Metal exchanged ferrierites as catalysts for the selective reduction of NOx with methane

Ll

Appked Catalysur B Enuuvnmental, 3 (1993) Ll-Lll Elsevler Science Publishers B V , Amsterdam APCAT B67

Metal exchanged ferrierites as catalysts for the selective reduction of NO, with methane

I

YueJm Li and John N. Armor Azr Products and Chemrcals, Inc , Allentown, PA 18195 (USA) (Recewed 5 August 1993, rewed manuscnpt recewed 15 September 1993)

Abstract Metal ion (Co’+, Mn2+ or N?+ ) exchanged fementes are very actwe catalystsfor the reduction of NO. Hnthmethane m the presence of excess oxygen Co-femente ylelda about tmce the mtnc oxide convemon compared to Co-ZSM-5 at T> 5OO”C,previously the most active cat&& reportedfor this reaction, and Co-femente 1smore aelectlvem the use of methane The temperaturefor maxunummtnc oxide convemon 18a function of xeohte type and the cation exchanged,and thle temperaturemcreases m the followmg order Co-ZSM-5 < Co-femente < Mn-femente < Nl-femente The turnover frequency (TOF) normahxedby Co*+ IEa function of Co’+ exchange level and increasesmth the Co’+ loadmg until it reachesan optunumvalue,whereasthe aekctmty for methanedecreasesv&h mcreasmg Cop+ loadmg The presenceof water vapor suppressesthe actmty of Co-femente, however,under wet condltlons, 1t.aactmty 1sstill much higher than Co-ZSM-5 at T> 500°C Key words fementes, methane, nltnc oxide reduction

INTRODUCTION

Recently, there has been a great deal of interest in selective reduction of NO, with hydrocarbons driven by the need to develop an effective lean burn catalyst for mobile emission sources and the desire for a practical alternative to the ammonia selective catalytic reduction (SCR) process for stationary emission controls Much of the research has focused on NO, reduction with non-methane hydrocarbons, e.g , propane and propene, while the most frequently studied catalyst is Cu-ZSM-5 This topic was recently reviewed by Truex et al. [ 1] and by Iwamoto and Mlzuno [ 21 (see references therein) More recently, methane was demonstrated to be an effective reducmg agent for NO, over metal exchanged zeohtes [ 3,4] and H-zeohtes [ 4,5]. The advantages of using methane as a selective NO, reducing agent are obvious for natural gas fueled utilities and engines, where the process integration is more convement compared to Correspondence to Dr J N Armor, Air Producta and Chemicals, Inc ,720l Ham&on Boulevard, Allentown, PA 18195, USA Tel ( + 1-215) 4815792, fax (+ 1-215) 4812989

0926-3373/93/$06 00 0 1993 Elaevler Science Publishers B V All nghte reserved

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using ammoma In addition, the use of methane avoids the hmltatlons associated with the ammonia SCR process, e.g , ammoma shp and equipment corrosion Previously, we reported that certain metal ion (Co*+, Mn*+ or Ni*+ ) exchanged ZSM-6 and mordenite are active for the selective reduction of NO, with methane in the presence of excess oxygen [ 3,4]. (Cu-ZSM-5 is not an effective catalyst for this reaction ) On metal-zeohte catalysts, e g Co-ZSM5, the presence of oxygen greatly enhances the nitric oxide conversion, and a large excess of oxygen does not cause excessive combustion of methane The NO, conversion is proportional to the level of methane added m the feed. We were able to obtain a 100% NO conversion to nitrogen at 400°C on a Co-ZSM5 catalyst The reactions mvolved are depicted by eqns. (1) and (2) Besides the mtrlc oxide reduction reaction (Reaction 1), methane also reacts with oxygen (Reaction 2) The novelty of our catalysts is their ability to catalyze the nitric oxide reduction while mimmlzmg Reaction 2 2NO+CHI+02-+Nz+C02+2Hz0

(1)

CH, +202-+C02 +2H20

(2)

To remove NO, from flue gas, catalysts need to be operated in a harsh environment We found that water vapor reversibly inhibited the nitric oxide conversion over Co-ZSM-5, and this effect was a function of temperature [6] By operating at a higher temperature we can mmimize the effect of water, however, the bendmg over of nitric oxide conversion with increasing temperature becomes a more serious problem. Ga-H-ZSM-5, on the other hand, suffers a much greater loss in nitric oxide conversion due to water addition, although it has a comparable activity and even higher selectlvlty under dry conditions compared to Co-ZSM-5 [ 71 H-zeohtes are much less active than certain metal exchanged zeohtes, e g , Co-zeohtes and they are extremely sensitive to water vapor [ 61. Among the catalysts reported, Co-ZSM-5 representedone active catalyst for this reaction Beyond the catalyst performance and compositions, the structural requirement for an effective catalyst and the detailed reaction mechanism are incompletely understood. On the other hand, the prior reported catalysts were still not active enough for practical applications especially in the presence of water vapor This technology requires improved catalysts. In this communication, we wish to report metal exchanged fenrerkes as catalysts for the selectivereduction of NO, with methane. Compared to Co-ZSM-5, this new catalyst has a significantly higher activity m both dry and wet feeds and alters the conversion-temperature profiles [ 81 EXPERIMENTAL

A mixed cation (K+, Na+ ) ferrlerlte (obtamed from Tosoh Corporation,

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Japan) was first converted to the NH4+ form by exchanging with NH,NO,; the NH,+ form ferrierite was then exchanged with Co2+ to obtam Co-ferrierite. For example, 16 g of the ferrierite #(inK+, Na+ form, Si/Alr8) were suspended m 180ml of a NHINOB solution ( [NH,+ ] = 1 M) w&h constant, vlgorous stirring. The exchange was carried out at room temperature overnight, and three exchanges were performed. After the final exchange, the preparation was filtered and washed with 11 of water. It was then filtered again and dried at 110°C overnight The elemental analyses of the sample showed that Na+ and K+ catrons were completely exchanged out by NH,+. Typically, 10 g of the NH,+-ferrrerrte was used for Co2+ exchange. The N&-ferrierrte was suspended m 600 ml H20, and 4.0 g CO(CH&O~)~ 4H20 (0.016 mol) was dissolved m another 600 ml H20. The Co2+ solution was slowly added mto the zeollte slurry with vigorous nnxmg Exchanges were carrmd out twice at 80 ’ C, each lasting for 24 h The product was washed with 11 of water after the exchange, and dried at 110 ’ C overnight. Elemental analyses showed a Si/Al rat10 of 8 35 and a Co/Al ratio of 0.37. Co-ferrierites with vlvlous exchange levels, Mn-ferrierites and a Ni-femerite were prepared with similar procedures. Two composltlons of the starting ferriente, %/Al = 8 (designated as FEX (8) ) and &/Al z 6 (designated as FRR (6) ) , were used for the ion exchange. All samples were analyzed by mductively coupled plasma-atomic spectroscopy (for Co, Mn, Nl, Si and Al) and flame-atomic adsorption spectroscopy (for K), and their elemental compositions are hsted m Table 1. ( Co-FER (8) -74 means S1/Al= 8 with Co2+ exchange level = 74% of the zeolite exchange capacity.) The experunental detarls for activity measurements were described elsewhere [ 3,4]. Briefly, the activities were measured using a nucro-catalytic reactor operating in a steady-state plug flow mode. The reactor was a U-shaped quartz tube with l/4 in. O.D. at the inlet and 3/8 m. O.D. at the outlet. The TABLE 1 Elemental compohon

of femerhe and ZSM-5 catalysta

All metal-zeolltee were preparedby a two-step process as descrdxtd m Expenmental

Catalyst

No of exchangea

!%/A1

Metal/Al

Metal loadmg (wt -%I)

Co-ZSM-5 Co-FER(8)-40 Co-FER(8)-55” Co-FER(8)-74 Co-FER(8)-100 Co-FER(6)-76 Mn-FER(8)-54 NGER(8)-74

2 1 2 2 3 2 3 2

10 9 85 85 83 s”;

053 020 027 037 050 038 027 037

41 18 27 32 43 46 23 32

84 82

’ Obtamed from a larger scale preparation but Wh procedures sun&r to Co-FIER(8) -74

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catalyst was pretreated m situ m flowmg helium at 506°C for 1 h at a rate of 5’ C/mm The catalyst was pelletized, crushed and then sieved to 60-80 mesh before use. A 0 10 g sample was used for activity measurement. The reaction mixture was obtained by blendmg four channels of flow, 1e NO/He, CH,/He, O.JHe and He, and each was controlled by an independent mass flow controller The reaction mixture typically conslated of 1610 ppm NO, 1006 ppm CH4 and 2 5% O2 wrth helium as the balancmg gas, at a total flow-rate of 100 cm”/ mm (ambient) [The typical gas hourly space velocity (GHSV) was 30600 based on the apparent bulk density of the seolite catalyst, ca. 0.5 g/cm3 ] An on-line gas chromatograph with a thermal conductlvlty detector (TCD) detector was used for the product analysis, and a molecular sieve 5A column was used to separate Nz, O2 and CH,. Formation of nitrogen was used to calculate the mtnc oxide conversion, and the methane conversion was obtained by followmg the change in the methane peak area The selectivity index (CX)is defined as the fraction of the reductant ( CH4) that 1sused to reduce nitric oxide to nitrogen, 1e., the ratio of the consumption rate of methane for the mtnc oxide reduction (reaction 1) to the total consumption rate of methane (reaction 1+ reaction 2); a! can be calculated based upon eqn. (3)) --=rl Cyrt

~~X[NO~OXC,,~~~~

[C&lo x GH,

(3)

where, r,=OJxF,X [NO],xCNo, Ft 1s the total flow-rate, [NO],, IS the inlet concentration of nrtnc oxide, and CNo 1s the conversron of NO, r,= FtX [CHAOXCCH,, [CH410ISthe inlet concentration of methane, and CCn, 1s methane conversion The value, 0.5, is the stolchlometrrc ratio of nltnc oxide to methane based on eqn. ( 1). [ 1 molecule of nitric oxide reacts with 0.5 molecule of methane (eqn 1) ] Consequently, (1- a) ISthe percentage of methane combusted RESULTS AND DISCUSSION

Fig. 1 compares the mtnc oxide conversions over two Co-fernentes, CoZSM-5 , Mn-FER(8) and NI-FER(8) as a function of reactlon temperature. Under the reaction condrtlons (GHSV= 30600, [NO] = 1610 ppm, [ CH,] = 1015ppm, and [O,] = 2 5% at the inlet), Co-ZSM-5 has a maximum nltrlc oxide conversion of 33% at 45O”C, and the conversron curve bends over with mcreasmg temperature beyond 450 oC This temperature effect 1s reversible upon decreasing temperature. The maximum mtnc oxide conversion on Co-FER(8)-74 1s 47% at 5OO”C,a 50°C shift upward Clearly, Co-FER(8)-74 1s much more active than Co-ZSM-5 for mtric oxide reduction at T> 450’ C This activity enhancement is even more pronouncedwith Co-FER (6) -76 (Co/ A1=0.38), which has a higher aluminum content and, therefore, more exchangeable cation &es. At 5OO”C,Co-FER(6) has a mtrlc oxide conversion

Y k and J N Armor /Appk Catal B 3 (1993) Ll -Lll

L5

60 _'~~'1~'~~1~~~~1"~~'~~"""'_ -+-Co-ZSM-S(ll)-106 -

_ - + 50 40

10

4-y /

-

\

Co FER(6)-76

: --• - Co-FFiR(8)-74 Mn-FER(8)-54 y q NI-FER(8)-76

//p-

+---+:

,'/

/'o , I

1

0

rl,.‘..,.l,...‘.lrl”‘.“.“’ 300

350

400

450

Temperature

500

550

600

(“C)

Fig 1 Companaon of nltnc oxide reduction actwbe of Co-ZSM-5 and femente catalysts as a function of temperature The reactions were run at GHSV=30 000 h-l, 1610 ppm NO, 1015 ppm CH, and 2 5% 0,

of 56%, a factor of two higher than Co-ZSM-5 at the same temperature. The difference m nitric oxide converslon between Co-FER (8) -74 and Co-FER (6) 76 seems to be related directly to the number of Co2+ m the zeolite (5.4 lop4 mol Co2+/g in Co-FRR(8) -74 and 7.8 10 -’ mol./g in Co-FER(G)-76). The shapes of their nitrrc oxide conversron curves are identical Interestingly, for Mn-FER (8) -54, the optimum temperature 1sshifted to ca 600°C with a maximum nitric oxide conversion of 47%, a conversion similar to Co-FER(8) -74 at 500°C This resembles the difference between Co-ZSM5 and Mn-ZSM-5, where the optimum temperature of Mn-ZSM-5 is shifted 50°C higher compared to Co-ZSM-5, while then maximum mtric oxide conversions are the same [ 41 Like Co-ferriente, Mn-FER (8) -54 offers a sqnificant activity increase compared to Mn-ZSM-5. Ni-FER(8)-74, on the other hand, has a much lower activity compared to Co-FER (8) -74 and Mn-FER (8) 54 (below 550°C). However, its actlvlty increases mth temperature rather steeply, and no bending over of the nitric oxide conversion occurs up to 600 ’ C Note, Nl-FER(8)-74 and Co-FER(8)-74 have the same metal loading The drastic difference m their catalytic activities suggests that both the type of zeohte and the kmd of catron are very important in deternumng the nitric oxide reduction a&v&y The higher mtrrc oxide conversions obtained on femerrte vs. ZSM-5 catalysts are closely related to their low actlvlties for methane combustion (Re-

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actlon 2) The methane conversions and nitric oxide conversions are compared m Table 2 as a function of temperature Co-ZSM-5 1smore active for methane combustion with 100% CH( conversion at 500’ C On Co-FER( 8) -74, methane was completely consumed at 550’ C, while Mn-FER (8) -74 and Nl-FER (8) -74 have much lower methane conversions, 70 and 58% at 550” C, respectively The higher mtrlc oxide conversion and lower methane conversion on Co-ferrrerrte catalyst compared to Co-ZSM-5 lead to a much mgher methane selectlvlty (a). The selectivity indices of methane on Co-ZSM-5 and Co-FER (8) -74 are compared m Fig 2. Carbon &oxide and water were the only products detected from methane oxldatlon At any temperature, Co-FER (8) -74 IS much more selective for nitric oxide reduction than Co-ZSM-5. The catalytic activity of Co-fernerrte catalysts was studied as a function of the level of Co2+ exchanged. Fig. 3 illustrates the effect of cobalt loading on the mtnc oxide conversion. Nltmcoxide conversion mcreasesdramaticallywkh the level of Co2+ at lower cobalt loadings, e.g., Co/Al=O.2-0.37, but remains essentmllyconstant at higher loadings, Co/Al = 0.37 vs. 0 50 Thussuggestthat not all Co2+ cations m ferrierrtehave an identical mtrlc oxide reductronactivity or selectlvlty Note, the overall activity (conversion) depends not only on the intrinsic activity of the catalyst but also on the avarlabllityof methane m the reactor (methane selectivity); the nitnc oxide conversion drops when methane ISdepleted. Balancing these two factors, the optunum Co2+ exchange level 1sbetween 70 and 90% Table 3 further compares the reaction rates, turnover frequencies (TOF) and selectivity as a function of cobalt level m xeohte (TOF ISdefined as the number of mtric oxide molecules converted to nitrogen per Co2+ per second.) For Co-FER(8), TOF increases with cobalt loading at low levels of Co2+ but TABLE 2 Catalytx actmtie8 of femente catalysts The reactionswerenm with a feed conelstmgof 1610 ppm NO, 1015 ppm CH, and 2 5% 0, at a GHSV of 30 000 h-’ CH, convemon (% )

NO convmlon

( %)

T (“C)

400

460

500

550

580

400

450

500

550

580

Co-ZSM-5 Co-FER@)-74 Co-FER(6)-76 Mn-FER(8)-54 NI-FER(8)-74

20 13 14 3 1

33 33 42 14 6

28 47 56 31 13

39 44 45 21

47 -

30 11 16 1 2

68 38 46 12 8

100 79 88 38 21

100 100 70 58

84 -

- Not tested

Y Lt and J N Armor / Appl Catal B 3 (1993) Ll -Lll 100

L7

““11’111”‘~1”1’1”1’ \

80

-

\

-

-

Co-ZSM-5(11)-106

9

Co-FER(8)-74

\ 60 -

\ \ \ \

40

'0 i

20

0 350

\

'1'1""""11"111'1111 400 450

500

Temperature

600

550

(“C)

Fig 2 Selectlvhes of methane over Co-ZSM-5 ( 11) -106 and Co-F’EX (8) -74 aa a function of temperature The reaction con&tlons are the eame ae m hg 1

50

““1”“1~‘~~I~‘~‘I~~”

- -Co/AI=020 d 40

-

Co/AI=018

- A- c0/Ako31

- --8- co/Ako5o 30

-

20

-

,' / ,' /

/

,

350

400

450 Temperature

500

550

600

(“C)

Fig 3 N&w oxide conversion ae a function of reaction temperature over Co-femente. catalyeta mth var~oue Coa+ exchange levels Reaction condltlone ere came ae m Rg 1

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TABLE 3 Nltnc ox&

reduction rates and TOF over ferremte catalysts

The reactions were run with a feed conslstmg of 1610 ppm NO, 1015 ppm CH, and 2 5% 0, at a GHSVof30OOOh-’ Catalyst

Co-ZSM-5(11)-106 Co-FER(8)-40 Co-FER(8)-55 Co-FER(8)-74 Co-FER@)-100

[Co2+]

0 70 031 0 46 0 54 0 73

NO conversion (%)

Rate (mmol/h g)

TOF x lo4 (l/s)

Selectivity mdex ((u) (%)

450°C

500°C

450°C

500°C

450°C

500°C

450°C

500°C

33 12 20 33 37

28 17 36 47 45

130 0 67 141 185 177

110 067 141 185 177

52 42 48 67 55

44 60 85 95 67

38 86 72 71 67

22 79 52 47 41

decreasesat high levels, whereasthe selectivity slightly decreaseswith mcreasmg Co2+ level. This suggests that the selectivity and activity of each Co2+ is dependent on its position within the zeolite It is conceivable that the first exchanged Co2+ is located at the site that gives low activities for nitric oxide reduction and methane combustion, and this site is thermodynamically most stable The last Co2+ exchanged appear to be more active for methane combustion. As reported earlier [ 41, on Co-ZSM-5 the TOF’s are constant (ca 10 10m3and 11 10s3 a- ’ at 450 and 500’ C, respectively) at Co/Al < 0.33 but are much lower on over-exchanged Co-ZSM-5, ca. 5 10B4a-’ at both 450 and 500°C TheTOF’sofCo-ZSM-5(U)-106 (Co/Al=0 53) andCo-FER(8)-100 (Co/Al=O.50) are comparable at 450°C At 5OO”C, Co-FRR(8)-100 has a higherTOF value than Co-ZSM-5,6 7 low4 vs 4 4 10m4At both temperatures Co-FER (8) catalysts are much more selective m their use of methane than CoZSM-5 The shape selectivity of the zeohte may not influence the NO, reduction in the same way as reactions between some large orgamc molecules (1 e , excluding certain molecules from entering or leaving the zeohte pores) because we are dealing with relativelysmall molecules (NO, 02, H20, and CH,) However, zeohte topology may affect the coordination of Co2+ and the relative diffuseities of these molecules The difference in activity and selectivity between CoZSM-5 and Co-ferrierite must relate to the characteristics of the two types of zeohte There must be some differences in Co2+ posltionmg and coordmatlon ZSM-5 exhibits a two-dimensional, lo-ring channel system with a ring size of 5 6x 5 3 A for the straight channels, [OOl] plane, and 5 1x5 5 A for the smusonlal channels, [lOO] plane [9] Ferrierite, on the other hand, has lo-ring channels (4 3 x 5 5 A) on the [ 0011 plane and 8-rmg channels (3.5 x 4 8 A) on the [ 0101 plane [ 91. In addition to the Co2+ positioning, smaller channels may

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Y Li and J N Armor / Appl Catal B 3 (1993) Ll -Lll

enhance the selectnnty of methane and therefore increase the nitric oxide conversion. This is cons&ent mth the fact that on Co-mordemte (12-nng channels, 6 5 x 7.0 A) even lower selectivity and activity were observed Restriction of the diffusion of methane (kmetlc diameter=33 A) molecules by small channels may control the selectivity of methane. On the other hand, smaller channels may provide Co2+ coordination that is less favorable for oxygen adsorption Fundamental studies on the role of the zeohte are contmuing [lo] All these Co-xeolites were obtamed by exchanging a NH,-femerite with Co2+ and, after high temperature treatment, contam H+ as balancing cation The presence of H+ m the Co-ferrierne may contribute to the catalysis, and yet Hferrlerite itself does not have appreciable activity under our standard reaction comhtlons (see Table 4) At this time we do not have evidence suggesting the direct mvolvement of H+ m this reaction. Alternatively, pre-exchange of NHd+ may Just simply control the cation positioning m the xeohte and the ion exchange stolchlometry The effect of pre-exchange and partial balance by NH,+ is further illustrated with a series Mn-FER catalysts which are shown m Table 4 A Mn-H-FER (8)-78 (Mn/Al=O39) and a Mn-Na-FRR(8)-80 (Mn/ Al=0 40) have sun&u exchange levels but made from different forms of fernerne, NHI-FER for the former and Na-FER for the latter The Mn-HFER (8) -78 is much more active for nitric oxide reduction at high temperatures (39% at 550’ C) than the Mn-Na-FER (8) -80 (26% at the same temperature) Conversely, Mn-Na-FER(8)-80 is more active for methane combustion. Interestingly, the high loading Mn-H-FRR (8) -78 (Mn/Al= 0.39) has a lower mtrlc oxide conversion than the lower loading Mn-H-FER( 8)-54 (Mn/ Al = 0 27)) whereas their methane conversions are in the opposite order. The presence of H+ may be a way to control the methane combustion rate, which TABLE 4 Converelons of mtnc oxide and methane ( % ) on Mn-Femente Reaction condltlons [NO] = 1610 ppm, [CH ,= 1015 ppm, [Oa=2 5%, GHSV=30 000 h-’ alysts were made by exchangmg Co2+ mt.o NH,-FER, except Mn-Na-F’ER(8)-80 Balance ion

Mn-H-FER(8)-54 Mn-H-FER(8)-78 Mn-Na-FER(8)-80” Mn-H-FER(G)-106 H-FER(8)

Compoeition

400°C

450°C

500°C

650°C

&/Al

Mn/Al

NO CH,

NO

NO CH,

NO CH, NO

84 85 8.2 6.0 83

027 039 040 053

3 2 4 4 _

13 17 13 18 15 22 20 30 <2 1

28 29 25 36 4

42 74 39 79 26 95 4094 7 7

3 2 2 7 _

CH,

51 48 64 66 2

Cat-

600°C CH,

45 96 39 100 ~23 100 nana 10 13

’ The startmg femer& wanNa-F’ER (8 ) [The Na,K-FER (8 ) was first converted to a NH,+ form, then exchanged mth Na+ ] Na/Al ratio remamed m Mn-FER(8)-80 u 0 19

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TABLE 5 Effect of water on nltnc oxide and methane converelons over Co-femente and Co-ZSM-5 Reactloncondltlone

[NO]=850ppm,

Catalyst

Condltlon

co-FXR(8)” Co-ZSM-5”

dry wet” dry wetc

[CH,] =lOOOppm, [O,] =2 5%,GHSV=30000 500°C

550°C

h-’

600°C

NO

CHI

NO

CH,

NO

CH,

60 28 40 29

60 23 91 38

50 40 27 28

93 75 100 86

40 32 21 22

100 100 100 100

a &/Al=8 3, Co/Al=0 39 (3 9% Co by weight) b Si/Al=ll, Co/Al=0 49 (3 8% Co by weight) e 2% H20 added m feed

m turn influences the mtnc oxide reduction rate. Alternatively, the presence of Na+ may serve to catalyze methane oxidation (eqn 2 ) Flue gas contams large amounts of water vapor as a combustion product. Therefore, tolerance to water vapor is a critical requirement for a practical catalyst [6] The effect of water on the nitric oxide conversion and methane conversion was studed over Co-fernente and Co-ZSM-5 (Table 5) Upon addition of 2% Hz0 in the feed, both nitric oxide and methane conversions over the Co-FER catalyst decreased. The effect of water decreases with mcreasmg temperature, and the optimum reaction temperatureunder the wet conditions is 550’ C. On the Co-ZSM-5 catalyst, the effect of water is more pronounced at 566”C, but at 550 and 666” C, 2% Hz0 does not affect the mtnc oxide conversion On both catalysts, the effect of water is reversible upon ehmmating water from the system Unlike Ga-ZSM-5 [ 71 which was very sensitiveto water, Co-ferriente was more active than Co-ZSM-5 in a wet stream. CONCLUSIONS

We have discovered that metal exchanged ferrierites, e.g., Co-fernerite, are very active catalysts for the selective reduction of NO, with methane Co-fernente doubles the mtnc oxide conversion compared to Co-ZSM-5 at T> 506oC and is more selective for the use of methane The temperaturedependence for mtnc oxide conversion is a function of seolite type and the cation exchanged. Substitution of Ni2+ or Mn2+ for Co2+ shifts the maximum nitric oxide conversion temperature. The turnover frequency (TOF) normalized by Co2+ is a function of Co2+ exchange level which increases with the exchange level until reaching an optimum value. In general, the selectivity of methane utilization decreases with mcreasing Co2+ loading. Exchange levels of 70-90% appear to

Y Ls and J N Armor / Appl Catal B 3 (1993) Ll -Lll

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be the optimum for the overall performance. Zeolite topology 1s emergmg as a key factor affecting catalyst activity and selectivity ACKNOWLEDGEMENT

Thanks are due to Paula Battavio for the actlvlty measurements and sample preparatlons We thank Air Products and Chemicals, Inc. for perrmsslon to publish this work.

REFERENCES

10

T J Truex, R A Searlee end D C Sun, Platmum Metals Rev ,36 (1992) 2 M Iwamoto and N Mlzuno, J Auto Eng ,207 (1993) 23 Y Ll and J N Armor, Appl Catal B, 1 (1992) L31 Y Ll and J N Armor, Appl Cati B, 2 (1993) 239 K Yogo, M Umeno, H Watanabe and E Klkuchl, Catal L&t, 19 (1993) 131 Y L1, P J Battawo and J N Armor, J Catal, 142 (1993) 561 Y LlandJN Armor,J Catal,mpress Y Ll and J N Armor, US Patent (allowed) 1993 W M Meler and D H Olson, Atlas of Zeol1t.eStructure Types, Butterworth-Hememann, London, 3rd titlon, 1992 J N Armor and Y 4 Prepnnte, Dw Petrol Chem , Am Chem Sot , San Diego, CA, March 1994