Atmospheric pollution is a societal problem which is receiving much attention. The major source of such pollution is the extensive use of fossil fuels, although industrial and chemical processes, such as the manufacture of nitric acid, also contribute. The principal pollutants are nitrogen oxides, carbon monoxide, and perhaps to a lesser extent hydrocarbons, sulfur oxides and other objectionable gases and vapors.
Although several nitrogen oxides are known which are relatively stable at ambient conditions, it is generally recognized that two of these, viz. nitric oxide (NO) and nitrogen dioxide (NO.sub.2), are the principle contributors to smog and other undesirable environmental effects when they are discharged into the atmosphere. These effects will not be discussed further herein since they are well recognized and have led various government authorities to restrict industrial and automotive emissions in an attempt to limit the level of the oxides in the atmosphere. Nitric oxide and nitrogen dioxide, under appropriate conditions, are interconvertible according to the equation: EQU 2NO+O.sub.2 =2NO.sub.2.
For purposes of the present invention, NO.sub.x will be used herein to represent nitric oxide, nitrogen dioxide, and mixtures thereof
Formation of nitrogen oxides from the elements occurs in the high temperature zones of combustion processes. The internal combustion engine, and coal or gas-fired furnaces, boilers and incinerators, all contribute to NO.sub.x emissions. In general, fuel-rich combustion mixtures produce exhaust gases with lower contents of NO.sub.x than do lean mixtures. Although the concentrations of NO.sub.x in the exhaust gases produced by combustion usually are low, the aggregate amounts discharged in industrial and/or highly populated areas is adequate to cause problems. Other industrial sources of pollution also exist. These are associated with the manufacture of nitric acid, with nitration of organic chemicals, and with 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. In these instances the exhaust gas may contain relatively high levels of NO.sub.x, such as from 0.1% up to 2% to 3%.
The so-called "stable" nitrogen oxides have in common the somewhat peculiar property that although they are thermodynamically extremely unstable at room temperature with respect to decomposition into elemental oxygen and nitrogen, no simple, economical method has been described for inducing this decomposition. It has been discovered, however, that adding a reductant such as ammonia to the exhaust gas can, under appropriate reaction conditions, convert NO.sub.x to elemental nitrogen and steam at moderate temperatures in the range of 200.degree. 600.degree.600.degree. C.
Unlike the objectionable nitrogen oxides, carbon monoxide is a combustible gas which is convertible to CO.sub.2 by burning. Also unlike the nitrogen oxides, for which relatively few effective conversion catalysts are known, the combustion of carbon monoxide is catalyzed by a fairly large number of catalysts among which are the platinum group metals and their alloys, gold, base metal oxides such as chromia and copper oxide, and certain rare earth oxides.
The technology for abatement of pollution by noxious gases began to be developed some years ago. The earliest methods for control of gaseous emissions included absorption, adsorption, condensation, chemical reaction and incineration. Some of these methods, such as absorption, require the disposal of recovered pollutants, which in itself may be a problem.
In recent years, increasing attention has been paid to developing catalytic technology for control of exhaust gas pollution. Such technology potentially offer the advantages of low cost and the possibility for converting the noxious gas to one or more innocuous substances such as carbon dioxide and water, which are free of disposal problems. In instances in which two or more substances such as NO.sub.x and CO are present in the exhaust, as is often the case with fossil fuel combustion, a single catalyst that is effective for converting more than one of the pollutants is highly desirable.
Certain zeolitic catalysts have been described as effective for control of pollution by one or more of NO.sub.x, CO, and hydrocarbons. The term "zeolite" or "zeolitic" as used herein refers to porous crystalline minerals or synthetic oxides, usually aluminosilicates, that have a rigid three dimensional framework structure, such as are described in "Zeolite Molecular Sieves" by Donald W. Breck, John Wiley & Sons, New York, N.Y. (1974), relevant portions of which are incorporated herein by reference for background.
U.S. Pat. No. 3,900,554 to Lyon describes a non-catalyzed homogeneous gas phase reaction to remove NO.sub.x from combustion effluent by adding 0.4 to 10 moles (preferably 0.5 to 1.5 moles) of ammonia followed by heating to 1600.degree. C. to 2000.degree. C. The NO.sub.x content is lowered as a result of its being reduced to nitrogen by reaction with ammonia. The method is reported to work best if hydrocarbon is also added to the mixture. The extremely high temperature required for NO.sub.x abatement is a disadvantage of the method.
U.S. Pat. No. 4,220,632 to Pence et al. discloses a process for reducing noxious nitrogen oxides from a fossil-fuel-fired power generation plant, or from other industrial plant off-gas stream, to elemental nitrogen and/or innocuous nitrogen oxides employing ammonia as reductant and, as catalyst, the hydrogen or sodium form of a zeolite having pore openings of about 3 to 10 Angstroms. The Pence et al. patent illustrates the so-called Selective Catalytic Reduction Process (hereinafter referred to as the "SCR Process") for removal of NO.sub.x from exhaust gas. While the SCR process usually operates at 200.degree.-600.degree. C., which is an advantage over the uncatalyzed conversion, an external reductant (ammonia) must be furnished which adds to the cost of the process, and control of the NH.sub.3 feed is required to avoid ammonia emissions.
During early commercial development of molecular sieves, it was found that copper, for example, could be introduced into the inner absorption regions of the crystalline zeolites by several different means, including simple ion-exchange with divalent copper ion, and impregnation with complexes such as copper acetylacetonate in which the metal is in the zero valence state. It was further found that copper, in the inner absorption regions of the zeolite, whether present as metal or cation, reacted readily with molecules that could enter the pores of the molecular sieve. U.S. Pat. No. 3,013,985 to Breck et al. is incorporated herein by reference for background purposes, and also for the description contained therein on methods for loading the molecular sieve with copper either by ion-exchange or by the use of complex decomposable compounds in which the metal is in the zero valence state.
The use of copper exchanged zeolites in the SCR reduction of NO.sub.x is mentioned in U.S. Pat. No. 4,046,088.
U.S. Pat. No. 3,346,328 to Sergeys et al. proposes to use as the catalyst for treating internal combustion engine exhaust gases a Cu.sup.++ exchanged zeolite such as Zeolite Y which has CuO loaded or held interstitially in its pore system. Since the copper ion is reported to play an important part in hydrocarbon conversion and copper oxide is reported to be an influential factor in carbon monoxide conversion, the activity of the catalyst is optimized for a particular exhaust gas by varying the proportion of zeolitic Cu.sup.++ cations and impregnated CuO.
In catalytic pollution control technology, the monetary cost is significant, and particularly so when the exhaust contains little or no economically recoverable values in terms of materials or heat. For this reason, it becomes very important to find catalysts that are relatively inexpensive, and that are not only effective for the desired conversion, but that also are adequately long lived in the process environment. As a general rule, a catalyst will deteriorate with time on stream for one or more reasons, such as loss of activity for conversion, for selectivity, or for both, until it becomes necessary to discard it. The term "aging", as used herein, will refer to such deterioration. One of the factors often responsible for excessive aging is exposure of the catalyst to steam at high temperature. Unfortunately, the burning of fossil fuels inherently forms a flue gas or exhaust gas that contains a significant content of steam as well as NO.sub.x and/or other pollutants such as CO and hydrocarbons. Catalytic removal of such pollutants necessarily include contact with steam.
It is an object of this invention to provide a steam-resistant zeolite copper catalyst for the catalytic decomposition of NO.sub.x. It is a further object of this invention to provide a method for preparing a steam-resistant zeolitic copper catalyst. These and other objects will become evident on reading this entire specification including the appended claims.
We now have found a surprising, simple way to make zeolitic copper catalysts that are highly resistant to aging in the presence of steam.