JP7739715B2 - Hydrocarbon adsorbent and hydrocarbon adsorption method - Google Patents

Description

This disclosure relates to a hydrocarbon adsorbent and a method for adsorbing hydrocarbons.

Exhaust gases emitted from internal combustion engines used in mobile vehicles such as automobiles and ships contain a large amount of hydrocarbons. These hydrocarbons are purified by a three-way catalyst. Because a three-way catalyst requires a temperature environment of 200°C or higher to function, hydrocarbons are adsorbed onto a hydrocarbon adsorbent at temperatures where the three-way catalyst does not function, such as during cold starts. The hydrocarbons are then released from the adsorbent at temperatures where the three-way catalyst begins to function, where they are decomposed and purified by the three-way catalyst. Compositions containing zeolite are commonly used as hydrocarbon adsorbents, but the higher the hydrocarbon desorption onset temperature, the more active the three-way catalyst will be in releasing hydrocarbons, which is advantageous for purifying hydrocarbons. Therefore, compositions with a high hydrocarbon desorption onset temperature are desired.

Patent Document 1 proposes a composition containing copper-containing FAU-type zeolite as a composition with a high hydrocarbon desorption onset temperature.

Japanese Patent Application Laid-Open No. 2019-150822

However, the zeolite described in Patent Document 1 had insufficient durability in hydrothermal environments, particularly at high temperatures of around 900°C. It is generally known that zeolites lose some of their framework atoms, such as aluminum, in high-temperature, high-humidity environments, reducing their crystallinity and resulting in a decrease in the amount of hydrocarbons they adsorb and desorb. As a result, the hydrocarbon adsorption rate tends to decrease in high-temperature environments such as automobile exhaust systems.

The present disclosure aims to provide a hydrocarbon adsorbent that exhibits a high hydrocarbon adsorption rate even in high-temperature environments, or a hydrocarbon adsorption method that uses such a hydrocarbon adsorbent.

The inventors have discovered the following hydrocarbon adsorbent, which exhibits a high hydrocarbon adsorption rate even in high-temperature environments.

In other words, the present invention is as defined in the claims, and the gist of this disclosure is as follows:

[1] A hydrocarbon adsorbent comprising a zeolite represented by _YO2 - _X2O3 and containing copper and _a rare earth element, characterized in that Y is a tetravalent element, X is a trivalent element, the copper content is 0.5% by mass or more and 4% by mass or less relative to 100% by mass of the zeolite, and the molar ratio of the rare earth element to X is 0.005 or more and 0.1 or less.

[2] The hydrocarbon adsorbent according to [1], wherein the molar ratio of _{YO2 to X2O3} ( _YO2 / _{X2O3 ratio} ) of the zeolite is 2 or more and 100 or less.

[3] The hydrocarbon adsorbent according to [1] or [2], wherein the rare earth is at least one selected from the group consisting of scandium, yttrium, lanthanum, cerium, neodymium, samarium, gadolinium, terbium, thulium, ytterbium, and lutetium.

[4] The hydrocarbon adsorbent according to any one of [1] to [3], wherein the zeolite has a ring structure of 10 or more members.

[5] The hydrocarbon adsorbent according to any one of [1] to [4], wherein the zeolite has one or more structures selected from the group consisting of a BEA structure, an MFI structure, an MOR structure, a YFI structure, and an FAU structure.

[6] A method for adsorbing hydrocarbons using the hydrocarbon adsorbent described in any one of [1] to [5] above.

The present disclosure aims to provide a hydrocarbon adsorbent that exhibits a high hydrocarbon adsorption rate even in high-temperature environments, or a hydrocarbon adsorption method that uses such a hydrocarbon adsorbent.

1 is a graph showing the relationship between the molar ratio of cerium to aluminum in a hydrocarbon adsorbent and the hydrocarbon adsorption rate.

The hydrocarbon adsorbent of the present disclosure will be described below using an example embodiment.

The hydrocarbon adsorbent of this embodiment is a hydrocarbon adsorbent containing a zeolite represented by YO ₂ -X ₂ O ₃ and containing copper and a rare earth element, wherein Y is a tetravalent element and X is a trivalent element, the copper content is 0.5% by mass or more and 4% by mass or less relative to 100% by mass of the zeolite, and the molar ratio of the rare earth element to X is 0.005 or more and 0.1 or less.

Based on its constituent requirements, the hydrocarbon adsorbent of this embodiment exhibits a high hydrocarbon adsorption rate even in high-temperature environments.

(Zeolite)
In this embodiment, the zeolite is a compound having a regular structure in which skeleton atoms (hereinafter also referred to as "T atoms") are connected via oxygen (O), and the T atoms are at least one of metal atoms, metalloid atoms, and other atoms. Examples of metal atoms include one or more selected from the group consisting of iron (Fe), aluminum (Al), gallium (Ga), tin (Sn), and titanium (Ti), as well as boron (B) and other transition metal elements. Examples of metalloid atoms include one or more selected from the group consisting of boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), and tellurium (Te), and examples of other atoms include phosphorus (P).

The zeolite contained in the hydrocarbon adsorbent of this embodiment is a crystalline aluminosilicate. Crystalline aluminosilicate has a crystal structure consisting of a repeating network of aluminum (Al) and silicon (Si) via oxygen ₍ O). That is, the zeolite contained in the hydrocarbon adsorbent of this embodiment contains _SiO2 as _YO2 and _Al2O3 as _X2O3 , where Y is Si and X is Al. In addition to aluminosilicate, the zeolite contained in the hydrocarbon adsorbent of this embodiment _may be composed of metallosilicates such as ferrosilicate and gallosilicate, or zeolite-related substances such as SAPO (silicoaluminophosphate) and AlPO (aluminophosphate).

In the following, the zeolite contained in the hydrocarbon adsorbent of this embodiment will be described as a crystalline aluminosilicate.

The skeletal structure of a zeolite (used interchangeably with crystalline structure, hereinafter also referred to as "zeolite structure") is a skeletal structure specified by the structure code (hereinafter also referred to simply as "structure code") established by the Structure Commission of the International Zeolite Association, and can be identified by comparing the XRD pattern of the target zeolite with the XRD pattern of each zeolite structure (hereinafter also referred to as "reference pattern") described in "Collection of Simulated XRD Powder Patterns for Zeolites, Fifth Revised Edition (2007)."

In this embodiment, the XRD pattern can be obtained by XRD measurement under the following conditions:

Acceleration current/voltage: 40mA/40kV
Radiation source: CuKα radiation (λ=1.5405Å)
Measurement mode: Continuous scan
Scan condition: 40°/min
Measurement range: 2θ = 3° to 43°
Divergence vertical limit slit: 10 mm
Divergence/entrance slit: 1°
Receiving slit: open
Receiving solar slit: 5°
Detector: D/teX Ultra
Use of Ni filter The molar ratio of silica to alumina in the zeolite contained in the hydrocarbon adsorbent of this embodiment (hereinafter also referred to as the "SiO ₂ /Al ₂ O ₃ ratio") preferably has a lower limit of 5, and upper limits of 35, 20, 18, 15, and 10, in that order, are more preferable, in terms of the high hydrocarbon desorption initiation temperature.

The zeolite contained in the hydrocarbon adsorbent of this embodiment has a high hydrocarbon desorption initiation temperature, and therefore the lower limit of the BET specific surface area is preferably 200 m ² /g, followed by 300 m ² /g, and the upper limit is preferably 700 m ² /g or less, followed by 800 m ² /g.

The zeolite contained in the hydrocarbon adsorbent of this embodiment may have, for example, one or more zeolite structures selected from the group consisting of BEA, MFI, FAU, FER, and MOR structures. In terms of the high hydrocarbon desorption onset temperature, it is preferable to have one or more zeolite structures selected from the group consisting of BEA, MFI, FAU, and MOR structures. In this embodiment, a zeolite having a specific zeolite structure is also referred to as "-type zeolite." For example, a zeolite having an FAU structure is also referred to as an FAU-type zeolite. Examples of FAU-type zeolites include zeolite X, zeolite LSX, zeolite Y, and zeolite USY, with at least one of zeolite Y and zeolite USY being preferred. The zeolite contained in the hydrocarbon adsorbent of this embodiment may be composed of an intergrowth having two or more of the above zeolite structures.

The zeolite contained in the hydrocarbon adsorbent of this embodiment has an average crystal size of 0.1 μm or more, preferably 0.3 μm or more. Because hydrocarbon adsorption properties tend to improve not only after exposure to a reducing atmosphere but also to a high-temperature, high-humidity oxidizing atmosphere, the average crystal size of the zeolite is more preferably 0.4 μm or more, and even more preferably 0.5 μm or more.

From the perspective of improving operability, such as ease of application to the adsorbent carrier, the average crystal size of the zeolite is preferably 2.5 μm or less, more preferably 1.5 μm or less, and even more preferably 1.0 μm or less. In order to exhibit high hydrocarbon adsorption properties even after exposure to both a reducing and an oxidizing atmosphere at high temperatures and humidity, the average crystal size of the zeolite is preferably 0.4 μm or more and 2.0 μm or less, and particularly preferably 0.6 μm or more and 0.9 μm or less.

In this embodiment, the average crystal size of zeolite is the average particle size of primary particles. The particle size of primary particles is the particle size of primary particles confirmed in SEM images obtained by observation with a scanning electron microscope (hereinafter also referred to as "SEM"), and the average crystal size is the average value of the particle sizes of the primary particles. One method for measuring the average crystal size is to extract 80 to 150 primary particles observed at a magnification of 3,000 to 20,000 times, measure the particle sizes of the primary particles, and use the average value as the average crystal size. When extracting primary particles for particle size measurement, it is sufficient to use one or more SEM images.

(copper rare earth-containing zeolite)
The zeolite contained in the hydrocarbon adsorbent of this embodiment contains copper and rare earths. Zeolite containing copper and rare earths (hereinafter also referred to as "copper-rare earth-containing zeolite") has superior hydrocarbon retention and adsorption rate after hydrothermal durability compared to zeolite not containing copper and rare earths. It is thought that the inclusion of copper and rare earths in the zeolite stabilizes the Al framework of the zeolite, thereby maintaining crystallinity and suppressing copper sintering during hydrothermal durability.

The zeolite contained in the hydrocarbon adsorbent of this embodiment contains copper. Copper-containing zeolite (hereinafter also referred to as "copper-containing zeolite") has superior hydrocarbon retention compared to zeolites that do not contain copper. Most hydrocarbons adsorbed by hydrocarbon adsorbents containing copper-free zeolites are easily released as the temperature of the hydrocarbon adsorbent increases. It is believed that the copper content of the zeolite strengthens the interaction between the hydrocarbons and the zeolite, making it more difficult for hydrocarbons adsorbed by hydrocarbon adsorbents containing such zeolite to be released from the hydrocarbon adsorbent.

In this embodiment, the copper contained in the zeolite is preferably in the form of divalent copper (Cu ²⁺ ), and more preferably in the form of highly dispersible divalent copper (hereinafter also referred to as "dispersed copper"). Examples of dispersed copper include at least one of Cu ²⁺ ions and CuO clusters, and Cu ²⁺ ions are preferred.

The copper content of the zeolite contained in the hydrocarbon adsorbent preferably has a lower limit of 0.5 mass%, 1.0 mass%, or 1.5 mass%, and an upper limit of 4.0 mass%, 3.0 mass%, or 2.8 mass%, respectively.

In this embodiment, the copper content of the zeolite contained in the hydrocarbon adsorbent is the mass ratio of copper to the mass of the metals and metalloids other than rare earths contained in the zeolite, converted into oxides. For example, the copper content of a hydrocarbon adsorbent containing copper (Cu) and an alkali metal (M) can be calculated using the following formula:

Copper content (mass%) = W' _Cu / (W _Al + W _Si + W _M + W _Cu ) x 100
In the above formula, W' _Cu is the content of copper (Cu) in the zeolite contained in the hydrocarbon adsorbent, and _{W Al} , W _Si , W _M , and W _Cu are the mass of aluminum (Al) converted into oxide (Al ₂ O ₃ ), the mass of silicon (Si) converted into oxide (SiO ₂ ), the mass of alkali metal (M) converted into oxide (M ₂ O), and the mass of copper converted into oxide (CuO), respectively, in the zeolite contained in the hydrocarbon adsorbent.

The zeolite contained in the hydrocarbon adsorbent of this embodiment contains rare earths. In this embodiment, the rare earths contained in the zeolite are preferably tetravalent or pentavalent rare earths, and more preferably highly dispersible rare earths (hereinafter also referred to as "dispersed rare earths"). Dispersed rare earths include at least one of rare earth ions and rare earth oxide clusters, and are preferably rare earth ions.

The rare earth contained in the zeolite contained in the hydrocarbon adsorbent of this embodiment refers to scandium, yttrium, lanthanum, cerium, neodymium, samarium, gadolinium, terbium, thulium, ytterbium, and lutetium. It is preferable to contain at least one selected from this group, and more preferably at least one of cerium and thulium. From the viewpoint of improving the hydrocarbon adsorption rate, it is increasingly preferable for the content of the rare earth to be in the molar ratio to X in the zeolite contained in the hydrocarbon adsorbent of 0.005 or more and 0.1 or less, 0.01 or more and 0.09 or less, 0.02 or more and 0.08 or less, or 0.03 or more and 0.06 or less.

The zeolite contained in the hydrocarbon adsorbent of this embodiment may contain an alkali metal. The alkali metal is not particularly limited, but examples include one or more selected from the group consisting of sodium, potassium, rubidium, and cesium. As an example, the zeolite contained in the hydrocarbon adsorbent may contain an alkali metal that is derived from the raw material composition and contained in the zeolite during hydrothermal synthesis.

The hydrocarbon adsorbent of this embodiment may contain components other than those described above. These components are not particularly limited, but examples include binders.

(Method for producing hydrocarbon adsorbent)
Next, a method for producing the hydrocarbon adsorbent of this embodiment will be described.

The hydrocarbon adsorbent of this embodiment can be obtained by a manufacturing method including a copper-containing process in which copper is contained in zeolite, and a rare earth-containing process in which rare earth elements are contained in the zeolite after the copper-containing process.

(Method for producing zeolite)
The method for producing the zeolite used in this embodiment is not particularly limited. For example, the zeolite may be obtained by a production method including a crystallization step of hydrothermally treating a composition containing a silica source, an alumina source, an alkali source, and water (hereinafter also referred to as a "raw material composition") to obtain a crystallized product.

The silica source may be at least one of a silicon-containing salt and compound, such as at least one selected from the group consisting of colloidal silica, amorphous silica, sodium silicate, tetraethyl orthosilicate, and aluminosilicate gel. Of these, at least one of colloidal silica and amorphous silica is preferred.

The alumina source may be at least one of an aluminum-containing salt and compound, such as at least one selected from the group consisting of aluminum nitrate, aluminum sulfate, sodium aluminate, aluminum hydroxide, aluminum chloride, aluminosilicate gel, and metallic aluminum. Of these, at least one of aluminum hydroxide and aluminum sulfate is preferred.

The alkalinity source may be one or more selected from the group consisting of various salts such as hydroxides, halides, and carbonates of sodium, potassium, and ammonium.

The raw material composition may contain a structure directing agent (hereinafter also referred to as "SDA") as needed. Examples of structure directing agents include N,N,N-trimethyl-(+)-cis-myrtanylammonium cation, hexamethonium cation, 1,1-dialkyl-4-alkylcyclohexylpiperazin-1-ium cation, 1,1-dialkyl-4-cyclohexylpiperazin-1-ium cation, 1,1'-((3as,6as)-octahydropentalen-2,5-diyl)bis(1-methylpiperidin-1-ium) cation, 1,1'-(buth ... 1,1'-(pentane-1,4-diyl)bis(1-methylpiperidin-1-ium) cation, 1,1'-(pentane-1,5-diyl)bis(1-methylpiperidin-1-ium), 1,1'-(hexane-1,6-diyl)bis(1-methylpiperidin-1-ium) cation, 3-hydroxy-1-(4-(1-methylpiperidin-1-ium-1-yl)butyl)quinukikudzin-1-ium cation, 3-hydroxy-1-(5-(1-methylpiperidin-1- (N-1-yl)pentyl)quinukikudzin-1-ium cation, N,N,N,N-tetraethylbicyclo[2.2.2]-oct-7-ene-dipyrrolidinium, N,N-dimethyl-N'-cyclohexylpiperazinium cation, dimethyldipropylammonium cation, tetraethylammonium cation, 1,6-bis(N-cyclohexylpyrrolidinium)hexane dication, 1,4-bis(N-cyclohexylpiperidinium)butane Examples of suitable anions include one or more selected from the group consisting of 1,4-bis(N-cyclohexylpyrrolidinium)butane dication, 1,4-bis(N-cyclopentylpiperidinium)butane dication, 1,5-bis(N,N-dimethylcyclohexylammonium)pentane dication, N,N,N-trimethyltricyclo[5.2.1.0]-decaneammonium, and (6R,10S)-6,10-dimethyl-5-azoniaspiro[4.5]decane cation. SDA may be contained in the raw material composition as a salt paired with one or more anions selected from the group consisting of fluoride, chloride, bromide, iodide, and hydroxide (hereinafter, the salt of SDA will also be referred to as "SDAX"). Preferably, the raw material composition has the following molar composition. In the following composition, SDAX represents the salt of SDA, and X represents an anion other than fluorine.

SiO ₂ /Al ₂ O ₃ ratio = 2 or more and 500 or less SDAX/SiO ₂ ratio = 0.00 or more and 0.80 or less
Na/SiO2 _ratio = 0.00 or more and 0.80 or less
K/SiO2 _ratio = 0.00 or more and 0.80 or less
X/SiO2 _ratio = 0.00 or more and 2.0 or less
HF/SiO2 _ratio = 0.00 or more and 1.0 or less
In the method for producing a zeolite according to the _present embodiment, the raw material composition preferably contains seed crystals. _The inclusion of seed crystals increases the crystallization rate of the zeolite, shortens the time required for zeolite production, and improves the yield.

The zeolite seed crystals are preferably zeolite having at least one structure selected from the group consisting of LTL structure, LTA structure, MOR structure, MFI structure, *BEA structure, FAU structure, CHA structure, and YFI structure.

The SiO ₂ /Al ₂ O ₃ molar ratio of the seed crystal is preferably 2 or more and 100 or less, and more preferably 3 or more and 60 or less.

The smaller the content of seed crystals, the better, but taking into consideration the reaction rate and the effect of suppressing impurities, a content of 0.1 to 60 mass% is preferred, and 0.5 to 40 mass% is even more preferred.

The content of the seed crystals in the raw material composition (hereinafter also referred to as "seed crystal content") can be determined from the mass of silicon (Si) contained in the raw material composition converted into _SiO2 by the formula: Seed crystal content (mass%) = ( _SiO2 mass of zeolite) / ( _SiO2 mass in the entire raw material composition) × 100.

(Crystallization process)
In the crystallization step, the raw material composition is crystallized by hydrothermal treatment. The conditions for the hydrothermal treatment are not particularly limited, but the following conditions can be mentioned, for example.

Temperature: 80°C to 200°C Time: 1 hour to 10 days Pressure: Autogenous pressure The raw material composition is crystallized by the hydrothermal treatment to obtain zeolite. The obtained zeolite may be subjected to any of the steps of recovery, washing, drying, and calcination, and may further be dealuminated to adjust the _SiO2 / _Al2O3 ratio to _any value.

When the zeolite contains SDA, the hydrocarbon desorption onset temperature tends to be high, so it is preferable to calcinate the zeolite after crystallization to remove the SDA from the zeolite. Calcination conditions are optional, but examples include an oxidizing atmosphere, a calcination temperature of 400°C to 800°C, and a calcination time of 0.5 hours to 12 hours.

(Copper addition process)
The copper addition step includes a copper source contacting step of contacting the zeolite with a copper source, and a calcining step of calcining the zeolite after the copper source contacting step.

In the copper source contacting step, the copper source is a compound containing copper (Cu), preferably a copper salt, more preferably one or more selected from the group consisting of copper-containing nitrates, sulfates, acetates, chlorides, complex salts, oxides, and composite oxides, and even more preferably one or more selected from the group consisting of copper nitrate, copper sulfate, and copper acetate.

It is increasingly preferred that the lower limit of the copper content of the zeolite be 0.5 mass%, 1.0 mass%, or 1.5 mass%, and it is increasingly preferred that the upper limit be 4.0 mass%, 3.0 mass%, or 2.8 mass%.

The zeolite can be contacted with the copper source by any known method, including at least one selected from the group consisting of ion exchange, impregnation, evaporation to dryness, precipitation, and physical mixing. At least one of the ion exchange and impregnation methods is preferred, with the impregnation method being more preferred.

After contacting the zeolite with the copper source, the zeolite may be washed and dried by any method. Examples of washing methods include washing with a sufficient amount of water, and examples of drying methods include treating the zeolite in air at 100°C to 150°C for 5 to 30 hours.

The firing conditions in the firing process are optional, but examples include the following conditions:

Firing atmosphere: oxidizing atmosphere, preferably air Firing temperature: 400°C or higher and 600°C or lower Firing time: 30 minutes or higher and 5 hours or lower The firing step is preferably carried out under air flow, and the air flowing preferably has a low moisture content. Firing in air with a low moisture content tends to strengthen the interaction between copper and the aluminum that constitutes the framework of the crystal structure, making it easier to improve heat resistance. The moisture content of the air flowing is preferably 0.7% by volume or lower, more preferably 0.5% by volume or lower, and even more preferably 0.3% by volume or lower.

(Rare earth addition process)
The rare earth adding step includes a rare earth source contacting step of contacting the zeolite after the copper adding step with a rare earth source, and a calcining step of calcining the zeolite after the rare earth source contacting step.

In the rare earth source contacting step, the rare earth source is a compound containing a rare earth, preferably a rare earth salt, more preferably at least one selected from the group consisting of nitrates, sulfates, acetates, chlorides, complex salts, oxides, and composite oxides containing rare earths, and even more preferably at least one selected from the group consisting of nitrates, sulfates, and acetates.

In order to improve the hydrocarbon adsorption rate, it is preferable that the molar ratio of the rare earth element to X in the zeolite contained in the hydrocarbon adsorbent be 0.005 or more and 0.1 or less, 0.01 or more and 0.09 or less, 0.02 or more and 0.08 or less, or 0.03 or more and 0.06 or less.

The zeolite can be contacted with the rare earth source by any known method, including at least one selected from the group consisting of ion exchange, impregnation, evaporation to dryness, precipitation, and physical mixing. At least one of the ion exchange and impregnation methods is preferred, with the impregnation method being more preferred.

After contacting the zeolite with the rare earth source, the zeolite may be washed and dried by any method. Examples of washing methods include washing with a sufficient amount of water, and examples of drying methods include treating the zeolite in air at 100°C to 150°C for 5 to 30 hours.

The firing conditions in the firing process are optional, but examples include the following conditions:

Firing atmosphere: oxidizing atmosphere, preferably air Firing temperature: 400°C or higher and 600°C or lower Firing time: 30 minutes or higher and 5 hours or lower The firing process is preferably carried out under air flow, and the air flowing preferably has a low moisture content. Firing in air with a low moisture content tends to strengthen the interaction between copper and the aluminum that constitutes the framework of the crystal structure, making it easier to improve heat resistance. The moisture content of the air flowing is preferably 0.7% by volume or lower, more preferably 0.5% by volume or lower, and even more preferably 0.3% by volume or lower.

(Hydrocarbon adsorbent)
The hydrocarbon adsorbent of this embodiment may have any shape depending on the application, and examples thereof include at least one of a powder and a molded body. Specific examples of the molded body shape include at least one selected from the group consisting of a spherical shape, a nearly spherical shape, an elliptical shape, a disk shape, a cylindrical shape, a polyhedral shape, an irregular shape, and a petal shape.

When using the hydrocarbon adsorbent as a powder, the above-mentioned hydrocarbon adsorbent can be mixed with a solvent such as water or alcohol to form a slurry, and the slurry can be coated onto a substrate to form an adsorption member.

When the hydrocarbon adsorbent of this embodiment is formed into a molded body, the hydrocarbon adsorbent may be mixed with a binder as needed and molded by any method. The binder is preferably at least one selected from the group consisting of silica, alumina, kaolin, attapulgite, montmorillonite, bentonite, aloene, and sepiolite. The molding method may be at least one selected from the group consisting of rolling granulation molding, press molding, extrusion molding, injection molding, slip casting, and sheet molding.

The hydrocarbon adsorbent of this embodiment can be used in a method for adsorbing hydrocarbons, preferably in a method for adsorbing hydrocarbons in an environment where the hydrocarbon adsorbent is exposed to high temperatures, more preferably in a method for adsorbing hydrocarbons from the exhaust gas of an internal combustion engine, and even more preferably in a method for adsorbing hydrocarbons from the exhaust gas of an internal combustion engine of a mobile vehicle.

The hydrocarbon adsorbent of this embodiment can adsorb hydrocarbons by a method including a step of contacting a hydrocarbon-containing fluid with the hydrocarbon adsorbent of this embodiment (hereinafter also referred to as the "contact step"). Examples of hydrocarbon-containing fluids include hydrocarbon-containing gases and hydrocarbon-containing liquids.

The hydrocarbon-containing gas that comes into contact with the hydrocarbon adsorbent of this embodiment is a gas containing at least one type of hydrocarbon. The hydrocarbon-containing gas is not particularly limited, but is preferably a combustion gas such as exhaust gas from a typical internal combustion engine, such as an automobile engine.

The hydrocarbons contained in the hydrocarbon-containing gas to be adsorbed are not particularly limited, but examples of those contained in the combustion gas include at least one of the group consisting of paraffins, olefins, and aromatic hydrocarbons. The hydrocarbons may have one or more carbon atoms, preferably between one and 15. Due to the strength of the interaction between the hydrocarbons and the zeolite, the hydrocarbon adsorbent of this embodiment has a high adsorption capacity for aromatic hydrocarbons, olefins, and paraffins, in that order.

Specific examples of hydrocarbons to be adsorbed include at least one selected from the group consisting of methane, ethane, ethylene, propylene, butane, linear paraffins with 5 or more carbon atoms, linear olefins with 5 or more carbon atoms, benzene, toluene, and xylene. These hydrocarbons are particularly abundant in combustion gases, and exhibit a strong electrical interaction with zeolite. Therefore, it is preferable to apply the hydrocarbon adsorbent of this embodiment to hydrocarbon-containing gases containing at least toluene as the hydrocarbon to be adsorbed.

The hydrocarbon-containing gas may further contain at least one member selected from the group consisting of carbon monoxide, carbon dioxide, hydrogen, oxygen, nitrogen, nitrogen oxides, sulfur oxides, and water.

The hydrocarbon adsorbent of this embodiment will be described in more detail in the following examples. However, this embodiment is not limited to these examples.

(Identification of crystal structure)
Using a general X-ray diffraction device (device name: UltimaIV Protectus, manufactured by Rigaku Corporation), XRD measurement of the sample was carried out under the following conditions.

Acceleration current/voltage: 40mA/40kV
Radiation source: CuKα radiation (λ=1.5405Å)
Measurement mode: Continuous scan
Scan condition: 40°/min
Measurement range: 2θ = 3° to 43°
Divergence vertical limit slit: 10 mm
Divergence/entrance slit: 1°
Receiving slit: open
Receiving solar slit: 5°
Detector: D/teX Ultra
The zeolite structure was identified by comparing the obtained XRD pattern with a reference pattern using a Ni filter.

(composition analysis)
A sample solution was prepared by dissolving the sample in a mixed aqueous solution of hydrofluoric acid and nitric acid. The sample solution was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a common ICP device (device name: OPTIMA 5300DV, manufactured by PerkinElmer). From the obtained measured values of Si, Al, Cu, and Ce, the _SiO2 / _{Al2O3 molar} ratio, Ce/Al molar ratio, and copper mass% of the sample were calculated.

Example 1
(Copper addition)
0.097 parts by mass of copper nitrate trihydrate and 0.38 parts by mass of pure water were added to 1 part by mass of FAU zeolite (product name: HSZ-341NHA, manufactured by Tosoh Corporation) and mixed, followed by drying in air at 110°C overnight. The dried FAU zeolite was calcined at 550°C for 2 hours in a stream of air with a water content of 0.1% by volume, thereby obtaining copper-containing FAU zeolite. The obtained copper-containing FAU zeolite had a _SiO2 / _{Al2O3 molar} ratio of 7.2 and a copper content of 2.42% by mass.

(Addition of rare earth (Ce))
To the copper-containing FAU zeolite (mass ratio: 1), cerium acetate (mass ratio: 0.0079) and pure water (mass ratio: 0.38) were added and mixed, and then dried overnight in air at 110°C. The dried FAU zeolite contained the entire amount of cerium acetate added. The dried FAU zeolite was further calcined at 550°C for 2 hours in a stream of air with a water content of 0.1% by volume to desorb acetate ions from the cerium acetate, thereby obtaining a copper- and cerium-containing FAU zeolite, which was used as the hydrocarbon adsorbent of Example 1. Calculated from the amount of cerium acetate added, the molar ratio of cerium to aluminum (Ce/Al molar ratio) in the resulting hydrocarbon adsorbent was 0.011.

Example 2
An adsorbent for hydrocarbons of Example 2 was obtained in the same manner as in Example 1, except that the mass ratio of the cerium acetate added in Example 1 was changed to 0.022.

Example 3
An adsorbent for hydrocarbons of Example 3 was obtained in the same manner as in Example 1, except that the mass ratio of the cerium acetate added in Example 1 was changed to 0.034.

Example 4
An adsorbent for hydrocarbons of Example 4 was obtained in the same manner as in Example 1, except that the mass ratio of the cerium acetate added in Example 1 was changed to 0.045.

Example 5
An adsorbent for hydrocarbons of Example 5 was obtained in the same manner as in Example 1, except that the mass ratio of cerium acetate added in Example 1 was changed to 0.067.

Comparative Example 1
An adsorbent for hydrocarbons of Comparative Example 1 was obtained in the same manner as in Example 1, except that cerium acetate was not added.

Comparative Example 2
An adsorbent for hydrocarbons of Comparative Example 2 was obtained in the same manner as in Example 1, except that the mass ratio of cerium acetate added in Example 1 was changed to 0.089.

The Ce/Al molar ratios for Examples 1 to 5 and Comparative Examples 1 and 2 are shown in Table 1.

Example 6
An adsorbent for hydrocarbons of Example 6 was obtained in the same manner as in Example 1, except that thulium acetate n-hydrate was added in a mass ratio of 0.046 to 1 mass of zeolite instead of cerium acetate.

Example 7
An adsorbent for hydrocarbons of Example 7 was obtained in the same manner as in Example 1, except that yttrium acetate tetrahydrate was added in a mass ratio of 0.042 to 1 mass of zeolite instead of cerium acetate.

The rare earth/Al molar ratios for Examples 6 and 7 are shown in Table 2.

(Preparation and pretreatment of measurement samples)
The hydrocarbon adsorption rates of the hydrocarbon adsorbents obtained in Examples 1 to 7 and Comparative Examples 1 and 2 were measured as follows.

The hydrocarbon adsorbents according to Examples 1 to 7 and Comparative Examples 1 and 2 were each pressure-molded and pulverized to form irregularly shaped compacts with an aggregate size of 20 to 30 mesh, and the resulting compacts were each used as measurement samples.

1 g of each measurement sample was packed into an atmospheric pressure fixed-bed flow reactor and treated at 500°C for 1 hour under nitrogen flow, then cooled to 50°C as a pretreatment.

(Measurement of Hydrocarbon Adsorption Rate)
A hydrocarbon-containing gas was passed through each of the hydrocarbon adsorbents that had been pretreated as described above, and the amount of hydrocarbon adsorbed was measured between 50° C. and 200° C., and this was taken as the amount of hydrocarbon adsorption. The composition of the hydrocarbon-containing gas and the measurement conditions are shown below.

Hydrocarbon-containing gas: toluene 3000 ppmC by volume (methane equivalent concentration)
Water 3% by volume
Nitrogen balance Gas flow rate: 200 mL/min Measurement temperature: 50 to 600°C
Heating rate: 10°C/min. Hydrocarbons in the gas after passing through the hydrocarbon adsorbent were continuously quantitatively analyzed using a hydrogen ionization detector (FID). The hydrocarbon concentration (methane equivalent concentration; hereinafter referred to as "inlet concentration") of the hydrocarbon-containing gas at the inlet side of the atmospheric pressure fixed-bed flow-type reactor and the hydrocarbon concentration (methane equivalent concentration; hereinafter referred to as "outlet concentration") of the hydrocarbon-containing gas at the outlet side of the atmospheric pressure fixed-bed flow-type reactor were measured. The ratio of the integral of the outlet concentration (methane equivalent concentration) to the integral of the inlet concentration was calculated as the hydrocarbon adsorption efficiency (also referred to as "hydrocarbon purification efficiency").

(Hydrothermal durability test)
The hydrocarbon adsorbent was treated by hydrothermal treatment in the same manner as in (Measurement of hydrocarbon adsorption rate), except that the treatment gas was passed through the pretreated hydrocarbon adsorbent under the following conditions.

Treated gas: 10% water by volume
Nitrogen balance Gas flow rate: 300 mL/min Space velocity: 6000 hr ^-1
Processing temperature: 900°C
Treatment time: 1 hour The hydrocarbon adsorption rates of the hydrocarbons of each measurement sample after the hydrothermal treatment are shown in Table 1. The difference between the hydrocarbon adsorption rate of each example and the hydrocarbon adsorption rate of Comparative Example 1 (hydrocarbon adsorption rate of each example - hydrocarbon adsorption rate of Comparative Example 1) is shown in the graph of Figure 1.

As shown in Table 1, the hydrocarbon adsorbents of Examples 1 to 5 exhibit hydrocarbon adsorption rates up to higher temperatures than the hydrocarbon adsorbents of Comparative Examples 1 and 2.

Figure 1 shows that the hydrocarbon adsorption rate reaches a maximum when the Ce/Al molar ratio is near 0.04, suggesting that there is a preferred range of Ce/Al molar ratios. Furthermore, Examples 1 to 5, in which the Ce/Al molar ratio is 0.005 or greater and 0.08 or less, exhibited higher hydrocarbon adsorption rates than Comparative Example 1, which contains only Cu and no Ce, and Comparative Example 2, in which the Ce/Al molar ratio was 0.106.

Table 2 shows the hydrocarbon adsorption rates after hydrothermal treatment for Example 6, which contains thulium, and Example 7, which contains yttrium, both of which have the same rare earth/Al molar ratio as Example 3.

As shown in Table 2, the hydrocarbon adsorbents of Examples 6 and 7, like those of Examples 1 to 5, maintain their hydrocarbon adsorption rates at higher temperatures than the hydrocarbon adsorbent of Comparative Example 1. Because Ce, Tm, and Y are rare earth elements with similar chemical properties, it is believed that the addition of rare earth elements will produce similar effects.

Claims (6)

A hydrocarbon adsorbent comprising a zeolite represented by YO ₂ -X ₂ O ₃ and containing copper and rare earth elements,
Y is Si and X is Al ;
The zeolite has an FAU structure,
The copper content is 0.5% by mass or more and 4% by mass or less relative to 100% by mass of the zeolite,
a molar ratio of the rare earth to X is 0.005 or more and 0.1 or less,
A hydrocarbon adsorbent that adsorbs at least one of the group consisting of paraffins, olefins, and aromatic hydrocarbons . The hydrocarbon adsorbent according to claim 1, wherein the molar ratio of _{YO2 to X2O3} ( _YO2 / _{X2O3 ratio} ) of the zeolite is 2 or more and 100 or less. The hydrocarbon adsorbent according to claim 1 or 2, wherein the rare earth is one or more elements selected from the group consisting of scandium, yttrium, lanthanum, cerium, neodymium, samarium, gadolinium, terbium, thulium, ytterbium, and lutetium. 4. The hydrocarbon adsorbent according to claim 1, wherein the rare earth element is at least one of cerium and thulium. 5. The adsorbent for hydrocarbons according to claim 1, wherein the molar ratio of said rare earth to said X is 0.03 or more and 0.06 or less. A method for adsorbing at least one of the group consisting of paraffins, olefins, and aromatic hydrocarbons , using the hydrocarbon adsorbent according to any one of claims 1 to 5.