CA2079176A1 - Selective reduction of nox - Google Patents

Abstract

Abstract of the Disclosure A catalyst system for the selective reduction of gaseous NOx in the presence of excess oxygen, having a copper-containing ZSM5 zeolite to which a NOx-containing gas is exposed, and means for introducing a metered volume of finely dispersed, water soluble, oxygen-containing organic compound (alcohol, aldehyde, ketone, ether) into the NOx-containing gas prior to exposure to the Cu-ZSM5 zeolite.
Also, a method of treating automotive exhaust gas emissions, having an excess of oxygen, by the steps of: (a) introducing a water soluble, oxygenated organic compound (steam/oxygenate mixture) into the exhaust gas emissions as a reductant and at a location closely adjacent the source of the exhaust gas generation (in an appropriate amount with respect to the NOx); (b) substantially immediately exposing the reductant/emission mixture to a transition metal-exchanged high silica ZSM5 zeolite (SiO2/A12O3 of between 10-50) catalyst at a space velocity in the range of 20-80 K hr-l; and (c) sequentially exposing the effluent of such zeolite catalyst to an oxidation catalyst.

Description

2~7~

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 :~
- :.

~7~1~6 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 , .

!

~079~ 7~

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 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 .

. .
,. " ,, ~37~ 76 ` 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 .~ . . .
, -.. ..

~7~7~

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 :

~7~ 7~

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 ~79~76 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' .. : . , ~ ;~ ; ., .

::

~79~76 450 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 , ' 9 2 ~

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 .

..

2~7~17~

; 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%.
;

' 35 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.

. . .

~ :.

. ,:, :. . .. ..
: ~
. . .
"'.'' ": .~ ' ' :

Claims (23)

1. A catalyst system for selective reduction of gaseous NOx in the presence of excess O2, comprising:
(a) a copper ion-exchanged ZSM5 zeolite to which a NOx-containing gas is exposed; and (b) means for introducing a metered liquid volume of distributed water soluble, oxygen-containing organic compound into such gas prior to exposure to said zeolite.

2. The system as in claim l, in which said oxygen-containing organic compound, introduced by means (b), is selected from a group consisting of alcohols, aldehydes, ketones, and ethers, provided they preferably produce a minimum conversion efficiency of at least 40%
when excess °2 varies between .l-.5%, of at least 35%
when excess °2 is .5-1.5%, and at least an average of 30% when °2 is greater than 1.5%.

3. The system as in claim 2, in which said compound is a low molecular weight compound selected from said group.

4. The system as in claim l, in which said NOx-containing gas additionally contains combustion by-products.

5. The system as in claim 4, in which said gas contains NOx in an amount of about 400 ppm.

6. The system as in claim l, in which said zeolite has an SiO2/A1203 ratio of 10-1000, and the copper is exchanged to a substantially maximum amount.

7. The system as in claim 1, in which said zeolite contains copper in a weight amount of 1-8%.

8. The system as in claim 1, in which said means (b) comprises an injector of a finely sprayed aqueous solution containing said oxygen-containing organic compound, said solution being introduced to provide a ratio of oxygen/reductant of about 5.

9. The system as in claim 1, in which said means (b) introduces said compound in a reductant to NOx ratio of about 3-5.

10. The system as in claim 1, in which said means (b) draws said liquid compound from an ambient pressure storage canister.

11. The system as in claim 1, in which said NOx-containing gas is the emission stream from a fossil-fueled internal combustion engine, and means (b) controls the injection of said water soluble organic compound as a function of the variance of NOx in such stream in a predetermined ratio.

12. The system as in claim 11, in which said means (b) control is carried out by use of a NOx sensor in said stream.

13. The system as in claim 1, in which said oxygen-containing compound is propanol.

14. The system as in claim 1, in which said oxygen-containing organic compound is a low molecular weight water soluble hydrocarbon selected from within the group consisting of alcohols, aldehydes, ketones, and ethers.

15. A method of treating automotive exhaust gas emissions from a lean-burn engine carrying excess oxygen, the treatment providing for three-way conversion, comprising the steps of:
(a) distributing a liquified, oxygenated organic compound into said emissions as a reductant and at a location adjacent the source of emission generation;
(b) substantially immediately exposing said reductant/emission mixture to a transition metal-exchanged high silica ZSM5 zeolite (SiO2/A12O3 of between 10-50) catalyst at a compatible space velocity; and (c) sequentially exposing the effluent from said zeolite catalyst to an oxidation catalyst.

16. The method as in claim 15, in which said reductant is distributed in a ratio to NOx in said emission of about 3-5.

17. The method as in claim 15, in which said compound is water soluble and is introduced at a predetermined metered volume proportionate to the variance of NOx in said emissions.

18. The method as in claim 17, in which said liquified compound is sprayed.

19. The method as in claim 15, in which said compound of step (a) is partially oxygenated, low molecular weight compound selected from within the group consisting of alcohols, aldehydes, ketones, and ethers.

20. The method as in claim 15, in which step (c) is carried out at a space velocity of 20-80 K hr-1

21. The method as in claim 15, in which the method results in a conversion efficiency of NOx in said emissions of at least 42% when O2 is .1-.5%.

22. The method as in claim 15, in which said water-soluble compound is propanol.

23. The method as in claim 15, in which step (a) introduces the reductant as a steam/organic oxygenate mixture.