JP7690763B2 - Composite material, nitrogen oxide adsorbent, nitrogen oxide adsorption method, and nitrogen oxide removal device - Google Patents

Description

The present invention relates to a composite material used as a nitrogen oxide adsorbent, and is used, for example, to purify various exhaust gases, particularly exhaust gases emitted from internal combustion engines of automobiles.

In the purification of exhaust gases containing nitrogen oxides emitted from internal combustion engines of automobiles and the like, a method has been put into practical use in which urea is used, and the ammonia produced by the decomposition reaction of urea is used as a reducing agent to bring the exhaust gas into contact with a selective catalytic reduction (SCR) catalyst. However, when the engine is started, the SCR catalyst has not yet reached its operating temperature, so there is a problem that the nitrogen oxides are not purified and are emitted as they are. For example, the temperature of the exhaust gas from a diesel engine is a low temperature of 100 to 200°C for about 800 seconds after the engine is started, and during this time, nitrogen oxides that could not be detoxified are contained in the exhaust gas (Non-Patent Document 1).

In the urea-SCR method currently installed in diesel engines, the temperature at the urea injection point generally needs to be about 150°C or higher in order for the urea to decompose into ammonia and activate the SCR catalyst. If the temperature is below 150°C, the temperature at which the catalyst is activated is not reached, and there is a problem that nitrogen oxides are discharged without being purified in low temperature regions such as during startup.
Incidentally, strict regulations are scheduled to come into effect in California from 2023 that will reduce emissions of nitrogen oxides in exhaust gases to about 10% of the current level, and so a method is needed to further reduce the amount of nitrogen oxides in the exhaust gases emitted when the engine is started.

As a method for reducing nitrogen oxides emitted when the engine is started, a system has been proposed in which an adsorbent such as zeolite is placed upstream of an exhaust gas purification catalyst as in Non-Patent Document 2, and nitrogen oxides are adsorbed in the low temperature range when the engine is started, and once the temperature reaches or exceeds the activation temperature of the exhaust gas purification catalyst, the nitrogen oxides are desorbed from the adsorbent and purified by the downstream exhaust gas purification catalyst.

Patent Document 1 also describes an exhaust gas purification catalyst in which a front-stage catalyst made of zeolite carrying rhodium is placed upstream of the exhaust gas, and a rear-stage catalyst carrying platinum or palladium is placed downstream of that. This exhaust gas purification catalyst proposes a method in which nitrogen oxides are adsorbed onto the front-stage catalyst in the low temperature range, and in the high temperature range, the nitrogen oxides desorbed from the front-stage catalyst are reduced and purified by the rear-stage catalyst.

Patent document 2 discloses a system in which nitrogen oxides from lean exhaust gas are adsorbed at temperatures below 200° C. and subsequently thermally released at temperatures above 200° C. The nitrogen oxide adsorbent is taught to consist of palladium and cerium oxide or mixed oxides, or composite oxides containing cerium and at least one other fourth row transition metal.

In Patent Document 3, it is reported that an adsorbent (Passive NOx Adsorber; PNA) in which palladium is supported on a small-pore zeolite in which the maximum pore diameter in the zeolite crystal structure is composed of an 8-membered oxygen ring, adsorbs nitrogen oxides at low temperatures and releases them at higher temperatures. Furthermore, it is taught that an adsorbent composed of zeolite has higher resistance to S components than an adsorbent using ceria oxide as a carrier.
Non-Patent Document 3 also reports that palladium-supported CHA-type zeolite can retain nitrogen oxides up to high temperatures because nitrogen oxides are released at temperatures of 150° C. or higher.

"Catalysts" (Catalysis Society of Japan), Vol. 45 (2003), pp. 236-240 Catal. Lett., 146 (2016) pp. 1706-1711. Applied Catalysis B: Environmental 212 (2017) pp. 140-149 Deactivation of Pd/Zeolites passive NOx adsorber induced by NO and H2O: Comparative study of Pd/ZSM-5 and Pd/SSZ-13, Catalysis Today (2020) (Journal Pre-proof)

JP 2000-312827 A International Publication No. 2008/047170 International Publication No. 2015/085303

In low-temperature nitrogen oxide adsorbents that use zeolite, such as the pre-catalyst described in Patent Document 1, there is a phenomenon in which the amount of nitrogen oxide adsorbed decreases due to the influence of moisture contained in exhaust gas. Therefore, although the cause is unclear, there is an issue that zeolites supporting rhodium have low stability.

It was thought that, depending on the type of metal supported on the zeolite, there may be a strong interaction with water, and when water coexists, such as in automobile exhaust gas, the water is preferentially adsorbed, resulting in a decrease in the nitrogen oxide adsorption performance.
For example, Patent Document 3 states that a nitrogen oxide adsorbent in which palladium is supported on an 8-membered ring zeolite with a small pore size (ring size), such as a CHA-type zeolite supported with palladium, is effective, and Non-Patent Document 4 reports that, as a result of comparing the adsorption characteristics of Pd/ZSM-5 and Pd/CHA, Pd/CHA with a small ring size is less affected by water in NO adsorption at 120°C. However, this is merely a result of comparing the adsorption performance in a high temperature range where the adsorption power of water is weak, and water is easily adsorbed by zeolite in a temperature range lower than 120°C, the temperature at which nitrogen oxides are emitted in exhaust gas from the start of the engine, and although the adsorption characteristics in a gas atmosphere containing water are important, the adsorption characteristics in the competitive adsorption of water and nitrogen oxides in that temperature range have not been clarified. In fact, as described in Reference Examples 1 and 2 of this patent, it can be seen that the amount of nitrogen oxide adsorption decreases when the adsorption temperature is lowered from 120°C to 80°C in an atmosphere in which water coexists.

In this patent, the adsorption performance was evaluated using Pd/CHA to examine the effect of water concentration on the adsorption of nitrogen oxides at 80°C. As a result, it was revealed that the amount of nitrogen oxide adsorption showed a decreasing trend as the water concentration increased (see Comparative Example 2). Therefore, Pd/CHA is considered to be a more effective adsorbent than Pd/ZSM-5 in the high temperature range, but the issue of being affected by water in the low temperature range was confirmed.

The present invention was made to solve the above problems, and its purpose is to provide a composite material that has high nitrogen oxide adsorption performance even at low temperatures and in a gas atmosphere containing water.

The present inventors have succeeded in developing a _{nitrogen oxide} adsorbent having the surprising effect that a composite material carrying a precious metal such as palladium on a zeolite having an lta in its framework, as defined by the International Zeolite Association (IZA) as a composite building unit (CBU) and having a _SiO2 / _Al2O3 molar ratio in a specific range of 11 or more, does not lose its nitrogen oxide adsorption performance even in the presence of water, but rather the amount of nitrogen oxide adsorbed is increased in the presence of water, thereby arriving at the present invention.
That is, the present invention provides the following [1] to [9].
[1] A composite material containing a precious metal and a zeolite, the composite material being characterized in that the zeolite contains an lta defined as a composite building unit by the International Zeolite Association (IZA) in the framework, and has a SiO ₂ /Al ₂ O ₃ molar ratio of 11 or more.
[2] The composite material described in [1] above, wherein the maximum ring size of the zeolite is 3.6 Å or less.
[3] The composite material according to [1] or [2] above, wherein the zeolite is an RHO type zeolite.
[4] The composite material according to any one of [1] to [3] above, wherein the precious metal is palladium and the content thereof is 0.1 mass% or more.
[5] The composite material according to any one of the above [1] to [4], wherein the nitrogen oxide adsorption amount change rate, expressed as the ratio (A2/A1), is 0.6 or more and 2.0 or less, where A1 is the nitrogen oxide adsorption amount when a gas containing 200 ppm by volume of nitrogen oxide and 5 vol.% of oxygen and having a water vapor content of 0 vol.% is passed through the composite material at 80°C, and A2 is the nitrogen oxide adsorption amount when a gas containing 200 ppm by volume of nitrogen oxide and 5 vol.% of oxygen and having a water vapor content of 5 vol.% is passed through the composite material at 80°C.
[6] A composite material comprising a precious metal and a zeolite, wherein, when a gas containing 200 ppm by volume of nitrogen oxide and 5 vol. % of oxygen and having a water vapor content of 0 vol. % is passed through the composite material at 80° C., the nitrogen oxide adsorption amount is A1, and when a gas containing 200 ppm by volume of nitrogen oxide and 5 vol. % of oxygen and having a water vapor content of 5 vol. % is passed through the composite material at 80° C., the nitrogen oxide adsorption amount change rate, expressed as a ratio (A2/A1), is 0.6 or more and 2.0 or less.
[7] A nitrogen oxide adsorbent comprising the composite material according to any one of [1] to [6] above.
[8] A method for adsorbing nitrogen oxides in a gas containing water and nitrogen oxides by using the composite material described in any one of [1] to [6] above.
[9] A nitrogen oxide removal device comprising the composite material described in any one of [1] to [6] above, the nitrogen oxide removal device removing nitrogen oxides from a gas containing water and nitrogen oxides by using the composite material.

The present invention provides a composite material that has high nitrogen oxide adsorption performance without being affected by water, even at low temperatures such as during engine start-up and in a gas atmosphere that contains water.

1 shows an XRD pattern of the RHO zeolite described in Example 1. 1 shows an XRD pattern of the RHO zeolite described in Comparative Example 1.

The following describes in detail the embodiments of the present invention, but the following description is one example (representative example) of the embodiment of the present invention, and the present invention is not limited to these contents in any way.

[Composite material]
A composite material according to one embodiment of the present invention is a composite material containing a precious metal and a zeolite, and is characterized in that the zeolite contains lta, which is defined as a composite building unit by the International Zeolite Association (IZA), in its framework and has a _SiO2 / _{Al2O3 molar} ratio of 11 or more. The composite material according to one embodiment of the present invention will be described in detail below.

The above zeolite has an lta framework and is therefore likely to support precious metals that are effective in adsorbing nitrogen oxides.
Zeolite contains at least oxygen, aluminum (Al) and silicon (Si) as atoms constituting the framework structure, and some of these atoms may be substituted with other atoms (Me). The zeolite is preferably an aluminosilicate zeolite rather than an AlPO-based or SAPO-based zeolite.
Among the aluminosilicate zeolites having lta, zeolites having an 8-membered oxygen ring structure are preferred. Zeolites having an RHO structure (hereinafter referred to as RHO-type zeolites) according to the code for defining the zeolite skeletal structure defined by the International Zeolite Association (IZA) are more preferred.

The structure of zeolite is characterized by X-ray diffraction data. However, when actually measuring a produced zeolite, the intensity ratio and peak position of each peak may vary slightly due to the influence of the zeolite growth direction, the ratio of constituent elements, adsorbed substances, the presence of defects, the drying state, the alkali cation species, etc., so the values obtained are not necessarily exactly the same as the parameters of the RHO structure described in the IZA regulations, and a margin of about 20% is allowed.
When CuKα radiation is used, major peaks of RHO zeolite include, for example, the peak of the 110 plane at 2θ=9.1°±0.2°, the peak of the 211 plane at 2θ=15.8°±0.2°, the peak of the 301 or 310 plane at 2θ=20.4°±0.2°, the peak of the 312 or 321 plane at 2θ=24.2°±0.2°, and the peak of the 330 or 411 plane at 2θ=27.5°±0.2°.

As described above, the composite material of the present invention contains a precious metal and a zeolite. In the composite material of the present invention, the precious metal is preferably supported on the zeolite. The precious metal used in the composite material is, for example, selected from the group consisting of platinum, palladium, rhodium, gold, silver, iridium, ruthenium, osmium, and mixtures thereof.
Among these, palladium is preferred because of its excellent affinity for nitrogen oxides. The valence of the precious metal is not particularly limited, and may be monovalent or divalent. In the composite material of the present invention, the precious metal is usually not included in the zeolite framework.

Since the precious metal serves as an adsorption site for nitrogen oxides in the adsorbent, a large amount of the precious metal is preferable in order to adsorb a large amount of nitrogen oxides. From this viewpoint, the content of the precious metal in the composite is usually 0.1 mass% or more, preferably 0.5 mass% or more, more preferably 0.7 mass% or more, particularly preferably 0.8 mass% or more, and most preferably 1.0 mass% or more.
Since a large amount of the precious metal is fixed to the zeolite and the gas to be treated can easily penetrate into the pores without the pores being blocked by the precious metal, the content of the precious metal in the composite is usually 10 mass% or less, preferably 8 mass% or less, more preferably 6.0 mass% or less, even more preferably 5.0 mass% or less, particularly preferably 4.0 mass% or less, and especially preferably 3.0 mass% or less.

The SiO ₂ /Al ₂ O ₃ molar ratio of the zeolite is equal to or greater than 11. Due to the high SiO ₂ /Al ₂ O ₃ molar ratio, even in a low-temperature gas atmosphere containing water, the zeolite is not affected by water and tends to maintain high nitrogen oxide adsorption performance.
When used as a catalyst or adsorbent, the more active sites and adsorption sites there are, the more preferable it is, and therefore the _SiO2 / _{Al2O3 molar} ratio is preferably small, specifically, preferably 100 or less, more preferably 50 or less, even more preferably 30 or less, particularly preferably 25 or less, particularly preferably 22 or less, and most preferably 20 or less.
On the other hand, since zeolites with a small amount of Al in the framework are less likely to suffer structural destruction due to the elimination of Al from the framework even when exposed to a gas containing water vapor, it is preferable that the _SiO2 / _{Al2O3 molar} ratio is large. In other words, the higher _{the SiO2} / _Al2O3 molar ratio, the higher the nitrogen oxide adsorption performance can be maintained without being affected by water. From these viewpoints, the _SiO2 / _{Al2O3 molar} ratio is usually 11 or more, preferably 12 or more, more preferably 13 or more, and particularly preferably 14 or more.

Zeolites containing lta as CBU in the zeolite structure usually have a low SAR (hereinafter may be referred to as "SAR") and, for example, as described in Microporous Materials Volume 4, Issues 2-3, June 1995, Pages 231-238, have a SiO ₂ /Al ₂ O ₃ molar ratio of at most 10. In contrast, the composite material according to the present embodiment has a SiO ₂ /Al ₂ O ₃ molar ratio of 11 or more while containing lta in the framework, and therefore can exhibit high nitrogen oxide adsorption performance without being affected by water even in a gas atmosphere containing water and at a low temperature.

The zeolite containing lta as a CBU in the zeolite structure of the composite material is not particularly limited, and for example, CLO, KFI, LTA, LTN, PAU, RHO, TSC, UFI, and MWF are preferable, KFI, LTA, LTN, PAU, RHO, and MWF are more preferable, MWF, LTA, PAU, and RHO are particularly preferable, MWF, PAU, and RHO are particularly preferable, and RHO is most preferable. The reason why lta is preferable as a CBU in the zeolite structure is that gas diffuses through the d8r contained in lta, but since d8r is larger in size than d6r, even if water is adsorbed, it has pores large enough for the gas to diffuse, and is therefore less susceptible to the effects of the adsorbed water. In addition, since the ring size through which gas penetrates is small, these zeolites have the effect of being less likely to cause pore blockage due to the large hydrocarbons contained in automobile exhaust gas.

The maximum ring size of the zeolite in the composite material is preferably 3.7 Å or less, and most preferably 3.6 Å or less, because the smaller the maximum ring size, the easier it is to prevent water and hydrocarbons from penetrating into the pores. On the other hand, the lower limit of the maximum ring size must be a size that does not affect the support of the precious metal that serves as the adsorption point or the penetration of gas, and is usually 2.0 Å or more, preferably 2.5 Å or more, more preferably 3.0 Å or more, particularly preferably 3.2 Å or more, particularly preferably 3.4 Å or more, and most preferably 3.5 Å or more. The maximum ring size in the crystal structure of the zeolite is the value described in ATLAS OF ZEOLITE FRAME WORK TYPES (Sixth Revis ed Edition, 2007, ELSEVIER) by Ch. Baerlocher et al.

The framework density of the zeolite in the present invention (hereinafter sometimes abbreviated as "FD") is not particularly limited, but is usually 12.0 T/nm ³ or more, preferably 12.5 T/nm ³ or more, more preferably 13.0 T/nm ³ or more. On the other hand, it is usually 14.4 T/nm ³ or less, preferably 14.3 T/nm ³ or less, particularly preferably 14.2 T/nm ³ or less, and most preferably 14.1 T/nm ³ or less. The framework density (T/nm ³ ) means the number of T atoms (atoms of elements other than oxygen constituting the zeolite framework) present per unit volume nm ³ of the zeolite, and this value is determined by the structure of the zeolite. When the FD is high, the structure is stable and sufficient durability tends to be obtained, while when the FD is low, a sufficient amount of adsorption is obtained and it is suitable for use as an adsorbent. The above framework density is as described in Ch. These values are described in ATLAS OF ZEOLITE FRAME WORK TYPES by Baerlocher et al. (Sixth Revis ed Edition, 2007, ELSEVIER).

The particle size of the zeolite is not particularly limited, but since the gas diffusion is likely to be high when the zeolite is used as an adsorbent, it is preferably 0.01 μm or more, more preferably 0.1 μm or more, particularly preferably 1.0 μm or more, and most preferably 2.0 μm or more. On the other hand, it is preferably 10 μm or less, more preferably 8 μm or less, particularly preferably 6 μm or less, particularly preferably 5.5 μm or less, and most preferably 5.0 μm or less. The particle size is an average particle size, and is specifically measured by the method described in the Examples section below.
The specific surface area of the zeolite is not particularly limited, but since the number of active sites present on the inner surface of the pores increases, it is preferably in the following range. That is, it is preferably 200 m ² /g or more, more preferably 300 m ² /g or more, and even more preferably 400 m ² /g or more. On the other hand, it is preferably 1000 m ² /g or less, more preferably 800 m ² /g or less, and even more preferably 700 m ² /g or less. The specific surface area of the zeolite is measured by the BET method described in the Examples section below.
The acid amount of the zeolite is preferably 0.5 mmol/g or more, more preferably 0.7 mmol/g or more, even more preferably 0.9 mmol/g or more, and particularly preferably 1.2 mmol/g or more. On the other hand, it is preferably 3.0 mmol/g or less, more preferably 2.5 mmol/g or less. The acid amount of the zeolite is measured by the method described in the Examples section below.

The composite material of the present invention may contain alkali metal atoms, such as sodium, potassium, cesium, etc., derived mainly from the raw materials, but the content thereof, as a molar ratio to the aluminum in the composite material, is preferably 0.001 or more and 1.0 or less. The low content makes it difficult for the alkali metal atoms to destroy the structure of the zeolite in a steam atmosphere. In addition, forcibly removing the alkali metal atoms from the composite material may damage the zeolite framework, so the molar ratio of the alkali metal atoms to the aluminum in the catalyst is preferably equal to or more than the above lower limit.

In the composite material of the present invention, when a gas containing 200 ppm by volume of nitrogen oxide (NO), 5% by volume of oxygen, and a water vapor content of 0% by volume is passed through the composite material at 80° C., the nitrogen oxide adsorption amount A1 is the nitrogen oxide adsorption amount A2 ...
The rate of change in the amount of adsorbed nitrogen oxides is specifically measured by the method described in the Examples section below.

In another embodiment of the present invention, the composite material contains a precious metal and a zeolite, and has a nitrogen oxide adsorption amount change rate, represented by the above-mentioned ratio (A2/A1), of 0.6 to 2.0. By setting the nitrogen oxide adsorption amount change rate within the above range, the composite material according to the another embodiment can have high nitrogen oxide adsorption performance even at low temperatures and in a gas atmosphere containing water.
In another embodiment, the composite material does not need to have a _SiO2 / _Al2O3 molar ratio of ₁₁ or more, and does not need to include lta as a CBU in the zeolite structure, but may have a _SiO2 / _{Al2O3 molar} ratio of 11 or more, and may include lta as a CBU in the zeolite structure.
Details of the composite material according to the other embodiment are as described in the composite material according to the above embodiment, and therefore description thereof will be omitted.

[Method of manufacturing composite material]
The method for producing the composite material according to each of the above embodiments is not particularly limited, and examples thereof include a method in which a zeolite is synthesized, and then a predetermined amount of a precious metal is supported through a metal supporting process, i.e., a zeolite containing Al and Si in its framework structure and having a _SiO2 / _{Al2O3 molar} ratio of 11 or more is produced, and a desired amount of a precious metal is supported on the zeolite relative to its mass.
Hereinafter, a manufacturing example of zeolite used in manufacturing a composite material will be described using an aluminosilicate zeolite as an example. In the following description of the manufacturing method, aluminosilicate zeolite may be simply referred to as "zeolite".

<Method of manufacturing zeolite>
Zeolite is usually obtained by mixing each raw material to obtain a pre-reaction mixture, and then performing hydrothermal synthesis. The raw materials for zeolite are usually an aluminum atom raw material, a silicon atom raw material, an alkali metal atom raw material, and water. In addition, an organic structure directing agent and a seed crystal may be further used.

<Aluminum atom source>
The aluminum atom raw material used to produce zeolite is not particularly limited, and examples of the aluminum atom raw material that can be used include amorphous aluminum hydroxide, aluminum hydroxide having a gibbsite structure, aluminum hydroxide having a bayerite structure, aluminum nitrate, aluminum sulfate, aluminum oxide, sodium aluminate, boehmite, pseudo-boehmite, aluminum alkoxides, and even compounds containing silica such as zeolites.
As the aluminum atom raw material used for producing zeolite, an aluminosilicate zeolite having a common building unit with RHO-type zeolite, such as FAU-type zeolite, is particularly preferred. In this case, the Si content of FAU-type zeolite is preferably high in order to add more building units to the pre-reaction mixture as the zeolite raw material, and specifically, the _SiO2 / _{Al2O3 molar} ratio is preferably 5 or more, more preferably 7 or more, even more preferably 10 or more, particularly preferably 20 or more, particularly preferably 25 or more, and most preferably 30 or more.
The aluminum atom source may be used alone or in any combination of two or more kinds in any ratio.

The amount of aluminum atom raw material used, in terms of ease of preparation of the pre-reaction mixture or the aqueous gel obtained by aging the pre-reaction mixture, and production efficiency, is usually 100 or less, preferably 50 or less, more preferably 40 or less, and even more preferably 30 or less, in terms of the SiO ₂ /Al ₂ O ₃ molar ratio contained in the pre-reaction mixture other than the seed crystals. In addition, the lower limit is not particularly limited, but in terms of uniformly dissolving the aluminum atom raw material in the aqueous gel, it is usually 10 or more, preferably 15 or more, more preferably 18 or more, and even more preferably 20 or more.
In the SiO ₂ /Al ₂ O ₃ molar ratio shown as the amount of aluminum atom raw material used, SiO ₂ is the molar amount of SiO ₂ when all of the silicon (Si) contained in the pre-reaction mixture is assumed to be SiO ₂ , and Al ₂ O ₃ is the molar amount of Al ₂ O ₃ when all of the aluminum (Al) contained in the pre-reaction mixture is assumed to be Al ₂ O ₃ .

<Silicon atom source>
The silicon atom raw material used to produce the zeolite used in the present invention is not particularly limited, and various known substances can be used. For example, zeolite may be used, and colloidal silica, amorphous silica, sodium silicate, trimethylethoxysilane, tetraethylorthosilicate, aluminosilicate gel, etc. can be used. These may be used alone or in any ratio and combination of two or more. In particular, aluminosilicate zeolites having a common building unit with RHO-type zeolites, such as FAU-type zeolites, are preferred.
The silicon atom source is used so that the ratio of the amount of the other source to the amount of the silicon atom source falls within the preferred ranges described above or below.

<Alkali metal atom source>
The alkali metal atoms contained in the alkali metal atom raw material used to produce zeolite are not particularly limited, and known ones used in the synthesis of zeolite can be used. The alkali metal atoms are preferably one or more selected from the group consisting of lithium, sodium, potassium, rubidium, and cesium, and are crystallized in the presence of alkali metal ions. Among these, it is particularly preferable that both sodium and cesium atoms are contained.
As the alkali metal atom source, inorganic acid salts such as hydroxides, oxides, sulfates, nitrates, phosphates, chlorides, bromides, etc., and organic acid salts such as acetates, oxalates, citrates, etc., of the above-mentioned alkali metal atoms can be used. One or more kinds of alkali metal atom sources may be contained.

By using an appropriate amount of the alkali metal atom raw material, the organic structure directing agent described later can be easily coordinated to aluminum in a suitable state, and the crystal structure can be easily formed. The molar ratio (R/Si) of the alkali metal atom raw material (R) to silicon (Si) contained in the pre-reaction mixture other than the seed crystal is usually 0.1 or more, preferably 0.2 or more, more preferably 0.25 or more, and even more preferably 0.3 or more, and is usually 2.0 or less, preferably 1.5 or less, more preferably 1.0 or less, even more preferably 0.8 or less, particularly preferably 0.6 or less, and most preferably 0.4 or less.
In addition, when an alkali metal atom source containing an alkali metal atomic component is used as an aluminum atom source or a silicon atom source, it is preferable to use the alkali metal atom source so that the total of the alkali metal atomic components in these sources falls within the above-mentioned R/Si range.

<Organic structure directing agent>
In the production of zeolite, it is preferable to use an organic structure directing agent (also called "template". Hereinafter, the organic structure directing agent may be referred to as "OSDA"). As the organic structure directing agent, a macrocyclic compound represented by crown ether is preferably used. The template is not particularly limited, but a macrocyclic compound having a heteroatom such as oxygen, nitrogen, or sulfur as an electron donor atom is preferable, and from the viewpoint of ease of formation of zeolite crystals, 12-crown-4-ether, 15-crown-5-ether, 18-crown-6-ether, 24-crown-8-ether, dibenzo-18-crown-6-ether, cryptand [2.2], and cryptand [2.2.2] are particularly preferable. Only one type of template may be used, or two or more types may be used in combination in any ratio. Among these, 18-crown-6-ether is particularly preferable.

Alternatively, a polymer such as polydiallyldimethylammonium chloride (hereinafter, PDADMAC) may be used as the template. In this case, as described in the non-patent literature Microporous and Mesoporous Materials 132 (2010) 352-356, it is possible to synthesize an RHO-type zeolite without using cesium as an alkali metal.
When these organic structure directing agents are used, one kind may be used alone, or two or more kinds may be used in any ratio and combination.

When an organic structure directing agent is used, the molar ratio of the organic structure directing agent to the silicon (Si) contained in the raw material mixture other than the seed crystals is usually 0.01 or more, preferably 0.03 or more, more preferably 0.05 or more, even more preferably 0.08 or more, and especially preferably 0.10 or more, from the viewpoint of ease of crystal formation. Also, in order to obtain a sufficient cost reduction effect, the molar ratio is usually 1 or less, preferably 0.8 or less, more preferably 0.5 or less, even more preferably 0.3 or less, and especially preferably 0.2 or less. Incidentally, as shown in Molecular Sieves, Adv. Chem. Ser., 121 (1973), p. 106, it is possible to obtain RHO-type zeolite without using an organic structure directing agent.

<Water>
From the viewpoint of kneadability of the raw material gel and ease of crystal formation, the amount of water used is, in terms of a molar ratio to silicon (Si) contained in the raw material mixture other than the zeolite added as seed crystals, usually 1 or more, preferably 3 or more, more preferably 5 or more, even more preferably 7 or more, and particularly preferably 10 or more. This range is preferable because it makes crystal formation easier. In addition, in order to fully obtain the effect of reducing the cost of waste liquid treatment, the amount of water used is usually 100 moles or less, preferably 80 or less, more preferably 50 or less, even more preferably 25 or less, and particularly preferably 15 or less.

<Seed crystal>
Seed crystals may be used to produce the zeolite used in the present invention. The zeolite used as the seed crystal in the present invention may be one type alone or two or more types in any ratio and combination.
The amount of seed crystals used is preferably large in terms of accelerating the reaction rate and suppressing the generation of impurities. Therefore, the amount of zeolite added as seed crystals is preferably 0.1 mass% or more, more preferably 0.5 mass% or _{more, even more preferably 1.0 mass% or more, particularly preferably 1.5 mass% or more, and most preferably 2.0 mass% or more, based on the SiO 2 when} all silicon (Si) contained in the raw material mixture other than the zeolite added as seed crystals is SiO 2. The upper limit of the amount of zeolite added in the present invention is not particularly limited, but in order to fully obtain the effect of reducing costs, it is usually 10 mass% or less, preferably 8 mass% or less, more preferably 5 mass% or less, and even more preferably 3 mass% or less.

The zeolite used as the seed crystal is not particularly limited as long as it is a zeolite. Zeolites containing at least one of d8r and lta in the building units constituting the crystal structure are preferred, zeolites containing at least lta are more preferred, and zeolites containing both are even more preferred. Zeolites containing building units constituting the crystal structure are preferred, and when producing RHO-type zeolites, RHO-type is most preferred. Zeolites containing d8r include MER-type, SBE-type, PAU-type, RHO-type, and TSC-type. Zeolites containing lta include KFI-type, LTA-type, LTN-type, UFI-type, MWF-type, PAU-type, RHO-type, and TSC-type. Zeolites containing both include PAU-type, RHO-type, and TSC-type.

The zeolite used as the seed crystals may be uncalcined, that is, not calcined after hydrothermal synthesis, or calcined after hydrothermal synthesis. However, in order for the zeolite to function as a crystal nucleus, it is better for it to be less soluble in alkali, and therefore uncalcined zeolite is preferred. However, depending on the composition or temperature conditions in the raw material mixture, uncalcined zeolite may be less soluble and less likely to function as a crystal nucleus. In such cases, it is preferable to use zeolite from which the OSDA has been removed by calcination in order to increase solubility.

<Mixing of ingredients>
In the method for producing zeolite, the raw materials are usually mixed and then hydrothermally synthesized.
The order of mixing the raw materials is not particularly limited, but it is preferable to add the silicon atom raw material and the aluminum atom raw material after preparing the alkaline solution, since the raw materials are more easily dissolved uniformly. Specifically, it is preferable to mix water and the alkali metal atom raw material, or, when an organic structure directing agent is used, to prepare an alkali metal aqueous solution by mixing water, an alkali metal atom raw material, and an organic structure directing agent, and then add the aluminum atom raw material and the silicon atom raw material to it, or to add the aluminum atom raw material and the silicon atom raw material in this order to mix them. In addition, when a seed crystal is used, it is preferable to add the seed crystal after adding the aluminum atom raw material and the silicon atom raw material.

<Preparation of crown ether-alkali aqueous solution>
When a crown ether is used as the organic structure directing agent, as described above, water, an alkali metal atom raw material, and a crown ether as an organic structure directing agent may be mixed to prepare a crown ether-alkali aqueous solution. In this case, the order of mixing the raw materials is not limited and may be appropriately selected depending on the conditions used, but usually, the alkali metal atom raw material is first dissolved in water, and then the crown ether is added and mixed. In this case, mixing may be performed at room temperature, but in order to make the raw materials easily dissolve, the mixing temperature is usually 50° C. or higher, preferably 60° C. or higher, and more preferably 80° C. or higher. In addition, the upper limit is not particularly limited, but is usually 200° C. or lower, preferably 150° C. or lower, and particularly preferably 100° C. or lower. Note that room temperature refers to the temperature when the material is placed in a laboratory without strict temperature control.

From the viewpoint of the ease of forming zeolite crystals, the concentration of crown ether in the crown ether-alkali aqueous solution is preferably 3.0% by mass or more, more preferably 5.0% by mass or more, and on the other hand, it is preferably 80% by mass or less, more preferably 60% by mass or less.

It is preferable that the crown ether-alkali aqueous solution does not substantially contain aluminum atom raw materials and silicon atom raw materials. Here, "substantially does not contain" preferably means that the content in the crown ether-alkali aqueous solution is 0.1 mass% or less, and more preferably, no raw materials are contained at all.

In addition to the above substances, in the production of zeolite, additives such as auxiliary agents that aid in the synthesis of zeolite, for example, acid components that promote the reaction, and metal stabilizers such as polyamines, may be added and mixed at any step as necessary to prepare a pre-reaction mixture.

<Aging>
The pre-reaction mixture prepared as described above may be hydrothermally synthesized immediately after preparation, but in order to obtain an RHO zeolite with few impurities or high crystallinity, it is preferable to age it for a certain period of time under a predetermined temperature condition. The aging temperature is usually 100° C. or less, preferably 80° C. or less, more preferably 50° C. or less, and although there is no particular lower limit, it is usually 0° C. or more, preferably 10° C. or more. The aging temperature may be constant during aging, or may be changed stepwise or continuously. The aging time is not particularly limited, but is usually 2 hours or more, preferably 3 hours or more, more preferably 5 hours or more, more preferably 10 hours or more, and particularly preferably 12 hours or more, and on the other hand, it is usually 10 days or less, preferably 5 days or less, more preferably 2 days or less, and even more preferably 1 day or less.

<Hydrothermal synthesis>
Hydrothermal synthesis is usually carried out by placing the pre-reaction mixture obtained as described above in a pressure-resistant container and maintaining a predetermined temperature under self-generated pressure or under gas pressure to an extent that does not inhibit crystallization, while stirring, rotating or rocking the container, or leaving it still.
The temperature of the hydrothermal synthesis is usually 110° C. or higher and 200° C. or lower, but is preferably 190° C. or lower, more preferably 180° C. or lower, and even more preferably 170° C. or lower. The reaction time is not particularly limited, but is usually 5 hours or higher, preferably 10 hours or higher, more preferably 12 hours or higher, and even more preferably 1 day or higher, and on the other hand, is usually 10 days or lower, preferably 6 days or lower, more preferably 5 days or lower, even more preferably 4 days or lower, and especially preferably 3 days or lower. The reaction temperature may be constant during the reaction, or may be changed stepwise or continuously.
The above-described method for producing zeolite is preferable because it improves the yield of the target zeolite and makes it difficult for a different type of zeolite to be produced.

<Zeolite Recovery>
After the hydrothermal synthesis, the produced zeolite is separated from the reaction solution. The obtained zeolite (hereinafter, sometimes referred to as "OSDA-containing zeolite") usually contains alkali metal atoms in the pores. When an organic structure-directing agent is used, it is also contained in the pores. The method for separating the OSDA-containing zeolite from the reaction solution is not particularly limited, but typically includes filtration, decantation, direct drying, and the like.
The OSDA etc.-containing zeolite separated from the reaction solution is washed with water, dried, and then calcined, if necessary, in order to remove the organic structure-directing agent etc., to obtain a zeolite that does not contain the organic structure-directing agent etc. Note that it is preferable to use the zeolite from which these have been removed as the adsorbent described below.

<Removal of organic structure-directing agents, etc. from zeolite containing OSDA, etc.>
As a method for removing the organic structure directing agent from the OSDA-containing zeolite, removal by calcination is preferred from the viewpoint of productivity. The calcination temperature is preferably 300° C. or higher, more preferably 400° C. or higher, even more preferably 450° C. or higher, and particularly preferably 500° C. or higher, and on the other hand, is preferably 800° C. or lower, more preferably 700° C. or lower, and even more preferably 600° C. or lower. The atmosphere is preferably a gas containing oxygen, but carbonization may be performed using an inert gas such as nitrogen.
In the method for producing zeolite, by selecting raw materials and adjusting the ratio thereof, etc., it is possible to produce a zeolite having a desired crystal type and a desired SiO ₂ /Al ₂ O ₃ molar ratio, etc.

(Zeolite counter cation species)
The counter cation held by the zeolite before the precious metal loading treatment is not particularly limited, but is preferably an alkali cation such as sodium cation, an ammonium cation, or a proton. The ion exchange reaction proceeds more easily with a cation having a smaller ionic radius. On the other hand, if a proton exists as an acid in the liquid after ion exchange, it may destabilize the structure of the zeolite, so that sodium cation and ammonium cation are more preferable, and ammonium cation is particularly preferable.

Zeolite can be subjected to ion exchange treatment before carrying a precious metal, thereby changing the type of counter cation held by the zeolite. For example, the alkali metal atom source, aluminum atom source, silicon atom source, organic structure directing agent, and alkali metal portion derived from the alkali metal atom contained in the zeolite added in the present invention can be converted to a proton type or an ammonium type and used, and a known technique can be adopted for this method. For example, this can be done by treating with an ammonium salt such as NH ₄ NO ₃ or NaNO ₃ or an acid such as hydrochloric acid at room temperature to 100° C., followed by washing with water.

<Method of supporting precious metal>
The aluminosilicate zeolite produced as described above may contain a precious metal by supporting the precious metal. Examples of the precious metal raw material used for supporting the precious metal include inorganic acid salts such as oxides, hydroxides, sulfates, nitrates, chlorides, and bromides containing the precious metal, and organic acid salts such as acetates.
Specific examples of methods for supporting a precious metal on a zeolite include impregnation support, liquid phase ion exchange, solid phase ion exchange, precipitation support, CVD, spray drying, etc. In addition, after supporting by the liquid phase ion exchange or solid phase ion exchange, supporting by the impregnation method may be combined.
Of the above methods, it is preferable to support the precious metal on the zeolite by the solid phase ion exchange method, the impregnation support method, or the spray drying method, and it is particularly preferable to support the precious metal on the zeolite by the impregnation support method.

(Impregnation support method)
The solvent used in the impregnation support method may be water or an organic solvent as long as the precious metal raw material dissolves therein, but water is usually preferred. The amount of the solvent used in the impregnation support treatment may be determined by the mass ratio to the zeolite. In general, the smaller the amount of water, the smaller the amount of waste liquid after the ion exchange treatment, so the mass ratio to the zeolite is preferably 100 or less, more preferably 50 or less, even more preferably 25 or less, particularly preferably 20 or less, especially preferably 15 or less, and most preferably 10 or less. On the other hand, since it is easy to stir and to perform uniform impregnation support, the mass ratio to the zeolite is usually 1 or more, preferably 3 or more, more preferably 5 or more, even more preferably 7 or more, and particularly preferably 9 or more.

The impregnation process is usually carried out by dissolving the precious metal raw material in a solvent to prepare a slurry liquid to which zeolite has been added, and then carrying out a reduced pressure heating process to remove the organic solvent. The heating temperature may be any temperature that allows the solvent to evaporate, and varies depending on the pressure, but is usually carried out at 100°C or lower, preferably 80°C or lower, more preferably 70°C or lower, and even more preferably 60°C or lower. On the other hand, since the solvent is easily evaporated, it is usually carried out at 0°C or higher, preferably 10°C or higher, more preferably 15°C or higher, even more preferably 20°C or higher, and particularly preferably 30°C or higher.

In the impregnation support process, after removing the solvent, the process of adding and removing the solvent again is repeated, so that the precious metal can be supported more uniformly on the zeolite surface. Therefore, there is no particular limit to the number of times the solvent addition and solvent removal processes are performed, and the process may be repeated until the desired effect is obtained.

(Steam treatment)
The precious metal impregnated and supported on the surface is usually supported in the vicinity of the zeolite surface as an oxide. Therefore, in order to support the precious metal cation in the pores of the zeolite, it is preferable to perform steam treatment. The higher the steam treatment temperature, the more effectively the precious metal cation can be dispersed in the pores, so that the steam treatment temperature is preferably 500°C or higher, more preferably 600°C or higher, even more preferably 650°C or higher, particularly preferably 700°C or higher, and most preferably 750°C or higher. On the other hand, a low temperature is preferable in terms of preventing structural destruction due to the detachment of Al from the zeolite acid site, and is preferably 900°C or lower, more preferably 875°C or lower, even more preferably 850°C or lower, particularly preferably 825°C or lower, and most preferably 800°C or lower.

In the steam treatment, the higher the concentration of water contained in the treatment gas, the more effectively the precious metal cations can be dispersed in the pores, so it is preferably 1 vol% or more, more preferably 5 vol% or more, even more preferably 8 vol% or more, particularly preferably 10 vol% or more, and most preferably 20 vol% or more. On the other hand, a low concentration is preferable from the viewpoint of preventing structural destruction due to desorption of Al from the zeolite acid sites, so the water concentration is preferably 100 vol% or less, more preferably 90 vol% or less, even more preferably 80 vol% or less, particularly preferably 70 vol% or less, particularly preferably 60 vol% or less, and most preferably 50 vol% or less.
The contact conditions for the steam treatment gas are not particularly limited, but the space velocity (SV) is usually 100/h or more, preferably 1000/h or more, and on the other hand, usually 500,000/h or less, preferably 300,000/h or less, more preferably 100,000/h or less.
The time for the water vapor treatment varies depending on factors such as temperature, water vapor concentration, SV, etc., and therefore the optimal time varies depending on the conditions, but is usually 1 hour or more, more preferably 3 hours or more, and even more preferably 5 hours or more. The upper limit of the time for the water vapor treatment is not particularly limited, but is, for example, 24 hours.

The zeolite may be calcined after impregnation with the precious metal. Specifically, the zeolite may be calcined at preferably 300°C to 900°C, more preferably 350°C to 850°C, and even more preferably 400°C to 800°C for 1 second to 24 hours, preferably 10 seconds to 8 hours, and even more preferably 30 minutes to 5 hours. Although this calcination is not necessarily required, calcination can increase the dispersibility of the precious metal supported in the zeolite framework, and is effective in improving the adsorption capacity. Calcination may be performed, for example, before the steam treatment.

However, the method of incorporating a precious metal into zeolite is not limited to the method of supporting a precious metal after synthesizing an aluminosilicate zeolite, and other methods may be used. For example, a precious metal (which may be a simple substance or a compound) may be added before the hydrothermal synthesis described above to directly synthesize a zeolite incorporating a precious metal.

[Application]
The use of the composite material of the present invention is not particularly limited, and it is preferably used as a catalyst, an adsorbent, a separation material, etc. It is more preferable to use the composite material of the present invention as an adsorbent for nitrogen oxides.

<Nitrogen oxide adsorbent>
The nitrogen oxide adsorbent includes the above-mentioned composite material. The nitrogen oxide adsorbent may be composed of the above-mentioned composite material alone, or may include a binder such as silica, alumina, or a clay mineral in addition to the composite material.
For example, the nitrogen oxide adsorbent may be prepared by mixing the above composite material with a binder as required, granulating the mixture, and molding the mixture into a predetermined shape, preferably a honeycomb shape.

When the nitrogen oxide adsorbent of the present invention is obtained by molding, it can usually be obtained by kneading a composite material containing a precious metal and zeolite with an inorganic binder such as silica or alumina, or inorganic fibers such as alumina fiber or glass fiber, molding the mixture by an extrusion method or compression method, and then firing the mixture. Preferably, the mixture is molded into a honeycomb shape at this time to obtain a honeycomb-shaped adsorbent.

The nitrogen oxide adsorbent may also be obtained by a coating method. When the nitrogen oxide adsorbent is obtained by a coating method, it is usually produced by mixing the composite material containing the precious metal and zeolite of the present invention with an inorganic binder such as silica or alumina to produce a slurry, which is then coated on the surface of a molded body made of an inorganic substance such as cordierite, and then sintered. Preferably, the slurry is coated on a honeycomb-shaped molded body at this time, thereby producing a honeycomb-shaped adsorbent. Note that an inorganic binder is used here because an adsorbent for exhaust gas treatment, which will be described later, is used as an example, but an organic binder may be used depending on the application and conditions of use.

When the composite material of the present invention is used as a nitrogen oxide adsorbent, the adsorbed nitrogen oxides are usually rendered harmless after adsorption. When used in an engine system, the composite material of the present invention may be installed in a stage preceding the SCR catalyst, or in a stage preceding the particle filter, or in a stage preceding the oxidation catalyst. The nitrogen oxide adsorbent of the present invention may also be mixed with the oxidation catalyst and installed.

When the composite material of the present invention is used as an adsorbent for nitrogen oxides, the composite material of the present invention is usually brought into contact with a gas containing nitrogen oxides to adsorb the nitrogen oxides contained in the gas. The gas to be treated may contain components other than nitrogen oxides, such as hydrocarbons, carbon monoxide, carbon dioxide, hydrogen, nitrogen, oxygen, sulfur oxides, water, etc.
The adsorbent of the present invention exhibits excellent nitrogen oxide adsorption ability when treating a gas containing water. Specifically, the adsorption of nitrogen oxides using the composite material of the present invention can be applied to the adsorption of nitrogen oxides contained in a wide variety of exhaust gases emitted from various diesel engines, boilers, gas turbines, etc. for diesel automobiles, gasoline automobiles, stationary power generation, ships, agricultural machinery, construction machinery, motorcycles, and aircraft.

The adsorbent of the present invention also exhibits particularly excellent nitrogen oxide adsorption ability when treating gas containing water in a low-temperature environment. Therefore, the adsorbent may be used to treat gas at, for example, 20 to 110°C, and adsorb nitrogen oxides in this low temperature range. After adsorption, the adsorbent may also be placed in an environment with a higher temperature than the low temperature range (for example, a temperature higher than 110°C), thereby releasing the adsorbed nitrogen oxides.

<Nitrogen oxide removal device>
The composite material (nitrogen oxide adsorbent) of the present invention can be used as a device for removing nitrogen oxides (nitrogen oxide removal device). The nitrogen oxide removal device includes at least the above-mentioned composite material, and the composite material adsorbs nitrogen oxides from, for example, a gas that flows through the device and contains water and nitrogen oxides. The gas may contain components other than water and nitrogen oxides, the details of which are as described above. The nitrogen oxide removal device can be used, for example, as a system for purifying automobile exhaust gas.

The nitrogen oxide removal device preferably includes a nitrogen oxide reduction catalyst in addition to the composite material, and the composite material is disposed in front of the nitrogen oxide reduction catalyst in the gas flow path. The nitrogen oxide reduction catalyst is preferably a selective catalytic reduction (SCR catalyst) that reduces nitrogen oxides with a reducing agent, and examples of the reducing agent include ammonia. The reducing agent may be supplied to the SCR catalyst from a reducing agent supply unit, and the supplied reducing agent may be ammonia itself, or a compound capable of generating ammonia. Examples of compounds capable of generating ammonia include urea. The SCR catalyst is not particularly limited, and known SCR catalysts can be used.

In the nitrogen oxide removal device having the SCR catalyst described above, when the temperature of the flowing gas (e.g. exhaust gas) is in the low temperature range (e.g. 20 to 110°C), the nitrogen oxides in the exhaust gas are adsorbed by the composite material. When the temperature of the exhaust gas is in the high temperature range (e.g. higher than 110°C), the high temperature exhaust gas usually causes the nitrogen oxides adsorbed by the adsorbent to be desorbed. On the other hand, the SCR catalyst is a catalyst that can reduce nitrogen oxides with a reducing agent in the high temperature range (e.g. higher than 110°C). Generally, exhaust gas is at a low temperature at the beginning of operation, but becomes hot after a certain time has passed since the start of operation.

Therefore, in a nitrogen oxide removal device having an SCR catalyst, for example, when low-temperature exhaust gas is discharged and passes through the nitrogen oxide adsorbent, the nitrogen oxides contained in the exhaust gas are adsorbed by the nitrogen oxide adsorbent, thereby reducing the amount of nitrogen oxides in the exhaust gas. On the other hand, when high-temperature exhaust gas is discharged and passes through the composite material, the nitrogen oxides are not adsorbed by the composite material, and the adsorbed nitrogen oxides are desorbed. Therefore, the exhaust gas including the desorbed nitrogen oxides is sent from the composite material to the SCR catalyst. In the SCR catalyst, the nitrogen oxides contained in the exhaust gas are reduced to nitrogen and the like by a reducing agent. This makes it possible for the nitrogen oxide removal device to reduce the amount of nitrogen oxides contained in the exhaust gas even when the exhaust gas is at a high temperature.
The nitrogen oxide removing device may also be appropriately equipped with a particle filter, an oxidation catalyst, and the like, and the composite material may be used, for example, by being mixed with an oxidation catalyst.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples as long as they do not exceed the gist of the invention.

[Analysis and Evaluation]
In the following Examples and Comparative Examples, the analysis and performance evaluation of the obtained zeolites were carried out by the following methods.

[Powder XRD Measurement]
<Sample preparation>
100 mg of zeolite crushed using an agate mortar was held in a sample holder.
<Apparatus specifications and measurement conditions>
The specifications of the powder XRD measurement device and the measurement conditions are as shown in Tables 1 and 2 below.

[Measurement of average particle size of zeolite]
In the present invention, the particle size is defined as the average value of the particle sizes of 30 or more randomly selected particles measured by particle observation using a scanning electron microscope. The particle size is defined as the diameter of a circle having an area equal to the projected area of the particle (equivalent circle diameter).

[Zeolite composition analysis]
The silicon atom and aluminum atom contents in the standard zeolite samples were determined as follows.
After the standard sample of zeolite was dissolved by heating in an aqueous hydrochloric acid solution, the silicon atom and aluminum atom contents (mass%) were determined by ICP analysis. Then, X-ray fluorescence analysis was performed on this standard sample, and a calibration curve of the fluorescent X-ray intensity and the element concentration of ICP analysis was created. From this calibration curve, the silicon atom and aluminum atom contents (mass%) in the zeolite sample were determined by X-ray fluorescence (XRF) analysis. The ICP analysis was performed using an ULTIMA 2C device manufactured by Horiba, Ltd. The XRF analysis was performed using an EDX-700 device manufactured by Shimadzu Corporation.

[Measurement of zeolite specific surface area]
20 mg of a composite material containing a precious metal and zeolite was placed in a quartz cell and subjected to reduced pressure heat treatment at 400°C for 2 hours. Then, the adsorption isotherm (adsorbed gas: nitrogen) was measured at liquid nitrogen temperature using a BELSORP-miniII manufactured by Microtrac-Bell.
The specific surface area was calculated from the nitrogen adsorption isotherm by the BET multipoint method. The calculation was performed at p/p0 = 0.01 to 0.10, where good linearity was obtained in the BET plot.
Pretreatment temperature: 400°C, pretreatment time: 2 hours, pretreatment atmosphere: vacuum, adsorption gas: nitrogen, BET analysis range: p/p0 = 0.01 to 0.10

[Measurement of the acidity of zeolite]
The maximum peak intensity of a composite material containing a noble metal and a zeolite, as determined by ammonia temperature programmed desorption (NH ₃ -TPD), is determined as follows.
A composite material containing precious metals and zeolite is spread on a glass petri dish and stored in a desiccator with a saturated magnesium nitrate aqueous solution at a relative humidity of 50% for 12 hours to absorb moisture. The thermogravimetric change of the moisture-absorbed composite material is measured from room temperature to 800°C under air circulation, and the weight change is taken as the moisture content. 50 mg of the moisture-absorbed composite material is filled into a quartz glass cell, and measurements are performed under the following conditions.
Measuring device: BELCAT-II (Microtrack Bell)
Pretreatment temperature: 450°C, pretreatment time: 1 hour, ammonia adsorption temperature: 160°C, ammonia adsorption time: 15 minutes, desorption temperature range: 160°C to 800°C

[Measurement of nitrogen oxide adsorption amount]
The amount of nitrogen oxides adsorbed by the nitrogen oxide adsorbent (the amount of adsorbed NO) was determined as follows: A gas having a water vapor content of 0 vol. % and a gas having a water vapor content of 5 vol. % were passed through the nitrogen oxide adsorbent, and the amount of adsorbed NO was measured for each case.
(Pretreatment conditions)
Gas composition: nitrogen Heating rate: 10 K/min
Drying time: 500°C
Retention time: 0.5 hours (NO adsorption conditions)
Adsorption temperature: 80℃, 120℃
Gas composition: NO = 200 ppm by volume, _O2 = 5% by volume, water = 0% or 5% by volume, balance _N2
Space velocity (SV): 70000/h
After NOx adsorption, the supply of only NOx was stopped and the gas was allowed to flow for 30 minutes. Thereafter, the temperature was raised under the following conditions to desorb the gas adsorbed by NOx, and the amount of nitrogen oxide adsorbed by the adsorbent was measured based on the amount of desorbed NOx.
(NO Release Conditions)
Heating rate: 10K/min
Desorption temperature range: 80°C to 600°C
(Measuring equipment)
NOx meter: ECL-88A Lite manufactured by Yanaco Technical Science Co., Ltd.

[Example 1]
[Production of RHO-type zeolite]
The RHO-type zeolite was synthesized as follows.
22.8 g of 18-crown-6-ether (Tokyo Chemical Industry Co., Ltd.), 5.8 g of NaOH (Kishida Chemical Industry Co., Ltd.), and 4.7 g of CsOH·H ₂ O (Mitsuwa Chemical Industry Co., Ltd.) were dissolved in 84.1 g of water, and the resulting solution was stirred at 80° C. for 3 hours to obtain a crown ether-alkali aqueous solution.

The above crown ether-alkali aqueous solution was dropped into 29.8 g of FAU zeolite ( _SiO2 / _{Al2O3 molar} ratio = 30, Zeolyst CBV720), and 0.6 g of RHO zeolite synthesized according to Microporous Materials 4 (1995) 231-238 was added as seed crystals (2.0 mass% relative to _SiO2 when all Si in the raw material mixture is _SiO2 ), and the mixture was stirred at room temperature for 2 hours to prepare a pre-reaction mixture. The composition of this mixture (molar ratio excluding seed crystals) is as follows:
_SiO2 / _Al2O3 / _NaOH /CsOH/ _H2O /18-crown-6-ether = 1/0.033/0.30/0.06/10/0.18

This pre-reaction mixture was aged at room temperature for 24 hours, then placed in a pressure-resistant container and placed in an oven at 160°C for 72 hours to carry out hydrothermal synthesis. After this hydrothermal synthesis reaction, the reaction liquid was cooled and filtered to recover the generated crystals. The recovered crystals were dried at 100°C for 12 hours, and the resulting powder was subjected to XRD measurement, confirming that an RHO-type zeolite having peaks and relative intensities at the positions shown in Figure 1 was obtained. The _SiO2 / _{Al2O3 ratio} by XRF analysis was 15, and the average particle size was 5.0 μm.

[OSDA Removal Processing]
In order to remove organic substances in the zeolite, the RHO type zeolite synthesized as described above was calcined in an air stream at 500° C. for 5 hours.

[Ion exchange treatment to ammonium form]
Next, in order to remove the alkali cations (sodium cations, cesium cations) contained in the fired RHO-type zeolite, the zeolite was dispersed in a 1 mol/L NH ₄ NO ₃ aqueous solution and subjected to ion exchange treatment at 80° C. for 2 hours. After the predetermined treatment time was completed, the zeolite was collected by filtration and washed three times with ion-exchanged water. Then, the ion exchange treatment using an NH ₄ NO ₃ aqueous solution, filtration collection, and three washings were repeated five times. The obtained zeolite powder was dried at 100° C. for 12 hours to obtain an ammonium-type RHO-type zeolite.

[Palladium Support Treatment]
The impregnation of zeolite with palladium was carried out under the following conditions. 0.075 g of palladium nitrate (manufactured by Aldrich) was dissolved in 10 g of ion-exchanged water as a solvent so that 3% by mass of palladium was supported on the ammonium-type RHO-type zeolite, and then 1 g of ammonium-type RHO-type zeolite was added to prepare a mixed slurry. The mixed slurry was distilled under reduced pressure at 50°C in an evaporator to remove water as a solvent. Thereafter, the process of adding 10 g of ion-exchanged water again to form a slurry and then distilling under reduced pressure in an evaporator was repeated three times in total to obtain an RHO-type zeolite impregnated with palladium.

[Steam treatment]
The palladium-supported RHO zeolite was subjected to steam treatment at 750° C. with 10% by volume of steam at a space velocity SV of 3000/h for 5 hours to obtain palladium-supported RHO zeolite. The specific surface area of the palladium-supported RHO zeolite after the steam treatment was 560 m ² /g.

[Example 2]
Palladium-supported RHO zeolite was obtained in the same manner as in Example 1, except that the steam concentration during the steam treatment of the palladium-supported RHO zeolite was changed to 20% by volume.

[Comparative Example 1]
(RHO type zeolite produced by a conventional method)
RHO-type zeolite was synthesized by the method described in Microporous Materials 4 (1995) pp. 231-238. Its _XRD pattern is shown in Figure 2. The _SiO2 / _Al2O3 ratio by XRF analysis was 10, and the average particle size was 1.9 μm.
The RHO zeolite was subjected to OSDA removal treatment, ion exchange treatment to convert to ammonium type, palladium support treatment, and steam treatment under the same conditions as in Example 1 to obtain a palladium-supported RHO zeolite.

[Comparative Example 2]
(Production of CHA-type zeolite)
First, CHA-type zeolite was synthesized as seed crystals. After preparing a pre-reaction mixture with the following composition, hydrothermal synthesis was performed at 160°C for 2 days, and the reaction liquid was cooled and filtered to recover the generated crystals. The recovered crystals were dried at 100°C for 12 hours to obtain CHA-type zeolite for seed crystals.
_Al2O3 / _SiO2 / _NaOH /KOH/OSDA/ _H2O =0.033/1/0.1/0.06/0.07/20

The raw materials for the seed crystals are as follows:
Al source: Al(OH) ₃ (Al ₂ O ₃ : 53.5% by mass, manufactured by Aldrich)
Na source: NaOH (Wako Pure Chemical Industries, Ltd.)
K source: KOH (manufactured by Wako Pure Chemical Industries, Ltd.)
OSDA: N,N,N-trimethyl-1-adamantanammonium hydroxide (TMADAOH) aqueous solution (containing 25% by mass of TMADAOH, manufactured by Seichem Co., Ltd.)
Silicon source: colloidal silica (silica concentration: 30% by mass, Cataloid Si-30, manufactured by JGC Catalysts and Chemicals)

Next, a raw material mixture for hydrothermal synthesis with the following composition was prepared, and the above-mentioned CHA-type zeolite for seed crystals was added thereto so that the amount was 2.0 mass% relative to _SiO2 when all of the Si in the raw material mixture was assumed to be _SiO2 . After performing hydrothermal synthesis at 160°C for 2 days, the reaction liquid was cooled and filtered to recover the generated crystals. The recovered crystals were dried at 100°C for 12 hours to obtain CHA-type zeolite. The _SiO2 / _{Al2O3 molar} ratio was 25 according to XRF analysis.
_Al2O3 / _SiO2 / _NaOH /KOH/OSDA/ _H2O =0.033/1/0.1/0.06/0.07/20

The raw materials are as follows:
Al source: Al(OH) ₃ (Al ₂ O ₃ : 53.5% by mass, manufactured by Aldrich)
Na source: NaOH (Wako Pure Chemical Industries, Ltd.)
K source: KOH (manufactured by Wako Pure Chemical Industries, Ltd.)
OSDA: N,N,N-trimethyl-1-adamantanammonium hydroxide (TMADAOH) aqueous solution (containing 25% by mass of TMADAOH, manufactured by Seichem Co., Ltd.)
Silicon source: colloidal silica (silica concentration: 30% by mass, Cataloid Si-30, manufactured by JGC Catalysts and Chemicals)

[OSDA Removal Processing]
In order to remove organic matter from the zeolite, the CHA-type zeolite was calcined in an air stream at 600° C. for 5 hours.

[Ion exchange treatment to ammonium form]
Next, in order to remove the alkali cations (sodium cations, potassium cations) contained in the fired CHA-type zeolite, the zeolite was dispersed in a 1 mol/L NH ₄ NO ₃ aqueous solution and subjected to ion exchange treatment at 80°C for 2 hours. After the treatment for a predetermined time was completed, the zeolite was recovered by filtration and washed with ion-exchanged water three times. After that, ion exchange treatment and washing using a 1 mol/L NH ₄ NO ₃ aqueous solution were performed twice in total. The obtained zeolite powder was dried at 100°C for 12 hours to obtain ammonium-type CHA-type zeolite.

[Palladium Support Treatment]
The impregnation treatment for supporting palladium was carried out under the following conditions. 0.075 g of palladium nitrate (manufactured by Aldrich) was dissolved in 10 g of ion-exchanged water as a solvent so that 3 mass% of palladium was supported on the ammonium-type CHA-type zeolite, and then 1 g of ammonium-type CHA-type zeolite was added to prepare a mixed slurry. The mixed slurry was distilled under reduced pressure in an evaporator to remove the water as the solvent. Thereafter, the process of adding 10 g of ion-exchanged water again to make a slurry and distilling under reduced pressure in an evaporator was repeated three times in total to obtain a CHA-type zeolite impregnated with palladium.

[Steam treatment]
The palladium-impregnated CHA-type zeolite was subjected to a steam treatment by passing 10% by volume of steam at 750° C. and a space velocity SV of 3000/h for 15 hours to obtain a palladium-supported CHA-type zeolite.

[Comparative Example 3]
(Production of AEI zeolite 1)
A raw material mixture for hydrothermal synthesis having the following composition was prepared, and CHA-type zeolite (uncalcined product) with SAR=25 synthesized in Comparative Example 2 was added thereto so that the amount was 5.0 mass% relative to _SiO2 when all Si in the raw material mixture was converted to _SiO2 . After performing hydrothermal synthesis at 180°C for 1 day, the reaction liquid was cooled and filtered to recover the generated crystals. The recovered crystals were dried at 100°C for 12 hours to obtain AEI-type zeolite 1. The _SiO2 / _{Al2O3 molar} ratio was 11 according to XRF analysis.
_Al2O3 / _SiO2 / _NaOH /OSDA/ _H2O =0.054/1/0.22/0.43/22

The raw materials are as follows:
Al source: Al(OH) ₃ (Al ₂ O ₃ : 53.5% by mass, manufactured by Aldrich)
Na source: NaOH (Wako Pure Chemical Industries, Ltd.)
OSDA: N,N-dimethyl-3,5-dimethylpiperidinium hydroxide (35% by mass, manufactured by Seichem Co., Ltd.)
Si source: colloidal silica (silica concentration: 40 mass%, ST-40, manufactured by Nissan Chemical Industries, Ltd.)

[OSDA Removal Processing]
In order to remove organic matter in the zeolite, AEI zeolite 1 was calcined in an air stream at 600° C. for 5 hours.

[Ion exchange treatment to ammonium form]
Next, in order to remove the alkali cations (sodium cations) contained in the fired AEI zeolite, the zeolite was dispersed in a 1 mol/L NH ₄ NO ₃ aqueous solution and subjected to ion exchange treatment at 80° C. for 2 hours. After the predetermined treatment time was completed, the zeolite was recovered by filtration and washed with ion-exchanged water three times. Then, ion exchange treatment and washing using a 1 mol/L NH ₄ NO ₃ aqueous solution were performed twice in total. The obtained zeolite powder was dried at 100° C. for 12 hours to obtain ammonium-type AEI zeolite 1.

[Palladium Support Treatment]
The impregnation treatment for supporting palladium was carried out under the following conditions. 0.075 g of palladium nitrate (manufactured by Aldrich) was dissolved in 10 g of ion-exchanged water as a solvent so that 3 mass % of palladium was supported on ammonium-type AEI zeolite 1, and then 1 g of ammonium-type AEI zeolite 1 was added to prepare a mixed slurry. The mixed slurry was distilled under reduced pressure in an evaporator to remove water as a solvent. Thereafter, the process of adding 10 g of ion-exchanged water again to form a slurry and distilling under reduced pressure in an evaporator was repeated three times in total to obtain AEI zeolite 1 impregnated with palladium.

[Steam treatment]
A steam treatment was carried out by passing 10% by volume of steam at 750° C. and a space velocity SV of 3000/h for 5 hours through the palladium-impregnated AEI zeolite 1, thereby obtaining palladium-supported AEI zeolite 1.

[Comparative Example 4]
[Palladium Support Treatment]
The impregnation treatment for supporting palladium was carried out under the following conditions: AEI zeolite 2 impregnated with palladium was obtained using the same method as in Comparative Example 3, except that 0.025 g of palladium nitrate (manufactured by Aldrich) was used so that 1 mass % of palladium was supported on ammonium-type AEI zeolite 1.

[Steam treatment]
A steam treatment was carried out by passing 10% by volume of steam at 750° C. and a space velocity SV of 3000/h for 5 hours through the palladium-impregnated AEI zeolite 2, thereby obtaining palladium-supported AEI zeolite 2.

[Comparative Example 5]
(Production of AEI zeolite 2)
AEI zeolite 2 was obtained by the method described in Example 2 of U.S. Patent No. 5,958,370. The details are as follows.
1.9 g of water, 5.5 g of N,N-dimethyl-3,5-dimethylpiperidinium hydroxide (manufactured by Seichem Co., Ltd.), and 20.5 g of 1 M (mol/L) NaOH aqueous solution (manufactured by Wako Pure Chemical Industries, Ltd.) were mixed, to which 2.4 g of Y-type zeolite (USY30 CBV720, manufactured by Zeolyst Co., Ltd.) (framework density: 12.7T/1000Å ³ ) was added and stirred to dissolve into a transparent solution. 1.9 g of Snowtex 40 (silica concentration: 40 mass%, manufactured by Nissan Chemical Industries, Ltd.) was added and stirred at room temperature for 2 hours to obtain a pre-reaction mixture. This pre-reaction mixture was placed in a pressure-resistant container and rotated (15 rpm) in an oven at 135° C. for 7 days to perform hydrothermal synthesis. After this hydrothermal synthesis reaction, the reaction solution was cooled and the generated crystals were collected by filtration. The collected crystals were dried at 100° C. for 12 hours to obtain AEI-type zeolite 2. The _SiO2 / _{Al2O3 molar} ratio was 15 according to XRF analysis.

[OSDA Removal Processing]
In order to remove organic matter in the zeolite, AEI zeolite 2 was calcined in an air stream at 600° C. for 5 hours.

[Ion exchange treatment to ammonium form]
Next, in order to remove the alkali cations (sodium cations) contained in the fired AEI zeolite, the zeolite was dispersed in a 1 mol/L NH ₄ NO ₃ aqueous solution and subjected to ion exchange treatment at 80° C. for 2 hours. After the predetermined treatment time was completed, the zeolite was recovered by filtration and washed with ion-exchanged water three times. Then, ion exchange treatment and washing using a 1 mol/L NH ₄ NO ₃ aqueous solution were performed twice in total. The obtained zeolite powder was dried at 100° C. for 12 hours to obtain ammonium-type AEI zeolite 2.

[Palladium Support Treatment]
The impregnation treatment for supporting palladium was carried out under the following conditions. 0.025 g of palladium nitrate (manufactured by Aldrich) was dissolved in 10 g of ion-exchanged water as a solvent so that 1 mass % of palladium was supported on ammonium-type AEI zeolite 2, and then 1 g of ammonium-type AEI zeolite 2 was added to prepare a mixed slurry. The mixed slurry was distilled under reduced pressure in an evaporator to remove water as a solvent. Thereafter, the process of adding 10 g of ion-exchanged water again to form a slurry and distilling under reduced pressure in an evaporator was repeated three times in total to obtain AEI zeolite 3 impregnated with palladium.

[Steam treatment]
A steam treatment was carried out by passing 10% by volume of steam at 750° C. and a space velocity SV of 3000/h through the palladium-impregnated AEI zeolite 3, thereby obtaining a palladium-supported AEI zeolite 3.

[Evaluation of nitrogen oxide adsorption performance]
For each of the composite materials in the Examples and Comparative Examples, the nitrogen oxide adsorption amounts (A1, A2) were measured when the moisture content in the gas was 0 vol % and 5 vol %, respectively, and the rate of change in the nitrogen oxide adsorption amount was calculated from the ratio (A2/A1), and is shown in Table 3.
In addition, in Reference Example 1, CHA-type zeolite carrying palladium as described in Comparative Example 2 was used, and in Reference Example 2, AEI-type zeolite carrying palladium as described in Comparative Example 3 was used, and the results are shown in Table 3 when the adsorption temperature was changed to 120°C.

It has become clear that the specific SAR RHO-type zeolite of the present invention carrying a precious metal does not exhibit a decrease in the amount of nitrogen oxide adsorption even in a gas atmosphere having a water concentration close to that actually contained in automobile exhaust gas, and rather exhibits the characteristic that the amount of nitrogen oxide adsorption is higher when water is contained.
The nitrogen oxide adsorbent of the present invention exhibits the characteristic that, even if it has the same RHO type structure, the higher the SAR, the higher the nitrogen oxide adsorption amount is without being affected by water. It is generally known that the higher the SAR, the fewer the acid sites of the zeolite that are water adsorption sites, making it difficult for water to penetrate into the zeolite pores. However, as described above, the zeolite of the present application example is less susceptible to the influence of water than Comparative Examples 2 and 5, which have the same or higher SAR. This does not mean that the zeolite is less susceptible to the influence of water simply by increasing the SAR, but rather that it is important that the zeolite has a specific structure and a specific SAR in the coexistence of nitrogen oxides and water.

On the other hand, it was revealed that when palladium was supported on a high SAR CHA-type zeolite, as in Comparative Example 2, it was affected by water. This is presumably because CHA-type zeolite has a small-sized d6r structure, and water is adsorbed by the d6r when the gas diffuses, reducing the diffusivity of the gas and reducing the adsorption performance of nitrogen oxides.
In contrast, the nitrogen oxide adsorbent of the present invention contains LTA as a CBU in the zeolite structure, and gas diffuses via the d8r contained in LTA. However, since d8r is larger in size than d6r, it is presumed that even if water is adsorbed, the adsorbent has pores large enough for the gas to diffuse through, and is therefore less susceptible to the effects of the adsorbed water.
On the contrary, since water has a smaller kinetic diameter than nitrogen oxide, it is believed that favorable coordination of water creates a state in which gas is more easily diffused than coordination of only nitrogen oxide at the adsorption point, and therefore the nitrogen oxide adsorbent of the present invention adsorbs a larger amount of nitrogen oxide when the gas contains water.
As described above, the present invention can provide a composite material in which a precious metal is supported on a zeolite having a specific CBU and a _SiO2 / _{Al2O3 molar} ratio, as a nitrogen oxide adsorbent that exhibits high nitrogen oxide adsorption performance even in the presence of water.

Claims (9)

A nitrogen oxide adsorbent comprising a composite material containing a precious metal and a zeolite, the nitrogen oxide adsorbent being characterized in that the zeolite contains lta, which is defined as a composite building unit by the International Zeolite Association (IZA), in its framework, and has a _SiO2 / _{Al2O3 molar} ratio of 11 or more. 2. The nitrogen oxide adsorbent of claim 1, wherein the maximum ring size of said zeolite is 3.6 Å or less . 3. The nitrogen oxide adsorbent according to claim 1, wherein the zeolite is an RHO type zeolite. 4. The nitrogen oxide adsorbent according to claim 1, wherein the noble metal is palladium and the content thereof is 0.1 mass % or more . 5. The nitrogen oxide adsorbent according to claim 1, wherein a rate of change in the nitrogen oxide adsorption amount, expressed as a ratio (A2/A1), is 0.6 or more and 2.0 or less, where A1 is the nitrogen oxide adsorption amount when a gas containing 200 ppm by volume of nitrogen oxide and 5 vol. % of oxygen and having a water vapor content of 0 vol. % is caused to flow through the composite material at 80° C., and A2 is the nitrogen oxide adsorption amount when a gas containing 200 ppm by volume of nitrogen oxide and 5 vol. % of oxygen and having a water vapor content of 5 vol. % is caused to flow through the composite material at 80° C. A nitrogen oxide adsorbent comprising a composite material containing a precious metal and a zeolite, wherein a rate of change in nitrogen oxide adsorption amount, expressed as a ratio (A2/A1) of 0.6 to 2.0, is defined as A1, the nitrogen oxide adsorption amount when a gas containing 200 ppm by volume of nitrogen oxide and 5 vol. % of oxygen and having a water vapor content of 0 vol. % is passed through the composite material at 80° C., and A2, the nitrogen oxide adsorption amount when a gas containing 200 ppm by volume of nitrogen oxide and 5 vol. % of oxygen and having a water vapor content of 5 vol. % is passed through the composite material at 80° C. 7. A nitrogen oxide adsorbent according to any one of claims 1 to 6 for adsorbing nitrogen oxides from a gas containing water. A method for adsorbing nitrogen oxides in a gas containing water and nitrogen oxides by using the nitrogen oxide adsorbent according to any one of claims 1 to 7 . 8. A nitrogen oxide removal device comprising the nitrogen oxide adsorbent according to claim 1 , the nitrogen oxide removal device removing nitrogen oxides from a gas containing water and nitrogen oxides by using the composite material.