JP7774252B2 - VOC removal catalyst, its manufacturing method, and VOC removal method - Google Patents

Description

The present invention relates to a VOC removal catalyst, a method for producing the same, and a method for removing VOCs.
Regarding.

VOC is an abbreviation for volatile organic compounds, and known examples include toluene, xylene, benzene, ethyl acetate, methanol, and dichloromethane. While such VOCs are widely used in solvents, adhesives, chemical raw materials, and more, they have been identified as a cause of photochemical oxidants and suspended particulate matter (SPM), and their emissions are therefore strictly regulated under the Air Pollution Control Act. Therefore, there is a need to establish technology to more efficiently remove VOCs in order to further reduce VOC emissions.

Catalytic oxidation is a known technique for removing VOCs. This method is considered the most promising because it removes VOCs at relatively low temperatures. Because catalytic oxidation primarily uses transition metal oxides, it offers cost advantages over precious metal catalysts. From this perspective, extensive research is being conducted into improving the catalytic performance of transition metal oxides. For example, Patent Document 1 discloses a technology for efficiently removing VOCs using a material containing manganese (IV) oxide.

Japanese Patent Application Laid-Open No. 2019-147131

However, conventional VOC removal catalysts still lack sufficient VOC removal performance in low-temperature environments, and their production also takes time, meaning that overall there are still challenges to overcome when considering practical application. From this perspective, there is currently a demand for the development of a catalyst that can be easily produced and can efficiently remove VOCs even at low temperatures.

The present invention was made in consideration of the above, and aims to provide a VOC removal catalyst that is easy to manufacture, has excellent VOC removal efficiency, and can efficiently remove VOCs even at low temperatures, as well as a method for manufacturing the same and a method for removing VOCs.

As a result of extensive research into achieving the above objective, the inventors discovered that the above objective can be achieved by using a specific Mn-based composite oxide as an essential component, or by using a Mn-based composite oxide produced through a specific process as an essential component, and thus completed the present invention.

That is, the present invention includes, for example, the subject matter described in the following sections.
Item 1
A method for producing a VOC removal catalyst, comprising:
Step 1: Mixing a raw material containing a metal element M with a gelling agent to obtain a gel precursor;
and step 2 of calcining a product obtained by mixing the gel precursor with an oxidizing agent containing Mn to obtain a composite oxide containing Mn and a metal element M,
The manufacturing method, wherein the metal element M is at least one element selected from the group consisting of transition metals, rare earth elements, and noble metal elements.
Section 2
Item 2. The method according to Item 1, wherein the metal element M is at least one selected from the group consisting of Co, Fe, Ni, Cu, Zn, Ti, V, Zr, Nb, Mo, Ir, Ru, and W.
Section 3
Item 3. The method according to Item 1 or 2, wherein the gelling agent is starch.
Section 4
Item 4. The method according to any one of Items 1 to 3, wherein the composite oxide contains 1 to 35 mol% of Mn element.
Section 5
A VOC removal catalyst,
a composite oxide containing Mn and a metal element M, wherein the metal element M is at least one element selected from the group consisting of transition metals, rare earth elements, and noble metal elements;
The VOC removal catalyst, wherein the composite oxide has an amorphous main phase.
Section 6
Item 6. The VOC removal catalyst according to Item 5, wherein the metal element M is at least one selected from the group consisting of Co, Fe, Ni, Cu, Zn, Ti, V, Zr, Nb, Mo, Ir, Ru, and W.
Section 7
Item 7. The VOC removal catalyst according to Item 5 or 6, wherein the composite oxide contains 1 to 35 mol % of Mn element.
Section 8
A method for removing VOCs, comprising a step of decomposing VOCs using a VOC removal catalyst obtained by the production method according to any one of Items 1 to 4 or the VOC removal catalyst according to any one of Items 5 to 7.

The VOC removal catalyst of the present invention is easy to manufacture, has excellent VOC removal efficiency, and can efficiently remove VOCs even at low temperatures. Furthermore, the method for manufacturing the VOC removal catalyst of the present invention allows for the simple manufacture of the aforementioned VOC removal catalyst of the present invention.

FIG. 1 is a schematic diagram showing the flow of an evaluation test method for a VOC removal catalyst. 1 shows SEM images of VOC removal catalysts obtained in Examples and Comparative Examples. 1 shows XRD spectra of VOC removal catalysts obtained in Examples and Comparative Examples. The results of VOC removal tests using the VOC removal catalysts obtained in the examples and comparative examples are shown below.

Embodiments of the present invention are described in detail below. Note that, in this specification, the expressions "contain" and "comprise" include the concepts of "contain," "comprise," "consist essentially of," and "consist only of."

1. Method for Producing a VOC Removal Catalyst The production method of the present invention comprises the following steps 1 and 2.
Step 1: A step of mixing a raw material containing a metal element M with a gelling agent to obtain a gel precursor.
Step 2: A step of calcining a product obtained by mixing the gel precursor with an oxidizing agent containing Mn to obtain a composite oxide containing Mn and a metal element M.

Here, the metal element M is at least one selected from the group consisting of transition metals, rare earth elements, and noble metal elements.

By using the manufacturing method comprising steps 1 and 2 described above, a composite oxide containing Mn and the metal element M can be obtained, and this composite oxide can be used in a VOC removal catalyst. Because such a VOC removal catalyst is obtained by a manufacturing method comprising steps 1 and 2, it has excellent VOC removal efficiency and can efficiently remove VOCs even at low temperatures.

(Step 1)
Step 1 is a step for obtaining a gel precursor by mixing a raw material containing a metal element M with a gelling agent.

As mentioned above, the metal element M is at least one element selected from the group consisting of transition metals, rare earth elements, and noble metal elements. More specifically, examples of the metal element M include transition metal elements such as Co, Fe, Ni, Cu, Zn, Ti, V, Zr, Nb, Mo, Ir, Ru, and W; rare earth metal elements such as La, Ce, Pr, Nd, Sm, Eu, Gd, Ho, and Yb; and noble metal elements such as Pt, Rh, Pd, Ag, and Au.

In particular, the metal element M is preferably at least one selected from the group consisting of Co, Fe, Ni, Cu, Zn, Ti, V, Zr, Nb, Mo, Ir, Ru, and W, and more preferably Co, in that it is easier to obtain a VOC removal catalyst that provides superior VOC removal efficiency. Therefore, the composite oxide containing Mn and the metal element M is preferably a composite oxide containing Mn and Co, and particularly preferably a composite oxide of Mn and Co.

The raw material containing metal element M is not particularly limited as long as it contains metal element M. For example, it can be a solution in which metal element M, a compound of metal element M, or a mixture thereof is dissolved or dispersed in a solvent.

The type of compound of metal element M is not particularly limited, and examples include inorganic compounds, chlorides, and organic compounds of metal element M. Examples of inorganic compounds of metal element M include oxides of metal element M, compounds containing oxoanions of metal element M (metallates), nitrates, sulfates, chlorides, chlorates, perchlorates, chloride complexes, carbonates, bicarbonates, phosphates, and hydrogen phosphates of metal element M, with nitrates being preferred as inorganic compounds of metal element M. Examples of organic compounds of metal element M include acetates, oxalates, formates, and succinates.

Examples of solvents for raw materials containing metal element M include water, but other alcohol-based solvents such as ethanol and isopropanol may also be used, and mixed solvents of water and alcohol may also be used.

The raw material containing metal element M is preferably an aqueous solution of an inorganic compound of metal element M, more preferably an aqueous solution of a nitrate of metal element M, and particularly preferably an aqueous solution of cobalt nitrate.

In raw materials containing metal element M, the concentration of metal element M is not particularly limited, but is preferably 0.01 M or more, more preferably 0.05 M or more, even more preferably 0.1 M or more, and is preferably 100 M or less, more preferably 50 M or less, even more preferably 10 M or less, and particularly preferably 5 M or less.

The gelling agent serves to gel raw materials containing the metal element M. There are no particular limitations on the type of gelling agent, as long as it can produce a gelling effect; for example, a wide range of known gelling agents can be used.

Gelling agents include starch, as well as various polysaccharides other than starch, such as agar and cellulose. Gelling agents may also be organic acids such as citric acid.

Starch is a preferred gelling agent for use in Step 1, as it has a strong gelling effect on raw materials containing the metal element M. In particular, when starch is used as a gelling agent, it can also function as a dispersant and reducing agent in Step 2, which will be described later. This means that the final composite oxide will have more voids and a lower degree of crystallinity, which will increase structural defects and tend to improve catalytic activity.

The type of starch is not particularly limited, and examples include potato starch, wheat starch, rice starch, corn starch, mung bean starch, etc.

The amount of gelling agent used is not particularly limited, as long as it gels the raw material containing the metal element M. For example, the gelling agent can be used so that the concentration of the gelling agent in the raw material containing the metal element M is 1 to 100 g/L, preferably 5 to 50 g/L, and more preferably 10 to 30 g/L.

In step 1, the method for mixing the raw material containing metal element M with the gelling agent is not particularly limited; for example, mixing can be carried out using a known mixer. The temperature during mixing is also not particularly limited; for example, it is 15 to 120°C, preferably 20 to 110°C, and more preferably 40 to 100°C.

In step 1, a raw material containing metal element M is mixed with a gelling agent, causing gelation to occur and producing a gel precursor.

(Step 2)
In step 2, the gel precursor obtained in step 1 is mixed with an oxidizing agent containing Mn.

The oxidizing agent containing Mn (hereinafter simply referred to as "oxidizing agent") can be, for example, a wide variety of compounds that contain Mn and have oxidizing properties, and specifically, permanganates can be used. Among them, the oxidizing agent containing Mn is preferably potassium permanganate ( _KMnO4 , _K2MnO4 , _etc. ), and more preferably _KMnO4 .

The oxidizing agent used in step 2 can be in the form of a solution, for example, an aqueous solution of the oxidizing agent. The concentration of the oxidizing agent is, for example, 0.01 M or more, more preferably 0.05 M or more, even more preferably 0.1 M or more, and preferably 100 M or less, more preferably 50 M or less, even more preferably 10 M or less, and particularly preferably 5 M or less.

In step 2, the gel precursor is mixed with an oxidizing agent, and the gel precursor is etched by the oxidizing agent. Therefore, the product obtained by mixing the gel precursor with the oxidizing agent in step 2 contains an oxide of manganese and an oxide of the metal element M.

In Step 2, the amount of oxidizing agent used is not particularly limited. For example, the number of moles of manganese per mole of metal element contained in the raw materials used in Step 1 is preferably 0.01 to 100 moles, more preferably 0.05 to 50 moles, even more preferably 0.1 to 10 moles, and particularly preferably 0.5 to 5 moles.

The product produced by mixing the gel precursor with the oxidizing agent can be recovered by an appropriate method. For example, the product can be recovered as a solid by known solid-liquid separation methods such as centrifugation or filtration. The recovered solid can also be washed by an appropriate method.

The product produced in step 2 is subjected to a calcination treatment. This results in a composite oxide containing Mn and the metal element M (Mn-based composite oxide).

The calcination method is not particularly limited, and a wide variety of known calcination methods can be used. For example, the calcination temperature can be 200°C or higher. Furthermore, since the crystallinity of the resulting composite oxide tends to be low, it is preferable that the calcination temperature be 600°C or lower. The preferred calcination temperature is 250 to 480°C, more preferably 290 to 450°C, and even more preferably 300 to 400°C.

The firing time can be selected appropriately depending on the firing temperature, and can be, for example, 1.5 to 5 hours. The temperature rise rate during firing in Step 2 is also not particularly limited and can be set appropriately, for example, at 1 to 10°C/min.

The calcination process may be carried out in air or in an inert gas atmosphere. Calcination is preferably carried out in air. For the calcination process, a known heating device such as a commercially available heating furnace can be used.

By firing, impurities are removed and a highly pure composite oxide containing Mn and the metal element M is formed.

The composite oxide obtained by the manufacturing method comprising steps 1 and 2 above can be used directly as a VOC removal catalyst, or, if necessary, other additives can be mixed with the composite oxide to produce a VOC removal catalyst. The VOC removal catalyst may consist solely of a composite oxide containing Mn and the metal element M.

In particular, when starch is used as the gelling agent in step 1, as mentioned above, the resulting composite oxide has more voids and a lower degree of crystallinity, which increases structural defects and makes it easier to improve catalytic activity. As a result, the VOC removal catalyst obtained by using starch as the gelling agent in step 1 has excellent VOC removal efficiency even at lower temperatures. From this perspective, it is preferable that the composite oxide obtained by the production method comprising steps 1 and 2 has an amorphous main phase.

The manufacturing method of the present invention comprises steps 1 and 2, and is particularly characterized by etching a gel precursor gelled using a gelling agent. This allows for the easy production of a catalyst with excellent VOC removal efficiency using only low-toxicity raw materials. While not intended to be limiting, the manufacturing method of the present invention includes etching the gel precursor, and it is believed that the resulting composite oxide has uniformly dispersed manganese and metal element M (e.g., cobalt). In particular, the etching target being gel-like makes it easier for metal element M to disperse, facilitating the uniform dispersion of manganese and metal element M (e.g., cobalt). As a result, the composite oxide has a high specific surface area and low crystallinity, resulting in numerous so-called structural defects, providing numerous oxygen vacancies and active species. It is therefore believed that the VOC removal catalyst can efficiently remove VOCs even at low temperatures.

The composite oxide containing Mn and the metal element M produced by the production method of the present invention preferably contains, for example, 1 to 35 mol% Mn. In this case, the VOC removal catalyst is likely to have a high VOC removal efficiency. The composite oxide containing Mn and the metal element M preferably contains 5 to 40 mol% M. In this case, the VOC removal catalyst is likely to have a high VOC removal efficiency. The composite oxide more preferably contains 6 mol% or more of the metal element M, even more preferably 7 mol% or more, and particularly preferably 8 mol% or more, and more preferably 30 mol% or less, and even more preferably 20 mol% or less. The composite oxide containing Mn and the metal element M produced by the production method of the present invention may further contain oxygen derived from the oxide and potassium derived from the oxidizing agent.

2. VOC Removal Catalyst The VOC removal catalyst of the present invention contains a composite oxide containing Mn and a metal element M, where the metal element M is at least one selected from the group consisting of transition metals, rare earth elements, and noble metal elements, and the composite oxide has an amorphous main phase. Such a VOC removal catalyst can be produced, for example, by the production method of the present invention comprising the above-mentioned steps 1 and 2.

More specifically, examples of the metal element M include transition metal elements such as Co, Fe, Ni, Cu, Zn, Ti, V, Zr, Nb, Mo, Ir, Ru, and W; rare earth metal elements such as La, Ce, Pr, Nd, Sm, Eu, Gd, Ho, and Yb; and noble metal elements such as Pt, Rh, Pd, Ag, and Au. Among these, in terms of the ease with which a VOC removal catalyst that provides superior VOC removal efficiency can be obtained, it is preferable that the metal element M be at least one selected from the group consisting of Co, Fe, Ni, Cu, Zn, Ti, V, Zr, Nb, Mo, Ir, Ru, and W, and Co is even more preferable.

The composite oxide has an amorphous main phase; specifically, the crystalline phase contained in the composite oxide is 10% by mass or less, preferably 5% by mass or less, and more preferably 1% by mass or less. When the composite oxide has an amorphous main phase, the VOC removal catalyst tends to have high VOC removal efficiency even at low temperatures. Whether the composite oxide has an amorphous main phase can be determined from the XRD spectrum.

The BET specific surface area of the composite oxide is preferably 80 m ² /g or more, more preferably 90 m ² /g or more, even more preferably 95 m ² /g or more, and particularly preferably 100 m ² /g or more.

The VOC removal catalyst of the present invention can be formed solely from the above-mentioned complex oxide, or it can contain components other than the above-mentioned complex oxide, as long as the effects of the present invention are not impaired.

The composite oxide preferably contains 1 to 35 mol% Mn element. In this case, the VOC removal catalyst is likely to have a high VOC removal efficiency. The composite oxide more preferably contains 5 mol% or more Mn element, even more preferably 10 mol% or more, particularly preferably 15 mol% or more, more preferably 30 mol% or less, and particularly preferably 25 mol% or less. The composite oxide also preferably contains 5 to 40 mol% metal element M. In this case, the VOC removal catalyst is likely to have a high VOC removal efficiency. The composite oxide more preferably contains 6 mol% or more metal element M, even more preferably 7 mol% or more, particularly preferably 8 mol% or more, more preferably 30 mol% or less, and even more preferably 20 mol% or less.

The composite oxide contains oxygen in addition to the Mn element and the metal element M. The oxygen content of the composite oxide is, for example, 50 mol % or more and 70 mol % or less. In addition to the Mn element, the metal element M, and oxygen, the composite oxide may also contain, for example, a metal element (e.g., a metal element such as potassium) derived from the oxidizing agent used during synthesis.

The composite oxide may be a mixture of an oxide of Mn and an oxide of the metal element M, a composite oxide in which Mn and the metal element M form a solid solution, or a composite oxide in which some of the Mn in the Mn oxide has been replaced with a metal element.

The complex oxide contained in the VOC removal catalyst of the present invention is, for example, in powder form. The shape of the complex oxide is not particularly limited, and it can be formed into various shapes such as particles, polygons, scales, fibers, and rods, among which nanoparticles are preferred.

3. VOC Removal Method The VOC removal method of the present invention comprises a step of burning VOCs using the above-described VOC removal catalyst or a VOC catalyst obtained by a production method comprising steps 1 and 2.

For example, a VOC removal catalyst is placed in a reactor, and VOCs such as toluene are introduced into the reactor and treated at a predetermined temperature to combust the VOCs. This allows the VOCs to be removed. If necessary, a mixed gas of nitrogen and oxygen is introduced into the reactor to combust the VOCs.

There are no particular restrictions on the type of reactor used to remove VOCs; for example, a wide range of known reactors used in catalytic combustion of VOCs can be used. The VOC treatment temperature in the reactor is also not particularly limited, and can be the same as the treatment temperature set for known VOC removal. In particular, the present invention achieves excellent VOC removal efficiency even at low temperatures by using the above-mentioned VOC removal catalyst.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

Example 1
A nitric acid solution prepared by dissolving 5 mmol of Co(NO ₃ ) ₂ ·6H ₂ O in 50 mL of deionized water was added to a beaker, and 2 g of potato starch was added. The mixture was stirred at room temperature for 10 minutes, followed by stirring in a 70°C water bath for 90 minutes to obtain a red gel precursor (gelatinous substance). The beaker was then cooled to room temperature (25°C), and 10 mmol of KMnO ₄ aqueous solution (40 mL) was added and stirred at room temperature until a black precipitate formed. The resulting black precipitate was washed several times with an ethanol-water mixed solvent, then collected by centrifugation and dried at 60°C for 12 hours. The black precipitate was then calcined in air at 350°C for 3 hours to obtain a composite oxide. This composite oxide was designated "MnCoαOβ" and used as a VOC removal catalyst.

(Comparative Example 1)
A nitric acid solution prepared by dissolving 5 mmol of Co( _NO3 ) _2.6H2O in 50 mL of deionized water was added to a beaker, and 2 g of potato starch was added thereto _. The mixture was stirred at room temperature for 10 minutes, followed by stirring in a water bath at 70°C for 90 minutes to obtain a red gel precursor (gelatinous substance). The gel precursor was dried at 60°C for 12 hours and then calcined in air at 350°C for 3 hours to obtain cobalt oxide. This cobalt oxide was designated "CoOx" and used as a VOC removal catalyst.

(Comparative Example 2)
In a beaker, 2 g of potato starch was dispersed in 50 mL of deionized water and stirred at room temperature for 10 minutes. The beaker was then placed in a 70°C water bath and stirred for 90 minutes to obtain a gel precursor (gelatinous substance). The beaker was then cooled to room temperature (25°C), and 10 mmol of _KMnO4 aqueous solution (40 mL) was poured into the dispersion and stirred at room temperature until a black precipitate formed. The resulting black precipitate was washed several times with an ethanol-water mixed solvent, then collected by centrifugation and dried at 60°C for 12 hours. The black precipitate was then calcined in air at 350°C for 3 hours to obtain an oxide. This oxide was designated "MnOx" and used as a VOC removal catalyst.

<Evaluation method>
(VOC removal test)
A toluene removal test was conducted on the VOC removal catalyst obtained in each example according to the schematic flow chart shown in Figure 1. In this test, a tubular reactor was filled with the VOC removal catalyst (Catalyst in the figure) sandwiched between quartz wool, and toluene was introduced into the reactor at a predetermined flow rate to cause a reaction, thereby removing the toluene. As shown in Figure 1, the reactor was connected to an oxygen cylinder and a nitrogen cylinder, allowing oxygen and nitrogen to flow into the reactor. The toluene removal test was conducted using a glass reactor with an inner diameter of 8 mm, with a VOC removal catalyst loading of 50 mg and a toluene concentration of 1,000 ppm by volume in the reactor. The flow rate of the nitrogen gas carrier into the reactor was 35 cm ³ /min, the nitrogen gas flow rate for introducing toluene was 5 cm ³ /min, and the oxygen gas flow rate was 10 cm ³ /min. The reaction temperature in the reactor was adjusted to various temperatures ranging from 130 to 300°C, and the toluene removal characteristics were evaluated. Sampling was performed three times every 10°C in the temperature range of 130 to 200°C, three times every 5°C in the temperature range of 200 to 250°C, and three times every 10°C in the temperature range of 260 to 300°C. A Shimadzu GC-2014 gas chromatograph was used to measure the VOC concentration. The carbon dioxide concentration discharged from the reactor outlet was measured using a HORIBA FG-120 FT-IR gas analyzer.

Figure 2 shows SEM images of the catalysts obtained in Example 1 and Comparative Examples 1 and 2. The catalyst in Example 1 was in the form of irregular nanoparticles, while Comparative Example 1 (CoOx) was composed of irregular flakes and blocks, and Comparative Example 2 (MnOx) was composed of a bulk with a smooth surface.

Table 1 shows the results of elemental analysis of the catalysts obtained in Example 1 and Comparative Examples 1 and 2, performed by EDS analysis.

3 shows the XRD spectra of the catalysts obtained in Example 1 and Comparative Examples 1 and 2. In _the XRD spectra, Comparative Example 1 (CoOx) was found to exhibit a sharp diffraction peak of _{the Co3O4} phase, while Comparative Example 2 (MnOx) was found to be a multiphase mixture containing _MnO2 and _{K2Mn4O8 phases} . On the other hand, MnCoαOβ of Example 1 exhibited a weak diffraction peak of _{the Co3O4} phase, and no diffraction peaks related to manganese species were observed. This result suggests that the MnCoαOβ of Example 1 contained a large amount of amorphous phase, had a crystallinity of 10% by mass or less, and had many structural defects.

Figure 4 shows the results of the VOC removal test using the catalysts obtained in Example 1 and Comparative Examples 1 and 2. Specifically, Figure 4 is a plot showing the relationship between temperature (X axis) and toluene removal rate (Y axis). The graph in Figure 4 also includes the values of the 50% decomposition temperature (T _50% ), 90% decomposition temperature (T _90% ), and 100% decomposition temperature (T _100% ) of toluene for each catalyst.

Table 2 summarizes the results of Figure 4, showing the results of VOC removal tests using the catalysts obtained in Example 1 and Comparative Examples 1 and 2. The results of Figure 4 and Table 2 show that the VOC removal catalysts obtained in each Example have better VOC removal performance than the Comparative Examples. Therefore, it was found that the VOC removal catalysts obtained in each Example can be suitably used as catalysts for the catalytic combustion of toluene, a typical VOC substance. In particular, it was found that this depends on the amount of potassium permanganate used in producing the catalyst.

Claims (4)

A method for producing a VOC removal catalyst, comprising:
Step 1: Mixing a raw material containing Co with a gelling agent to obtain a gel precursor;
and step 2 of calcining a product obtained by mixing the gel precursor with an oxidizing agent containing Mn to obtain a composite oxide containing Mn and Co.
The method for producing the gelling agent is at least one selected from the group consisting of starch, agar, and cellulose . The method of claim 1 , wherein the gelling agent is starch. 3. The method according to claim 1 , wherein the composite oxide contains 1 to 35 mol % of Mn element. A method for removing VOCs, comprising a step of decomposing VOCs using a VOC removal catalyst obtained by the production method according to any one of claims 1 to 3 .