Method for reduction of nitrogen oxides with ammonia using promoted zeolite catalysts

A method in accordance with the invention comprises passing through a zeolite catalyst as described below, a gaseous stream containing nitrogen oxides, ammonia and oxygen to selectively catalyze the reduction of nitrogen oxides and, if excess or unreacted ammonia is present, to oxidize the excess of unreacted ammonia with oxygen to hydrogen and water. The method includes the use of a zeolite catalyst composition which comprises a metal (e.g., iron or copper) promoted zeolite, the zeolite being characterized by having a silica to alumina ratio of at least about 10 and a pore structure which is interconnected in all three crystallographic dimensions by pores having an average kinetic pore diameter of at least about 7 Angstroms. Promoted zeolites of the above type have demonstrated high tolerance for sulfur poisoning, good activity for the selective catalytic reduction of nitrogen oxides with ammonia, good activity for the oxidation of ammonia with oxygen, and the retention of such good activities even under high temperature operations, e.g., 400.degree. C. or higher, and hydrothermal conditions.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The present invention is concerned with a method of catalyzing the 
reduction of nitrogen oxides with ammonia, especially the selective 
reduction of nitrogen oxides with ammonia in the presence of oxygen, using 
zeolite catalysts, especially metal-promoted zeolite catalysts. 
2. The Related Art 
Both synthetic and natural zeolites and their use in promoting certain 
reactions, including the selective reduction of nitrogen oxides with 
ammonia in the presence of oxygen, are well known in the art. Zeolites are 
aluminosilcate crystalline materials having rather uniform pore sizes 
which, depending upon the type of zeolite and the type and amount of 
cations included in the zeolite lattice, range from about 3 to 10 
Angstroms in diameter. 
Japanese Patent Publication (Kokai) No. 51-69476, published Jun. 16, 1976 
on Application No. 49-142463, filed Dec. 13, 1974, discloses a method for 
reducing nitrogen oxides in waste gases by reaction with ammonia in the 
presence of a metal-promoted, dealuminized synthetic or natural mordenite 
zeolite. The resistance of the catalyst to sulfurous poisons, particularly 
sulfur trioxide and sulfuric acid mist, is said to be enhanced by 
dealuminizing the mordenite to increase the silica to alumina ratio to 
more than 12, preferably to more than 15. The zeolite is promoted with 0.5 
to 30 weight percent of at least one of a number of promoters including 
copper, vanadium, chromium, iron, cobalt or nickel and used at a reaction 
temperature of 200.degree. to 500.degree. C. with from 0.5 to three times 
the stoichiometric amount of ammonia reductant. Example 1 of the 
Publication illustrates an iron-promoted mordenite ore as being effective 
for the reduction of nitrogen oxides. In connection with Example 2, it is 
stated that a slight decrease of the activity of a high silica to alumina 
ratio, copper-promoted mordenite catalyst is recognized when sulfur 
trioxide is included in the gas stream. However, an "extreme improvement" 
of resistance to sulfur trioxide poisoning is noted in comparison with a 
copper mordenite which has not been dealuminized to increase the silica to 
alumina ratio. 
UK patent application No. 2,193,655A discloses a catalyst containing a low 
surface area titania and a copper-promoted zeolite for use in the 
reduction of nitrogen oxides with ammonia. The zeolite has an average pore 
diameter of 10 Angstroms or less, preferably 8 Angstroms or less, and a 
silica to alumina molar ratio of 10 or more, preferably 20 or more; the 
resultant titania/promoted zeolite catalysts having these characteristics 
are stated to have good mechanical strength and to be resistant to 
volatile catalyst poisons such as arsenic, selenium, tellurium, etc., 
contained in exhaust gases. Examples of suitable zeolites are mordenite, 
ZSM-5, and ferrierite. 
U.S. Pat. No. 4,297,328 discloses a "three-way conversion" catalytic 
process for the simultaneous catalytic oxidation of carbon monoxide and 
hydrocarbons and reduction of nitrogen oxides for purifying the exhaust 
gas of automobile engines operated within a prescribed range of air to 
fuel ratio (column 4, lines 63-68). The disclosed catalyst is a 
copper-promoted zeolite having a silica to alumina ratio greater than 10, 
preferably greater than 20 (column 6, lines 23-28). Representative 
high-silica zeolites are described at columns 6-8 of the patent and 
include (column 6, lines 29-33) silicalite (as described in U.S. Pat. No. 
4,061,724), ZSM-5, ZSM-8, ZSM-11, ZSM-12, hyper Y, ultrastabilized Y, 
Beta, mordenite and erionite. Ultrastabilized Y is described (column 7, 
lines 22-25) as "a form of zeolite Y which has been treated to give it the 
organophilic characteristic of the adsorbents of the present invention." 
Example 6 of the patent is stated to show no measureable loss in 
combustion activity of the copper-promoted zeolite catalyst due to sulfur 
poisoning (exposure of the catalyst to methylmercaptan in the gaseous 
stream). The patent thus discloses the utility of the copper-promoted 
specified zeolites for three-way conversion in an exhaust gas generated by 
a lean air to fuel ratio combustion mixture. 
The art thus shows an awareness of the utility of metal-promoted zeolite 
catalysts including, among others, iron-promoted and copper-promoted 
zeolite catalysts, for the selective catalytic reduction of nitrogen 
oxides with ammonia. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a method for 
the reduction of nitrogen oxides with ammonia, the method comprising the 
following steps. A gaseous stream containing nitrogen oxides and ammonia, 
and which may also contain oxygen, is contacted at a temperature of from 
about 250.degree. C. to 600.degree. C. with a sulfur-tolerant catalyst 
composition. The catalyst composition comprises a zeolite having a silica 
to alumina ratio of at least about 10, and a pore structure which is 
interconnected in all three crystallographic dimensions by pores having an 
average kinetic pore diameter of a least about 7 Angstroms, e.g., from 
about 7 to 8 Angstroms, and one or both of an iron and a copper promoter 
present in the zeolite, for example, in the amount of from about 0.1 to 30 
percent by weight, preferably from about 1 to 5 percent by weight, of the 
total weight of promoter plus zeolite. 
Another aspect of the invention provides that the promoter is an iron 
promoter. 
Still another aspect of the present invention provides that the zeolite 
comprises one or more of USY, Beta and ZSM-20. A refractory binder may be 
admixed with the zeolites. 
The gaseous stream may contain from about 0.7 to 2 moles of ammonia per 
mole of nitrogen oxides. Oxygen may also be present in the gaseous stream 
in an amount of from about 0.5 to 30 volume percent of the gaseous stream.

References herein and in the claims to a zeolite catalyst containing a 
percent "by weight" promoter means a percentage calculated as the weight 
of promoter, as the metal, divided by the combined weights of promoter (as 
the metal) plus the zeolite. 
Reference herein and in the claims to "metal", "iron" and "copper" with 
respect to the promoters should not be taken to imply that the promoter is 
necessarily in the elemental or zero valence state; the terms enclosed in 
quotes should be understood to include the presence of promoters as they 
exist in the catalyst compositions, e.g., as exchanged ions and/or 
impregnated ionic or other species. 
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF 
In order to reduce the emissions of nitrogen oxides from flue and exhaust 
gases, such as the exhaust generated by gas turbine engines, ammonia is 
added to the gaseous stream containing the nitrogen oxides and the gaseous 
stream is then contacted with a suitable catalyst at elevated temperatures 
in order to catalyze the reduction of nitrogen oxides with ammonia. Such 
gaseous streams often inherently contain substantial amounts of oxygen. 
For example, a typical exhaust gas of a turbine engine contains from about 
2 to 15 volume percent oxygen and from about 20 to 500 volume parts per 
million nitrogen oxides, the latter normally comprising a mixture of NO 
and NO.sub.2. Usually, there is sufficient oxygen present in the gaseous 
stream to oxidize residual ammonia, even when an excess over the 
stoichiometric amount of ammonia required to reduce all the nitrogen 
oxides present is employed. However, in cases where a very large excess 
over the stoichiometric amount of ammonia is utilized, or wherein the 
gaseous stream to be treated is lacking or low in oxygen content, an 
oxygen-containing gas, usually air, may be introduced between the first 
catalyst zone and the second catalyst zone, in order to insure that 
adequate oxygen is present in the second catalyst zone for the oxidation 
of residual or excess ammonia. The reduction of ammonia with nitrogen 
oxides to form nitrogen and H.sub. 2 O can be catalyzed by metal-promoted 
zeolites to take place preferentially to the oxidation of ammonia by the 
oxygen, hence the process is often referred to as the "selective" 
catalytic reduction ("SCR") of nitrogen oxides, and is sometimes referred 
to herein simply as the "SCR" process. 
The catalysts employed in the SCR process ideally should be able to retain 
good catalytic activity under high temperature conditions of use, for 
example, 400.degree. C. or higher, under hydrothermal conditions and in 
the presence of sulfur compounds. High temperature and hydrothermal 
conditions are often encountered in practice, such as in the treatment of 
gas turbine engine exhausts. The presence of sulfur or sulfur compounds is 
often encountered in treating the exhaust gases of coal-fired power plants 
and of turbines or other engines fueled with sulfurcontaining fuels such 
as fuel oils and the like. 
Theoretically, it would be desirable in the SCR process to provide ammonia 
in excess of the stoichiometric amount required to react completely with 
the nitrogen oxides present, both to favor driving the reaction to 
completion and to help overcome inadequate mixing of the ammonia in the 
gaseous stream. However, in practice, significant excess ammonia over the 
stoichiometric amount is normally not provided because the discharge of 
unreacted ammonia from the catalyst would itself engender an air pollution 
problem. Such discharge of unreacted ammonia can occur even in cases where 
ammonia is present only in a stoichiometric or sub-stoichiometric amount, 
as a result of incomplete reaction and/or poor mixing of the ammonia in 
the gaseous stream. Channels of high ammonia concentration are formed in 
the gaseous stream by poor mixing and are of particular concern when 
utilizing catalysts comprising monolithic honeycomb-type carriers 
comprising refractory bodies having a plurality of fine, parallel gas flow 
paths extending therethrough because, unlike the case with beds of 
particulate catalysts, there is no opportunity for gas mixing between 
channels. It is therefore also desirable that the catalyst employed to 
catalyze the selective catalytic reduction of nitrogen oxides, be 
effective to catalyze the reaction of oxygen and ammonia, in order to 
oxidize excess or unreacted ammonia to N.sub.2 and H.sub.2 O. 
The present invention is predicated on the discovery that a certain class 
of zeolites, especially when promoted with a promoter such as iron or 
copper, especially iron, exhibits desired characteristics as described 
above by providing a sulfur tolerant catalyst which shows good activity 
for both (1) the selective catalytic reduction of nitrogen oxides by 
reaction with ammonia, even in the presence of oxygen, and (2) the 
oxidation of ammonia with oxygen when nitrogen oxides are at very low 
concentrations. The catalysts of the present invention retain such 
activity even after prolonged exposure to high temperatures, hydrothermal 
conditions, and sulfate contamination of the type often encountered in 
use, e.g., in the treatment of coal-fired power plants or turbine engine 
exhaust gases. 
Generally, in accordance with the practices of the present invention, a 
catalyst is provided which comprises a zeolite having specific properties 
as described below, and which is promoted by a metal, preferably iron, in 
order to enhance its catalytic activity. The zeolite may be provided in 
the form of a fine powder which is admixed with or coated by a suitable 
refractory binder, such as bentonite or silica, and formed into a slurry 
which is deposited upon a suitable refractory carrier. Typically, the 
carrier comprises a member, often referred to as a "honeycomb" carrier, 
comprising one or more refractory bodies having a plurality of fine, 
parallel gas flow passages extending therethrough. Such carriers are, of 
course, well known in the art and may be made of any suitable material 
such as cordierite or the like. The catalysts of the present invention may 
also be provided in the form of extrudates, pellets, tablets or particles 
of any other suitable shape, for use as a packed bed of particulate 
catalyst, or as shaped pieces such as plates, saddles, tubes or the like. 
The catalysts of the present invention show a marked resistance to 
poisoning by sulfates (or other sulfur compounds) which are often 
contained in the gas streams which are treatable by the catalysts of the 
present invention. Without wishing to be bound by any particular theory, 
it appears that SO.sub.2 poisoning has both short term and long term 
effects. For example, flowing a gas stream containing 2,000 parts per 
million by volume ("Vppm") SO.sub.2 through catalysts comprising 
copper-promoted small to medium pore zeolites such as ZSM-5, naturally 
occurring chabazite and clinoptilolite, resulted in 10 to 40 percent 
reduction in SCR process activity. Even at SO.sub.2 levels as low as 130 
Vppm SO.sub.2, significant activity reduction for the SCR process was 
noted for such catalysts. On the other hand, larger pore zeolites such as 
Y, L and USY exhibited no short-term SO.sub.2 susceptibility. With 
operating temperatures at about 350.degree. C., the short-term SO.sub.2 
poisoning effect on a copper-promoted mordenite was shown to be 
reversible. Thus, when the supply of SO.sub.2 to the test gas stream 
passing through the copper-promoted mordenite catalyst was turned off, the 
activity for catalytic reduction of NO immediately returned to the same 
level attained by the catalyst prior to introducing the SO.sub.2. 
Apparently, SO.sub.2 is absorbed, but not tightly bound in the zeolite 
pores. In the case of the small to medium pore zeolites, this competition 
absorption with NH.sub.3 and NO probably results in a physical blockage 
and/or diffusional restriction. 
On the other hand, when zeolite catalysts are subjected to higher SO.sub.2 
concentrations for longer periods of time, such as 5,000 Vppm SO.sub.2 for 
protracted periods, such as overnight, a 15 to 25 percent activity 
reduction for the SCR process was noted for copper promoted, synthetic 
iron-free zeolites. A 60 percent reduction in SCR process activity is 
typical for Fe.sub.2 O.sub.3 containing natural chabazite. Similar results 
were sustained with iron promoted mordenite catalysts. 
Even at lower levels of SO.sub.2 concentration, similar to those likely to 
be encountered in commercial operations, a permanent activity loss for the 
SCR process is shown by many zeolite catalysts. For example, a 
copper-promoted mordenite catalyst was subjected overnight to passage 
through it of a gaseous stream containing 540 Vppm SO.sub.2, and showed a 
permanent activity loss comparable to that described above for the 
catalysts subjected to the 5000 Vppm SO.sub.2 -containing gas. 
For zeolites with silica-to-alumina ratios of less than 10, the activity 
loss appears to be associated with insufficient stability under the 
simulated acidic aging conditions. As indicated by the prior art noted 
above, the utilization of high ratios of silica to alumina is known to 
enhance acid resistance of the zeolite and to provide enhanced resistance 
of the zeolite to acid sulfur poisoning. Generally, silica to alumina 
ratios well in excess of the minimum of 10 may be employed. Conversion 
efficiencies of 90 to 93% for NO.sub.x reduction with ammonia have been 
attained with fresh copper promoted Beta zeolites having silica to alumina 
ratios of 20, 26, 28, 37 and 62. A conversion efficiency of 77% was 
attained by a fresh copper promoted ZSM-5 zeolite having a silica to 
alumina ratio of 46. However, fresh copper promoted USY zeolites with 
silica to alumina ratios of, respectively, 8 and 30 provided 85% and 39% 
conversions of NO.sub.x, suggesting that at least for USY, silica to 
alumina ratios should be significantly less than 30. 
However, resistance to short term sulfur poisoning and the ability to 
sustain a high level of activity for both the SCR process and the 
oxidation of ammonia by oxygen has been found to be provided by zeolites 
which also exhibit pore size large enough to permit adequate movement of 
the reactant molecules NO and NH.sub.3 in to, and the product molecules 
N.sub.2 and H.sub.2 O out of, the pore system in the presence of sulfur 
oxide molecules resulting from short term sulfur poisoning, and/or sulfate 
deposits resulting from long term sulfur poisoning. The pore system of 
suitable size is interconnected in all three crystallographic dimensions. 
As is well known to the those skilled in the zeolite art, the crystalline 
structure of zeolites exhibits a complex pore structure having more or 
less regularly recurring connections, intersections and the like. Pores 
having a particular characteristic, such as a given dimension diameter or 
cross-sectional configuration, are said to be one dimensional if those 
pores do not intersect with other like pores. If the pores intersect only 
within a given plane with other like pores, the pores of that 
characteristic are said to be interconnected in two (crystallographic) 
dimensions. If the pores intersect with other like pores lying both in the 
same plane and in other planes, such like pores are said to be 
interconnected in three dimensions, i.e., to be "three dimensional". It 
has been found that zeolites which are highly resistant to sulfate 
poisoning and provide good activity for both the SCR process and the 
oxidation of ammonia with oxygen, and which retain good activity even when 
subject to high temperatures, hydrothermal conditions and sulfate poisons, 
are zeolites which have pores which exhibit a pore diameter of at least 
about 7 Angstroms and are interconnected in three dimensions. Without 
wishing to be bound by any specific theory, it is believed that the 
interconnection of pores of at least 7 Angstroms diameter in three 
dimensions provides for good mobility of sulfate molecules throughout the 
zeolite structure, thereby permitting the sulfate molecules to be released 
from the catalyst to free a large number of the available adsorbent sites 
for reactant NO.sub.x and NH.sub.3 molecules and reactant NH.sub.3 and 
O.sub.2 molecules. Any zeolites meeting the foregoing criteria are 
suitable for use in the practices of the present invention; specific 
zeolites which meet these criteria are USY, Beta and ZSM-20. Other 
zeolites may also satisfy the aforementioned criteria. 
A number of tests were conducted in order to evaluate the catalytic 
activity and selectivity for the SCR process and ammonia oxidation, of 
both fresh and aged catalysts comprising iron promoted zeolites and copper 
promoted zeolites. All the catalysts employed in these tests were prepared 
from the same NH.sub.4 +form of Beta zeolite powder, which was synthesized 
as described in the following Example 1. 
Reference is made below to the weights of solids being on a "vf basis". The 
term in quotes means a volatiles-free basis, and is used to indicate the 
weight that the solid in question would have if it were calcined at 
1000.degree. C. to drive off volatiles. Thus, if 10.1 grams of a substance 
contains 0.1 gram of such volatiles, the 10.1 grams is reported as "10 
grams (vf basis)". Unless specifically otherwise stated, all percentages 
by weight herein and in the claims are on a vf basis. 
EXAMPLE 1 
I. Synthesis of Batch 1: 
A. The following materials were combined in a 100 gallon, titanium lined, 
autoclave reactor and stirred sufficiently to maintain the solids in 
suspension: 
1. 18.28 Kg of Hi-sil.RTM. #233 silica powder 
2. Sufficient amounts of each of the following to result in molar ratios of 
SiO.sub.2, Na.sub.2 O, H.sub.2 O, and (Tetraethylammonium).sub.2 O to 
Al.sub.2 O.sub.3 of 23.1, 1.94, 767, and 1.62, respectively: 
a. Nalco sodium aluminate solution (20.9% Al.sub.2 O.sub.3, 24.7% Na.sub.2 
O, 54.0% H.sub.2 O) 
b. 40% solution of Tetraethylammonium hydroxide (TEAOH) 
c. Deionized water 
B. To the mixture obtained in step A was added 1.38 Kg (vf basis) of 
zeolite Beta powder. 
C. The reactor was sealed and heated to 150.degree. C. with continued 
stirring. 
D. After 6 days at 150.degree. C. the reactor was cooled to room 
temperature and three separate batches as the slurry contained in the 
reactor were filtered in a twelve inch square filter press to separate the 
solids from the reaction liquor. The solids from the first two filter 
batches were not washed, while the solids from the third batch was washed 
with several gallons of deionized water. 
E. The resultant filter cakes were combined and dried at 100.degree. C. 
Next, a 13.0 Kg batch of the dried solids was calcined for 1 hour at 
316.degree. C. followed by 1 hour at 593.degree. C. The resultant calcined 
powder, which was designated Batch 1, had a molar SiO.sub.2 /Al.sub.2 
O.sub.3 ratio of 17/1 (by chemical analysis) and a BET surface area of 562 
m.sup.2 /g. Analysis by x-ray diffraction showed the characteristic peaks 
associated with Beta zeolite. 
II. Synthesis of Batch 2: 
A. Step A of the procedure used to prepare Batch 1 was repeated, except 
that 36.56 Kg of Hi-sil.RTM. #233 silica powder and sufficient amounts of 
the same materials as used in Step A of the preparation of Batch 1 were 
used to result in molar ratios of SiO.sub.2, Na.sub.2 O, H.sub.2 O, and 
(Tetraethylammonium).sub.2 O to Al.sub.2 O.sub.3 of 23.1, 1.94, 383, and 
1.62 respectively. 
B. To the mixture obtained in Step A was added 2.76 Kg (vf basis) of 
zeolite Beta powder of Batch 1. 
C. The reactor was sealed and heated to 150.degree. with continued 
stirring. 
D. After 6 days at 150.degree. C. the reactor was cooled to room 
temperature and batches of the slurry contained therein were filtered in a 
twelve square inch filter press to separate the solids from the reaction 
liquor. All the solids obtained were washed by passing deionized water 
through the filter cake. 
E. The resultant filter cake solids were combined and dried at 100.degree. 
C. A 26.4 Kg (vf basis) batch of the dried solids was calcined for 1 hour 
at 316.degree. C., followed by 1 hour at 593.degree. C. The resultant 
calcined powder, which was designated Batch 2, had a molar SiO.sub.2 
/Al.sub.2 O.sub.3 ratio of 18/1 (by chemical analysis) and a BET surface 
area of 577 m.sup.2 /g. Analysis by x-ray diffraction showed the 
characteristic peaks associated with Beta zeolite. 
III. A master lot of zeolite Beta was made by combining 7.7 Kg of the Batch 
1 powder and 26.4 Kg of the Batch 2 powder. The resultant 34.1 Kg master 
lot of zeolite Beta was NH.sub.4 +ion exchanged, as follows. 
A. A solution was prepared by mixing 51.1 Kg of 54% NH.sub.4 NO.sub.3 
solution with 68.1 Kg of deionized water. 
B. To the solution of Step A was added the master lot of Beta powder, with 
stirring sufficient to suspend the solids. 
C. The pH of the suspension of Step B was adjusted from 3.9 to 3.15 using 
484 g of concentrated HNO.sub.3, and the slurry was heated to 82.degree. 
C. 
D. After 30 minutes at 82.degree. C., the slurry was cooled, and then 
filtered on a vacuum filter to separate the solids from the spent exchange 
solution and provide an NH.sub.4.sup.+ Beta powder, designated 
NH.sub.4.sup.+ Beta. Na.sub.2 O analysis was 0.47% by weight, vf basis. 
The resultant NH.sub.4.sup.+ Beta was used to prepare iron promoted zeolite 
catalysts, as shown in the following Example 2. 
EXAMPLE 2 
I. A portion of the NH.sub.4.sup.+ Beta powder of Example 1 was promoted 
with iron as follows. 
A. 8.35 Kg (vf) of the NH.sub.4.sup.+ Beta exchanged powder was combined 
(with stirring) with an aqueous solution of Fe.sub.2 (SO.sub.4).sub.3 that 
contained 3% by weight Fe. A ratio of 2.5 parts by weight of solution per 
part by weight of NH.sub.4.sup.+ Beta powder (on a vf basis) was used. 
B. With continued stirring, the slurry of Step A was heated to 82.degree. 
C. for one hour, and then cooled. 
C. The cooled slurry of Step B was then vacuum filtered and washed with a 
equal volume of deionized water. 
D. The filter cake of Step C was dried at 100.degree. C. to provide an 
iron-promoted Beta powder, designated Fe Beta 1. 
E. Chemical analysis of Fe Beta 1 showed that it contained 2.78% iron 
(expressed as the metal on a vf basis). 
F. One half of the Fe Beta 1 had additional Fe added to it, using the same 
procedure described in steps A-D above, except that the iron sulfate 
solution contained only 1.5% by weight Fe. After drying, this material was 
calcined for 2 hours at 538.degree. C. to provide an iron promoted Beta 
powder designated Fe Beta 2. 
G. Chemical analysis of Fe Beta 2 showed that it contained 4.42% iron 
(expressed as the metal on a vf basis). 
The NH.sub.4.sup.+ Beta of Example 4 was used to prepare copper promoted 
zeolite catalysts, as shown in the following Example 3. 
EXAMPLE 3 
A portion of the NH.sub.4.sup.+ Beta powder of Example 1 was promoted with 
copper as follows: 
A. 25.0 Kg (vf) of the NH.sub.4.sup.+ Beta powder was added to 56.25 Kg of 
Cu(SO.sub.4) solution containing 5% by weight Cu, with stirring to suspend 
the solids and disperse the lumps. 
B. With continued stirring, the slurry of Step A was heated to 82.degree. 
C. for one hour, and then cooled. 
C. The cooled slurry of Step B was vacuum filtered to separate the solids 
from the liquid, and the solids were washed with a volume of deionized 
water equal to the volume of the separated liquid. 
D. The powder of Step C was dried at 100.degree. C. to provide a copper 
promoted Beta powder, designated Cu Beta 1. Chemical analysis showed that 
the Cu Beta 1 powder contained 3.42% by weight Cu (expressed as the metal 
on a vf basis). 
E. Two-thirds by weight of the Cu Beta 1 dried powder was reslurried (with 
continuous stirring) in deionized water in a ratio of 3 parts by weight of 
water to one part by weight (vf basis) of the Cu Beta 1 powder. 
F. After one hour at room temperature, the slurry of Step E was vacuum 
filtered to remove the water and allowed to air dry overnight. 
G. The powder obtained from Step E was again subjected to the re-slurrying 
filtering and drying of Steps E and F, but with a weight ratio of 
water/powder of 2.5/1 instead of 3/1. 
H. The air dried filter cake obtained from Step G was oven dried at 
100.degree. C., and then calcined for 2 hours at 538.degree. C. to provide 
a copper promoted Beta designated Cu Beta 2. 
I. Chemical analysis of Cu Beta 2 showed that this powder contained 2.56% 
Cu (expressed as the metal on a vf basis). 
The NH.sub.4.sup.+ Beta obtained in Example 1 and the iron (Fe Beta 1 and 
Fe Beta 2) and copper (Cu Beta 1 and Cu Beta 2) promoted catalysts 
obtained in Examples 2 and 3 were prepared for testing as described in the 
following Example 4. 
EXAMPLE 4 
I. Small portions of each powder (Fe Beta 1, Fe Beta 2, Cu Beta 1 and Cu 
Beta 2) were separately granulated into a -40+80 mesh screen fraction for 
testing. This was done as follows: 
A. Several disks were made at nominally 10,000-15,000 lb/in.sup.2 pressure 
from each powder, using a tool steel die in a hydraulic press. 
B. Each disk was gently ground with a porcelain mortar and pestle, and the 
resultant granules screened through 40 and 80 mesh screens. 
C. The size fraction that passed through 40 mesh and was retained upon the 
80 mesh screen was used for testing. 
II. Samples of NH.sub.4.sup.+ Beta powder, Fe Beta 1, Fe Beta 2, Cu Beta 1 
and Cu Beta 2 powders were aged for 840 hours at 520.degree. C. using the 
following procedure. 
A. Several grams of each -40+80 mesh powder were placed in separate 
compartments of glazed porcelain refractory boats (nominally 1.5 cm 
wide.times.1.5 cm deep.times.10 cm long, with each compartment being 2 cm 
long). 
B. The boats were placed in the hot zone of a 5 cm diameter horizontal tube 
furnace, and the furnace was sealed. 
C. A gas containing 10% O.sub.2, 20% H.sub.2 O, and the balance N.sub.2, 
was passed through the furnace tube at a rate of 22 liters per minute 
("1/min") and the furnace was heated to a hot zone temperature of 
520.degree. C. As used herein "1/min" means liters per minute based on 
standard conditions of temperature and pressure, i.e., 20.degree. C. and 
1.0 atmospheres. 
D. After the furnace had reached temperature, sufficient water was injected 
into the entrance of the hot zone to provide a 10% steam environment. 
E. After 840 hours the furnace was cooled and the samples removed. The 
refractory boat containing the NH.sub.4.sup.+ Beta sample and the Fe Beta 
2 sample (4.42% Fe) failed during aging, and those samples were lost. 
The prepared samples were tested for catalytic activity, as described in 
the following Example 5. 
EXAMPLE 5 
I. The aged samples obtained from Example 4 were tested as catalysts for 
selective catalytic reduction of NO.sub.x activity ("SCR Testing") and for 
NH.sub.3 oxidation activity, using nominally 3 millimeter inside diameter, 
"U" shaped Vycor reactors having two vertical legs nominally 20 cm long, 
and a semicircular bottom section nominally 5 cm in diameter. The 
procedure used was as follows: 
A. A plug of fused silica wool was placed at the base of the vertical 
section of the inlet leg of one of the reactors. 
B. One tenth of a gram (0.1 g) of the -40+80 mesh Beta powder to be tested 
was placed on the silica wool to serve as the catalyst bed, and a Vycor 
thermocouple well was positioned just above the catalyst bed. 
C. Between 1 and 3 reactors were placed in a reactor furnace and connected 
to the gas supply system. 
D. N.sub.2 and air were mixed into a gas containing 10% O.sub.2 and the 
balance N.sub.2, and this was passed through a furnace where it was 
preheated to 350.degree. C. 
E. The heated gas stream of Step D was then divided among the reactors such 
that each reactor received a flow rate of 2 l/min. (for a space velocity 
of 1.2.times.10.sup.6 ccg.sup.-1 hr.sup.-1). 
F. The reactor furnace was then heated to a temperature nominally 
50.degree. C. above the test temperature, such that the reactor 
thermocouples read the nominal test temperature. 
G. The reaction gases were than added to the inlet gas stream in the 
following amounts: 
1. For SCR testing, 200 parts per million parts by volume "Vppm" each of NO 
and NH.sub.3 were added to the gas. 
2. For NH.sub.3 oxidation activity testing, 200 Vppm of NH.sub.3 was added 
to the gas. 
H. After all the flows and temperatures had stabilized, the inlet and 
outlet concentrations of NO.sub.x and NH.sub.3 were measured using a 
Thermoelectron Model 10 NO.sub.x analyzer for both NO.sub.x and NH.sub.3 
analysis. Periodic NH.sub.3 measurements were verified using the Draeger 
tube method. 
I. The gas temperature was then changed, and the measurements repeated as 
in Step H above. 
The results obtained by the tests of Example 5 are plotted in FIGS. 1-4. 
In each of FIGS. 1, 2 and 3 the percentage conversion of nitric oxide (NO) 
in the test gas is plotted on the vertical axis, and the test gas inlet 
temperature (to the catalyst bed) is plotted on the horizontal axis. The 
nitric oxide (NO) content of the test gas is representative of nitrogen 
oxides (NO.sub.x) generally, and so reference below is made to NO.sub.x 
conversion. 
FIG. 1 compares the NO.sub.x SCR process conversion in the test gas flowed 
through beds comprised of fresh samples of Cu Beta.sup.-2, Fe Beta.sup.-1 
and Fe Beta 2. In FIG. 1, data points for Cu Beta 2 are shown by diamonds, 
for Fe Beta 1 by rectangles and for Fe Beta 2 by Xs. The data of FIG. show 
that the copper and iron promoted Beta powders have similar SCR activities 
and selectivities although, as evidenced by the slight conversion decrease 
with Cu Beta 2 above about 450.degree. C., the iron promoted catalysts, Fe 
Beta 1 and Fe Beta 2, may have less of a tendency than copper promoted 
Beta to oxidize NH.sub.3 in the presence of NO.sub.x. 
FIG. 2 compares the NO.sub.x SCR process conversion in the test gas flowed 
through beds comprised of fresh and aged samples of Cu Beta.sup.-1. In 
FIG. 2, data points for aged Cu Beta 1 are shown by diamonds and for fresh 
Cu Beta 1 by reactangles. The data of FIG. 1 show that Cu Beta.sup.-1 
experienced substantial deactivation during aging. 
FIG. 3 compares the NO.sub.x SCR process conversion in the test gas flowed 
through beds comprised of fresh and aged samples of Fe Beta.sup.-1. In 
FIG. 3, data points for fresh Fe Beta 1 are shown by diamonds, and data 
points for aged (840 hours) Fe Beta 1 are shown by reactangles. The data 
of FIG. 3 show that Fe Beta.sup.-1 did not deactivate after 840 hours of 
aging. 
In FIG. 4, the percentage conversion on NH.sub.3 in the test gas is plotted 
on the vertical axis, and the test gas inlet temperature (to the catalyst 
bed) is plotted on the horizontal axis. FIG. 4 compares the NH.sub.3 
conversion in the test gas flowed through beds comprised of fresh samples 
of Fe Beta.sup.-2 and NH.sub.4.sup.+ Beta. In FIG. 4, data points for Fe 
Beta 2 are shown by diamonds and data points for NH.sub.4 + Beta are shown 
by rectangles. The data of FIG. 4 show excellent conversion of NH.sub.3 by 
Fe Beta 2 and no conversion by NH.sub.4.sup.+ Beta. Analysis showed that 
the NH.sub.3 oxidation was selective to N.sub.2 and H.sub.2 O, and there 
was no evidence of NO.sub.x formation with either tested catalyst. The Fe 
Beta.sup.-2 showed substantial NH.sub.3 oxidation activity in the absence 
of NO.sub.x, producing about 80+% conversion even at the exceptionally 
high space velocity of 1.2.times.10.sup.6 ccg.sup.-1 hr.sup.-1. 
NH.sub.4.sup.+ Beta produced no detectable NH.sub.3 conversion under these 
conditions. 
The results above show that iron promoted Beta is a highly active and 
selective bifunctional catalyst that is particularly well suited for the 
SCR process and excess or residual ammonia oxidation at temperature above 
about 400.degree. C. It is extremely active and selective for the SCR 
process reaction when NH.sub.3 and NO.sub.x are both present. However, 
under conditions where there is excess NH.sub.3 present, the iron promoted 
catalyst is extremely active for the selective oxidation of NH.sub.3 by 
oxygen to N.sub.2 and water. In addition, iron promoted Beta does not 
deactivate during exposure to hydrothermal conditions similar to those 
expected in a high temperature SCR process environment, such as the 
exhaust of a gas turbine. Copper promoted Beta, while exhibiting catalytic 
performance similar to that of iron promoted Beta in the fresh state, 
deactivated noticably during aging. 
In order to demonstrate the enhanced sulfur resistance provided by zeolite 
catalysts made in accordance with the present invention, a series of 
metal-promoted zeolite catalysts were prepared by techniques similar to 
those described above and subjected to aging in both SO.sub.2 containing 
and SO.sub.2 free gaseous streams, as described in the following Example 
6. 
EXAMPLE 6 
Three catalyst samples in accordance with the present invention were 
prepared as clay bound extrudates by the following general procedure: -100 
mesh dried zeolite powder was combined with Georgia Kaolin GK 129 
bentonite clay and FMC Avicel.RTM. microcrystalline cellulose in a ratio 
of 4 parts by weight zeolite powder (vf basis) to 1 part by weight clay 
(vf basis) plus 3% by weight cellulose (based on the total vf weight of 
zeolite and clay). To this dried mixture was added a minimum amount of 
deionized water to produce a paste having a consistency of putty. This 
paste was then extruded through a 60 cc plastic syringe having an aperture 
of about 0.063 inches in diameter. The resultant extrudates were dried at 
100.degree. C. then calcined for 2 hours at 538.degree. C. The three 
catalyst samples so made were designated as: Catalyst 1, comprising a 
ZSM-20 zeolite promoted with 3.76 weight percent copper; Catalyst 2, a 
Beta zeolite catalyst promoted with 4.11 percent iron; and Catalyst 3, a 
Beta zeolite catalyst promoted with 3.23 percent copper. The ZSM-20 
zeolite powder (SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 14/1 by 
chemical analysis) and Beta zeolite powder (SiO.sub.2 /Al.sub.2 O.sub.3 
molar ratio of about 20/1 by chemical analysis) were synthesized in the 
laboratory and were identified as such by x-ray diffraction and surface 
area analysis. The as-synthesized zeolites were then ammonium exchanged to 
less than 0.75% by weight Na.sub.2 O (vf basis) in a manner similar to 
that described in Example 1. 
A comparative extrudate catalyst, designated Catalyst C, comprised a 
hydrogen mordenite catalyst promoted with 2.86 percent copper. The 
mordenite zeolite powder used was Linde LZM-8 (SiO.sub.2/l /Al.sub.2 
O.sub.3 molar ratio of 18/1 by chemical analysis). The addition of Cu and 
Fe was accomplished via standard impregnation techniques or by exchange 
procedures similar to those described in Examples 2 and 3. Mordenite does 
not possess the pore structure (about 7 Angstroms diameter pores 
interconnected in all three crystallographic dimensions) which 
characterizes the zeolites of the present invention. 
All the promoting metal loadings given above are calculated on the basis of 
the copper or iron being taken as the metal, and are based on the weight 
of the promoter, as metal, plus zeolite. Two samples of each catalyst were 
prepared and one sample was aged in a 0.625 inch (1.59 cm) diameter 
stainless steel reactor in an SO.sub.2 environment by having flowed 
therethrough a gas containing 5,000 Vppm SO.sub.2, 1,000 Vppm NO, 10 
volume percent H.sub.2 O, and 10 volume percent oxygen, balance N.sub.2. 
This aging gas was passed through a 4.5 inch (11.4 cm) deep bed of the 
catalyst samples at a temperature of 350.degree. C. and a volumetric 
velocity of 12,500 volumes of gas, measured at standard conditions of 
temperature and pressure, per volume of catalyst per hour. The aged 
samples plus a second sample of each of the four unaged catalysts were 
placed into a 0.625 inch (1.59 cm) diameter stainless steel reactor in 
beds about 3 inches (7.6 cm) deep. A gas containing 400 Vppm NO plus 400 
Vppm NH.sub.3, 10 percent H.sub.2 O and 10 percent O.sub.2, balance 
N.sub.2 was passed through the beds of catalyst samples at a temperature 
of 350.degree. C. and a volumetric flow rate of 150,000 volumes of gas, 
measured at standard temperature and pressure, per volume of catalyst per 
hour. The following Table I shows the activity of the catalysts in terms 
of the percent of NO.sub.x, i.e., NO, converted by the respective unaged 
samples and the samples aged for the indicated aging periods. The 
"SO.sub.2 added" column shows the conversion efficiencies attained with 
the same test gas to which 2,000 Vppm SO.sub.2 have been added. 
TABLE I 
______________________________________ 
Aging Time % NO.sub.x Conversion 
Catalyst (hrs) NO SO.sub.2 
SO.sub.2 Added 
______________________________________ 
Catalyst 1 
0 87 86 
64 75 75 
192 66 66 
Catalyst 2 
0 88 90 
162 70 
Catalyst 3 
0 88 85 
64 70 
129 64 
Catalyst C 
0 90 80 
64 57 
129 37 
______________________________________ 
The results of Table I show that the activity of fresh Catalyst 1, Catalyst 
2 and Catalyst 3, each being a catalyst in accordance with the teachings 
of the present invention, was practically the same whether or not SO.sub.2 
was present in the gas being treated. On the other hand, fresh Catalyst C, 
a copper promoted mordenite catalyst which lies outside the scope of the 
present invention, showed a significant reduction in conversion 
efficiency, from 90% to 80%, immediately upon the introduction of SO.sub.2 
to the gas stream being treated. 
The data of Table I also shows that Catalysts 1, 2 and generally retained 
higher activities, i.e., higher percent conversion of NO.sub.x, in the SCR 
process upon aging than did comparative Catalyst C. Thus, Catalyst 1 
showed an activity decline from 87% to 66% after 192 hours, Catalyst 2 
showed an activity decline from 88% to 70% after 162 hours, and Catalyst 3 
showed an activity decline from 88% to 64% after 129 hours. These declines 
are much smaller than that of comparative Catalyst C, which showed an 
activity decline from 90% to 37% after 129 hours. 
The data of Table I clearly show that catalysts made in accordance with the 
teaching of the present invention are substantially more resistant to 
sulfate poisoning than the comparative sample. 
As described above, the zeolites useful in the present invention have a 
silica to alumina ratio in excess of 10 in order to enhance their 
resistance to acidic conditions and therefore their resistance to acidic 
sulfur poisoning. A number of catalysts in accordance with the present 
invention were prepared with different silica to alumina ratios and tested 
for their conversion activity in the SCR process, in an attempt to 
determine if changes in the silica to alumina ratio affected activity. The 
catalysts were prepared and tested as described in the following Example 
7. 
EXAMPLE 7 
The catalyst samples were prepared as clay bound extrudates according to 
the procedure described in Example 6. The Beta zeolites were synthesized 
in the laboratory and were identified as such by x-ray diffraction and 
surface area analysis. Beta zeolites with increased SiO.sub.2 /Al.sub.2 
O.sub.3 molar ratios (as determined by chemical analysis) were prepared by 
corresponding increases in the reagent SiO.sub.2 /Al.sub.2 O.sub.3 molar 
ratios used. As-synthesized Beta zeolites were then ammonium exchanged to 
less than 0.50%5 by weight Na.sub.2 O (vf basis) in a manner similar to 
that described in Example 1. The USY zeolite with a SiO.sub.2 /Al.sub.2 
O.sub.3 molar ratio of 8/1 (by x-ray unit cell size determination) was 
prepared by a standard steam ultrastabilization/ammonium exchange of Linde 
LZY062 (NH.sub.4.sup.+ /Na.sup.+ -Y zeolite). The USY zeolite used with a 
SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 30/1 (by x-ray unit cell size 
determination) was Toyo Soda #TSZ-360 HUA. The addition of Cu and Fe was 
accomplished via standard impregnation techniques or by exchange 
procedures similar to those described in Examples 2 and 3. 
The extrudate catalysts were tested in a 0.625 inch (1.59 cm) diameter 
stainless steel reactor by flowing through a 3 inch (7.6 cm) deep bed of 
the fresh catalyst a test gas having an inlet temperature of 350.degree. 
C. and containing 400 Vppm NO, 400 Vppm NH.sub.3, 10 percent by volume 
O.sub.2 and 10 percent by volume H.sub.2 O, balance nitrogen. The test gas 
was flowed through the fresh catalyst bed at a volumetric velocity of 
150,000 volumes of gas, measured at standard temperature and pressure, per 
volume of catalyst per hour and the percentage of NO originally in the gas 
which was converted to N.sub.2 and H.sub.2 O was measured. The results set 
forth in the following table were attained. 
TABLE II 
______________________________________ 
Catalyst SiO.sub.2 /Al.sub.2 O.sub.3 
% NO.sub.x Conversion 
______________________________________ 
1.73% Cu/Beta 
(20) 90 
1.63% Cu/Beta 
(26) 93 
1.17% Cu/Beta 
(28) 93 
1.80% Cu/Beta 
(37) 90 
1.66% Cu/Beta.sup.3 
(62) 93 
1.97% Cu/USY (8) 85 
2.20% Cu/USY (30) 39 
______________________________________ 
The data of Table II show that increasing the silica to alumina ratio of 
Beta zeolite had no effect on the conversion efficiency provided by the 
catalyst, whereas an increase of the silica to alumina ratio of the USY 
zeolite to 30 caused a significant reduction in the conversion, from 85% 
(for a USY zeolite catalyst with a silica to alumina ratio of 8) to 39%. 
The data of the table suggests that at least for USY zeolite, the silica 
to alumina ratio should be maintained well below 30, probably close to 10.