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`JM 1034
`Johnson Matthey v BASF
`IPR2015-01267
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`JM 1034
`Johnson Matthey v BASF
`IPR2015-01267
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`WO 02/41991
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`PCT/US01/45377
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`HYDROTHERMALLY STABLE METAL PROMOTED
`
`ZEOLITE BETA FOR NOX REDUCTIONV
`
`BACKGROUND OF THE INVENTION
`
`Field of the Invention
`
`The present invention is concerned with a method of
`catalyzing the reduction of nitrogen oxides with ammonia,
`
`especially the selective reduction of nitrogen oxides, with
`ammonia in the presence of oxygen, using zeolite catalysts,_
`especially metal—promoted zeolite catalysts.V The invention‘
`
`is also‘directed to hydrothermally stable zeolite catalysts
`
`and methods of making same.
`
`i The Related Artd
`
`Both synthetic and natural ieolites and their use in
`promoting certain reactions,
`including the selective
`reduction of nitrogen oxides with ammonia in the presence of
`oxygen, are well known in the art. Zeolites are
`T
`aluminosilicate crystalline materials having rather uniform‘
`pore sizes which, depending upon the type of zeolite and the’
`type and amount of cations included in the zeolite lattice,
`range from about 3 to 10 Angstroms in diameter.
`C«,
`,Japanese Patent Publication (Kokai) No. 51-69476,
`published June 16, 1976 on Application No. 49—l4246s, filed
`December 13, 1974, discloses_a method for reducing nitrogen
`
`oxides in waste gases by reaction with ammonia in the
`‘presence ofia metalapromoted, dealuminized synthetic or
`
`natural mordenite zeolite.
`
`The resistance of the catalyst
`
`to sulfurous poisons, particularly sulfur trioxide and
`sulfuric acid mist,
`is said to be enhanced by dealuminizing
`
`the mordenite to increase the silica to alumina ratio to
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`The zeolite is
`more than 12, preferably to more than 15.
`promoted with 0.5 to 30 wt. % of at least one of a number of’
`
`promoters including copper, vanadium, chromium,
`iron, cobalt
`or nickel and used at a reaction temperature of 200°C.
`to
`
`500°C. with from 0.5 to three times the stiochiometric
`
`amount of ammonia reductant.
`
`Example 1 of the Publication
`
`illustrates an iron—promoted mordenite ore as being
`effective for the reduction of nitrogen oxides.
`In‘
`
`connection with Example 2, it is stated that a slight
`
`decrease of the activity ofia high silica to alumina ratio,‘
`copperepromoted mordenite catalyst is recognized when sulfur
`trioxide is included in the gas stream. However, an
`Vextreme improvement” of resistance to sulfur trioxide
`, poisoning is noted in comparison with a copper mordenite
`which has not been dealuminized to increase the silica to
`
`alumina ratio.
`A
`UK Patent Application No. 2,l93,655A discloses a
`
`catalyst containing a low surface area titania and a copper-.
`
`'.promoted zeolite for use in the reduction“of nitrogen oxides
`with ammonia.
`The zeolite has an average pore diameter of
`
`10 Angstroms or less, preferably 8 Angstroms or less, and a
`silica to alumina molar ratio of 10 or more, preferably 20»
`or more;
`the resultant titania/—promoted zeolite catalysts
`having these characteristics are stated to have good
`mechanical strength and to be resistant to volatile catalyst
`poisons such as arsenic, selenium,
`tellurium, etc.,
`contained in exhaust gases. ‘Examples of suitable zeolites
`
`.are mordenite, ZSM—5, and ferrierite.
`
`U.S. Patent No. 4,297,328 discloses a “three—way
`
`conversion” catalytic process for the simultaneous catalytic
`
`oxidation of carbon monoxide and hydrocarbons and reduction
`
`of nitrogen oxides for purifying the exhaust gas of
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`automobile engines operated within a prescribed range of air
`to fuel ratio (Column 4,
`lines 63-68).
`The disclosed
`.
`
`catalyst is a copper—promoted zeolite having a silica to
`alumina ratio greater than 10, preferably greater than 20)
`(column 6,
`lines 23-28).
`iRepresentative high~silica
`zeolites are described at-columns 6-8 of the patent and
`
`include (column 6,
`
`lines 29~33) silicalite (as described in
`
`Patent NO. 4,061,724), ZSM—8, ZSM—ll, ZSIM-12, hyper Y,
`' ultrastabilized Y, Beta, mordenite and erionite.
`Ultrastabilized Y is described (column 7,
`lines 22-25) as “a
`form ofrzeolite Y which has been treated to give it the
`organophilic characteristic of the'adsorbents of the present
`invention.” Example 6 of the patent is stated to show no
`‘measurable loss in combustion activity of the copper-
`promoted zeolite catalyst due to sulfur poisoning (exposure
`‘of the catalyst to methylmercaptan in the-gaseous stream).
`
`The patent thus discloses the utility of the copper—promoted
`
`'specified zeolites for three-way conversion in an exhaust
`gas generated by a lean air to fuel ratio*combustion
`
`-
`
`p
`mixture.
`The art thus shows an awareness of the utility of
`
`metal~promoted zeolite catalysts including, among others,
`iron~promoted and copper—promoted zeolite catalysts, for the
`selective catalytic reduction of nitrogen oxides with
`
`ammonia.
`
`‘
`
`there is
`In accordance with U.S. Patent No. 4,96l9l7,
`provided an improred method for the reduction of nitrogen
`oxides with ammonia.
`The method described in this commonly
`
`A
`assigned U.S. patent comprising the following steps.
`gaseous stream containing nitrogen oxides and ammonia, and
`
`which may also contain oxygen,
`
`is contacted at a temperature
`
`of from about 250°C.
`
`to 600°C. with a sulfur—tolerant
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`The catalyst composition comprises a
`catalyst composition.
`zeolite having a silica to alumina ratio of at least about
`
`10, and a pore structure which is interconnected in all
`three crystallographic dimensions by pores having an average
`
`kinetic pore diameter of at least about 7 Angstroms, e.g.
`
`‘from about 7 to 8 Angstroms, and one or both of an iron and
`
`in
`
`a copper promoter present in the zeolite, for example,
`’lthe amount of from about 0.1 to 30 percent by weight,
`"preferably from about 1 to 5 percent by weight, of the total
`weight of promoter plus zeolite. The‘zeolite comprises one
`or more of USY, Beta and ZSM-20.
`A refractory binder may bed
`admixed with the zeolites. An iron—promoted zeolite beta is
`preferred and has been commercialired for removing Nox by
`selective catalytic reduction such as from gas turbine
`Vexhaust.
`I
`I
`V
`
`The ironepromoted zeolite beta has been an effective
`V catalyst for the selective reduction of nitrogen oxides such
`‘as by the reduction of nitrogen oxides with ammonia.‘
`Unfortunately,
`it-has been found that under harsh
`hydrothermal conditions, such as reduction of NOX from gas
`.turbine exhaust at temperatures exceeding 500°C.,
`the
`, activity of the iron—promoted zeolite beta begins to
`"decline.
`‘This decline in activity is believed to be due to
`destabilization of the zeolite such as by dealumination and
`consequent reduction of.metal—containing catalytic sites
`‘within the zeolite.
`To maintain the overall activity of Nox
`
`increased levels of the iron—promoted zeolite
`,reduction,
`catalyst must be provided. As the levels of the zeolite
`
`catalyst increase so as to provide adequate Nox removal,
`
`there is an obvious reduction in the cost efficiency of the
`
`process for NOX removal as the costs of the catalyst rise.
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`‘Accordingly,
`
`there is a need to improve the process for
`
`the selective catalytic reduction of NOX by ammonia so as to
`
`maintain catalytic activity; even under harsh hydrothermal"
`conditions.
`A
`A
`There is a further general need for improving the
`
`.hydrothermal stability of aluminosilicate zeolite catalysts,
`especially metal—promoted zeolites so as to stabilize such
`materials from dealumination and loss of catalytic sites
`
`‘during use.
`
`’sUMMARY OF THE INVENTION
`In accordance with the present invention, a metal-
`
`promoted zeolite catalyst useful in the selective catalytic
`.reduction of nitrogen oxides with ammonia is premtreated so
`‘as to provide the zeolite with improved hydrothermal"
`.
`
`The improved stability is believed to manifest
`stability.
`in an.improved resistance to dealumination and consequent
`resistance to removal of catalytic sites from within the
`
`zeolite.
`.In another aspect ofi the invention, aluminosilicate
`seolite catalysts,
`in general, are stabilized such as
`against hydrothermal conditions by treating the
`aluminosilicate zeolites in a manner heretofore not known in
`ithe prior art;~~~
`the present invention is directed to a
`Still further,
`stable aluminosilicate zeolite as well as a metal—promoted
`aluminosilicate zeolite which is stabilized against loss of
`catalytic sites.
`
`The stabilized aluminosilicate zeolites in accordance
`with this invention are provided by incorporating into the
`
`zeolite structure non—framework aluminum oxide chains, which
`are believed to be associated with or even linked to the
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`aluminosilicate framework of the zeolite.
`
`The presence-of
`
`the non-framework aluminum oxide chains is manifest by a
`unique peak found in the FT—IR spectrum.v The presence of
`this peak at 378li2 cm4 is associated with the improved.
`stability of the zeolite.
`The non—framework aluminum oxide
`
`chains can be incorporated into the zeolite structure by.
`several processes.known at this time,
`including via a unique
`
`steaming regimen or by treatment with rare earth metals,
`
`such as cerium. While not wishing to be bound by any
`theory, it is believed that the treatment of the.
`
`aluminosilicate zeolite decouples aluminum oxide temporarily.
`
`The decoupled aluminum oxide
`from the zeolitic framework.
`r molecules are then recombined and linked as a chain, which
`‘is reattached to or otherwise associated with the zeolite
`framework.
`‘The treatment process is unlike well-known
`methods of.dealuminizing zeolites for the purpose of
`increasing the silica to alumina ratio.
`In the present-
`invention,
`the alumina is.not removed from the zeolite but
`is believed to be rearranged and otherwise attached or '
`associated with the aluminosilicate framework.
`The non-
`
`framework aluminum oxide chains associated with the FT—IR ,
`
`absorption peak at‘3781i2 cm” appear to stabilize the
`zeolite against further dealumination such as under
`i
`oxidizing and harsh hydrothermal conditions.
`The aluminosilicate zeolites which can be stabilized in '
`
`accordance with this invention are not known to be limited.
`
`Those zeolites which have known catalytic activity,
`in
`particular, medium to large pore zeolites appear to be most
`usefully treated.
`In general, zeolites having an average
`
`pore diameter of at least about 5 A are believed to be
`
`effectively treated in accordance with this invention.
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`Catalytic processes which involve oxidizing and/or
`
`hydrothermal conditions can be operated very effectively
`_with the stabilized aluminosilicate zeolites,
`including‘
`
`metal—promoted aluminosilicate zeolites treated in
`
`accordance with this invention. More specifically, it has
`
`been found that iron—promoted zeolite beta which has been
`treated to provide the non—framework aluminum oxide chains
`associated with the zeolite framework has increased
`
`I hydrothermal stability than the iron promoted zeolite beta
`catalyst which has not been so treated. An iron—promoted
`zeolite beta catalyst treated in accordance with this
`invention yields improved activity in the selective
`
`«catalytic reduction of Non with ammonia, especially-when,
`operated under high temperatures of at least about 5QO°Q.
`and high.water vapor environments of 10% or more.»
`
`BRIEF. DESCRIPTION OF THE DRAWINGS
`
`Figure 1 is a schematic of the aging of the stabilized
`zeolite of this invention.
`V
`I
`Figure 2 is a FT—IR Spectra of a stabilized zeolite»
`beta of this invention and a standard zeolite beta.
`.
`Figure 3 is a plot of activity for NOX conversion at
`430°C. comparing the activity of stabilized zeolite_beta
`- catalysts in accordance with the present invention and a
`non—treated zeolite beta catalyst.
`}
`Figure 4 is a plot of activity for NOX conversion at
`550°C. comparing the activity of stabilized zeolite beta
`
`catalysts in accordance with the present invention and a
`
`nonrtreated zeolite beta catalyst.
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`DETAILED DESCRIPTION OF THE INVENTION
`
`In order to reduce the emissions of nitrogen oxides
`
`~from flue and exhaust gases, such as the exhaust generated
`by gas turbine engines, ammonia is added to the gaseous
`stream containing the nitrogen oxides and the gaseous stream
`is then contacted with a suitable catalyst at elevated
`
`‘temperatures in order to catalyze the reduction of nitrogen
`'oxides with ammonia.
`Such gaseous streams often inherently
`
`For example, a
`contain substantial amounts of oxygen.
`typical exhaust gas of a turbine engine contains from about.
`V2 to 15 volume percent oxygen and from about 2b to 500
`“volume parts per million nitrogen oxides,
`the latter
`. normally comprising a mixture of NO and N02! Usually,
`’.is sufficient oxygen present in the gaseous stream to
`‘oxidize residual ammonia, even when an excess over the
`
`there
`
`" stoichiometric amount of ammonia required to reduce all the
`
`in cases
`nitrogen oxides present is employed: "However,
`where a very large excess over the stoichiometric amount of
`ammonia is utilized, or wherein the gaseous stream to be
`.
`treated is lacking or low in oxygen content, an oxygen-
`, containing gas, usually air, may be introduced between the-
`first catalyst zone and the second catalyst zone,
`in order
`Vto insure that adequate oxygen is present in the second
`~.catalyst zone for the oxidation of residual or excess
`ammonia.
`The reduction of nitrogen oxides with ammonia to_
`‘form nitrogen and H40 can be catalyzed by metal—promoted
`zeolites to take place preferentially to the oxidation of
`
`ammonia by the oxygen, hence the process is often referred
`to as the “selective” catalytic reduction (“SCR") of
`
`nitrogen oxides, and is sometimes referred to herein simply
`
`was the “SCR" process.
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`The catalysts employed in the SCR process ideally
`
`should be able to retain good catalytic activity under high
`‘. temperature conditions of use, for example, 400°C. or
`‘higher, under hydrothermal conditions and in the presence of
`
`sulfur compounds. High temperature and hydrothermal
`
`conditions are often encountered in practice, such as in the
`treatment of gas turbine engine exhausts.
`The presence of
`
`sulfur or sulfur compounds is often encountered in treating
`the exhaust gases of coal—fired power plants and of turbines
`or other engines fueled with sulfur—containing fuels such as
`fuel oils and the like.
`"
`Theoretically, it would be desirable in the SCR process
`to provide ammonia in excess of the stoichiometric amount
`required to react completely with the nitrogen oxides
`present, both to favor driying the reaction to completion
`and to help overcome adequate mixing of the ammonia in the_
`gaseous stream. VHowever,
`in.practice, significant excess
`"ammonia over the stoichiometric amount is normally not
`provided because the discharge of unreacted ammonia from the
`catalyst would itself engender an air pollution problem.
`Such discharge of unreacted ammonia can occur even in cases
`where ammonia is present only inpa stoichiometric or sub-
`stoichiometric amount, as a result of incomplete,reaction
`.and/or‘poor mixing of the ammonia in the gaseous stream.
`
`Channels of high ammonia concentration are formed in the
`ivgaseous stream by poor mixing and are of particular concern
`when utilizing catalysts comprising monolithic honeycomb-
`
`type carriers comprising refractory bodies having a
`
`plurality of fine, parallel gas flow paths extending
`
`therethrough because, unlike the case with beds of
`
`particulate catalysts,
`
`there is no opportunity for gas
`
`mixing between channels.
`
`It is,
`
`therefore, also desirable
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`that the catalyst employed to catalyze the selective
`
`catalytic reduction of nitrogen oxides, be effective to
`
`in order to
`catalyze the reaction of oxygen and ammonia,
`oxidize excess or unreacted ammonia to N5 and H20.
`.
`
`Commonly assigned U.S. Patent No. 4,961,917 is
`predicated on the discovery that a certain class of
`
`zeolites, especially when promoted with a promoter such as
`iron or copper, especially iron, exhibits desired
`‘characteristics as described above by providing a‘sulfur
`
`tolerant catalyst which shows good activity for both (1)
`
`the
`
`selective catalytic reduction of nitrogen oxides by reaction
`
`the
`with ammonia, evenhin the presence of oxygen, and (2)
`oxidation of ammonia with oxygen when nitrogen oxides_are at
`very low concentrations. The catalysts disclosed in the
`above referenced patent retain such activity even after
`prolonged exposure to high temperatures, hydrothermal
`conditions, and sulfate contamination of the type often
`encountered in use, e.g.,
`in the treatment of coal—fired d
`
`power plants or turbine engine exhaust gases.
`Generally,
`in accordance with the practices of the
`present invention and as disclosed in U.S. Patent No.
`4,961,917, a catalyst is provided which comprises a zeolite
`
`having specific properties as described below, and which is
`promoted byva metal, preferably iron,
`in order-to enhance
`its catalytic activity.
`The zeolite may be provided in the
`form of a fine powder which is admixed with or coated by a
`suitable refractory binder, such as bentonite or silica, and
`
`formed into a slurry which is deposited upon a suitable
`
`refractory carrier. Typically,
`
`the carrier comprises a
`
`member, often referred to as a “honeycomb” carrier,
`
`comprising one or more refractory bodies having a plurality
`
`of fine, parallel gas flow passages extending therethrough.
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`bsuch carriers are, of course, well known in the art and may
`
`be made of any suitable material such as cordierite or the
`
`like. The_catalysts of the present invention may also be
`
`provided in the form of extrudates, pellets, tablets or
`
`particles of any other suitable shape, for use as a packed
`bed of particulate catalyst, or as shaped pieces such as
`plates, saddles,
`tubes, or the like.
`Useful catalysts show a marked resistance to poisoning
`by sulfates (or other sulfur compounds) which are often
`contained in the gas streams which are treatable by the V
`
`catalysts of the present invention.“ Without wishing to be
`' bound by any particular theory, it appears that 802
`poisoning has both short term and long term effects. For
`example,
`flowing a gas stream containing 2,00Q parts per
`million_by volume (“Vppm”) S02 through catalysts comprising.
`copper—promoted small to medium pore zeolites such as ZSM—5,
`naturally occurring chabazite and clinoptilolite, resulted
`in 10 to 40 percent reduction in SCR process activity.
`iEven
`at S02 levels as low as 130 Vppm S02, significant activity '
`reduction for the SCR process was noted for such catalysts.
`
`On the other hand, larger pore zeolites such as Y, L and USY ”
`exhibited no short-term SO; susceptibility. With operating
`temperatures at about 350°C.,
`the short-term S02 poisoning
`eeffect on a copper-promoting mordenite was shown to be
`»reversible. Thus, when the supply of S02 to the test gas
`stream passing through the copper—promoted mordenite
`
`catalyst was turned off,
`
`the activity for catalytic
`
`reduction of NO immediately returned to the same level
`attained by the catalyst prior to introducing the S02.
`Apparently, S02 is absorbed, but not tightly bound in the
`
`zeolite pores.
`
`In the case of the small to medium pore
`
`zeolites,
`
`this competition absorption with NH3 and NO
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`probably results in_a physical blockage and/or diffusional
`
`restriction.
`
`On the other hand, when zeolite catalysts are subjected.
`to higher S02 concentrations for longer periods of time,
`such as 5,000 Vppm S0; for protracted periods, such as
`overnight, a 15 to 25 percent activity reduction for the SCR
`process was noted for copper—promoted, synthetic iron—free i
`
`zeolites.‘ A 60 percent reduction in SCR process activity is
`typical for Fe2O3 containing natural chabazitel' Similar
`results were sustained with iron—promoted mordenite
`catalysts.
`.
`Even at lower levels of Sozconcentration, similar to
`
`those likely to be encountered in commercial operations, a
`permanent activity loss for the SCR process is shown by many
`zeolite catalysts; For example, a copper-promoted mordenite
`catalyst was subjected overnight to passage through it of a
`gaseous stream containing 540 Vppm S01, and showed a
`I
`
`‘permanent activity loss comparable to that described above
`for the catalysts subjected to the 5,000 Vppm SO2—containing
`.gas.
`
`For zeolites with silica—alumina ratios of less than 8,
`
`»
`
`the activity loss appears to be associated with insufficient
`stability under the simulated acidic aging conditions. As @
`indicated by the prior art noted above,
`the utilization of
`high ratios of silica to alumina is known to enhance acid
`
`resistance of the zeolite and to provide enhanced resistance
`of the neolite to acid sulfur poisoning. Generally, silica
`
`to alumina ratios well in excess of the minimum-of 8 may be
`
`employed. Conversion efficiencies of 90 to 93% for NOX
`
`reduction with ammonia have been attained with fresh copper—
`
`promoted Beta zeolites having silica to alumina ratios of
`
`20, 26, 28, 37 and 62.
`
`A conversion efficiency of 77% was
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`attained byda fresh copper—promoted ZSM-5 zeolite having a’
`
`silica to alumina ratio of 46. However, fresh oopper4
`
`promoted USY zeolites with silica to alumina ratios of,
`respectively, 8 and 30 provided 85% and 39% conversions of
`NO; suggesting that at least USY, silica to alumina ratios,“
`
`‘should be significantly less than 30.
`
`However, resistance to short term sulfur poisoning and
`the ability to sustain a high level of activity for both the
`S¢R process and the oxidation of ammonia by oxygen has been
`
`found to be provided by zeolites which also exhibit pore
`size large enough to permit adequate movement of the
`reactant molecules NO and NH3 in to, and the product -
`molecules N2 and H40 out of,
`the pore system in the presence
`
`of sulfur oxide molecules resulting.from short term sulfur
`’ poisoning and/or sulfate deposits resulting from long term
`sulfur poisoning.
`The pore system of suitable size is
`
`interconnected in all three crystallographic dimensions. As
`
`is well known to those skilled in the zeolite art,
`the
`crystalline structure of zeolites exhibits a complex pore
`structure having more or less regularly recurring
`connections,
`intersections and the like, Pores having a
`particular characteristic, such as a given dimension
`' diameter or crossesectional configuration, are said to be
`_one dimensional if those pores do not intersect with other
`like pores.
`If the pores intersect only within a given
`
`-‘
`
`the pores of that
`plane with other like pores,
`characteristics are said to be interconnected in two
`
`If the pores intersect with
`(crystallographic) dimensions.
`other like pores lying both in the same plane and in other
`
`planes, such like pores are said to be interconnected in
`
`three dimensions, i.e., to be “three dimensional”.
`
`It has
`
`been found that zeolites which are highly resistant to
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`sulfate poisoning and provide good activity for both the SCR
`process and the oxidation of ammonia with oxygen, and which
`
`retain good activity even when subject to high temperatures,
`‘hydrothermal conditions and sulfate poisons, are zeolites
`
`which have pores which exhibit a pore diameter of at least
`Vabout 7 Angstroms and are interconnected in three
`dimensions. Without wishing to be bound by any specific
`
`‘theory, it is believed that the interconnection of pores of
`at least 7 Angstroms diameter in three dimensions provides
`for good mobility of sulfate molecules throughout'thetl
`‘zeolite structure;
`thereby permitting the sulfate molecules
`to be released from the catalyst to free a large number of
`the available adsorbent sites for reactant NOX and NH3
`
`molecules and reactant NH3 and O2_molecules._ Any zeolites
`'meeting the foregoing criteria are suitable for use in the
`practice of the present invention; specific zeolites which
`
`.
`
`rmeet these criteria are USY, Beta and ZSM—20. Other.
`zeolites may also satisfy the aforementioned criteria.
`The above—described zeolite catalysts have been very
`effective for the selective catalytic reduction of NOx with.‘
`bammonia.
`In particular, an iron—promoted zeolite beta hast
`
`‘been found most useful in the SCR process for removing Nox
`from gas turbine exhaust streams. Unfortunately, at the
`higher temperatures, e.g. 500°C. or more, provided by recent
`
`gas turbine technology, it has been found that the
`
`hydrothermal stability of such catalyst is reduced as
`manifest by a reduced catalytic activity over time.‘ Thus,
`the present invention is directed to improving the stability
`of catalysts described in U.S. Patent No, 4,961,917 for use
`
`in SCR processing;
`
`Importantly, a further discovery has
`
`been made which is believed to be relevant to all zeolite
`catalysts.
`A novel zeolite structure has been found which
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`is more resistant to dealumination such as under oxidizing p
`or hydrothermal conditions and the like. Thus, while the
`
`treatment of zeolite beta to improve stability is a
`
`- preferred embodiment of the invention inasmuch as such
`
`zeolite catalyst has been proven to be effective in the SCR
`process,
`the present invention is also directed to the
`improvement in stability under oxidizing and/or hydrothermal
`
`The improvement in
`conditions for any zeolite catalyst.
`stability is provided by incorporating non—framework‘
`aluminum oxide units into a xeolite catalyst.
`The non-
`framework aluminum oxide units should be present in amounts
`of at least 10 wt.% relative to total aluminum oxide content
`
`in the zeolite to provide the desired stability.
`
`.Accordingly, examples of zeolite catalysts which can be”
`treated in accordance with this invention include but are
`not so limited to ZSM—5, ZSM-8, ZSM—ll, ZSM—l2, zeolite X,
`‘zeolite Y, beta, mordenite, erionite.
`A
`q
`The stabilized aluminosilicate Zeolites of this
`
`‘invention formed, for example, by the processes as described
`below, are believed-to be characterized as containing non-
`framework aluminum oxide chains which are attached or
`
`otherwise associated with the aluminosilicate framework of.
`the zeolite. Figure 1 schematically illustrates what is
`believed-to be the structure of the stabilized zeolites
`containing the aluminosilicate zeolite framework which has
`
`attached thereto an aluminum oxide chain 10 comprising
`alternating aluminum and oxygen atoms.
`Each end of the
`aluminum oxide chain 10 is shown as linked to the
`
`‘aluminosilicate framework of the zeolite. ‘It is possible
`
`that a portion of the aluminum oxide chains formed may have
`
`only one end linked to the zeolite framework and still
`
`provide improved stability. This structure, which is
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`the invention is‘
`illustrated is only theorized and as such,
`not to be strictly limited to the structure shown in Figure
`
`'
`that at least 10% of they
`It is believed, however,
`l.
`.aluminum oxide present in the zeolite should be present in
`the non—framework aluminum oxide units to provide the
`noticeable improvements in resistance to dealumination
`during catalyst use. As a consequence of the improved
`[resistance to dealumination, metal promoters such as iron
`(Fekl as shown in Figure 1 remain coordinated to the
`aluminosilicate tetrahedra of the zeolite framework even
`
`upon use under harsh hydrothermal conditions.
`Regardless of the exact association of the aluminum
`oxide chain to the zeolite framework,
`the non~framework
`‘aluminum oxide chains have been found to have a
`0
`Characteristic FT—IR adsorption peak at 378l§2 cm‘. This
`
`characteristic peak 12 is shown in Figure 2 for zeolite
`beta, which has either been pre—steamed or which has been
`exchanged with cerium under acidic conditions.
`The FT—IR
`
`absorption band at 3781 cm*,is a characteristic of non¥
`
`framework aluminum in the zeolite beta, but is not present
`in F$7IR of untreated or dealuminized zeolite beta,
`(ZNX)see'
`Figure 2. Moreover, a zeolite beta which has been
`pretreated by exchange with aluminum cations or by the
`vincorporation of aluminum oxide such as by slurry into the
`
`pores of the zeolite also do not show the characteristic
`
`absorption FT—IR band which is believed to represent extra
`
`framework aluminum oxide units linked to or otherwise
`associated with the aluminosilicate framework found with the
`
`to
`Importantly,
`stabilized zeolites of this invention.
`provide the enhanced stability of this invention,
`the FT—IR
`
`peak at 3781 cm“ should have a peak area of at least 0.05
`
`absorbance unit x cm”, preferably at least 0.1 absorbance
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`unit X cmd, and, most preferably, at least 0.2 absorbance
`unit x cm‘.
`The improved stability provided to aluminosilicate
`
`zeolites has so far been achieved by two distinct processes.
`In the first process,
`the zeolite is presteamed under
`..specific conditions prior to the inclusion of the metal
`promoters.
`The zeolite to be presteamed can be in the
`,hydrogen, ammonium, or metal cationic form other than the
`‘sodium form.
`It has been found that the sodium form (Na?
`,of the zeolite will not form the non-framework aluminum
`oxide by either of the treatments of this invention.
`The
`steaming conditions are such as to provide improved
`
`resistance to dealumination during use under high
`temperature, oxidizing conditions, and harsh hydrothermal,
`environments. (It is believed that the steaming conditions
`are such as to provide the non—framework aluminum oxide
`
`chains and are not such as to merely dealuminate the zeolite
`so as to increase the silica to alumina-ratio.
`In accordance with this invention, zeolite beta can be
`.provided with improved stability for catalyzing the
`-
`selective catalytic reduction of NOX with ammonia by pre-
`steaming the catalyst at temperatures of greater than 600°C.
`to 800°C. for a period of time of 0.25 to 8 hours.
`The 0
`A ’preferred steam temperature is 650°C.
`to 750°C.
`The length
`of the pre—steaming treatment is preferably from 0.5 to 4
`‘hours and most preferably from l-to 2 hours.
`The
`temperatures for the steaming treatment of this-invention
`
`are generally lower than those used for removing aluminum
`
`from the framework of zeolites, and the length of treatment
`is generally shorter than that usually provided for
`
`dealumination of the zeolite framework.
`
`Steaming conditions
`
`used to provide stability for other aluminosilicate zeolites
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`other than‘zeolite beta should be similar to the conditions
`
`set forth.
`
`Such conditions can be readily determined by
`
`steaming the zeolite at conditions such as to provide the
`
`peak at 378li2 cm“ and peak area observed by FT—IR as
`discussed above.
`the zeolite can
`Subsequent to the steaming treatment,
`be promoted with various metals.
`For the use of zeolite‘
`beta as in the selective catalytic reduction of NOX with
`ammonia,
`the pre~steamed zeolite beta can be promoted with
`birch and copper as described in U.S; Patent No. 4,961,917,
`the entire contents of which are herein incorporated by
`reference. ‘In general,
`the iron or copper promoter,
`iron
`
`is added in amounts of from about 0.1 to
`being preferred,
`A3O% by wtl calculated as metal based on the total weight of
`the metal and the zeolite. Preferred levels of the iron
`promoter ranges from 0.5 to 2.5 wt.%, and most preferred
`from about 0.7 to 1.5 wt.%.
`4
`
`The second method which has been found to provide
`zeolite beta with hydrothermal stability during the
`. ‘selective catalytic reduction of NOX with ammonia is to pre~
`treat the zeolite beta with a compound of the lanthanide
`~
`.series, such as cerium, prior to exchange with the promoter
`i metal such as iron. Again, it is theorized that the
`'
`
`‘
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`25
`
`lanthanide being slightly acidic results in the scission of’
`the aluminum oxide from the zeolite framework which aluminum
`oxide is then recombined as aluminum oxide chains, which are
`linked to or associated with the zeolite framework.
`The
`
`lanthanides such as cerium are not so acidic as to cause the
`
`complete dealumination and removal of the aluminum oxide
`
`from the zeolite.
`
`In the lanthanide exchange, an aqueous
`
`solution of a lanthanide salt at a pH of 2 to 4 is first
`
`exchanged into a hydrogen or ammonium zeolite beta to
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`provide a level of lanthanide of approximately 0.25 to 1
`wt,% on the zeolite.
`A metal cationic form other than
`
`sodium can also be treated with the lanthanide salt.
`
`Subsequent exchange with the metal promoter such as iron is
`
`_aChieved by conventional methods by use of an aqueous metal
`salt to provide the level of metal promoter as described
`
`above.. Again, although improved stability has been found
`
`with zeolite beta when used to catalyze the selective 4
`. catalytic reduction of NOx
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