Molecular sieve abstract
This invention relates to a process for treating an exhaust gas
stream from an engine, especially during cold start. The process
involves a molecular sieve bed over which the cold exhaust is flowed
before flowing over a catalyst bed. Pollutants such as hydrocarbons
are adsorbed on the molecular sieve bed. When the molecular sieve
bed reaches a temperature of about 150.degree. C., the pollutants
are desorbed from the adsorbent bed and converted by the catalyst
to innocuous compounds. The molecular sieves used in the process
are those that selectively adsorbs pollutants, e.g., hydrocarbons
versus water, and are hydrothermally stable.
Molecular sieve claims
I claim as my invention:
1. A process for treating a cold-start engine exhaust gas stream
containing hydrocarbons and other pollutants consisting of flowing
said engine exhaust gas stream over a molecular sieve bed which
preferentially adsorbs the hydrocarbons over water to provide a
first exhaust stream, and flowing the first exhaust gas stream over
a catalyst to convert any residual hydrocarbons and other pollutants
contained in the first exhaust gas stream to innocuous products
and provide a treated exhaust stream and discharging the treated
exhaust stream into the atmosphere, the molecular sieve bed characterized
in that it comprises at least one molecular sieve selected from
the group consisting of molecular seives which have: 1) a framework
Si:Al ratio of at least 2.4; 2) are hydrothermally stable; and 3)
have a hydrocarbon selectivity (.sup..alpha. HC-H.sub.2 O) greater
than 1 where .sup..alpha. HC-H.sub.2 O is defined by the following
equation: ##EQU3## where X.sub.HC is the hydrocarbon co-loading
on the molecular sieves in equilibrium with the hydrocarbon water
vapor mixture in the gas phase over the molecular sieve adsorbent;
X.sub.H.sbsb.2.sub.O is the water co-loading on the molecular sieve
in equilibrium with the water and hydrocarbon vapor mixture in the
gas phase over the molecular sieve adsorbent; [H.sub.2 O] is the
concentration of water and [HC] is the concentration of hydrocarbon.
2. The process of claim 1 where the molecular sieve is selected
from the group consisting of silicalite, faujasite, clinoptilolites,
mordenites, chabazite, zeolite ultrastable Y, zeolite Y, ZSM-5 and
mixtures thereof.
3. The process of claim 2 where the molecular sieve is faujasite.
4. The process of claim 2 where the molecular sieve is zeolite
ultrastable Y.
5. The process of claim 1 wherein the molecular sieve bed is a
honeycomb monolithic carrier having deposited thereon a molecular
sieve.
6. The process of claim 1 where the engine is an internal combustion
engine.
7. The process of claim 6 where the internal combustion engine
is an automobile engine.
8. The process of claim 1 where the engine is fueled by a hydrocarbonaceous
fuel.
9. The process of claim 8 where the fuel is an alcohol.
10. The process of claim 8 where the fuel is a hydrocarbon.
11. The process of claim 1 where the molecular sieve has deposited
thereon a metal selected from the group consisting of platinum,
palladium, rhodium, ruthenium and mixtures thereof.
12. The process of claim 11 where the metal is platinum.
13. The process of claim 11 where the metal is palladium.
14. The process of claim 11 where the metal is a mixture of platinum
and palladium.
Molecular sieve description
BACKGROUND OF THE INVENTION
Gaseous waste products resulting from the combustion of hydrocarbonaceous
fuels, such as gasoline and fuel oils, comprise carbon monoxide,
hydrocarbons and nitrogen oxides as products of combustion or incomplete
combustion, and pose a serious health problem with respect to pollution
of the atmosphere. While exhaust gases from other carbonaceous fuel-burning
sources, such as stationary engines, industrial furnaces, etc.,
contribute substantially to air pollution, the exhaust gases from
automotive engines are a principal source of pollution. Because
of these health problem concerns, the Environmental Protection Agency
(EPA) has promulgated strict controls on the amounts of carbon monoxide,
hydrocarbons and nitrogen oxides which automobiles can emit. The
implementation of these controls has resulted in the use of catalytic
converters to reduce the amount of pollutants emitted from automobiles.
In order to achieve the simultaneous conversion of carbon monoxide,
hydrocarbon and nitrogen oxide pollutants, it has become the practice
to employ catalysts in conjunction with air-to-fuel ratio control
means which functions in response to a feedback signal from an oxygen
sensor in the engine exhaust system. Although these three component
control catalysts work quite well after they have reached operating
temperature of about 300.degree. C., at lower temperatures they
are not able to convert substantial amounts of the pollutants. What
this means is that when an engine and in particular an automobile
engine is started up, the three component control catalyst is not
able to convert the hydrocarbons and other pollutants to innocuous
compounds. Despite this limitation, current state of the art catalysts
are able to meet the current emission standards. However, California
has recently set new hydrocarbon standards (these standards most
probably will be promulgated nationwide) which can not be met with
the current state of the art three component control catalysts.
Applicant has found a solution to this problem which involves the
use of an adsorbent bed to adsorb the hydrocarbons during the cold
start portion of the engine. Although the process will be exemplified
using hydrocarbons, the instant invention can also be used to treat
exhaust streams from alcohol fueled engines as will be shown in
detail. Applicant's invention involves placing an adsorbent bed
immediately before the catalyst. Thus, the exhaust stream is first
flowed through the adsorbent bed and then through the catalyst.
The adsorbent bed preferentially adsorbs hydrocarbons over water
under the conditions present in the exhaust stream. After a certain
amount of time, the adsorbent bed has reached a temperature (about
150.degree. C.) at which the bed is no longer able to remove hydrocarbons
from the exhaust stream. That is, hydrocarbons are actually desorbed
from the adsorbent bed instead of being adsorbed. This regenerates
the adsorbent bed so that it can adsorb hydrocarbons during a subsequent
cold start.
The adsorbents which may be used to adsorb the hydrocarbons may
be selected from the group consisting of molecular sieves which
have 1) a Si:Al ratio of at least 2.4; 2) are hydrothermally stable;
and 3) have a hydrocarbon selectivity greater than 1. Examples of
molecular sieves which meet these criteria are silicalite, faujasites,
clinoptilolites, mordenites and chabazite. The adsorbent bed may
be in any configuration with a preferred configuration being a honeycomb
monolithic carrier having deposited thereon the desired molecular
sieve.
The prior art reveals several references dealing with the use of
adsorbent beds to minimize hydrocarbon emissions during a cold start
engine operation. One such reference is U.S. Pat. No. 3699683
in which an adsorbent bed is placed after both a reducing catalyst
and an oxidizing catalyst. The patentees disclose that when the
exhaust gas stream is below 200.degree. C. the gas stream is flowed
through the reducing catalyst then through the oxidizing catalyst
and finally through the adsorbent bed, thereby adsorbing hydrocarbons
on the adsorbent bed. When the temperature goes above 200.degree.
C. the gas stream which is discharged from the oxidation catalyst
is divided into a major and minor portion, the major portion being
discharged directly into the atmosphere and the minor portion passing
through the adsorbent bed whereby unburned hydrocarbon is desorbed
and then flowing the resulting minor portion of this exhaust stream
containing the desorbed unburned hydrocarbons into the engine where
they are burned.
Another reference is U.S. Pat. No. 2942932 which teaches a process
for oxidizing carbon monoxide and hydrocarbons which are contained
in exhaust gas streams. The process disclosed in this patent consists
of flowing an exhaust stream which is below 800.degree. F. into
an adsorption zone which adsorbs the carbon monoxide and hydrocarbons
and then passing the resultant stream from this adsorption zone
into an oxidation zone. When the temperature of the exhaust gas
stream reaches about 800.degree. F. the exhaust stream is no longer
passed through the adsorption zone but is passed directly to the
oxidation zone with the addition of excess air.
Finally, Canadian Patent No. 1205980 discloses a method of reducing
exhaust emissions from an alcohol fueled automotive vehicle. This
method consists of directing the cool engine startup exhaust gas
through a bed of zeolite particles and then over an oxidation catalyst
and then the gas is discharged to the atmosphere. As the exhaust
gas stream warms up it is continuously passed over the adsorption
bed and then over the oxidation bed.
The problem with the prior art processes is that the adsorbents
which were used are not selective. That is, water is adsorbed as
easily as the pollutants which necessitates the use of large beds,
which in turn means that the large bed acts as a heat sink, thereby
cooling the exhaust and lengthening the time required to warm up
the catalyst bed. Another problem with the adsorbents used in the
prior art is that they have poor thermal durability and would not
be able to meet the EPA durability requirements of at least 50000
miles or 5 years. For these reasons, adsorbent beds have not been
used in conjunction with catalysts to treat automotive exhaust streams.
Applicants have recognized this longfelt need and have solved the
problems found in the prior art. This has been accomplished by the
use of molecular sieves which selectively adsorb hydrocarbons and
other pollutants over water. What this means is that the molecular
sieve bed does not have to be very large in order to adsorb sufficient
quantities of hydrocarbons and other pollutants during engine startup.
Accordingly, the size of the adsorbent bed can be minimized so that
the catalyst bed warms up as quickly as possible. Additionally,
the molecular sieves which are used as the adsorbent are hydrothermally
stable at temperatures of at least 750.degree. C. This means that
the adsorbent bed can meet the EPA requirements that catalysts have
a durability of at least 50000 miles or 5 years.
SUMMARY OF THE INVENTION
This invention generally relates to a process for treating an engine
exhaust stream and in particular to a process for minimizing emissions
during the cold start operation of an engine. Accordingly, one embodiment
of the invention is a process for treating an engine exhaust gas
stream containing pollutants comprising flowing said engine exhaust
gas stream over a molecular sieve bed which preferentially adsorbs
the pollutants over water to provide a first exhaust stream, and
flowing the first exhaust gas stream over a catalyst to convert
the pollutants contained in the first exhaust gas stream to innocuous
products and provide a treated exhaust stream and discharging the
treated exhaust stream into the atmosphere, the molecular sieve
bed characterized in that it comprises at least one molecular sieve
selected from the group consisting of molecular sieves which have:
1) a framework Si:Al ratio of at least 2.4; 2) are hydrothermally
stable; and 3) have a hydrocarbon selectivity (.sup..alpha. HC-H.sub.2
O) greater than 1 where .sup..alpha. HC-H.sub.2 O is defined by
the following equation: ##EQU1## where X.sub.HC is the hydrocarbon
co-loading on the molecular sieves in equilibrium with the hydrocarbon
water vapor mixture in the gas phase over the molecular sieve adsorbent;
X.sub.H.sbsb.2.sub.O is the water co-loading on the molecular sieve
in equilibrium with the water and hydrocarbon vapor mixture in the
gas phase over the molecular sieve adsorbent; [H.sub.2 O] is the
concentration of water and [HC] is the concentration of hydrocarbon.
Other objects and embodiments will become more apparent after a
more detailed description of the invention.
DETAILED DESCRIPTION OF THE INVENTION
As stated this invention generally relates to a process for treating
an engine exhaust stream and in particular to a process for minimizing
emissions during the cold start operation of an engine. The engine
consists of any internal or external combustion engine which generates
an exhaust gas stream containing noxious components or pollutants
including unburned or thermally degraded hydrocarbons or similar
organics. Other noxious components usually present in the exhaust
gas include nitrogen oxides and carbon monoxide. The engine may
be fueled by a hydrocarbonaceous fuel. As used in this specification
and in the appended claims, the term "hydrocarbonaceous fuel"
includes hydrocarbons, alcohols and mixtures thereof. Examples of
hydrocarbons which can be used to fuel the engine are the mixtures
of hydrocarbons which make up gasoline or diesel fuel. The alcohols
which may be used to fuel engines include ethanol and methanol.
Mixtures of alcohols and mixtures of alcohols and hydrocarbons can
also be used. The engine may be a jet engine, gas turbine, internal
combustion engine, such as an automobile, truck or bus engine, a
diesel engine or the like. The process of this invention is particularly
suited for hydrocarbon, alcohol, or hydrocarbon-alcohol mixture,
internal combustion engine mounted in an automobile. For convenience
the description will use hydrocarbon as the fuel to exemplify the
invention. The use of hydrocarbon in the subsequent description
is not to be construed as limiting the invention to hydrocarbon
fueled engines.
When the engine is started up, it produces a relatively high concentration
of hydrocarbons in the engine exhaust gas stream as well as other
pollutants. Pollutants will be used herein to collectively refer
to any unburned fuel components and combustion byproducts found
in the exhaust stream. For example, when the fuel is a hydrocarbon
fuel, hydrocarbons, nitrogen oxides, carbon monoxide and other combustion
byproducts will be found in the engine exhaust gas stream. The temperature
of this engine exhaust stream is relatively cool, generally below
500.degree. C. and typically in the range of 200.degree. to 400.degree.
C. This engine exhaust stream has the above characteristics during
the initial period of engine operation, typically for the first
30 to 120 seconds after startup of a cold engine. The engine exhaust
stream will typically contain, by volume, about 500 to 1000 ppm
hydrocarbons.
The engine exhaust gas stream which is to be treated is now flowed
over a molecular sieve bed which contains at least one molecular
sieve that has a high selectivity for hydrocarbons versus water
to provide a first exhaust stream. The properties of the molecular
sieves will be more fully explained herein. The first exhaust stream
which is discharged from the molecular sieve bed is now flowed over
a catalyst to convert the pollutants contained in the first exhaust
stream to innocuous components and provide a treated exhaust stream
which is discharged into the atmosphere. It is understood that prior
to discharge into the atmosphere, the treated exhaust stream may
be flowed through a muffler or other sound reduction apparatus well
known in the art.
The catalyst which is used to convert the pollutants to innocuous
components is usually referred to in the art as a three-component
control catalyst because it can simultaneously oxidize any residual
hydrocarbons present in the first exhaust stream to carbon dioxide
and water, oxidize any residual carbon monoxide to carbon dioxide
and reduce any residual nitric oxide to nitrogen and oxygen. In
some cases the catalyst may not be required to convert nitric oxide
to nitrogen and oxygen, e.g., when an alcohol is used as the fuel.
In this case the catalyst is called an oxidation catalyst. Because
of the relatively low temperature of the engine exhaust stream and
the first exhaust stream, this catalyst does not function at a very
high efficiency, thereby necessitating the molecular sieve bed.
When the molecular sieve bed reaches a temperature of about 150.degree.-200.degree.
C., the pollutants which are adsorbed in the bed begin to desorb
and are carried by the first exhaust stream over the catalyst. At
this point the catalyst has reached its operating temperature and
is therefore capable of fully converting the pollutants to innocuous
components.
As stated, the molecular sieves which are used in the invention
are those molecular sieves which meet the following criteria: 1)
have a framework Si:Al ratio of at least 2.4; 2) are hydrothermally
stable and 3) have a hydrocarbon selectivity (.sup..alpha. HC-H.sub.2
O) greater than 1.0. By hydrothermally stable is meant the ability
of the molecular sieve to maintain its structure after thermal cycling
in the exhaust gas stream. One method of measuring hydrothermal
stability is to look at the temperature at which 50% of the structure
is decomposed after heating for 16 hours in air. The temperature
is referred to as T(50). Accordingly, as used in this application,
by hydrothermally stable is meant a molecular sieve which has a
T(50) of at least 750.degree. C. The hydrocarbon selectivity .alpha.
is defined by the following equation: ##EQU2## X.sub.HC =the hydrocarbon
co-loading on the molecular sieve in equilibrium with the hydrocarbon
water vapor mixture in the gas phase over the zeolite adsorbent;
X.sub.H.sbsb.2.sub.O =the water co-loading on the molecular sieve
in equilibrium with the water and hydrocarbon vapor mixture in the
gas phase over the molecular sieve adsorbent;
[H.sub.2 O]=the concentration of water vapor in the exhaust gas
stream; and
[HC]=the concentration of the hydrocarbon species in the exhaust
gas.
The above definitions show that the selectivity of molecular sieves
for hydrocarbons over water is dependent upon the exhaust gas stream
temperature, the particular hydrocarbon species of interest and
the relative concentrations of water vapor and hydrocarbon.
In order to calculate X.sub.HC and X.sub.H.sbsb.2.sub.O one needs
to first determine the intrinsic adsorption strength of the molecular
sieve. Intrinsic adsorption strength can be described by reference
to the Dubinin Polanyi model for adsorption. The model says that
the sorption expressed as the volume of the sorbent structure occupied
by the sorbate is a unique function of the Gibbs Free Energy change
on adsorption. Mathematically this relationship takes the form of
a Gaussian distribution with Gibbs free energy change as follows:
where X is the loading expected, VO is the pore volume (cc/g),
B is a constant that is dependent on the sorbent and sorbate, and
G is the Gibbs Free Energy change. The product of liquid density
and VO equates to the saturation loading, XO, for any pure compound
by the Gurvitsch Rule. (see Breck, Zeolite Molecular Sieves, page
426.)
The constant B is then inversely related to the intrinsic adsorption
strength. For example, if the hydrocarbon is benzene, a value of
B of 0.04 for both benzene and water gives good results. The estimates
of water and hydrocarbon co-loadings are made in the following way:
1) each individual component loading is estimated by use of the
Dubinin Polanyi model as outlined above. For each compound present
one needs to know the liquid phase density (approximating the sorbed
phase density), the vapor pressure as a function of temperature,
and the actual concentration of the species in the gas.
2) Once each pure component loading is calculated the function
.PHI. is calculated as,
where X/XO is the loading ratio or fraction of the pore volume
filled by each component if it were present alone. .PHI. then represents
the ratio of occupied pore volume to unoccupied pore volume.
3) The co-loadings are then calculated, accounting for each species
present, by the formula,
X.sub.mc is the co-loading of each component on the zeolite. This
procedure follows the Loading Ratio Correlation, which is described
in "Multicomponent Adsorption Equilibria on Molecular Sieves",
C. M. Yon and P. H. Turnock AlCHE Symposium Series, No. 117 Vol.
67 (1971).
Both natural and synthetic molecular sieves may be used as adsorbents.
Examples of natural molecular sieves which can be used are faujasites,
clinoptilolites, mordenites, and chabazite. Examples of synthetic
molecular sieves which can be used are silicalite, Zeolite Y, ultrastable
zeolite Y, ZSM-5. Of course mixtures of these molecular sieves both
nautral and synthetic can be used.
The adsorbent bed used in the instant invention can be conveniently
employed in particulate form or the adsorbent can be deposited onto
a solid monolithic carrier. When particulate form is desired, the
adsorbent can be formed into shapes such as pills, pellets, granules,
rings, spheres, etc. In the employment of a monolithic form, it
is usually most convenient to employ the adsorbent as a thin film
or coating deposited on an inert carrier material which provides
the structural support for the adsorbent. The inert carrier material
can be any refractory material such as ceramic or metallic materials.
It is desirable that the carrier material be unreactive with the
adsorbent and not be degraded by the gas to which it is exposed.
Examples of suitable ceramic materials include sillimanite, petalite,
cordierite, mullite, zircon, zircon mullite, spondumene, alumina-titanate,
etc. Additionally, metallic materials which are within the scope
of this invention include metals and alloys as disclosed in U.S.
Pat. No. 3920583 which are oxidation resistant and are otherwise
capable of withstanding high temperatures.
The carrier material can best be utilized in any rigid unitary
configuration which provides a plurality of pores or channels extending
in the direction of gas flow. It is preferred that the configuration
be a honeycomb configuration. The honeycomb structure can be used
advantageously in either unitary form, or as an arrangement of multiple
modules. The honeycomb structure is usually oriented such that gas
flow is generally in the same direction as the cells or channels
of the honeycomb structure. For a more detailed discussion of monolithic
structures, refer to U.S. Pat. Nos. 3785998 and 3767453.
The adsorbent material, e.g., molecular sieve, is deposited onto
the carrier by any convenient way well known in the art. A preferred
method involves preparing a slurry using the molecular sieves and
coating the monolithic honeycomb carrier with the slurry. The slurry
can be prepared by means known in the art such as combining the
appropriate amount of the molecular sieve and a binder with water.
This mixture is then blended by using means such as sonification,
milling, etc. This slurry is used to coat a monolithic honeycomb
by dipping the honeycomb into the slurry, removing the excess slurry
by draining or blowing out the channels, and heating to about 100.degree.
C. If the desired loading of molecular sieve is not achieved, the
above process may be repeated as many times as required to achieve
the desired loading. The size of the adsorbent bed is chosen such
that at least 40% of the hydrocarbons in the exhaust stream discharged
from the engine is adsorbed. Generally, this means that the size
of the adsorbent bed varies from about 1 to about 3 liters. When
the adsorbent is deposited on a monolithic honeycomb carrier, the
amount of adsorbent on the carrier varies from about 100 to about
450 grams. It is desirable to optimize the volume of the adsorbent
bed such that the catalyst downstream from the adsorbent bed is
heated as quickly as possible while at the same time ensuring that
at least 40% of the hydrocarbons in the exhaust stream are adsorbed
on the adsorbent bed. It is preferred that the adsorbent be deposited
on a monolithic honeycomb carrier in order to minimize the size
of the adsorbent bed and the back pressure exerted on the engine.
Instead of depositing the molecular sieve onto a monolithic honeycomb
structure, one can take the molecular sieve and form it into a monolithic
honeycomb structure by means known in the art.
The adsorbent may optionally contain one or more catalytic metals
dispersed thereon. The metals which can be dispersed on the adsorbent
are the noble metals which consist of platinum, palladium, rhodium,
ruthenium, and mixtures thereof. The desired noble metal may be
deposited onto the adsorbent, which acts as a support, in any suitable
manner well known in the art. One example of a method of dispersing
the noble metal onto the adsorbent support involves impregnating
the adsorbent support with an aqueous solution of a decomposable
compound of the desired noble metal or metals, drying the adsorbent
which has the noble metal compound dispersed on it and then calcining
in air at a temperature of about 400.degree. to about 500.degree.
C. for a time of about 1 to about 4 hours. By decomposable compound
is meant a compound which upon heating in air gives the metal or
metal oxide. Examples of the decomposable compounds which can be
used are set forth in U.S. Pat. No. 4791091 which is incorporated
by reference. Preferred decomposable compounds are chloroplatinic
acid, rhodium trichloride, chloropalladic acid, hexachloroiridate
(IV) acid and hexachlororuthenate. It is preferable that the noble
metal be present in an amount ranging from about 0.01 to about 4
weight percent of the adsorbent support. Specifically, in the case
of platinum and palladium the range is 0.1 to 4 weight percent,
while in the case of rhodium and ruthenium the range is from about
0.01 to 2 weight percent.
These catalytic metals are capable of oxidizing the hydrocarbon
and carbon monoxide and reducing the nitric oxide components to
innocuous products. Accordingly, the adsorbent bed can act both
as an adsorbent and as a catalyst.
The catalyst which is used in this invention is selected from any
three component control or oxidation catalyst well known in the
art. Examples of catalysts are those described in U.S. Pat. Nos.
4528279; 4791091; 4760044; 4868148; and 4868149 which
are all incorporated by reference. Preferred catalysts well known
in the art are those that contain platinum and rhodium and optionally
palladium, while oxidation catalysts usually do not contain rhodium.
Oxidation catalysts usually contain platinum and/or palladium metal.
These catalysts may also contain promoters and stabilizers such
as barium, cerium, lanthanum, nickel, and iron. The noble metals
promoters and stabilizers are usually deposited on a support such
as alumina, silica, titania, zirconia, alumino silicates, and mixtures
thereof with alumina being preferred. The catalyst can be conveniently
employed in particulate form or the catalytic composite can be deposited
on a solid monolithic carrier with a monolithic carrier being preferred.
The particulate form and monolithic form of the catalyst are prepared
as described for the adsorbent above.
The following examples are presented in illustration of this invention
and are not intended as undue limitations on the generally broad
scope of the invention as set out in the appended claims.
EXAMPLE 1
A slurry was prepared using Y-54 and Ludox AS-40 binder. Y-54 is
an ultrastable sodium Y zeolite with a SiO.sub.2 /Al.sub.2 O.sub.3
ratio of 5 an A.sub.o of 24.68 .ANG. and a Na/Al ratio of 0.93.
Y-54 is produced and was obtained from UOP. Ludox AS-40 is an ammonium
stabilized colloidal silica containing 40 weight percent solids
with 20 micron spherical SiO.sub.2 particles and is available from
DuPont Corp. To 141 grams of distilled water, there was added 100
grams of Ludox AS-40. To this mixture there were added 191 grams
of Y-54 zeolite and then 551 grams of water. This mixture was sonified
for 10 minutes using a Sonifier Cell Disruptor 350.
A ceramic monolithic honeycomb carrier manufactured by Corning
Glass Works measuring 28 mm in diameter by 50 mm in length was dipped
into the slurry, pulled out and allowed to drain. The wet honeycomb
was air dried and then heated at 100.degree. C. for 1 hour. The
monolith contained 4.1 grams of zeolite plus binder. This sample
was designated sample A.
EXAMPLE 2
A monolithic honeycomb was prepared as in Example 1 except that
the adsorbent used was Y-84. Y-84 is the ammonium form of stabilized
Y zeolite with an A.sub.o of 24.55 .ANG. , an NH.sub.4 /Al of 0.3
and a Na/Al of less than 0.01. Y-84 was also obtained from UOP.
This sample contained 4.2 grams of zeolite plus binder and was designated
sample B.
EXAMPLE 3
A monolithic honeycomb was prepared as in Example 1 except that
the adsorbent used was SA-15. Sa-15 is a steamed form of Y-84 with
an A.sub.o of 24.29 .ANG. and NH.sub.4 /Al and a Na/Al ratio of
less than 0.01. This sample contained 5.5 grams of zeolite plus
binder and was designated sample C.
EXAMPLE 4
Samples A, B and C were tested to determine their hydrocarbon adsorption
properties by using the following test procedure. The sample to
be tested, measuring 28 mm in diameter by 50 mm in length and having
a volume of 30.8 cc was placed in a tubular glass reactor. Over
this adsorbent bed there was flowed a gas stream containing 998
ppm of propylene, 17570 ppm of water and the remainder nitrogen.
The test was run by starting with a cold (room temperature) adsorbent
bed and gas stream flowing the gas stream at a flow rate of 7 Standard
Liters Per Minute (SLPM) over the adsorbent while heating the gas
stream from about 25.degree. C. to about 360.degree. C. in approximately
400 seconds.
The hydrocarbon retention was calculated by integrating the difference
between the instantaneous mass flow of hydrocarbons into and out
of the adsorbent. The percentage of the hydrocarbons retained was
calculated by dividing the net hydrocaron retention by the integral
of the hydrocarbons flowed into the bed. Plots of hydrocarbon retention
versus time for samples A, B and C are presented in FIG. 2.
The results presented in FIG. 2 shows that sample A has the largest
initial value of hydrocarbon retention, but the retention falls
off quickly. Samples B and C have lower initial retention but fall
off more slowly with sample B being the best. It is clear from this
test that any of the three zeolites tested can be used to selectively
adsorb hydrocarbons during the cold-start phase of an automobile
engine.
Thus, having described the invention in detail, it will be understood
by those skilled in the art that certain variations and modifications
may be made without departing from the spirit and scope of the invention
as defined herein and in the appended claims. |