Nitrogen-containing compounds as a reductant for the selective catalytic reduction of NO

Article abstract of DOI:10.1246/cl.2004.924

When acrylonitrile and other nitrogen-containing compounds are passed over Cu- and H-ZSM-5, they can act as efficient reductants in the selective catalytic reduction of NO by forming NH₃ and carbonaceous deposits through hydrolysis.

Full text of DOI:10.1246/cl.2004.924

924
Chemistry Letters Vol.33, No.7 (2004)
Nitrogen-containing Compounds as a Reductant for the Selective Catalytic Reduction of NO
Tetsuya Nanba,ꢀ Shouichi Masukawa, Junko Uchisawa, and Akira Obuchi
Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology (AIST),
16-1 Onogawa, Tsukuba, Ibaraki, 305-8569
(Received May 26, 2004; CL-040599)
When acrylonitrile and other nitrogen-containing com-
200
150
100
50
2
pounds are passed over Cu- and H-ZSM-5, they can act as effi-
cient reductants in the selective catalytic reduction of NO by
forming NH3 and carbonaceous deposits through hydrolysis.
1.5
1
New techniques for decreasing the amount of reductants that
are required in the selective catalytic reduction of NO (SCR)
are needed for its economical operation. Although SCR with
NH3 is a practical reaction, equivalent amounts of NH3 and
NO are necessary because of the stoichiometry of the reaction.
In the case of SCR with hydrocarbons, one hydrocarbon mole-
cule can react with two NO molecules.1 The rate of NO conver-
sion, however, is competitive with the rate of hydrocarbon oxi-
dation by O2. We focused on certain nitrogen-containing
compounds (NC), such as acetonitrile (AN), acrylonitrile
(ACN), and N,N-dimethylformamide (DMF) for use as reduc-
tants. These NCs are expected to exhibit dual character as reduc-
tants, because they contain both a nitrogen atom and carbon
atoms in a reducing state in one molecule. Miyadera reported
high reactivities of AN and HCN with NOx over Ag/Al2O3.2
We report here high performances of Cu- and H-ZSM-5 for
the reduction of NOx with NCs, which have a bifunctional role
as a reductant.
Cu-exchanged ZSM-5 was prepared by aqueous ion ex-
change (IE) using Cu(CH3COO)2. These catalysts were denoted
by the format Cu-ZSM-5-57, where the suffix number indicates
the ion exchange ratio. The catalytic reactions were carried out
under conventional flow reactor conditions with a flow rate of
160 mL minꢁ1. The NCs were introduced into the reactant gas
by passing a predetermined flow rate of He through liquid sam-
ples of the NCs, which were maintained at 273 K for AN and
DMF and 255 K for ACN. The feed gas was composed of ca.
200 ppm NC, 1150 ppm NO, 0.5% H2O, and 5% O2. Tempera-
ture-programmed desorption of NH3 (NH3-TPD) was carried
out for some zeolite materials. The details of the procedure are
described elsewhere.3
0.5
0
300
0
800
400
500
600
700
Temperature / K
Figure 1. Temperature dependence of the activity of Cu-ZSM-
5-57 for NO reduction by ACN. The weight of catalyst was 0.1 g.
The feed gas composition was ca. 200 ppm ACN, 1150 ppm NO,
0.5% H2O and 5% O2. The total flow rate was 160 mL minꢁ1
The symbols indicate ACN conversion ( ) and the ratios of
NOx consumption ( ) and N2 formation ( ) to ACN inlet.
.
was obtained with ACN over H-ZSM-5.
The formation of acrylic acid was observed over H-ZSM-5
during SCR with ACN, and the formation of NH3 was observed
during the reaction of ACN þ O2 þ H2O. These results clearly
suggest that ACN was hydrolyzed to acrylic acid and NH3.
The occurrence of SCR with NH3 over Cu-ZSM-5 and H-
ZSM-5 has been reported4,5 and it has been suggested that
NH3 adsorbed on H-ZSM-5 exhibits selective reduction activity
at temperatures as low as 373 K.6 Consequently, it is suspected
that the NH3 formed during the hydrolysis of ACN might act
as a reductant.
We examined SCR with acrylic acid over Cu- and H-ZSM-5
by using a feed gas containing 1180 ppm NO, 150 ppm acrylic
acid and 5% O2. The maximum amounts of NO that were con-
Table 1. Activity for SCR with nitrogen-containing compounds
Figure 1 shows the activity for SCR with ACN over Cu-
ZSM-5-57. The reaction proceeded above 400 K, and the con-
version of ACN reached 100% at 623 K. N2 was a dominant ni-
trogen-containing product, and a small amount of N2O, HCN,
and HNCO were observed as byproducts. HCN and HNCO were
produced only below 623 K. The ratio of NOx consumption and
N2 formation to ACN consumption was greater than unity above
623 K.
Table 1 lists the activity for SCR of various NCs over Cu-
and H-ZSM-5. The temperature where NO/NC was a maximum
decreased as the IE ratio of the Cu increased. AN and ACN ex-
hibited higher NO/NC and N2/NC ratios than DMF. However, it
was noted that Cu-ZSM-5-165 exhibited the most significant de-
cline in NO/NC at higher temperature. The highest N2/NC ratio
Activity
Temp./Ka NC Conv./% NO/NCb N2/NCc
Catalyst
Reductant
Cu-ZSM-5-165
AN
623
573
523
623
623
673
673
723
773
100
100
99
100
100
100
97
1.7
1.5
1.2
1.7
1.8
1.5
1.9
1.7
1.3
1.2
1.2
1.1
1.2
1.2
1.3
1.2
1.4
1.0
ACN
DMF
AN
ACN
DMF
AN
Cu-ZSM-5-57
H-ZSM-5
ACN
DMF
99
100
aAt the temperature for the highest ratio of NOx consumption/NC consump-
tion.
bRatio of NO consumption (ppm) to NC consumption (ppm).
cRatio of N2 formation (ppm N) to NC consumption (ppm).
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.7 (2004)
925
verted were 46 ppm (579 K), 132 ppm (723 K) and 82 ppm
(723 K) for Cu-ZSM-5-165, Cu-ZSM-5-57, and H-ZSM-5, re-
spectively. Although the conversion of acrylic acid was 100%
at the same temperatures, the conversion rates of acrylic acid
to COx (¼ CO2 þ CO) remained at 98, 98, and 78%, correspond-
ing to ratios of 0.3, 0.9, and 0.7 for the conversion of NO with
respect to the conversion of acrylic acid to COx. The low carbon
balance for H-ZSM-5 indicates that significant carbonaceous
deposition occurred. Considering the high activity of H-ZSM-
5, carbonaceous deposits might be involved as reductants for
SCR. One of the co-authors has suggested that carbonaceous de-
posits can be an active species for NO reduction.7
These results suggest that ACN has a bifunctional role as a
reductant, i.e., it works for conversion to NH3 and it works for
producing active carbonaceous deposits derived from acrylic
acid. A speculative reaction pathway is shown in Scheme 1.
The fact that the NO/NC ratios are above unity suggests that
the bifunctional role is exhibited by all of the NCs, though there
should be differences in the rate of formation of NH3 and in fea-
tures of the carbonaceous deposits.
previously reported a promotion effect due to the clearance of
NH3 in the decomposition of nitroethene over H-ferrierite.8
The differences in T50 for each reaction condition decreased as
the Cu loading increased, probably as a result of improvements
in the oxidation activity. Too much Cu loading may result in an
increase in the oxidation activity and a decrease in the active car-
bonaceous deposits. On the other hand, a lack of Cu loading may
result in the rapid accumulation of carbonaceous deposits and
the covering of the active sites. An appropriate loading of Cu,
probably an IE ratio of ca. 50%, is thought to be the most favor-
able for achieving a bifunctional role of reductant.
Table 2 lists the NH3-TPD results and the activities of three
H-form zeolites. H-ZSM-5 showed the highest activity, whereas
it is mid-range in terms of the NH3-TPD results. It may be nec-
essary to establish the relationship between the acid properties
and the carbonaceous deposits to clarify the role of the acid sites
in this reaction.
Table 2. NH3-TPD results and activity of H-form zeolite for
SCR with ACN
NH3 desorptiona
Activityb
Si/Al
ratio
Catalyst
H-FER
H-ZSM-5 23.8
H-MOR 18.7
amount
mmol gꢁ1
Temp. Temp. Conv. NO/ACN N2/ACN
K
CH2=CH−C≡N + H2O NH3 + CH2CHCOOH
K
%
Carbonaceous
deposits
17
0.57
0.28
0.17
698
637
743
648
723
748
63
99
67
1.3
1.7
1.3
0.9
1.4
1.2
NO
aThe amount of NH3 desorption for a higher peak.
bCalculated in the same manner as those in Table 1.
N2
AN is better than ACN as a reductant in terms of their rela-
tive toxicities.9 However, we confirmed that Cu-ZSM-5 can
completely convert ACN in the reaction of ACN þ O2 þ H2O
at 623 K and can run for 100 h without any change in selectivity.
Furthermore, there will be no release of NH3 as a by-product,
due to the strong acidity of ZSM-5 and the rapid reaction with
NO. The NCs used in this study are disposed of at ca. 21000 t/
year into the atmosphere and in waste-treatment sites in Japan.
The incineration of these compounds induces almost an equiva-
lent amount of NOx formation. The work in this paper implies
that these wastes could be utilized in the NOx abatement process.
Studies in widening the active temperature range, especially
toward lower temperature, are in progress.
Scheme 1. Speculated reaction pathway.
Figure 2 shows the temperature of 50% ACN conversion,
T50, under various conditions. All of the catalysts exhibited a de-
crease in T50 after the addition of H2O and H2O þ NO to the
feed gas. It is suggested that H2O and NO enhanced the reactiv-
ity of the ACN by promoting hydrolysis and by the clearance of
acid sites that were covered with NH3, respectively. We have
800
750
700
650
600
550
500
450
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0
57
165
Ion-exchange ratio / %
Figure 2. The effect of co-existing gas and ion-exchange ratio
on the temperature for 50% ACN conversion. The weight of cat-
alyst was 0.1 g. The feed gas composition was ca. 200 ppm ACN,
5% O2, 0 or 0.5% H2O and 0 or 1150 ppm NO. The total flow
rate was 160 mL minꢁ1. The shaded patterns indicate the gas
composition: ACN þ O2 (heavy shading), ACN þ O2 þ H2O
(light shading), and ACN þ O2 þ H2O þ NO (no shading).
Published on the web (Advance View) June 21, 2004; DOI 10.1246/cl.2004.924