Method for reducing automotive NO.sub.x emissions in lean burn internal combustion engine exhaust using a transition metal-containing zeolite catalyst which is in-situ crystallized

A method is provided for reducing NO.sub.x for high flow applications such as NO.sub.x abatement in an exhaust gas from an internal combustion engine operating under lean burn conditions wherein NO.sub.x is reduced by hydrocarbon reductants. The method employs a hydrothermally stable catalyst comprising transition metal-containing ZSM-5 which is prepared by in-situ crystallization of an aggregate comprising ZSM-5 seeds, silica, and a crystalline silicate.

FIELD OF THE INVENTION 
This invention is concerned with a method for reduction of nitrogen oxides 
contained in a gaseous stream such as lean burning internal combustion 
engine exhaust. The method employs a hydrothermally stable catalyst 
comprising transition metal-containing ZSM-5 which is prepared by in-situ 
crystallization of a preformed aggregate. 
BACKGROUND OF THE INVENTION 
Atmospheric pollution is a societal problem which is receiving much 
attention. The major source of such pollution is the extensive use of 
fossil fuels, although industrial and chemical processes, such as the 
manufacture of nitric acid, also contribute. The principal pollutants are 
nitrogen oxides, carbon monoxide, and perhaps to a lesser extent 
hydrocarbons, sulfur oxides and other objectionable gases and vapors. 
Although several nitrogen oxides are known which are relatively stable at 
ambient conditions, it is generally recognized that two of these, viz., 
nitric oxide (NO) and nitrogen dioxide (NO2), are the principal 
contributors to smog and other undesirable environmental effects when they 
are discharged into the atmosphere. These effects will not be discussed 
further here since they are well recognized and have led various 
government authorities to restrict industrial emissions in an attempt to 
limit the level of the nitrogen oxides in the atmosphere. Nitric oxide and 
nitrogen dioxide, under appropriate conditions, are interconvertible 
according to the equation 
EQU 2NO+O2=&gt;2NO2. 
For purposes of the present invention NO.sub.x will be used herein to 
represent nitric oxide, nitrogen dioxide, and mixtures thereof. 
Formation of man-made nitrogen oxides from the elements occurs in the high 
temperature zones of combustion processes. The internal combustion engine, 
and coal-, oil-, and gas-fired furnaces, boilers and incinerators, all 
contribute to NO.sub.x emissions. In general, fuel-rich combustion 
mixtures produce exhaust gases with lower contents of NO.sub.x than do 
lean mixtures. Although the concentrations of NO.sub.x in the exhaust 
gases produced by combustion usually are low, the aggregate amounts 
discharged in industrial and/or highly populated areas is adequate to 
cause problems. 
The so-called "stable" nitrogen oxides have in common the somewhat peculiar 
property that although they are thermodynamically very unstable with 
respect to decomposition into elemental oxygen and nitrogen, no simple, 
economical method has been described for inducing this decomposition. A 
variety of catalysts are known which reduce NO.sub.x to N.sub.2, using 
carbon monoxide, hydrogen or hydrocarbons in a net reducing environment. 
Since all three of these reductants are present in normal automobile 
emissions, this would appear to be a simple matter. Unfortunately, oxygen 
is also present in such emissions and most catalysts which reduce NO.sub.x 
will not operate effectively in an oxidizing atmosphere. Instead of 
reducing NO.sub.x the reductants reduce oxygen. One class of materials, 
copper-exchanged zeolites, have been used to overcome this problem, and 
have been shown to be suitable catalysts for reduction of NO.sub.x in 
automobile engine exhaust containing hydrocarbons which act as reductants. 
For example, U.S. Pat. No. 4,297,328 discloses concurrent catalytic 
reduction of oxides of nitrogen and the oxidation of carbon monoxide and 
hydrocarbons in a gas stream containing a stoichiometric excess of 
oxidant, over a copper-containing ZSM-5 zeolite. U.S. Pat. No. 5,041,270 
discloses NO.sub.x reduction in the presence of hydrocarbons acting as 
reductant in an oxidizing atmosphere, over a catalyst containing copper 
loaded on a support. U.S. Pat. No. 5,041,272 discloses NO.sub.x reduction 
in the presence of excess oxygen, in the presence of organic reductant, 
e.g., using hydrocarbons over hydrogen form zeolites such as ZSM-5 which 
are impregnated with a metal such as copper. 
Despite the initial effectiveness of such NO.sub.x reducing 
copper-containing catalysts employed in lean burn exhaust operations, 
their ultimate service life is severely limited during operations under 
hydrothermal conditions. Hydrothermal stability of NO.sub.x reduction 
catalysts is considered in U.S. Pat. No. 4,157,375. This reference 
discloses the preparation of a zeolite prepared from a calcined honeycomb 
preform such as kaolin with an aqueous solution of base (e.g., water, NaOH 
and tetrapropylammonium) to form a monolith containing ZSM-5. However, the 
resulting catalyst is used for reduction of nitrogen oxides in exhaust 
gases in the presence of "suitable reducing gas, ammonia, carbon monoxide, 
hydrogen or the like . . . added in an amount such that the added gas 
together with any reducing agent present (e.g., carbon monoxide) will be 
about equal to the stoichiometric amount required for a desired reduction 
of NO.sub.x " (column 8, lines 14 to 21). The resulting catalyst is suited 
to operation at temperatures up to 800.degree. C. in the absence of water 
and up to 700.degree. C. in the presence of substantial amounts of water 
due to the sensitivity of zeolites to elevated temperature in the presence 
of steam. No mention is made of utilizing such a catalyst in NO.sub.x 
reduction employing a hydrocarbon reductant. 
All of the above patents are incorporated herein by reference in their 
entirety. 
BRIEF SUMMARY OF THE INVENTION 
It has now been found that nitrogen oxides contained in an exhaust gas from 
an internal combustion engine operating under lean burn conditions can be 
reduced by contacting the exhaust gas at a temperature of at least 
300.degree. C. with a hydrothermally stable catalyst comprising a 
transition metal and a zeolite having the structure of ZSM-5 which is 
prepared by in-situ crystallization of an aggregate comprising ZSM-5 
seeds, silica, and a crystalline silicate. The exhaust gas has a molar 
ratio of hydrocarbons to nitrogen oxides of at least the stoichiometric 
ratio, and the reduction of NO.sub.x is substantially effected by 
hydrocarbon reductant. It has been found that the catalysts employed in 
this invention are more heat and/or steam stable in this particular use 
than corresponding zeolite-containing catalysts prepared by other methods 
which enhance thermal and/or hydrothermal stability.

DETAILED DESCRIPTION AND SPECIFIC EMBODIMENTS 
The present invention provides a catalyst which is significantly less 
susceptible to deactivation resulting from NO.sub.x reduction in an 
exhaust gas stream from a lean burning internal combustion engine. The 
catalyst employed is prepared by in situ crystallization of an aggregate, 
e.g, a preformed clay aggregate. As noted above, the aggregate comprises 
three inorganic components: ZSM-5 seeds, silica, e.g., a colloidal silica 
such as Ludox.TM. as available from DuPont, and a crystalline silicate. 
Optionally, the aggregate can also include alumina. The silicate can be a 
layered material or other crystalline component which is convertible, as a 
component of the aggregate, upon high temperature calcination and 
hydrothermal treatment to ZSM-5. The layered silicates are also known as 
phyllosilicates and are divided into a number of groups and subgroups 
according to their structure and chemical composition. The six main groups 
are: kaolinite-serpentine, pyrophyllite-talc, mica, chlorites, 
smectites-vermiculites, and polygorskites-sepiolites. The 
kaolinite-serpentine group is the preferred source of the crystalline 
silicate for the preparation of the aggregate; however, as mentioned 
previously, any layered silicate and many other crystalline silicates have 
utility in this area. Various techniques for preparing a suitable catalyst 
for such in situ crystallization are set out below. 
U.S. Pat. 4,522,705 discloses a ZSM-5 of enhanced hydrothermal stability 
suitable for cracking hydrocarbons which is prepared by in-situ 
crystallization of preformed aggregates. 
U.S. Pat. No. 4,091,007 discloses a method for the preparation of ZSM-5 
prepared by in-situ crystallization of clay aggregates in the presence of 
tetraalkylammonium ions. The resulting ZSM-5 can be a discrete particle 
having a crystallinity of greater than 40 percent by preforming the 
reaction mixture oxides into pellets or extrudates which retain their 
shape and acquire substantial strength during the calcination process. In 
addition to the oxides, the reaction mixture contains a source of alkali 
metal cations and tetraalkylammonium cations, and water. The crystallized 
product can be handled in subsequent chemical processing, such as ion 
exchange, without necessitating cumbersome processes such as filtration. 
Further, these discrete particles can be used directly as catalysts after 
appropriate processing but without the need of any reformulation or 
pelletizing since the non-crystalline portion of the discrete particle 
serves as the porous matrix of the prior art compositions. One variation 
on the method of this disclosure which is well-suited to use in the 
present invention substitutes tetraalkylammonium cations with alkylamine, 
e.g. n-propylamine. Another variation employs high silica zeolite seeds in 
preparing the preformed composite particles whereby a highly crystalline 
product is obtained in the absence of organic compounds ordinarily 
employed in high silica zeolite in situ syntheses. Following the 
preforming operation, the discrete particles are calcined and then 
contacted with an alkali metal hydroxide or other hydroxide solution to 
achieve the desired degree of crystallization. The integrity of the 
composite particles is retained during the crystallization to provide a 
zeolite composition in particulate form which is attrition resistant and 
highly stable. 
U.S. Pat. No. 4,800,187 discloses a method for crystallizing strongly bound 
zeolite such as ZSM-5 on the surface of a sintered monolithic ceramic by 
hydrothermal treatment with an aqueous base solution. 
All of the above references disclose methods for preparing zeolites by in 
situ crystallization of a clay aggregate which are suited to use in the 
present invention and are accordingly incorporated herein by reference. In 
one embodiment, the zeolite is prepared from a clay aggregate which 
comprises a non-clay added source of silica. 
The clay component which is treated to form the zeolite-containing catalyst 
can be selected from the group consisting of kaolin, halloysite, 
montmorillonite, illite, and dickite, with kaolin preferred. 
The aggregate can be in the form of a monolith, e.g., a honeycombed 
monolith, or in the form of spheroids, cylinders, or other conventional 
catalyst shapes. Preferably the aggregate is in the form of a structure 
suitable for high flow applications, e.g., applications wherein the linear 
gas velocity is at least 3 meters per second. In an alternative 
embodiment, the zeolite prepared from in-situ crystallization is applied 
as a wash coat on a suitable support. 
Generally, the catalyst of the present invention contains at least one 
transition metal. Preferred transition metals include those selected from 
the group consisting of copper, zinc, vanadium, chromium, manganese, iron, 
cobalt, nickel, rhodium, palladium, platinum, and molybdenum. Copper is 
especially preferred. 
In addition to the transition metal, alkali or alkaline earth metals may be 
present in the catalyst in order to facilitate NO.sub.x reduction. Such 
metals include Na, K, Rb, Cs, Mg, Ca, and Ba. 
The catalysts of the present invention are prepared by introducing 
transition metal or transition metal ions into the zeolite framework or 
within the zeolite pores by any suitable technique. The zeolite can be 
ion-exchanged, ion-doped, or loaded sufficiently so as to provide an 
efficient amount of catalytic metal within or on the zeolite. 
Alternatively, the metals or metal ions can be introduced to the 
non-zeolitic support, or to both the zeolite and the support. 
The resulting catalyst exhibits thermal and/or hydrothermal stability at 
temperatures of at least 500.degree. C., preferably at least 750.degree. 
C., or even more preferably at least 800.degree. C., under the conditions 
and test protocol set out in Example 6 of the specification, especially 
with regard to aging conditions and gas composition. For present purposes, 
thermal or hydrothermal stability can be measured in terms of percent of 
NO.sub.x conversion activity remaining after exposure to a temperature of 
800.degree. C. for 5 hours relative to the fresh acid activity of the 
zeolite. Preferably, the catalyst employed retains at least 70%, 
preferably at least 80 or even 90% of its original activity. 
The present invention comprises contacting the above-described catalyst 
with an exhaust gas having hydrocarbons present at levels of at least the 
stoichiometric amount required to reduce the nitrogen oxides present. The 
hydrocarbons are preferably present in at least 3 times, more preferably 
at least 10 times the stoichiometric amount. Generally, the hydrocarbons 
can be present in amounts ranging from 1 to 10 times the stoichiometric 
amount. 
The exhaust gases thus treated are derived from combustion occurring under 
lean burn conditions, i.e., above the soichiometric value of 14.7. Such 
conditions can comprise an air-to-fuel ratio greater than 14.7, say, 18 to 
23, preferably 18 to 20. The contacting of exhaust gases with the catalyst 
occurs at a temperature of at least 300.degree. C., preferably ranging 
from 350.degree. to 800.degree. C. 
The present invention is operated under conditions wherein the catalyst is 
contacted with the exhaust gas at a space velocity no greater than 500000 
vol/vol per hour, preferably no greater than 250000 vol/vol per hour. 
Suitable ranges include 20000 to 120000 vol/vol per hour, preferably 30000 
to 80000 vol/vol per hour, on a gas hourly basis. 
As noted above, the catalytic reduction of nitrogen oxides in the present 
invention is substantially effected by hydrocarbon reductant. By 
substantially effected is meant at least 90, 95, or even 99% of the 
observed NO.sub.x reduction is effected by hydrocarbon as reductant. The 
measurement of the effect of hydrocarbon as reductant can be determined by 
monitoring NO.sub.x conversion as a function of temperature whereby 
selected hydrocarbon reductants, such as propylene are added one at a time 
to an otherwise constant simulated exhaust gas containing primarily 
N.sub.2, NO, H.sub.2 O, O.sub.2, CO.sub.2 and CO. The relative efficacy of 
each hydrocarbon for effecting NO.sub.x conversion can thus be determined. 
The most effective hydrocarbon reductants are those yielding the largest 
NO.sub.x reduction at the lowest temperature for a given (molar) 
concentration. Virtually 100% of the observed NO.sub.x reduction achieved 
is effected by hydrocarbon as reductant. 
This invention will now be illustrated by examples. The examples are for 
illustrative purposes only and are not to be construed as limiting the 
scope of the invention, which scope is defined by this entire 
specification including the appended claims. 
EXAMPLES 
Example 1 (Comparative) 
Preparation of High Activity ZSM-5 
A ZSM-5 catalyst is prepared by a method which is known to provide a 
product of enhanced hydrothermal stability. 3.66 parts quantity, by 
weight, of a commercial precipitated silica, e.g., Ultrasil VN3 (available 
from Nasilco) were added to a mixture containing 1.00 parts Al.sub.2 
(SO.sub.4).sub.3 .multidot.14H.sub.2 O, 1.54 parts 50% NaOH, and 10.08 
parts water. The mixture was heated to 160.degree. C. in a stirred 
autoclave and held at that temperature for crystallization. After full 
crystallinity was achieved, the resulting crystals were separated from the 
remaining liquid by filtration, washed with water, and dried. 
Example 2 (Comparative) 
Preparation of Large Crystal ZSM-5 
A ZSM-5 catalyst is prepared by a method which is known to provide a 
product of enhanced hydrothermal stability. 
A sodium silicate solution was prepared by mixing 16.9 parts, by weight, of 
deionized water with 30.0 parts N-Brand sodium silicate (available from PQ 
Corp.). A 26% sodium chloride solution was prepared by dissolving 3.54 
parts NaCl in 2.85 parts deionized water. An acid alum solution was 
prepared by mixing 18.6 parts deionized water, 1.04 parts aluminum 
sulfate, 3.12 parts 93% H.sub.2 SO.sub.4, 3.5 parts 50% 
tetramethylammonium chloride, and 4.89 parts 35% tetrapropylammonium 
bromide. 
0.5 parts deionized water were added to an autoclave and the sodium 
silicate and acid alum solutions were mixed and added to the autoclave 
through a nozzle which insured the formation of a homogeneous gel. The gel 
was then heated to 160.degree. C. and that temperature was maintained 
under agitation for approximately 20 hours at which time the autoclave was 
cooled to room temperature. The crystallized slurry was removed from the 
autoclave and the ZSM-5 recovered by filtration and washing with deionized 
water and dried. 
Example 3 (Comparative) 
Preparation of High SiO.sub.2 /Al.sub.2 O.sub.3 ZSM-5 
A ZSM-5 catalyst is prepared by a method which is known to provide a 
product of enhanced hydrothermal stability. 
A mixture of 653 parts, by weight, deionized water, 48.8 parts 50% NaOH, 
6.77 parts aluminum sulfate, and 1.00 parts ZSM-5 crystals was added to a 
stirred autoclave. 227 parts of a precipitated silica, Ultrasil VN3, were 
slowly added to the solution. After the addition of 39.7 parts 
n-propylamine, the autoclave was heated, with stirring, to 104.degree. C. 
After 44 hours at 104.degree. C., the temperature was raised to 
110.degree. C. in order to remove the n-propylamine by flashing the 
organic into a collection vessel. After the removal of the organic and 
cooling the autoclave to room temperature, the ZSM-5 crystals were 
recovered by filtration, washing with deionized water, and drying. 
Example 4 
Preparation of Aggregate Based ZSM-5 
A ZSM-5 catalyst of the present invention was prepared by adding a 1.00 
parts quantity, by weight, of ammonium exchanged, small crystal ZSM-5 to 
3.97 parts water. The resulting mixture was ball milled to reduce the mean 
particle size to less than 2 microns. The resultant slurry was added to 
3.76 parts water and 12.5 parts Ludox.TM. AS-40, a commercial colloidal 
silica containing 40 wt % SiO.sub.2 available from DuPont. The slurry was 
homogenized with a high speed mixer, and 16.14 parts Kaopaque.TM. 10 S 
clay available from Georgia Kaolin Co. were added and the mixing was 
continued to produce a slurry which was then spray dried. The spray dried 
material was calcined at 1000.degree. C. for three hours. 
A 16.83 parts quantity, by weight, of the calcined aggregate were added to 
a solution of 1.00 parts n-propylamine, 2.69 parts 50% NaOH, and 64.64 
parts water. The mixture was then heated to 150.degree. C. in a stirred 
autoclave. After crystallization was complete, the n-propylamine was 
removed from the autoclave by flashing and the zeolite crystals were 
removed from the remaining liquid by filtration, washed with water, and 
dried. 
Example 5 
Copper-Exchange Procedures 
The ZSM-5 crystals prepared in the previous examples were readied for 
copper exchange by calcination at 480.degree. C. in flowing nitrogen for 
three hours followed by a 540.degree. C. treatment in air for three hours. 
Copper was incorporated into the zeolite samples of Examples 1 to 6 by 
excess solution, room temperature exchange with 1 M solutions of 
Cu(NO.sub.3).sub.2. After four two-hour exchange procedures, the slurries 
were filtered and dried. The copper loading for each catalyst is supplied 
below. 
______________________________________ 
Example 1 3.8 wt % Cu 
Example 2 2.6 wt % Cu 
Example 3 0.7 wt % Cu 
Example 4 3.6 wt % Cu 
______________________________________ 
Example 6 
NO.sub.x Reduction Testing 
A 0.3 g sample of copper-exchanged ZSM-5 of Example 1 is mixed with 0.75 cc 
of 12/60 mesh Vycor.TM. chips and loaded into a glass reactor. A simulated 
lean burn engine exhaust whose composition is set out in Table 1 below is 
introduced to the reactor at 0-8 psig and 80 WHSV. The temperature is then 
ramped to 500.degree. C. to condition the catalyst. After cooling the 
catalyst to 200.degree. C., the temperature is then raised in a step 
fashion so that the NO.sub.x reduction activity of the catalyst can be 
evaluated at each 50.degree. C. increment. After the initial fresh 
activity cycle, the catalyst undergoes an accelerated aging test by 
holding the temperature at 800.degree. C. for five hours. The catalyst 
temperature is again reduced to 200.degree. C. and the aged activity of 
the catalyst is again determined in stepwise fashion at 50.degree. C. 
increments. This process is repeated for the catalysts of Examples 2 to 4. 
The fresh activities of the catalysts of Examples 1, and 4 are similar. 
However, as shown in Table 2, the catalyst employed in the present 
invention (Example 4) retains 90% of its maximum NO.sub.x reduction 
activity after aging compared to 70% for the catalyst of Example 1. 
TABLE 1 
______________________________________ 
Test Gas Composition 
______________________________________ 
Nitric Oxide, ppm 680 
Propylene, ppm 400 
Hydrogen, ppm 330 
Carbon Monoxide, ppm 
1020 
Carbon Dioxide, % 10 
Oxygen, % 4 
Water, % 3 
Nitrogen, % Balance 
______________________________________ 
TABLE 2 
______________________________________ 
Retention of NO.sub.x Abatement Activity 
Maximum Fresh 
Retention of Activity 
NO.sub.x Conversion, % 
After Aging, % 
______________________________________ 
Example 1 34 70 
Example 2 20 62.5 
Example 3 22.5 100 
Example 4 35 90 
______________________________________ 
The catalyst of the present invention (Example 4) retaining 90% of its 
fresh activity represents an improvement over the high silica-to-alumina 
ratio sample (Example 3) despite the retention by the latter of 100% 
NO.sub.x reduction activity because the absolute NO.sub.x reduction 
activity of the former is significantly greater (31.5% vs 22.5%). The 
conversion activity of the catalyst of Example 3 is significantly lower 
than that for the catalyst of the present invention. This activity 
difference can not be overcome by addition of catalyst because the 
hydrocarbons reducing NO.sub.x are fully depleted by the time the gas 
passes through the original catalyst bed fill. Those species that do not 
react with the NO.sub.x are combusted to CO.sub.x so the catalyst of 
Example 3 is of limited utility due to low absolute NO.sub.x reduction 
activity under the specific example conditions. In contrast, the catalyst 
of the present invention can have an absolute NO.sub.x reduction activity 
of at least 25, or even 30% or more after aging.