqueous solution. When the aqueous benzene solution is passed through the filter, the absorption derived from chlorobenzene as shown in spectrum B has its intensity reduced to about 1/4 of that before the passage. This indicates that the nanotubes removed 75% of the chlorobenzene in the aqueous solution. After the experiment, the nanotubes were subjected to heat treatment at 60 ° C. for 1 hour under reduced pressure, and the same experiment was performed. As a result, the same adsorption activity as the above result was exhibited. Therefore, it was found that the nanotubes can be reused as an adsorbent when adsorbing chlorobenzene as in the case of adsorbing benzene.

[0031]

From the above examples, it is demonstrated that it is possible to purify, collect, fractionate or recover the substance in the liquid phase and the liquid phase itself by using the nanotubes as an adsorbent or a filtering agent. It was Similar results can be obtained by using a gas phase instead of the liquid phase used in the examples. Furthermore, due to the unique properties of the nanotubes, the following characteristics, which are superior to those of conventional adsorbents and filter agents, have been proved. That is, the nanotubes have physical and chemical stability, and have a very fine structure, but are difficult to scatter. Since the nanotube is a perfect crystal composed of 100% carbon, it does not contain any low molecular weight organic matter. It has excellent adsorption capacity and is very easily wet with liquid. Nanotubes are adsorbents,
It can be regenerated as a filtering agent. The nanotubes self-assemble into high-density, minute three-dimensional pores by themselves accumulating, and have a function of filtering fine particles of the order of micrometers or nanometers.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the present invention.

FIG. 2 is a diagram for explaining an embodiment of the present invention.

FIG. 3 is a diagram for explaining an example of the present invention.

FIG. 4 is a diagram for explaining an example of the present invention.

Claims (5)

[Claims] 1. An adsorbent comprising carbon nanotubes. 2. An adsorbent composed of carbon nanotubes is used as a fixed bed, and a gas phase or a liquid phase is passed through the fixed bed to adsorb a substance in the gas phase or the liquid phase to the adsorbent. The method of using the adsorbent according to claim 1. 3. The method of using an adsorbent according to claim 2, wherein the substance is an organic substance or a halogen compound. 4. A filter agent comprising carbon nanotubes. 5. A filter agent comprising carbon nanotubes is used as a fixed layer, a gas phase or a liquid phase is passed through the fixed layer, and a substance in the gas phase or the liquid phase is filtered. Use of the described filtering agent.