AU687582B2 - Catalytic system for the reduction of nitrogen oxides - Google Patents

Description

WO 96/01689 PCTUS95/08253 -1- CATALYTIC SYSTEM FOR THE REDUCTION OF NITROGEN OXIDES This invention is concerned with the abatement of nitrogen oxides and, optionally, other undesirable compounds, in industrial and engine exhaust gases. In particular, it is concerned with a catalytic method for efficiently eliminating these undesirable compounds before discharge to the atmosphere. It is more particularly concerned with the use of a specially prepared catalyst comprising an intermediate pore size zeolite that has been treated or modified to contain iron or an iron compound for the selective catalytic reduction of the NO, present in the exhaust gas.

The present invention concerns a catalyst that is useful for treating an exhaust gas comprising NO. and ammonia, the catalyst comprising an intermediate pore size zeolite and iron or an iron compound that has been hydrothermally treated, the hydrothermal treatment and the iron concentration being effective to produce a catalyst that is capable of converting at least 75 percent of the

NO

X and ammonia present in the exhaust gas after the catalyst has been treated in 100 percent steam at a temperature of 700°C for 10 hours.

The present invention also concerns a process for treating exhaust gas comprising NO. and ammonia comprising contacting the exhaust gas with the catalyst described -above under conditions effective for the conversion of at least a portion of the NO, and ammonia to innocuous compounds.

An embodiment of the invention is a method for treating a gas mixture comprising NO,, ammonia, and, optionally, at least one of CO and a hydrocarbon and mixtures thereof, said method comprising directing the gas mixture along with a source of oxygen, such as air, over a catalyst under conditions effective for the selective catalytic reduction of NO., said catalyst comprising an WO 96/01689 PCT/US95/08253 -2intermediate pore size zeolite which has been treated to contain iron or an iron compound, has been hydrothermally treated at least once, and optionally further comprises a binder.

As mentioned above, catalysts useful in this invention comprise intermediate pore size zeolites that have been treated or modified with iron or an iron compound. The iron may be added by ferrocene impregnation, ion-exchange under specified conditions, contacting the zeolite with a water soluble iron-containing salt or salt precursor, contacting the zeolite with another type of inorganic ironcontaining compound, iron oxide, or the iron may be incorporated into the zeolite during the in-situ production of the zeolite from zeolite seeds, silica, and clay. Each of these methods will be further described below.

The term "exhaust gas" as used herein means any waste gas which is formed in an industrial process or operation and which is normally disposed of by discharge to the atmosphere, with or without additional treatment. "Exhaust gas" also includes the gas produced by internal combustion engines. The composition.of such a gas varies and depends on the particular process or operation which leads to its formation.

The conversion of NO x to N 2 is believed to proceed generally according to equations and 2 N0 2 4 NH 3 3 N 2 6 H20 (1) 4 NO 4 NH 3 4 N 2 6 H 2 0 (2) This invention is effective for treating exhaust gas containing the approximate stoichiometric amount of ammonia. The ammonia may be present in the gas, may be added to the gas, or may be produced by an upstream process. As used herein, the expression "approximate stoichiometric amount of ammonia" is intended to mean 0.75 to 1.25 times the molar amount of ammonia indicated in equations and when excess oxygen is present.

Carbon monoxide and hydrocarbons present in the exhaust gas may be oxidized to carbon dioxide and water WO 96/10689 PCT/US95/08253 -3over the catalyst. Additionally, hydrocarbons may be selectively absorbed/adsorbed on the catalyst.

Feeds This invention is effective to treat industrial and engine exhaust gases to remove NOx, and optionally other undesirable compounds, such as CO and hydrocarbons, if present. These exhaust gases are typically produced in internal combustion engines, and coal or gas-fired or oilfired furnaces, boilers and incinerators, and by the manufacture of nitric acid, by the nitration of organic chemicals, and by other chemical operations such as the reprocessing of spent nuclear fuel rods by dissolution in nitric acid to recover uranyl nitrate followed by calcination to convert the nitrate to uranium oxide.

Process Conditions The exhaust gas may be treated in the catalytic system of this invention at a temperature of 200"C to 1,000"C or more, within the range of 225"C to 900"C, of 225°C to 750-C, of 250°C to 600°C and at a gas hourly space velocity, GHSV, (vols. of gas at STP per volume of catalyst per hour) adjusted to provide the desired conversion. The GHSV can be from 1,000 to 500,000 hr within the range of 2,500 to 250,000 hr"i, from 5,000 to 150,000 hr from 10,000 to 100,000 hr The process of this invention may be operated at .subatmospheric to superatmospheric pressure, e.g. at 34.5 to 3,450 kPaa (5 to 500 psia), at 68.9 to 345 kPaa to 50 psia), i.e. near or slightly above atmospheric pressure.

The gas mixture directed over the catalyst may contain at least a stoichiometric amount of oxygen as indicated by equations and above. Excess levels of oxygen above the stoichiometric amount may be desirable. In the method of this invention, a source of oxygen, such as air, is cofed over the catalyst along with the exhaust gas. If WO 96/01689 PCT/US95/08253 -4sufficient oxygen is not present in the exhaust gas, a source of oxygen, e.g. air, may be added to the exhaust gas, and if sufficient oxygen is present in the exhaust gas, no air need be added to the exhaust gas.

Adequate conversion may be readily achieved with a simple stationary fixed-bed of catalyst. However, other contacting means are also contemplated, such as contacting with a fluid bed, a transport bed, and a monolithic catalyst structure -h as a honeycomb. Suitable mixing may be used in the exhaust gas before the treatment according to the present invention to produce a homogeneous gas mixture for treatment.

Catalyst Composition Catalysts useful in this invention may comprise an active material and a support or binder. The support for the catalysts of this invention may be the same as the active material and further can be a synthetic or naturally occurring substance as well as an inorganic material such as clay, silica, zirconia, titania and/or one or more other metal oxides. One binder that is suitable is a low acidity titania prepared from a mixture comprising a low acidity titanium oxide binder material and an aqueous slurry of titanium oxide hydrate. The preferred support is one that is a high surface area material that also possesses a high temperature stability and further possesses a high oxidation stability.

The binder may be prepared according to methods disclosed in U.S. Patent Nos. 5,430,000; 4,631,267; 4,631,268; 4,637,995; and 4,657,880. Also, the catalysts described herein may be combined with any of the binder precursors described in the above application and patents, and then may be formed, such as by extrusion, into the shape desired, and then hydrothermally treated and/or calcined as hereinafter described. The preferre binder may include less than 50 weight percent alumina, less than 2 weight percent alumina, is substantially free WO 96101689 PCT/US95/08253 of alumina. By the term "substantially free of alumina" is meant that no alumina is intentionally added to the binder, however, it is recognized that trace amounts of alumina may be present.

When low acidity titania is used as a binder, it may be desirable that the formable, extrudable, mass prepared by combining the zeolite, the iron salt, and the titania binder precursors contain at least 0.5 e.g., 1 wt.% to 20 2 to 8 wt.% of the aqueous slurry of titanium oxide hydrate.

The low acidity titania may be added in dry particulate form, titanium oxide hydrate, so as to control the moisture content of the binder/dispersant mixture at a level to promote satisfactory forming, e.g., extrusion.

The catalysts may also contain stabilizers such as alkaline earth oxides, phosphates and combinations thereof.

Catalysts useful in invention are frequently used with a substrate. A material can be both substrate and part of the catalyst. The catalyst may be combined with the substrate in any method that ensures that the catalyst will remain intact during the catalytic reaction, the catalyst may be present as a coating on the subst:ate, or it can be present as an integral part of the substrate.

The catalyst useful in this invention will now be described in detail. It comprises an intermediate pore size zeolite less than 7 Angstroms pore size, such .as from 5 to less than 7 Angstroms) having a silica to alumina molar ratio of at least 5, at least 20, e.g., between 40 and 1000, 50 to 500, a Constraint Index of 1 to 12, said zeolite having been treated or modified to contain iron or a iron compound. The Constraint Index qualifies it as having an intermediate pore size. The method by which Constraint Index is determined is described fully in U.S. Pat. No. 4,016,218.

Examples of such zeolites include ZSM-5 Patent No. 3,702,886 and Re. 29,948); ZSM-11 Patent No.

WO 96/01689 PCTUS5/08253 -6- 3,709,979); ZSM-12 Patent No. 3,832,449); ZSM-21 Patent No. 4,046,859); ZSM-22 Patent No.

4,556,477); ZSM-23 Patent No. 4,076,842); Patent No. 4,016,245); ZSM-38 Patent No.

4,406,859); ZSM-48 Patent No. 4,397,827); ZSM-57 Patent No. 4,046,685); and ZSM-58 Patent No.

4,417,780).

A catalyst useful in the method of this invention may be prepared by combining a zeolite, such as HZSM-5, an iron salt or other iron containing compound, a high molecular weight, hydroxy functional silicone, such as Dow Corning Q6-2230 silicone resin, a suitable extrusion aid, such as methyl cellulose, and a suitable polar, water soluble carrier, such as methanol, ethanol, isopropyl alcohol, Nmethyl pyrrolidone or a dibasic ester along with water as needed, then forming the mixture into the desired shape, such as by extrusion, then simultaneously calcining and hydrothermally treating the formed material. One particular methyl cellulose that is effective as an extrusion aid in the method of this invention is a hydroxypropyl methyl cellulose, such as K75M MethocelT, available from Dow Chemical Co. Dibasic esters that are useful in this invention include dimethyl glutarate, dimethyl succinate, dimethyl adipate, and mixtures thereof, one example of which is DuPont Chemical Co. DBE, which typically comprises 50 to 75 percent dimethyl glutarate, to 25 percent dimethyl adipate, 19 to 26 percent dimethyl .succinate and less than 0.2 wt.% methanol. Other silicone resins that may be used in the method of this invention include those described in U.S. Patent No. 3,090,691.

The relative proportions of zeolite component and the support material on an anhydrous basis may vary widely with the zeolite content ranging from between 5 to 99 percent by weight and more usually in the range of 10 to 95 percent by weight, specifically from 20 to 90 percent by weight of the dry composite.

WO 96101689 PCT/US95/08253 -7- Original ions, alkali or alkaline earth metal, of the as-synthesized intermediate pore size material and any found in the zeolite/support material can be replaced in accordance with techniques welL known in the art, at least in part, by ion-exchange with other ions. For the present catalyst coApositio", preferred replacing ions include hydrogen ions and hydrogen precursor, e.g., ammonium ions. ZSM-5 in the hydrogen exchanged form is referred to herein as HZSM-5. Representative ion-exchange techniques are disclosed in a wide variety of patents including U.S. Patents 3,140,249; 3,140,251; and 3,140,253.

The desired iron loading on the zeolite component of the catalyst is 0.01 to 5 at least 0.4 wt.%, at least 0.6 at least 1 at least 1.5 preferably 2 iron into the zeolite.

The catalyst may also optionally include another metal, such as a transition metal, preferably a noble metal, the combination of the metals being able to oxidize other undesirable compounds present in the exhaust gas along with allowing the SCR of NO.. The metal may be at least one of copper, zinc, vanadium, chromium, manganese, cobalt, nickel, palladium, platinum, molybdenum, tungsten, sodium, potassium, magnesium, calcium, barium, cerium and mixtures thereof, with the noble metals, platinum, palladium and combinations of these, al with cerium, particularly preferred. The terms "metal" and "iron" as used herein are .intended to include the elemental metal as well as metal oxides, metal sulfides, and other metal containing compounds.

After the intermediate pore size zeolites have been treated or modified to contain iron or an iron compound, they may be washed with water and dried at a temperature ranging from 65°C to 315°C. They may also be calcined or thermally treated in air, or in an inert gas, at temperatures ranging from 260°C to 925°C for periods of time ranging from 1 to 48 hours or more, typically at 538°C WO 96101689 PCTIUS95/08253 -8for 4 to 6 hours. While subatmospheric or superatmospheric pressure can be employed for the thermal treatment, atmospheric pressure is preferred simply for reasons of convenience.

Catalysts of improved selectivity and other beneficial properties, such as improved resistance to ageing in an atmosphere containing steam, can be obtained by subjecting the iron containing zeolite to at least one treatment with streams containing steam (hydrothermally treating the catalysts) at elevated temperatures ranging from 260°C to 900 0 C, 400"C to 850°C, 500°C to 750*C. The hydrothermal treatment may be accomplished in an atmosphere containing at least 20 ppm, 5, 10%, 20%, even up to 99% steam in air or some other suitable gas stream or in an atmosphere consisting of steam or nitrogen and some other gas which is essentially inert to the zeolite. Optionally, more than one hydrothermal treatment may be used, specifically two, three, or more hydrothermal treatments at different temperatures, increasing temperatures, may be used. Examples of steaming conditions include 600°C with 10 percent steam in air for 10 hours, 700 to 750 C with 5 to 10 percent steam in air for 6 to 10 hours, and 850"C with 20 percent steam in air for 6 hours. Typical steaming conditions are described in U.S. Patent Nos.

4,429,176; .4,522,929; 4,594,146; and 4,663,492. The calcination and hydrothermal treatments of the catalysts are preferably combined into one treatment step and -conducted simultaneously.

As mentioned above, the iron may be added to the catalyst by ferrocene impregnation, ion-exchange under specified conditions, contacting the zeolite with an inorganic iron-containing salt or salt precursor, contacting the zeolite with another type of inorganic ironcontaining compound, iron powder or iron oxide, or the iron may be incorporated into the zeolite during the in-situ production of the zeolite from zeolite seeds, silica, and clay.

WO 96/01689 PCT/US95/08253 -9- Ferrocene impregnated catalysts may be prepared by contacting the zeolite with ferrocene which has been dissolved in a suitable solvent. Suitable solvents are those which dissolve the ferrocene and may then be removed from the impregnated zeolite under conditions sufficiently mild as to avoid causing the ferrocene to sublimate off the zeolite. Examples of suitable organic solvents include benzene, toluene, xylenes, and hexane among others. The impregnation is typically conducted for more than 4 hours, specifically from 4 hours to several days, more specifically from 6 hours to 100 hours. The impregnation is generally conducted at conditions effective to achieve the desired iron loading or iron concentration on the zeolite.

The impregnation may occur either before or after the zeolite is combined with the support. For example, if the zeolite is first combined with the support and then impregnated, ferrocene that is deposited on the surface of the zeolite and on the support may be washed off using a solvent having an effective atomic diameter that is larger than the zeolite pore size, an organic solvent such as Tetralin®.

The impregnation may be conducted after first drying or dehydrating the zeolite. Also, the ferrocene solution may be prepared using a water-free or dry solvent.

The impregnated zeolite is recovered by removing the solvent from the zeolite. It is desired to remove the -solvent without causing the ferrocene to sublimate off the zeolite. One possible way is to evaporate the solvent at a temperature less than the ferrocene sublimation temperature, 100°C. This solvent removal may be done at superatmospheric pressure, if desired, or pressures ranging from subatmospheric to atmospheric. The recovered ferrocene impregnated zeolite then may be calcined at 450"C to 550°C to oxidize the ferrocene and to convert the iron to its oxide.

WO 96/01689 PCT/US95/010253 Another catalyst comprises an intermediate pore size zeolite which has been exposed to at least one ion-exchange sequence under specific conditions, wherein the ionexchange sequence comprises the steps of contacting the intermediate pore size zeolite with an aqueous solution of a ferrous, Fe(II), salt under suitable conditions, recovering the zeolite, and calcining the zeolite. The conditions include those effective to substantially prevent oxidation of the ferrous ion to the ferric form, such as using nitrogen, argon and the like inert gases to blanket the solution. The conditions also include stirring or mixing at a temperature of above 55*C, specifically above or those temperatures effective to reduce the hydration sphere of the ferrous cation to a size small enough to enter the pores of the zeolite. The ferrous cation is reported to have an ionic radius in an aqueous solution of about 0.0000000006 meters (6 A) at 25'C, which is too large to enter ZSM-5 pores, which are slightly smaller than this. Increasing temperature is believed to reduce the size of the hydration sphere associated with the ferrous cation. For example, with increasing temperature, a ferrous compound, ferrous sulfate, changes from the heptahydrate form to the tetrahydrate form (at 56.6*C) and then to the monohydrate form (at The ion-exchange may be conducted for more than 4 hours, from 4 hours to several days, from 6 hours to 100 hours. During the ion-exchange, the pH of the .aqueous solution may be maintained between 1 and 4.5 or at a pH level effective to prevent precipitation of the ferrous salt.

It has been found that ferrous, Fe(II), ions are more readily exchanged into the zeolite than ferric, Fe(III), ions.

Ferrous, Fe(II), salts useful in this invention include the water soluble salts, such as, ferrous ammonium sulfate, ferrous chloride, ferrous fluosilicate, ferrous hyposulfite, ferrous iodide, ferrous lactate, ferrous WO 96/01689 PCT/US95/08253 -11nitrate, ferrous perchlorate, ferrous sulfate, and ferrous thiocyanate.

After the iron has been ion-exchanged into the zeolite, the zeolite may be recovered by rooling the aqueous solution below 55°C to 65 0 C, filtering the zeolite from the aqueous solution and washing the filtered zeolite with a neutral or slightly basic solvent, for example, an aqueous solvent with a pH of from 6 to 9, such as distilled or deionized water. The solvent washing may be conducted at conditions effective to avoid leaching the iron from the zeolite. The aqueous solution and the ion-exchanged zeolite need not be maintained under inert conditions after the solution has been cooled. The recovered zeolite may be dried, calcined and hydrothermally treated as more fully described herein.

The ion-exchange sequence described above may be conducted at least once, at least twice, at least three times, four or more times.

Another method to incorporate iron onto the zeolite is to contact an intermediate pore size zeolite with an iron salt or salt precursor. In this method, the iron salt may first be dissolved in water or another suitable solvent and then the zeolite may be contacted with the solution.

Alternatively, the iron salt and the zeolite, and any binder material desired, may be physically combined and then water or another suitable solvent added and the mixture recovered and formed, as desired. The formed material may be dried, calcined, or hydrothermally treated as is more fully described herein. As will be apparent to one skilled in the art, any method that is effective to contact the zeolite with the iron salt and to achieve a high degree of iron distribution on and inside the zeolite may be used, including, but not limited to an incipient wetness technique.

Suitable iron salts include all of the ferrous salts mentioned above, as well as ferric acetate, iron(III) benzoate, iron(III) cacodylate, iron(III) dichromate, WO 96/01689 PCT/US95/08253 -12iron(III) citrate, iron(III) fluoride, iron(III) fluosilicate, iron(III) formate, iron(III) glycerophosphate, iron(III) hydrogen cyanide, iron(III) hydrosulfate, iron(III) lactate, iron(III) malate, iron(III) oxalate, iron(III) orthosphosphate, iron(III) hypophosphite, iron(III) sulfide, iron(III) thiocyanate, ferric acetylacetonate, ferric ammoniumchloride, ferric chloride, iron(III) nitrate nanohydrate, iron(III) sulfate pentahydrate, ammonium ferric sulfate, ferric bromide, ferric iodide, and any other ferrous, ferric, or other iron salts that are water soluble. Also included are precursors of all of the salts mentioned above.

The incorporation of iron onto the zeolite may be accomplished by contacting an intermediate pore size zeolite with an iron salt or salt precursor, such as those mentioned above. In this method, the iron salt may first be dissolved in water or another suitable carrier or solvent and then the zeolite may be contacted with the solution, recovered from the solution, dried and then bound if desired. Alternatively, the iron salt and the zeolite, and any binder material desired, may be physically combined with water or another suitable carrier or solvent to produce a mixture and the mixture recovered and formed, such as by extrusion. The formed material may be dried, calcined, or hydrothermally treated as is more fully described herein. As will be apparent to one skilled in the art, any method that is effective to contact the zeolite with the iron salt may be used, including, but not limited to mulling or an incipient wetness technique.

Yet another method that is suitable to add iron to the zeolite is to physically contact an intermediate pore size zeolite with an inorganic iron-containing compound, e.g., iron oxide. In this method, it is preferred to use a finely divided iron-containing compound, such as a fine pigment grade red iron oxide. It is also preferred to add a binder precursor to the physical mixture of the ironcontaining compound and the zeolite and to form the WO 96/01689 PCT/US95/08253 -13physical mixture. After the desired shape has been formed, the iron-contacted zeolite may be calcined and hydrothermally treated as is more full' described herein.

Another catalyst may be prepared by in-situ crystallization of an aggregate, a preformed clay aggregate. The aggregate comprises four components: seeds of an intermediate pore size zeolite, ZSM-5 seeds, silica, a colloidal silica such as Ludox M available from E. I. DuPont de Nemours Co., clay, and iron oxide.

Optionally, the aggregate can also include alumina.

Various techniques for preparing an in-situ crystallized zeolite are described in U.S. Patent Nos. 4,522,705; 4,091,007; and 4,800,187.

The zeolite may be prepared from a clay aggregate which comprises a non-clay added source of silica. The sources of silica in the reaction mixture may include both a clay and a non-clay source of silica, or a clay alone.

Non-clay sources of silica that can be employed in the synthesis are LudoxT

M

an aqueous dispersion of colloidal silica, water glass, sand, silica gel, fused silica, and finely-divided precipitated silicas, such as Hi-Sil, Quso, and Zeosil 100.

The clay component which is treated to form the zeolite-containing catalyst can be selected from the group 2- consisting of kaolin, halloysite, montmorillonite, illite, and dickite, with kaolin preferred.

The preformed aggregate may be treated for one or more hours at a temperature sufficient to convert the clay into the metakaolin phase, e greater than 927C to 1000*C.

After the thermal treatment of the aggregate, the zeolite may be crystallized by treatment at a temperature of greater than 135*C, 149°C, in the presence of water, alkali metal cations, and optionally, a directing agent such as tetraalkylammonium ions, a betraalkylammonium halide, such as tetrapropylammonium bromide, or a nalkylamine, such as n-propylamine, with no additional nutrients for zeolite formation.

-U-

WO 96/01689 PCT/US95/08253 -14- The in-situ formed zeolite may be prepared from a aggregate comprising about 3-5% zeolite seeds, seeds; 23-25% silica, e.g. colloidal silica; 68-70% clay, kaolin clay; and 0.01-5% metal oxide, iron oxide.

As noted above, the catalytic reduction of nitrogen oxides is substantially effected by the use of the present process. By substantially effected is meant a conversion of greater than 40, 75, 80, 85, 90, 95, or even 99% or more of the nitrogen oxides and the ammonia in the exhaust gas to innocuous compounds, such as nitrogen, through the use of this process. This i also referred to herein as conversion of a substantial portion of the NO x and ammonia in the exhaust gas to innocuous compounds.

Example 1 Base A HZSM-5 sample, prepared according to U.S. Patent No.

3,702,886, was used as the reference sample and was used as the basis for all of the other examples.

Example 2 Preparation of an iron containing An iron containing ZSM-5 sample was prepared by the following method: 100 g of distilled water was heated to about 85°C with constant stirring under a blanket of dry nitrogen. Approximately 0.11 g of ferrous sulfate and 5.1 g of the same HZSM-5 as Example 1 were added to the heated water. A solution pH of about 2 was maintained by dropwise addition of nitric acid or ammonium hydroxide, as needed.

The exchange solution was maintained at about 85°C under a nitrogen blanket with continued stirring for approximately 17 hours. The solution was subsequently cooled to room temperature, filtered and washed with distilled water. The filtered solid was calcined in air for 8 hours at 538"C to produce the iron containing catalyst, Catalyst A. The iron loading of this catalyst as prepared is about 0.5 wt.%.

WO 96/01689 PCTIUS95/08253 Example 3 Comparative testing against the base In this example, the SCR activity of Catalyst A is compared with the base HZSM-5 catalyst from Example 1. The catalyst samples were evaluated using a fixed bed quartz reactor operating between 250-550'C. The reactor inlet contained 500 ppm NO, 500 ppm NH 3 and 5 vol.% 02 in a He carrier flowing at a constant gas hourly space velocity (GHSV) of 12,000 hr-1. The effluent from the reactor was continuously monitored by non-dispersive infrared spectroscopy (NDIR) detectors. Catalyst activity results are reported below in Table 1.

Table 1 Percent N in Feed Converted to N 2 Temperature, "C Base ZSM-5 Catalyst Catalyst A 550 89% 93% 455 100% 98% 400 100% 98% 345 71% 97% 250 26% 92% Example 4 Preparation of an Iron Containing An iron containing ZSM-5 sample was prepared by the following method: a solution containing 25 g of the same HZSM-5 as Example 1 and 500 g of distilled water was heated to about 77 0 C with constant stirring under a blanket of dry argon. Approximately 0.3 g of ammonium ferrous sulfate was added to the heated solution. A solution pH of 3 was maintained by dropwi'e addition of nitric acid or ammonium hydroxide, as needed. The exchange solution was maintained at about 77*C under an argon blanket with continuous stirring for approximately 9 hours. The solution was subsequently cooled to room temperature, filtered and WO 96/01689 PCT/US95/08253 -16washed with distilled water. The filtered solid was calcined in air for 8 hours at 538*C to produce the iron containing catalyst, Catalyst B. The iron loading of this catalyst as prepared is about 0.5 wt.%.

Example Comparative testing against a base In this example, the SCR activity of Catalyst B is compared with the base HZSM-5 catalyst from Example 1. The catalyst samples were evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 2.

Table 2 Percent N in Feed Converted to N 2 Temperature, °C Base ZSM-5 Catalyst Catalyst B 550 89% 100% 455 100% 100% 400 100% 100% 345 71% 100% 250 26% Example 6 Preparation of an Iron Containing An iron containing ZSM-5 sample was prepared by the following method: a solution containing 0.2354 g of ferrous acetate in 30 g distilled water was added to a flask containing 15 g of the same HZSM-5 as Example 1 at room temperature. The catalyst slurry was mixed for approximately 2 hours and was air dried at room temperature for about 16 hours. The dried catalyst was calcined in air for 8 hours at 538 0 C to produce the iron containing catalyst, Catalyst C. The iron loading of this catalyst as prepared is about 0.5 wt.%.

WO 96/01689 PCIT/US95/08253 -17- Example 7 Comparative testing against a base ZSM-5 and Catalyst A In this example, the SCR activity of Catalyst C is compared with the base HZSM-5 catalyst from Example 1 and Catalyst A as prepared in Example 2. The catalyst samples were evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 3.

Table 3 Percent N in Feed Converted to N 2 Temperature,°C Base Catalyst Catalyst A Catalyst C 550 89% 93% 89% 455 100% 98% 91% 400 100% 98% 91% 345 71% 97% 250 26% 92% Example 8 Preparation of an iron containing An iron containing ZSM-5 catalyst was prepared by the following method: 7.6 g of the same HZSM-5 as that of Example 1 was heated in a stream of dry nitrogen to 375°C.

After this temperature was maintained for 4 hours, the catalyst was cooled, in a stream of dry nitrogen, to room temperature. A solution of 0.5 g f ferrocene (dicyclopentadienyliron) in 30 g dr benzene was then added to the calcined zeolite and the slurry allowed to mix on a roller for about 16 hours. The benzene was allowed to evaporate at room temperature to give a benzene/ferrocene wetted zeolite. This wet solid was then heated to a temperature that did not exceed 100*C in a vacuum oven so as to remove the residual benzene. This dry catalyst was then calcined in a stream of air at about 450 0 C for about 6 WO 96/01689 I'CT/US95/08253 -18hours to produce the desired iron containing catalyst, Catalyst D. The iron loading of this catalyst is about 1.8 wt.%.

Example 9 Catalytic evaluation of Catalyst D In this example, the SCR activity of Catalyst D from Example 8 was evaluated. The catalyst sample was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 4.

Table 4 Percent N in Feed Converted to N 2 Temperature, °C Catalyst D 550 455 97% 400 98% 345 98% 250 Example Preparation of an iron containing An iron containing ZSM-5 catalyst was prepared by the following method: about 10 g of the same HZSM-5 as that of Example 1 was heated in a stream of dry air to 375"C for 4 hours and then was cooled to room temperature. A solution of 0.1667 g of ferrocene (dicyclopentadienyliron) in 30 g dry toluene was then added to the dried zeolite and the slurry was mixed at room temperature overnight. Excess toluene was allowed to evaporate at room temperature to give a toluene/ferrocene wetted zeolite. The recovered solid was then calcined in a stream of dry air at about 538 0 C for about 6 hours to produce the desired iron containing catalyst, Catalyst E. The iron loading of this catalyst is about 0. ,5 wt.%.

WO 96/01689 PCTUS95/08253 -19- Example 11 Catalytic evaluation of Catalyst E In this example, the SCR activity of Catalyst E from Example 10 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table Table Percent N in Feed Converted to N 2 Temperature, *C Catalyst E 550 98% 455 99% 400 99% 345 98% 250 81% Example 12 Preparation of an iron containing An iron containing ZSM-5 catalyst was prepared by the following method: about 10 g of the same HZSM-5 as that of Example 1 was heated in a stream of dry air to 375 0 C for 4 hours and then was cooled to room temperature. A solution of 0.3331 g of ferrocene (dicyclopentadienyliron) in 30 g dry toluene was then added to the dried zeolite and the slurry was mixed at room temperature overnight. The slurry was filtered and washed once with 100 ml of Tetralin® to remove excess ferrocene from the external surface of the zeolite. Excess Tetralin® was allowed to evaporate at room temperature to give a Tetralin®/ferrocene wetted zeolite.

The recovered solid was then calcined in a stream of dry air at about 538*C for about 6 hours to produce the desired iron containing catalyst, Catalyst F. The iron loading of this catalyst is about 0.66 wt.%.

WO 96/01689 PCT/US95/08253 Example 13 Catalytic evaluation of Catalyst F In this example, the SCR activity of Catalyst F from Example 12 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 6.

Table 6 Percent N in Feed Converted to N 2 Temperature, °C Catalyst F 550 96% 455 98% 400 98% 345 96% 250 88! Example 14 Preparation of an iron containing An iron containing ZSM-5 catalyst was prepared by the following method: about 10 g of the same HZSM-5 as that of Example 1 was heated in a stream of dry air to 375*C for 4 hours and then was cooled to room temperature. A solution of 0.3331 g of ferrocene (dicyclopentadienyliron) in 30 g dry toluene was then added to the dried zeolite and the slurry was mixed at room temperature overnight. Excess toluene was allowed to evaporate at room temperature to give a toluene/ferrocene wetted zeolite. The recovered solid was then calcined in a stream of dry air at about 538*C for about 6 hours to produce the desired iron containing catalyst, Catalyst G. The iron loading of this catalyst is about 1.05 wt.%.

WO 96/01689 PCT/US95/08253 -21- Example Catalytic evaluation of Catalyst G In this example, the SCR activity of Catalyst G from Example 14 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 7.

Table 7 Percent N in Feed Converted to N 2 Temperature, 'C Catalyst G 550 455 98% 400 98% 345 97% 250 91% Example 16 Aging'of the HZSM-5 catalysts The base HZSM-5 of Example 1 and Catalysts D through G as prepared above in Examples 8, 10, 12, and 14 were hydrothermally aged. Approximately 5 g of each catalyst was placed in individual identical crucibles and put in a controlled atmosphere furnace. The furnace was continuously purged with air containing 20 wt.% water vapor. The samples were treated in this moist atmosphere at 850"C for 6 hours. The treated samples will be referred to as aged base HZSM-5, and aged Catalysts D through G.

Example 17 Catalytic evaluation of aged base In this example, the SCR activity of the aged base from Example 16 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 8.

WO 96/01689 PCT/US95/08253 -22- Table 8 Percent N in Feed Converted to N 2 Temperature, °C Aged base 550 82% 455 56% 400 36% 345 250 9% Example 18 Catalytic evaluation of aged Catalyst D In this example, the SCR activity of the aged Catalyst D from Example 16 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 9.

Table 9 Percent N in Feed Converted to N, Temperature. °C Aged Catalyst D 550 98% 455 100% 400 100% 345 98% 250 96% Example 19 Catalytic evaluation of aged Catalyst E In this example, the SCR activity of the aged Catalyst E from Example 16 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table WO 96(i01689 PCT/US95/08253 -23- Table Percent N in Feed C ,averted to N, Temperature, "C Aged Catalyst E 550 98% 455 100% 400 99% 345 88% 250 37% Example Catalytic evaluation of aged Catalyst F In this example, the SCR activity of the aged Catalyst F from Example 16 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 11.

Table 11 Percent N in Feed Converted to N 2 Temperature, °C Aged Catalyst F 550 98% 455 99% 400 92% 345 57% 250 27% Example 21 Catalytic evaluation of aged Catalyst G In this example, the SCR activity of the aged Catalyst G from Example 16 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 12.

WO 96/01689 PC'T/US95/08253 -24- Table 12 Percent N in Feed Converted to N 2 Temperature, "C Aged Catalyst G 550 97% 455 100% 400 99% 345 250 33% Example 22 Preparation of an iron containing An iron containing ZSM-5 catalyst was prepared by the following method: about 20 g of ammonium iron(III) sulfate was dissolved in 74 g distilled water. This solution was added to about 120 g of the base HZSM-5 and the slurry was mixed at about room temperature and was then dried at about 100*C overnight. Approximately 10 g of the dried, impregnated solid was then^calcined in air at about 600°C with about 10% steam for about 10 hours to produce the desired iron containing catalyst, Catalyst H. The iron loading of this catalyst is about 2 wt.%.

Example 23 Catalytic evaluation of Catalyst H In this example, the SCR activity of Catalyst H from Example 22 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 13.

WO 96(/01689 PC'TUS95/08253 Table 13 Percent N in Feed Converted to N 2 Temperature, °C Catalyst H 550 76% 455 400 99% 345 98% 250 Example 24 Preparation of an iron containing An iron containing ZSM-5 catalyst was prepared by the following method: about 1.5 g of iron(III) nitrate was dissolved in 6.5 g distilled water. This solution was added to about 10 g of the base HZSM-5 and the slurry was mixed at about room temperature and was then dried at about 100°C overnight. The dried, impregnated solid was then calcined in air at about 600*C with about 10% steam for about 10 hours to produce the desired iron containing catalyst, Catalyst I. The iron loading of this catalyst is about 2 wt.%.

Example Catalytic evaluation of Catalyst I In this example, the SCR activity of Catalyst I from Example 24 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 14.

L

WO 96/01689 PC1/US95/08253 -26- Table 14 Percenp in Feed Converted to N 2 Temperatuve. *C Catalyst I 5500 86% 455 97% 400 99% 345 99% 250 92% Example 26 Preparation of a silica bound-iron containing A silica bound iron containing ZSM-5 catalyst was prepared by the following method: about 100 g of the dried, iron impregnated HZSM-5 catalyst prepared in Example 22 was dry blended with about 22 g of Dow Corning silicone resin (Q6-2230) and about 6.8 g of K75M Methocelm (available from Dow Chemical Approximately 89 ml of a 1:1 by volume mixture of water:isopropyl alcohol was then added to the dry blended powder. The slurry was mulled for a minimum of 5 minutes until the mixture reached the proper consistency for extrusion. The mixture was then extruded into 1/16 inch diameter cylindrical strands in a screw extruder and was allowed to dry. Approximately 10 g of the dried extrudate was calcine( in air at 600"C with 10% steam .for 10 hours to produce the desired iron containing catalyst, Catalyst J. The iro:. 'oading of this catalyst is about 2 wt.%, Example 27 Catalytic evaluation of Catalyst J In this example, the SCR activity of Catalyst J from Example 26 was evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table WO 96101689 PCT/US95/08253 -27- Table Percent N in Feed Converted to N 2 Temperature. *C Catalyst J S550 74% 455 97% 400 100% 345 99% 250 88% Example 28 Comparative testing after steaming In this example, the SCR activity of the base and Catalysts H, I, and J are compared after each of the catalyst were treated with 100% steam at 70 0 °C for 7 hours.

The catalyst samples were evaluated using similar equipment and the same procedure as Example 3. Catalyst activity results are reported below in Table 16.

Table 16 Percent N in Feed Converted to N 2 Steamed Catalysts Temp., "C Base Catalyst Catalyst H Catalyst I Catalyst J 550 56% 92% 98% 92% 455 46% 100% 98% 100% 400 36% 100% 97% 100% 345 20% 100% 99% 100% 250 6% 96% 77% Example 29 An iron containing ZSM-5 catalyst was prepared by the following method: Approximately 1.5g of iron (III) nitrate was dissolved in 6.5g distilled water. This solution was WO 96/01689 PCT/US95/08253 -28added to about 10g of the base HZSM-5 and the resulting slurry was mixed and dried at room temperature overnight.

The dried inpregnated solid was then calcined in air at 600 0 C with 10% steam for 10 hours to produce the desired catalyst, Catalyst K. The nominal iron loading of this sample is about 2 wt%.

Example In this example, the SCR activity of Catalyst K from Example 29 was evaluated using similar equipment and the same reaction conditions as Example 3. See Table 17 below.

Table 17 Percent N in Feed Converted to N 2 Temperature, *C Catalyst K 550 86% 455 97% 400 99% .345 99% 250 92% Example 31 In this example Catalyst K was hydrothermally treated at 850*C in air containing 20% steam for 6 hours.

This catalyst is referred to as Catalyst L.

Example 32 In this example, the SCR activity of the hydrothermally treated catalyst, Catalyst L, from Example 31 was evaluated using similar equipment the same reaction conditions as Example 3. Catalyst activity results are reported below in Table 18.

WO 96/01689 PCT/US95/08253 -29- Tablt 18 Percent N in Feed Converted to N 2 Temperature, *C Hydrothermally treated Catalyst L 550 96% 455 99% 400 99% 345 99% 250 96% Example 33 In this example the hydrothermally treated catalyst, Catalyst L, from Example 31 was aged at 700*C in 100% steam for 7 hours. This catalyst is referred to as Catalyst M.

Example 34 In this example, the SCR activity of the aged, hydrothermally treated catalyst, Catalyst M, from Example 33 was evaluated using similar equipment and the same reaction conditions as Example 3. Catalyst activity Sresults are reported below in Table 19.

Table 19 Percent N in Feed Converted to N 2 Temperature, 0 C Aged Catalyst of Example 33 550 99% 455 96% 400 345 93% 250 82% WO 96/01689 PCT/US95/08253 Example In this example the aged, hydrothermally treated catalyst, Catalyst M, from Example 33 was further aged at 900°C in 100% steam for 7 hours. This catalyst is referred to as Catalyst N.

Example 36 In this example, the SCR activity of the aged, hydrothermally treated catalyst, Catalyst N, from Example was evaluated using similar equipment and the same reaction conditions as Example 3. Catalyst activity results are reported below in Table Table Percent N in Feed Converted to N 2 Temperature, *C Aged Catalyst of Example 550 86% 455 71% 400 53% 345 28% 250 11% Example 37 In this example Catalyst K was aged at 700C in 100% steam for 7 hours without the intermediate hydrothermal treatment. This catalyst will be referred to as Catalyst 0.

Example 38 In this example, the SCR activity of t',e aged catalyst, Catalyst 0 from Example 37 was evaluated using similar equipment and the same reaction conditions as Example 3. Catalyst activity results are reported below in Table 21.

WO 96/01689 PCT/US95/08253 -31- Table 21 Percent N in Feed Converted to N 2 Temperature, °C Aged Catalyst of Example 37 550 98% 455 98% 400 97% 345 99% 250 77% Example 39 In this example the aged catalyst, Catalyst 0, from Example 37 was further aged at 900°C in 100% steam for 7 hours. This catalyst will be referred to as Catalyst P.

Example In this example, the SCR activity of the twice-aged catalyst, Catalyst P, from Example 39 was evaluated using similar equipment and the same reaction conditions as Example 3. Catalyst activity results are reported below in Table 22.

Table 22 Percent N in Feed Converted to N 2 Temperature, °C Aged Catalyst of Example 39 550 56% 455 32% 400 21% 345 14% 250 8% WO 96/01689 PCT/US95/08253 -32- Example 41 A ZSM-5 catalyst was prepared by the following method: 99 grams of calcined ZSM-5 were mixed in a muller with grams of Dow Corning Q6-2230 silicone resin, and 6.5 grams of Dow Chemical Co. K75M Methocel

T

To this dry blend, 52.9 grams of distilled water and 23.1 grams of E. I.

DuPont de Nemours Co. DBE (dibasic ester) were added while mulling. The mixture was then extruded to form 1/16 inch cylindrical extrudates. The extrudates were dried overnight at 120°C and then calcined at 600"C in 10% steam for 10 hours to produce a ZSM-5 containing catalyst. This catalyst is referred to herein as Catalyst Q.

Example 42 An iron containing ZSM-5 sample was prepared by the following method: 99 grams of calcined ZSM-5 were mixed in a muller with 20 grams of Dow Corning Q6-2230 silicone resin, 6.5 grams of Dow K75M Methocelm, and 3.2 grams of iron oxide, Fe 2 0 3 To this dry blend, 52.9 grams of distilled water and 23.1 grams of DuPont DBE (dibasic ester) were added while mulling. The mixture was then formed into 1/16 inch cylindrical extrudates. The extrudates were dried overnight at 120*C and then calcined at 600*C in 10% steam for 10 hours to produce an iron containing catalyst, Catalyst R.

Example 43 In this example, the SCR activity of Catalyst Q is compared with the SCR activity of Catalyst R. The catalyst samples were evaluated using a fixed-bed quartz reactor operating between 250 and 550 0 C. The reactor was loaded with 2.75 grams of catalyst with inlet gases consisting of 500 ppm NO, 500 ppm NH 3 and 5% 02 in a N 2 carrier flowing at a constant flow rate of 1,000 cc/min. The effluent from WO 96/01689 PCT/US95/08253 -33the reactor was continuously monitored by FTIR (Fourier Transform Infrared) analysis. Catalyst activity results are summarized below in Table 23.

Table 23 Net NO Conversion, Temperature, "C Catalyst Q Catalyst R 550 71% 92% 454 74% 96% 398 72% 96% 343 60% 96% 250 25% Example 44 An iron containing ZSM-5 sample was prepared by the following method: 99 grams of calcined ZSM-5 were mixed in a muller with 20 grams of Dow Corning Q6-2230 silicone resin, 6.5 grams of Dow K75M MethocelT, and 0.32 grams of iron oxide, Fe 2

O

3 To this dry blend, 52.9 grams of distilled water and 23.1 grams of DuPont DBE (dibasic ester) were added while mulling. The mixture was then formed into 1/16 inch cylindrical extrudates. The extrudates were dried overnight at 120°C and then calcined at 600 0 C in 10% steam for 10 hours to produce an iron containing catalyst, Catalyst S.

Example In this example, the SCR activity of Catalyst Q is compared with the SCR activity of Catalyst S. The catalyst samples were evaluated using a fixed-bed quartz reactor operating between 250 and 550°C. The reactor was loaded with 2.75 grams of catalyst with inlet gases consisting of WO 96/01689 PCT/US95/08253 -34- 500 ppm NO, 500 ppm NH 3 and 5% 0, in a N 2 carrier flowing at a constant flow rate of 1,000 cc/min. The effluent from the reactor was continuously monitored by FTIR (Fourier Transform Infrared) analysis. Catalyst activity results are summarized below in Table 24.

Table 24 Net NO Conversion, Temperature, °C Catalyst Q Catalyst S 550 71% 84% 0 454 74% 86% 398 72% 87% 343 60% 82% 250 25% Example 46 A component of the in-situ grown ZSM-5 clay aggregate catalyst precursor was prepared by mixing 140 grams of calcined ZSM-5 seeds with 1860 grams of kaolin clay (Kaopaque 10S, a Georgia kaolin clay, Dry Branch Chemical Co., Dry Branch, GA). The mixed components will be referred to herein as Component A.

Example 47 An in-situ grown ZSM-5 clay aggregate catalyst precursor was prepared by mixing 75.9 grams of Component A from Example 46 with 7.6 grams of Dow Chemical Co. Methocelm in a muller. To this dry blend, 60.3 grams of E.

I. DuPont de Nemours Co. Ludox M AS-40 colloidal silica were added while mulling. The mixture was then extruded into 1/16 inch cylindrical extrudates. The extrudates were dried overnight at 120°C and then calcined at 1010"C for 3 hours to produce an in-situ grown ZSM-5 catalyst precursor, Precursor A.

WO 96/01689 PCT/US95/08253 Example 48 An iron containing in-situ grown ZSM-5 clay aggregate catalyst precursor was prepared by mixing 75.9 grams of Component A from Example 46 with 7.6 grams of Dow Methocel T m and 3.6 grams of Fe 2 O in a muller. To this dry blend, 60.3 grams of DuPont Ludox T AS-40 colloidal silica were added while mulling. The mixture was then extruded into 1/16 inch cylindrical extrudates. The extrudates were dried overnight at 120*C and then calcined at 1010*C for 3 hours to produce an in-situ grown ZSM-5 catalyst precursor, Precursor B.

Example 49 An in-situ grown ZSM-5 catalyst was prepared by the following method: 100 grams of Precursor A from Example 47 were placed in the bottom of a 1 liter autoclave with 384 grams of water, 16.1 grams of a 50% NaOH solution, and 5.9 grams of n-propylamine. Crystallization was completed under autogenous pressure at 149*C for 6 hours with no additional nutrients. After synthesis, the extrudates were cooled to room temperature, washed and dried at 120'C. The extrudates were NH 4 exchanged three times. A hybrid calcination followed which consisted of three hours in nitrogen at 482°C, with air slowly bled in to minimize the exotherm, then the temperature was raised to 537°C and held for six hours to produce an in-situ grown ZSM-5 catalyst, Catalyst T (greater than about 30% crystallinity).

Example An in-situ grown iron-containing ZSM-5 catalyst that should be effective in the method of this invention may be prepared by the following method: about 100 grams of Precursor B from Example 48 may be placed in the bottom of a 1 liter autoclave with about 384 grams of water, about 16.1 grams of a 50% NaOH solution, and about 5.9 grams of n-propylamine. Crystallization should be possible by treating the mixture under autogenous pressure at about 36 1490C for about 6 hours with no additional nutrients. After synthesis, the extrudates may be cooled to room temperature, and then may be washed and may be dried at 1200C. The extrudates should be NH4/ exchanged three times.

The ammonium exchanged extrudates may be hybrid calcined, by treating them for about three hours in nitrogen at about 4820C, and then by slowly bleeding air in to minimise the exotherm that may be developed, then raising the temperature to 53700 and holding the extrudates at that temperature for about six hours.

Throughout the description and claims of this specification, the word "comprise" and variations of that word, such as "comprises" and "comprising", are not intended to exclude other additives or components or integers.

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Claims (12)

1. A catalyst that is useful for treating an exhaust gas comprising NOx and ammonia, the catalyst comprising an intermediate pore size zeolite and iron or an iron compound that has been hydrothermally treated, the hydrothermal treatment and the iron concentration being effective to produce a catalyst that is capable of converting at least 75 percent of the NOx and ammonia present in the exhaust gas after the catalyst has been treated in 100 percent steam at a temperature of 7000°C for 10 hours.

2. A catalyst according to claim 1, wherein the catalyst is bound with silica, zirconia, titania, and combinations thereof.

3. A catalyst according to claim 1 or claim 2, wherein the catalyst has been prepared by the impregnation of the intermediate pore size zeolite with ferrocene in a solvent.

4. A catalyst according to claim 1 or claim 2, wherein the catalyst has been 15 prepared by ion-exchanging iron in an aqueous solution onto the intermediate pore size zeolite under conditions effective to allow the iron to enter the pores of o* 'the zeolite, the ion-exchange conditions being further effective to maintain the iron in solution during the ion-exchange treatment, the conditions still further being effective to maintain the iron at a valence of 2 during the ion-exchange treatment.

5. A catalyst according to claim 1 or claim 2, wherein the catalyst has been prepared by contacting the intermediate pore size zeolite with an iron salt or salt precursor in a suitable solvent for the salt or salt precursor.

6. A catalyst according to claim 2, wherein the catalyst has been prepared by preparing a physical mixture of dry intermediate pore size zeolite, binder, and iron oxide and then forming the physical mixture into the desired shape,

7. A catalyst according to claim 2, wherein the catalyst has been prepared by combining ZSM-5 seeds, silica, clay, and iron oxide into an aggregate; calcining the aggregate at a temperature of at least 1000°C; and crystallising zeolite in the aggregate by heating the calcined aggregate to a temperature greater than 1350°C (275 0 F) in the presence of caustic, and optionally, at least one organic directing S agent. C:\WINWORDU(YLIE\VARIOUS\NODEL\P29149. OC I I 38

8. A catalyst according to any one of the preceding claims, wherein the catalyst has been hydrothermally treated in an atmosphere of up to 100 percent steam at a temperature of from 400°C to 850°C.

9. A process for treating exhaust gas comprising NOx and ammonia comprising contacting the exhaust gas with a catalyst according to any one of the preceding claims under conditions effective for the conversion of at least a portion of the NOx and ammonia to innocuous compounds.

A process according to claim 9, wherein the contacting conditions comprise a temperature of from 2000C to 6000C, a pressure of from 34.5 to 3450 kPaa, and at a gas hourly space velocity (GHSV) of 1,000 to 100,000 hr'

11. A catalyst according to claim 1 substantially as hereinbefore described with reference to any of the examples.

12. A process according to claim 9 substantially as hereinbefore described with reference to any of the examples. DATED: 29 October, 1997 MOBIL OIL CORPORATION By ti,'ir Patent Attorneys 20 PHILLIPS ORMONDE FITZPATRICK C: WINWORD KYUE\ARIOUS\NODEL29149.DOC INTERNATIONAL SEARCH REPORT International application No. PCT/US95/08233 A. CLASSIFICATION OF SUBJECT MATTER IPC(6) :B01J 29/06, 8/00 US CL :502/66, 58, 71, 85; 423/239.3 According to International Patent Classification (IPC) or to both nationll classification and IPC B. FIELDS SEARCHED Minimum documentation searched (classification system followed by classification symbols) U.S. 502/66, 58, 71, 85; 423/239.3 Documentation searched other than minimum documentation to the extent that such documents are included in the fields searched Electronic data base consulted during the international search (name of data base and, where practicable, search terms used) C. DOCUMENTS CONSIDERED TO BE RELEVANT Category* Citation of document, with indication, where appropriate, of the relevant passages Relevant to claim No. Y US, A, 5,271,913 (IIDA ET AL.) 21 DECEMBER 1993, SEE 1-10 COLUMN 1, LINE 37 COLUMN 3, LINE 6. Y US, A, 5,248,643 (PATIL ET AL.) 28 SEPTEMBER 1993, 1-10 SEE COLUMNS 2-6. Y US, A, 5,223,236 (INOUEETAL.) 29 JUNE 1993, SEE THE 1-10 CLAIMS Y US, A, 5,077,026 (NAIR ET AL.) 31 DECEMBER 1991, SEE 1-10 THE ENTIRE DOCUMENT. Further documents are listed in the continuation of Box C. See patent family annex. Special categories of cited documents: T" later document published after the international filing date or priority date and not in connict with the application but cited to understand the doceunentdefining the general state of the art which i not considred principle or theory underlying the invention to be of particular relevance S earlier donent publed on or ater te inte onal filin docL "ent of particular relevance; the claimned invention cannot be E' earlier douent published on or after the in aional filing date consiocre novel or cannot be coniered to involve an inventive tp document which may throw doubts on priority claim(s) or which is when the document is taken alone cited to establish the publication date of another citation or other special reason (an specified) Y" docunent of particular rrevance; the clanimed invention cannot be considered to involve in inventive step when the document is document referring to an oral disclosure, use, exhibition or other combined with one or r ore other such docunents, such combination means being obvious to a perran skilled in the art document published prior to the international filing date but later than document member of the aine patent riunily the priority date claimed Date of the actual completion of the international search Date of mailing of the international search report 07 AUGUST 199522SEP Name and mailing address of the ISA/US Aut Commissioner of Patents and Trademarks Box PCT Washington, D.C. 20231 Facsimile No. (703) 305-3230 Telephone No. (703) 308-0661 Form PCT/ISA/210 (second sheet)(July 1992)*