Note: Descriptions are shown in the official language in which they were submitted.
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Fe-BEA/Fe-MFI MIXED ZEOLITE CATALYST AND PROCESS FOR TREATING NOX IN GAS
STREAMS USING THE SAME
TECHNICAL FIELD
The present invention relates to a catalyst which is preferably for use in
selective catalytic
reduction (SCR), as well as to an exhaust gas treatment system comprising said
catalyst, and
to a process for the treatment of a gas stream comprising NOR. In particular,
the present in-
vention is concerned with a method of catalyzing the reduction of nitrogen
oxides, and espe-
LO cially with the selective reduction of nitrogen oxides with ammonia in the
presence of oxygen,
using metal-promoted zeolite catalysts.
BACKGROUND
L5 The emissions present in the exhaust gas of a motor vehicle can be divided
into two groups.
Thus, the term "primary emission" refers to pollutant gases which form
directly through the
combustion process of the fuel in the engine and are already present in the
untreated emis-
sion before it passes through an exhaust gas treatment system. Secondary
emission refers to
those pollutant gases which can form as by-products in the exhaust gas
treatment system.
The exhaust gas of lean engines comprises, as well as the customary primary
emissions of
carbon monoxide CO, hydrocarbons HC and nitrogen oxides NON, a relatively high
oxygen
content of up to 15% by volume. In the case of diesel engines, there is
additional particulate
emission in addition to the gaseous primary emissions, which consists
predominantly of soot
residues, with or without organic agglomerates, and originates from partially
incomplete fuel
combustion in the cylinder.
In diesel engine applications, the use of specific diesel particulate filters
is unavoidable for the
removal of the particulate emissions. Furthermore, complying with the
emissions limits pre-
scribed by legislation in Europe and the United States requires nitrogen oxide
removal from
the exhaust gas ("denitrification"). Thus, although carbon monoxide and
hydrocarbon pollutant
gases from the lean exhaust gas can easily be rendered harmless by oxidation
over a suitable
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oxidation catalyst, the reduction of the nitrogen oxides to nitrogen is much
more difficult owing
to the high oxygen content of the exhaust gas stream.
Known methods for removing nitrogen oxides from exhaust gases are firstly
methods using
nitrogen oxide storage catalysts (NSCs) and secondly methods for selective
catalytic reduc-
tion (SCR) by means of ammonia over a suitable catalyst, SCR catalyst for
short.
The cleaning action of nitrogen oxide storage catalysts is based on the
nitrogen oxides being
stored in a lean operating phase of the engine by the storage material of the
storage catalyst,
LO predominantly in the form of nitrates. When the storage capacity of the NSC
is exhausted, the
catalyst has to be regenerated in a subsequent rich operating phase of the
engine. This
means that the nitrates formed beforehand are decomposed and the nitrogen
oxides released
again are reacted with the reducing exhaust gas components over the storage
catalyst to give
nitrogen, carbon dioxide and water.
L5
Since the implementation of a rich operating phase in diesel engines is not
straightforward
and the establishment of the rich exhaust gas conditions required for
regeneration of the NSC
frequently entails auxiliary measures such as fuel postinjection into the
exhaust gas line, the
alternative SCR method is preferably used for denitrification of diesel motor
vehicle exhaust
?O gases. In this method, according to the engine design and construction of
the exhaust gas
system, a distinction is made between "active" and "passive" SCR methods,
"passive" SCR
methods involving use of ammonia secondary emissions generated deliberately in
the ex-
haust gas system as a reducing agent for denitrification.
?5 For example, U.S. Pat. No. 6,345,496 61 describes a method for cleaning
engine exhaust
gases, in which repeatedly alternating lean and rich air/fuel mixtures are
established and the
exhaust gas thus produced is passed through an exhaust gas system which
comprises, on
the inflow side, a catalyst which converts NO, to NH3 only under rich exhaust
gas conditions,
while a further catalyst arranged on the outflow side adsorbs or stores NO,,
in the lean ex-
30 haust gas, and releases it under rich conditions, such that it can react
with NH3 generated by
the inflow-side catalyst to give nitrogen. As an alternative, according to
U.S. Pat. No.
6,345,496 131, an NH3 adsorption and oxidation catalyst may be arranged on the
outflow side,
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which stores NH3 under rich conditions, desorbs it under lean conditions and
oxidizes it with
oxygen to give nitrogen and water. Further disclosures of such methods are
known. Like the
use of the nitrogen oxide storage catalysts, however, such "passive" SCR
methods have the
disadvantage that one of their essential constituents is the provision of rich
exhaust gas con-
ditions, which are generally required for in situ generation of ammonia as a
reducing agent.
Compared to this, in "active" SCR methods, the reducing agent is metered into
the exhaust
gas line from an addition tank carried in the vehicle by means of an injection
nozzle. Such a
reducing agent used may, apart from ammonia, also be a compound readily
decomposable to
LO ammonia, for example urea or ammonium carbamate. Ammonia has to be supplied
to the
exhaust gas at least in a stoichiometric ratio relative to the nitrogen
oxides. Owing to the.
greatly varying operation conditions of the motor vehicles, the exact metered
addition of the
ammonia is not straightforward. This leads in some cases to considerable
ammonia break-
throughs downstream of the SCR catalyst. To prevent secondary ammonia
emission, an oxi-
L5 dation catalyst is usually arranged downstream of the SCR catalyst, which
is intended to oxid-
ize ammonia which breaks through to nitrogen. Such a catalyst is referred to
hereinafter as an
ammonia slip catalyst.
To remove particulate emissions from the exhaust gas of diesel motor vehicles,
specific diesel
?0 particulate filters are used, which may be provided with an oxidation
catalyst-containing coat-
ing to improve their properties. Such a coating serves to lower the activation
energy for oxy-
gen-based particulate burnoff (soot combustion) and hence to lower the soot
ignition tempera-
ture on the filter, to improve the passive regeneration performance by
oxidation of nitrogen
monoxide present in the exhaust gas to nitrogen dioxide, and to suppress
breakthroughs of
?5 hydrocarbon and carbon monoxide emissions.
If compliance with legal emissions standards requires both denitrification and
removal of par-
ticulates from the exhaust gas of diesel motor vehicles, the described
measures for removing
individual pollutant gases are combined in a corresponding conventional
exhaust gas system
30 by connection in series. For example, WO 99/39809 describes an exhaust
aftertreatment sys-
tem wherein an oxidation catalyst for oxidation of NO in NO,, to NO2, a
particulate filter, a me-
tering unit for a reducing agent and an SCR catalyst follow on each other. To
prevent ammo-
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nia breakthroughs, an additional ammonia slip catalyst is generally required
downstream of
the SCR catalyst, and continues the series of catalysts on the outflow side of
the SCR cata-
lyst.
In this respect, both synthetic and natural zeolites and their use in
promoting certain reac-
tions, including the selective reduction of nitrogen oxides with ammonia in
the presence of
oxygen, are well known in the art. Zeolites are aluminosilicate crystalline
materials having
rather uniform pore sizes which, depending upon the type of zeolite and the
type and amount
of cations included in the zeolite lattice, may range from about 3 to 10
angstroms in diameter.
L0
EP 1 961 933 Al, for example, relates to a diesel particulate filter for
treating exhaust gas
comprising a filter body having provided thereon an oxidation catalyst
coating, an SCR-active
coating, and an ammonia storage material. Among the materials which may be
used as the
catalytically active component in the SCR reaction, said document mentions the
use of zeo-
L5 lites selected from beta zeolite, Y-zeolite, faujasite, mordenite and ZSM-5
which may be ex-
changed with iron or copper.
EP 1 147 801 Al, on the other hand, relates to a process for reducing nitrogen
oxides present
in a lean exhaust gas from an internal combustion engine by 5CR using ammonia,
wherein
20 the reduction catalyst preferably contains ZSM-5 zeolite exchanged with
copper or iron. Said
document further concerns an SCR catalyst having a honeycomb substrate and
deposited
thereon a coating containing ZSM-5 zeolite exchanged with iron.
EP 2 123 614 A2 for its part concerns a honeycomb structure containing
zeolites and an inor-
25 ganic binder. In particular, a first zeolite included in said structure is
ion-exchange with a met-
al including Cu, Mn, Ag, and V, and a second zeolite is further included which
is exchanged
with a metal including Fe, Ti, and Co. Regarding the types of zeolites used
for the first and
second zeolite, these include zeolite beta, zeolite Y, ferrierite, ZSM-5
zeolite, mordenite, fau-
jasite, zeolite A, and zeolite L.
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Finally, US 7,332,148 B2 describes a stabilized aluminosilicate zeolite
containing copper or
iron, wherein the stabilized zeolite includes ZSM-5, ZSM-8, ZSM-11, ZSM-12,
zeolite X, zeo-
lite Y, zeolite beta, mordenite, and erionite.
Accordingly, the prior art relates an awareness of the utility of metal-
promoted zeolite cata-
lysts including, among others, iron-promoted and copper-promoted zeolite
catalysts, in partic-
ular for the selective catalytic reduction of nitrogen oxides with ammonia.
Presently, however, increasingly strict legislature with respect to emissions,
and in particular
LO regarding motor vehicle exhaust gas emissions, requires improved catalysts
and exhaust
treatment systems using such catalysts for the treatment thereof. Thus,
exhaust gas emission
legislation in the European Union for exhaust gas emission stage Euro 6 now
requires reduc-
tion of NOX emissions for most passenger cars powered by diesel engines. For
this purpose,
exhaust gas emissions are tested using the New European Driving Cycle (NEDC),
also re-
L5 ferred to as the MVEG (Motor Vehicle Emissions Group) cycle, which is laid
down in Euro-
pean Union Directive 70/220/EEC. One way of meeting this requirement includes
the applica-
tion of SCR catalyst technology to the exhaust gas systems of the vehicles in
question.
As opposed to the old European driving cycle (ECE-15) driving cycle, a
particular feature of
?O the NEDC is that it integrates a so-called extra-urban driving cycle, such
that testing may bet-
ter represent the typical usage of a car in Europe, and, accordingly, the
typical emission pat-
tern linked thereto. More specifically, in the NEDC, the old European driving
cycle ECE-15 is
performed in the time period of 0 to 800 seconds, after which the extra-urban
driving cycle is
conducted in the time period up to 1200 seconds.
5
It is therefore the object of the present invention to provide an improved
catalyst, in particular
for use in selective catalytic reduction, wherein said catalyst is, for
example, better adapted to
the actual emission conditions encountered in motor vehicle use, such as for
example those
encountered in the NEDC.
DESCRIPTION
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In this respect, it has surprisingly been found that according to the present
invention as out-
lined in the following, an improved catalyst may be provided. In particular,
it has unexpectedly
been found that a catalyst comprising zeolites of both the MFI and of the BEA
structure type,
wherein both the MFI-and BEA-type zeolites respectively contain iron, display
clearly im-
proved catalytic properties, in particular when used in SCR applications.
Thus, the present invention relates to a catalyst, preferably for use in
selective catalytic reduc-
tion (SCR), said catalyst comprising
one or more zeolites of the MFI structure type, and
to one or more zeolites of the BEA structure type,
wherein at least part of the one or more zeolites of the MFI structure type
and at least part of
the one or more zeolites of the BEA structure type respectively contain iron
(Fe).
Within the meaning of the present invention, the term "selective calatytic
reduction" abbre-
viated as "SCR" refers to any catalytic process involving the reaction of
nitrogen oxides NO,
with a reductant. In particular, SCR refers to reduction reactions, wherein
NO, is transformed
to a reduction product thereof, which is preferably N2. Regarding the term
"reductant", said
term refers to any suitable reducing agent for the SCR process, wherein
preferably ammonia
and/or any ammonia precursor such as urea and/or. ammonium carbamate is
preferred, urea
being preferably comprised in the ammonia precursor. Even more preferably, the
term "reduc-
tant" refers to ammonia. The term "reductant" may, however, further include
hydrocarbons
and/or hydrocarbon derivatives such as oxygenated hydrocarbons, such as for
example those
which may be found in motor vehicle fuels and/or in motor vehicle exhaust gas,
in particular in
diesel fuel and/or diesel exhaust gas.
?5
According to the present invention, any conceivable zeolite of the MFI or of
the BEA structure
type may be used, respectively, provided that it displays the typical
structural characteristics
of that structure-type. With respect to the one or more zeolites of the MFI
structure, these may
for example comprise one or more zeolites selected from the group consisting
of ZSM-5, [As-
Si-O]-MFI, [Fe-Si-O]-MFI, [Ga-Si-O]-MFI , AMS-1B, AZ-1, Bor-C, Boralite C,
Encilite, FZ-1,
l._Z-105, Monoclinic H-ZSM-5, Mutinaite, NU-4, NU-5, Silicalite, TS-1, TSZ,
TSZ-Ill, TZ-01,
USC-4, USI-108, ZBH, ZKQ-1B, ZMQ-TB, organic-free ZSM-5, and mixtures of two
or more
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thereof. According to preferred embodiments of the present invention, the one
or more zeo-
lites of the MFI structure type include ZSM-5.
Concerning the one or more zeolites of the BEA structure, these may comprise
one or more
zeolites selected from the group consisting of, Beta,
[B-Si-O]-BEA, [Ga-Si-O]-BEA, [Ti-Si-O]-BEA, Al-rich beta, CIT-6,
Tschernichite, pure silica
beta and mixtures of two or more thereof. According to preferred embodiments
of the present
invention, the one or more zeolites of the BEA structure type include zeolite
Beta.
According to embodiments of the present invention which are further preferred,
the one or
more zeolites of the MFI structure type include ZSM-5 and the one or more
zeolites of the
BEA structure type include zeolite Beta, wherein according to particularly
preferred embodi-
ments, the one or more zeolites of the MFI structure type is ZSM-5 and the one
or more zeo-
lites of the BEA structure type is zeolite Beta.
According to the present invention, at least part of the one or more MFI-type
zeolites and at
least part of the one or more BEA-type zeolites respectively contain iron .
With respect to the iron contained in at least part of the one or more MR-type
zeolites and at
least part of the one or more BEA-type zeolites, said metal may respectively
be contained
therein in any conceivable fashion and in any conceivable state. Thus,
according to the
present invention, there is no particular limitation with respect to the
oxidation state of iron
contained in the catalyst, nor with respect to the way in which it is
contained in the respective
type of zeolite. Preferably, however, iron displays a positive state of
oxidation in the respec-
tive zeolite. Furthermore, iron may be contained on the zeolite surface and/or
within the por-
ous structure of the respective zeolite framework. Alternatively or in
addition to being sup-
ported on the zeolite surface and/or within the porous structure thereof, iron
may be included
in the zeolite framework, for example by isomorphous substitution. According
to preferred
embodiments, the iron is supported on the respective zeolite surface and/or
within the porous
structure thereof, and even more preferably both on the respective zeolite
surface and within
the porous structure thereof, According to particularly preferred embodiments
of the present
invention, iron is respectively contained in at least part of the one or more
zeolites of the MFI
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and BEA structure type in a positive oxidation state, wherein said iron is
supported on the
surface of the respective zeolite, including being contained within the porous
structure thereof.
The catalyst according to the present invention may comprise the one or more
zeolites of the
MFI structure type and the one or more zeolites of the BEA structure type in
any conceivable
weight ratio, wherein it is preferred that the weight ratio of the one or more
zeolites of the MFI
structure type relative to the one or more zeolites of the BEA structure type
ranges from 1 : 10
to 10 : 1, more preferably from 1 : 5 to 5 : 1, more preferably form 1 : 2 to
2 : 1, more prefera-
bly from 0.7 : I to 1 : 0.7, more preferably from 0.8 : 1 to 1 : 0.8, and even
more preferably
LO from 0.9:1to1 :0.9.
According to particularly preferred embodiments of the present invention, the
weight ratio of
the MFI-type zeolites to the BEA-type zeolites is approximately 1:1.
According to the present invention, it is preferred that the one or more
zeolites of the MFI
L5 structure type and/or the one or more zeolites of the BEA structure type
respectively comprise
both Al and Si in their frameworks, wherein it is more preferred that both the
zeolites of the
MFI structure type and the zeolites of the BEA structure type respectively
comprise both Al
and Si in their frameworks. Thus, according to the present invention, it is
preferred that one or
more of the zeolites, and more preferably all of the zeolites, comprise both
Al and Si in their
?O respective zeolite frameworks.
With respect to embodiments of the present invention wherein one or more of
the zeolites
comprise both Al and Si in their respective frameworks, said zeolites may in
principle display
any possible ratio of Al to Si. In embodiments of the present invention
wherein one or more
?5 zeolites of the MFI structure type comprise both Al and Si in their
framework, it is however
preferred that the molar ratio of silica to alumina (SAR) in the one or more
zeolites of the MFI
structure type ranges from 5 to 150, more preferably from 15 to 100, more
preferably from 20
to 50, more preferably from 23 to 30, and even more preferably from 25 to 27.
Furthermore, in
embodiments of the present invention wherein one or more zeolites of the BEA
structure type
30 comprise both Al and Si in their framework, it is preferred that the SAR in
the one or more
zeolites of the BEA structure type ranges from 5 to 150, preferably from 20 to
100, more pre-
ferably from 30 to 70, more preferably from 35 to 45, and even more preferably
from 38 to 42.
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According to particularly preferred embodiments of the present invention
wherein one or more
zeolites of both the MFI and the BEA structure type respectively comprise Al
and Si in their
framework, it is further preferred that the SAR in the one or more MFI-type
zeolites ranges
from 5 to 150, and the one or more BEA-type zeolites ranges from 5 to 150,
more preferably
that the SAR in the one or more MFI-type zeolites ranges from 15 to 100, and
the one or more
BEA-type zeolites ranges from 20 to 100, more preferably that the SAR in the
one or more
MFI-type zeolites ranges from 20 to 50, and the one or more BEA-type zeolites
ranges from
30 to 70,more preferably that the SAR in the one or more MFI-type zeolites
ranges from 23 to
30, and the one or more BEA-type zeolites ranges from 35 to 45, and even more
preferably
LO that the SAR in the one or more MFI-type zeolites ranges from 25 to 27, and
the one or more
BEA-type zeolites ranges from 38 to 42.
Regarding the iron respectively contained in the MFI- and BEA-type zeolites,
there is no par-
ticular limitation according to the present invention as to their respective
amounts. It is, how-
L5 ever, preferred according to the present invention, that the amount of iron
(Fe) in the one or
more zeolites of the MFI structure type is comprised in the range of from 0.1
to 15 wt.-%
based on the weight of said one or more zeolites of the MFI structure type,
wherein more pre-
ferably the amount of Fe ranges from 0.5 to 10 wt.-%, more preferably from 1.0
to 7.0 wt.-%,
more preferably from 2.5 to 5.5 wt.-%, more preferably from 3.5 to 4.2 wt.-%,
and even more
?0 preferably from 3.7 to 4.0 wt.-%. Furthermore, it is preferred according to
the present inven-
tion that the amount of iron (Fe) in the one or more zeolites of the BEA
structure type ranges
from 0.05 to 10 wt.-% based on the weight of said one or more zeolites of the
BEA structure
type, wherein more preferably the amount of Fe ranges from 0.1 to 5 wt.-%,
more preferably
from 0.5 to 2 wt.-%, and even more preferably from 1.0 to 1.6 wt.-%. According
to particularly
?5 preferred embodiments of the present invention, the amount of iron in the
one or more MFI-
type zeolites ranges from 0.1 to 15 wt.-%, and the amount of iron in the one
or more BEA-type
zeolites ranges from 0.05 to 10 wt.-%, wherein more preferably the amount of
iron in the one
or more MFI-type zeolites ranges from 1.0 to 7.0 wt.-%, and the amount of iron
in the one or
more BEA-type zeolites ranges from 0.1 to 5 wt.-%, more preferably the amount
of iron in the
30 one or more MFI-type zeolites ranges from 2.5 to 5.5 wt.-%, and the amount
of iron in the one
or more BEA-type zeolites ranges from 0.5 to 2 wt.-%, more preferably the
amount of iron in
the one or more MFI-type zeolites ranges from 3.5 to 4.2 wt.-%, and the amount
of iron in the
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one or more BEA-type zeolÃtes ranges from 0.5 to 2 wt.-%, and even more
preferably, the
amount of iron in the one or more MFI-type zeolites ranges from 3.7 to 4.0 wt.-
%, and the
amount of iron in the one or more BEA-type zeolites ranges from 1.0 to 1.6 wt.-
%.
According to the present invention, the catalyst may be provided in any
conceivable form,
such as by way of example in the form of a powder, a granulate, or a monolith.
In this respect,
it is particularly preferred that the catalyst further comprises a substrate,
onto which the one or
more zeolites are provided. In general, the substrate can be made from
materials commonly
known in the art. For this purpose, porous materials are preferably used as
the substrate ma-
[0 terial, in particular ceramic and ceramic-like materials such as
cordierite, a-alumina, an alumi-
nosilicate, cordierite-alumina, silicon carbide, aluminum titanate, silicon
nitride, zirconia, mul-
lite, zircon, zircon mullite, zircon silicate, sillimanite, a magnesium
silicate, petalite, spodu-
mene, alumina-silica-magnesia and zirconium silicate, as well as porous
refractory metals and
oxides thereof. According to the present invention, "refractory metal" refers
to one or more
L5 metals selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo,
W, and Re. The
substrate may also be formed of ceramic fiber composite materials. According
to the present
invention, the substrate is preferably formed from cordierite, silicon
carbide, and/or from alu-
minum titanate, and even more preferably from cordierite and/or silicon
carbide.
?0 The substrates useful for the catalysts of embodiments of the present
invention may also be
metallic in nature and be composed of one or more metals or metal alloys. The
metallic sub-
strates may be employed in various shapes such as corrugated sheet or
monolithic form.
Suitable metallic supports include the heat resistant metals and metal alloys
such as titanium
and stainless steel as well as other alloys in which iron is a substantial or
major component.
?5 Such alloys may contain one or more of nickel, chromium and/or aluminum,
and the total
amount of these metals may advantageously comprise at least 15 wt.-% of the
alloy, e.g., 10-
25 wt.-% of chromium, 3-8 wt.-% of aluminum and up to 20 wt.-% of nickel. The
alloys may
also contain small or trace amounts of one or more other metals such as
manganese, copper,
vanadium, titanium and the like. The surface or the metal substrates may be
oxidized at high
t0 temperatures, e.g., 1000 C and higher, to improve the resistance to
corrosion of the alloys by
forming an oxide layer on the surfaces the substrates.
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Furthermore, the substrate according to the present invention may be of any
conceivable
shape, provided that it allows for the fluid contact with at least a portion
of the respective one
or more zeolites of the MFI and BEA structure types present thereon.
Preferably, the sub-
strate is a monolith, wherein more preferably the monolith is a flow-through
monolith. Suitable
substrates include any of those materials typically used for preparing
catalysts, and will usual-
ly comprise a ceramic or metal honeycomb structure. Accordingly, the
monolithic substrate
contains fine, parallel gas flow passages extending from an inlet to an outlet
face of the sub-
strate, such that passages are open to fluid flow (referred to as honeycomb
flow through sub-
strates). The passages, which are essentially straight paths from their fluid
inlet to their fluid
outlet, are defined by walls onto which the one or more zeolites of the MFI
and BEA structure
types are respectively disposed, so that the gases flowing through the
passages may contact
them. The flow passages of the monolithic substrate are thin-walled channels,
which can be
of any suitable cross-sectional shape and size such as trapezoidal,
rectangular, square, sinu-
soidal, hexagonal, oval, or circular. Such structures may contain up to 900
gas inlet openings
(i.e., cells) per square inch of cross section, wherein according to the
present invention struc-
tures preferably have from 50 to 600 openings per square inch, more preferably
from 300 to
500, and even more preferably from 350 to 400.
Thus, according to a preferred embodiment of the present invention, the
catalyst comprises a
substrate which is a monolith, and preferably a honeycomb substrate.
According to further preferred embodiments of the present invention, the
substrate is a wall
flow monolith. For these embodiments, the substrate is preferably a honeycomb
wall flow fil-
ter, wound or packed fiber filter, open-cell foam, or sintered metal filter,
wherein wall flow fil-
ters are particularly preferred. As for the equally preferred flow through
monoliths, useful wall
flow substrates have a plurality of fine, substantially parallel gas flow
passages extending
along the longitudinal axis of the substrate. Typically, each passage is
blocked at one end of
the substrate body, with alternate passages blocked at opposite end-faces.
Particularly pre-
ferred wall flow substrates for use in the present invention include thin
porous walled honey-
comb monoliths, through which a fluid stream may pass without causing too
great an increase
in back pressure or pressure across the catalyst. Ceramic wall flow substrates
used in the
present invention are preferably formed of a material having a porosity of at
least 40%, pre-
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ferably from 40 to 70%, and having a mean pore size of at least 5 microns,
preferably from 5
to 30 microns. Further preferred are substrates having a porosity of at least
50% and having a
mean pore size of at least 10 microns.
Thus, according to the present invention, the substrate preferably comprised
in the catalyst is
preferably selected from the group consisting of flow-through substrates and
wall-flow sub-
strates, more preferably from the group consisting of cordierite flow-through
substrates and
wall-flow substrates, and silicon carbide flow-through substrates and wall-
flow substrates.
In general, according to embodiments of the present invention which further
comprise a sub-
strate, the zeolites may be provided thereon in any conceivable fashion,
wherein they are
preferably provided thereon in the form of one or more layers which are
preferably washcoat
layers. In preferred embodiments of the present invention, wherein the
catalyst comprises a
substrate and two or more layers provided thereon, the zeolites may be
provided in said two
or more layers in any possible manner. Accordingly, the present invention
includes, for exam-
ple, such preferred embodiments wherein the zeolites are contained in only a
single of the two
or more layers, as well as embodiments wherein the zeolite is contained in
more than one of
the two or more layers. Preferably, however, the zeolites are contained in a
single layer, irres-
pective of the number of layers present on the substrate.
Thus, according to preferred embodiments of the present invention wherein the
catalyst com-
prises a substrate, it is further preferred that the catalyst comprises one or
more layers, pre-
ferably washcoat layers, provided on the substrate, the zeolites being
contained in one single
layer or two or more separate layers, wherein preferably the zeolites are
contained in one
single layer.
In further embodiments of the present invention comprising a substrate and two
or more lay-
ers provided thereon, wherein the zeolites are contained in more than one of
said layers,
there is no particular limitation as to the distribution of the one or more
zeolites of the MFI and
the BEA structure type among said more than one layers which comprise said
zeolites. Thus,
it is principally possible according to the present invention, that, for
example, the MFI- and
BEA-type zeolites are respectively contained in each of the layers which
contain zeolites, or
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that, alternatively, only part of the layers containing zeolites contain both
MFI- and BEA-type
zeolites. Furthermore, it is possible according to said further embodiments of
the present in-
vention that no single layer contains both MFI- and BEA-type zeolites, said
zeolites being ac-
cordingly contained in separate layers of the catalyst. According to the
present invention it is,
however, preferred that at least one of the layers in such embodiments
contains both MFl-
and BEA-type zeolites, wherein it is even more preferred that each of the two
or more layers
of said embodiments containing the zeolites also contains both the MFI- and
BEA-type zeo-
lites.
LO In principle, the one or more zeolites of the MFI and of the BEA structure
type may be respec-
tively present in the catalyst in any conceivable amount, provided that an
improved catalyst
according to the present invention may be obtained. Thus, either the one or
more zeolites of
the MFI structure type, or the one or more zeolites of the BEA structure type,
or both the one
or more zeolites of the MFl structure type and the one or more zeolites of the
BEA structure
type, may respectively be present in the catalyst in a loading ranging from
0.1 to 5.0 glin3,
wherein their loading preferably ranges from 0.7 to 2.0 glin3, more preferably
from 1.0 to 1.7
g/in3, more preferably from 1.15 to 1.55 glin3, more preferably from 1.25 to
1.45 g/in3, more
preferably from 1.32 to 1.38 g/in3, and even more preferably from 1.34 to 1.36
ghn3. In particu-
lar, the respective loadings of the MFI- and BEA-type zeolites may be
independent from one
another, in the sense that the preferred loading ranges may apply either to
the MFl- or to the
BEA-type zeolites, wherein the loading of the one or more zeolites belonging
to the other
structure type is respectively not particularly limited, and may therefore be
present in any
loading, or may be limited to a different range of loadings. Thus, the present
invention also
comprises embodiments wherein, for example, the loading of the MFI-type
zeolites ranges
from 0.1 to 5.0 g/in3, and the loading of the BEA-type zeolites ranges from
1.34 to 1.36 g/in3,
or embodiments wherein, for example, the loading of the MFI-type zeolites
ranges from 0.7 to
2.0 glin3, and the loading of the BEA-type zeolites ranges from 1.32 to 1.38
g/in3, or embodi-
ments wherein, for example, the loading of the MFI-type zeolites ranges from
1.0 to 1.7 g/in3,
and the loading of the BEA-type zeolites ranges from 1.25 to 1.45 g/in3, or
embodiments
wherein, for example, the loading of the MFI-type zeolites ranges from 1.15 to
1.55 glin3, and
the loading of the BEA-type zeolites ranges from 1.15 to 1.55 g/in3, or
embodiments wherein,
for example, the loading of the MFI-type zeolites ranges from 1.25 to 1.45
glin3, and the load-
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ing of the BEA-type zeolites ranges from 1.0 to 1.7 g/in3, or embodiments
wherein, for exam-
ple, the loading of the MFI-type zeolites ranges from 1.32 to 1.38 g/in3, and
the loading of the
BEA-type zeolites ranges from 0.7 to 2.0 g/in3, or embodiments wherein, for
example, the
loading of the MFI-type zeolites ranges from 1.34 to 1.36 g/in3, and the
loading of the BEA-
type zeolites ranges from 0.1 to 5.0 g/in3.
In addition to the above-mentioned catalyst, the present invention also
relates to a treatment
system for an exhaust gas stream. In particular, the treatment system of the
present invention
comprises an internal combustion engine which is preferably a lean burn
engine, and even
more preferably a diesel engine. According to the present invention, it is
however also possi-
ble to use a lean burn gasoline engine in said treatment system.
Furthermore, the treatment system according to the present invention comprises
an exhaust
gas conduit which is in fluid communication with the internal combustion
engine. In this re-
L5 spect, any conceivable conduit may be used, provided that it is capable of
conducting exhaust
gas from an internal combustion engine, and may sufficiently resist the
temperatures and the
chemical species encountered in the exhaust gas of an internal combustion
engine, in particu-
lar of a lean burn engine such as a diesel engine. Within the meaning of the
present invention,
the fluid communication provided between the exhaust gas conduit and the
internal combus-
?0 tion engine signifies that the treatment system allows for the constant
passage of exhaust gas
from the engine to the conduit.
According to the exhaust gas treatment system of the present invention, the
catalyst is
present in the exhaust gas conduit. In general, the catalyst may be provided
in the exhaust
?5 gas conduit in any conceivable fashion, provided that it is present within
the exhaust gas con-
duit in the sense that it may be contacted by the exhaust gas passing through
said conduit.
Preferably, the catalyst is provided in the exhaust gas conduit on a substrate
as outlined in
the present application, and in particular on a honeycomb substrate, which is
preferably either
a flow-through or a wall-flow honeycomb substrate.
Thus, the present invention also relates to an exhaust gas treatment system
comprising an
internal combustion engine and an exhaust gas conduit in fluid communication
with the inter-
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nal combustion engine, wherein the catalyst according to the present invention
is present in
the exhaust gas conduit, and wherein the internal combustion engine is
preferably a lean burn
engine, and more preferably a diesel engine.
In this respect and independently thereof, the present invention also relates
to embodiments
wherein the inventive catalyst is comprised in an exhaust gas treatment system
comprising an
internal combustion engine and an exhaust gas conduit in fluid communication
with the inter-
nal combustion engine, wherein said catalyst is present in the exhaust gas
conduit, and
wherein the internal combustion engine is preferably a lean burn engine, and
more preferably
LO a diesel engine.
According to preferred embodiments of the present invention, the exhaust gas
treatment sys-
tem further comprises a means of introducing a reductant into the exhaust gas
stream, where-
in said means is located upstream from the inventive MFI/BFA-zeolite catalyst.
In particular, it
L5 is preferred that a means of introducing ammonia and/or urea into the
exhaust gas conduit is
provided. In this respect, any means known to the skilled person may be
provided, in particu-
lar those commonly applied to exhaust gas treatment systems operating with
active SCR me-
thods necessitating the direct introduction of said reductants. According to
particularly pre-
ferred embodiments, the reductant which preferably comprises ammonia and/or
urea is intro-
?0 duced by the means of an injection nozzle provided in the exhaust gas
conduit upstream from
the inventive catalyst.
Within the meaning of the present invention, the exhaust gas treatment system
may suitably
further comprise any further components for the effective treatment of an
exhaust gas. In par-
?5 titular, said system preferably further comprises an oxidation catalyst or
a catalyzed soot filter
(CSF) or both an oxidation catalyst and a CSF. According to said embodiments,
the oxidation
catalyst and/or the CSF are also present within the exhaust gas conduit.
In the present invention, any suitable CSF can be used, provided that it may
effectively oxid-
30 ize soot which may be contained in the exhaust gas. To this effect, the CSF
of the present
invention preferably comprises a substrate coated with a washcoat layer
containing one or
more catalysts for burning off trapped soot and/or oxidizing exhaust gas
stream emissions. In
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general, the soot burning catalyst can be any known catalyst for combustion of
soot. For ex-
ample, the CSF can be coated with a one or more high surface area refractory
oxides (such
as e.g. alumina, silica, silica alumina, zirconia, and zirconia alumina)
and/or with an oxidation
catalyst (such as e.g. a ceria-zirconia) for the combustion of unburned
hydrocarbons and to
some degree particulate matter. However, preferably the soot burning catalyst
is an oxidation
catalyst comprising one or more precious metal catalysts, said one or more
precious metal
catalysts preferably comprising one or more metals selected from the group
consisting of pla-
tinum, palladium, and rhodium.
LO Regarding the oxidation catalyst preferably comprised in the exhaust gas
treatment system
instead of or in addition to a CSF, any oxidation catalyst may be used to this
effect which is
suitable for oxidizing unburned hydrocarbons, CO, and/or NOx comprised in the
exhaust gas.
In particular, oxidation catalysts are preferred which comprise one or more
precious metal
catalysts, and more preferably one or more precious metals selected from the
group consist-
t5 ing of platinum, palladium, and rhodium. According to particularly
preferred embodiments of
the present invention, wherein the internal combustion engine of the exhaust
gas treatment
system is a diesel engine, the oxidation catalyst is preferably a diesel
oxidation catalyst. In
particular, within the meaning of the present invention, a "diesel oxidation
catalyst" refers to
any oxidation catalyst which is particularly well adapted to the oxidation of
diesel exhaust gas,
?0 in particular with respect to the temperatures and to the composition of
diesel exhaust gas
encountered in the treatment thereof.
According to particularly preferred embodiments, the exhaust gas treatment
system further
comprises a CSF, and even more preferably both a CSF and an oxidation
catalyst. Even
?5 more preferably, the exhaust gas treatment system further comprises a CSF
and a diesel oxi-
dation catalyst.
In principle, in embodiments of the exhaust gas treatment system which further
comprise an
oxidation catalyst and/or a CSF, said further components may be present in the
exhaust gas
30 conduit in any order and at any emplacement therein, provided that the
effective treatment of
an exhaust gas may be provided. In particular, however, the presence and/or
order and/or
location of said further components may depend on the type, on the state, in
particular with
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respect to the temperature and pressure thereof, and on the average
composition of the ex-
haust gas which is treated. Thus depending on the application of the exhaust
gas treatment
system, the present invention includes preferred embodiments wherein the
oxidation catalyst
and/or the CSF are located upstream or downstream from the inventive MFI/BEA-
zeolite cata-
lyst, as well as preferred embodiments comprising both an oxidation catalyst
and a CSF,
wherein the oxidation catalyst is located upstream and the CSF downstream
thereof, or
wherein, vice versa, the CSF is located upstream, and the oxidation catalyst
downstream the-
reof. According to particularly preferred embodiments of the present
invention, the oxidation
catalyst and/or the CSF are located upstream from the inventive MFI/BEA-
zeolite catalyst,
LO wherein even more preferably, the exhaust gas treatment system comprises
both an oxidation
catalyst and a CSF upstream from the inventive MFI/BEA-zeolite catalyst.
Within the meaning
of the present invention, "upstream" and "downstream" relates to the direction
of flow of the
exhaust gas through the exhaust gas conduit in fluid communication with the
internal combus-
tion engine.
L5
Thus, the present invention also relates to an exhaust gas treatment system as
defined in the
foregoing, said exhaust gas treatment system further comprising an oxidation
catalyst and/or
a catalyzed soot filter (CSF), wherein the oxidation catalyst and/or the CSF
are preferably
located upstream from the inventive MFI/BEA-zeolite catalyst, and wherein the
oxidation cata-
?0 lyst is a diesel oxidation catalyst (DOC) in instances where the internal
combustion engine is
a diesel engine.
Furthermore, as outlined in the foregoing, the exhaust gas treatment system
preferably further
includes a means of introducing a reductant into the exhaust gas conduit, said
means being
?5 located upstream from the inventive MFI/BEA-zeolite catalyst. In
particular, said means
enables the introduction of a reductant comprising ammonia and/or urea into
the exhaust gas
conduit. Accordingly, the present invention also relates to an exhaust gas
treatment system
wherein in addition to or instead of further comprising an oxidation catalyst
and/or a catalyzed
soot filter (CSF) respectively preferably located upstream from the inventive
MFI/BEA-zeolite
30 catalyst, the oxidation catalyst being a diesel oxidation catalyst (DOC) in
instances where the
internal combustion engine is a diesel engine, said system further comprises a
means of in-
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traducing a reductant preferably comprising ammonia and/or urea into the
exhaust gas con-
duit, said means being located upstream of the inventive MFI/BEA-zeolite
catalyst.
According to further preferred embodiments of the present invention, the
exhaust gas treat-
ment system further comprises an ammonia slip catalyst located downstream of
the MFI/BEA-
zeolite catalyst for oxidizing excess ammonia and/or urea which has not
reacted in the SCR.
Regarding the preferred ammonia slip catalyst, said catalyst may be provided
in the exhaust
gas conduit in any manner commonly known in the art, provided that it may
effectively oxidize
said excess ammonia and/or urea. In particular, said preferred embodiments
involve an ex-
haust gas treatment systems according to the present invention which include a
means of
introducing a reductant into the exhaust gas conduit as defined in the
foregoing.
In addition to a catalyst and to an exhaust gas treatment system comprising
said catalyst, the
present invention further concerns a process for the treatment of a gas stream
comprising
NO,, In general, in the process of the present invention, any suitable gas
stream comprising
NOx may be employed, provided that its state and composition are both suited
for being
treated when contacted with a MFIIBEA-zeolite catalyst according to the
present invention,
wherein preferably said treatment at least in part involves the selective
catalytic reduction of
at least part of the NOx contained in said gas. For this purpose, the gas
stream used in the
?0 inventive process preferably contains at least one reductant, which is
preferably ammonia
and/or any ammonia precursor such as urea and/or ammonium carbamate, urea
being pre-
ferably comprised in the ammonia precursor. According to further embodiments
of the inven-
tive process, however, the gas stream used may also contain hydrocarbons
and/or hydrocar-
bon derivatives such as oxygenated hydrocarbons, such as for example those
which may be
?5 found in motor vehicle fuels and/or in motor vehicle exhaust gas, in
particular in diesel fuel
and/or exhaust gas. Said further reductants may be contained in the gas
treated in the inven-
tive process either in addition to ammonia, or, according to further
embodiments, may also be
contained therein instead of ammonia. According to the present invention, it
is however par-
ticularly preferred that the gas comprises ammonia and/or urea as a reducing
agent for the
i0 treatment of exhaust gas emissions, in particular via SCR.
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Thus, the present invention also relates to a process for the treatment ,of a
gas stream com-
prising NO,, as defined in the present application, wherein the gas stream
comprises ammonia
and/or urea.
Regarding the content of reductant in the gas stream, said reductant
preferably comprising
ammonia and/or urea, there is no particular limitation in this respect,
provided that at least
part of the NO, in said gas may be reduced by SCR when contacting the MFI/BEA-
zeolite
catalyst of the present invention. It is however preferred, that said content
does not consider-
ably derive from the amount of reductant necessary for the maximal conversion
of NO, by the
l0 catalyst. In this respect, the maximal conversion reflects the maximum
amount of NOx which
may be converted by SCR at a given time point in the inventive process, i.e.
relative to the
actual state and condition of both the catalyst and the gas to be treated upon
contacting the-
reof, and in particular depending on the content of the reductant and,
preferably, depending
on the amount of ammonia and/or urea contained therein. Accordingly, the
maximal conver-
l5 sion of NOx directly reflects the maximum amount of reductant, and
preferably of ammonia
and/or urea, which may react with NOx in the SCR process at a given time
point.
According to preferred embodiments of the present invention, the gas stream
used in the in-
ventive process is preferably an exhaust gas stream comprising NOR. In this
respect, there is
?0 no particular limitation as to the process which leads to such an exhaust
gas stream, provided
that it is suited for treatment with the MFI/BEA-zeolite catalyst according to
the present inven-
tion, or may be processed to a gas stream suited for treatment with such a
catalyst. According
to the inventive process it is further preferred that the exhaust gas stream
is an exhaust gas
stream resulting from an internal combustion engine, and even more preferably
from a lean
?5 burn engine. According to particularly preferred embodiments, the exhaust
gas stream is a
diesel engine exhaust gas stream.
In the process according to the present invention, the gas stream is contacted
with the inven-
tive MFIIBEA-zeolite catalyst for treatment thereof, wherein said contacting
is achieved by
30 either conducting the gas stream over the catalyst, or conducting the gas
stream through the
catalyst. Said contacting may, however, also be achieved by conducting the gas
stream both
over and through the inventive catalyst. According to preferred embodiments,
the gas stream
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is either conducted over the catalyst, wherein the catalyst preferably
comprises a flow-through
substrate for this purpose, or the gas stream is conducted through the
catalyst, wherein in this
case the catalyst preferably comprises a wall-flow substrate. When using a
wall-flow sub-
strate, however, there are instances wherein, depending on the process
conditions and the
specific form and dimensions of the catalyst, at least a portion of the gas
stream may also be
conducted over the catalyst. According particularly preferred embodiments of
the inventive
process, the catalyst used in the inventive process either comprises a wall-
flow honeycomb
substrate or a flow-through honeycomb substrate.
LO Thus, the present invention also relates to a process for the treatment of
a gas stream com-
prising NO,, comprising conducting said gas stream over and/or through an
MFI/BEA-zeolite
catalyst according to the present invention, wherein the gas stream is
preferably an exhaust
gas stream, more preferably an exhaust gas stream resulting from an internal
combustion
engine, and even more preferably a diesel exhaust gas stream.
L5
In the inventive process, there is no particular limitation as to the amount
of NOX contained in
the gas stream, wherein preferably, the amount thereof in the gas streams used
in the inven-
tive process does not exceed 10 wt.-% based on the total weight of the exhaust
gas, and
more preferably does not exceed 1 wt.-%, more preferably 0.5 wt.-%, more
preferably 0.1 wt-
?0 %, more preferably 0.05 wt-.%, more preferably 0.03 wt-.%, and even more
preferably does
not exceed 0.01 wt.-%.
Regarding the specific composition of the NO, fraction contained in the gas
stream treated in
the inventive process, there is no limitation as to the type or to the content
of specific nitrous
?5 oxide gases NON contained therein. According to specific embodiments of the
present inven-
tion, it is however preferred that the N02-content relative to the total NON-
content is 90 wt.-%
or less based on 100 wt.-% of NO,, wherein more preferably, the NO2 content is
comprised in
the range of from 10 to 80 wt.-%, more preferably of from 30 to 70 wt.-%, more
preferably of
from 35 to 65 wt.-%, more preferably of from 40 to 60 wt.-%, and even more
preferably of
M from 45 to 55 wt.-%.
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In general, the composition of the gas stream used in the inventive process as
defined in the
present application refers to the gas stream prior to its use in the inventive
process, and in
particular prior to the contacting thereof with the catalyst. Preferably,
however, said composi-
tion refers to the gas stream's composition immediately prior to contacting
the catalyst, i.e.
immediately before treatment thereof begins by catalyzed chemical conversion
thereof.
Thus, the present invention also relates to a process for the treatment of a
gas stream com-
prising NO, as defined in the present application, wherein prior to the
contacting of the cata-
lyst with the gas stream, the NO2 content thereof is 90 wt.-% or less based on
100 wt.-% of
l0 NO,, wherein more preferably, the NO2 content is comprised in the range of
from 10 to 80 wt.-
%, more preferably of from 30 to 70 wt.-%, more preferably of from 35 to 65
wt.-%, more pre-
ferably of from 40 to 60 wt.-%, and even more preferably of from 45 to 55 wt.-
%.
The catalyst according to the present invention can be readily prepared by
processes well
l5 known in the prior art. A representative process is set forth below. As
used herein, the term
"washcoat" has its usual meaning in the art of a thin, adherent coating of a
catalytic or other
material applied to a substrate carrier material, such as a honeycomb-type
carrier member,
which is preferably sufficiently porous to permit the passage there through of
the gas stream
being treated.
Z0
The several zeolite components of the catalyst may be applied to the substrate
as mixtures of
one or more components in sequential steps in a manner which will be readily
apparent to
those skilled in the art of catalyst manufacture. A typical method of
manufacturing the catalyst
of the present invention is to respectively provide the at least one zeolite
of the MFI structure
?5 type, and the at least one further zeolite of the BEA structure type as a
coating or washcoat
layer on the walls of a particularly preferred flow-through or wall-flow
honeycomb substrate.
According to certain preferred embodiments of the present invention, the
zeolites are provided
in a single washcoat on the substrate.
30 The catalyst according to the present invention is however preferably
prepared by further us-
ing at least one binder, wherein any conceivable binder used in the art of
catalyst manufac-
ture, and in particular in the art of automotive SCR catalyst manufacture, may
be used. In this
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respect, a silica-alumina binder is for example preferably used for the
preparation of the in-
ventive catalyst, wherein said binder may be provided together with one or
more of the zeolite
components, and is preferably provided together with the zeolite components in
one or more
coatings on a substrate, more preferably in one or more washcoat layers.
For preparing the inventive catalyst, the components of one or possibly more
washcoat layers
may respectively be processed to a slurry, preferably to an aqueous slurry.
The substrate may
then be sequentially immersed into the respective slurries for applying the
individual wash-
coats, after which excess slurry is removed to provide a thin coating of the
two or more slur-
ries on the walls of the substrate. The coated substrate is then dried and
preferably calcined
to provide an adherent coating of the respective component to the walls of the
substrate.
Thus, for example, after providing a first washcoat layer on the substrate,
and preferably dry-
ing and/or calcining the coated substrate, the resulting coated substrate may
then be im-
mersed into a further slurry to form a second washcoat layer deposited over
the first washcoat
layer. Again, the substrate may then be dried and/or calcined and eventually
coated with a
third washcoat, which again may subsequently be dried and/or calcined to
provide a finished
catalyst in accordance with one embodiment of the present invention. Regarding
the steps of
drying, washing, and calcining of the catalyst coated in this fashion, these
may be respectively
performed in the manner well known in the art of catalyst manufacture, in
particular regarding
the solvents andlor solutions used for washing the coated catalyst, as well as
regarding the
temperature, duration, and the atmosphere employed in the steps of drying and
calcination,
respectively. Concerning the step of calcination, any possible temperature may
be used there-
in, provided that the process leads to the desired transformations in the
catalyst without caus-
ing any notable or substantial deterioration of the catalysts stability, in
particular with regard to
?5 its use in SCR. Thus, in certain cases, the temperature of calcination will
not exceed 700 C,
preferably 650 C, more preferably 600 C, and even more preferably will not
exceed 550 C.
Thus, calcination may for example be conducted at a temperature comprised in
the range of
from 500 C to 650 C, preferably 550 C to 600 C, more preferably 570 C to 590
C, more pre-
ferably , and even more preferably at a temperature comprised in the range of
from 575 C to
585 C.
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When preparing the inventive catalyst in the above-mentioned manner, it is
however preferred
that no washing of the washcoat layers is performed after the application and
optional drying
thereof.
Accordingly, the catalyst of the present invention may be prepared according
to a process
comprising
(a) providing at least one zeolite of the MFI structure type, and at least one
further zeolite
selected from zeolites of the BEA structure type, wherein at least part of the
one or more zeo-
lites of the MFI structure type and at least part of the one or more further
zeolites of the BEA
structure type contain iron;
(b) preparing one or more washcoat compositions respectively comprising one or
more of
the zeolites;
(c) applying the one or more woashcoat compositions in one more respective
layers onto
the substrate, wherein a step of drying is optionally conducted after the
respective application
of one or more of the individual layers;
(d) optionally washing and/or drying the coated substrate, wherein the coated
substrate is
preferably not washed; and
(e) optionally subjecting the coated substrate to a calcination process.
Figures
Figure 1 displays results from NEDC testing of the catalyst compositions
according to
Example 1 and Comparative Example 2, wherein the testing period in seconds
?5 is plotted on the x-axis, and the NO, emissions in grams NOX is plotted on
the
y-axis, and wherein the background displays the legally prescribed course of
NEDC testing over the time period in terms of the variation of the motor ve-
hicle's speed as laid down in the European Union Directive 70/220/EEC.
Examples
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Example 1
A catalyst composition was prepared comprising 1.35 glin3 of a zeolite of the
BEA structure
type, said BEA-type zeolite having a silica to alumina ratio (SAR) of
approximately 40 and
containing 1.3 wt.-% of iron based on the total weight of the BEA-type
zeolite, 1.35 glin3 of a
zeolite of the MFI structure type, said MFI-type zeolite having a silica to
alumina ratio of ap-
proximately 26 and containing 3.8 wt.-% of iron based on the total weight of
the MFI-type zeo-
lite, and 0.3 g/in3 of a silica-alumina binder.
Comparative Example 2
A catalyst composition was prepared comprising 2.7 g/in3 of a zeolite of the
BEA structure
type, said BEA-type zeolite having a silica to alumina ratio (SAR) of
approximately 40 and
containing 1.3 wt.-% of iron based on the total weight of the BEA-type
zeolite, and 0.3 g/in3 of
a silica-alumina binder.
L5 SCR Performance Testing
DeNOx Performance of the SCR Catalysts were evaluated in transient conditions
using the
New European Driving Cycle, also referred to as the MVEG (Motor Vehicle
Emissions Group)
cycle. In particular, testing conditions were such, that the NOx fraction of
the exhaust gas
stream contained approximately 50 wt.-% of NO2 based on the total NO, -
content.
!0
For testing, the catalyst compositions according to Examples 1 and Comparative
Example 2
were respectively coated onto a 5.66"x5:66"x6" flow-through honeycomb
substrate having a
volume of 2.5 L, a cell density of 400 cells per square inch, and a wall
thickness of approx-
imately 100pm (4 mil). The catalyst samples prepared in this fashion were then
tested in an
!5 exhaust gas treatment system with a diesel oxidation catalyst (DOC) and a
catalyzed soot
filter (CSF) respectively located upstream from the tested catalyst.
The results from the NEDC catalyst testing is shown in Figure 1, Thus, as may
be taken from
said figure, the inventive catalyst according to Example 1 which contains a
combination of
10 BEA- and MFI-type zeolites displays a clearly improved performance compared
to the catalyst
sample of Comparative Example 2, which only contains BEA-type zeolite. In
particular, when
considering the results displayed in Figure 1, wherein the level of NOx
emissions is plotted as
CA 02795522 2012-10-04
WO 2011/125050 PCT/IB2011/051526
-25-
a function of the NEDC testing period, the inventive catalyst shows a superior
conversion per-
formance compared to Comparative Example 2 during the period from 0 to 800s
correspond-
ing to the old European driving cycle (ECE-1.5). However, when considering the
testing period
from 800 to 1200s, corresponding to the extra-urban part of the driving cycle
involving higher
space velocity and higher NO, mass flow, the advantage of the inventive
catalyst of Example
1 is crucial.
Consequently, the catalyst according to the present invention shows a clearly
superior per-
formance in SCR compared to a catalyst according to the prior art represent by
Comparative
Example 2, in particular with respect to the actual driving conditions
encountered in motor
vehicle use, as reflected in NEDC testing. In particular, these excellent
results may be attri-
buted to the use of a specific combination of zeolite materials as defined by
the catalyst of the
present invention.
