Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs
Patent
1999-01-14
2000-08-01
Wood, Elizabeth D.
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
502 61, 502 64, 502 71, 502232, 502233, 502234, 502235, 502355, 502407, B01J 2906, B01J 2987
Patent
active
060966742
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a heat-resistant HC (hydrocarbon) adsorbent for use in an exhaust system in an automobile or the like.
BACKGROUND ART
Various configurations of exhaust emission control catalysts disposed in an exhaust system of an automobile are known. However, the usual exhaust emission control catalyst exhibits a purifying ability by oxidation in a high temperature range of an exhaust gas, e.g., about 180.degree. C. or more, and exhibits an extremely low purifying ability with respect to an exhaust gas having a low temperature immediately after an engine is started and containing HC at a high concentration.
Therefore, to catch HC in the low-temperature exhaust gas, it is contemplated that an HC adsorbent is disposed in the exhaust system downstream of the catalyst. In this case, zeolite is used as the HC adsorbent.
The temperature of the exhaust gas flowing in the exhaust system is generally on the order of 1,000.degree. C., and for this reason, a heat resistance accommodating such temperature is required for the HC adsorbent.
There has been proposed a zeolite having a high silica/alumina, ratio and containing a reduced amount of aluminum which becomes a starting point of thermal deterioration, in order to enhance the heat resistance. However, even the zeolite suffers from a problem that the heat resistance thereof is not improved sufficiently, and the HC adsorbing and desorbing ability is also low.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide an HC adsorbent of the above-described type, which has a sufficiently improved heat-resistance and a good HC adsorbing/desorbing ability.
To achieve the above object, according to the present invention, there is provided a heat-resistant HC adsorbent comprising an MFI-type metallo-silicate including at least one of gallium (Ga) and indium (In) as a framework forming element, and having aluminum (Al) in a content of Al.ltoreq.0.05% by weight.
The MFI-type metallo-silicate has a three-dimensional framework structure, representatively as does an MFI-type aluminosilicate, e.g., a ZSM-5 zeolite. The Ga site and In site in such structure correspond to the Al site of the ZSM-5 zeolite. Moreover, gallium (Ga) and indium (In) have a high bonding force in the three-dimensional framework structure and exhibit a characteristic that they are difficult to be released from the structure.
The HC adsorbent comprising the MFI-type metallo-silicate having such a three-dimensional framework structure and having the aluminum content set in the above-described range has a high heat-resistance and a good HC adsorbing/desorbing ability.
However, if the aluminum content is larger than 0.05% by weight, the heat resistance is lowered. In the MFI-type metallo-silicate, it is desirable from the viewpoint of enhancement in heat resistance that the aluminum content is zero. However, it is difficult to suppress the aluminum content to zero, because aluminum is contained as an impurity in a starting material in the synthesis of the MFI-type metallo-silicate. Therefore, it can be said that the lower limit of the aluminum content is a value extremely near zero.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the relationship between a gas temperature and an adsorption rate of C.sub.5 H.sub.10 as well as a desorption rate of C.sub.5 H.sub.10 in an MFI-type gallium-silicate in a new state or the like;
FIG. 2 is a graph showing the relationship between the gas temperature and the adsorption rate of C.sub.5 H.sub.10 as well as the desorption rate of C.sub.5 H.sub.10 in the MFI-type gallium-silicate after being subjected to an aging treatment or the like;
FIG. 3 is a graph showing the relationship between an aluminum content and the adsorption rate of C.sub.5 H.sub.10 for one minute in the MFI-type gallium-silicate;
FIG. 4 is a graph showing the relationship between the gas temperature and the adsorption rate of C.sub.5 H.sub.10 as well as the desorption rate of C.sub.5 H.sub.10 in an MFI-type indium-silicate
REFERENCES:
patent: 5010048 (1991-04-01), Petit et al.
patent: 5281566 (1994-01-01), Marcilly et al.
patent: 5306684 (1994-04-01), Itoh et al.
patent: 5354719 (1994-10-01), Gabelica et al.
patent: 5741948 (1998-04-01), Kirishiki et al.
Agemoto Kenji
Endo Tetsuo
Fujisawa Yoshikazu
Mita Muneo
Naka Takahiro
Honda Giken Kogyo Kabushiki Kaisha
Nippon Chemical Industrial Co. Ltd.
Wood Elizabeth D.
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