Catalytic Reduction of NOx and Carbon Monoxide Using Methane in the Presence of Oxygen

The present invention describes a process for conveniently destroying NOx and carbon monoxide which may be present in combustion products, such as flue gases and engine exhaust, wherein such components are catalytically converted in the presence of methane and oxygen to the corresponding reduction products of nitrogen gas, carbon dioxide and water. The process utilizes metal-exchanged crystalline zeolite catalysts having a silicon to aluminum ratio of greater than or equal to about 2.5.

Emissions of nitrogen oxide, principally nitrogen dioxide (NO.sub.2) and nitric oxide (NO), collectively referred to as NOx, have been linked to urban smog, acid rain and numerous harmful health effects. Of all NOx emissions produced in the United States, an estimated 55 percent is attributed to stationary sources such as utility boilers, industrial boilers, gas turbines and stationary engines.

The U.S. Environmental Protection Agency has promulgated New Source Performance Standards (NSPS) to define limits on allowable NOx emissions permitted from such stationary sources in order to abate the harmful effects caused by such emissions. However, NOx emission levels exceeding NSPS occur in many combustion facilities rendering the point source susceptible to fine and/or interruption of business.

In order to enhance the air quality surrounding such point sources and to promote cleaner burning combustion processes, three major approaches have been undertaken to reduce NOx emissions: (1) making modifications before combustion; (2) making modifications during combustion; and (3) adding controls after combustion. Typical precombustion modifications include switching fuel stocks, emulsifying the fuel with water, and denitrifying the fuel. Typical combustion modifications include changing reaction stoichiometry, reducing combustion temperature and reducing process residence time. Flue-gas or exhaust gas treatment generally contemplates adding controls downstream of combustion.

NOx reduction during combustion has been employed since the early 1970's and has achieved a limited degree of success in reducing NOx emissions. However, flue-gas treatment is typically required to obtain higher levels of NOx reduction and to meet increasingly stringent environmental regulations. Flue-gas treatment consists of dry processes and wet processes. Dry flue-gas treatment processes are typically preferred over wet processes because they typically require less equipment and produce less waste that requires disposal.

Selective Catalyst Reduction (SCR) of nitrogen oxides using ammonia is currently considered one of the most efficient processes for removing NOx from flue gases. The SCR process is typically carried out on a titania supported vanadia catalyst employing NH.sub.3 /NO stoichiometric ratios near 1.0 and temperatures ranging from 300.degree. to 400.degree. C. to achieve conversions of up to 90%. NOx conversion increases with NH.sub.3 /NO ratio but higher ratios typically result in ammonia slip (ammonia breakthrough) which causes a secondary environmental problem.

The reaction pathway of SCR processes employing ammonia involves the oxidation of SO.sub.2 to SO.sub.3 by the vanadia catalyst followed by formation of NH.sub.4 HSO.sub.4 and (NH.sub.4).sub.2 S.sub.2 O.sub.7 which can cause corrosion and plugging of reactor components and catalyst deactivation. These problems coupled with equipment and operating costs associated with the storage, delivery and use of ammonia in SCR processes have led to a search for improved processes which do not utilize ammonia. However, such improved processes for removing NOx from oxygen-containing flue gases have eluded researchers.

Researchers have been investigating the use of hydrocarbons in the place of ammonia in SCR processes. Adlhart, et. al., R. E. Chem. Eng. Pro. 76, 73 (1971) studied the catalytic reduction of NOx in nitric acid tail gas using natural gas over alumina-supported platinum, palladium and rhodium catalysts. Results demonstrated that methane was the most difficult fuel to ignite among the fuels studied, requiring preheat temperatures of 480.degree. to 510.degree. C. Moreover, additional fuel in excess of the stoichiometric equivalent of total oxygen was required to completely remove NOx from the tail gas. For example, 1.7% methane was required to remove 0.23% NOx in tail gas having 3.0% oxygen at temperatures higher than 500.degree. C.

Limitations associated with the use of methane in processes for removing NOx from flue gas were confirmed in subsequent studies. Ault and Ayen, R. J., AlChE J. 17, 265 (1977), investigated the catalytic reduction of NOx in a substantially oxygen-free combustion stream. NOx-containing flue gas was reacted in the presence of hydrocarbons including methane, ethane, ethylene, acetylene, propane, propylene, octane, benzene and cyclohexane over a barium-promoted copper chromite catalyst in an oxygen-free atmosphere. Under reaction temperatures ranging from 225.degree. to 525.degree. C., an increase in the number of carbon atoms comprising the hydrocarbon reducing agent generally resulted in a decrease in the temperature required to effect the subject nitric oxide reduction. For example, about 10% NO was converted to the corresponding reduction products using methane as the reducing agent at 500.degree. C. wherein the nitric oxide inlet concentration was 1.0% and an amount of hydrocarbon 10% in excess of the stoichiometric requirement was employed.

Hamada and coworkers, Appl. Catal. 64, L1 (1990), Catal. Lett. 6, 239 (1990) studied the catalytic reduction of NOx in oxygen-containing flue gas using H-form zeolite and alumina catalysts and small amounts of propane and propene as reductants. The most active catalyst of three H-form zeolites studied was H-mordenite which gave the maximum nitric oxide conversion of 65% at 673K followed by H-ZSM-5 and HY. Na-ZSM-5 provided a nitric oxide conversion of 32% at 573K.

The above-mentioned results suggest that NOx reduction efficiency depends not only on process operating temperatures but also on the type of catalyst and hydrocarbon employed as well as the amount of oxygen present in the NOx-containing flue gas. These factors have greatly impaired the ability to predict optimum catalysts and operating conditions in processes for removing NOx in combustion products such as flue gas.

Japanese Patent Application No. 291258/1987 discloses a zeolite catalyst for estroying NOx in automotive exhaust gas. The zeolite is ionically exchanged with a transition metal and carried on a refractory support. Preferred transition metals are copper, cobalt, chromium, nickel, iron and manganese. Copper is the most preferred. Preferred zeolites have a pore size ranging from 5 to 10 angstroms which is slightly larger than the molecular diameter of NOx. Methane was not disclosed as a reducing agent.

Iwamoto and coworkers, Shokubai 32, 6 430 (1990) demonstrated the effectiveness of NO reduction over copper-exchanged zeolite catalysts using H.sub.2, CO.sub.2, C.sub.2 H.sub.4, C.sub.3 H.sub.6 and C.sub.3 H.sub.8 as reductants. However, no data was presented for NO reduction over the enumerated catalysts using methane as a reductant. The rate of conversion to N.sub.2 increased with increasing O.sub.2 concentration and maximum conversion was obtained when O.sub.2 was between 0.8 and 2.0%. The authors concluded that the presence of oxygen was indispensable to the progress of the reaction but that a large excess of oxygen resulted in a decline in the NO removal rate. Further, during a public meeting, Iwamoto stated that methane was not an effective reducing agent for converting NOx in the presence of oxygen when Cu-ZSM-5 served as a catalyst.

Iwamoto and coworkers, Catalysis Today 10 57 (1991) classified methane a non-selective reductant for NO, which classification means that methane is ineffective for NOx reduction in the presence of excess oxygen. Truex and coworkers, Platinum Metals Review, Vol. 36, 1, (1992) reported that Cu-ZSM-5 was completely ineffective for NO reduction with methane in the presence of oxygen (the conversion was reported at 0% below 500.degree. C.).

U.S. Pat. No. 5,017,538 discloses an improved method for producing an exhaust gas purification catalyst which comprises a ZSM-5 catalyst carried on a refractory support. The ZSM-5 catalyst is ion-exchanged with a copper carboxylate and ammonia solution. Although no details are known about the reaction mechanism, preliminary studies suggest that catalyst activity and reaction selectivity vary with the hydrocarbon utilized with C.sub.3 H.sub.6 being preferable to C.sub.3 H.sub.8. The method is improved by a small amount of oxygen.

Operators of natural gas (methane) fired power stations, industrial boilers and combustion processes have been searching for an efficient and inexpensive catalytic reduction process for removing NOx from oxygen-containing flue gases. However, a catalytic process for destroying NOx in oxygen-rich combustion products utilizing methane as a reductant has not been reported.

The present invention relates to a process for simultaneously catalytically removing NOx and carbon monoxide from oxygen-rich combustion products which process utilizes a unique and unobvious combination of catalyst and reducing agent. Under appropriate process conditions, the claimed process provides conversion of NOx and carbon monoxide to the desired products, namely, nitrogen gas, water and carbon dioxide.

The process comprises combusting a fuel source in the presence of oxygen to form combustion products comprising nitrogen oxides, carbon monoxide and oxygen; introducing methane into the combustion products in an amount such that the total amount of methane to nitrogen oxides, expressed as a ratio, by volume is greater than 0.1; and reacting the nitrogen oxides, carbon monoxide, methane and oxygen in the presence of the subject catalysts under conditions sufficient to convert the nitrogen oxides and carbon monoxide to gaseous nitrogen, water and carbon dioxide.

As will be further defined in the specification, carbon monoxide may not always be present in the combustion products. Nevertheless, the process performs well irrespective of whether carbon monoxide is present in the combustion products. Likewise, the amount of methane to be added to the combustion products will depend upon the amount of methane used as a fuel source and the efficiency of the combustion process. If methane is used as a fuel source and a portion of the methane is not combusted thereby remaining in the combustion products, then methane requirements may be diminished by the amount of methane already residing in the combustion product.

The catalysts of the present process comprise crystalline zeolites having a silicon to aluminum ratio of greater than or equal to about 2.5 wherein the zeolite is exchanged with a cation selected from the group consisting of gallium, cobalt, nickel, iron, chromium, rhodium and manganese. In an alternate embodiment the metal-exchanged zeolite catalysts are subjected to further metal-exchange treatments to exchange the catalyst with additional cations. Such additional cations include those metals represented by the third period transition metals and members of Groups 8, 9 and 10 of the Periodic Table of the Elements as defined by the notation presented in Pure & Appl. Chem., 60, 3, pp. 431-436 (1988).

In another alternate embodiment the ion-exchanged zeolites of the present invention are impregnated with various anionic and neutral species. Suitable impregnating moieties include oxidizing metals selected from Group 5, 6, 7 and 11 of the Periodic Table of the Elements as defined above. A preferred metal is niobium. In another alternate embodiment the exchanged zeolites are further impregnated with from 0.1 to 40% alumina based upon the total weight of the exchanged zeolite.

Suitable fuel sources to be combusted in the present process comprise combustible fuels which do not contain methane and combustible fuels which contain methane. Suitable fuels which do not contain methane include petroleum based fuels such as gasoline, diesel fuel and gaseous and liquid hydrocarbons, excluding methane, and mixtures thereof Suitable fuels which contain methane include natural gas, gaseous and liquid hydrocarbons containing methane, refuse and mixtures thereof.

The combusting of the above-mentioned fuels can take place in a wide variety of apparatus including internal combustion engines and external combustion devices. Following combustion of the fuel source, methane is introduced into the combustion products in an amount such that the total amount of methane to nitrogen oxides, expressed as a ratio, by volume is greater than about 0.1. Preferably, the ratio of methane to nitrogen oxides ranges from about 0.1 to about 800, and most preferably, between 0.1 and about 400.

In the case of combustion products formed by combusting a fuel source which does not contain methane, the entire portion of methane to be introduced to catalytically destroy NOx and carbon monoxide may be taken from a conventional external tank or source such as a pipeline. Alternatively, when the combustion products are formed by combusting a fuel source containing methane, the fuel to air ratio of the combustion process can be controlled by conventional steps to regulate the amount of methane, if any, remaining in the combustion products. Consequently, an additional amount of methane is added to the combustion products such that the total amount of methane to nitrogen oxides, calculated as a ratio, by volume is greater than about 0.1.

The process according to this invention is capable of affording high conversion of NOx and carbon monoxide to environmentally safe products and can be conveniently incorporated into a wide variety of powered devices including mobile conveyances such as automobiles, trains, boats and the like and stationary devices such as boilers, turbines and the like. The advantages afforded over conventional SCR processes include high NOx removal efficiency; use of a cost efficient reducing agent, methane; elimination of ammonia as a reducing agent, as taught in conventional SCR processes; the ability to operate the process in oxygen-rich flue gas and operation under mild temperatures and ambient pressure.

The FIGURE shows conversion of NO as a function of temperature.

The present invention relates to a process for catalytically removing NOx and carbon monoxide from oxygen-rich combustion products. The process utilizes a unique and unobvious combination of catalyst namely, metal-exchanged crystalline zeolites, and a novel reducing agent, methane, to yield a process which is capable of simultaneously removing NOx and carbon monoxide from oxygen-containing combustion products. This process represents a significant advance over prior art de-NOx processes which typically are incapable of destroying carbon monoxide.

The process comprises combusting a fuel source in the presence of oxygen to form combustion products comprising nitrogen oxides, carbon monoxide and oxygen; introducing methane into the combustion products in an amount such that the total amount of methane to nitrogen oxides, expressed as a ratio, by volume is greater than about 0.1; and reacting the nitrogen oxides, carbon monoxide, methane and oxygen in the presence of the subject catalysts under reaction conditions sufficient to convert the nitrogen oxides and carbon monoxide to gaseous nitrogen, water and carbon dioxide.

Throughout this specification and appended claims, the term, NOx, as used herein shall refer to one, or a mixture of two or more nitrogen oxides, including NO, NO.sub.2, and the like, formed during typical combustion processes. Applicants have filed a separate patent application relating to removing of N.sub.2 O from combustion products and such application is identified as U.S. patent application Ser. No. 07/790,611, now U.S. Pat. No. 5,171,553.

The term, combustion products, shall be given a definition which is broader than the conventional definition known in the art. The term shall include reaction products formed from burning or a chemical change accompanied by the liberation of heat, sound and/or light. Combustion products shall include products formed from burning fuels such as flue gases and engine exhaust as well as products formed from chemical reactions wherein NOx is present. A example of such a chemical reaction includes nitric acid production which is known to produce substantial amounts of NOx.

Applicants' process and the results obtained therefrom are most unexpected in view of the collective teachings of the prior art. Prior to this invention, no long lived, catalytic process employing crystalline zeolite catalysts was known for reducing NOx and carbon monoxide in an oxygen-containing combustion product wherein methane functioned as the reducing hydrocarbon.

For example, prior art processes using supported metal catalysts such as rhodium on alumina in the presence of methane were reported to be highly active for reducing nitric oxide present in substantially oxygen-free flue gas but the catalyst was substantially deactivated in the presence of oxygen. The poor performance of methane as a reducing agent in oxygen-containing combustion products was also demonstrated by Adlhart, et al, R. E. Chem. Eng. Pro. 76, 73 (1971) who studied the catalytic reduction of NOx in nitric acid tail gas using natural gas as a reductant over platinum, palladium and rhodium catalysts.

Adlhart demonstrated that methane was the most difficult fuel to ignite, among a group of fuels studied, requiring preheat temperatures of 480.degree. to 510.degree. C. Moreover, additional fuel in excess of the stoichiometric equivalent of total oxygen was required to completely remove NOx from the tail gas. For example, 1.7% methane was required to remove 0.23% NOx in tail gas having 3.0% oxygen at temperatures higher than 500.degree. C.

A zeolite catalyst for destroying NOx in automotive exhaust gas was disclosed in Japanese Patent Application No. 291258/1987. The zeolite was ionically exchanged with a transition metal and carried on a refractory support. Preferred transition metals were copper, cobalt, chromium, nickel, iron and manganese with copper being most preferred. Methane was not disclosed as a reducing agent.

Based upon the collective teachings of the cited prior which state that methane is a poor reductant for destroying NOx in the presence of oxygen, one of ordinary skill in the art would conclude that the ion-exchanged zeolites disclosed in Japanese Patent Application No. 29128/1987, would be ineffective in reducing NOx in oxygen-containing combustion products when methane is used as the reductant.

Applicants have unexpectedly discovered that methane is an effective reductant for destroying NOx and carbon monoxide in the presence of oxygen when certain catalysts are employed. More particularly, Applicants have discovered that various metal exchanged zeolite catalysts simultaneously destroy NOx and carbon monoxide present in combustion products when methane is used as the reductant and oxygen is present in the combustion product.

The combination of the defined metal-exchanged zeolite catalysts and use of methane as a reductant overcomes problems associated with prior art processes wherein the presence of oxygen deactivates the catalyst. Unexpectedly, the present process is activated by the presence of oxygen and is not substantially adversely affected by the presence of a substantial amount of oxygen.

The advantages afforded by the present invention over conventional SCR processes include high NOx removal efficiency; use of a cost effective and facile reducing agent, methane; elimination of ammonia, used as a reducing agent in conventional SCR processes; and the ability to operate the process using oxygen-rich NOx sources under moderate temperatures and ambient pressure. Additionally, the process can be readily adapted to vehicles powered by internal combustion engines as will be discussed in greater detail.

The process of this invention is suited for destroying NOx and carbon monoxide generated by internal combustion engines and external combustion devices such as utility boilers, industrial boilers and gas turbines which utilize any available fuel source. Suitable fuel sources to be combusted in the present process comprise combustible fuels which do not contain methane and combustible fuels which contain methane. Consequently, methane-containing fuel sources such as natural gas can be advantageously used to form combustion products containing a residual amount of unburned methane which serves as a reductant thereby reducing the overall amount of methane which must be added to the combustion products to destroy NOx.

Alternatively, combustion products produced by fuel sources which do not contain methane can be treated according to the present process in order to destroy NOx and carbon monoxide, if present in the combustion product. Since essentially no methane is present in the combustion products formed by burning a fuel source which does not contain methane, total methane requirements must be supplied by external methane sources such as fuel tanks, pipelines or the like. Suitable fuels which do not contain methane include petroleum based fuels such as coal, gasoline, diesel fuel and gaseous and liquid hydrocarbons, excluding methane, and mixtures thereof. Suitable fuels which contain methane include, but are not limited to, natural gas, refuse, and gaseous and liquid hydrocarbons containing methane and mixtures thereof. Those skilled in the art will recognize that some non-methane-containing fuels may produce some methane under conventional combustion conditions.

The combusting of the above-mentioned fuels can take place in a wide variety of apparatus including internal combustion engines such as diesel and gasoline engines and gas turbines, and external combustion devices such as boilers and process heaters. Gas turbines or combustion turbines are capable of burning both gaseous and liquid hydrocarbon fuels. Following combustion of the desired fuel source, methane is introduced into the combustion products in an amount such that the total amount of methane to nitrogen oxides, expressed as a ratio by volume, is greater than about 0.1, preferably between 0.1 and 800 and most preferably, between 0.1 and 400.

In the case of combustion products formed by combusting a fuel source which does not contain methane, the entire portion of methane to be introduced may be taken from a conventional external tank or source such as a pipeline. Alternatively, when the combustion products are formed by combusting a fuel source containing methane, or able to produce methane, the fuel to air ratio of the combustion process can be controlled by conventional steps to regulate the amount of methane, if any, remaining in the combustion products. Consequently, an additional amount of methane is added to the combustion products to reach the desired level of methane to be used as a reductant in the resulting steps of the process.

The process of the present invention is particularly suited toward being integrated into conveyances powered by internal combustion engines such as automobiles, trains and the like. Following combustion of the desired fuel, methane stored in a conventional storage tank is transferred into the combustion exhaust containing oxygen, NOx and carbon monoxide prior to making contact with the enumerated catalysts. Alternatively, internal combustion engines can be effectively powered by natural gas or other methane-containing fuels wherein the methane provides a dual role as a combustion fuel and a reductant for destroying NOx and carbon monoxide. As stated previously, additional amounts of methane can be added as necessary to control the desired methane to NOx ratio.

The present process is equally effective in removing NOx from gaseous mixtures formed during any chemical reaction, including but not limited to the preparation of nitric acid. In this embodiment the combustion product is a reaction product which contains NOx or alternatively a gaseous stream consisting essentially of NOx. The process of this invention does not have to be modified to remove NOx present in combustion products formed from a chemical reaction.

Applicants' process comprises contacting the combustion products containing NOx and oxides of carbon with a desired amount of methane and oxygen in the presence of metal-exchanged natural or synthetic crystalline zeolites having a silicon to aluminum ratio of greater than or equal to about 2.5 under combustion conditions sufficient to convert NOx to gaseous nitrogen, water and carbon oxides.

The zeolites of the present invention can be used either in the alkali metal form, e.g., the sodium or potassium form; the ammonium form; the hydrogen form or another univalent or multivalent cationic form to the extent that such zeolites are capable of being exchanged with the metals discussed herein. Suitable crystalline zeolites include those materials which are stable under the described reaction conditions and which have a pore size sufficient to effect the subject reaction. While catalysts having a Si/Al ratio less than 2.5 prior to treatment appear to demonstrate only limited activity, such catalysts may be activated by subjecting the catalyst to dealumination according to methods well known in the art.

Zeolite catalysts to be metal-exchanged include MOR, MFI and FER structured zeolites. Representative zeolites under the MOR designation include mordenite, Na-D, Ptilolite and Zeolon. Representative zeolites under the MFI designation include ZSM-5, Silicalite-1, Silicalite, Zeta-1, Zeta-3 and AZ-1. Representative zeolites under the FER designation include ferrierite, SR-D, Fu-9, NU-23 and ZSM-35.

Typically, the pore size of the base zeolite will range from about 5 to 15 angstroms although such a range is not to be construed as limiting the scope of this invention. The sodium form of ZSM-5 can be prepared by the procedures disclosed in U.S. Pat. No. 3,702,886, I & EC 24, 507 (1985) and Shiralkar, et. al., A. Zeolite, 9, 363, (1989), the disclosures which are specifically incorporated by reference herein.

LZ-M-5 zeolite, a synthetic material residing in the non-acid, sodium cation form and an example of a MOR structure type zeolite, is commercially available from Union Carbide Corporation, Chickasaw, Ala. LZM-5 has the following chemical composition (wt % anhydrous)

SiO.sub.2 78.7

Al.sub.2 O.sub.3 12.5

Na.sub.2 O 7.33

SiO.sub.2 /AT.sub.2 O.sub.3 (molar ratio) 10.7

Na.sub.2 O/Al.sub.2 O.sub.3 (molar ratio) 0.96

The term, mordenite, is meant to include those synthetic and naturally occurring zeolites having the mordenite topology as included under the general IUPAC structural code of mordenite (MOR). While naturally occurring mordenites vary widely in purity, the synthetic zeolites tend to have higher purity and controlled pore structure thereby rendering the synthetic mordenites preferable for catalytic applications.

Mordenite can be synthesized from a wide variety of starting materials of both chemical and natural origins. Synthetic mordenites are typically produced with Si/A]ratios ranging from 5 to about 12.5. Mordenite is a porous crystalline catalyst having a rigid three-dimensional anionic network with intracrystalline channels whose narrowest cross-section has essentially a uniform diameter. Mordenite is distinguished over crystalline alumino-silicate clays such as bentonite which have a two-dimensional layered structure and over alumino-silicates which are amorphous.

The original alkali metal cations of the zeolites according to this invention are preferably replaced in accordance with techniques well known in the art such as ion-exchange, acid-base and solid state reactions. For example, the alkali metal cations of the zeolite can be replaced, at least in part, by ion-exchange with from about 0.1 wt % to about 15 wt % (based upon the total weight of the catalyst) of one or more cations selected from gallium, niobium, cobalt, nickel, iron, chromium, chromium, rhodium and manganese. Alternately, ion exchange can be effected by solid state or vapor phase whereby the H.sup.+ form of the zeolite is reacted with a metal halide salt (MX) or a metal oxide to liberate HX or water and to place the subject metal into the exchange site.

In a preferred embodiment, the zeolites of this invention can be exchanged with a precious metal selected from the group consisting of platinum, palladium, ruthenium, rhodium and iridium. A suitable metal-exchange technique comprises contacting the zeolite with a solution which contains the salt of the desired replacing cation or cations. Examples of suitable salts include the halides such as chlorides, nitrates, carboxylates and sulfates. A preferred exchange solution is cobalt(II) acetate.

In an alternate embodiment the metal-exchanged zeolite catalysts can be subjected to further metal-exchange treatments to exchange sites on the catalyst with additional cations. Such additional cations include those metals represented by the third period transition metals and members of Groups 8, 9 and 10 of the Periodic Table of the Elements as defined by the notation presented in Pure & Appl. Chem., 60, 3, pp. 431-436 (1988). Preferred cations include cobalt, nickel, iron, manganese and silver. The amount of second metal to be exchanged ranges from about 0.01 wt % to about 2 wt % based upon the total weight of the catalyst with the remaining portion of the exchanged metal comprising cobalt.

In another alternate embodiment the metal-exchanged zeolites of the present invention are impregnated with various anionic and neutral species. Suitable species may be selected from oxidizing metals or their oxides formed from metals selected from Group 5,6,7 and 11 of the Periodic Table of the Elements as defined by the notation presented in Pure & Appl. Chem., 60, 3, pp. 431-436 (1988). The term, oxidizing metal, refers to a metal which is capable of catalyzing oxidation reactions and which affords enhanced catalytic activity when impregnated onto the metal-exchanged zeolites of the present invention. Preferred species include silver and oxides of niobium, molybdenum, vanadium and manganese. The amount of metal to be impregnated onto the metal-exchanged zeolite catalyst is that amount which is sufficient to achieve the desired selectivity and conversion of NOx and carbon monoxide to the reduction products.

Generally, the amount of metal moiety impregnated onto the metal-exchanged zeolite catalyst ranges from about 0.01 to 15 wt % , and preferably between about 0.1 to 8 wt % based upon the total weight of the impregnated metal-exchanged zeolite catalyst. However, the level of impregnation should not be such that substantially all of the pores on the zeolite catalyst become clogged thereby rendering the catalyst inactive for the subject process.

In another alternate embodiment, the exchanged zeolites are further impregnated with alumina. Applicants have discovered that a small amount of alumina impregnated onto the subject metal-exchanged catalysts provides a substantial Unprovement in catalyst activity. In fact, the minimum amount of alumina required to provided the desired result is difficult to ascertain because minute amounts of alumina provide the desired improvement. The maximum amount of alumina to be impregnated onto the catalyst is that amount which begins to adversely affect catalyst performance. General amounts of alumina to be used range from 0.1 to 40% alumina based upon the total weight of the exchanged zeolite.

The metal-exchanged and impregnated metal-exchanged catalysts of this invention may be subjected to thermal treatment prior to use in the process although such treatment is not required to practice the invention. The thermal treatment may be conducted in the reactor prior to contacting the reactants with the catalyst or as a separate step.

The thermal treatment comprises heating the catalysts of this invention to above ambient temperature, preferably between about 80.degree. and 150.degree. C. while under an inert atmosphere of about 1 to 220 atmospheres for a period ranging from about 0.5 to 12 hours to remove residual moisture. The catalyst may be dried during one or more periods utilizing one or more discrete temperatures or temperature ramping techniques known in the art. The amount of time and temperature regime employed to dry the catalyst is not critical to the invention.

The amount of catalyst to be utilized in the present process varies depending upon the reaction conditions (i.e., temperature, pressure and the like), and the type and distribution of components comprising the NOx. An effective amount of catalyst is used, i.e., that amount which causes a reaction involving the oxygen, methane and NOx to selectively produce the desired reduction products.

The catalysts of the invention can be fabricated onto ceramic or metal supports known in the art including those customarily used in the automotive industry. A preferred support has a honey-comb design whereby surface area is maximized to enhance catalytic activity. The catalysts of this invention can be used as a component in catalytic converters such as conventional three-way catalytic converters. The catalysts of this invention can be used in conjunction with conventional platinum catalysts wherein the platinum catalyst is used to reduce residual methane present in the combustion product following removal of NOx and/or carbon monoxide.

Combustion products containing NOx and carbon monoxide can be catalytically reacted in the presence of the subject catalysts under a broad range of conditions. Typically, such reaction is run at temperatures ranging from about 250.degree. C. to 700.degree. C. and pressures between about 0.5 and 300 atmospheres. More particularly, the process can be advantageously run under fixed bed conditions at temperatures ranging from about 350.degree. C. to 600.degree. C. and a gas hourly space velocity ranging from 1,000 to 100,000 hr.sup.-1, preferably 7,500 hr.sup.-1 to 30,000 hr.sup.-1. Some combustion products are produced or changed to sub-atmospheric pressure in the exhaust or flue gases.

The amount of methane to be added to the NOx-containing combustion product such as flue gas is important in achieving satisfactory NOx reduction. While NOx reduction is obtained by using a stoichiometric equivalent or less of methane with respect to NOx, a stoichiometric excess of methane is preferred to ensure complete removal of NOx and carbon monoxide from the combustion product. Generally, the methane/NOx ratio (by volume) is greater than about 0.1 to 100, although preferably, the methane/NOx ratio is maintained between about 0.1 and 800 and most preferably, between 0.1 and 400.

The amount of oxygen to be added to the NOx-containing combustion product is not critical to this invention. Amounts substantially below stoichiometric amounts have been found sufficient to effect the present process although a stoichiometric excess with respect to NOx is preferred to ensure complete removal of NOx from the effluent stream. For example, the process will operate using a lean-burn internal combustion engine wherein the fuel is typically burned with an excess of air in ratios up to 24 parts of air to one part of fuel.

To address concerns regarding venting methane into the atmosphere, a multi-stage catalyst system can be used wherein a first stage contains the above-mentioned catalysts and a second stage contains a catalyst specifically selected to destroy methane. Such methane-destroying catalysts include platinum and palladium based catalysts such as palladium on ZSM-5. NOx and carbon monoxide are destroyed in the first stage of the catalyst and residue methane is destroyed in the second stage prior to venting the products to the atmosphere.

The following examples are provided to further illustrate various embodiments of this invention and to provide a comparison between the enumerated catalysts of this invention and prior art catalysts for destroying NOx in oxygen-containing flue gases. These examples are provided to illustrate the nature of the process described herein and are not intended to limit the scope of the claimed invention. Unless otherwise stated, parts and percentages in the examples are given by volume.

The metal-exchanged mordenite catalysts of the present invention are prepared according to the following general procedure. By way of example, Co-LZ-M-5 was prepared by submersing fifteen grams of LZ-M-5, obtained from Union Carbide Company, Chickasaw, Ala., in a 2 liter Co(II)acetate solution (0.02M) with stirring at 80.degree. C. for 24 hours. The resulting cobalt exchanged catalyst was washed with 2 liters of distilled water for 1 hour and filtered followed by drying at 110.degree. C. overnight.

The metal-exchanged MFI structure type catalysts of the present invention were prepared according to the following general procedure. For example, ZSM-5 was prepared according to the general procedure described in I & EC 24, 507 (1985) wherein a gel was prepared containing 30% silica solution, sodium hydroxide and aluminum hydroxide in the molar ratio of 3.9 Na.sub.2 O/36 SiO.sub.2 /Al.sub.2 O.sub.3 /720 water. The resulting gel was stirred at 165.degree. C. in a PARR mini-reactor, filtered and washed with de-ionized water. The composition was verified by X-ray diffraction and elemental analysis. Fifteen grams of the resulting Na-ZSM-5, (Si/Al=14) were submersed in a 3.5 liter Co(II)acetate solution (0.01M) with stirring at room temperature for 18 hours followed at 40.degree. C. and 80.degree. C. for 20 and 24 hours, respectively. The resulting cobalt-exchanged catalyst was washed with 3.5 liters of distilled water for 1 hour and filtered followed by drying at 110.degree. C. for 5 hours. Elemental analysis demonstrated that the catalyst contained 4.0 wt % cobalt with a Co/Al ratio of 0.70 which corresponds to 140% of the theoretical exchange level.

The following general procedure can be used to prepare metal exchanged Co-ZSM-5 catalysts. By way of example, Mn-exchanged Co-ZSM-5 catalyst was prepared by submersing five grams of Co-ZSM-5, prepared by the procedure according to Example 2, in 40 ml of a manganese acetate solution (0.01M) with stirring at room temperature overnight (The exchange temperature for preparing copper- and chromium-exchanged Co-ZSM-5 was room temperature while manganese- and nickel-exchange reactions were conducted at 80.degree. C.). The resulting manganese-cobalt-exchanged ZSM-5 catalyst was washed with 2 liters of distilled water for 1 hour and filtered followed by drying at 110.degree. C. for 5 hours.

0.18 g. Nb(HC.sub.2 O.sub.4).sub.5 obtained from Kaweoki Berylco Industries, Inc., Boyertown, Pa., was dissolved in a solution of 0.55 g H.sub.2 C.sub.2 O.sub.4 2H.sub.2 O in 10 ml water via ultrasonic vibration. 2 ml of the resulting solution was added dropwise with constant stirring to 3.04 g of Co-exchanged ZSM-5 catalyst prepared according to Example 2. The catalyst was dried overnight followed by re-impregnation under the same conditions. The resulting catalyst was dried overnight at room temperature followed by drying at 110.degree. C. for 24 hours. The niobium loading was determined to be 0.4 wt % based upon total catalyst weight.

Alumina-impregnated zeolite catalysts can be conveniently prepared using the following general preparation. A desired zeolite, in this case, Co-ZSM-5, was suspended in a minimum amount of deionized water. A desired amount of Al(NO.sub.3).sub.3 was then dissolved in the zeolite slurry such that the concentration of Al.sub.2 O.sub.3 in the Co-ZSM-5 after calcination was 14% by weight. The Co-ZSM-5/Al(NO.sub.3).sub.3 slurry was vigorously stiffed with a magnetic stiffer until the excess water in the slurry evaporated. The solid was dried at 110.degree. C. for 3 hours and then calcined in air at 525.degree. C. for 1 hour.

Ferrierite in the K.sup.+ and Na.sup.+ form, obtained from TOSOH corporation (Japan) was first converted to the NH.sub.4.sup.+ form by exchanging with NH.sub.4 NO.sub.3. Ferrierite in the NH.sub.4.sup.+ form was then exchanged with Co.sup.2+ to obtain Co-ferrierite. For example, 15 g of ferrierite (in K.sup.+, Na.sup.+ form) was suspended in 180 ml NH.sub.4 NO.sub.3 solution ([NH.sub.4.sup.+ ]=1M) with constant vigorous stirring. Each exchange was carried out at room temperature overnight and three exchanges were performed. After the final exchange, the preparation was filtered and washed with 1 liter water, filtered again and dried at 110.degree. C. overnight. The elemental analyses of the sample demonstrated that Na.sup.+ and K.sup.+ cations were completely exchanged out by NH.sub.4.sup.+. 10 g of the NH.sub.4.sup.+ ferrierite was taken for Co.sup.2+ exchange. The NH.sub.4 -ferrierite was suspended in 500 ml water and 4.0 g Co(C.sub.2 O.sub.2 H.sub.3).sub.2 4H.sub.2 O was dissolved in another 500 ml water. The Co.sup.2+ solution was slowly added into the zeolite slurry while vigorously stirring with a magnetic stirrer. Each exchange was carried out at 80.degree. C. for 24 hours with two exchanges being performed. The resulting preparation was washed with 1 liter water and dried at 110.degree. C. overnight. Analytical analysis confirmed that 74% cobalt was exchanged. Mn-ferrierite was made by the same procedure except that the Mn.sup.2+ exchange was carried out three times.

NH.sub.4 -ZSM-5 was prepared by converting Na-ZSM-5 using the method according to Example 6. The Na-ZSM-5 was obtained from Vereinigte Aluminum-Werke A.G. (Germany). 10 g of NH.sub.4 -ZSM-5 was suspended in 500 ml deionized water with constant stirring. 3 g of Ga(NO.sub.3).sub.3 9H.sub.2 O was dissolved in 500 ml of water. The Ga.sup.3+ containing solution was then slowly added into the zeolite slurry. The exchange was carried out at 80.degree. C. for 24 h. The preparation was then filtered, washed in 1 liter of water and filtered again. Finally, the sample was dried at 110.degree. C. overnight.

The subject catalyst was prepared according to the following general procedure. 20 grams of Pd(NO.sub.3).sub.2 solution (1.44 wt % Pd) containing 2.72.times.10.sup.-3 moles Pd, which is equivalent to the base exchange capacity of 5 g of Na-ZSM-5, was prepared. Pd.sup.2+ was exchanged in a 500 ml solution [Pd.sup.2+ ]=0.005M with 5 g of Na-ZSM-5 for 24 hours. The resulting Pd-ZSM-5 catalyst was filtered and dried at 100.degree. C. overnight. The catalyst contained 3.7% palladium.

The following general procedure was utilized for effecting the catalytic reduction of NOx and carbon monoxide with methane and oxygen in the presence of the subject catalysts.

A reactor was constructed having a 4 mm i.d. glass tube with an expanded section (8-13 mm i.d.) as a catalyst bed. A separate inlet for admixing oxygen with the NO was provided at a position to enable thorough mixing just prior to contacting the reactants with the desired catalyst. The weight of catalyst used in these tests varied from a few tenths of a gram to one gram. The GHSV can be varied between 100 and 100,000 to achieve desired conversion levels. The reactor was surrounded by a temperature-controlled furnace. The temperature was monitored by a chromel-alumel thermocouple which was in contact with the catalyst bed.

The activity measurements were made with a microcatalytic reactor in a steady-state flow mode. Product analysis was obtained using an on-line gas chromatograph with a thermal conductivity detector. The separation column was packed with 5A molecular sieve (80/100 mesh) and was 4 feet long having a 1/8" outer diameter. Chromatograph temperature was 25.degree. C. and the flow rate of the carrier gas was 30 cm.sup.3 /min.

The catalysts according to Examples 1 through 8 and various prior art catalysts were tested according to the procedure of Example 9 for catalytic reduction of NO in the absence of methane and oxygen, in the presence of methane, in the presence of methane and oxygen and in the presence of propene and oxygen. Reaction conditions were constant using the following feedstream: [NO]=0.16%; [CH.sub.4 ]=0.10%; [C.sub.3 H.sub.6 ]=0.055%; [O.sub.2 ]=2.5% and a GHSV of 30,000 h.sup.-1.

The results presented in Table 1 demonstrate that while Cu-ZSM-5 (Run 2) provides good conversion of NO in the presence of methane (55% conversion) and propene (52% conversion), conversion decreases to 9% when NO is reacted in the presence of methane and oxygen over the Cu-ZSM-5 catalyst. In contrast, Co-ZSM-5 (Run 4) provides a conversion of 9% in the absence of oxygen but yields a 300% increase in conversion when a stoichiometric excess of oxygen is introduced in combination with methane as the reducing agent.

Runs 1 and 2 demonstrate that the prior art catalysts exhibit very high activity using methane as a reductant in the absence of oxygen. However, activity of the prior art catalysts plummets when oxygen is introduced into the system. These results demonstrate the unexpected nature of Applicants' catalytic process for destroying NOx in oxygen-containing combustion products wherein methane is used as a reductant in the presence of the enumerated metal-exchanged catalysts. Contrary to catalysts disclosed in prior art processes wherein the presence of oxygen decreases catalyst activity, the catalysts of the present invention exhibit a substantial increase in activity when oxygen is admixed with the methane reductant.

Table 2 provides a comparison of conversions achieved by reacting NO in the presence of methane and oxygen over various catalysts at defined temperatures. Table 2 demonstrates that cobalt, nickel, iron, chromium and manganese exchanged ZSM-5 (Runs 8, 10 and 11) and cobalt-exchanged LZ-M-5 (Run 9) exhibit significant activity for the subject reaction over a temperature range of 350.degree. to 500.degree. C.

In Runs 17-21, some of the cobalt which was previously ion-exchanged into the ZSM-5 catalyst was exchanged with manganese, copper, nickel, chromium or silver. The activity of the catalyst appears directly related to the cobalt loading level.

Under the specified reaction stoichiometry, each of the catalysts according to Runs 8 through 11 and 14 through 21, inclusive provided maximum conversion at about 450.degree. C. Run 12 demonstrates that cobalt metal supported on alumina is inactive for the reduction of NO in the presence of oxygen using methane as a reductant. Moreover, Cu-ZSM-5 (Run 13) provided relatively poor conversion to the desired reduction products compared to the Co-exchanged zeolite catalysts of the present invention when oxygen is present in the reactant stream.

Table 3 demonstrates the effect of the methane/NO ratio on NO conversion for the reaction of NO in the presence of methane and an excess of oxygen at a temperature of 400.degree. C. over Co-ZSM-5 prepared according to Example 2. Runs 22 through 25 demonstrate that the Co-ZSM-5 zeolites of the present invention provide a high degree of NO conversion over methane/NO ratios ranging from 0.6 to 2.4. In particular, Run 25 (methane/NO=2.4) provides a 95% conversion of NO to nitrogen gas. Overall, an increase in the methane/NO ratio provides higher conversion to the desired reduction products.

Table 4 demonstrates the effect of temperature on the conversion of NO over Co-ZSM-5 wherein the methane/NO ratio is 0.6 and 2500 ppm oxygen was present in the feedstream. Runs 26 through 30 illustrate that conversion reaches a maximum at approximately 425.degree. C.

Table 5 demonstrates the effect of the ratio of oxygen to methane on NO conversion for the reaction of NO over Co-ZSM-5 at 400.degree. C. and a GHSV of 30,000 hr.sup.-1. The oxygen/methane ratio was varied from 97/1 to 3/1. The results demonstrate that only a modest decrease in NO conversion is caused by increasing the oxygen/methane ratio from 97/1 (Run 31) to 3/1 (Run 36). These results show that the present process can effectively reduce NO in combustion products having a broad range of oxygen content.

Table 6 demonstrates the effect of oxygen content on reduction of NO at 400.degree. C. over Co-ZSM-5 in the presence of methane as the reductant. The reaction stream contained equal amounts of NO and methane (820 ppm) and the oxygen content was varied. Runs 37 through 41 illustrate that NO conversion is enhanced and maintained essentially constant so long as some oxygen is present in the reaction.

Stated alternately, the presence of oxygen actually enhances the conversion of NO to the desired reduction products whereas the catalysts of the prior art are typically deactivated by the presence of oxygen. For example, Run 38 summarizes the reaction wherein 5000 ppm oxygen was utilized which provides 60% conversion to nitrogen gas. In contrast use of over 5 times the stoichiometric amount of oxygen (Run 41) results in essentially no diminution of conversion. At much higher oxgyen levels, some diminution in conversion is observed.

Table 7 provides a comparison of the activities achieved by using Co-ZSM-5 and Mn-ZSM-5 as a function of methane concentration at a temperature of 400.degree. C. and a GHSV of 7,500 h.sup.-1. Runs 42 through 46 demonstrate that when an excess of oxygen is utilized, use of greater than a stoichiometric amount of methane results in increased conversion of NO to nitrogen gas. In particular, Co-ZSM-5 (Run 46) provides a 97% conversion of NO to nitrogen gas when the methane/NO ratio is 2.5.

The effect of metal loading on catalyst activity is presented in Table 8. The results demonstrate that catalyst activity is directly related to cobalt loading to a level of about 100% (Co/Al=0.5) for conversion of NO in the presence of methane and oxygen under the specified reaction conditions. Run 52 demonstrates that over-exchanging the zeolite with cobalt (Co/Al=1.05) does not cause a substantial decrease in catalyst activity.

Table 9 discloses the results for conversion of NOx in the presence of methane and oxygen over rhodium exchanged ZSM-5 catalyst prepared according to the procedure in Example 2. More particularly, 5 grams of Na-ZSM-5 zeolite was exchanged in a 1-liter Rh(NO.sub.3).sub.2 solution ([Rh.sup.2+ ]=0.006M twice at 25.degree. C. and 70.degree. C., respectively. The resulting preparation was dried at 110.degree. C. overnight. The elemental analysis showed a rhodium loading of 0.6% by weight. Run 55 demonstrates that 55% conversion to nitrogen gas was obtained when the reaction was conducted at 450.degree. C.

Table 10 discloses the results for conversion of NOx and methane in the presence of alumina impregnated Co-ZSM-5 prepared according to Example 7. Runs 57 through 60 demonstrate that NOx conversions were substantially lower on the alumina-occluded samples at low temperatures (400.degree. and 450.degree. C.) but surprisingly, NOx conversion was much greater at high temperatures (500.degree. and 550.degree. C.). Unlike Co-ZSM-5, the bend over of conversion with temperature was not observed on the occluded samples at temperatures up to 500.degree. C. These samples are the most active catalysts at high temperatures, i.e., greater than 500.degree. C. among Co-ZSM-5 catalysts. The high NO conversion on the occluded sample at high temperatures is apparently due to the availability of methane (relatively low conversion of methane) while on a normal Co-ZSM-5, the depletion of methane at 500.degree. C. resulted in the activity bend-over. Alumina occlusion provides a way to modify properties of the subject zeolite catalysts and this method can be extended to other catalyst types, e.g., ferrierite-based catalysts and the like. Run 61 demonstrates that a physical mixture of Co-ZSM-5 powder and Al.sub.2 O.sub.3 does not provide enhanced NOx conversion.

The Figure shows NO conversions on ferrierite based catalyst compared to Co-ZSM-5 as a function of temperature. Elemental compositions of the subject catalysts are summarized in Table 11. The reactions were run under identical conditions: [NO]=1600 ppm, [O2]=2.5 % with 0.1 g sample and a total flow rate of 100 cc/min.] Ferrierite based catalysts significantly enhanced NO reduction activity at temperatures greater than 450.degree. C. For example, the NO conversion at 500.degree. C. achieved by Co-ferrierite is doubled compared to Co-ZSM-5. Ferrierite, with a Si/Al ratio of 6 has higher conversions than that with 8. On Mn-ferrierite, the conversion bend-over was not observed even up to 590.degree. C., a striking contrast to Co-ZSM-5. The superior performance of ferrierite catalysts over Co-ZSM-5 is mainly due to its ability to limit the methane combustion rate at high temperatures. The NH.sub.4.sup.+ pre-exchange is necessary to obtain stoichiometric Co.sup.2+ or Mn.sup.2+ exchange.

Table 12 discloses catalyst activities achieved for conversion of NOx using Co-ZSM-5 and Ga-ZSM-5. Although the NO conversions obtained using both catalysts were comparable at temperatures less than about 450.degree. C., the NO conversion obtained using Ga-ZSM-5 at temperatures greater than about 500.degree. C. are much higher than those obtained using Co-ZSM-5. More importantly, Ga-ZSM-5 is much more selective toward NOx reduction; the catalyst uses much less methane to achieve similar or even greater NO conversions. The highest methane conversion on Ga-ZSM-5 is only 32% while 100% conversion was obtained on Co-ZSM-5. The efficiency of the Ga-ZSM-5 catalyst is further demonstrated in Table 13 wherein the NO conversion is shown as a function of methane level. Even with only 195 ppm methane (methane/NO=0.25) the Ga-ZSM-5 catalyst demonstrates remarkable reduction activity.

Table 14 present results for a dual-bed catalyst system comprising a NO reduction catalyst as the first layer and a methane oxidation catalyst as a second layer. The results demonstrate that the multilayer catalyst system can effectively reduce NO to N.sub.2 and completely oxidize the unreacted methane to carbon dioxide. Therefore, reaction conditions (i.e., space velocity and temperature) can be controlled to achieve a desirable NO conversion and avoid methane slip. The second layer is a palladium based catalyst prepared by exchanging palladium onto an inorganic oxide support such as Al.sub.2 O.sub.3, SiO.sub.2, Al.sub.2 O.sub.3 --SiO.sub.2, TiO.sub.2 and MgO or onto conventional zeolites.

Further experiments demonstrate that Pd-ZSM-5, alone is not a good catalyst for NO reduction but is an excellent methane oxidation catalyst. Like Pd-ZSM-5, the bimetallic cation exchanged ZSM-5 has a very low NO reduction activity but a very high methane oxidation activity. The order of the catalyst layers for the dual bed is very important, namely the combustion product should first contact the Co-ZSM-5 layer followed by the Pd-ZSM-5 layer. The reverse combination (Pd-ZSM-5 as the first layer and Co-ZSM-5 as the second layer) would be similar to using Pd-ZSM-5 alone, because Pd-ZSM-5 would deplete methane before it reached the Co-ZSM-5 catalyst thereby substantially diminishing the effectiveness of NO reduction.

An additional set of experiments was run to determine whether the subject catalysts generate carbon monoxide during the reduction of NO by methane and whether the subject catalysts can oxidize CO to carbon dioxide if carbon monoxide is deliberately added to the feed stream. As shown in Tables 15 and 16, carbon monoxide (CO) was not detected as a product during the NO reduction at 400.degree. C., and carbon dioxide was formed in an amount equivalent to methane consumption (the carbon balance was greater than 96%). When 1025 ppm of CO was added to the feed, NO conversion increased slightly from 33% to 36% and the CO added was completely oxidized to carbon dioxide (carbon was 100% balanced). These results clearly demonstrate that Co-ZSM-5 is bifunctional, meaning that the catalyst can simultaneously reduce NO via methane as a reductant and oxidize CO to carbon dioxide.

The enumerated catalysts of the present invention provide several improvements over prior art processes for removing NOx from combustion processes wherein methane is used as a reductant. First, the claimed catalysts are unexpectedly more active and selective than prior art catalysts in converting NO as well as carbon monoxide in the presence of oxygen and methane; second, the catalysts are not deactivated in the presence of a substantial stoichiometric excess of oxygen; and third, the subject catalysts can be used in conjunction with conventional platinum or palladium catalysts such that the platinum or palladium catalyst removes residual methane not consumed in removing NOX and carbon monoxide.

Having thus described the present invention, what is now deemed appropriate for letters patent is set forth in the following claims.