Title: Hydrothermally stable catalyst for improved lean NOx reduction
Abstract: Certain metal-exchanged SUZ-4 zeolites have been prepared that have catalytic activity for the reduction of NOx in the exhaust of a hydrocarbon or alcohol fueled engine operated under fuel lean conditions. Initially the SUZ-4 zeolite contains alkali metal cations such as Li+, Na+, K+ and/or Cs+. These alkali metal cation-containing zeolites are partially exchanged with at least one of copper (II), silver (I), iron (III) or cobalt (II) ions. The resulting partially exchanged SUZ-4 zeolites display such activity and are stable under extreme hydrothermal aging conditions.
Patent Number: 6,936,562 Issued on 08/30/2005 to Cho, et al.
| Inventors:
|
Cho; Byong Kwon (Rochester Hills, MI);
Blint; Richard J. (Shelby Township, MI);
Subbiah; Ayyappan (Warren, MI)
|
| Assignee:
|
General Motors Corporation (Detroit, MI)
|
| Appl. No.:
|
654800 |
| Filed:
|
September 4, 2003 |
| Current U.S. Class: |
502/64; 502/60 |
| Intern'l Class: |
B01J 029/06 |
| Field of Search: |
502/60,66,74,64
|
References Cited [Referenced By]
U.S. Patent Documents
| 5118483 | Jun., 1992 | Barri.
| |
| 6514470 | Feb., 2003 | Ott et al.
| |
| 6645448 | Nov., 2003 | Cho et al.
| |
| Foreign Patent Documents |
| 0 706 984 | Apr., 1996 | EP.
| |
Other References
J. W. Kim, J. E. Yie, Synthesis of SUZ-4 and its application with Lean-Burn deNOx
Catalyst, Mar. 6, 2002.
M. A. Asensi, M. A. Camblor, A. Martinez, Zeolite SUZ_4: reproducible synthesis
. . . , Microporous and Mesoporous Materials, 1999, pp. 427-436, Elsevier Science
B. V.
|
Primary Examiner: Johnson; Christina
Attorney, Agent or Firm: Marra; Kathryn A.
Parent Case Text
This application is a divisional of application Ser. No. 09/982,583, filed Oct.
18, 2001 now U.S. Pat. No. 6,645,448.
Claims
1. A NOx reduction catalyst for a hydrocarbon-fueled automotive engine operated
at a fuel lean air-to-fuel ratio, said catalyst consisting essentially of a cation
exchanged SUZ-4 zeolite, said zeolite having a Si/Al ratio of 5.1-6 and having
potassium cations that have been partially exchanged with Cu (II).
2. The NOx reduction catalyst as recited in claim 1 in which the metal cation
content consists essentially of 29 to 42% copper ions on the ion exchange capacity
basis of said zeolite, and the balance said potassium cations.
Description
TECHNICAL FIELD
This invention pertains to the catalytic reduction of nitrogen oxide and dioxide
(collectively NOx) in the exhaust gas from a lean burn hydrocarbon-fueled engine
or under similar oxygen and water containing atmosphere. More particularly, this
invention pertains to the use of a hydrothermally stable zeolite in cation-exchanged
form for such purpose.
BACKGROUND OF THE INVENTION
In order to further improve the fuel efficiency of hydrocarbon fuel engines there
is interest in operating the engine in a fuel-lean combustion mode. For gasoline
engines this means introducing an air/fuel mixture at a ratio of about seventeen
to twenty three parts by weight of air per part of gasoline. For diesel engines
the air to fuel mass ratio is even higher. The purpose of fuel lean operation is
to obtain more complete combustion of the fuel.
Attractive as the lean-burn engines have become lately for their superior
fuel efficiency, there remains a major technical barrier to the automotive application
of lean-burn engine technology. It is associated with NOx emission in the engine
exhaust. The exhaust gas from a lean-burn gasoline engine is typically at a temperature
of 300° to 600° C. during warmed up engine operation. And the exhaust
contains water, small amounts of carbon monoxide and unburned hydrocarbons (e.g.,
ethylene), nitrogen, and nitrogen oxides (NO and NO
2). The challenge
is to promote the reduction of NOx in this chemically oxidizing environment.
The traditional three-way catalysts while active for NOx reduction under stoichiometric
exhaust conditions, are not effective in reducing NOx under highly oxidizing conditions
prevailing in the lean-burn engine exhaust. Lean-NOx reduction technologies currently
available are not sufficiently effective to meet future stringent emission standards
either. This has prompted intensive and extensive R&D activities around the world
for improved lean-NOx reduction technology.
Among a few different approaches for lean-NOx reduction, the selective catalytic
reduction of NOx using unburned hydrocarbons (HC-SCR) as reductants has been attracting
the most attention. There are quite a few known lean-NOx reduction catalysts for
the HC-SCR process. Among those reported in the literature, Cu/ZSM-5 zeolite is
probably the most studied catalyst for high temperature applications, whereas Pt/ZSM-5
is for low temperature applications. In most lean-NOx catalysts, zeolites are used
as catalyst support on which the active metals are ion exchanged. Among many different
zeolites, the ZSM-5 zeolites with high silica content have been preferentially
used for lean-NOx catalysts. Unfortunately, however, all those catalysts suffer
from the combination of the narrow effective operating temperature window and insufficient
catalytic activity and hydrothermal stability.
All zeolite-based catalysts, Cu/ZSM-5 in particular, have major problems due
both to hydrothermal degradation and negative sensitivity towards water vapor and
SO
2. In general, the permanent loss of activity has been attributed
by investigators to (a) degradation of the support, (b) irreversible loss of Cu
2+
from the zeolite framework or (c) combination of the above. The Cu
1+
is known to be the active catalytic site for both NO decomposition and NO reduction
with hydrocarbon. The inter-conversion between Cu
1+ and Cu
2+
depends on the reaction conditions including temperature and the types of reductant.
Hydrothermal de-alumination of the zeolite framework has been a major issue in
the deactivation of the catalyst. It appears that deactivation is mainly caused
by migration of Cu
2+ ions to locations inside ZSM-5 where their reduction
to Cu
1+ is more difficult. The above mentioned studies clearly reveal
that Cu/ZSM-5 deactivates substantially even under relatively mild conditions and
indicate that a dramatic increase in hydrothermal stability is required for the
catalysts if they are to be used in the automotive application. Thus, the search
continues for better lean-NOx catalysts, which requires both more stable supports
and more active catalytic chemical ingredients.
Accordingly, it is an object of this invention to provide a stable and
effective catalyst for reduction of NOx in a lean burn exhaust such as from a hydrocarbon-fueled
automotive vehicle engine.
SUMMARY OF THE INVENTION
This invention utilizes certain metal ion exchanged SUZ-4 zeolites to catalyze
the reduction of nitrogen oxides in a high temperature gas mixture also containing
nitrogen, water and small amounts of carbon monoxide and unburned hydrocarbons.
U.S. Pat. No. 5,118,483 to Barri, entitled Crystalline (Metallo) Silicates and
Germanates-SUZ-4 describes the synthesis of a family of materials having a porous
crystalline structure. Some of the synthetic aluminosilicate members (zeolites)
of this family are useful in the practice of this invention.
The suitable SUZ-4 zeolite starting materials are crystalline aluminosilicates.
They have an empirical formula in their dehydrated form of M
2O:Al
2O
3:ySiO
2.
The cation M in the SUZ-4 starting material is preferably an alkali metal cation
selected from Li
+, Na
+, K
+ or Cs
+ and
y has a value such that the ratio of Si to Al is in the range of about 5.1 to 6.
In general, zeolites have complex crystalline structures with pores and/or channels
of specific and uniform dimensions. These structures are largely dependent upon
the synthesis of the zeolite. And retention of these structures during NOx reduction
operating conditions is necessary for stability of a catalyst. Thus the '483 patent
states the process for the preparation of SUZ-4 materials requires the presence
of tetraethylammonium hydroxide or halide or its precursor or reaction product
as a template, and that other nitrogenous materials may be present in the reaction
mixture. These SUZ-4 synthesis components are, of course, in addition to suitable
quantities of alumina and silica precursors and the M cation(s).
SUZ-4 zeolites have been found to be surprisingly stable at temperatures up
to 800° C. in a flowing nitrogen atmosphere containing five percent oxygen
and 2.5 percent water. Since all other known zeolites, including ZSM-5, have experienced
degradation under these extreme hydrothermal conditions it was decided to further
evaluate certain cation exchanged SUZ-4 zeolites as catalysts for the reduction
of nitrogen oxides under like conditions representative of the exhaust of a lean
burn automotive engine.
Potassium SUZ-4 zeolite was synthesized. Copper (II), silver (I), iron
(III) and cobalt (II) ion exchanged SUZ-4 zeolites were prepared, each by the aqueous
ion exchange method. In each case a portion of the potassium ion content in the
SUZ-4 zeolite was replaced with one of these cations. Each of these cation exchanged
materials was tested for catalytic activity in synthetic C
2H
4-NO-O
2
feedstreams (helium background) using a packed bed reactor under steady state conditions
over a wide temperature range from 200 to 650° C.
Each of these cation exchanged SUZ-4 zeolites was effective in reducing nitrogen
oxides in the oxygen containing gas that simulated a lean burn exhaust. The copper
exchanged SUZ-4 material was especially effective in reducing nitrogen oxides over
a broad range of operating temperatures even in the presence of water and/or SO
2.
The optimum range of copper (II) ion exchange was from 29 to 42% of the ion exchange
capacity of the potassium SUZ-4 zeolite. Potassium ions remained as the balance,
58 to 71%, of the cation content.
In addition to their catalytic activity in reducing nitrogen oxides in a hydrocarbon
containing but oxygen rich environment, these cation substituted SUZ-4 zeolites
retained their effectiveness after hydrothermal aging.
Other objects and advantages of the invention will become apparent from a description
of preferred embodiments which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a graph of data showing NO conversion over a temperature range from
300 to 650° C. for 2.3 weight % Cu/SUZ-4, 1.3% Fe/SUZ-4, 5.9% Ag/SUZ-4 and
0.11% Co/SUZ-4.
FIG. 1B is a graph of data showing NOx conversion over a temperature range from
300 to 650° C. for 2.3 weight % Cu/SUZ-4, 1.3% Fe/SUZ-4, 5.9% Ag/SUZ-4 and
0.11% Co/SUZ-4.
FIG. 1C is a graph of data showing ethylene conversion over a temperature range
from 300 to 650° C. for 2.3% (by weight) Cu/SUZ-4, 1.3% Fe/SUZ-4, 5.9% Ag/SUZ-4
and 0.11% Co/SUZ-4.
FIG. 2 is a graph of data showing the effect of Cu loading on SUZ-4 for NO and
NOx conversion.
FIG. 3 is a graph of data showing a comparison of NO conversion between fresh/aged
Cu/ZSM-5 and Cu/SUZ-4 catalysts.
FIG. 4 is a graph of data showing the effect of H
2O/SO
2
on fresh 2.3 weight % Cu/SUZ-4 for NO and NOx conversion.
DESCRIPTION OF PREFERRED EMBODIMENTS
Preparation of the Catalysts
SUZ-4 was synthesized according to the following procedure. 33.55 g KOH pellets
(86% KOH, 14% water by weight) were dissolved in 170 g distilled water in a plastic
bottle. 1.888 g of Al pellets were added with the bottle loosely capped (H
2
gas is evolved), and the solution was stirred overnight so that the Al pellets
were completely dissolved to form a clear solution. A second solution containing
91.13 g tetraethylammonium hydroxide (35% aqueous solution by weight), 175 g Dupont
Ludox AS-40 (40% colloidal SiO
2 by weight), and 145.2 g distilled water
was prepared, and then the two solutions were mixed slowly with stirring to form
an easily flowing gel. The gel was transferred to a one-liter autoclave and the
solution was heated to 150° C. under autogenous pressure with stirring for
four days. The product was retrieved by filtration, washed to pH<10, and dried
at 120° C. for two hours. Finally, the product was calcined by first treating
in flowing argon gas by heating at 10° C./min to 550° C., then switching
in 20% O
2 and holding at 550° C. for 4 hours. The elemental analysis
of a typical SUZ-4 zeolite sample resulted in Si/Al ratio of 5.3 and potassium
of 6.6 wt %.
Copper (II)-, silver (I)-, iron (III)-, and cobalt (II)-exchanged SUZ-4 zeolites
were prepared by the aqueous ion exchange method. In the case of silver ion exchange,
special precaution was taken to avoid the interaction of Ag
+ ions with
light. For the exchange of iron, the ion exchange mixture was purged with helium
gas to remove any dissolved oxygen gas prior to the ion exchange, thereby avoiding
the oxidation of iron (II) to iron (III) during the ion exchange process. (Note
that the ion exchange of Fe was carried out in the form of Fe
2+ which
subsequently converted to Fe
3+ on calcination.) The exchange was carried
out under vigorous stirring, using a diluted aqueous solution of copper acetate,
silver nitrate, iron (II) chloride or cobalt sulfate respectively in a solid-to-liquid
ratio of 1 g/100 ml at room temperature for 24 hours. The ion-exchanged zeolites
were then filtered and washed thoroughly with de-ionized water at room temperature
before drying at 120° C. overnight. The dried ion-exchanged zeolites were
then calcined in air at 500° C. for 6 h. The benchmark 2.3 wt % Cu/ZSM-5 was
obtained by copper acetate ion exchange on Na-ZSM-5 (obtained from the PQ Corporation)
followed by drying at 120° C. and calcination at 500° C. The standard
aging was done by treating the sample at 800° C. for 4 hours with a flowing
gas mixture containing 5% oxygen and 2.5% water vapor in nitrogen.
Testing and Evaluation
Hydrothermal Stability
Magic angle spinning (MAS) NMR experiments were done for fresh and aged ZSM-5,
SUZ-4 and Cu/SUZ4 materials. De-alumination can easily be detected by measuring
the loss of tetrahedral aluminum from the framework using MAS NMR measurements.
For these NMR experiments, each zeolite sample was spread onto a thin layer in
an aluminum drying pan and re-hydrated in a humidifier at 100% RH at atmospheric
pressure for 48 hours. Solid state
27Al MAS NMR spectra were obtained
in 400 MHz Bruker instrument using a direct polarization pulse sequence (automation
program "ZG"). About 3,000 to 10,000 scans were signal-averaged to produce a spectrum.
Each sample was first examined neat and then subsequently examined after addition
of ˜2 wt % aluminum nitride (AlN) as an internal standard. In order to reduce
the effects of artifacts due to MAS spinning speed, all of the analyses were run
using the same spinning speed (10 KHz). In addition, a previously prepared "reference"
sample made with bulk ZSM-5 of known Al content plus AlN was also analyzed. This
"reference" sample was used to calculate the Al saturation coefficient for AlN
under the current spectrometer experimental conditions.
Each sample was analyzed twice, using two different experimental methods. The
first method was an attempt to collect data in "quantitative" mode (i.e., with
no saturation). Thirty-two scans were collected with a delay time of 2 minutes
between scans. No dummy scans were used with this method. The second method is
a "non-quantitative" mode, which depends on calibrating the saturation coefficient
for AlN. A delay time of 1 second was used, with 100 dummy scans introduced to
establish equilibrium saturation conditions prior to collection of the analytical
data. The power setting used was PL1=2.0 dB with a 90 degree pulse (P1=2.5 μsec).
With respect to the fresh and aged ZSM-5, SUZ-4 and Cu/SUZ-4 materials, chemical
shifts of 60 ppm and 0 ppm were assigned to the tetrahedral and octahedral aluminum respectively.
The NMR spectra showed a slight decrease (˜29% loss) in the MAS NMR tetrahedral
aluminum peak intensity between the fresh and aged SUZ-4, with no octahedral aluminum
peaks. The fresh and aged 2.3 wt. % Cu/SUZ-4 catalysts did not lose any tetrahedral
Al. However, the aged ZSM-5 lost 65% of its tetrahedral Al compared to the fresh
ZSM-5 material.
XRD patterns of the fresh/aged SUZ-4 and Cu/SUZ-4 were prepared. They showed
no detectable change in the background due to degradation of the material as well
as no detectable formation of CuO in the XRD patterns of the fresh/aged Cu/SUZ-4
catalysts. All the four XRD patterns could be indexed in orthorhombic crystal system.
The cell parameters are for the fresh SUZ-4: a=18.914, b=14.246, c=7.443 Å,
aged SUZ-4: a=18.918, b=14.233, c=7.452 Å, fresh 2.3 wt % Cu/SUZ-4: a=18.858,
b=14.198, c=7.442 Å and aged 2.3 wt. % Cu/SUZ-4: a=18.855, b=14.201, c=7.490
Å. The XRD results on the fresh/aged SUZ-4 and Cu/SUZ-4 materials reveal
that there were no measurable structural changes on hydrothermal aging. These results
were surprising considering the instability of other known zeolites like Cu/ZSM-5.
Catalytic Activity
The NO reduction activity of the catalyst was measured in laboratory C
2H
4-NO-O
2
feedstreams (He background) using a packed-bed reactor under steady-state conditions.
Catalytic activity was typically characterized by monitoring reactor outlet concentrations
as a function of temperature (increased from 200 to 600° C.) in a feedstream
of fixed composition. Each catalyst sample powder was compressed into a disk using
a die at 10-ton pressure, annealed in air at 500° C. for 20 h. Each catalyst
disk was then crushed, ground and screened to 70-80 mesh sizes for use in the reactor experiments.
The reactor was made of 0.635 cm (o.d.) quartz tube packed with the catalyst
particles. Before each experimental run for the activity measurement, the catalyst
was pre-treated with the reaction mixture for 2 h at 500° C. The reactor temperature
was measured at the inlet of the catalyst bed and controlled electronically with
a typical precision of +/-1° C. The temperature rise within the reactor was
minimal (<2° C.) under all experimental conditions, indicating that the gas-phase
temperature of the reactor remained essentially isothermal. The gas flow rate through
the reactor was measured and controlled by electronic mass flow controllers, with
the total gas flow rate fixed at 50 cm
3/min. The effluent gas analysis
was made with the Agilent M-Series Micro-GC equipped with a Mol Sieve 5A column
(for the analysis of N
2, O
2, CO) and PoraPlot Q column (for
the analysis of C
2H
4, N
2O, CO
2). N
2O
production was negligible in the present study. A chemiluminescence analyzer was
used to measure the concentrations of NO and NOx (NO+NO
2).
The amount of Cu/SUZ-4 was adjusted to have the same amount of Cu as in 2.3 wt.
% Cu/ZSM-5 (benchmark) samples for the comparison purposes. Among the SUZ-4 samples,
the amount of metal concentration was adjusted for each experiment such that every
sample had the same amount of metal exchanged. The ion exchange level is calculated
assuming that one copper (II) ion can be exchanged with two potassium (I) or sodium
(I) ions or other alkali metal cations.
In order to study the influence of metals on lean-NOx catalysis, various metal-ions
such as Cu (1.9-5.5 wt %), Ag (5.9 wt %), Co (0.11 wt. %) and Fe (1.3 wt. %) were
ion-exchanged onto SUZ-4 in place of a portion of the potassium ions. The level
of exchange was in the range of 24-70, 23, 28 and 2% for Cu, Ag, Fe and Co respectively.
(For equitable comparison of catalytic activity, the level of ion exchange for
Ag, Fe and Co was aimed at 29-42% of the total exchange capacity which exhibited
the maximum NOx conversion performance for Cu. However, the actual exchange levels
obtained were slightly off the target level; 23% for Ag and 28% for Fe. The actual
exchange level of Co (2%) could not even come close to the target level.)
FIG. 1A shows the NO conversion as a function of temperature for different cations
exchanged. A major difference in the NO conversion performance between 2.3 wt %
Cu/SUZ-4 and the other metal-exchanged SUZ-4 is in the reaction lightoff temperature
(RLT) as defined as the temperature required for 50% conversion: The lightoff temperature
for NO conversion over Cu/SUZ-4 is around 325° C., while it is 450-475°
C. for the Ag, Co and Fe-exchanged SUZ-4 catalysts. Another noticeable difference
is that the NO conversion activity of Cu/SUZ-4 maintains a much wider temperature
(300-600° C.) window compared with the other metal-containing SUZ-4 catalysts.
Interestingly, Fe, Co and Ag can be more active than Cu for NO conversion in the
high temperature range above 500° C.
FIG. 1B compares the NOx (NO/NO
2) conversion. Similar to its NO conversion
performance, the NOx conversion performance of 2.3 wt. % Cu/SUZ-4 maintains a wide
temperature window ranging from 350 to 600° C., with a maximum conversion
of 68% at 450° C. Surprisingly, the NOx conversion performance of the other
metal-exchanged SUZ-4 catalysts is almost negligible except for Co around 550° C.
Presented in FIG. 1C is the ethylene (hydrocarbon, HC) conversion. The
lightoff temperature for HC conversion over Cu/SUZ-4 is 325° C., much lower
than that over the other catalysts, which is consistent with the earlier observations
for both NO and NOx conversions in FIGS. 1A and 1B.
Different amounts of Cu were loaded by successive aqueous ion exchange.
Chemical analyses show that 1.9, 2.3, 2.8, 3.3 and 5.5 wt. % Cu loadings (corresponding
to 24, 29, 36, 42 and 70% ion exchange level) were obtained on SUZ-4.
Activity measurements with these samples (see FIG. 2) show that increasing
the copper loading on SUZ-4 lowers the lightoff temperature for NO conversion.
The lightoff temperature is around 475° C. for 1.9 wt % Cu/SUZ-4, while it
is around 325° C. for both 2.3 wt % and 2.8 wt % Cu/SUZ-4. The lightoff temperature
does not change beyond 2.3% copper loading. For the loading of 2.3-3.3 wt % (29-42%
ion-exchange capacity) copper on SUZ-4, the activity is maintained over a wide
temperature window (i.e., 300-600° C.) reaching a maximum NO conversion of
70-84% at around 450° C. In the 5.5 wt % (70% ion-exchange) Cu/SUZ-4 catalyst,
the activity drops after reaching a maximum of 55% NO conversion at around 350°
C. Clearly, there is an optimum level of metal exchange (and metal loading) in
the zeolite SUZ-4. For copper containing SUZ-4 the optimum level of ion exchange
lies between 29 and 42% of the ion-exchange capacity of SUZ-4.
FIG. 3 compares NO conversion activity of the fresh and aged 2.3 wt % Cu/ZSM-5
and 2.8 wt % Cu/SUZ-4 catalysts. For the Cu/ZSM-5 catalyst, the maximum conversion
is 64% at 350° C. Aging the Cu/ZSM-5 catalyst severely depresses the NO conversion
activity, reaching a maximum of only 24% at 450° C. On the other hand, the
fresh Cu/SUZ-4 catalyst has higher activity than the fresh Cu/ZSM-5 especially
in the high temperature regime above 350° C. The fresh Cu/SUZ-4 catalyst maintains
a wide temperature window from 350 to 600° C. with a maximum NO/NOx conversion
of 84% at 450° C. Interestingly, aging the Cu/SUZ-4 catalyst decreases the
low temperature activity (i.e., <500° C.) while increasing the high temperature
activity. For both the fresh and aged, it is clear that Cu/SUZ-4 is much more active
for NO conversion than Cu/ZSM-5.
The combined effect of both SO
2 and H
2O on NO and NOx conversions
for the fresh 2.3 wt % Cu/SUZ-4 as a function of temperature is shown in FIG.
4.
The effect of H
2O/SO
2 resembles very much the effect due
to either H
2O or SO
2 alone on the catalyst; the NO/NOx conversion
decreases by 25% from 68% to 43% at 500° C. in the presence of both H
2O
and SO
2.
Thus, a family of new metal-exchanged SUZ-4 zeolite catalysts have been provided
for the chemical reduction of NOx in an hot oxidizing gas stream such as the exhaust
from a hydrocarbon fuelled engine operated under fuel lean conditions. Suitable
members of the family include Co/SUZ-4, Cu/SUZ-4, Fe/SUZ-4 and Ag/SUZ-4 zeolites.
Each has the capability of reducing NO in a hydrocarbon and oxygen containing gas.
All of these materials are stable under extreme hydrothermal conditions. The copper
exchanged SUZ-4 zeolites are particularly effective in reducing all NOx over a
broad temperature range even when the NOx containing gas also contains water and
sulfur dioxide.
The invention has been described in terms of certain preferred embodiments. But
other forms of the invention could be devised by those skilled in the art and the
scope of the invention is to be limited only by the following claims.
*
