Highly heat resistant &bgr;-zeolite and absorbent for...

Chemistry of inorganic compounds – Zeolite – With change of synthesized zeolite morphology

Reexamination Certificate

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C423SDIG002, C502S064000

Reexamination Certificate

active

06294150

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a highly heat-resistant and hydrothermal-resistant &bgr;-zeolite and an adsorbent for automobile exhaust gas purification using the &bgr;-zeolite.
2. Description of the Related Art
In order for a catalyst (used for purification of the exhaust gas emitted from an automobile or the like) to exhibit its catalytic activity, the catalyst must be heated to an activation temperature or higher by, for example, the heat of the exhaust gas. When the temperature of the exhaust gas is low as in the cold start of an engine, the harmful substances in the exhaust gas, such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) are hardly purified. The purification of, in particular, HC which is discharged in a large amount during the cold start of an engine, is an important task to be achieved.
In order to improve the efficiency of HC purification during the cold start, there are known techniques of using, as an HC adsorbent, a molecular sieve made of a crystalline aluminosilicate such as zeolite or the like and allowing the adsorbent to adsorb HC while a catalyst used together reaches its activation temperature.
For example, in Japanese Patent Application Kokai (Laid-Open) No. 75327/1990 is disclosed an apparatus for automobile exhaust gas purification, using a Y type zeolite or mordenite as an HC adsorbent. Also in Japanese Patent Application Kokai (Laid-Open) No. 293519/1992 is disclosed use of an HC adsorbent obtained by subjecting a H
+
/ZSM-5 zeolite to ion exchange with Cu and Pd, in order to alleviate the adverse effect of water adsorption, attain improved HC adsorption capability, and widen the temperature range for HC adsorption. For the same purpose, use, as an HC adsorbent, of a pentasil type metallosilicate subjected to ion exchange with H, Cu or Pd is proposed in Japanese Patent Application Kokai (Laid-Open) No. 63392/1994.
Further in Japanese Patent Application Kokai (Laid-Open) No. 99217/1997 is proposed use, as an HC adsorbent, of an H
+
/&bgr;-zeolite having a SiO
2
/Al
2
O
3
ratio of 100 or more, superior in HC adsorption capability and capable of maintaining its pore structure even when subjected to an exhaust gas of 750° C. or higher.
The SiO
2
/Al
2
O
3
ratio indicative of the composition of zeolite framework and/or the ion (e.g. H
+
, Na
+
or Cu
2+
) presence close to the Al of zeolite framework for electric charge compensation, which as the yardstick for the heat resistance of zeolite, have heretofore been used mainly.
It was discovered, however, that two zeolites having the same SiO
2
/Al
2
O
3
ratio or the same ion have greatly different heat resistances when they are produced from different raw materials or different processes.
When a zeolite insufficient in heat resistance, particularly hydrothermal resistance (e.g. heat resistance in automobile exhaust gas) is used for purification of the exhaust gas emitted from an internal combustion engine of an automobile or the like, the pore structure of the zeolite is gradually collapsed; therefore, such it is found that a zeolite will deteriorate in purifiability when used in a high temperature exhaust gas such as emitted during continuous high-speed engine operation or the like.
Also, when such a zeolite is used in a catalyst and subjected to a heating and regeneration treatment for removal of formed coke or the like, there have been cases that the pore structure of the zeolite is impaired.
Thus, it has been a task how to define a highly heat-resistant and hydrothermal-resistant zeolite by its properties and produce such a zeolite.
SUMMARY OF THE INVENTION
In view of the above-mentioned problems of the prior art, the present inventors made an intensive study on the properties of zeolite relating to the heat resistance of zeolite. As a result, the present inventors discovered that the heat resistance of a zeolite has close relations with the average particle diameter, particle size distribution and particle shape of particles constituting the zeolite, and/or with the crystal structure of the zeolite. The finding has led to the completion of the present invention.
According to the present invention, there is provided, as a first invention, a highly heat-resistant &bgr;-zeolite having a SiO
2
/Al
2
O
3
ratio of 80 or more, which is constituted by primary particles having an average particle diameter of 30 nm or more.
The highly heat-resistant &bgr;-zeolite is preferably constituted by primary particles having an average particle diameter of 50 nm or more, more preferably by primary particles having an average particle diameter of 80 nm or more.
According to the present invention, there is also provided, as a second invention, a highly heat-resistant &bgr;-zeolite having a SiO
2
/Al
2
O
3
ratio of 80 or more, which is constituted by primary particles having such a particle size distribution that the 10% particle diameter is 20 nm or more.
The highly heat-resistant &bgr;-zeolite is preferably constituted by primary particles having such a particle size distribution that the 10% particle diameter is 40 nm or more.
According to the present invention, there is also provided, as a third invention, a highly heat-resistant &bgr;-zeolite having a SiO
2
/Al
2
O
3
ratio of 80 or more, which is constituted by primary particles having at least one angular portion.
According to the present invention, there is also provided, as a fourth invention, a highly heat-resistant &bgr;-zeolite having a SiO
2
/Al
2
O
3
ratio of 80 or more, which is constituted by particles giving a full width at half maximum (FWHM) in X-ray diffraction pattern at (h,k,l)=(3,0,2), of 2&thgr;=0.5° or smaller when the X-ray diffraction pattern is obtained from an X-ray diffractometer using Cu=K&agr; as the X-ray source.
Generally, a heat-resistant zeolite having crystallinity shows a sharp X-ray diffraction pattern. In the pattern, crystallinity was determined by the use of full width at half maximum of reflection peak for (h,k,l)=(3,0,2).
Preferably, the highly heat-resistant &bgr;-zeolite of the present invention has at least two elements of the above-mentioned particular primary particle diameter, particular particle size distribution, particular particle shape and particular X-ray diffraction pattern (crystal structure).
According to the present invention, there is also provided an adsorbent for automobile exhaust gas purification, comprising any of the above-mentioned highly heat-resistant &bgr;-zeolites.


REFERENCES:
patent: 5171556 (1992-12-01), Caullet et al.
patent: 5228979 (1993-07-01), Ward
patent: 5427765 (1995-06-01), Inoue et al.
patent: 5558851 (1996-09-01), Miller
patent: 5853566 (1998-12-01), Kraushaar-Czarnetzki
patent: 6004527 (1999-12-01), Murrell et al.
patent: 0 419 334 (1991-03-01), None
patent: 0 523 829 (1993-01-01), None
patent: 0 593 898 (1994-04-01), None
patent: 609 304-B (1994-08-01), None
patent: 0 614 853 (1994-09-01), None
patent: 0 854 114 (1998-07-01), None
patent: 2-75327 (1990-03-01), None
patent: 4-293519 (1992-10-01), None
patent: 6-63392 (1994-03-01), None
patent: 9-99217 (1997-04-01), None
patent: 9-175818 (1997-07-01), None
patent: 94/11623 (1994-05-01), None
patent: 94/26663 (1994-11-01), None
Culity, Elements of X-Ray Diffraction, 1978, p. 100.

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