Molecular sieve abstract
The present invention provies an adsorbent which, as compared with
an adsorbent comprising a zeolite (an aluminosilicate) of at least
about equal Si/(Fe+Al) molar ratio (Fe is substantially absent),
has high heat resistance and high hydrothermal resistance and can
maintain adsorption capability even when exposed to high temperatures.
This adsorbent comprises a .beta.-silicate molecular sieve containing
at least Fe besides H, O and Si and, when Al is contained, containing
Fe in a molar amount at least equal to that of Al.
Molecular sieve claims
What is claimed is:
1. An adsorbent comprising a .beta.-zeolite molecular sieve, which
.beta.-zeolite consists essentially of Fe, H, O, Si, and Al with
the Fe in a molar amount at least equal to that of Al and the molar
ratio of Si/Al being 80 or more.
2. An automotive exhaust gas purification system having therein
the adsorbent of claim 1.
Molecular sieve description
TECHNICAL FIELD
The present invention relates to an adsorbent suitably used for
hydrocarbon adsorption and purification of the exhaust gas emitted
from an automobile.
BACKGROUND ART
Catalysts used for purification of the exhaust gas emitted from
automobiles, etc. must be heated to a desired temperature or higher
by, for example, the heat of exhaust gas, in order to exhibit their
catalytic activities. Therefore, when the temperature of exhaust
gas is low as experienced during the cold start of engine, the harmful
substances (HC, CO and NOx) in exhaust gas are hardly purified.
HC, in particular, is discharged in a large amount during the cold
start, and its purification is an important task to be achieved.
To improve the efficiency of HC purification during the cold start,
a technique is known which comprises using, as a HC adsorbent, a
molecular sieve made of a crystalline aluminosilicate (e.g. a zeolite)
and allowing the adsorbent to adsorb and store HC until a catalyst
reaches its operating temperature.
For example, an apparatus for purification of automobile exhaust
gas, using Y zeolite or mordenite as a HC adsorbent is disclosed
in Japanese Patent Application Kokai (Laid-Open) No. 75327/1990.
Also in Japanese Patent Application Kokai (Laid-Open) No. 293519/1992
is asserted use of an adsorbent obtained by subjecting H.sup.+ /ZSM-5
zeolite to ion exchange with Cu and Pd, in order to alleviate the
adverse effect by water adsorption and achieve higher HC adsorption
capability and wider temperature range allowing for HC adsorption.
For the same purpose, use, as an adsorbent, of a pentasil type metalosilicate
subjected to ion exchange with H, Cu or Pd is proposed in Japanese
Patent Application Kokai (Laid-Open) No. 63392/1994.
Zeolites, etc. heretofore used as a HC adsorbent, however, are
inferior in heat resistance, particularly in heat resistance in
a moisture-containing atmosphere (e.g. an automobile exhaust gas),
i.e. hydrothermal resistance, as compared with alumina, etc. used
as a carrier of ordinary catalyst for automobile exhaust gas purification;
therefore, there has been a fear that they deteriorate when used
at high exhaust gas temperatures such as experienced in continuous
high speed driving of automobile.
Use of zeolite particularly in an in-line type exhaust gas purification
system (which uses no bypass for high temperature exhaust gas and
has a simple structure) has been fairly severe to ordinary zeolite.
The present invention has been completed in view of the above-mentioned
problem of the prior art. The object of the present invention is
to provide an adsorbent suitably usable in applications where the
adsorbent must have high heat resistance and high hydrothermal resistance,
for example, a HC adsorbent used in a system for hydrocarbon adsorption
or for purification of exhaust gas from internal combustion engine,
such as an in-line type system for exhaust gas purification.
DISCLOSURE OF THE INVENTION
According to the present invention, there is provided an adsorbent
comprising a .beta.-silicate molecular sieve, which silicate contains
at least Fe besides H, O and Si and, when Al is contained, contains
Fe in a molar amount at least equal to that of Al.
Preferably, the .beta.-silicate molecular sieve has a Si/Al molar
ratio of 80 or more.
The adsorbent of the present invention is suitably used for adsorption
of hydrocarbons and for purification of automobile exhaust gas.
BRIEF DESCRIPTION OF DRAWING
FIG. 1 is a drawing showing the constitution of the testing apparatus
used for measurement of hydrocarbon adsorption capability.
BEST MODE FOR CARRYING OUT THE INVENTION
The adsorbent of the present invention comprises a .beta.-silicate
molecular sieve which contains at least Fe besides H, O and Si and,
when Al is contained, contains Fe in a molar amount at least equal
to that of Al.
Comprising the above .beta.-silicate molecular sieve, the present
adsorbent, as compared with an adsorbent comprising a zeolite (an
aluminosilicate) of at least about equal Si/(Fe+Al) molar ratio
(Fe is substantially absent), has high heat resistance and high
hydrothermal resistance and can maintain adsorption capability even
when exposed to high temperatures.
Therefore, the present adsorbent can be used suitably as a HC adsorbent
of a system for hydrocarbon adsorption and purification of exhaust
gas from internal combustion engine, for example, an in-line type
exhaust gas purification system.
When the adsorbent of the present invention is used for purification
of exhaust gas from internal combustion engine, it is preferable
that the .beta.-silicate molecular sieve is used by being loaded
on a carrier such as honeycomb structure or the like.
The .beta.-silicate molecular sieve used in the present adsorbent
has a .beta. type zeolite structure and contains larger pores than
pentasil type silicate; therefore, it can adsorb even hydrocarbons
of large molecules and moreover has a larger adsorption capacity.
Generally in pores of one-dimensional structure (having no branching),
when two places of one pore are plugged by coking, the portion of
the pore between the two places does not contribute to adsorption.
In the .beta.-silicate molecular sieve used in the present adsorbent,
however, the pore structure is two-dimensional or three-dimensional
and significant reduction in adsorptivity can be prevented.
Zeolite refers to an inorganic crystalline molecular sieve having
uniform pores, made of aluminosilicate (Al silicate), in particular.
Part or the whole of the Al can be substituted by other particular
element.
In the present invention, Fe is allowed to be present in a zeolite
mainly by adding a Fe compound (or Fe simple substance) to raw materials
and then conducting zeolite synthesis.
This method makes it possible to synthesize a .beta.-silicate molecular
sieve which is a zeolite of relatively uniform composition.
In the .beta.-silicate molecular sieve used in the present adsorbent,
not only Fe is contained but also a Si/Al molar ratio and a Si/Fe
molar ratio have been specified based on the study made on the effect
of these molar ratios on the hydrothermal resistance, etc. of the
molecular sieve.
The effect becomes noticeable from around the contents at which
the Si/Al molar ratio and the Si/Fe molar ratio become equal. In
order to allow the .beta.-silicate molecular sieve to have an hydrothermal
resistance required for a HC adsorbent for purification of exhaust
gas from internal combustion engine (such an adsorbent is the primary
usage of the present invention), the .beta.-silicate molecular sieve
preferably has a Si/Al molar ratio of 80 or more and a Si/Fe molar
ratio of 80 or less.
Since a lower Al content generally gives higher hydrothermal resistance,
it is preferable that the Si/Al molar ratio is 80 or more and is
as large as possible.
Below is described the merits and effects brought about by the
presence of Fe in the .beta.-silicate molecular sieve of the present
adsorbent.
The Fe used in the present invention has a valency of 3 (same as
Al) or 4 (same as Si) which is most stable at room temperature,
in the form of oxide; has an ionic radius relatively close to that
of Al; and can be easily substituted for Al.
Nevertheless, the Fe, as compared with Al, tends to be released
from the crystal lattice of .beta.-silicate molecular sieve.
However, the Fe, even when released, does not substantially promote
destruction of zeolite structure, which is different from Al.
Rather, it is likely that the Fe released from the lattice remains
in the particles, functions as an adhesive, and prevents the crystal
structure from being destroyed.
Further, it is presumed that the Fe remaining in the particles
makes a solid solution with the Al released from the lattice, prevents
the movement of Al, and prevents the destruction of crystal structure.
As is clear from the above, the .beta.-silicate molecular sieve
used as the adsorbent of the present invention, as compared with
conventional silicate molecular sieves, is improved in heat resistance
and hydrothermal resistance of structure and adsorption capacity.
EXAMPLES
The present invention is described in detail below by way of Examples.
However, the present invention is in no way restricted to these
Examples.
Examples 1 to 3 and Comparative Examples 1 to 3
There were mixed desired amounts of fumed silica (SiO.sub.2 :>99.9
wt. %), NaOH, Al(NO.sub.3).sub.3.9H.sub.2 O, Fe(NO.sub.3).sub.3.9H.sub.2
O, NaCl, TEAOH [=(C.sub.2 H.sub.5).sub.4 NOH] and water, to prepare
a raw material gel. The gel was placed in a fluororesin container,
transferred into in an autoclave, heated therein to 135.degree.
C. in 1 hour under an autogenic pressure, kept at 135.degree. C.
for 144 hours, and allowed to cool to synthesize a .beta.-(Fe,Al)silicate.
The silicate was subjected to water washing, drying, calcination,
ion exchange, drying and calcination to obtain H.sup.+ /.beta.-(Fe,Al)silicates
of Examples 1 to 3 and Comparative Examples 1 to 2.
In Comparative Example 3 was used a H.sup.+ /type .beta.-zeolite
having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of about 110 produced
by THE PQ CORPORATION.
The chemical analysis results of the above silicates and zeolite
are shown in Table 1.
TABLE 1 Si/Fe Si/Al Na.sub.2 O Specific surface molar ratio molar
ratio (wt. %) area (m.sup.2 /g) Example 1 17 97 <0.01 575 Example
2 16 135 <0.01 580 Example 3 18 83 <0.01 573 Comparative 19
55 <0.01 571 Example 1 Comparative 18 76 <0.01 574 Example
2 Comparative >2500 57 0.02 574 Example 3
Comparison of Hydrocarbon Adsorption Capability Before and After
Durability Test
(1) Production of Adsorbents for Durability Test
To each of the powders of the above 6 kinds of zeolites of Examples
1 to 3 and Comparative Examples 1 to 3 were added water and an alumina
sol having an alumina content of 2.5% by weight. Each mixture was
pulverized in a ball mill for 20 hours to prepare 6 different slurries.
In each slurry was dipped a mini honeycomb structure (diameter:
25.4 mm, length: 50.8 mm, apparent volume: about 26 cc) molded from
a cordierite honeycomb structure produced by NGK Insulators, Ltd.
(square cells, cell density: 400 cells/in..sup.2 rib thickness:
6 mil), followed by drying. This dipping and drying was repeated
to conduct wash coating so that the amount of the slurry loaded
became 0.16 g/cc. The resulting material was dried sufficiently
and calcinated in air at 550.degree. C. for 1 hour to produce adsorbents
for durability test. The number of adsorbents produced was 3 for
each zeolite, and one of the three was not subjected to any durability
test and the remaining two were subjected to the following durability
test. Then, the three adsorbents were measured for hydrocarbon adsorption
capability.
(2) Durability Test Using Engine Exhaust Gas
The adsorbent produced above was disposed in the exhaust gas line
of an internal combustion engine (an in-line four-cylinder type
gasoline engine of 2.0 L displacement), and the engine was operated
for 100 hours while the exhaust gas temperature at the inlet of
the adsorbent was controlled to become 850.degree. C. This test
was conducted two times for each kind of adsorbent, that is, two
adsorbents of each kind were used for the test.
(3) Measurement of Hydrocarbon Adsorption Capability
The hydrocarbon adsorption capability of each adsorbent before
and after the above durability test were measured as follows. A
testing apparatus shown in FIG. 1 was used. A gas simulating the
exhaust gas emitted from an internal combustion engine during the
cold start, which consisted of 16 volume % of CO.sub.2 10 volume
% of H.sub.2 O, 0.77 volume % of O.sub.2 2 volume % of CO, 0.33
volume % of H.sub.2 2000 ppm (by volume) of NO, 5000 ppm (by
volume) of hydrocarbons (toluene) (this value is obtained by multiplying
the carbon number of the hydrocarbon by the volume of the hydrocarbon)
and the remainder of N.sub.2 was passed through an adsorbent to
measure hydrocarbon adsorption capability at a rate of 17 NL/min;
the amount of hydrocarbons in the gas after having been passed through
the adsorbent was measured for 150 seconds from the start of gas
passing; thereby, the adsorption efficiency (%) of the adsorbent
was determined. The gas temperature at the inlet of adsorbent was
controlled at 60.degree. C., 100.degree. C., 140.degree. C. or 180.degree.
C.
The adsorption efficiency (%) was calculated from the following
equation. The results obtained are shown in Table 2.
Adsorption efficiency (%)=[(B-A)/B].times.100 wherein
A: the amount of hydrocarbons measured using an adsorbent formed
by loading a zeolite on a honeycomb structure, and
B: the amount of hydrocarbons measured using a honeycomb structure
loading no zeolite thereon.
TABLE 2 Adsorption efficiency (%) at adsorbent inlet Durability
gas temperature of: test 60.degree. C. 100.degree. C. 140.degree.
C. 180.degree. C. Example 1 Before 96 93 60 30 After 91 71 20 6
Example 2 Before 95 94 64 33 After 91 75 30 10 Example 3 Before
93 90 53 24 After 91 68 19 6 Comparative Before 93 88 52 23 Example
1 After 86 60 22 7 Comparative Before 94 92 54 25 Example 2 After
88 62 18 5 Comparative Before 92 82 46 18 Example 3 After 85 59
20 6
INDUSTRIAL APPLICABILITY
As described above, the adsorbent of the present invention, as
compared with an adsorbent comprising a zeolite (an aluminosilicate)
of at least about equal Si/(Fe+Al) molar ratio (Fe is substantially
absent), has high heat resistance and high hydrothermal resistance
and can maintain adsorption capability even when exposed to high
temperatures.
Therefore, the present adsorbent is suitable as a hydrocarbon adsorbent
used in a system for hydrocarbon adsorption or for purification
of exhaust gas from internal combustion engine, for example, an
in-line type system for exhaust gas purification. |