604
`
`Chemistry Letters 2001
`
`Simultaneous Catalytic Removal of Nitrogen Oxides and Soot by Copper-Loaded MFI Zeolites
`
`Yasutake Teraoka,* Kazunori Kanada, Hiroshi Furukawa, Isamu Moriguchi, and Shuichi Kagawa
`Department ofApplied Chemistry, Faculty ofEngineering, Nagasaki University, Nagasaki 852—8521
`
`(Received March 12, 2001; CL-010217)
`
`Cu—loaded MFI zeolites showed the catalytic activity for
`the oxidation of soot and reduction of NOx simultaneously in
`the sootiNOxfiz reaction system.
`Ion-exchanged and impreg-
`nated catalysts showed the comparable activity, but the latter
`was decidedly superior to the former with respect to the selec—
`tivity to NO reduction into N2.
`
`Nitrogen oxides (NOX) and soot particulates emitted from
`diesel exhaust have been greatly contributing to the environ—
`mental pollution, and currently the regulation of diesel emis—
`sions becomes tightened with respect to nitrogen oxides (NOX)
`and particulate matters (PM): PM consists mainly of soot and
`soluble organic fraction (SOF). Apart from the catalytic after-
`treatrnents so far investigated actively, such as the selective cat—
`alytic reduction of NOx by hydrocarbons and the oxidation of
`CO, gaseous hydrocarbons and SOF,' another possible option is
`the simultaneous catalytic removal of NOx and PM (soot).2 We
`have been studying the simultaneous NOxisoot removal reac—
`tion with respect to catalyst development3‘8 and reaction mech—
`anismw’10 and revealed that mixed metal oxides with per—
`ovskite-related3""7=8 and spinels’7 structures are promising cata-
`lysts for this reaction. This paper reports the catalytic property
`of Cu—loaded MFl for the simultaneous NOxesoot removal
`reaction.
`It has turned out that Cu-loaded MFI catalysts pre-
`pared by an impregnation method are good candidates showing
`high activity and selectivity to N2 formation.
`Na-MFI (SiOZ/A1203=23.3, MF12) and NH4—MFI (39.5,
`MFI4) were kindly supplied by Tosoh Corporation. Cu ion—
`exchanged MFI (Cu-MFI) was prepared as follows. The parent
`zeolite was treated with 0.1 M aq NaNO3 at 60 °C for 1 day,
`followed by a conventional ion—exchange procedure using aq
`Cu(ll) acetate at 60 °C for 1 day.
`In the preparation of Cu-
`impregnated catalyst (Cu/Na—MFIZ), Na—MFI2 powder was put
`in an aq solution of Cu(ll) acetate, and immediately the suspen—
`sion was evaporated to dryness. H Both Cu-MFl and Cu/Na—
`MF12 were finally air-calcined at 550 0C for 1 h. The Cu load—
`ing was expressed by wt% of Cu:
`1 wt% Cu loading corre—
`sponds to 27% and 42% ion—exchange levels for MF12 and
`MFI4, respectively.
`The catalytic activity for the simultaneous NOxisoot
`removal was evaluated by a technique of the temperature pro—
`grammed reaction (TPR)?10 A catalyst and activated carbon12
`(ca. 5 wt%) was well mixed by mortar and pestle. The tight
`mixture (0.33 g) thus obtained was packed in a reactor and
`heated at a rate of 1 oC min’1 under flowing NO(0.5%)7
`02(5%)—He(balance) at 20 cm3 min‘1 and the outlet gas was
`analyzed with intervals of about 15 min by a TCD gas chro—
`matograph (Shimadzu GC-8A).
`TPR result over 3.1 wt% Cu—MFI2 in the NO—OZ—He
`atmosphere is shown in Figure l. The formation of C02, N2
`and N20 at the same temperature range evidenced the occur-
`
`25
`
`o\° 20
`1
`ON
`E 15
`x
`
`E 10
`2.
`z 5
`2'
`
`0
`
`
`
`100
`
`200
`
`300
`
`400
`
`500
`
`800
`
`Temperature I DO
`
`15
`
`g.
`Q
`10 E
`g
`N
`O
`o
`5 a
`3
`O
`
`0
`
`of
`reaction
`programmed
`Temperature
`1.
`Figure
`simultaneous NOx-soot
`removal over 3.1wt% Cu-MFIZ
`(closed symbols) and Na—MFIZ (open symbol). Only the
`C02 formation curve is Shown for Na—MF12. X[N2] and
`XINZO] are conversions of NO into N2 and N30, respectively.
`
`rence of the simultaneous NOx—soot removal reaction: the soot
`pre—mixed with the catalyst was oxidized by either NOx or 02 to
`produce C02, and NOx was reduced by the soot into N2 and
`N20. The C02 formation over Na—MFIZ took place at higher
`temperature region than that over 3.1 wt% Cu—MF12 (Figure 1).
`In addition, the soot was completely oxidized into CO2 over all
`the Cu—loaded catalysts, while CO amounting about 18% of
`C02 was formed over N a—MF1 (not shown in Figure 1). These
`results indicate that Cu introduced in MFI zeolites effectively
`works as a catalyst for the simultaneous NOx—soot removal
`reaction. From the TPR result, ignition temperature (Tig)7
`which was used as a measure of activity, was obtained by
`extrapolating the steeply ascending portion of the C02 forma—
`
`tion curve to zero C02 concentration (estimation error; :5 °C),
`and total amounts of C02, N2 and N20 formed throughout the
`TPR run, V[C02], V[N2] and V[N20], were obtained by inte—
`grating the respective curves. The selectivity to N2 formation
`(S[N2]) was defined by V[N2]/V[C02], which corresponds to a
`fraction of soot used for the reduction of NO into N2.
`In Figure 2, Tig value (A) and the selectivity to N2 forma—
`tion (B) are plotted as a function of the Cu loading. For all the
`catalyst systems, the Tig values decreased, or the soot ignition
`activity increased, with increasing the Cu loading and reached
`the constant activity above ca.
`1 wt%. These results indicate
`that the soot ignition activity of Cu-loaded MFI in the simulta-
`neous NOx—soot removal reaction is almost exclusively deter—
`mined by the Cu loading (wt%) irrespective of the Si/Al ratio of
`MFI, preparation methods and existing state of Cu. As for the
`selectivity to N2 formation, both the ion—exchanged catalysts,
`Cu-MFl2 and Cu—MFl4, showed nearly the same tendency that
`the S[N2] gradually decreased with increasing the Cu loading
`and reached the steady value above ca. 2.3 wt%. On the other
`hand, impregnated Cu/Na—MFIZ showed higher S[N2] values
`
`Copyright © 2001 The Chemical Society of Japan
`
`JM 1009
`
`1
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`JM 1009
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`

`605
`
`investigated, the most active catalyst was the Cu—exchanged
`catalyst, 3.5 wt% Cu—MFIZ. The impregnated 3.8 wt% Cu/Na—
`MFIZ was more selective to the N2 formation than Cu/A1203,
`CuO and any of the metal exchanged catalysts. As compared
`with mixed metal oxide catalysts so far reported, 3.8 wt%
`Cu/Na—MF12 is medium in activity but the most selective to
`the N2 formation: the highest selectivity was about 6%
`attained by La0_9K0_1Cu0_7V0_3O,,,3 La1_9K0_1Cu0_95V0_0504,4 and
`Cu0_95K0_05Fe2046 (see Figure 3). Moreover, the Cu-loaded
`MFI catalysts have an advantage over the other NOx—soot
`removal catalysts that they catalyze the HC—SCR reaction: it
`was experimentally confirmed that when the mixture of soot
`and 4.6 wt% Cu—MFIZ was heated in an 02—NO—C2H4 stream,
`C2H4—SCR reaction proceeded in addition to the NOx—soot
`removal.
`
`In conclusion, it has turned out that Cu/Na—MF12 prepared
`by impregnation method is a promising N Oxisoot removal cata—
`lyst showing medium soot ignition activity and high selectivity
`to N2 formation. This letter reports only the evaluation of the
`catalytic performance. There remain many subjects to be inves—
`tigated in future studies such as reasons why impregnation
`method or CuO on the outer surface of MFI affords high selec—
`tivity to N2 formation and why Cu ions exchanged in zeolite
`pores catalyze the reaction involving soot, which is present on
`the outer surface.
`
`References and Notes
`
`4
`
`7
`
`8
`
`1
`
`2
`
`3
`
`P. Zelenka, W. Cartellieri, and P. Herzog, App]. Cata]. B,
`10, 3 (1996).
`K. Yoshida, S. Makino, S. Sumiya, G. Muramatsu, and R.
`Helferich, SAE Paper, 1989, 892046.
`Y. Teraoka, K. Nakano, S. Kagawa, and W.F. Shangguan,
`App]. Cata]. B, 5, L181 (1995).
`Y. Teraoka, K. Nakano, W. F. Shangguan, and S. Kagawa,
`Cata]. Today, 27, 107 (1996).
`5 W. F. Shangguan, Y. Teraoka, and S. Kagawa, App]. Cata].
`B, 8, 217 (1996).
`6 W. F. Shangguan, Y. Teraoka, and S. Kagawa, App]. Cata].
`3,16, 149 (1998).
`Y. Teraoka and S. Kagawa, Cara]. Surveys from Jpn., 2,
`155 (1998).
`Y. Teraoka, W. F. Shangguan, K. Jansson, M. Nygren, and
`S. Kagawa, Bull. Chem. Soc. Jpn, 72, 133 (1999).
`9 W. F. Shangguan, Y. Teraoka, and S. Kagawa, App]. Cata].
`B, 12, 237 (1997).
`10 Y. Teraoka, W.F. Shangguan, and S. Kagawa, Res. Chem.
`Interment, 26, 201 (2000).
`11 During the evaporation—to—dryness process, a small portion
`of Cu ions might be ion—exchanged. However, a majority
`of them were deposited on the outer surface of MN and
`were present as CuO after calcination, which was con—
`firmed by HTTPR as well as color of the catalysts.
`12 Activated carbon (Nakalai Chemicals) was used as a sub-
`stitute for the diesel soot, because under the present experi—
`mental condition its reactivity was almost the same as that
`of the dry soot obtained by the incomplete combustion of
`diesel oil and so far used in our laboratory.3‘10
`
`Chemistry Letters 2001
`
`
`500
`
`400
`
`o Cu/NaMFIZ
`o Cu—MFIZ
`1: Cu-MFI4
`
`
`
`O Cu/NaMFI2
`O Cu-MFI2
`D Cu-MFI4
`
`U
`
`1
`
`2
`
`5
`4
`3
`Cu loading / wt%
`
`5
`
`7
`
`8
`
`ignition temperature. T,,,, and (B)
`(A) Soot
`Figure 2.
`selectivity to N2 formation. SD12], of Cu—loaded MFl
`catalysts as a function of Cu loading.
`
`than the ion—exchanged catalysts in the whole Cu—loading range
`examined, with a moderate maximum at 1.2 wt% loading.
`Furthermore, Cqua-MFIZ were superior to Cu-MF12 with
`respect to less amount of N20 formation. The selectivity to
`N20, V[N20]/(V[N2]+V[N20]), over catalysts with >1 wt% Cu
`loadings were 7.1712.2% and 14.9723.9% for Cu/Na—MF12 and
`Cu—MFIZ, respectively. As a result, impregnated Cu/Na—MF12
`with the Cu loadings higher than ca. 1 wt% are good catalysts
`showing both high activity and selectivity to N2 formation.
`Figure 3 shows the relation between Tig and S[N2] in order
`to compare the catalytic performance for the simultaneous
`NOx—soot removal. Among ion-exchanged MFI catalysts
`
`
`8
`
`D,
`7
`101
`6
`
`10
`BN1”
`11
`
`KH
`4
`
`1K
`
`M
`
`810+
`
`3101
`2101
`
`D1
`12
`
`l
`I
`l
`I
`.3.
`I
`l
`250
`300
`350
`400
`
`Ti,/°c
`
`a
`
`E 4
`a"
`m
`
`2
`
`0
`
`200
`
`Figure 3. Relation between soot ignition temperature. Tig,
`and selectivity to N2 formation. S[N2].
`1. 3.8 wt% Cu/Na-MFI2
`7. Na-MF12
`2. 3.5 wt% Cu—MFIZ
`8. Clio 9SK0_05F€204
`3.
`1.2 wt% Cu-MFIZ
`9. Lao.9K01Cu0 7V0.30x
`4.
`12 wt% Pt‘MFIZ
`10.
`lrangmCqugVn 0504
`5. 2.8 wt% Co-MFI2
`11.
`3.5 wt% Cu/A1203
`6
`3.3 wt% Pd-MF12
`12. CuO
`
`2
`
`