JP7234765B2 - Nitrogen oxide adsorbent and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a novel nitrogen oxide adsorbent, and for example, to a nitrogen oxide adsorbent that is applied to purify various exhaust gases, particularly exhaust gases emitted from internal combustion engines such as automobiles.

In the purification of exhaust gas containing nitrogen oxides emitted from internal combustion engines such as automobiles, urea is used, ammonia generated by the decomposition reaction of urea is used as a reducing agent, and an SCR (Selective Catalytic Reduction) catalyst is added to the exhaust gas. has been put into practical use. However, when the engine is started, the SCR catalyst has not reached the operating temperature, so there is a problem that the nitrogen oxides are discharged as they are without being purified. For example, the exhaust gas temperature of a diesel engine is a low temperature of 100 to 200 ° C. for about 800 seconds after the engine starts, and during this time the exhaust gas contains nitrogen oxides that could not be detoxified (Non-Patent Document 1 reference).

The urea-SCR system installed in the current diesel engine generally requires a temperature of 150°C or more at the urea injection position in order to decompose urea into ammonia and activate the SCR catalyst. there is If the temperature is less than 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 a low temperature range such as at the time of starting.

By the way, from 2023, strict regulations are scheduled to start in California to reduce the amount of nitrogen oxides contained in the exhaust gas to about 10% of the current level. Further reduction methods are needed.

As a method of reducing nitrogen oxides emitted when the engine is started, an adsorbent such as zeolite is placed upstream of the exhaust gas purifying catalyst to adsorb nitrogen oxides in a low temperature range and activate the exhaust gas purifying catalyst. Nitrogen oxides are desorbed from the adsorbent after reaching a temperature higher than that, and are purified by an exhaust gas purifying catalyst on the downstream side.

For example, Patent Document 1 describes an exhaust gas purifying catalyst in which a front-stage catalyst in which rhodium is supported on zeolite is arranged on the upstream side of the exhaust gas, and a rear-stage catalyst in which platinum or palladium is supported is arranged on the downstream side. According to this exhaust gas purifying catalyst, a method has been proposed in which nitrogen oxides are adsorbed to the front-stage catalyst in a low temperature range, and nitrogen oxides desorbed from the front-stage catalyst are reduced and purified by the rear-stage catalyst in a high temperature range.

In US Pat. No. 5,200,000, a system is disclosed in which nitrogen oxides from lean exhaust gases are adsorbed at temperatures below 200.degree. C. and subsequently thermally desorbed at temperatures above 200.degree. Nitrogen oxide adsorbents are taught to consist of palladium and cerium oxides or mixed oxides or composite oxides containing cerium and at least one other four-row transition metal.

In Patent Document 3, an adsorbent in which palladium is supported on a small-pore zeolite whose maximum pore diameter in the zeolite crystal structure is composed of an eight-membered oxygen ring can release nitrogen oxides adsorbed at a low temperature at a higher temperature. It has been reported. Furthermore, it is taught that an adsorbent composed of zeolite is more resistant to sulfur components than an adsorbent using ceria oxide as a carrier.
Non-Patent Document 2 reports that CHA-type zeolite supporting palladium desorbs nitrogen oxides at a temperature of 150° C. or higher, and can retain nitrogen oxides at high temperatures.

Non-Patent Document 3 reports, as adsorbents for nitrogen oxides using metals other than noble metals, adsorbents in which iron is supported on FAU-type, MFI-type, and LTA-type zeolites.
In Non-Patent Document 4, when comparing the adsorption amount of nitrogen oxides with respect to divalent iron in the beta-type supporting iron, it is shown that the higher the aluminum content in the zeolite skeleton, the higher the nitrogen oxide adsorption amount. It is In particular, it has been reported that the iron-supported beta type has a higher nitrogen oxide adsorption amount than the MFI type, FAU type, and LTA type at a Si/Al ratio of 5 when compared with the same amount of iron supported. .

JP-A-2000-312827 WO2008/047170 WO2015/085303

"Catalyst" (published by Catalysis Society of Japan) Vol. 45 (2003), pp. 236-240 Applied Catalysis B: Environmental 212 (2017) pp. 140-149 "Zeolite" (published by the Zeolite Society), 2015 Vol. Pages 32, 43-52 Journal of Catalysis 315 (2014) 1-5 Journal of Chemistry C (2017), 121, 3404-3408

In low-temperature nitrogen oxide adsorbents using zeolite, such as the pre-catalyst described in Patent Document 1, a phenomenon is observed in which the amount of adsorbed nitrogen oxides decreases due to the influence of moisture contained in the exhaust gas. Therefore, although the cause is not clear, the rhodium-supporting zeolite has a problem of low stability.

Further, as described above, Patent Document 2 describes a nitrogen oxide adsorbent such as cerium oxide supporting palladium. Therefore, it is suggested that the poisoning effect is greater than that of nitrogen oxide adsorbents in which palladium is supported on zeolite. This is because zeolite, which is a porous material, is poisoned by gas that enters the pores, while oxides have a bare surface, so gases easily adsorb to the surface, and the surface area of the adsorbent is small. Therefore, it is considered that the degree of influence of poisoning is high.

Focusing on the resistance of zeolite to sulfur components described in Patent Document 3, even in zeolite, the amount of nitrogen oxides adsorbed after treatment with a gas containing sulfur components is reduced. Therefore, the essential problem that palladium is susceptible to poisoning by sulfur components remains unchanged, and if an adsorbent supporting palladium is used in the exhaust gas of automobiles, etc., the problem is that the performance will decrease due to the effects of poisoning. remains.

Non-Patent Document 2 introduces a method for supporting palladium on CHA-type zeolite. Palladium does not exhibit the ability to adsorb nitrogen oxides simply by subjecting zeolite to ion exchange treatment or impregnation treatment, and requires a hydrothermal treatment step, making the production process complicated.
Furthermore, precious metals including palladium are expensive, and there still remain essential problems such as a decrease in adsorption performance when sintering occurs under high temperature conditions.

The iron-supporting zeolite introduced in Non-Patent Document 3 is expected as a nitrogen oxide adsorbent that overcomes the above-mentioned problems of palladium. However, as described in the text, the nitrogen oxide adsorbent in which iron is supported on LTA-type zeolite has a large amount of aluminum contained in the skeleton of LTA-type zeolite, so it has heat resistance and hydrothermal stability. is low. In addition, the FAU type and MFI type disclosed in Non-Patent Document 3 have a problem that the amount of nitrogen oxides adsorbed is small.

The nitrogen oxide adsorbent of beta-type zeolite supporting iron described in Non-Patent Document 4 has a low Fe loading of 0.01% by weight or less, and does not have performance that can be put to practical use as a nitrogen oxide adsorbent. In addition, the beta type with a Si/Al ratio of 5, which shows a relatively high adsorption capacity, has an aluminum content compared to beta type zeolite (with a Si/Al ratio of 10 or more) used as a catalyst for SCR. It is considered that the heat resistance and hydrothermal stability are low like the above-mentioned LTA type.

SUMMARY OF THE INVENTION The present invention was made to solve the above-mentioned problems, and its object is to provide a nitrogen oxide adsorbent having a large amount of nitrogen oxide adsorption and sufficient heat resistance. It is in.

The present inventors have found that aluminosilicate zeolites containing eight-membered oxygen rings contain a large amount of iron, which is effective as a nitrogen oxide adsorbent when the SiO ₂ /Al ₂ O ₃ molar ratio is in the specific range of 3 to 15. As a result, the inventors succeeded in developing a nitrogen oxide adsorbent having a high nitrogen oxide adsorption capacity and excellent thermal stability, and arrived at the present invention.

That is, the present invention provides the following [1] to [14].
[1] A nitrogen oxide adsorbent containing an iron-supporting aluminosilicate zeolite, wherein in the iron-supporting aluminosilicate zeolite, the skeleton of the aluminosilicate zeolite has an oxygen eight-membered ring structure,
Nitrogen oxidation wherein the SiO ₂ /Al ₂ O ₃ molar ratio of the iron-supporting aluminosilicate zeolite is 3 or more and 15 or less, and the iron content of the iron-supporting aluminosilicate zeolite is 0.5 wt% or more and 10 wt% or less. material adsorbent.
[2] The nitrogen oxide adsorbent according to [1] above, wherein the average oxidation number of the supported iron is 1.5 or more and less than 2.3.
[3] The nitrogen oxide adsorbent according to [1] or [2] above, wherein the aluminosilicate zeolite is AEI zeolite or AFX zeolite.
[4] A method for producing a nitrogen oxide adsorbent according to any one of [1] to [3] above,
A method for producing a nitrogen oxide adsorbent, comprising the step of mixing an aluminosilicate zeolite and an antioxidant with a solution in which a divalent iron raw material is dissolved in an inert gas atmosphere to obtain an iron-supporting aluminosilicate zeolite.
[5] The method for producing a nitrogen oxide adsorbent according to [4] above, wherein the aluminosilicate zeolite contains alkali cations.
[6] A method of using the nitrogen oxide adsorbent according to any one of [1] to [3] above for exhaust gas purification.
[7] In an exhaust gas purification system comprising an adsorbent and a nitrogen oxide reduction catalyst, wherein the adsorbent is arranged upstream of the nitrogen oxide reduction catalyst, the adsorbent is the adsorbent according to [6] above. A method using a nitrogen oxide adsorbent.
[8] The method according to [7] above, wherein a reducing agent is supplied to the nitrogen oxide reduction catalyst.
[9] The method according to any one of [7] or [8] above, wherein the nitrogen oxide reduction catalyst is a selective reduction catalyst that reduces nitrogen oxides with a reducing agent.
[10] The method according to [8] or [9] above, wherein the reducing agent is at least one selected from the group consisting of ammonia and compounds capable of generating ammonia.
[11] An exhaust gas purifying system comprising an adsorbent and a nitrogen oxide reduction catalyst, wherein the adsorbent is arranged upstream of the nitrogen oxide reduction catalyst, wherein the adsorbent is the above [1] to An exhaust gas purification system, which is the nitrogen oxide adsorbent according to any one of [3].
[12] The exhaust gas purification system according to [11] above, which includes a reducing agent supply unit that supplies a reducing agent to the nitrogen oxide reduction catalyst.
[13] The exhaust gas purification system according to the above [11] or [12], wherein the nitrogen oxide reduction catalyst is a selective reduction catalyst that reduces nitrogen oxides with a reducing agent.
[14] The exhaust gas purification system according to [12] or [13] above, wherein the reducing agent is at least one selected from the group consisting of ammonia and compounds capable of generating ammonia.

According to the present invention, it is possible to provide a nitrogen oxide adsorbent that adsorbs a large amount of nitrogen oxides and has sufficient heat resistance.

1 is a chart showing the XRD pattern of AEI zeolite synthesized in Example 1. FIG. 4 is a chart showing the XRD pattern of AFX-type zeolite synthesized in Example 3. FIG. 4 is a chart showing the XRD pattern of the AEI zeolite synthesized in Comparative Example 1. FIG. 4 is a graph showing the NO desorption behavior of nitrogen oxide adsorbents 1, 2 and 4. FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below, but the following description is an example (representative example) of embodiments of the present invention, and the present invention is not limited to these contents.

The nitrogen oxide adsorbent of the present invention is a nitrogen oxide adsorbent containing iron-supporting aluminosilicate zeolite. The iron-supporting aluminosilicate zeolite contained in the nitrogen oxide adsorbent will first be described in detail below.

[Iron-supporting aluminosilicate zeolite]
The skeleton of the aluminosilicate zeolite in the iron-supporting aluminosilicate zeolite has an oxygen eight-membered ring structure. The iron-supporting aluminosilicate zeolite in the present invention has a SiO ₂ /Al ₂ O ₃ molar ratio of 3 or more and 15 or less and an iron content of 0.5% or more and 10% or less by weight.

The aluminosilicate zeolite used in the nitrogen oxide adsorbent of the present invention should easily support iron, which is effective for adsorbing nitrogen oxides. Therefore, the skeleton of the aluminosilicate zeolite has an oxygen eight-membered ring structure. .
The aluminosilicate 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). However, the skeleton has an oxygen eight-membered ring structure.
The composition ratio (molar ratio) of Me, Al and Si constituting the framework structure of the aluminosilicate zeolite is not particularly limited, but the molar ratio of Me to the total of Me, Al and Si is x, Al When y is the molar ratio of Si and z is the molar ratio of Si, x is usually 0 or more and 0.3 or less. It is preferable that x is less than or equal to this upper limit because impurities are less likely to be mixed during synthesis.
The y is usually 0.001 or more, preferably 0.005 or more, more preferably 0.01 or more, still more preferably 0.05, usually 0.5 or less, preferably 0.4 or less, It is more preferably 0.3 or less, still more preferably 0.25 or less.
The z is usually 0.5 or more, preferably 0.6 or more, more preferably 0.7 or more, still more preferably 0.75 or more, and usually 0.999 or less, preferably 0.995 or less. , more preferably 0.99 or less, and still more preferably 0.98 or less.
When y and z are within the above ranges, synthesis is easy, and when used as a catalyst, sufficient acid sites are present and sufficient activity is easily obtained.
One or two or more other atoms Me may be contained. Preferred Me is an element belonging to period 3 or 4 of the periodic table.

Examples of the aluminosilicate zeolite having an oxygen eight-membered ring structure include ABW, ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, AFT, AFX, ANA, APC, and APD, which are codes that define the framework structure of zeolite defined by the International Zeolite Association (IZA). ATN, ATT, ATV, AWO, AWW, BCT, BIK, BRE, CAS, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, ESV, GIS, GOO, IHW, ITE, ITW, JBW, KFI, LEV, LTA, MER, MON, MTF, MWF, NSI, OWE, PAU, PHI, RHO, RTE, RTH, RWR, SAS, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG, YFI, ZON structure, and ZSM-25, PST-20, and ECR-18 (see Non-Patent Document 5), which are similar in skeleton structure to RHO, can be mentioned. Among them, AEI, AFX, CHA, and ERI are preferred. , KFI, LEV, LTA, MER, MWF, RHO, more preferably AEI, AFX, ERI, KFI, LEV, LTA, MWF, RHO, more preferably AEI structure zeolite (hereinafter referred to as "AEI zeolite" ), or zeolite having an AFX structure (hereinafter referred to as “AFX-type zeolite”), particularly preferably AEI-type zeolite. By using AEI zeolite and AFX zeolite, coupled with a SiO ₂ /Al ₂ O ₃ molar ratio of 3 to 15, it becomes easier to coordinate divalent iron to aluminum, and the amount of iron carried can be increased. can be easily increased. In addition, the durability and hydrothermal stability of the nitrogen oxide adsorbent can be easily increased.

The structure of zeolites is characterized by X-ray diffraction data. However, when measuring the zeolite actually produced, it is affected by the growth direction of the zeolite, the ratio of the constituent elements, the adsorbed substances, the presence of defects, the drying state, etc., and the intensity ratio of each peak and the peak Since there is some deviation in the position, it is not possible to obtain exactly the same numerical values as the parameters of the AEI and AFX structures described in the IZA regulations, and a width of about 10% is allowed.

The major peaks of the AEI zeolite are, for example, the 110 plane peak at 2θ = 9.5° ± 0.2° and the 202 plane peak at 2θ = 16.1° ± 0.2° when CuKα rays are used. and −202 plane peaks (very close and often overlap), 022 plane peaks at 16.9°±0.2°, 310 plane peaks at 20.6°±0.2°, and the like. .
AFX-type zeolite has a peak of 101 planes at 2θ = 8.6° ± 0.2°, a peak of 102 planes at 2θ = 11.6° ± 0.2°, and a peak of 110 planes at 12.9° ± 0.2° and the like.

In the present invention, the iron-supporting aluminosilicate zeolite has a SiO ₂ /Al ₂ O ₃ molar ratio of 3 or more and 15 or less. If the SiO ₂ /Al ₂ O ₃ molar ratio exceeds 15, it becomes difficult to increase the amount of iron supported, and the amount of nitrogen oxides adsorbed by the nitrogen oxides adsorbent decreases. The SiO ₂ /Al ₂ O ₃ molar ratio is preferably 14 or less, more preferably 13 or less, still more preferably 12 or less, and particularly preferably 11, in order to increase the adsorption points and the amount of nitrogen oxides adsorbed. It is below. On the other hand, if the molar ratio is less than 3, the amount of Al in the framework increases, and when exposed to a gas containing water vapor, Al in the framework is likely to detach and cause structural destruction. From these points of view, the SiO ₂ /Al ₂ O ₃ molar ratio is preferably 4 or more, more preferably 5 or more.

Nitrogen oxide adsorbents are required to have purification performance for exhaust gas containing nitrogen oxides and hydrothermal durability at 600° C. or higher. If there is no difference in crystallinity, an iron-supporting aluminosilicate zeolite with a low SiO ₂ /Al ₂ O ₃ molar ratio has more adsorption points than an adsorbent with a high SiO ₂ /Al ₂ O ₃ molar ratio. It is advantageous to have high adsorption performance for exhaust gas containing For example, when the exhaust gas is at a relatively low temperature of 700° C. or less, such as in a truck, even if a nitrogen oxide adsorbent is used in a gas atmosphere containing water vapor, dealumination of Al in the skeleton by water vapor does not progress easily. Therefore, the adsorption performance of exhaust gases containing nitrogen oxides is given priority, and the use of an iron-supporting aluminosilicate zeolite having a relatively low SiO ₂ /Al ₂ O ₃ molar ratio and having many active sites in the zeolite skeleton is desired.
On the other hand, an iron-supporting aluminosilicate zeolite with a high SiO ₂ /Al ₂ O ₃ molar ratio has a small amount of Al in the zeolite framework, and therefore has the advantage that the structure is less likely to collapse even in a high-temperature gas atmosphere containing water vapor. In the case of diesel passenger cars and gasoline cars, it is used as a nitrogen oxide adsorbent in a gas atmosphere containing water vapor at a temperature of 800° C. or higher, so high water vapor resistance is required. Therefore, it is desirable to use an iron-supporting aluminosilicate zeolite with a relatively high SiO ₂ /Al ₂ O ₃ molar ratio. Specifically, it is preferable that the SiO ₂ /Al ₂ O ₃ molar ratio exceeds 7.5, and it is also desirable to use a zeolite with a SiO 2 /Al 2 O 3 molar ratio of 10 or more.

The iron content in the iron-supporting aluminosilicate zeolite of the present invention is 0.5% by weight or more and 10% by weight or less. In iron-supporting aluminosilicate zeolite, iron exists in two states: one is coordinated with aluminum and not included in the zeolite framework structure, and the other is included in the framework structure without being coordinated with aluminum. It is preferably not included in the structure.
When the iron content in the iron-supporting aluminosilicate zeolite is less than 0.5% by weight, it becomes difficult to increase the nitrogen oxide adsorption amount of the nitrogen oxide adsorbent. On the other hand, if it exceeds 10% by weight, the amount of iron that is not coordinated with aluminum increases, and it becomes difficult to exhibit the adsorption performance commensurate with the iron content.
In addition, by increasing the iron content, the temperature at which the adsorbed nitrogen oxides are desorbed tends to increase, and the nitrogen oxide adsorbent of the present invention has an iron content of 0.5% by weight or more. This makes it difficult for the adsorption performance to decline even in a relatively high temperature environment.
The iron content in the iron-supporting aluminosilicate zeolite is preferably 1.0% by weight or more and 8.0% by weight or less, more preferably 1.5% by weight or more and 7.0% by weight or less, and particularly preferably 2 0% by weight or more and 6.0% by weight or less, and particularly preferably 2.3% by weight or more and 5.0% by weight or less. By making it equal to or higher than these lower limits, the adsorption amount of nitrogen oxides is further increased, and the temperature at which the adsorbed nitrogen oxides are desorbed becomes higher, thereby improving the adsorption performance. Moreover, by making it below these upper limits, it becomes easy to exhibit the adsorption performance commensurate with iron content.

The iron supported in the iron-supporting aluminosilicate zeolite may be divalent or trivalent, but is more preferably divalent. Moreover, the average oxidation number of the supported iron is preferably 1.5 or more and less than 2.3. Hereinafter, iron ions having an average oxidation number of 1.5 or more and less than 2.3 may be referred to as "divalent iron ions". Normally, when a divalent iron source is used, it tends to exist in the divalent state in the zeolite.

The ion exchange rate of the nitrogen oxide adsorbent of the present invention is not particularly limited, but is usually 13% or higher, preferably 15% or higher, more preferably 17% or higher, and particularly preferably 20% or higher. On the other hand, there is no upper limit, but it can be 100% or less, or 90% or less.
Ion exchange rate refers to the rate at which H + on the ion exchange site (acid point) in the catalyst support is replaced by the ion of the catalyst metal. A substantially equivalent amount of one divalent iron ion will be replaced.
Specifically, the ion exchange rate can be calculated by the formula (number of metal ion atoms)×(metal ion valence)/(number of acid sites in the catalyst carrier)×100.

Further, the iron-supporting aluminosilicate zeolite of the present invention may contain alkali metal atoms, such as sodium, potassium, cesium, etc., mainly derived from raw materials. The content of alkali metal atoms in the iron-supporting aluminosilicate zeolite is preferably 0.001 or more and 1.0 or less as a molar ratio to aluminum. By setting the content to 1.0 or less, it is possible to prevent the alkali metal atoms from destroying the structure of the zeolite in a water vapor atmosphere, thereby preventing the performance from deteriorating. Further, when the ratio is 0.001 or more, the fear of forcibly removing the alkali metal atoms from the adsorbent and damaging the skeleton of the zeolite or the like is reduced.
Also, the content of alkali metal atoms may be 0.02 or more as a molar ratio to silicon atoms.

[Method for producing iron-supporting aluminosilicate zeolite]
The method for producing the iron-supporting aluminosilicate zeolite of the present invention is not particularly limited. An aluminosilicate zeolite that is included in the framework structure and has a SiO ₂ /Al ₂ O ₃ molar ratio of 3 to 15 is produced, and iron is supported on the aluminosilicate zeolite so that the weight is 0.5 to 10% by weight. method.
Production by such a production method makes it possible to mass-produce the iron-supporting aluminosilicate zeolite of the present invention.

(Method for producing aluminosilicate zeolite)
The aluminosilicate zeolite used in the nitrogen oxide adsorbent of the present invention can be produced by the method described in the literature published by the International Zeolite Association (IZA). Also, AEI zeolite and AFX zeolite can be produced, for example, by the following production method.

An example of a method for producing AEI-type and AFX-type zeolites includes an aluminum atom source, a silicon atom source, an alkali metal atom source, and an organic structure directing agent (also referred to as a “template”. Hereinafter, the organic structure directing agent is referred to as “OSDA”. and a method of hydrothermally synthesizing AEI-type or AFX-type zeolite using seed crystals if necessary.

<Aluminum atom raw material>
The aluminum atom raw material used for producing the AEI-type or AFX-type zeolite used in the present invention is not particularly limited, and includes 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 alkoxide and the like can be used.
Furthermore, zeolite containing silica, such as zeolite, can also be used as the raw material for aluminum atoms. The zeolite used as the source of aluminum atoms can be an aluminosilicate zeolite, such as FAU zeolite, which has a common building unit with AEI or AFX zeolite.
These may be used individually by 1 type, and may be used in mixture of 2 or more types. In addition, each of the above raw materials can be easily obtained with stable quality, which can bring about cost reduction.

Preferred aluminum atom raw materials for AEI zeolite include amorphous aluminum hydroxide, aluminum hydroxide having a gibbsite structure, and aluminum hydroxide having a bayerite structure. Among these, amorphous aluminum hydroxide is particularly preferred. It is preferable to use an aluminosilicate zeolite as the aluminum atom raw material for the AFX-type zeolite.

The amount of aluminum atom raw material used is included in the raw material mixture other than zeolite added as seed crystals in the present production method from the viewpoint of ease of preparation and production efficiency of the raw material mixture or the aqueous gel obtained by aging it. The molar ratio of aluminum (Al) in the aluminum atom raw material to silicon (Si) is usually 0.02 or more, preferably 0.04 or more, more preferably 0.06 or more, and still more preferably 0.08 or more. Although the upper limit is not particularly limited, it is usually 2 or less, preferably 1 or less, more preferably 0.4 or less, and still more preferably 0.2 or less from the viewpoint of uniformly dissolving the aluminum atom raw material in the aqueous gel.

<Silicon atom raw material>
The silicon atom raw material used in this production method is not particularly limited, and various known substances can be used, including colloidal silica, amorphous silica, sodium silicate, trimethylethoxysilane, tetraethylorthosilicate, and aluminosilicate gel. etc. can be used. Also, an aluminosilicate zeolite having a common building unit with AEI or AFX zeolite, such as FAU zeolite, may be used.
These may be used individually by 1 type, and may be used in mixture of 2 or more types.
Among these, materials that are in a form that can be sufficiently uniformly mixed with other components and that are particularly easily soluble in water are preferred, and colloidal silica, sodium silicate, trimethylethoxysilane, tetraethylorthosilicate, and aluminosilicate gel are preferred.
The silicon atom raw material is used so that the amount of the other raw material to be used relative to the silicon atom raw material is within the preferred range described above or below.

<Alkali metal atom raw material>
Alkali metal atoms contained in the alkali metal atom raw material used in the present production method are not particularly limited, and known ones used in the synthesis of zeolite can be used, but the group consisting of lithium, sodium, potassium, rubidium and cesium At least one alkali metal ion selected from is preferred. That is, the AEI or AFX zeolite is preferably obtained by crystallization in the presence of these alkali metal ions.

Alkali metal atoms preferably include sodium atoms. As will be described later, the alkali metal atoms incorporated into the crystal structure of zeolite during the synthesis process may be removed from the crystal by ion exchange. The removal process can be simplified.
Therefore, it is preferable that 50 mol % or more of the alkali metal atoms contained in the alkali metal atom raw material are sodium atoms, and more preferably 80 mol % or more of the alkali metal atoms contained in the alkali metal atom raw material are sodium atoms. and most preferably substantially all (ie, 100 mol %) are sodium atoms.

Alkali metal atom raw materials include inorganic acid salts such as hydroxides, oxides, sulfates, nitrates, phosphates, chlorides and bromides of the above alkali metal atoms, acetates, oxalates, citrates, etc. can be used. One or two or more alkali metal atom sources may be contained.

In the production of AEI zeolite, the molar ratio of the alkali metal atom raw material to the organic structure directing agent in the raw material mixture is preferably 0.1 or more and 2.0 or less, more preferably 0.2 or more and 1.5 or less. .
On the other hand, in the production of AFX-type zeolite, the molar ratio of the alkali metal atom raw material to the organic structure-directing agent in the raw material mixture is preferably 0.1 or more and 20 or less, more preferably 0.2 or more and 10 or less.
By blending the alkali metal atom raw material within the above range, the organic structure-directing agent, which will be described later, is easily coordinated to the aluminum in a suitable state, thereby facilitating formation of a crystal structure.

<Organic Structure Directing Agent>
As the organic structure-directing agent, a nitrogen-containing organic structure-directing agent, a phosphorus-containing organic structure-directing agent, or the like can be used. Various known substances such as tetraethylammonium hydroxide (TEAOH) and tetrapropylammonium hydroxide (TPAOH) can be used as the nitrogen-containing organic structure-directing agent for producing the AEI zeolite. As the nitrogen-containing organic structure-directing agent for producing the AEI zeolite, for example, the following substances can be used.

N,N-diethyl-2,6-dimethylpiperidinium cation, N,N-dimethyl-9-azoniabicyclo[3.3.1]nonane cation, N,N-dimethyl-2,6-dimethylpiperidinium cation , N-ethyl-N-methyl-2,6-dimethylpiperidinium cation, N,N-diethyl-2-ethylpiperidinium cation, N,N-dimethyl-2-(2-hydroxyethyl)piperidinium cations, N,N-dimethyl-2-ethylpiperidinium cation, N,N-dimethyl-3,5-dimethylpiperidinium cation, N-ethyl-N-methyl-2-ethylpiperidinium cation, 2, 6-dimethyl-1-azonium[5.4]decane cation, N-ethyl-N-propyl-2,6-dimethylpiperidinium cation and the like.
Among these, a particularly preferable nitrogen-containing organic structure-directing agent is N,N-dimethyl-3,5-dimethylpiperidinium cation, specifically N,N-dimethyl-3,5-dimethylpiperidinium cation. Preference is given to using nium hydroxide. In addition, N,N-dimethyl-3,5-dimethylpiperidinium hydroxide has cis- and trans-forms, and each of them may be used alone or mixed in an arbitrary ratio. .

Various known substances such as tetraethylammonium hydroxide (TEAOH) and tetrapropylammonium hydroxide (TPAOH) can be used as the nitrogen-containing organic structure-directing agent for producing the AFX-type zeolite. Further, for example, 1,1′-(1,4-butanediyl)bis(1-azonia-4-azabicyclo[2.2.2]octane (hereinafter referred to as DABCO), N,N,N′,N′ -tetraethylbicyclo[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidinium hydroxide (hereinafter referred to as TEBPOH), 1,3-di(1-adamantyl)imidazolium A cation or the like may also be used.

Also, substances such as tetrabutylphosphonium and diphenyldimethylphosphonium can be used as phosphorus-containing organic structure-directing agents for producing AEI zeolites. However, phosphorus compounds may generate diphosphorus pentoxide, which is a hazardous substance, when the synthesized zeolite is calcined to remove OSDA. The agent is preferably a nitrogen-containing organic structure-directing agent.
The organic structure-directing agent may be used alone or in combination of two or more.

From the viewpoint of ease of crystal formation, the amount of the organic structure directing agent used is usually 0.01 in terms of molar ratio to silicon (Si) contained in the raw material mixture other than zeolite as a seed crystal added in this production method. Above, it is preferably 0.03 or more, more preferably 0.05 or more, still more preferably 0.08 or more, and still more preferably 0.10 or more. In order to sufficiently obtain the effect of cost reduction, it is usually 1 or less, preferably 0.8 or less, more preferably 0.6 or less, still more preferably 0.5 or less.

<Water>
The amount of water used is usually 3 or more, preferably 5 or more, more preferably 7 or more in terms of molar ratio to silicon (Si) contained in the raw material mixture other than the zeolite as the seed crystal, from the viewpoint of easy formation of crystals. It is more preferably 10 or more. This range is preferable because crystals are more likely to form. In order to sufficiently obtain the effect of reducing the cost of waste liquid treatment, it is usually 50 moles or less, preferably 40 moles or less, more preferably 30 moles or less, and even more preferably 25 moles or less.

<Seed crystal>
A zeolite may be added as a seed crystal to the raw material mixture in order to produce the AEI-type or AFX-type zeolite used in the present invention. The zeolites added as seed crystals in the present invention may be used singly or in combination of two or more.

The amount of seed crystals added is 0.1% by weight or more with respect to _SiO2 when all silicon (Si) contained in the raw material mixture other than zeolite added as seed crystals is _SiO2 , and the reaction is preferably 0.5% by weight or more, more preferably 2% by weight or more, still more preferably 3% by weight or more, and particularly preferably 5% by weight or more. In addition, although the upper limit of the amount of zeolite used as seed crystals added in the present invention is not particularly limited, it is usually 20% by weight or less, preferably 10% by weight or less, more preferably 8% by weight in order to sufficiently obtain the effect of cost reduction. % or less, more preferably 6 wt % or less.

The zeolite added as a seed crystal is not particularly limited as long as it is a zeolite, but is preferably a zeolite containing d6r in the building unit constituting the crystal structure, particularly AEI type, AFT type, AFX type, CHA type, EAB type, EMT type, ERI type, FAU type, GME type, KFI type, LEV type, LTL type, LTN type, MOZ type, MSO type, MWW type, OFF type, SAS type, SAT type, SAV type, SBS type, SBT type and SZR type are preferable, and AEI type, CHA type and FAU type are more preferable.

The zeolite added as a seed crystal may be an unfired product that has not been fired after hydrothermal synthesis or a fired product that has been fired after hydrothermal synthesis. It is preferable to use an uncalcined product rather than a calcined product because it is preferable that it is difficult to dissolve in an alkali. However, depending on the composition or temperature conditions in the raw material mixture, uncalcined zeolite may not dissolve and may not function as crystal nuclei. In such a case, it is preferable to use zeolite from which OSDA has been removed by calcination in order to increase solubility.

<Mixing of raw materials (preparation of raw material mixture)>
In the method for producing the AEI-type and AFX-type zeolites used in the present invention, the above-described raw materials for aluminum atoms, silicon atoms, alkali metal atoms, organic structure directing agents, and water are mixed, and if necessary, seeds are mixed. Zeolite is added as crystals and thoroughly mixed, and the obtained raw material mixture is hydrothermally synthesized.
The mixing order of these 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 uniformly dissolved. , and an alkali metal atom source are mixed to prepare an alkaline solution, and then an aluminum atom source, a silicon atom source, and a seed crystal are added in this order and mixed.

In addition to the above aluminum atom source, silicon atom source, alkali metal atom source, organic structure directing agent, water and zeolite as a seed crystal, additives and the like are further added to the raw material mixture as necessary. good too. Such additives include, for example, auxiliary agents that serve as components for assisting synthesis of zeolite, more specifically auxiliary agents that accelerate the reaction with an acid component. In addition, metal stabilizers such as polyamines, and metals such as copper that act as catalysts during hydrothermal synthesis, which will be described later, can also be used.

<Aging>
The raw material mixture prepared as described above may be hydrothermally synthesized immediately after preparation, but in order to obtain a highly crystalline zeolite, it is preferably aged under predetermined temperature conditions for a certain period of time. Particularly in the case of scale-up, the agitation is deteriorated and the mixing state of the raw materials tends to be insufficient. Therefore, it is preferable to improve the raw material to a more uniform state by aging the raw material while stirring it for a certain period of time. The aging temperature is usually 100° C. or lower, preferably 95° C. or lower, more preferably 90° C. or lower, and although there is no particular lower limit, it is usually 0° C. or higher, preferably 10° C. or higher. 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, and usually 30 days or less, preferably 10 days or less, more preferably 4 days or less.

<Hydrothermal synthesis>
In the hydrothermal synthesis, the raw material mixture prepared as described above or the aqueous gel obtained by aging it is placed in a pressure vessel and stirred under self-generating pressure or under gas pressure to the extent that crystallization is not hindered. This is carried out by maintaining a predetermined temperature while rotating or rocking the container, or in a stationary state.

The reaction temperature during hydrothermal synthesis is usually 100° C. or higher, preferably 120° C. or higher, and usually 230° C. or lower, preferably 220° C. or lower, more preferably 200° C. or lower, and still more preferably 190° C. or lower. is. Although the reaction time is not particularly limited, it is usually 2 hours or more, preferably 3 hours or more, more preferably 5 hours or more, and usually 30 days or less, preferably 10 days or less, more preferably 7 days or less, and still more preferably 7 days or less. 5 days or less. The reaction temperature may be constant during the reaction, or may be changed stepwise or continuously.
By carrying out the reaction under the above conditions, the yield of the desired AEI-type or AFX-type zeolite is improved, and different types of zeolite are less likely to be produced.

<Recovery of Zeolite>
After the above hydrothermal synthesis, the product AEI or AFX zeolite is separated from the hydrothermal synthesis reaction solution.
The obtained zeolite (hereinafter referred to as "OSDA-containing zeolite") contains an organic structure-directing agent and/or an alkali metal atom in pores. The method for separating the zeolite containing OSDA and the like from the hydrothermal synthesis reaction solution is not particularly limited, but methods such as filtration, decantation, or direct drying are usually used.

The zeolite containing OSDA, etc. separated and recovered from the hydrothermal synthesis reaction solution is washed with water and dried, if necessary, in order to remove the organic structure directing agent, etc. It is possible to obtain a zeolite that does not contain
In order to use the AEI-type or AFX-type zeolite used in the present invention as an adsorbent, the zeolite should be used after removing the organic structure-directing agent and the like as described below.

<Removal of organic structure-directing agent, etc. from zeolite containing OSDA, etc.>
In order to remove the organic structure-directing agent and the like from the OSDA-containing zeolite, it is preferable to carry out calcination in terms of manufacturability. In this case, the firing temperature is preferably 400° C. or higher, more preferably 450° C. or higher, still more preferably 500° C. or higher, preferably 900° C. or lower, more preferably 850° C. or lower, further preferably 800° C. or lower. be. Firing may be performed in the atmosphere or under an inert gas. Nitrogen or the like can be used as the inert gas.

In the method for producing AEI-type and AFX-type zeolites described above, AEI-type and AFX-type zeolites having a wide range of SiO ₂ /Al ₂ O ₃ molar ratios can be produced by changing the charge composition ratio.
_The AEI-type and AFX-type zeolites used in the present invention have a SiO ₂ /Al ₂ O ₃ molar ratio of ₃ or more and 15 or less as described above _. It may be adjusted according to the desired SiO ₂ /Al ₂ O ₃ molar ratio in the aluminosilicate zeolite. Therefore, the SiO ₂ /Al ₂ O ₃ molar ratio in the AEI-type and AFX-type zeolites used in the present invention is preferably 14 or less, more preferably 13 or less, even more preferably 12 or less, and particularly preferably 11 or less. Also, the SiO ₂ /Al ₂ O ₃ molar ratio is preferably 4 or more, more preferably 5 or more.

The average primary particle size of the AEI-type and AFX-type zeolite used in the present invention is not particularly limited. 0.02 to 8 μm, more preferably 0.05 to 5 μm. The average primary particle size in the present invention corresponds to the particle size of primary particles. The average primary particle size is obtained by measuring 30 or more particles with arbitrarily selected particle sizes in observation of particles with a scanning electron microscope and averaging the particle sizes of the primary particles. The particle diameter was defined as the diameter of a circle having an area equal to the projected area of the particle (equivalent circle diameter).

The specific surface area of the ^AEI -type and AFX-type zeolite used in the present invention is not particularly limited. ² /g, more preferably 450 to 750 m ² /g. In addition, the specific surface area of the present invention is measured by the BET method. For example, it is preferable to measure by a flow-type one-point method using a fully automatic powder specific surface area measuring device (device name: AMS1000) manufactured by Okura Riken Co., Ltd.

The acid amount of the AEI-type and AFX-type zeolite used in the present invention is preferably 0.5 to 3.0 mmol/g, more preferably 0.7 to 3.0 mmol/g, still more preferably 0.9 to 3.0 mmol/g. g, particularly preferably 1.2-3.0 mmol/g, most preferably 1.2-2.5 mmol/g. The acidity of zeolite can be obtained by measuring the ammonia adsorption amount.

The ion-exchange capacity of the AEI-type or AFX-type zeolite used in the present invention is determined from the alkali metal atom source, the aluminum atom source, the silicon atom source, the organic structure directing agent, and the alkali metal atom contained in the zeolite added in the present production method. can also be used by converting the alkali metal portion of to H-type or _NH4 -type, and a known technique can be employed for this method. For example, it can be treated with an ammonium salt such as NH ₄ NO ₃ or NaNO ₃ or an acid such as hydrochloric acid at a temperature ranging from room temperature to 100° C., followed by washing with water.

In addition, AEI zeolite can also be produced by methods such as those disclosed in JP-A-2017-81809 and US-A-2017/0128921. According to the production method described in US Publication No. 2017/0128921, it is possible to reduce the amount of the expensive structure-directing agent used by reducing the amount of water used as a reaction raw material.
In addition, as a method for producing AFX-type zeolite, US Pat. It can also be produced according to the method described in JP-A-2016-147801 using a cation.

[Iron raw material]
Examples of the iron raw material used to support the above iron include oxides containing iron, hydroxides, sulfates, nitrates, phosphates, chlorides, inorganic acid salts such as bromides, acetates, oxalates, Organic acid salts such as citrates can be used. 1 type or 2 types or more may be contained in the iron raw material.

[Method for supporting iron]
Specific examples of the method for supporting iron on the aluminosilicate zeolite include an impregnation method, an ion exchange method, and the like. The ion exchange method may be either a liquid phase ion exchange method or a solid phase ion exchange method. The ion exchange method may be performed, for example, by mixing an aluminosilicate zeolite and an iron raw material. For example, the liquid-phase ion exchange method may be carried out by mixing an aluminosilicate zeolite with a solution in which iron raw materials are dissolved. As the impregnation method, a spray drying method or the like is also possible. Supporting by a liquid phase ion exchange method or a solid phase ion exchange method may be combined with supporting by an impregnation method.

[Counter cation species of aluminosilicate zeolite]
The counter cations possessed by the aluminosilicate zeolite to be treated with iron support are not particularly limited, but specific examples include alkali cations, ammonium cations, and protons. Cations with smaller ionic radii facilitate the ion exchange reaction, but protons may destabilize the zeolite structure if they exist as acids in the liquid after ion exchange. Therefore, alkali cations or ammonium cations are preferred, and alkali cations are more preferred.
Alkali cations are not limited as long as they are cations of alkali metal atoms such as lithium, sodium, potassium, rubidium and cesium, but sodium cations are particularly preferred.

[Iron ion exchange treatment]
In the method for producing a nitrogen oxide adsorbent of the present invention, iron may be supported on zeolite preferably by an ion exchange treatment, more preferably a liquid phase ion exchange method. The valence of the iron supported on the zeolite is preferably bivalent or trivalent, and more preferably bivalent from the viewpoint of NO adsorption amount per unit amount of iron and high temperature maintenance. Generally, divalent iron ions are easily oxidized to trivalent iron ions, so a method for preventing oxidation in the supporting treatment is desired.

Hereinafter, a preferred treatment method for supporting iron in the present invention will be described in more detail. A preferred treatment method of the present invention is to obtain an iron-supporting aluminosilicate zeolite by mixing an aluminosilicate zeolite and an antioxidant with a solution in which a divalent iron raw material is dissolved in an inert gas atmosphere.

<Gas Atmosphere>
A preferred treatment method of the present invention is carried out under an inert gas atmosphere such as nitrogen, helium, argon, etc., in order to prevent oxidation of divalent iron. By using an inert gas atmosphere, the gas atmosphere becomes a state in which oxygen does not exist. The inert gas is preferably nitrogen, which is inexpensive. At this time, since the dissolved oxygen contained in the solution also oxidizes the divalent iron, it is desirable to perform a deaeration treatment or the like in advance to reduce the dissolved oxygen.

<Raw material for iron>
The iron raw material used for supporting divalent iron on zeolite is not particularly limited as long as it is divalent, but among the above iron raw materials, iron hydroxide, iron chloride, iron sulfate, iron nitrate, phosphorus Iron acid, iron acetate, iron oxalate and the like may be used, preferably iron sulfate and iron nitrate, more preferably iron sulfate. However, the solution in which the ion exchange treatment is performed may contain a trivalent iron raw material within a range that does not hinder the function of the divalent iron raw material.

The concentration of the divalent iron raw material in the solution is usually 0.1 in terms of the molar ratio (Fe/Al molar ratio) of the divalent iron raw material to the amount of Al contained in the zeolite in order to increase the supported amount. or more, more preferably 0.2 or more, still more preferably 0.25 or more, and particularly preferably 0.3 or more.
On the other hand, by reducing the iron concentration to a certain level or less, the amount of ion exchange that occurs near the zeolite pore entrance is suppressed, and iron ions penetrate into the pores, making it easier to increase the amount of iron carried. Therefore, the molar ratio of the divalent iron raw material (Fe/Al molar ratio) to the amount of Al contained in the zeolite is usually 3 or less, preferably 2 or less, and 1.5 or less. is more preferably 1.0 or less, particularly preferably 0.8 or less, particularly preferably 0.7 or less, and most preferably 0.6 or less.

<Antioxidant>
In this treatment method, by adding an antioxidant to the solution, the divalent iron becomes more resistant to oxidation. Antioxidants used include ascorbic acid (vitamin C), tocopherol (vitamin E), dibutylhydroxytoluene, butylhydroxyanisole, sodium erythorbate, propyl gallate, sodium sulfite, potassium sulfite, potassium pyrosulfite, sulfur dioxide, and chlorogen. Acids, catechins and the like are preferred, ascorbic acid (vitamin C) and tocopherol (vitamin E) are more preferred, and ascorbic acid (vitamin C) is even more preferred.

When an antioxidant is used in the ion exchange treatment, the molar ratio of the antioxidant to the divalent iron raw material is usually 1 or more, but from the viewpoint of further preventing oxidation, it is better to increase the amount of the antioxidant. Well, 2 or more is more preferable, 3 or more is more preferable, 5 or more is particularly preferable, 7 or more is particularly preferable, and 8 or more is most preferable. On the other hand, if the amount of the antioxidant added is too large, the raw material cost will increase, so the molar ratio of the antioxidant to the divalent iron raw material is usually preferably 100 or less, more preferably 50 or less, and even more preferably 20 or less. , 15 or less are particularly preferred, 13 or less are particularly preferred, and 11 or less are most preferred.

<Solvent>
The solvent used in the above treatment method may be either water or an organic solvent as long as the ion-exchange salt (that is, iron raw material) is dissolved therein, but water is usually preferred.
The amount of solvent to be used may be set in terms of weight ratio to zeolite. In general, the smaller the amount of solvent, the smaller the amount of waste liquid after ion exchange treatment, so the weight ratio to zeolite is preferably 1000 or less, more preferably 500 or less, more preferably 250 or less, and even more preferably 200 or less. 150 or less is particularly preferred, and 120 or less is most preferred. On the other hand, if the amount of the solvent is too small, the stirrability will deteriorate and uniform ion exchange will become difficult. , more preferably 25 or more, particularly preferably 50 or more, particularly preferably 80 or more, most preferably 100 or more.

<Ion exchange treatment temperature>
The ion exchange rate is determined by factors such as treatment temperature and treatment time. Also, the ion exchange treatment temperature should be lowered from the viewpoint of making the divalent iron less susceptible to oxidation. From these viewpoints, the ion exchange treatment is usually performed at 150° C. or lower, preferably 100° C. or lower, more preferably 80° C. or lower, more preferably 60° C. or lower, particularly preferably 50° C. or lower, and most preferably 40° C. or lower. preferable. On the other hand, by increasing the temperature, the ion exchange reaction rate is increased, and ion exchange can be performed in a short time. Therefore, the temperature is usually 0° C. or higher, preferably 5° C. or higher, more preferably 10° C. or higher, even more preferably 15° C. or higher, and particularly preferably 20° C. or higher.

[Ion exchange treatment time]
The ion-exchange treatment may normally be carried out for about 10 minutes to 48 hours, and these treatment conditions may be appropriately set according to the iron raw material, amount of water, temperature, and type of solvent used.

Since the ion exchange rate increases as the ion exchange treatment is repeated, the number of ion exchange treatments is not particularly limited, and the treatment may be repeated until the intended effect is obtained.
The ion-treated zeolite may be filtered or centrifuged to separate the zeolite from the ion-exchanged liquid, washed with water, and then dried.

Furthermore, the iron-supported aluminosilicate zeolite may be calcined. Due to the above ion exchange treatment, residual substances derived from the ion exchange treatment raw material may exist in the zeolite pores and block the pores. Residues can be removed. In addition, the calcination can enhance the dispersibility of the metal supported on the zeolite skeleton structure, which is effective in improving the adsorption capacity.
The firing temperature is preferably 300°C to 900°C, more preferably 350°C to 800°C, still more preferably 400°C to 700°C. Further, the baking is preferably carried out for 1 second to 24 hours, preferably 10 seconds to 8 hours, more preferably 30 minutes to 4 hours. Furthermore, when divalent iron is supported, it is preferable to carry out a calcination treatment in an inert atmosphere.

The method for producing the iron-supporting aluminosilicate zeolite is not limited to the above method, and it may be produced by direct synthesis. Specifically, an iron-supporting aluminosilicate zeolite may be obtained by further adding an iron raw material to a raw material mixture before hydrothermal synthesis or a gel obtained by aging it, and hydrothermally synthesizing the raw material mixture. Direct synthesis may be performed by a conventionally known method, for example, according to the method disclosed in International Publication No. 2017/134001.

[Nitrogen oxide adsorbent]
The nitrogen oxide adsorbent of the present invention may consist of the iron-supporting aluminosilicate zeolite alone, but may also contain a binder such as silica, alumina, or clay mineral in addition to the iron-supporting aluminosilicate zeolite.
The nitrogen oxide adsorbent may be obtained by adding a binder or the like to iron-supporting aluminosilicate zeolite as necessary, granulating it, and molding it into a predetermined shape, or it may be obtained by, for example, a coating method. good too. Also, the nitrogen oxide adsorbent is preferably honeycomb-shaped.

When the nitrogen oxide adsorbent of the present invention is produced by a coating method, usually, an iron-supporting aluminosilicate zeolite and an inorganic binder such as silica or alumina are mixed to prepare a slurry, which is prepared from an inorganic material such as cordierite. It is preferably produced by applying it to the surface of the substrate and baking it. At this time, a honeycomb-shaped nitrogen oxide adsorbent can be obtained by preferably applying it to a honeycomb-shaped substrate. Here, an inorganic binder is used because an exhaust gas purifying adsorbent, which will be described later, is used as an example, but it is needless to say that an organic binder may be used depending on the application and conditions of use.

Further, the molded article of the nitrogen oxide adsorbent of the present invention can be obtained, for example, by kneading an iron-supporting aluminosilicate zeolite with an inorganic binder such as silica or alumina, or an inorganic fiber such as alumina fiber or glass fiber, followed by an extrusion method or a compression method. It can be obtained by carrying out molding using a method such as the above, followed by firing. Preferably, at this time, a honeycomb-shaped nitrogen oxide adsorbent can be obtained by molding into a honeycomb shape.

[Use of nitrogen oxide adsorbent]
The nitrogen oxide adsorbent of the present invention is used as an adsorbent for adsorbing nitrogen oxides, preferably as an exhaust gas purifying adsorbent, more preferably as an automobile exhaust gas purifying adsorbent. The nitrogen oxide adsorbent of the present invention is effective, for example, as an adsorbent for adsorbing nitrogen oxides contained in exhaust gas.

The nitrogen oxide adsorbent of the present invention is used, for example, in an exhaust gas purification system. Exhaust gas purification systems are systems for purifying exhaust gases from automobiles, for example. An exhaust gas purification system includes the nitrogen oxide adsorbent of the present invention and a nitrogen oxide reduction catalyst, and the nitrogen oxide adsorbent may be arranged upstream of the nitrogen oxide reduction catalyst in the exhaust gas discharge path. preferable.
Further, the nitrogen oxide reduction catalyst is preferably a selective reduction catalyst (hereinafter referred to as "SCR catalyst") that reduces nitrogen oxides with a reducing agent, and the reducing agent in this case includes ammonia and the like. .
An exhaust gas purification system using an SCR catalyst has a reducing agent supply section that supplies a reducing agent, and the reducing agent is supplied to the SCR catalyst from the reducing agent supply section. The reducing agent supply unit may supply the reducing agent to the SCR catalyst by a known means, for example, spray the reducing agent and supply it to the SCR catalyst. Further, the supplied reducing agent may be ammonia itself, or may be a compound capable of generating ammonia. Compounds capable of generating ammonia include urea.

As the SCR catalyst, a known SCR catalyst can be used, and it is not particularly limited, but examples thereof include a catalyst in which zeolite contains metals other than Si and Al. As the metal element used in the SCR catalyst, transition metals are preferred. ), gold (Au), cerium (Ce), lanthanum (La), praseodymium (Pr), titanium (Ti), zirconium (Zr) and the like.

The nitrogen oxide adsorbent of the present invention adsorbs nitrogen oxides in exhaust gas when the temperature of the exhaust gas is in a low temperature range (for example, less than 150°C). Moreover, when the temperature of the exhaust gas is in a high temperature range (for example, 150° C. or higher), the nitrogen oxides adsorbed on the adsorbent are desorbed by the high temperature exhaust gas. On the other hand, the SCR catalyst is a catalyst that can reduce nitrogen oxides with a reducing agent in a high temperature range (for example, 150° C. or higher). In general, the exhaust gas is at a low temperature at the beginning of operation, but becomes high temperature after a certain period of time has passed since the start of operation.

Therefore, in this exhaust gas treatment system, 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 nitrogen content of the exhaust gas. The amount of oxides is reduced.
On the other hand, when high-temperature exhaust gas is discharged and passes through the nitrogen oxide adsorbent, the nitrogen oxide adsorbent does not adsorb the nitrogen oxides, and the adsorbed nitrogen oxides are desorbed. Therefore, the exhaust gas, which also contains desorbed nitrogen oxides, is sent from the nitrogen oxide adsorbent to the SCR catalyst. In the SCR catalyst, nitrogen oxides contained in the exhaust gas are reduced to nitrogen or the like by a reducing agent. As a result, the present exhaust gas treatment system can reduce nitrogen oxides contained in the exhaust gas even when the exhaust gas is at a high temperature.
Moreover, the exhaust gas may reach a temperature of, for example, 600° C. or higher, but the nitrogen oxide adsorbent of the present invention has high heat resistance. Therefore, the nitrogen oxide adsorbent can maintain high adsorption performance over a long period of time without deteriorating performance over a long period of time.

The exhaust gas treatment system may have various purifying means capable of purifying pollutants in the exhaust gas in addition to the nitrogen oxide adsorbent and the nitrogen oxide reducing catalyst described above. Specific examples include a particle filter for removing fine particles contained in exhaust gas, an oxidation catalyst for oxidizing carbon monoxide (CO), hydrocarbons (HC), and the like. The exhaust gas treatment system may also have a reducing agent slip catalyst or the like that oxidizes the reducing agent such as ammonia that has not been used in the nitrogen oxide reduction catalyst.
The arrangement position of the particle filter is not particularly limited. or upstream of the nitrogen oxide adsorbent. An oxidation catalyst may also be placed downstream or upstream of the nitrogen oxide adsorbent. Also, the reducing agent slip catalyst may be arranged downstream of the nitrogen oxide reduction catalyst.
Furthermore, the nitrogen oxide adsorbent of the present invention may be supported on a filter such as a particle filter, or may be used by mixing with other catalysts. For example, it may be used by mixing with an oxidation catalyst.

The exhaust gas may contain components other than nitrogen oxides, such as hydrocarbons, carbon monoxide, carbon dioxide, hydrogen, nitrogen, oxygen, sulfur oxides, and water.
In addition to automobiles such as diesel and gasoline vehicles, nitrogen oxide adsorbents are also emitted from stationary generators, ships, agricultural machinery, construction machinery, motorcycles, various diesel engines for aircraft, boilers, and gas turbines. It can be used as an adsorbent for treating a wide variety of exhaust gases.

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited by the following examples as long as the gist thereof is not exceeded.

[Analysis/Evaluation]
Analysis and performance evaluation of the zeolites obtained in the following examples and comparative examples were performed by the following methods.

[Measurement of powder XRD]
<Sample preparation>
About 100 mg of a zeolite sample manually pulverized using an agate mortar was placed in a sample holder of the same shape so that the amount of the sample was constant.
<Equipment specifications and measurement conditions>
The powder XRD measurement device specifications and measurement conditions are as follows.

[Analysis of zeolite composition]
The elemental analysis of the content of silicon atoms and aluminum atoms in the zeolite standard sample was carried out as follows.
After the zeolite sample was heated and dissolved in an aqueous solution of hydrochloric acid, the contents (% by weight) of silicon atoms and aluminum atoms were determined by ICP analysis. Then, a calibration curve was prepared between the fluorescent X-ray intensity of the analytical element in the standard sample and the atomic concentration of the analytical element. Based on this calibration curve, the content (% by weight) of silicon atoms and aluminum atoms in the zeolite sample was determined by X-ray fluorescence analysis (XRF). The ICP analysis was performed using an apparatus name: ULTIMA 2C manufactured by Horiba, Ltd. XRF was performed using an apparatus name: EDX-700 manufactured by Shimadzu Corporation.

[Measurement of Fe content]
After the zeolite sample was treated with an aqueous hydrofluoric acid solution and then heated and dissolved in an aqueous hydrochloric acid solution, the content of Fe atoms was determined by inductively coupled plasma (ICP) emission spectrometry.

[Measurement of NO adsorption amount and desorption temperature]
Using NO-TPD, nitrogen oxides (NO) adsorbed on the nitrogen oxide adsorbent were desorbed under the following conditions, and the desorption behavior of the adsorbed NO with respect to temperature was confirmed, and nitrogen oxide adsorption was performed. The amount of NO adsorbed on the material was determined.

(Measurement condition)
Pretreatment conditions: 500°C, 0.5 hours, NO adsorption conditions in air atmosphere: 50°C, 1% NO/He
NO adsorption time: 30 minutes Heating rate: 10 K/°C (under He atmosphere)
Desorption temperature range: 50°C to 600°C

[Ion exchange rate]
The ion exchange rate is determined by the number of metal ion atoms of the metal supported on the zeolite, the number of metal ion valences, and further The number of acid sites in the zeolite was obtained and calculated by the following formula 1.
Formula 1: (Number of metal ion atoms) x (metal ion valence) / (number of acid sites in the catalyst support) x 100

[Synthesis of Zeolite]
[Example 1]
1.1 g of water and 13.6 g of N,N-dimethyl-3,5-dimethylpiperidinium hydroxide 20% aqueous solution (mixture of cis and trans isomers) as an organic structure directing agent (OSDA) (Sechem Co., Ltd.) ) and 0.3 g of NaOH (manufactured by Wako Pure Chemical Industries, Ltd.), 0.4 g of amorphous Al(OH) ₃ (containing 53.5% by weight of Al ₂ O ₃ , manufactured by Aldrich) is added and stirred. and dissolved to give a clear solution. After adding 5.9 g of Snowtex 40 (silica (SiO ₂ ) concentration: 40% by weight, manufactured by Nissan Chemical Industries, Ltd.) and stirring at room temperature for 5 minutes, 0.1 g of uncalcined CHA-type zeolite (SiO ₂ /Al ₂ O ₃ molar ratio=22) was added and stirred at room temperature for 2 hours to obtain a raw material mixture.

This raw material mixture was placed in a pressure vessel, and hydrothermal synthesis was performed for 1 day while rotating (15 rpm) in an oven at 180°C. After this hydrothermal synthesis reaction, the reaction solution was cooled and the crystals produced were collected by filtration. After drying the collected crystals at 100° C. for 12 hours, the XRD of the obtained zeolite powder was measured. rice field. Next, in order to remove organic substances in the zeolite, AEI zeolite 1 was calcined at 580° C. for 6 hours under an air stream to obtain AEI zeolite 2. The SiO ₂ /Al ₂ O ₃ molar ratio of AEI zeolite 2 was 10 by XRF analysis.

[Fe carrying treatment]
An aqueous solution containing iron (II) sulfate heptahydrate, AEI zeolite 2, and ascorbic acid in the following proportions was prepared as an ion-exchange treatment liquid for supporting Fe.
Iron (II) sulfate heptahydrate: AEI zeolite 2: H ₂ O = x: 0.25: 25 (weight ratio, where x is an amount adjusted so that Fe/Al has the following molar ratio and x has the same meaning below.)
Fe/Al=0.2
Ascorbic acid: iron (II) sulfate = 10 mol: 1 mol
Under a nitrogen atmosphere, ion exchange is performed by stirring at room temperature (23° C.) for 24 hours, followed by filtration by centrifugation, washing and drying to obtain a nitrogen oxide adsorbent 1 made of iron-supporting AEI zeolite. Obtained. The SiO ₂ /Al ₂ O ₃ molar ratio by XRF analysis was 10, and the amount of Fe supported by ICP was 1.8% by weight.

[Example 2]
A nitrogen oxide adsorbent 2 composed of iron-supporting AEI zeolite was produced in the same manner as in Example 1, except that in the treatment for supporting Fe on zeolite, the treatment was performed under the condition of Fe/Al=0.3. The produced iron-supporting AEI zeolite had a SiO ₂ /Al ₂ O ₃ molar ratio of 10 by XRF analysis, and an Fe-supporting amount of 2.7% by weight by ICP.

[Example 3]
31.4 g of water, 28.7 g of DABCO 17% by weight aqueous solution (manufactured by Seichem Co., Ltd.) as an organic structure directing agent (OSDA), 0.7 g of NaOH (manufactured by Wako Pure Chemical Industries), and 27.5 g of water glass 3 (manufactured by WAKO), 2.5 g of NaY-5 (SiO ₂ /Al ₂ O ₃ molar ratio = 5: manufactured by Nikki Shokubai Kasei Co., Ltd.) is added and stirred at room temperature for 2 hours to form a raw material mixture. Obtained.

This raw material mixture was placed in a pressure vessel, and hydrothermal synthesis was performed for 3 days while rotating (15 rpm) in an oven at 150°C. After this hydrothermal synthesis reaction, the reaction solution was cooled and the crystals produced were collected by filtration. After drying the collected crystals at 100° C. for 12 hours, the XRD of the obtained zeolite powder was measured. was done. Next, in order to remove organic substances in the zeolite, the AFX zeolite 1 was calcined at 500° C. under an air stream for 6 hours to obtain an AFX zeolite 2. The SiO ₂ /Al ₂ O ₃ molar ratio of AFX-type zeolite 2 was 8 by XRF analysis.

[Fe carrying treatment]
An aqueous solution containing iron (II) sulfate heptahydrate, AFX-type zeolite 2, and ascorbic acid in the following proportions was prepared as an ion exchange treatment liquid for supporting Fe.
Iron (II) sulfate heptahydrate: AFX type zeolite 2: H O = x: 0.25: 25 (weight ratio)
Fe/Al=0.4
Ascorbic acid: iron (II) sulfate = 10 mol: 1 mol
Under a nitrogen atmosphere, ion exchange is performed by stirring at room temperature (23° C.) for 24 hours, followed by filtration by centrifugation, washing and drying to obtain a nitrogen oxide adsorbent 3 made of iron-supporting AFX-type zeolite. Obtained. The SiO ₂ /Al ₂ O ₃ molar ratio was 8 by XRF analysis, and the amount of Fe supported by ICP was 1.5% by weight.

[Comparative Example 1]
2.2 g of water and 9.2 g of an organic structure directing agent (OSDA) of N,N-dimethyl-3,5-dimethylpiperidinium hydroxide 20% aqueous solution (mixture of cis and trans isomers) (Sechem Co., Ltd.) ) and 1.0 g of NaOH (manufactured by Wako Pure Chemical Industries), USY-30 (SiO ₂ /Al ₂ O ₃ molar ratio = 30: manufactured by Zeolyst) 2.2 g, Snowtex 40 (silica Concentration: 40% by weight, manufactured by Nissan Chemical Industries, Ltd.) (5.2 g) was added and stirred at room temperature for 2 hours to obtain a raw material mixture.

This raw material mixture was placed in a pressure vessel, and hydrothermal synthesis was performed for 4 days while rotating (15 rpm) in an oven at 160°C. After this hydrothermal synthesis reaction, the reaction solution was cooled and the crystals produced were collected by filtration. After drying the collected crystals at 100° C. for 12 hours, the XRD of the obtained zeolite powder was measured. was done. Next, in order to remove organic substances in the zeolite, AEI zeolite 3 was calcined at 580° C. for 6 hours under an air stream to obtain AEI zeolite 4 . The SiO ₂ /Al ₂ O ₃ molar ratio of AEI zeolite 4 was 20 by XRF analysis.

[Fe carrying treatment]
An aqueous solution containing iron (II) sulfate heptahydrate, AEI zeolite 4, and ascorbic acid in the following proportions was prepared as an ion exchange treatment liquid for supporting Fe.
Iron (II) sulfate heptahydrate: AEI zeolite 4: H O = x: 0.25: 25 (weight ratio)
Fe/Al=0.2
Ascorbic acid: iron (II) sulfate = 10 mol: 1 mol
After stirring for 24 hours at room temperature in a nitrogen atmosphere, the mixture was filtered by centrifugation, washed and dried to obtain a nitrogen oxide adsorbent 4 composed of Fe-supporting AEI zeolite. The SiO ₂ /Al ₂ O ₃ molar ratio by XRF analysis was 20, and the amount of Fe supported by ICP was 0.4% by weight.

[Comparative Example 2]
A nitrogen oxide adsorbent 5 made of Fe-supporting AEI zeolite was produced in the same manner as in Comparative Example 1, except that in the treatment for supporting Fe on zeolite, the treatment was performed under the condition of Fe/Al=0.4. The SiO ₂ /Al ₂ O ₃ molar ratio was 20 by XRF analysis, and the amount of Fe supported by ICP was 0.3% by weight.

[NO adsorption performance]
Table 2 shows the measurement results of the NO adsorption amounts of the nitrogen oxide adsorbents 1 to 5.

From the results of Examples 1 to 3 and Comparative Examples 1 and 2 above, nitrogen oxide adsorbents 1 to 3 were treated under the same Fe loading treatment conditions, but nitrogen oxide adsorbents 4 and 5 It was found that the amount of iron supported was larger than that of The amount of iron supported is obviously large even when compared with the ion exchange rate. This result suggests that iron loading is dependent on the _SiO2 / _Al2O3 molar ratio within _a specific range. This is because AEI zeolite or AFX zeolite contains d6r defined as a composite building unit by IZA in the skeleton as a crystal structure unit, but the SiO ₂ /Al ₂ O ₃ molar ratio is 15 or less, that is, 2 in d6r This is probably because iron is easily coordinated because there are many adjacent Al atoms. On the other hand, AEI zeolite with a SiO ₂ /Al ₂ O ₃ molar ratio of 20 has only about one Al in d6r, so a divalent metal is coordinated only at the coordination sites having two Al in the vicinity. It is considered that the loading amount did not increase. Moreover, as the amount of Fe supported increased, the amount of NO adsorption also tended to increase.

Furthermore, when comparing the NO adsorption amounts of the nitrogen oxide adsorbents 1 to 3 with the MFI type, which is a 10-membered ring zeolite, and the FAU type, which is a 12-membered ring zeolite described in Non-Patent Document 3, the nitrogen oxide adsorption of the present invention The agent clearly resulted in a large amount of NO adsorption. This result is also considered to be due to the fact that the small pore zeolite has smaller pore diameters, so there is more adjacent Al, and iron, which is effective for NO adsorption, can be supported.
From the above results, it was found that, in the 8-membered ring zeolite with a small pore size, the zeolite supporting iron at a SiO ₂ /Al ₂ O ₃ molar ratio within a specific range has high performance as an adsorbent for nitrogen oxides. It can be understood.

FIG. 4 shows the NO desorption behavior of nitrogen oxide adsorbents 1, 2 and 4. FIG. The NO desorption peak tended to shift to higher temperatures as the amount of Fe supported increased. It is considered that this is because NO is adsorbed in a stable coordination state that is more difficult to detach due to the proximity of the adsorbed NOs.

[Thermal stability evaluation]
In order to evaluate the thermal stability of the nitrogen oxide adsorbent 2, the amount of NO adsorbed before and after the heat treatment under the conditions described below was measured. Table 3 shows the results.
Heat treatment 1: 650°C, 15 hours, under helium atmosphere Heat treatment 2: 650°C, 15 hours, under air atmosphere

A zeolite having a large amount of Al in its framework, such as the LTA type, has low thermal stability, and thus heat treatment causes collapse of the crystal structure, resulting in a decrease in adsorption performance. Therefore, in heat treatment 1, the stability of the zeolite crystal structure due to heat was evaluated by heat treatment in a He atmosphere. Before and after the heat treatment, the amount of NO adsorbed decreased by only about 10%, confirming that the structure of the zeolite crystal did not collapse.
Next, in heat treatment 2, the stability of Fe supported on zeolite was evaluated in an oxidizing atmosphere. As in heat treatment 1, it was confirmed that a high NO adsorption amount was maintained even after the treatment. Therefore, it is clear that the nitrogen oxide adsorbent of the present invention has high thermal stability.

Claims (4)

A nitrogen oxide adsorbent comprising an iron-supporting aluminosilicate zeolite, wherein in the iron-supporting aluminosilicate zeolite, the skeleton of the aluminosilicate zeolite has an oxygen eight-membered ring structure,
The SiO ₂ /Al ₂ O ₃ molar ratio of the iron-supporting aluminosilicate zeolite is 3 or more and 15 or less, and the iron content of the iron-supporting aluminosilicate zeolite is 0.5 wt% or more and 10 wt% or less , A nitrogen oxide adsorbent , wherein the average oxidation number of supported iron is 1.5 or more and less than 2.3 . 2. The nitrogen oxide adsorbent according to claim 1, wherein said aluminosilicate zeolite is AEI zeolite or AFX zeolite. A method for producing the nitrogen oxide adsorbent according to claim 1 or 2,
A method for producing a nitrogen oxide adsorbent, comprising the step of mixing an aluminosilicate zeolite and an antioxidant with a solution in which a divalent iron raw material is dissolved in an inert gas atmosphere to obtain an iron-supporting aluminosilicate zeolite. 4. The method for producing a nitrogen oxide adsorbent according to claim 3, wherein said aluminosilicate zeolite contains alkali cations.

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