Molecular sieve abstract
A process for synthesizing a high-silica aluminosilicate molecular
sieve, includes: subjecting, to hydrothermal synthesis, a raw material
containing Al, H, O and Si and, as other elements, at least Fe and
having a Si/Al molar ratio of 50 or more and a Si/Fe molar ratio
of 80 or less (45 or less when a high-silica aluminosilicate molecular
sieve having a .beta. type zeolite structure is produced) and then
subjecting the resulting material to heat treatment in an oxidizing
atmosphere at 300.degree. C. or more. The above process can synthesize
a high-silica zeolite low in Al content directly without dealumination
operation, and can provide process simplification and a higher yield.
Molecular sieve claims
What is claimed is:
1. A process for synthesizing a high-silica aluminosilicate molecular
sieve having a .beta. type zeolite structure, which comprises subjecting,
to hydrothermal synthesis, a raw material containing Al, H, O and
Si and, as other elements at least Fe, and having a Si/Al molar
ratio of 50 or more and a Si/Fe molar ratio of 45 or less and then
subjecting the resulting material to heat treatment in an oxidizing
atmosphere at 300.degree. C. or more, wherein the Si/Al and Si/Fe
molar ratios are such that the resultant zeolite retains at least
60.9% of its surface area upon heating at 1000.degree. C. for 4
hours in an atmosphere containing 10% steam.
2. A process according to claim 1 wherein the raw material contains
a Na.sup.+ source.
3. A process according to claim 2 wherein the Na.sup.+ source
contains NaOH.
4. A process according to claim 2 wherein the Na.sup.+ source
contains NaOH with and without NaCl and the NaOH:NaCl molar ratio
is 3:4 to 7:0.
5. A process according to claim 1 wherein the heat treatment is
conducted in an oxidizing atmosphere at 650-1050.degree. C.
Molecular sieve description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a process for synthesizing a high-silica
aluminosilicate molecular sieve which can be preferably used as
a hydrocarbon adsorbent of exhaust gas purification system.
(2) Description of Related Art
In order for an exhaust gas purification system for an automobile
or the like to be able to exhibit its catalytic activity, the catalyst
used therein must be heated to at least its lowest operating temperature
by, for example, the heat of exhaust gas. Therefore, when the temperature
of exhaust gas is low as seen in the cold start of engine, the harmful
components (e.g. HC, CO and NOx) of exhaust gas are hardly purified.
HC, in particular, is generated in a large amount during the cold
start and its purification is an important task.
In order to improve the efficiency of HC purification during the
cold start, there have heretofore been known techniques of using,
as a HC adsorbent, a molecular sieve made of a crystalline aluminosilicate
(e.g. zeolite) and allowing the adsorbent to adsorb HC until the
exhaust gas purification system used reaches its operating temperature.
For example, Japanese Patent Application Laid-Open No. 75327/1990
discloses an exhaust gas purification system for an automobile,
using a Y type zeolite or mordenite as an HC adsorbent. Japanese
Patent Application Laid-Open No. 293519/1992 proposes use of an
adsorbent obtained by subjecting a H.sup.+ type ZSM-5 zeolite to
ion exchange with Cu and Pd, in order to alleviate the effect of
water adsorption, improve HC adsorbability and widen the temperature
range of adsorption. Japanese Patent Application Laid-Open No. 63392/1994
proposes, for the same purpose, use of an H-, Cu- or Pd-ion exchanged
pentasil type metallosilicate as an adsorbent.
With respect to the above pentasil (pentasil is a generic name
for structures similar to ZSM-5) type zeolite, a relatively high-silica
product can be synthesized easily and even a product having a Si/Al
molar ratio of substantially infinity (containing no Al) can be
synthesized by using a template.
However, direct synthesis of pentasil type zeolite of high Si/Al
molar ratio (>50) by using no template has heretofore been difficult.
Meanwhile, .beta. type zeolite and mordenite type zeolite are known
to have large lattice strain when containing no Al, as compared
with pentasil type zeolite. Therefore, in synthesizing .beta. type
zeolite and mordenite type zeolite, it has been necessary to add
Al (which has an ionic radius different from that of Si) in a certain
amount to alleviate said lattice strain. As a result, the .beta.
type zeolite and mordenite type zeolite synthesized had a Si/Al
molar ratio of about 50 or less.
While zeolites differ in heat resistance and hydrothermal resistance
depending upon the crystal structures, among zeolites of the same
crystal structure, a zeolite of higher Al content has lower heat
resistance and lower hydrothermal resistance.
Therefore, in order to allow a zeolite to have higher heat resistance
and higher hydrothermal resistance, it is necessary to reduce the
Al content in zeolite.
However, there has been no assurance that a zeolite having a pore
structure suited for HC molecules to be adsorbed can be synthesized
directly so as to have a low Al content at which desired heat resistance
can be obtained.
Hence, when the Al content is higher than desired, it has been
necessary to remove excessive Al (dealumination operation) by steam
treatment, acid treatment or the like and repeat the operation until
a desired Al content is obtained.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned
problems, and an object of the present invention is to provide a
process for making a high-silica silicate molecular sieve by directly
synthesizing a low-Al content zeolite without a dealumination operation,
which process can provide process simplification and a higher yield.
The high-silica aluminosilicate molecular sieve synthesized by
the present process can be preferably used in applications where
high heat resistance or high hydrothermal resistance is required,
for example, the HC adsorbent used in the exhaust gas purification
system (e.g. in-line type exhaust gas purification system) of an
internal combustion engine.
According to the present invention there can be provided a process
for directly synthesizing a high-silica aluminosilicate molecular
sieve, which comprises subjecting, to hydrothermal synthesis, a
raw material containing Al, H, O and Si and, as other elements,
at least Fe and having a Si/Al molar ratio of 50 or more and a Si/Fe
molar ratio of 80 or less and then subjecting the resulting material
to heat treatment in an oxidizing atmosphere at 300.degree. C. or
more.
According to the present invention there can also be provided a
process for synthesizing a high-silica aluminosilicate type molecular
sieve having a .beta. type zeolite structure, which comprises subjecting,
to hydrothermal synthesis, a raw material containing Al, H, O and
Si and, as other elements, at least Fe and having a Si/Al molar
ratio of 50 or more and a Si/Fe molar ratio of 45 or less and then
subjecting the resulting material to heat treatment in an oxidizing
atmosphere at 300.degree. C. or more.
In the present process, it is preferable that the raw material
contains a Na.sup.+ source and the Na.sup.+ source contains NaOH;
and it is also preferable that the Na.sup.+ source contains NaOH
and NaCl and the NaOH:NaCl molar ratio is 3:4 to 7:0.
In the present process, it is preferable to conduct the heat treatment
in an oxidizing atmosphere at 650-1050.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
The process for synthesizing a high-silica silicate molecular sieve
according to the present invention comprises subjecting, to hydrothermal
synthesis, a raw material containing Al, H, O and Si and, as other
elements, at least Fe and having a Si/Al molar ratio of 50 or more
and a Si/Fe molar ratio of 80 or less and then subjecting the resulting
material to heat treatment in an oxidizing atmosphere at 300.degree.
C. or more, preferably 650-1050.degree. C.
Herein, "hydrothermal synthesis" refers to an operation
in which a raw material is heated to room temperature or higher
in the presence of water or steam. It is not restricted to an operation
in which an ordinary aqueous gel is heated in an autoclave.
The raw material may take various forms such as liquid, gas, solid,
solution, colloid and the like.
The pressure employed may be atmospheric pressure or reduced pressure
as in chemical vapor deposition.
Zeolite refers to an inorganic molecular sieve having a regular
pore structure, made of aluminosilicate, in particular. Part or
the whole of the Al atoms present in the zeolite framework may be
substituted by particular elements.
In the present invention, a Fe compound or Fe itself is added to
a raw material to synthesize a Fe-containing zeolite.
In this way, there can be synthesized a high-silica aluminosilicate
molecular sieve which is a zeolite of relatively uniform composition.
In zeolite synthesis, since optimum synthesis conditions need be
employed depending upon the addition amount of Fe compound or Fe
itself and the intended pore structure of zeolite to be synthesized,
it is preferable to set up the synthesis conditions for said zeolite
beforehand by using at least one raw material composition.
It is thought that by including Fe in the crystal structure of
zeolite, the strain between crystal lattices is alleviated and crystallization
of zeolite becomes easy; as a result, a zeolite of lower Al content
than usual can be synthesized directly with relative ease.
Thereby, the control of Al content in zeolite becomes easy and
a molecular sieve having properties not seen heretofore can be synthesized.
Further in the present invention, in-depth study was made to examine
the effects of Si/Al molar ratio, Si/Fe molar ratio and Na compound
(kind and amount) of raw material, on the specific surface area,
hydrothermal resistance, etc. of aluminosilicate molecular sieve
synthesized; and the levels of Si/Al molar ratio and Si/Fe molar
ratio and the kind and amount of Na compound were specified.
In order for the high-silica aluminosilicate molecular sieve produced
by the present process to have a hydrothermal resistance required
for the HC adsorbent used for purification of exhaust gas from internal
combustion engine (the HC adsorbent is a primary use of the molecular
sieve), it is preferable to use, in the present process, a raw material
having a Si/Al molar ratio of 50 or more and a Si/Fe molar ratio
of 80 or less.
Since a lower Al content generally gives a higher hydrothermal
resistance, the Si/Al molar ratio of raw material is preferably
50 or more and yet as large as possible.
In many zeolites such as .beta. type and the like, the presence
of Fe in a certain amount or more is thought to be necessary to
alleviate the lattice strain during crystallization due to the low
content of Al and produce an intended pore structure. Fe shortage
invites non-crystallization of corresponding amount of Si and a
reduced yield.
Hence, the Si/Fe molar ratio of raw material is preferably 80 or
less. When the pore structure is, in particular, .beta. type, the
Si/Fe molar ratio is preferably 45 or less, more preferably 30 or
less, particularly preferably 20 or less.
Preferably, the raw material further contains a Na.sup.+ source.
The Na.sup.+ source preferably contains NaOH, or NaOH and NaCl.
In the latter case, the NaOH:NaCl molar ratio is preferably 3:4
to 7:0 more preferably 4:3 to 7:0.
The merits and effects brought about by the presence of Fe in silicate
molecular sieve are as follows.
The Fe used in the present invention has a valency of 3 which gives
an oxide most stable at room temperature and which is the same as
that of Al. This Fe has an ionic radius relatively close to that
of Al and can substitute Al easily.
Nevertheless, Fe, as compared with Al, is easily eliminated from
the crystal lattices of aluminosilicate molecular sieve by calcination
or ion-exchange operation.
Even when Fe goes out of framework, however, Fe does not seem to
promote the destruction of zeolite structure, differently from Al.
Rather, the Fe out of the zeolite framework is presumed to stay
in the pores and function as an adhesive, thereby suppressing the
destruction of crystal structure.
It is further presumed that the Fe staying in the pores makes a
solid solution with eliminated Al, thereby preventing the movement
of Al and the destruction of crystal structure.
For the above reasons, in the present process, heat treatment is
conducted in an oxidizing atmosphere at 300.degree. C. or higher,
preferably 650-1050.degree. C. so that Fe goes out of the zeolite
framework, whereby a high-silica aluminosilicate molecular sieve
can be produced.
Further, a previous heat treatment in a temperature range in which
a product is actually used can contribute to stabilization of a
crystal structure of a high-silica aluminosilicate molecular sieve.
Thus, the present process for synthesis of a high-silica aluminosilicate
molecular sieve can directly synthesize a zeolite low in Al content
without dealumination operation and therefore can provide process
simplification and a higher yield.
Further, the high-silica aluminosilicate molecular sieve synthesized
by the present process can maintain its crystal structure up to
at least a high temperature to which a zeolite (aluminosilicate)
having about the same Si/Al molar ratio is stable. Therefore, the
present high-silica aluminosilicate molecular sieve can be preferably
used in applications where high heat resistance or high hydrothermal
resistance is required, for example, the HC adsorbent used in the
exhaust gas purification system (e.g. in-line type exhaust gas purification
system) of internal combustion engine.
The preferable pore structure of the high-silica aluminosilicate
molecular sieve synthesized by the present process is determined
in view of the following matters when the molecular sieve is used
particularly as an HC adsorbent for purification of exhaust gas
from internal combustion engine.
Incidentally, in this specification, types of zeolite are indicated
by typical names, which includes corresponding isomorphisms or the
like.
First, the pore structure preferably has a high framework density
[in the case of zeolites, the framework density is represented by
the number of T atoms (the atoms surrounded by oxygen atoms and
forming a TO.sub.4 type tetrahedron) in 1 nm.sup.3 ].
For example, pentasil type zeolite and ferrierite type zeolite
have high framework densities for the pore diameters.
When a molecular sieve is used for HC adsorption, the pore diameter
of the molecular sieve is required to have a size through which
the HC molecules to be adsorbed can enter the molecular sieve. The
size, however, is preferably close to the size of the HC molecules
to be adsorbed because it allows strong adsorption of HC molecules
by molecular sieve.
.beta. type zeolite and mordenite type zeolite have pores larger
than those of pentasil type zeolite and can adsorb even HC components
of larger molecular sizes.
.beta. type zeolite is known to have excellent adsorption capacity
for C.sub.6-8 aromatic hydrocarbons. Chabazite type and sigma-1
type having small pore diameters are suitable for adsorption for
C.sub.2-3 alkenes.
Coke formed from HC may deposit in the pores of molecular sieve.
When the pores are linear and have no branches and when the two
locations of one-dimensional pore are plugged by coke, the portion
of the pore between the two plugged portions has no adsorbability.
Therefore, the molecular sieve of the present invention preferably
has a two-dimensional or three-dimensional pore structure.
All of the above-mentioned pentasil type, ferrierite type, .beta.
type, mordenite type, chabazite type and sigma-1 type have a two-dimensional
or higher pore structure.
.beta. type is particularly preferable because it can adsorb even
larger HC components than pentasil type can, has a large HC adsorption
capacity, and can be produced so as to have sufficient heat resistance
by the present process.
The present invention is hereinafter described specifically by
way of Examples. However, the present invention is not restricted
to the Examples. |