Note: Descriptions are shown in the official language in which they were submitted.
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SELECTIVE REDUCTION OF NX
Backaround of the Invention
Technical Field
This invention relates to the technology of
purifying exhaust gases or emissions of stationary and
mobile sources, and more particularly to the technology
of cleansing such emissions of NO~ from an oxygen-rich
stream.
Discussion of Related Art
Emissions from large scale stationary sources,
containing NO~ and excess 2~ are generall~ treated
with ammonia as a reductant over a catalyst containing
V2O5 on TiO2 (see H. Bosch and F. Janssen,
"Catalysis Today", Vol. 2 (4), 1987). Emissions from
mobile sources that do not have excess oxygen but contain
NOX (automotive vehicles) are removed by reaction with
in-situ reductants, such as carbon monoxide or
hydrocarbons ~HC), ~hen passed over a catalyst, often
containing rhodium. Such a catalyst would be ineffective
in the presence of a large excess of 2 (see K.C.
Taylor, "Automobile Catalyst Converters", Springer,
Berlin, 1984).
Recently, copper-exchanged zeolites have been
found to reduce NOX in the presence of excess oxygen
(see U.S. patent 4,934,1~2 and Japanese patent
application publication No. Hèi 3-52644, 3/6/913, but to
attain substantial conversion efficiencies at the
moderate temperatures of an exhaust produced by a
lean-burn engine, a temporary rich A/F condition is
required to provide a residual HC reductant.
Unfortunately, it is not desirable to operate an
automotive engine or other emission source under
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artificially rich conditions simply to facilitate
catalytic conversion of the engine emissions (see M.
Iwamoto et al, Applied Catalysis, Vol. 69, L 15-19,
1991). To facilitate conversion of NOX under
conditions of excess oxygen, without the need for
stimulating high HC in the exhaust, alkane and alkene
additions have been suggested as reductants injectable
into the emissions ahead of the zeolite (see S. Sato et
al, Applied Catalysis, Vol. 70, L 1-5 (1991)). However,
it is difficult to meter small doses of such gaseous
reductants to match accurately the varying NO~ content
of the exhaust gas and to reliably and safely store such
gaseous reductants on~board a vehicle.
It is therefore an object of this invention to
provide a catalyst system for selective and efficient
reduction of NO2 accompanied by excess oxygen by use of
a liquid reductant that provides several advantages not
attainable by the prior art: (i) the reductant is easy to
meter and match to the NOX variability, (ii) the
reductant is nontoxic and safe to store for periodic use,
and (iii) the reductant provides substantial enhancement
of NOX conversion over a copper-exchanged zeolite at
small excess oxygen conditions.
Summary of the Invention
In a first aspect, the invention is a catalyst
system for selective reduction of gaseous NOX in the
presence of excess oxygen, the system comprising: (a) a
copper-containing ZSM~ zeolite to which a
NOx-containing gas is exposed; and ~b) means ~or
introducing a metered volume of distributed
water-soluble, oxygen-containing organic compound into
the NOx-containing gas prior to exposure to the Cu-ZSM5
zeolite.
The compounds are partially oxygenated
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reductants selected from the group consisting of small
molecular weight, water soluble alcohols, aldehydes,
ketones, and ethers. The oxygenated compounds are
capable of providing enhanced conversion ~fficiency of
NOX, particularly at .1-.5~ excess oxygen.
Another aspect of the invention is a method of
treating automotive exhaust gas emissions, having an
excess of oxygen, by the steps comprising: (a)
introducing a water-soluble, oxygenated organic compound
into the exhaust gas emissions as a reductant and at a
location adjacent to the source of the exhaust yas
generation; (b) substantially immediately exposing the
reductant/emission mixture to a transition
metal-exchanged high silica zeolite catalyst
(SiO /A12O of between 10-50) at a space velocity
2 3
in the range of 20-80 K hr ~; and (c) sequentially
exposing the effluent of such zeolite catalyst to an
oxidation catalyst.
Preferably, the water-soluble, oxygenated
compound is introduced by injection of a water/oxygenate
solution correlated in an appropriate amount with respect
to the NOX in the instantaneous e~haust gas.
Brief Description of the Drawings
Figures 1 and 2 are graphical illustrations of
the percènt conversion efficiency for reduction of nitric
oxide as a function of the percent oxygen in th~ gas
mixture being converted, Figure 1 using a nonoxygenated
reductant (propylene), and Figure 2 shows the effect when
using an oxygenated hydrocarbon compound, in this case,
propanol.
Detailed Description and Best Mode
The catalyst system of this invention is
operative to cleanse the exhaust gas generated by a
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` fossil-fueled engine, particularly a gasoline-fueled
internal combustion engine, operated under lean-burn
conditions. Lean-burn is generally used herein to mean:
for a gasoline engine, an A/F ratio between about 15-23,
or a lambda of 1.02-1.7; for a diesel enyine, an A/F of
between 15.4 and 30, and a lambda of 1.1-2.0; and for a
compressed natural gas-fueled vehicle, an A/F ratio of
about 17.~-25, and a lambda of about 1.02-1.7.
The catalyst system of this invention comprises
essentially (i) a copper ion-exchanged ZSM5 zeolite
catalyst, and (ii) means for introducing a metered volume
of sprayed water-soluble, oxygen-containing organic
compound into an NOx-containing gas mixture prior to
exposure to the ZSM5 zeolite.
Zeolites, in general, are aluminosilicates with
a framework containing cations such as those of alkali
metals and alkaline earth metals. The framework of a
zeolite is based on the combination of A104~SiO4
tetrahedrons. Only synthetically produced zeolites are
suitable for this invention.
ZSM5 is a crystalline zeolite and is disclosed
in U.S. patent 3,702,886, the disclosure of which is
incorporated herein by reference. ZSM5 can have a
SiO2/A12O3 ratio ranging from about 10 to 1000.
The copper ion-exchanged version of such zeolite may be
obtained by stirring a proportion of copper acetate
solution tpreferably about .05M) with the ZSM5 zeolite.
The material is filtered, washed, and preferably
ion-exchanged three times. It is then dried at about
120C for about three hours and ~alcined at about 600C
for about three hours. The resulting material will
contain copper exchanged for cation(s) of an alkali metal
or of a proton of the zeolite as well as copper
impregnated onto the zeolite, i.e., about 3% by weight.
For instance, the copper ions will replace the sodium
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ions in the zeolite. The limit is set by the amount ofA12O3 in the zeolite. It is advantageous to provide
as much transition metal (copper) in the zeolite as
possible since the amount of copper present in the
zeolite is directly related to the catalytic activity of
the catalyst. The copper-exchanged zeolite may contain
copper in the weight percent range of 1-8% and will be
stable up to a temperature of about 600C. The zeolite
catalyst is advantageously coated onto a monolithic or
pelleted support which is placed in the exhaust stream to
flow therethrough.
The partially oxygenated compound is selected
from the group consisting of water soluble alcohols,
aldehydes, ketones, and ethers of small molecular weight
1~ with more than two carbon atoms, provided that the
selected compounds from this ~lass preferably produce
NOX conversion efficiency of at least 40% when the
excess oxygen in the gas to be treated varies between
.1-.5% (weight percent of oxygen), of at least 35~ when
the e~cess oxygen is between .5-1.5%, and at least an
average of 30% when the excess oxygen is greater than
1.5%. NOX is used herein to mean ~O, NO2, and
mixtures thereof. The NOx-containing gas usually
contains NOX in an amount of at least 400 ppm; the gas
may also contain water vapor, carbon dioxide, and other
combustion products. Specific e~amples of the partially
oxygenated compounds include ethanol, propanol,
acetaldehyde, acetone, methyl ethyl ketone, and
dioxane.
The oxygenated compound, preferably propanol, is
metered into the exhaust gas stream immediately ahead of
the ZSM5 zeolite, and preferably immediately downstream
of the combustion zone for generating the emissions. The
- metering may be carried out by a suitable injection
device, such as a well calibrated in~ector, to atomize
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the aqueous solution and achieve a steam/oxygenate
mixture at the exhaust gas temperature which is typically
about 700-1100F. It is desired that appropriate amounts
o~ the oxygenated organic compounds be injected in
accordance with the variations of NOX present in the
exhaust gas. This will usually require a reductant to
NOX ratio of about 3-5. The variability of the NOX
may be instantaneously measured either as a function of
exhaust gas temperature or by use of a direct sensor.
The supply of partially oxygenated organic
compounds can be stored in plastic or metal canisters.
The pressure of such liquid compounds will be around
ambient pressure conditions. This mode of storage is
considerably simpler than that required for the injection
of aiternative gaseous reductants and is considerably
safer than the use of urea, ammonia, or gaseous
reductants.
The method aspect of this invention for treating
automotive exhaust gas emissions from a lean-burn fueled
en~ine, having an excess of oxygen, comprises: (a)
introducing a water-soluble, partially oxygenated organic
compound into the emissions as a reductant and at a
location closely adjacent the generating source for said
emissions; (b) substantially immediately exposing the
reductant/emission mixture to a transition
metal-exchanged, high silica zeolite (SiO2~Al~O3
ratio between 10-50) catalyst at a compatible space
velocity; and (c) sequentially exposing the effluent from
the zeolite catalyst to an oxidation catalyst.
The zeolite is of the transition metal exchanged
type; the transition metal can be selected from the group
consisting of copper, cobalt, nickel, chromium, iron,
manganese, silver, zinc, calcium, and compatible mixtures
thereof.
The compound is preferably propanol but can be a
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water soluble, low molecular weight, at least two carbon
atom containing hydrocarbon, selected from the group
consisting of alcohols, aldehydes, ketones, and ethers.
The oxidation catalyst is arranged advantageously to have
a space velocity of 20-80 K hr 1. The compound is
sprayed to provide a reductant/NOx ratio of about 3-5
and a HC concentration in the emissions of about 4800 ppm.
The oxidation catalyst can be of the Pt/A12O3
type or may ba a 1% Pd/10% La2O3/A1203 formulation.
The method results in an enhancement of
conversion efficiency at .5% 2 for N0x of at least
42%, for HC of at least 57%, and for C0 of at least 12%.
Flow reactor studies were carried out to
corroborate the scope of this invention. Catalyst
samples were prepared using a cylindrical cordierite
monolithic substrate (400 cells/inch2, 1" diameter,
1.5" length) coated with 17% by weight of a washcoat
consisting of 85% ZSM5 zeolite and 15% alumina. The ZSM5
had a silica/alumina ratio of 30. This sample was
ion-exchanged in a 0.05M copper-acetate solution
overnight, washed in distilled water, and then calcined
in 5% oxygen. The samples were analyzed by x-ray
fluorescence and found to have a total copper loading of
1.6% by weight.
Two sets of experiments were conducted. The
first series consisted of characterizing the sxtent of
reaction between N0 and various oxygenated hydrocarbons,
and comparing these to the NO-propene reaction used as a
baseline test. The second series of tests characterized
the effect of oxygen on both the NO-propene and the
N0-propanol reactions.
For the first series, the sample was tested in a
quartz flow reactor under the following steady-state
conditions: space velocity 50,000 hr ; temperature
482C; base gas blend 1~% CO2, 10% X20, 3.9% 2'
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450 ppm NO, reductant (stoichiometric equivalent of 1620
ppm C3H6), balance N2 (no SO2 present).
As shown in Table I, the reductants included a
variety of alcohols, ethers, aldehydes, and ketones.
Baseline runs using 1620 ppm C3H6 (not an oxygenated
hydrocarbon) as the reductant were used for calibration
to confirm that no change in activity had occurred
throuyhout the experiment. When using other
hydrocarbons, the concentration was adjusted so as to be
the stoichiometric equivalent of this concentration of
propylene. The oxygenated compounds selected, all water
soluble, were injected from aqueous solutions into the
gas stream, as a steam/oxygenate mixture. Downstream of
the reaction, water vapor was e~tracted by two condensers
before sample gases entered the analytical train, and in
the condensation process the unreacted water soluble
oxygenates were likewise trapped out. For this reasons, `
it was not possible to analyze the post-catalyst
oxygenate concentrations and hydrocarbon conversions are
therefore given for the baseline C3H6 runs only.
Results given in Table I show that low molecular weight,
water soluble HC's tested with two or more carbon atoms
all provided a satisfactory conversion efficiency for
NOX, at an extremely high oxygen content, which was
within 15% of the propylene reductant.
The second series of tests were also conducted
on the above flow reactor under the following
` steady-state conditions: space velocity 50,000 hr 1;
temperature 482C; base gas blend 12% CO2, 10% H~O,
450 ppm NO, reductant (either 1620 ppm propanol or 1620
ppm C3H6), balance N2 (no SO2 present). Oxygen
concentration was varied between approximately 0-4%.
Results are given in Figures 1 and 2.
`` Water soluble, partially oxygenated reductants
were employed; representative compounds of the most
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common classes: alcohols--methanol, ethanol, propanol;
aldehydes--acetaldehyde; ketones--acetone, methyl ethyl
ketone; ethers--1,4-dio~ane. The conditions described in
the experimental section were dictated by the perceived
conditions of practical exploitation, either in use for
stationary source NOX abatement or for lean-burn
combustion automotive use, hence the large oxygen excess
of 3.9%. This amount of oxygen exceeded by approximately
two orders of magnitude the NO to be reduced. The
concentration of reductant was determined empirically at
a level where its further increase does not affect NO
conversion. This is a bit more than a thirty-fold excess
needed to reduce the NO and, conversely, the
oxygen/reductant ratio is appro~imately five. The
conversion of the reductant, as seen in the case of
propylene, is 95%. The conversion in the case of the
partially oxygenated reductants was not measured, but a
similar temperature rise of approximately 3SC measured
at the catalyst inlet attests that it is comparable to
that of propylene. The four base runs with propylene are
quite reproducible both with respect to its own oxidation
and to the NO conversion.
None of the partially o~idized reductants were
quite as proficient in NO conversion as propylene itself
at very large oxygen excess, although propanol came very
close (34% versus an average of 39% for the four runs
with propylene). Methanol was completely inactive for NO
reduction. The temperature rise when using methanol was
comparable to that of other reductants showing that it
was itself oxidized by the o~ygen present. All the other
compounds with more than two carbon atoms gave ~O
conversions of 25-30%.
Data from these experiments are plotted in
Figures 1 and 2. NOX and hydrocarbon conversions are
based on the disappearance of the respective species, and
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; conversion to CO is defined as the percentage of the
inlet carbon in the reductant. Propylene is used in
Figure 1, and propanol in Figure 2. Figure 1
demonstrates the initial acceleration of the N0~
reduction reaction by the addition of small amounts of
oæygen. Figure 2, however, shows the exact opposite
behavior in the NOX reduction. In the case of
propanol, the initial addition of small concentrations of
oxygen drastically inhibits the reduction of NOX. This
is not unreasonable if the oxygenated species is indeed
an active intermediate in the NOX reduction over
Cu-ZSM5. With the addition of further oæygen, the
difference between propylene and propanol becomes less.
At a large excess of oxygen (>3.0%), the selectivity of
~æ reduction is somewhat subdued by oxygen in the case
of the partially oæygenated reductant/propanol. The data
indicate conclusively that the use of water-soluble
reducing agents provides a superior mode of operation
under certain automotive operating conditions, i.e.,
eæcess 2 of .1-.5%.
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TABLE I
_
¦ Reductant Reductant inReductant out % NO in NO out %
l (as ppm C) (as ppm C) Conv. ppm ppm Conv.
~ ~ __ I_ _~ __
¦ C3H6 4790 65 98.7 454 281 38.1 l
. I
¦ Methanol 4860 _ _ 480 479 0.2
¦ Ethanol ¦ 4860 _ _ 470 354 24.7 l
~_ I__ __ ~_ __ __ _I_ I
C,H6 4941 162 96.7 470 300 36.2
I . l ' I
¦ Propanol 4860 _ 486 322 33.7
~ I I .
¦ Acetaldehyde 5832 _ _ 490 351 28.3
~ I____ _ __ __ ~_ .
C3H6 4957 297 94.0 477 273 42.8
¦ Acetone 5468 _ _ 480 342 28.7
l l _ l
¦ Methyl Ethyl Ketone 5303 459 331 27.8
~: I_ I_ __ __ I_ __ _
¦ C~H6 4860 110 93.2 450 270 40.0
I I
¦ 1,4-Dioxa~e ¦ 5832 _ _ L~ 35t 25.7
, / ~ote-successive day's runs are grouped within thick horizontal lines.
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