WO2023042783A1

WO2023042783A1 - Core-shell zeolite

Info

Publication number: WO2023042783A1
Application number: PCT/JP2022/034031
Authority: WO
Inventors: 優太郎辻; 泰之武田; 伸司岩本
Original assignee: ユミコア日本触媒株式会社
Priority date: 2021-09-15
Filing date: 2022-09-12
Publication date: 2023-03-23

Abstract

Provided is a means for increasing the hydrocarbon desorption temperature in a hydrocarbon adsorbent made of zeolite. Provided is a core-shell zeolite having a core made of a first zeolite and a shell made of a second zeolite, wherein the channel diameter of the first zeolite is larger than the channel diameter of the second zeolite.

Description

core-shell zeolite

The present invention relates to core-shell zeolites.

In recent years, automobile exhaust gas regulations have been tightened. Gasoline engine vehicles are required to reduce the amount of hydrocarbons (HC) emitted in a low-temperature region immediately after the engine is started, where the three-way catalyst has not yet been activated. To meet this demand, a catalyst containing a hydrocarbon trap (HCT) is used, which is capable of adsorbing hydrocarbons in a low temperature range and desorbing them in a high temperature range. As a result, it is possible to temporarily adsorb hydrocarbons discharged in a low temperature range up to a high temperature range where purification is possible, so that the amount of hydrocarbons discharged can be reduced.

Conventionally, zeolites have been widely used as hydrocarbon adsorbents. For example, Japanese Patent Application Laid-Open No. 2-135126 discloses that an exhaust system is provided with an exhaust gas purification catalyst, and one or more kinds of catalyst metals are supported on a part of a zeolite-coated monolith carrier on the upstream side of the catalyst. An automobile exhaust gas purifier is disclosed, which is characterized by comprising an adsorbent of The document also describes that H-type mordenite (MOR) or HY-type zeolite (FAU) is preferable as the zeolite from the viewpoint of adsorption performance. According to this document, it is possible to improve the purification performance of HC at a relatively low temperature at which HC begins to desorb from the adsorbent by using the automobile exhaust gas purifier.

However, although the FAU zeolite has a relatively large amount of hydrocarbon adsorption, the temperature range in which hydrocarbons are desorbed is lower than the temperature range in which the three-way catalyst is sufficiently activated, and the amount of hydrocarbon emissions is sufficiently reduced. There is a problem that it may not be possible to reduce it.

Therefore, the object of the present invention is to provide means for increasing the desorption temperature of hydrocarbons in a hydrocarbon adsorbent made of zeolite.

The inventors have conducted intensive research to solve the above problems. As a result, the present inventors have found that the above problems can be solved by a core-shell zeolite having a zeolite with a large channel diameter as a core and a zeolite with a small channel diameter as a shell, and have completed the present invention.

That is, one aspect of the present invention has a core made of a first zeolite and a shell made of a second zeolite, wherein the channel diameter of the first zeolite is larger than the channel diameter of the second zeolite. , which are core-shell zeolites.

FIG. 1 is an SEM image of powder d. FIG. 2 is an SEM image of powder h.

Embodiments of the present invention will be described below. The numerical range "A to B" in this specification means "A or more and B or less". In addition, "A and/or B" means "either A or B" or "both A and B." In addition, unless otherwise specified, various physical properties in the present specification mean values measured by the methods described in Examples described later.

<Core-shell type zeolite>
One form of the invention is a core-shell having a core made of a first zeolite and a shell made of a second zeolite, wherein the channel diameter of the first zeolite is greater than the channel diameter of the second zeolite. type zeolite. According to the core-shell type zeolite according to the present invention, the desorption temperature of hydrocarbons can be increased in the hydrocarbon adsorbent made of zeolite.

Although it is not clear why the core-shell zeolite according to the present invention can desorb hydrocarbons at a higher temperature than conventional zeolites, the present inventors speculate as follows. In addition, the present invention is not limited to the following mechanism.

Zeolite is a crystalline aluminosilicate, and more than 245 types of skeletal structures are known. A zeolite has a specific channel diameter due to its framework structure, and hydrocarbons enter and exit through these channels (tubular pores), thereby exerting its function as a hydrocarbon adsorbent. It is known that there is a correlation between channel diameter and hydrocarbon adsorption and desorption temperatures. That is, the smaller the channel diameter, the smaller the adsorption amount of hydrocarbons, and the more difficult it is for hydrocarbons to enter and exit, so the desorption temperature is higher. On the other hand, the larger the channel diameter, the larger the adsorption amount of hydrocarbons, and the easier the entrance and exit of hydrocarbons, and the lower the desorption temperature.

According to the core-shell zeolite according to the present invention, the core with a large channel diameter adsorbs a large amount of hydrocarbons, while the shell with a small channel diameter suppresses the desorption of hydrocarbons. Also, the existence of crystal interfaces between the core and the shell makes the channels discontinuous and prevents the hydrocarbons adsorbed on the core from migrating to the shell. As a result, it is possible to increase the desorption temperature of hydrocarbons. In addition, since the hydrocarbon adsorbent consisting only of zeolite with a small channel diameter adsorbs a small amount of hydrocarbons as described above, all hydrocarbons are desorbed within a short time after the start of desorption when the exhaust gas temperature rises. detached. However, according to the core-shell type zeolite according to the present invention, hydrocarbons are desorbed over a longer period of time from the start of desorption. can be supplied to the catalyst. As a result, according to the catalyst using the core-shell type zeolite according to the present invention, it is possible to improve the purification performance of hydrocarbons.

The core-shell zeolite according to the present invention contains at least two types of zeolite (first zeolite and second zeolite) with different channel diameters. Hereinafter, the present invention will be described mainly taking as an example a core-shell zeolite in which the core is composed of one type of first zeolite and the shell is composed of one type of second zeolite. However, the present invention is not limited to such a form, and is a core-shell type zeolite in which the core is composed of two or more first zeolites and/or the shell is composed of two or more second zeolites. It also includes certain forms.

In this specification, the channel diameter of each zeolite is determined by identifying the type of zeolite contained in the core-shell zeolite by X-ray diffraction analysis, and then identifying the identified zeolite indicated by the International Zeolite Association (IZA). Adopt the value of Maximum diameter of a sphere that can be included. In this specification, of the two zeolites, the zeolite with the larger channel diameter is defined as the first zeolite, and the zeolite with the smaller channel diameter is defined as the second zeolite. When the core is composed of two or more first zeolites and/or the shell is composed of two or more second zeolites, the smallest channel diameter of the first zeolites is the second larger than the largest channel diameter among the zeolites.

In the present invention, the specific types and combinations of the first zeolite and the second zeolite are not particularly limited, but the following types and combinations are preferred. In this specification, the type of zeolite means the skeleton structure of the zeolite. The types of zeolites (skeletal structures) in this specification are expressed in three-letter codes defined by the International Zeolite Association (IZA).

The first zeolite preferably has a channel diameter larger than any hydrocarbon species contained in the exhaust gas and retains a large amount of hydrocarbons in a low temperature range of less than 110 ° C., such as FAU, LEV, MWW and It is preferably at least one selected from the group consisting of LTA, more preferably FAU and/or LEV, and even more preferably FAU.

The second zeolite preferably has a channel diameter equal to or smaller than that of any hydrocarbon species contained in the exhaust gas, and is capable of retaining the adsorbed hydrocarbon species even in a high temperature range of 110°C or higher. , BEA, CHA, MFI, MOR, SZR, FER and TON, preferably at least one selected from the group consisting of BEA, CHA, MFI, MOR and SZR. more preferably, BEA and/or MFI, and particularly preferably BEA.

As a combination of the first zeolite and the second zeolite, when described in the notation of "first zeolite (channel diameter) / second zeolite (channel diameter)", FAU (11.24 Å) / BEA (6 .68 Å), FAU (11.24 Å)/MFI (6.36 Å), FAU (11.24 Å)/MOR (6.70 Å), and the like. Among them, from the viewpoint of further improving the desorption temperature of hydrocarbons, the combination is FAU (11.24 Å)/BEA (6.68 Å) or FAU (11.24 Å)/MFI (6.36 Å). Preferred is FAU(11.24 Å)/BEA(6.68 Å). That is, in the core-shell zeolite according to a preferred embodiment of the present invention, the first zeolite is FAU and the second zeolite is BEA.

The core-shell zeolite according to the present invention has a core-shell structure in which the first zeolite (the zeolite with the larger channel diameter) is arranged in the core and the second zeolite (the zeolite with the smaller channel diameter) is arranged in the shell. . In this specification, whether or not zeolite has a core-shell structure is determined by the following method.

First, perform X-ray diffraction analysis of the produced zeolite and confirm that at least two different zeolites are detected. The zeolite is identified by confirming whether the 2θ of each peak seen in the XRD pattern after analysis matches the diffraction angle described in the JCPDS card. When all the diffraction angles match, it can be identified as the zeolite, and at least the 2θ of the three peaks in descending order of intensity can be seen at an angle of 0.02° before and after the diffraction angle. , can be identified as the zeolite. This is because error factors and the like are included when the goniometer moves.

Next, the respective zeta potentials at pH=3 for the first zeolite-only particles and the second zeolite-only particles are measured. Then, in a similar manner, the respective zeta potentials at pH=3 are measured for the zeolite particles of interest. If these values satisfy the relationship of the formula: |P ₂ −P _x |/|P ₂ −P ₁ |<0.10 (0.1), it is assumed to have a core-shell structure. . Here, P ₁ represents the zeta potential (mV) of the particles composed of the first zeolite. _P2 represents the zeta potential (mV) of the particles consisting of the second zeolite. P _x represents the zeta potential (mV) of the zeolite particles of interest. In addition, in this specification, the zeta potential adopts the value obtained by the measuring method described in the examples below.

Since the zeta potential is a value that depends on the charge on the surface of the particles, the zeta potential of the particles when the first zeolite is completely covered with the second zeolite is theoretically equal to that of particles consisting only of the second zeolite. (|P ₂ −P _x |/|P ₂ −P ₁ |=0). On the other hand, the value of the zeta potential of the particles when the first zeolite is not covered with the second zeolite has a value of The zeta potential value of the particle shifts toward the value of A larger shift ratio (|P ₂ −P _x |/|P ₂ −P ₁ |) means a higher proportion of the first zeolite exposed to the surface. Further, when |P ₂ −P _x |/|P ₂ −P ₁ |<0.10 is satisfied, the first zeolite is completely covered with the second zeolite, or there is a portion that is not covered with the second zeolite. Even so, the percentage is likely to be low. Therefore, such a case is regarded as "the zeolite has a core-shell structure".

The value of |P ₂ −P _x |/|P ₂ −P ₁ | is essentially 0 or more and less than 0.10 (0.00 or more and less than 0.10), preferably 0 or more and 0.09 or less. , more preferably 0 or more and 0.05 or less, and still more preferably 0 or more and 0.03 or less. If the value is within the above range, it is considered that the first zeolite is well coated with the second zeolite (the proportion of the first zeolite exposed is low), so the effect of the present invention (carbonization increasing the desorption temperature of hydrogen) is further exhibited.

In the present invention, the proportions of the first zeolite and the second zeolite contained in the core-shell zeolite are not particularly limited. However, considering that the first zeolite is well coated with the second zeolite, the mass (percentage) of the second zeolite with respect to the total mass of the core-shell zeolite is preferably more than 62% by mass and 95% by mass or less. , more preferably 71% by mass or more and 91% by mass or less, and still more preferably 71% by mass or more and 87% by mass or less. Within this range, the second zeolite is present in a sufficient amount, so that the first zeolite can be satisfactorily coated with the second zeolite. As a result, the effect of the present invention (increasing the desorption temperature of hydrocarbons) is exhibited even more. The mass (percentage) of the first zeolite with respect to the total mass of the core-shell zeolite is a value obtained by subtracting the mass (percentage) of the second zeolite from the total mass of 100% by mass.

In the core-shell zeolite, the core may contain components other than the first zeolite, and the shell may contain components other than the second zeolite, as long as the effects of the present invention can be exhibited. However, from the viewpoint of further increasing the desorption temperature of hydrocarbons, the amount of the first zeolite contained in the core is preferably 90% by mass or more with respect to the total mass of the core, and 95% by mass or more. It is more preferably 98% by mass or more, particularly preferably 99% by mass or more, and most preferably 100% by mass. Similarly, the amount of the second zeolite contained in the shell is preferably 90% by mass or more, more preferably 95% by mass or more, and 98% by mass or more with respect to the total mass of the shell. is more preferable, 99% by mass or more is particularly preferable, and 100% by mass is most preferable.

<Method for producing core-shell zeolite>
The method for producing the core-shell type zeolite according to the present invention is not particularly limited, and methods for producing zeolite that can be used in this technical field can be appropriately combined. According to a preferred example, after preparing the particles made of the first zeolite as the core, the particles made of the first zeolite are coated with the precursor of the second zeolite, and the precursor of the second zeolite is crystallized. A method for producing a core-shell type zeolite is exemplified. That is, a method for producing a core-shell zeolite according to a preferred embodiment of the present invention comprises a step of preparing particles made of a first zeolite (hereinafter also referred to as “step 1”), and preparing a precursor of a second zeolite. a step of coating the first zeolite particles with the second zeolite precursor, and then crystallizing the second zeolite precursor (hereinafter also referred to as “step 2”); hereinafter also referred to as “step 3”). Each step will be described in detail below.

[Step 1]
In step 1, particles of the first zeolite are prepared. Since the specific method for preparing the particles made of the first zeolite differs depending on the type (skeletal structure) of the first zeolite, the following description will be given for particles made of FAU zeolite when the first zeolite is FAU zeolite ( Hereinafter, a method for preparing FAU particles) will be described as an example. As for the method for preparing particles made of zeolite other than FAU zeolite, known techniques can be appropriately adopted.

A method for preparing FAU particles includes, for example, a method of hydrothermally crystallizing a mixture of a silica source, an alumina source and an alkali source (hereinafter also referred to as a "raw material mixture").

Examples of silica sources include colloidal silica (silica sol), amorphous silica, sodium silicate, tetraethylorthosilicate, aluminosilicate gel, and the like.

Examples of alumina sources include aluminum sulfate, sodium aluminate, aluminum hydroxide, aluminum chloride, aluminosilicate gel, and metal aluminum.

Alkali sources include, for example, various salts such as sodium, potassium and ammonium hydroxides, halides, sulfates, nitrates and carbonates, alkali components in aluminates, silicates and aluminosilicate gels. etc. can be used.

The method of mixing these raw materials is also not particularly limited, but a method of dissolving the alumina source and alkali source in water as a solvent and then adding the silica source dropwise to the aqueous solution and mixing is preferred.

As a method of crystallizing the raw material mixture under hydrothermal conditions, for example, a method using an autoclave can be mentioned. The first zeolite may be crystallized or partially amorphous in the crystallized zeolite, but is preferably crystallized zeolite only. Crystallization conditions vary depending on raw materials, scales, and the like, and can be appropriately set by those skilled in the art. The temperature for crystallization is preferably 70-250°C, more preferably 120-180°C. The crystallization time varies depending on the type of first zeolite. For example, FAU can be crystallized preferably in 15 hours to 6 days, more preferably in 2 to 4 days. Crystallization can be performed either by standing still or under stirring.

After crystallization, solid-liquid separation is performed, and excess alkaline solution is washed with pure water or warm water. After that, the FAU particles can be obtained by drying the water adhering to the particles.

The average particle size of the particles made of the first zeolite (eg, FAU zeolite) is not particularly limited, but is preferably 3-15 μm, more preferably 5-8 μm. If the average particle diameter is within such a range, the particles made of the first zeolite can be satisfactorily coated with the precursor of the second zeolite (for example, the precursor of BEA zeolite) in step 2 described later. . As a result, it is possible to obtain a core-shell zeolite having excellent hydrocarbon adsorption/desorption performance. In this specification, the median diameter (D50) measured by a laser diffraction/scattering particle size distribution analyzer is used as the average particle size of particles.

[Step 2]
In step 2, a precursor of the second zeolite is prepared. As used herein, the precursor of the second zeolite means a substance in an amorphous state before becoming the second zeolite by crystallization. Since the second zeolite is amorphous, it can be coated on the surface of the first zeolite. When the second zeolite is crystallized, the surface of each crystal of the first and second zeolites is stable, so the surface of the first zeolite is not coated and the second zeolite exists alone as the second zeolite. I don't like it because it's easy. Whether or not the target substance is a precursor (amorphous state) can be determined by X-ray diffraction analysis. Since the specific method for preparing particles made of the second zeolite differs depending on the type (skeletal structure) of the second zeolite, the precursor of BEA zeolite ( Hereinafter, a method for preparing (also referred to as "BEA precursor") will be described as an example. As for the method for preparing a precursor made of zeolite other than BEA zeolite, a known technique can be appropriately adopted.

As a method for preparing the BEA precursor, for example, a mixture of a silica source, an alumina source, an alkali source, and an optional template molecule (also referred to as a structure-directing agent (OSDA)) (hereinafter referred to as a “raw material mixture ”) is hydrothermally treated.

Specific examples of the silica source, alumina source, and alkali source are the same as those described in Step 1, so descriptions are omitted here.

Examples of template molecules include tetraethylammonium hydroxide, tetraethylammonium bromide, hexamethyleneimine, and the like.

The method of mixing these raw materials is also not particularly limited, but after dissolving the alumina source, the alkali source, and the optionally used template molecule in water as a solvent, the silica source is added dropwise to the aqueous solution and mixed. A method is preferred.

Examples of methods for amorphizing the raw material mixture under hydrothermal treatment include a method using an autoclave. Amorphization conditions vary depending on raw materials, scales, and the like, and can be appropriately set by those skilled in the art. The temperature for amorphization is preferably 70-250°C, more preferably 120-150°C. Amorphization time is preferably from 2 hours to less than 6 days, more preferably from 1 to 3 days. Amorphization can be carried out either by standing still or under stirring. Thereby, a BEA precursor, which is a white gel-like product, can be obtained.

[Step 3]
In step 3, after the particles of the first zeolite are coated with the precursor of the second zeolite, the precursor of the second zeolite is crystallized. Hereinafter, a method for producing a core-shell zeolite in which the first zeolite is FAU zeolite and the second zeolite is BEA zeolite will be described as an example.

As a method of coating FAU zeolite particles with a BEA zeolite precursor, there is a method of adding FAU particles to the BEA precursor, which is the aforementioned white gel-like product, and stirring if necessary. The mixing ratio of the FAU particles and the BEA precursor is such that the mass of the first zeolite and the mass of the second zeolite in the core-shell zeolite are within the range described above.

At this time, if necessary, the FAU particles may be subjected to an ion exchange treatment in advance in order to align the cations of the FAU particles with the cations of the BEA precursor. In Examples described later, proton-type FAU particles are subjected to an ion exchange treatment with tetraethylammonium ions in order to align them with tetraethylammonium ions, which are cations of the BEA precursor.

After that, the BEA precursor coating the FAU particles is crystallized under hydrothermal conditions. A method of crystallization under hydrothermal conditions includes, for example, a method using an autoclave. Crystallization conditions vary depending on raw materials, scales, and the like, and can be appropriately set by those skilled in the art. The temperature for crystallization is preferably 70-250°C, more preferably 120-180°C. Crystallization time is preferably 6-14 days, more preferably 8-10 days. Crystallization can be performed either by standing still or under stirring.

After crystallization, solid-liquid separation is performed, and excess alkaline solution is washed with pure water or warm water. After that, the core-shell type zeolite according to the present invention can be obtained by drying the water adhering to the particles.

The average particle size of the core-shell zeolite is not particularly limited, but is preferably 4-20 μm, more preferably 7-10 μm. If the average particle size is within such a range, the cordierite three-dimensional structure can be washcoated without any problem.

<Catalyst for purifying exhaust gas>
Since the core-shell type zeolite according to the present invention can raise the desorption temperature of hydrocarbons, it can improve the purification performance of hydrocarbons in exhaust gas by applying it to an exhaust gas purification catalyst. Therefore, according to another aspect of the present invention, there is provided an exhaust gas purifying catalyst in which the core-shell type zeolite according to the present invention and a noble metal are supported on a three-dimensional structure.

This form will be described below. The exhaust gas purifying catalyst (hereinafter also referred to as "catalyst") according to the present invention can appropriately employ known techniques, except that it contains the core-shell type zeolite according to the present invention. Therefore, the present invention is not limited to the following embodiments.

[Core-shell type zeolite]
The catalyst according to the present invention essentially contains core-shell zeolite. Here, the content of the core-shell type zeolite is preferably 10 to 200 g, more preferably 30 to 120 g, and even more preferably 55 to 85 g per liter of the three-dimensional structure. By including the core-shell type zeolite in such a content, the hydrocarbon purification performance is further improved.

[Precious metals]
The catalyst according to the present invention essentially contains a noble metal. Precious metals catalyze oxidation-reduction reactions to purify exhaust gases. Here, the type of noble metal is not particularly limited, but platinum (Pt), palladium (Pd), rhodium (Rh) and the like can be mentioned. These noble metals may be used alone or in combination of two or more. Among these, the noble metal is preferably at least one selected from platinum, palladium and rhodium, more preferably palladium alone; a combination of platinum and/or palladium and rhodium, particularly preferably palladium alone, palladium and rhodium.

The content of platinum (in terms of metal) is preferably 0.01 to 20 g, more preferably 0.05 to 10 g, and more than 0.5 g and less than 5 g per 1 L of the three-dimensional structure, considering the exhaust gas purification performance. is more preferred.

The content of palladium (in terms of metal) is preferably 0.01 to 20 g, more preferably 0.05 to 5 g, more preferably 0.3 to 3 g is more preferred.

The content of rhodium (in terms of metal) is preferably 0.01 to 20 g, more preferably 0.05 to 5 g, more preferably 0.1 to 3 g is more preferred.

[Refractory inorganic oxide]
The catalyst according to the present invention may optionally contain a refractory inorganic oxide (excluding core-shell zeolite). A refractory inorganic oxide has a function as a carrier for supporting catalyst components such as noble metals, rare earth metals, and other metal elements. The refractory inorganic oxide has a high specific surface area, and by supporting the catalyst component on this, it is possible to increase the contact area between the catalyst component and the exhaust gas and to adsorb the reactant. . As a result, it is possible to further increase the reactivity of the catalyst as a whole.

Examples of refractory inorganic oxides include alumina, zeolite (excluding core-shell zeolite), titania, zirconia, and silica. These refractory inorganic oxides may be used alone or in combination of two or more. Among these, alumina and zirconia are preferred, and alumina is more preferred, from the viewpoint of high-temperature durability and high specific surface area. Here, the alumina that is preferably used as the refractory inorganic oxide is not particularly limited as long as it contains an oxide of aluminum. silica-containing alumina, silica-titania-containing alumina, silica-titania-zirconia-containing alumina, and the like. These aluminas may be used alone or in combination of two or more. Of these, γ, δ, or θ-alumina and lanthana-containing alumina are preferred from the viewpoint of high-temperature durability and high specific surface area.

The content of the refractory inorganic oxide is preferably 10-300 g, more preferably 40-200 g, per 1 L of the three-dimensional structure. When the content of the refractory inorganic oxide is 10 g/L or more, the precious metal can be sufficiently dispersed in the refractory inorganic oxide, resulting in a catalyst with more sufficient durability. On the other hand, when the content of the refractory inorganic oxide is 300 g/L or less, the state of contact between the noble metal and the exhaust gas is improved, and exhaust gas purification performance can be exhibited more fully.

[Ceria-Zirconia Composite Oxide]
The catalyst according to the present invention may optionally contain a ceria-zirconia composite oxide (CeO ₂ —ZrO ₂ ) as an oxygen storage material. Here, the oxygen storage material (also referred to as "oxygen storage/release material") stores oxygen in an oxidizing atmosphere (lean) in response to fluctuations in the air-fuel ratio (A/F) that changes according to operating conditions. In a reducing atmosphere (rich), it has the function of stably advancing the oxidation/reduction reaction by releasing oxygen.

The ceria-zirconia composite oxide may contain at least one metal selected from the group consisting of lanthanum (La), yttrium (Y), neodymium (Nd), and praseodymium (Pr). Specific examples include ceria-zirconia-lanthana composite oxides, ceria-zirconia-lanthana-yttria composite oxides, and the like.

The content of the ceria-zirconia composite oxide (in terms of oxide) is not particularly limited, but is preferably 5 to 200 g, more preferably 5 to 100 g, and even more preferably 10 to 90 g per 1 L of the three-dimensional structure. By including the ceria-zirconia composite oxide in such a content, the oxidation/reduction reaction can proceed stably.

[Other ingredients]
The catalyst according to the invention may further contain other components. Other components include Group 2 elements such as magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These elements can be contained in the exhaust gas purifying catalyst in the form of oxides, nitrates or carbonates. Among them, barium and/or strontium are preferred, and strontium oxide (SrO), barium sulfate (BaSO ₄ ) and/or barium oxide (BaO) are more preferred. These other components may be used individually by 1 type, and may be used in combination of 2 or more types.

When the catalyst according to the present invention contains other components, the content of the other components (especially SrO, BaSO ₄ , BaO) (in terms of oxide) is preferably 0 to 50 g per liter of the three-dimensional structure. Yes, more preferably 0.1 to 30 g, still more preferably 0.5 to 20 g.

[Three-dimensional structure]
The three-dimensional structure functions as a carrier that supports core-shell zeolite, noble metals, refractory inorganic oxides, ceria-zirconia composite oxides, and other components. The three-dimensional structure can appropriately adopt a fire-resistant three-dimensional structure known in this technical field. As the three-dimensional structure, for example, a heat-resistant carrier such as a honeycomb carrier having triangular, quadrangular, or hexagonal through-holes (gas passage openings, cell shape) can be used. A cell density (number of cells/unit cross-sectional area) of 100 to 1200 cells/square inch is sufficiently usable, preferably 200 to 900 cells/square inch, more preferably 400 to 900 cells/square inch ( 1 inch = 25.4 mm).

The catalyst according to the present invention can be produced by appropriately referring to known knowledge. A method for producing a catalyst according to the present invention will be briefly described below.

First, a core-shell zeolite, a noble metal source, and, if necessary, other components such as those described above (e.g., refractory inorganic oxides, ceria-zirconia composite oxides, other components) and an aqueous medium are added to a desired composition. Depending on the situation, the materials are appropriately weighed, mixed, and stirred at 5 to 95° C. for 0.5 to 24 hours (if necessary, wet pulverization is performed after stirring) to prepare a slurry. Here, as the aqueous medium, water (pure water, ultrapure water, deionized water, distilled water, etc.), lower alcohols such as ethanol and 2-propanol, organic alkaline aqueous solutions, and the like can be used. Among them, it is preferable to use water and lower alcohols, and it is more preferable to use water. The amount of the aqueous medium is not particularly limited, but it is preferably such that the proportion of solids in the slurry (mass concentration of solids) is 10 to 60% by mass, more preferably 30 to 50% by mass.

Next, the slurry prepared above is applied to the three-dimensional structure. As a method for applying the slurry onto the three-dimensional structure, a known method such as wash coating can be appropriately employed. The amount of slurry to be applied can be appropriately determined by those skilled in the art according to the amount of solid matter in the slurry and the thickness of the catalyst layer to be formed. The amount of the slurry to be applied is preferably such that each component has the content (supported amount) as described above.

Next, the three-dimensional structure to which the slurry has been applied is dried in the air, preferably at a temperature of 70-200°C, for 5 minutes to 5 hours. Next, the dried slurry coating film (catalyst precursor layer) thus obtained is calcined in air at a temperature of 400° C. to 900° C. for 10 minutes to 3 hours. Under such conditions, catalyst components (core-shell zeolite, noble metals, etc.) can be efficiently adhered to the three-dimensional structure. Through the steps described above, the catalyst according to the present invention can be obtained.

As described above, the catalyst according to the present invention may have only one catalyst layer, or may have a structure in which two or more catalyst layers are laminated, as long as it has a core-shell type zeolite and a noble metal. may have. When the catalyst has a structure in which two or more catalyst layers are laminated, the core-shell zeolite and the noble metal may be contained in the same layer or in different layers. Preferably, the core-shell zeolite and the noble metal are contained in different layers, more preferably with the core-shell zeolite in the lower layer and the noble metal in the upper layer. With such arrangement, the ability of the core-shell zeolite according to the present invention can be maximized.

<Method for Purifying Exhaust Gas>
The catalyst according to the present invention can exhibit high purification performance for exhaust gas containing hydrocarbons. Therefore, according to still another aspect of the present invention, there is provided a method for purifying exhaust gas, comprising bringing an exhaust gas purifying catalyst according to the present invention into contact with exhaust gas containing hydrocarbons.

The temperature of the exhaust gas may be the temperature of the exhaust gas during normal gasoline engine operation, preferably 0 to 1500°C, more preferably 25 to 700°C. As used herein, the term "exhaust gas temperature" means the temperature of the exhaust gas at the catalyst inlet. Here, the "catalyst inlet" refers to a portion 15 cm from the exhaust gas inflow side end face of the catalyst.

The catalyst according to the present invention can exhibit sufficient catalytic activity by itself, but a similar or different exhaust gas purifying catalyst may be added to the front stage (inflow side) or rear stage (outflow side) of the catalyst according to the present invention. may be placed. That is, the catalyst according to the present invention is arranged alone, or the catalyst according to the present invention is arranged both in the front stage (inflow side) and the rear stage (outflow side), or the catalyst according to the present invention is arranged in the front stage (inflow side) and the rear stage. (outflow side), and it is preferable to arrange a conventionally known exhaust gas purifying catalyst on the other side.

While the embodiments of the invention have been described in detail, it is clear that this is to be considered illustrative and exemplary rather than limiting, and the scope of the invention is to be construed by the appended claims. is.

The present invention includes the following aspects and forms:
1. having a core made of a first zeolite and a shell made of a second zeolite,
a core-shell zeolite, wherein the channel diameter of the first zeolite is greater than the channel diameter of the second zeolite;
2. 1. The first zeolite is at least one selected from the group consisting of FAU, LEV, MWW and LTA. Core-shell zeolite according to;
3. 1. above, wherein the second zeolite is at least one selected from the group consisting of BEA, CHA, MFI, MOR, SZR, FER and TON; or 2. above. Core-shell zeolite according to;
4. Formula: |P ₂ −P _x |/|P ₂ −P ₁ |<0.1 (P ₁ represents the zeta potential (mV) of the particles composed of the first zeolite, P ₂ represents the second zeolite where P _x represents the zeta potential (mV) of the zeolite particle of interest. ~ above 3. The core-shell zeolite according to any one of;
5. 1. above, wherein the first zeolite is FAU and the second zeolite is BEA; ~ above 4. The core-shell zeolite according to any one of;
6. 1. The second zeolite is contained in more than 62% by mass and not more than 95% by mass with respect to the core-shell zeolite. to the above 5. The core-shell zeolite according to any one of;
7. preparing particles of the first zeolite;
preparing a precursor of the second zeolite;
After coating the particles of the first zeolite with the precursor of the second zeolite, crystallizing the precursor of the second zeolite;
A method for producing a core-shell zeolite having
8. 1 above. to the above 6. A catalyst for purifying exhaust gas, wherein the core-shell type zeolite according to any one of 1 and a noble metal are supported on a three-dimensional structure;
9. 8 above. A method for purifying exhaust gas, comprising contacting the exhaust gas purifying catalyst according to 1 to exhaust gas containing hydrocarbons.

The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples. Unless otherwise specified, each operation was performed under the conditions of room temperature (25° C.)/relative humidity of 40 to 50% RH. Also, unless otherwise specified, ratios represent mass ratios.

<Production of zeolite>
[Comparative Example 1-1] Synthesis of powder a (BEA zeolite (6.68 Å)) 0.975 g of sodium hydroxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., special grade, the same shall apply hereinafter) was dissolved in 25 mL of purified water. To this, sodium aluminate (Al / NaOH (molar ratio) = 0.78, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., first grade) 1.73 g and tetraethylammonium bromide (manufactured by Tokyo Chemical Industry Co., Ltd., special grade, the same applies hereinafter) 14.50 g were added and allowed to dissolve. To this solution, 12.86 g of aqueous ammonia (28%, special grade manufactured by Fuji Film Wako Pure Chemical Industries, Ltd., the same applies hereinafter) was added and stirred. After that, 42 mL of silica sol (LUDOX® HS-30, manufactured by Sigma-Aldrich, the same applies hereinafter) was added slowly with a pipette, and the solution was stirred for 2 hours. 30 g of the resulting white gel-like product was placed in a Teflon (registered trademark) container and sealed. The Teflon (registered trademark) container was heated in a constant temperature bath at 150° C. for 8 days to obtain powder a (average particle size: 5.25 μm).

(XRD measurement)
The crystal structure of powder a was confirmed using an X-ray diffraction (XRD) method. The measurement was performed using an X-ray diffraction measurement device (RINT2200VF, manufactured by Rigaku Corporation). The measurement conditions are as follows: measurement angle range (2θ): 3° to 80°, step interval: 0.02°, measurement time: 1.2 seconds/step, radiation source: CuKα ray, tube voltage: 40 kV, current: 20mA.

From the XRD spectrum of powder a, BEA zeolite was detected, and other types of zeolite were not detected. Therefore, powder a was confirmed to be BEA zeolite.

[Comparative Example 2-1] Synthesis of powder h (FAU zeolite (11.24 Å)) 5.50 g of sodium hydroxide was dissolved in 42.40 mL of purified water. To this solution, 4.685 g of sodium aluminate (Al/NaOH (molar ratio) = 0.78, manufactured by Fuji Film Wako Pure Chemical Industries, Ltd., grade 1) was added and dissolved. After that, 22 mL of silica sol (LUDOX® HS-40, manufactured by Sigma-Aldrich) was slowly added and the solution was stirred for 2 hours. This solution was placed in a Teflon (registered trademark) container and heated in a constant temperature bath at 150° C. for two days. The resulting white powdery product was collected by suction filtration. Thereafter, suction filtration was performed while washing with water until the filtrate reached pH=7. The filtered powder was air-dried for 3 days to obtain powder h (average particle size: 6.72 µm).

When the crystal structure of powder h was confirmed using the X-ray diffraction (XRD) method, FAU zeolite was detected from the XRD spectrum of powder h, and no peaks derived from other types of zeolites were detected. rice field. Therefore, powder h was confirmed to be FAU zeolite.

[Example 1-1] Synthesis of Powder b (BEA/FAU=91/9) 47.25 g of tetraethylammonium bromide was weighed into a 500 mL beaker. 450 mL of purified water was added to the beaker to completely dissolve the tetraethylammonium bromide. 19.997 g of powder h (FAU zeolite (proton type, Si/Al (molar ratio) = 6.1)) synthesized in Comparative Example 2-1 was added to this solution and stirred for 2 hours. After that, the FAU zeolite was collected by filtration. After repeating this operation three times, it was dried overnight in a constant temperature bath at 100°C. As a result, the FAU zeolite was ion-exchanged from the proton type to the tetraethylammonium type.

0.992 g of sodium hydroxide was dissolved in 25 mL of purified water. To this, 1.74 g of sodium aluminate (Al/NaOH (molar ratio) = 0.78, manufactured by Fuji Film Wako Pure Chemical Industries, Ltd., first class) and 14.52 g of tetraethylammonium bromide were added and dissolved. 12.9 g of ammonia water (28%, Wako Pure Chemical, special grade) was added to this solution and stirred. After that, 42 mL of silica sol (LUDOX® HS-30) was slowly added by pipette and the solution was stirred for 2 hours. 30 g of the resulting white gel-like product was placed in a Teflon (registered trademark) container and sealed. This Teflon (registered trademark) container was heated in a constant temperature bath at 150° C. for 3 days. As a result, 9.746 g of an amorphous BEA zeolite precursor was obtained. X-ray diffraction analysis of the precursor confirmed that it was amorphous as no BEA zeolite was detected and an overall baseline increase was observed. To this, 0.98 g of the FAU zeolite after ion exchange was added, and the mixture was further heated in a constant temperature bath at 150° C. for 8 days. The resulting white powdery product was collected by suction filtration. Thereafter, suction filtration was performed while washing with water until the filtrate reached pH=7. The filtered powder was air-dried for 3 days to obtain powder b (average particle size: 8.36 μm) having a mass ratio of BEA/FAU=91/9.

When the crystal structure of powder b was confirmed using the X-ray diffraction (XRD) method, both BEA zeolite and FAU zeolite were detected from the XRD spectrum of powder b, and other types of zeolite were detected. it wasn't. Therefore, it was confirmed that the powder b produced above was composed of BEA zeolite and FAU zeolite.

[Example 2-1] Synthesis of powder c (BEA/FAU=87/13) A powder c (average particle size: 8.25 μm) having a BEA/FAU ratio by mass of 87/13 was obtained in the same manner as in Example 1-1.

When the crystal structure of powder c was confirmed using the X-ray diffraction (XRD) method, both BEA zeolite and FAU zeolite were detected from the XRD spectrum of powder c, and other types of zeolite were detected. it wasn't. Therefore, it was confirmed that the powder c produced above was composed of BEA zeolite and FAU zeolite.

[Example 3-1] Synthesis of powder d (BEA/FAU = 71/29) Powder d (average particle diameter 7.94 μm) having a BEA/FAU=71/29 mass ratio was obtained in the same manner as in Example 1-1.

When the crystal structure of powder d was confirmed using the X-ray diffraction (XRD) method, both BEA zeolite and FAU zeolite were detected from the XRD spectrum of powder d, and other types of zeolite were detected. it wasn't. Therefore, it was confirmed that the powder d produced above was composed of BEA zeolite and FAU zeolite.

[Comparative Example 3-1] Synthesis of powder e (BEA/FAU=62/38) A powder e (average particle size: 7.32 μm) having a mass ratio of BEA/FAU=62/38 was obtained in the same manner as in Example 1-1.

When the crystal structure of powder e was confirmed using the X-ray diffraction (XRD) method, both BEA zeolite and FAU zeolite were detected from the XRD spectrum of powder e, and other types of zeolite were detected. it wasn't. Therefore, it was confirmed that the powder e produced above was composed of BEA zeolite and FAU zeolite.

[Comparative Example 4-1] Preparation of powder f (mixed powder (BEA/FAU = 80/20)) 0.801 g of powder a (BEA zeolite) obtained in Comparative Example 1-1 and Comparative Example 2- By mixing with 0.203 g of the powder h (FAU zeolite) obtained in 1, a powder f in which BEA zeolite and FAU zeolite were mixed at a mass ratio of BEA/FAU=80/20 was obtained.

When the crystal structure of powder f was confirmed using the X-ray diffraction (XRD) method, both BEA zeolite and FAU zeolite were detected from the XRD spectrum of powder f, and other types of zeolite were detected. it wasn't. Therefore, it was confirmed that the powder f produced above was composed of BEA zeolite and FAU zeolite.

[Comparative Example 5-1] Preparation of powder g (mixed powder (BEA/FAU = 70/30) 0.705 g of powder a (BEA zeolite) obtained in Comparative Example 1-1, and By mixing 0.302 g of the powder h (FAU zeolite) obtained in 1. in a mortar, BEA zeolite and FAU zeolite were mixed at a mass ratio of BEA/FAU = 70/30 to obtain powder g. Obtained.

When the crystal structure of powder g was confirmed using the X-ray diffraction (XRD) method, both BEA zeolite and FAU zeolite were detected from the XRD spectrum of powder g, and other types of zeolite were detected. it wasn't. Therefore, it was confirmed that the powder g produced above was composed of BEA zeolite and FAU zeolite.

(Zeta potential)
For each powder, the surface state of the particles was confirmed by measuring the zeta potential. The measurement was performed in accordance with JIS Z8836:2017 using a zeta potential measuring device (ELSZ-2Plus, manufactured by Otsuka Electronics Co., Ltd.). Measurement conditions were a liquid temperature of 25° C. and a pH of 3, and the laser Doppler method was used. As the measurement solution, a solution obtained by dispersing 0.5% of various powders in purified water was used. The pH was adjusted using 0.1 mol/L hydrochloric acid and 0.1 mol/L sodium hydroxide aqueous solution. The results are shown in Table 1 below.

As shown in Table 1, powder a (BEA only) has a zeta potential of −25.75 mV and powder h (FAU only) has a zeta potential of 32.06 mV. In the column of "Theoretical value of zeta potential (mV)" in Table 1, the measured values of zeta potential (mV) for powder a and powder h, and the ratios of BEA and FAU contained in each powder (mass %), the theoretical value obtained by the following formula is described.
Theoretical value (mV) = -25.75 (mV) x percentage of BEA (% by mass)/100 + 32.06 (mV) x percentage of FAU (% by mass)/100.

The measured values of zeta potential for powders f and g (mixed powder) were close to their respective theoretical values.

The measured zeta potentials of powders c and d were both similar to the zeta potential of powder a (only BEA) (|P ₂ −P _x |/|P ₂ −P ₁ |<0. 1). Although not shown in Table 1, similar results were obtained with powder b. Therefore, it was determined that the surfaces of powders b to d were composed of BEA. Furthermore, considering the above XRD measurement results, powders b to d were confirmed to be core-shell zeolites having a core made of FAU zeolite and a shell made of BEA zeolite.

The measured zeta potential for powder e was significantly different from the zeta potential for powder a (BEA only) (|P ₂ −P _x |/|P ₂ −P ₁ |<0.1 ). From the results, it was determined that the powder e did not have a core-shell structure because both BEA and FAU were present on the surface.

Further, the powders f and g consisted of particles consisting only of the first zeolite (FAU) and particles consisting only of the second zeolite (BEA) at ratios of FAU:BEA = 20:80 and 30:70, respectively. The values of |P ₂ -P _x |/|P ₂ -P ₁ | are 0.23 and 0.30, respectively. From this result as well, the higher the ratio of particles composed only of the first zeolite (FAU) (the larger the area of the first zeolite (FAU) on the surface of the particles), the more |P ₂ −P _x |/|P It can be seen that there is a correlation that the value of ₂ −P ₁ | increases.

[HC adsorption/desorption performance]
The HC adsorption/desorption performance of each powder was evaluated by toluene TPD (Temperature Programmed Desorption). The measurement was carried out using a catalyst analyzer (BELCAT II manufactured by Microtrac Bell). As the measurement gas, helium gas containing 3000 ppmC of toluene gas and water vapor (3% by volume) was used. The sample amount was 0.050 g, and the measurement was performed from 50°C to 400°C at a temperature elevation rate of 10°C/min. The gas after the reaction was analyzed using a quadrupole mass spectrometer (BELMASS, manufactured by Microtrack Bell). At this time, the temperature at which the detected amount of toluene first coincides with 3000 ppmC, which is the toluene concentration in the circulation, was defined as the desorption start temperature, and the temperature at which the detected amount of toluene was maximized was defined as the peak top temperature for analysis. The higher the desorption start temperature and peak top temperature, the better the zeolite's ability to adsorb and desorb hydrocarbons. In addition, since it was judged that the powder e did not have a core-shell structure as described above, this evaluation was not performed. The results are shown in Table 2 below.

Table 2 shows that powders b to d according to the present invention have significantly high desorption start temperatures and peak top temperatures. Therefore, it was shown that the core-shell zeolite according to the present invention can increase the desorption temperature of HC.

[SEM image]
The powder surfaces of the core-shell zeolite powder d and the FAU zeolite powder h were observed with an FE-SEM (field emission scanning electron microscope). The observation was performed using FE-SEM JSM-6700FS manufactured by JEOL Ltd. under the conditions of an emission current of 10 μA and an acceleration voltage of 12.5 kV. SEM images of powder d and powder h are shown in FIGS. 1 and 2, respectively. As shown in FIG. 2, the surface of the FAU zeolite powder h is smooth, but since the core-shell type zeolite powder d in FIG. It can be seen that the appearance is different from FAU zeolite.

<Preparation of Exhaust Gas Purifying Catalyst>
[Comparative Example 1-2]
70.4 parts by mass of the powder a and 9.6 parts by mass of the alumina sol were weighed so as to have a solid content of 9.6 parts by mass. Alumina sol was added to distilled water and stirred, then powder a was added and stirred for 10 minutes. Next, wet pulverization was performed to obtain slurry a1.

36 parts by mass of La-containing alumina (La ₂ O ₃ content of 4% by mass, average particle size D50 of 5 μm, BET surface area of 172.4 m ² /g), CeZrLa composite oxide (CeO ₂ :ZrO ₂ :La ₂ 18.5 parts by mass of O ₃ =47:47:6 (mass ratio) and 4.62 parts by mass of barium sulfate were weighed. After adding these to distilled water and stirring for 10 minutes, 2.19 parts by mass of an aqueous palladium nitrate solution (concentration: 21% by mass) was added dropwise while stirring for 10 minutes. After stirring, wet pulverization was performed to obtain slurry a2.

Slurry a1 was wash-coated on a cordierite three-dimensional structure (diameter 25.4 mm, length 30 mm, cylindrical, 0.0157 L, 400 cells/square inch) so that the amount supported after firing was 80 g/L. bottom. Next, after drying in the air at 150° C. for 5 minutes, it was calcined in the air at 550° C. for 30 minutes.

Next, the slurry a2 was wash-coated on the three-dimensional structure that had been wash-coated with the slurry a1 so that the load after firing was 59.58 g/L. Next, after drying in the air at 150° C. for 5 minutes, the catalyst A was obtained by calcining in the air at 550° C. for 30 minutes.

[Comparative Example 2-2]
Catalyst H was obtained in the same manner as in Comparative Example 1-2, except that powder h was used instead of powder a.

[Example 3-2]
A catalyst D was obtained in the same manner as in Comparative Example 1-2, except that powder d was used instead of powder a.

[Comparative Example 5-2]
Catalyst G was obtained in the same manner as in Comparative Example 1-2, except that powder g was used instead of powder a.

<Evaluation of exhaust gas purifying catalyst>
(HC desorption temperature)
For catalysts A, D, G, and H, a gas having a composition shown in Table 3 below (space velocity: 50000 hr ⁻¹ , gas linear velocity: 0.42 m/sec) was passed through the catalyst at a distance of 1 cm from the end surface of the catalyst on the gas inlet side. The temperature of the position was raised from 50°C to 400°C at a heating rate of 40°C/min. At this time, the temperature at which the detected amount of toluene first coincides with 840 ppmC, which is the toluene concentration in the circulation, was defined as the desorption start temperature, and the temperature at which the detected amount of toluene is maximized was defined as the peak top temperature for analysis. The higher the desorption start temperature and the peak top temperature, the better the hydrocarbon adsorption/desorption capacity of the catalyst. The results are shown in Table 4 below.

(HC purification rate)
In addition, for catalysts A, D, G, and H, a gas having a composition shown in Table 3 below (spatial velocity: 50000 hr ⁻¹ , gas linear velocity: 0.42 m/sec) was passed through the end face of the catalyst on the gas inflow side. Table 4 shows the HC purification rate when the temperature at the position 1 cm from the center is set to 110°C.

Table 4 shows that catalyst D according to the present invention has a significantly higher desorption start temperature and peak top temperature. According to the catalyst containing the core-shell type zeolite according to the present invention, HC can be desorbed in a temperature range where HC can be purified by Pd by increasing the desorption temperature of HC. As a result, it is possible to improve the HC purification performance of the catalyst. It can be seen that the catalyst containing the core-shell zeolite according to the present invention has a high HC purification rate even at a low temperature of 110°C.

This application is based on Japanese Patent Application No. 2021-149970 filed on September 15, 2021, the disclosure of which is incorporated herein by reference.

Claims

having a core made of a first zeolite and a shell made of a second zeolite,
A core-shell zeolite, wherein the channel diameter of the first zeolite is greater than the channel diameter of the second zeolite.
The core-shell zeolite according to claim 1, wherein the first zeolite is at least one selected from the group consisting of FAU, LEV, MWW and LTA.
The core-shell zeolite according to claim 1 or 2, wherein the second zeolite is at least one selected from the group consisting of BEA, CHA, MFI, MOR, SZR, FER and TON.
Formula: |P 2 −P x |/|P 2 −P 1 |<0.1 (P 1 represents the zeta potential (mV) of the particles composed of the first zeolite, P 2 represents the second zeolite 3. Core-shell zeolite according to claim 1 or 2, wherein P x represents the zeta potential (mV) of the zeolite particle in question.
The core-shell zeolite according to claim 1 or 2, wherein the first zeolite is FAU and the second zeolite is BEA.
The core-shell zeolite according to claim 1 or 2, wherein the second zeolite is contained in more than 62% by mass and not more than 95% by mass with respect to the core-shell zeolite.
preparing particles of the first zeolite;
preparing a precursor of the second zeolite;
After coating the particles of the first zeolite with the precursor of the second zeolite, crystallizing the precursor of the second zeolite;
A method for producing a core-shell zeolite.
An exhaust gas purifying catalyst in which the core-shell type zeolite according to claim 1 or 2 and a noble metal are supported on a three-dimensional structure.
A method for purifying exhaust gas, comprising bringing the exhaust gas purifying catalyst according to claim 8 into contact with exhaust gas containing hydrocarbons.