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`US0055l6497A
`
`United States Patent
`
`[191
`
`5,516,497
`May 14, 1996
`[45] Date of Patent:
`Speronello et al.
`
`[11]
`
`Patent Number:
`
`[54]
`
`[75]
`
`[73]
`
`[21]
`
`[22]
`
`[62]
`
`[51]
`[52]
`[53]
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`[56]
`
`STAGED 1\/IETAL-PROMOTED ZEOLITE
`CATALYSTS AND METHOD FOR
`CATALYTIC REDUCTION OF NITROGEN
`OXIDES USING THE SAME
`
`Inventors: Barry K. Speronello, Belle Mead;
`John W. Byme, Edison; James M.
`Chen, Edison, all of N.J.
`
`Assignee: Engelhard Corporation, lselin, N.J.
`
`Appl. No.: 678,777
`
`Filed:
`
`Apr. 1, 1991
`
`Related U.S. Application Data
`
`Division of Ser. No. 340,992, Apr. 20, 1989, Pat. No.
`5,024,981.
`
`Int. Cl.5 ............................ B01D 53/56; B0lD 53/58
`U.S. Cl.
`...................... .. 423/235; 423/237; 423/239.2
`Field of Search .............................. 423/213.2, 213.5,
`423/213.7, 235, 237, 239, 246, 239.2; 502/43
`
`References Cited
`
`U.S. PATENT DOCUMENTS 5
`
`3,970,739
`4,297,328
`4,961,917
`5,041,270
`
`7/1976 Shiraishi et al. ................... .. 423/235 S
`10/1981 Ritscher et al.
`. 423/213.2
`10/1990 Byme .............
`423/239
`B/1991 Fujitaniet al.
`...................... 423/213.2
`
`FOREIGN PATENT DOCUMENTS
`
`2133271
`2208190
`51-69476
`62-97629
`2-4453
`
`423/2112
`1/1973 Germany
`8/1973 Germany ............................ 423l213.7
`6/1976
`Japan .
`5/1987
`Iapan ..................................... 423/237
`1/1990
`Japan ..................................... 423/237
`
`Primary Exam:'ner—Ga1y P. Straub
`Assistant Examiner—Tirnothy C. Vanoy
`
`[57]
`
`ABSTRACT
`
`A zeolite catalyst composition is provided in which a first or
`upstream zone of the catalyst has a lower metal (e.g., iron or
`copper) promoter loading than the metal promoter rnoter
`loading of the second or downstream zone of the catalyst.
`The first zone may contain from none up to about 1 percent
`by weight of the promoter and the second zone may contain
`from about 1 to 30 percent by weight promoter. The zeolite
`may be any suitable zeolite, especially one having a silica-
`to-alumina ratio of about 10 or more, and a kinetic pore size
`of about 7 to about 8 Angstrorns with such pores being
`interconnected in all three crystallographic dimensions. The
`method -of the invention provides for passing a gaseous
`stream containing oxygen, nitrogen oxides and ammonia
`sequentially through first and second catalysts as described
`above,
`the first catalyst favoring reduction of nitrogen
`oxides and the second catalyst favoring the oxidation or
`other decomposition of excess ammonia.
`
`12 Claims, 3 Drawing Sheets
`
`so
`
`so
`
`40
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`CONVERSION(°/o)
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`
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`
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`
`D
`
`REACTION/REAGENT
`D SCR/NOx
`+ SCR/NH3
`
`as NH3 Ox./NH3
`
`0.1
`I
`I0
`
`Cu LOADING ON ZEOLITE (wf.°/cl
`
`JM 1008
`
`1
`
`JM 1008
`
`

`
`U.S. Patent
`
`May 14, 1996
`
`Sheet 1 of 3
`
`5,516,497
`
`mxzxxoMIZum
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`

`
`U.S. Patent
`
`May 14, 1996
`
`Sheet 2 of 3
`
`5,516,497
`
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`x 4.4 FE BETA
`
`250
`300
`350
`400
`450
`500
`550
`600
`
`INLET GAS TEMPERATURE-°C
`
`3
`
`Z O 50
`
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`Lu
`
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`
`250
`
`300
`
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`
`400
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`450
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`500
`
`550
`
`lNLET GAS TEMPERATURE'°C
`
`F|G.Z’A
`
`3
`
`

`
`U.S. Patent
`
`May 14, 1996
`
`Sheet 3 of 3
`
`5,516,497
`
`100
`
`90
`
`80
`
`TO
`
`60
`
`50
`
`40
`
`HOURS OF
`
`
`
`300
`
`350
`
`400
`
`450
`
`500
`
`INLET GAS TEM PERATURE‘°C
`
`FIG. 4
`
`°/oNOCONVERSION
`
`
`
`°/.NH3CONVERSION
`
`0 4.4 FE BETA
`
`13 NH4 BETA
`
`300
`550
`600
`250
`350
`400
`450
`500
`
`INLET GAS TEMPERATURE'°C
`
`FIG. 5
`
`4
`
`

`
`5,516,497
`
`1
`STAGED METALPROMOTED ZEOLITE
`CATALYSTS AND METHOD FOR
`CATALYTIC REDUCTION OF NITROGEN
`OXIDES USING THE SAIVIE.
`
`This is a divisional of application Ser. No. 07/340,992
`filed on Apr. 20, 1989, now U.S. Pat. No. 5,024,981.
`
`BACKGROUND OF THE INVENTION
`
`1. The present invention is concerned with mcta.l-pro-
`moted zeolite catalysts and a method for the catalytic
`reduction of nitrogen oxides with ammonia using the cata-
`lysts, including carrying out such catalytic reduction selec-
`tively ‘in the presence of oxygen. 2. The Related Art
`Both synthetic and natural zeolites and their use in
`promoting certain reactions, including the selective reduc-
`tion of nitrogen oxides with ammonia in the presence of
`oxygen, are well known in the art. Zeolites are aluminosili—
`cate crystalline materials having rather uniform pore sizes
`which, depending upon the type of zeolite and the type and
`amount of cations included in the zeolite lattice, range from
`about 3 to 10 Angstroms in diameter.
`Japanese Patent Publication (Kokai) No. 51-69476, pub-
`lished Jun. 16, 1976 on Application No. 49-142463, filed
`Dec. 13, 1974, discloses a method for reducing nitrogen
`oxides in waste gases by reaction with ammonia in the
`presence of a metal-promoted, dealuminizcd synthetic or
`natural mordenite zeolite. The resistance of the catalyst to
`sulfurous poisons, particularly sulfur trioxide and sulfuric
`acid mist, is said to be enhanced by dealuminizing the
`mordenite to increase the silica to alumina ratio to more than
`12, preferably to more than 15. The zeolite is promoted with
`0.5 to 30 weight percent of at least one of a number of metals
`including copper, vanadium, chromium,
`iron, cobalt or
`nickel, and is used at a reaction temperature of 200° to 500°
`C. with from 0.5 to three times the stoiehiometric 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
`of the Publication, it is stated that a slight decrease of the
`activity of a dealuminizcd, i.e., high silica to alumina ratio,
`copper-promoted mordenite catalyst
`is recognized when
`sulfur trioxide is included in the gas stream. However, an
`“extreme improvement” of resistance to sulfur trioxide
`poisoning is noted in comparison with a copper mordenite
`which has not been dealuminizcd to increase its silica to
`alumina ratio.
`
`20
`
`25
`
`30
`
`35
`
`45
`
`UK Patent Application 2,193,655A discloses a catalyst
`containing a low surface area titania and a copper-promoted
`zeolite for use in the reduction of nitrogen oxides with ,
`ammonia. The zeolite has an average pore diameter of 10
`Angstroms or less, preferably 8 Angstrorns 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 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.
`
`50
`
`55
`
`60
`
`U.S. Pat. No. 4,297,328 discloses a “three way conver-
`sion” catalytic process for the simultaneous catalytic oxida-
`tion of carbon monoxide and hydrocarbons and reduction of
`nitrogen oxides for purifying the exhaust gas of automobile
`engines operated within a prescribed range of air to fuel ratio
`(column 4, lines 63-68). The disclosed catalyst is stated to
`be a copper-promoted zeolite having a silica to alumina ratio
`
`2
`greater than 10, preferably greater than 20 (column 6, lines
`2348). Representative high-silica zeolites are described at
`columns 6-8 of the patent and include (column 6, lines
`29-33) silicalite (as described in U.S. Pat. No. 4,061,724),
`ZSM—5, ZSM-8, ZSM-ll, ZSM-12, hyper Y, ultrastabilized
`Y, Beta, mordenite and erionite. Ultrastabilized Y is
`described (column 7, lines 22-25) as “a form of zeolite Y
`which has been treated to give it the organophilic charac-
`teristic of the adsorbents of the present invention.” Example
`6 of the patent is stated to show no measureable loss in
`combustion activity of the copper-promoted zeolite catalyst
`due to sulfer poisoning (exposure of the catalyst to meth-
`ylmereaptan in the gaseous stream). The patent thus dis-
`closes the utility of the copper-promoted specified zeolites
`for three-way conversion in an exhaust gas generated by a
`lean air to fuel ratio combustion mixture.
`
`U.S. Pat. No. 4,302,431 discloses method and apparatus
`for controlling the content of nitrogen oxides in gases,
`including a first stage of high temperature, uncatalyzed
`reduction of nitrogen oxides with ammonia, followed by a
`second, catalyzed stage in which decomposition of residual
`nitrogen oxides and ammonia (column 4, lines 44-49) is
`carried out. Example 1 discloses the use of calcium silicate
`plates impregnated with ferric sulfate as the catalyst, and the
`patentee notes (column 6, lines 36-42) that other catalysts
`having denitrifying capacity, such as chromium and vana-
`dium, may also be utilized.
`U.S. Pat. No. 3,970,739 discloses (colunm 3, lines 32-46)
`mixing gases obtained from an ammonia synthesis plant
`waste water stream with flue gases so as to provide about 0.3
`to 10 moles of ammonia per mole of nitrogen oxides. The
`resultant gaseous mixture is contacted in a first stage with a
`metal catalyst to reduce nitrogen oxides and any unreacted
`ammonia is then decomposed in a second stage in the
`presence of a suitable catalyst; the process is carried out at
`a temperature of from 150° to 700° C. The first stage catalyst
`may be platinum or palladium, or oxides of copper, vana-
`dium, molybdenum or nmgsten, or a metal complex oxide
`such as an iron-chromium complex oxide (column 5, line 53
`et seq). The second stage catalyst may be any suitable
`catalyst (colunm 7,
`lines 1-7) such as iron-chromium,
`chromium-mangnesia, and chromium plus one or more of
`tin, antimony vanadium, cobalt phosphorus zinc, nickel,
`titanium, molydbenum and tungsten (column 6, line 59).
`Separate catalytic reactors or a single reactor containing the
`first and second stage catalysts in sequence, may be used.
`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 ammo-
`nia. The art also shows an awareness of providing a two-
`stage process in which, in a first stage, a thermal or catalyzed
`process may be used for the reduction of nitrogen oxides
`with ammonia and, in a second stage, residual ammonia is
`decomposed to nitrogen.
`
`SUMMARY OF THE INVENTION
`
`Generally, the present invention provides a metal-pro-
`moted zeolite catalyst and a method for using the same in the
`selective catalytic reduction of nitrogen oxides with ammo-
`nia, in which the promoter loading on the catalyst is staged
`so that the promoter loading in a first or upstream zone of the
`catalyst is lower than the promoter loading in a second or
`downstream zone of the catalyst. It has been found that by
`‘thus staging the promoter loading on a suitable zeolite
`
`5
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`5,516,497
`
`3
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`catalyst, the first zone catalyst favors the selective catalytic
`reduction of nitrogen oxides with ammonia whereas the
`second catalyst zone favors the oxidation of (excess or
`residual) ammonia to nitrogen.
`More specifically, in accordance with the present inven-
`tion there is provided a method for reacting nitrogen oxides
`with ammonia in a gaseous stream, the method comprising
`the following steps. A gaseous stream containing nitrogen
`oxides and ammonia is passed through a first catalyst zone
`containing a first zeolite catalyst which is optionally pro-
`moted with not more than about 1% by weight of an iron
`and/or copper promoter, calculated as the metal and based on
`the weight of metal plus the first zeolite. The gaseous stream
`is contacted within the first zone with the first catalyst under
`conditions etfective to reduce nitrogen oxides with ammonia
`and leave residual ammonia in the gaseous stream. The
`nitrogen oxides-depleted gaseous stream containing residual
`ammonia obtained as above is then passed through a second
`catalyst zone containing a second zeolite catalyst which is
`promoted with more than about 1% by weight of an iron
`andlor copper promoter, calculated as the metal and based on
`the weight of metal plus the second zeolite. The nitrogen
`oxides-depleted gaseous stream is contacted within the
`second zone with the second catalyst in the presence of
`oxygen and under conditions effective to oxidize ammonia
`to nitrogen, and the resultant ammonia-depleted gaseous
`stream is then withdrawn.
`
`In accordance with another aspect of the invention, the
`first zeolite catalyst may contain, for example, from about
`0.1 to about 1% by weight of the promoter, e.g., iron; the
`second zeolite catalyst may contain from more than about
`1% to 30% by weight of the promoter, e.g., iron, preferably
`from about 2% to 5% by weight promoter, both calculated
`as the metal and based on the weight of promoter (as metal)
`plus the respective zeolite.
`Other aspects of the invention include introducing ammo-
`nia into the gaseous stream, for example, in an amount to
`provide from about 0.7 to 2 moles of anmionia per mole of
`nitrogen oxides. In another aspect of the invention, the
`reaction is carried out at a temperature of from about 200°
`C. to 600" C., e.g., from about 300° C. to 550° C.
`Still other aspects of the invention include utilizing cata-
`lysts as described below in the methods described above.
`
`Compositional aspects of the present invention provide a
`catalyst composition effective for reducing nitrogen oxides
`with ammonia in a gaseous stream. The composition has a
`first catalyst and a second catalyst, as sensed relative to the
`sequence of flow of the gaseous stream through the com-
`position, and comprises the following components. A first
`catalyst comprises a first zeolite which is optionally pro-
`moted with not more than about 1%»by weight of an iron
`and/or copper promoter dispersed therein, calculated as the
`metal and based on the weight of metal plus the first zeolite.
`A second catalyst comprises a second zeolite promoted with
`more than about 1% by weightof an iron and/or copper
`promoter dispersed therein, calculated as the metal and
`based on the weight of metal plus second zeolite.
`In another aspect of the invention, the first zeolite catalyst
`comprises an iron and/or copper-promoted zeolite catalyst,
`preferably an iron-promoted zeolite catalyst.
`In yet another aspect of the invention, at least one, and
`preferably both, of the first zeolite catalyst and the second
`zeolite catalyst have a silica to alumina ratio of 0 or higher
`and an average pore kinetic diameter of from about 7 to
`about 8 Angstroms. The zeolite pore structure may be
`interconnected in all three crystallographic dimensions by
`
`10
`
`20
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`25
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`30
`
`35
`
`40
`
`45
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`50
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`55
`
`60
`
`65
`
`4
`pores having a kinetic diameter of about 7 to about 8
`Angstroms.
`As used herein and in the claims, reference to a “first”
`catalyst or catalyst zone and a “second” catalyst or catalyst
`zone is made with reference to the sequence of introduction
`therein of a gaseous stream to be treated. Thus, the “first”
`catalyst is the upstream catalyst and the “second” catalyst is
`the downstream catalyst, with “upstream" and “down-
`stream” being as sensed in the direction of flow of the treated
`gaseous steam therethrough.
`References herein and in the claims to a zeolite catalyst
`containing a percent “by weight” promoter means a percent-
`age calculated as the weight of promoter, as the metal,
`divided by the combined weights of promoter (as the metal)
`plus the zeolite.
`Reference herein and in the claims to “metal", “iron” and
`“copper” with respect to the promoters should not be taken
`to imply that the promoter is necessarily in the elemental or
`zero.valence state; the terms enclosed in quotes should be
`understood to include the presence of promoters as they
`exist in the catalyst compositions, e.g., as exchanged ions
`andlor impregnated ionic or other species.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 of the drawings is a plot showing the percent
`conversion of nitrogen oxides and ammonia in a treated
`gaseous stream, versus the copper loading of a zeolite
`catalyst;
`FIG. 2 is a plot showing the percent conversion of NO
`versus inlet temperature for various metal-promoted zeolite
`catalysts;
`FIG. 3 is a plot showing the percent conversion of NO
`versus the inlet temperature of a gaseous stream to be treated
`for aged and fresh copper-promoted zeolite catalysts;
`FIG. 4 is a plot showing the percent conversion of N0
`versus inlet temperature of a gas stream passed through aged
`and fresh iron promoted beta zeolite catalysts; and
`FIG. 5 is a plot showing the percent conversion of
`anunonia versus inlet temperature of a gas stream fed to
`different beta zeolite catalysts.
`
`DETAILED DESCRIPTION OF THE
`INVENTION AND PREFERRED
`EMBODIMENTS ‘THEREOF
`
`In order to reduce the emissions of nitrogen oxides from
`flue and exhaust gases, such as the exhaust generated by gas
`turbine engines, anunonia is added to the gaseous stream
`containing the nitrogen oxides and the gaseous stream is
`then contacted with a suitable catalyst at elevated tempera-
`tures in order to catalyze the reduction of nitrogen oxdes
`with ammonia. Such gaseous streams, for example,
`the
`products of combustion of an internal combustion engine or
`of a gas-fueled or oil-fueled turbine engine, often inherently
`also contain substantial amounts of oxygen. A typical
`exhaust gas of a turbine engine contains from about 2 to 15
`volume percent oxygen and from about 20 to 500 volume
`parts per million nitrogen oxides, the latter normally com-
`prising a mixture of NO and N02. Usually, there is suflicient
`oxygen present in the gaseous stream to oxidize residual
`ammonia, even when an excess over the stoichiometric
`amount of anunonia required to reduce all
`the nitrogen
`oxides present is employed. However, in cases where a very
`large excess over the stoichiometric amount of ammonia is
`utilized, or wherein the gaseous stream to be treated is
`
`6
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`5,516,497
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`5
`
`lacking or low in oxygen content, an oxygen-containing gas,
`usually air, may be introduced between the first catalyst zone
`and the second catalyst zone, in order to insure that adequate
`oxygen is present in the second catalyst zone for the oxi-
`dation of residual or excess ammonia. Metal-promoted zeo-
`lites can be used to promote the reaction of ammonia with
`nitrogen oxides (“NO,,”) to form nitrogen and H20 selec-
`tively over the‘ competing reaction of oxygen and ammonia.
`The catalyzed reaction of ammonia and nitrogen oxides is
`therefore sometimes referred to as the selective catalytic
`reduction (“SCR”) of nitrogen oxides or, as sometimes
`herein, simply as the “SCR process”.
`Theoretically, it would be desirable in the SCR process to
`provide ammonia in excess of the stoiehiometric amount
`required to react completely with the nitrogen oxides
`present, both to favor driving the reaction to completion and
`to help overcome inadequate mixing of the ammonia in the
`gaseous stream. However, in practice, significant excess
`ammonia over such stoichiometric amount is normally not
`provided because the discharge of unreacted ammonia from
`the catalyst to the atmosphere would itself engender an air
`pollution problem. Such discharge of unreacted ammonia
`can occur even in cases where ammonia is present only in a
`stoichiometric or sub-stoichiomettic amount, as a result of
`incomplete reaction and/or poor mixing of the ammonia in
`the gaseous stream, resulting in the formation therein of
`channels of high ammonia concentration. Such channeling is
`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 of beds of
`particulate catalyst, there is no opportunity for gas mixing
`between channels.
`
`The present invention is predicated on the surprising
`discovery that the selectivity of a zeolite catalyst can be
`tailored to favor either (1) the SCR process, i.e., the reduc-
`tion of nitrogen oxides with ammonia to form nitrogen and
`H20, or (2) the oxidation of ammonia with oxygen to form
`nitrogen and H20, the selectivity being tailored by control-
`ling the promoting metal content of the zeolite. More
`specifically, it has been found that at metal loading levels,
`e.g., iron or copper, on zeolites which do not exceed about
`1 percent by weight metal, calculated as the metal and based
`on the combined weight of metal plus the zeolite, selectivity
`for the SCR reaction, even in the presence of oxygen, is
`significantly favored over the oxidation of ammonia by
`oxygen. It has also been discovered that at metal loadings on
`the zeolite above about 1 percent by weight metal, same
`basis as above, selectivity of the catalyst is shifted towards
`the oxidation of ammonia by oxygen at the expense of the
`SCR process, thereby improving ammonia removal.
`The above principles are utilized by providing a staged or
`two-zone catalyst in which a first catalyst zone with no or a
`low metal promoter (iron or copper) loading on a zeolite,
`that is, fiom 0 to not more than about 1 percent by weight
`metal, is followed by a second catalyst zone comprising a
`zeolite having thereon a metal loading in excess of about 1
`percent by weight. The resultant catalyst composition thus
`has a first (upstream) zone which favors the reduction of
`nitrogen oxides with ammonia, and a second (downstream)
`zone which favors the oxidation of ammonia. In this way,
`when ammonia is present in excess of the stoichiometric
`amount, whether throughout the fiow cross section of the
`gaseous stream being treated or in localized channels of high
`ammonia concentration, the oxidation of residual ammonia
`by oxygen is favored by the downstream or second catalyst
`zone. The quantity of ammonia in the gaseous stream
`
`10
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`20
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`25
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`30
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`35
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`40
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`45
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`50
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`60
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`65
`
`6
`discharged from the catalyst is thereby reduced or elimi-
`nated.
`'
`
`Generally, any suitable zeolite material may be utilized in
`the catalyst compositions of the invention. Many zeolites
`demonstrate good thermal stability and so stand up well to
`high temperatures, thereby enabling the treatment of rela-
`tively high temperature gaseous streams without precooling.
`For example, turbine engine exhausts typically have a tem-
`perature in the range of about 450° to 600° C. This tem-
`perature range is too high for selective catalytic reduction
`catalysts such as those based on the anatase form of titania
`because, at such elevated temperatures, the -anatase converts
`to a less catalytically active rutile form. It thus becomes
`necessary, when using such temperature-sensitive catalysts,
`to invest in heat exchange equipment to cool the gaseous
`stream before treating it catalytically to reduce the nitrogen
`oxides. However, by utilizing suitable zeolite catalysts in
`accordance with the teachings of the present invention, high
`temperature gaseous streams, for example, gaseous streams
`at temperatures up to about 600° C., may be treated without
`seriously adversely affecting the life or efiiciency of the
`catalyst. Suitable promoted zeolite materials demonstrate
`sufiicient
`thermal and hydrothermal stability to survive
`turbine exhaust conditions and provide an acceptably long
`life and efliciency of the catalysts. The zeolite materials
`employed should have a silica to alumina molecular ratio of
`greater than loin order to enhance their resistance to acidic
`conditions, as discussed in more detail below. Preferably, the
`zeolite materials are medium to large pore zeolites having
`pore openings of at least about 4 Angstroms in diameter.
`Such openings are large enough to admit the reactants and
`products and to catalyze or otherwise facilitate the desired
`reactions.
`
`The gaseous streams containing nitrogen oxides may also
`contain sulfur oxides, especially sulfur dioxide. For
`example, the exhaust of a turbine engine which is operating
`on a liquid fuel, such as a number 2 fuel oil, may contain
`from about 10 to 150 parts per million of sulfur dioxide. The
`tolerance of the catalyst material for such sulfurous con-
`taminants is increased, i.e., the catalyst is rendered more
`resistant to sulfur poisoning, by selecting a zeolite of a larger
`average pore size than that which is necessary to admit the
`reactants and products. Specifically, an average pore size of
`about 7 Angstroms or more, e.g., about 7 to about 8
`Angstroms is preferred for enhanced resistance to sulfur
`poisoning. The most preferred types of zeolite for resistance
`to sulfur poisoning are those which have a pore system in
`which the 7 to 8 Angstrom diameter pores are interconnected
`in all three crystallographic dimensions. Such zeolite mate-
`rials are described in detail in co-pending and commonly
`owned patent application Ser. No. 341,405, filed Apr. 20,
`1989, now U.S. Pat. No. 4,961,917, of John W. Byme,
`entitled “Zeolite Catalysts and Method For Reduction of
`Nitrogen Oxides VV1th Ammonia Using the Same”,
`the
`disclosure of which is hereby incorporated herein and made
`part hereof. As disclosed in the aforesaid co-pending patent
`application, a particularly suitable class of such sulfur-
`resistant zeolite materials is comprised of Beta zeolites,
`ultrastable Y (“USY”) zeolites and ZSM-20 zeolites. Gen-
`erally, silica to alumina ratios well in excess of the minimum
`of 10 may be employed. Conversion eflieiencies of 90 to
`93% for NO, reduction with ammonia have been attained
`with fresh copper promoted zeolite having silica to alumina
`ratios of 20, 26, 28, 37 and 62. A conversion efficiency of
`77% was attained by a fresh copper promoted ZSM-5 zeolite
`having a silica to alumina ratio of 50. However, fresh copper
`promoted USY zeolites with silica to alumina ratios of,
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`7
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`5,516,497 .
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`7
`respectively, 8 and 30 provided 85% and 39% conversions
`of NO,,, suggesting that at least for USY, silica to alumina
`ratios should be significantly less than 30.
`The catalysts of the present invention comprise one or
`more suitable zeolite materials arranged in at least a first and
`second zone in which the first zone is either not promoted
`with a metal or has a promoter loading not in excess of 1
`percent by weight metal, and the second zone has a greater
`promoter loading than the first zone, the second zone con-
`taining more than about 1 percent by weight metal. Any
`suitable physical form of the catalyst may be utilized, such
`as a monolithic honeycomb-type body containing a plurality
`of fine, parallel gas flow passages extending therethrough,
`the walls of which are coated with the zeolite catalytic
`material. Typically, such monolithic bodies are made of a
`refractory ceramic material such as cordierite, mullite or
`alumina, and the catalytic material coating the fine gas flow
`passages is contacted by the gaseous stream as it flows
`through the gas flow passages. The first or inlet section or
`zone of such a monolith body will be prepared with a lower
`copper loading than the second or downstream zone of the
`same monolith body. This can be readily accomplished by
`dipping one end if the monolith into a slurry of a low- or
`no-copper containing zeolite, and the other end of the
`monolith into a slurry of a more heavily copper-loaded
`zeolite. Alternatively, separate monolith bodies may be used
`for the first and second zones.
`'
`
`The catalyst may also take the form of a packed bed of
`pellets,
`tablets, extrudates or other particles or shaped
`pieces, such as plates, saddles, tubes or the like. The physical
`configuration of the catalyst used in a given case will depend
`on a number of factors such as the space available for the
`catalytic reactor, the activity of the catalyst material utilized,
`and the permitted or desired amount of pressure drop across
`the catalyst bed. When catalysts are used to treat engine
`exhausts, such as the exhaust gas of a turbine engine, it is
`usually desired to minimize pressure drop in order to
`enhance the efliciency of the engine. In such cases, the
`preferred physical configuration of the catalyst is one which
`provides parallel flow passageways for the gas, such as those
`found in the above-described honey—comb-type catalysts.
`Other arrangements providing such parallel flow passage-
`ways include the use of parallel plates or stacked tubes.
`Because of its ease of handling and installation as well as
`good mass transfer characteristics relative to other parallel
`passage configurations, a highly preferred physical configu-
`ration of the catalysts of the invention is a monolithic
`honeycomb member having a relatively high cell (flow
`passageway) density of approximately 60 cells or more per
`square inch of end face of the honeycomb member. The
`walls defining the gas flow passages (or cells) are desirably
`as thin as possible consistent with the requisite mechanical
`strength of the honeycomb. Thus, the catalysts of the inven-
`tion may take the form of a monolithic honeycomb canier,
`the gas flow passages of which comprise or are coated with
`a zeolitic catalyst material with staged copper loadings as
`described above. For example, a catalytically inert honey-
`comb carrier member, such as a cordierite carrier, may be
`coated with a washcoat of fine particles of copper-promoted
`zeolite. Alternatively, a powder of copper-promoted zeolite
`may be mixed with a binder and extruded into the honey-
`comb configuration. In another approach, the catalytic mate-
`rial may be formed in situ by preparing the honeycomb
`structure from a zeolitic precursor raw material which is
`then treated to form the zeolitic material as part of the
`honeycomb structure, followed by copper impregnation. In
`this regard, see U.S. Pat. No. 4,157,375, assigned to the
`
`10
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`20
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`25
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`30
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`35
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`40
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`45
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`50
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`55
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`60
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`65
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`the disclosure of which is
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`8
`assignee of this application,
`hereby incorporated herein.
`Reference is made below to the weights of solids being on
`a “vf basis”. The term in quotes means a volatiles-free basis,
`and is used to indicate the weight that the solid in question
`would have if it were calcined at 1000° C. to drive off
`volatiles. Thus, if 10.1 grams of a substance contains 0.1
`gram of such volatiles, the 10.1 grams is reported as “l0
`grams (vf basis)”. Unless specifically otherwise stated, all
`weight percents herein and in the claims are stated on a vf
`basis.
`
`The following examples demonstrate the efficacy of cer-
`tain embodiments of the present invention.
`
`Example 1
`
`In order to demonstrate the effect of the promoter loading
`on zeolite catalyst selectivity as between (1) selective cata-
`lytic reduction of nitrogen oxides with ammonia and (ii)
`ammonia oxidation with oxygen, a series of catalysts were
`prepared as follows.
`I. A catalyst consisting of a honeycomb ceramic support
`coated with a washcoat of synthetic mordenite zeolite con-
`taining 0.12% by weight Cu (expressed as the metal), was
`prepared as follows:
`1. 100 g (vf basis) of Linde LZM-8 mordenite zeolite
`powder was added to 200 g of an aqueous Cu(S04)
`solution containing 0.6 g of Cu.
`2. This slurry was heated to 82° C. with stirring to suspend
`the solids, and maintained at 82° C. for about 30
`minutes.
`
`3. It was then vacuum (filtered to separate the solids from
`the liquid.
`4. The solid was washed with an equal volume of water,
`and then dried at 100° C.
`
`5. Chemical analysis showed the solid to contain 0.12%
`by weight Cu.
`6. 48.5 g (vi) of the dried powder was added to 81 g of
`deionized water and milled for 1 hour at about 50 rpm
`in a 500 ml polyethylene jar filled 1/2 full of alumina
`cylinders (ca. 0.5 in. diax0.5 in. long).
`7. 3.5 ml of glacial acetic acid was then added to the mill,
`and milling was continued for another 15 hours.
`8. The milled slurry was coated on to a cordierite mono-
`lithic support comprising a one inch (2.5 cm) diameter
`by three inches (7.6 cm) long cylindrical core drilled
`from a cell/in: cordierite support purchased from
`Applied Ceramics Co. The coating was carried out by
`immersing the weighed support into the slurry, gently
`agitating the support to remove entrapped air from the
`channels, removing the saturated support from the
`slurry, and removing excess slurry from the channels by
`blowing with compressed air.
`9. After drying at 110° C. and calcining for 1 hour at 450°
`C., the coated support was weighed, and the washcoat
`loading was calculated to be about 1.6 g/ina. The
`resultant catalyst is designated Catalyst I.
`II. A catalyst consisting of a honeycomb ceramic support
`coated with a washcoat of pentasil zeolite having the ZSM—5
`crystal structure, a silica to alumina molar ratio of 46/1 (by
`chemical analysis), and containing 0.27% by weight Cu
`(expressed as the metal) was prepared as follows:
`1. 168 g (vf basis) of the pentasil zeolite powder was first
`calcined for 2 hours at 316° C. followed by 2 hours at
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`5,516,497
`
`9
`
`593° C. then added to 504 g of an aqueous Cu(SO4)
`solution containing 15.1 g of Cu.
`2. This slurry was heated to 82° C. with stirring to suspend
`the solids then maintained at 82° C. for 1 hour.
`
`3