Note: Descriptions are shown in the official language in which they were submitted.
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P/S-TM-comprising zeolites for decomposition of N20
Description
The present invention relates to the use of a zeolite catalyst comprising at
least one
transition metal and in addition sulfur and/or phosphorus atoms for reducing
the con-
tent of nitrogen oxides in a gas, and also to a process for reducing the
content of nitro-
gen oxides in a gas by bringing this gas into contact with such a zeolite
catalyst.
The use of metal-doped catalysts in processes for the catalytic removal of
nitrogen oxi-
des is known from the prior art.
DE 101 12 396 Al discloses a process for reducing the content of N20 in gases.
Here,
a selected zeolite catalyst is used. This is present in the H form and/or
comprises
exchanged iron and is characterized by the presence of nonlattice aluminum in
addition
to the lattice aluminum in a molar ratio of from 1:2 to 20:1. Furthermore,
this document
discloses that dealumination or demetallation can be carried out by means of a
mineral
acid treatment. This is carried out using acids selected from among HCI, HF,
H2SO4,
HNO3 and H3PO4. The acid treatment as described in DE 101 12 396 Al is not
carried
out to introduce sulfur and/or phosphorus atoms onto the catalyst. No content
of sulfur
and/or phosphorus atoms in the finished catalyst is disclosed in this
document.
WO 03/084646 Al discloses a process for reducing the content of NO and N20 in
ga-
ses, in particular in process gases and offgases, which comprises addition of
at least
one nitrogen-comprising reducing agent to the NOR- and N20-comprising gas in
an
amount not less than that required for complete reduction of the NON, addition
of a hyd-
rocarbon, carbon monoxide, hydrogen or a mixture of one or more of these gases
to
the NON- and N20-comprising gas to reduce the N20 and introduction of the gas
mix-
ture into at least one reaction zone which has temperatures of up to 450 C and
compri-
ses one or more iron-laden zeolites. According to this process, catalysts
which are ba-
sed on zeolites into which iron has been introduced by means of solid-state
ion
exchange are used. For this purpose, commercially available ammonium zeolites
are
usually treated with appropriate iron salts, e.g. FeSO4 = 7 H20. After
calcination, the
iron-comprising zeolites are thoroughly washed in distilled water, filtered
off and dried.
Thus, the document cited discloses zeolite catalysts which are doped with
iron. How-
ever, the sulfate anions which are likewise applied together with the iron
cations are
removed again by means of the thorough washing, so that no sulfur is present
on the
iron-doped catalyst.
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2
DE 102 15 605 Al likewise discloses a process for reducing the content of NO
and
N20 in gases, in particular in process gases and offgases, where the gas to be
treated
is brought into contact with a catalyst which is based on a zeolite and is
doped with
iron. According to this document, the doping with iron can likewise be
achieved by
applying FeSO4 = 7 H20 to the zeolite. Moreover, here too, the sulfate anions
are remo-
ved again by thorough washing, so that no sulfur and/or phosphorus atoms are
present
in the final catalyst.
DE 10 2005 022 650 Al also discloses a process for reducing the content of
nitrogen
oxides in gases. For this purpose, the gas to be treated is brought into
contact with a
zeolite which is doped with copper and/or iron atoms. The presence of sulfur
or phos-
phorus atoms on the zeolite catalyst is likewise not disclosed in this
document.
The catalysts known from the prior art, in particular the iron-doped zeolites,
have an
activity for the degradation of nitrogen oxides in gases which is still
capable of impro-
vement. Furthermore, there is a need for an improved zeolite catalyst which
has the
same activity as the systems known from the prior art even at low
temperatures, or
displays a correspondingly higher activity at the same temperature. A catalyst
which
displays a sufficiently high activity even at a relatively low reaction
temperature would
be advantageous because the offgas from many industrial plants has a low
tempera-
ture and heating of this offgas before reaction over the appropriate catalyst
is unattrac-
tive for ecological and economic reasons.
The objects mentioned in the light of the available prior art are achieved,
according to
the invention, by the use of a zeolite catalyst for reducing the content of
nitrogen oxides
in a gas, where the zeolite catalyst comprises at least one transition metal
and in addi-
tion sulfur and/or phosphorus atoms.
The objects are also achieved by a process for reducing the content of
nitrogen oxides
in a gas by bringing the gas into contact with a zeolite catalyst as defined
above.
The zeolite catalyst used according to the invention will be described in
detail below:
The basis of the zeolite catalyst used according to the invention is a
zeolite. Zeolites
are known per se to those skilled in the art and are disclosed, for example,
in Catalysis
and Zeolites, Fundamentals and Applications, J. Weitkamp, I. Puppe, (eds),
Springer-
Verlag, Berlin, Heidelberg 1990.
In general, all zeolites known to those skilled in the art are suitable for
the zeolite cata-
lyst used according to the invention. These are named in the following using
the three
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letter nomenclature of the IZA (international zeolite association) structure
commission
known to those skilled in the art.
Zeolites which are particularly suitable for the purposes of the invention are
selected
from the group consisting of BEA, CHA, FAU, FER and MFI and mixtures thereof.
According to the invention, the zeolite catalyst comprises at least one
transition metal.
The term transition metal is known per se to those skilled in the art and
describes the
group of elements in transition groups 3 to 12 of the Periodic Table of the
Elements
(new IUPAC nomenclature).
In a preferred embodiment, the catalyst used according to the invention
comprises at
least one transition metal selected from the fourth period and/or groups 8 to
11 of the
Periodic Table of the Elements.
The present invention therefore relates particularly to the use according to
the invention
where the at least one transition metal is selected from the fourth period
and/or groups
8 to 11 of the Periodic Table of the Elements.
The catalyst used according to the invention more preferably comprises at
least one
transition metal selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe,
Co, Ni, Cu,
Zn, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au and mixtures thereof.
The present invention therefore relates particularly to the use according to
the invention
where the at least one transition metal is selected from the group consisting
of Sc, Ti,
V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au and mixtures
thereof.
In a particularly preferred embodiment, the catalyst used according to the
invention
comprises Fe, Cu, Co and/or Ni, very particularly preferably Fe, as at least
one transiti-
on metal.
The present invention therefore very particularly preferably relates to the
use according
to the invention where the at least one transition metal is Fe, Cu, Co and/or
Ni.
Examples according to the invention of nitrogen oxides are preferably selected
from the
group consisting of dinitrogen monoxide N20, nitrogen oxides NON, where x is 1
or 2,
and mixtures thereof. In a preferred embodiment, the gas to be treated
comprises a
little (NO/N20 < 0.5) and in particular no nitrogen oxides NOR. In a preferred
embodi-
ment, a stage for decreasing the amount of NO therefore precedes the use
according
to the invention. Methods of decreasing the amount of NO are known to those
skilled
in the art.
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The content of the nitrogen oxide N20 is particularly preferably reduced by
means of
the use according to the invention.
The at least one transition metal which is present according to the invention
can gene-
rally be comprised in the zeolite catalyst used according to the invention in
any amount
which gives the catalyst used according to the invention a particularly high
activity, for
example in the degradation of nitrogen oxides, in particular dinitrogen
monoxide N20.
In a preferred embodiment, the at least one transition metal is present in the
catalyst
used according to the invention in a concentration of from 0.1 to 10.0% by
weight, par-
ticularly preferably from 0.25 to 5.0% by weight, very particularly preferably
from 0.5 to
2.5% by weight, for example 0.7 or 2.5% by weight, in each case based on the
total
zeolite catalyst.
In a preferred embodiment, the present invention therefore relates to the use
according
to the invention where the at least one transition metal is present in a
concentration of
from 0.1 to 10.0% by weight, particularly preferably from 0.25 to 5.0% by
weight, very
particularly preferably from 0.5 to 2.5% by weight, for example 0.7 or 2.5% by
weight,
in each case based on the total zeolite catalyst.
The at least one transition metal which is present according to the invention
can be
present in either cationic or elemental form in the zeolite catalyst used
according to the
invention. If the transition metal is present in cationic form, it is
preferably present in
oxidation numbers which are typical of the respective transition metal as a
result of its
position in the Periodic Table. In the preferred case of iron being present as
transition
metal in the zeolite catalyst used according to the invention, the oxidation
number
thereof is preferably +2 or +3. If the at least one transition metal is
present in elemental
form, it has the oxidation number 0. The at least one transition metal can
also be
present as a mixture of various oxidation numbers.
It is possible, according to the invention, for the at least one transition
metal present to
be incorporated into the lattice of the respective zeolite and/or to be
present outside
this lattice structure as nonlattice transition metal.
Furthermore, the zeolite catalyst used according to the invention additionally
comprises
sulfur and/or phosphorus atoms.
In the zeolite catalyst used according to the invention, the sulfur and/or
phosphorus
atoms can generally be present in any amount which, in combination with the at
least
one transition metal present, gives the zeolite catalyst used according to the
invention
CA 02814409 2013-04-11
a particularly high activity in the degradation of nitrogen oxides, in
particular dinitrogen
monoxide N20.
In a preferred embodiment, sulfur and/or phosphorus atoms are present in the
catalyst
5 used according to the invention in a concentration of less than 10% by
weight, based
on the total catalyst.
In a further preferred embodiment, sulfur and/or phosphorus atoms are present
in the
catalyst according to the invention in a concentration of less than 3% by
weight, very
particularly preferably from 0.2 to 2.5% by weight, in each case based on the
total cata-
lyst.
The present invention therefore also preferably provides for the use according
to the
invention in which the sulfur and/or phosphorus atoms are present in a
concentration of
less than 10% by weight, preferably less than 3% by weight, particularly
preferably
from 0.2 to 2.0% by weight, based on the total catalyst.
The sulfur and/or phosphorus atoms which are present according to the
invention can
be present in a uniform oxidation state or in combinations of various
oxidation states in
the zeolite catalyst used according to the invention. In the embodiment of the
invention
in which sulfur is present in the zeolite catalyst according to the invention,
this sulfur is
preferably present in the oxidation state +6 or +2 or a combination of these
side by
side, but in particular the oxidation state +6.
In the embodiment of the invention in wich phosphorus is present in the
zeolite catalyst
according to the invention, this phosphorus is preferably present in the
oxidation state
+5 or +3 or a combination of these side by side, but in particular the
oxidation state +5.
Sulfur and/or phosphorus can be incorporated into the lattice of the
respective zeolite,
or sulfur and/or phosphorus are present as atoms, cations or anions outside
the lattice
of the zeolite, or sulfur and/or phosphorus are present both in the lattice
and also outsi-
de the lattice of the respective zeolite.
The zeolite catalysts used according to the invention generally comprise
aluminum in
cationic form which is present in the lattice. The zeolite catalyst used
according to the
invention can, in a further embodiment, comprise not only the aluminum cations
present in the lattice but also corresponding cations which are present
outside the lat-
tice as nonlattice aluminum cations.
Steaming of the zeolites, i.e. hydrothermal treatment of the zeolites by
passing steam
over them at elevated temperatures, or else treatment with acids are
particularly useful
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for setting a preferred ratio of nonlattice aluminum to lattice aluminum. A
combination
of various methods can also be employed.
In a treatment with H20 vapor and/or acid, as is known to those skilled in the
art,
dealumination or, if the zeolite comprises other metals such as Fe, Ga, etc.,
in addition
to Al, demetallation, i.e. removal of the aluminum or these metals from the
lattice of the
zeolite, takes place. The aluminum or the metals migrate from their lattice
positions into
the pores of the zeolite and remain there as amorphous constituents in oxidic
or hydro-
xidic form as extralattice metal. The degree of dealunnination or
demetallation can be
set via the duration of the treatment and the reagent concentration. Part of
the extralat-
tice metal produced can also be removed from the pores during the treatment.
As a
result, the metal content of the catalyst can change.
The treatment of the zeolite with steam can, for example, be carried out at
tempera-
tures of from 300 to 800 C for a period of from 0.5 to 48 hours. The zeolite
can be ex-
posed to pure steam or a mixture of nitrogen and/or air and water vapor having
a pro-
portion of water vapor of from 1 to 100% by weight at total pressures of up to
100 bar.
A carrier gas can optionally be added to the steam or the water vapor mixture.
Suitable
carrier gases are inert under the treatment conditions; examples are N2, Ar,
He, H2 or a
mixture thereof.
The zeolites can be dealuminated/demetallated further by means of an
additional mine-
ral acid treatment, optionally in addition to the steam treatment. The acid
treatment can
both remove extralattice metal from the pores and lead to further
demetallation of the
lattice. This step can, for example, be carried out in a batch reactor at
temperatures of
from 0 to 120 C at a lattice/zeolite ratio of from 1 to 100 cm3/g and acid
concentrations
of from 0.001 M to the maximum concentration of the acid. Examples of acids
which
can be used for this step are HCI, HF, H2SO4, HNO3 and H3PO4. After the acid
treat-
ment, the zeolite is separated off from the reaction mixture by conventional
methods,
e.g. by filtration or centrifugation.
According to the present invention, amorphous metal oxides or hydroxides are
pro-
duced at extralattice sites by the above-described treatment of the zeolite
and it is as-
sumed that they act as catalytic sites to increase the activity in respect of
the degrada-
tion of N20.
The zeolite catalyst used according to the invention can comprise, in addition
to the
above-described zeolite, the at least one transition metal and sulfur and/or
phosphorus
atoms, further customary components known to those skilled in the art, for
example
binders such as aluminum oxide or silicon oxide and mixtures thereof.
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The zeolite catalyst used according to the invention can be used in any form
which
appears to be suitable to a person skilled in the art, for example as shaped
bodies, e.g.
extrudates or honeycomb bodies, crushed material, particles or powder. In
industry, the
zeolite catalyst used according to the invention is preferably used in the
form of shaped
bodies, for example having a particle diameter of from 1 to 10 mm, preferably
from 1.5
to 5 mm.
The catalyst used according to the invention can, for example, be produced by
a pro-
cess which comprises the following steps:
(A) application of the at least one transition metal or a precursor
compound thereof to
a zeolite,
(B) calcination of the zeolite from step (A) to convert, if applicable, the
precursor
compound into the at least one transition metal and to obtain a zeolite
comprising
the at least one transition metal,
(C) application of the sulfur and/or phosphorus atoms or a precursor compound
thereof to the doped zeolite from step (B) and
(D) calcination of the zeolite from step (C) to obtain the catalyst to be
used according
to the invention.
The individual steps of the process for producing the catalyst used according
to the
invention are described in detail below:
Step (A):
Step (A) comprises application of the at least one transition metal or a
precursor com-
pound thereof to a zeolite.
According to the invention, it is generally possible to use all zeolites which
have been
mentioned above. In a preferred embodiment, zeolites selected from the group
consis-
ting of BEA, FAU, FER, MFI and mixtures thereof are used.
In a preferred embodiment, precursor compounds of the abovementioned
transition
metals, particularly preferably the metals Fe, Cu, Co, Ni or mixtures thereof,
are used
for this purpose.
Particularly preferred precursor compounds for the transition metal iron which
is very
particularly preferably used are Fe(NO3)2 and Fe(SO4).
Step (A) of the process is particularly preferably carried out by dissolving a
suitable
amount of the appropriate precursor compounds in water or an aqueous solution
and
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impregnating the appropriate zeolites with this aqueous solution. The aqueous
soluti-
ons which are preferably used can, in one embodiment, comprise water as
solvent. In a
further embodiment, the aqueous solutions can comprise not only water but also
further, polar and water-miscible solvents, for example alcohols such as
methanol,
ethanol, propanols and mixtures thereof.
Impregnation of a solid with an aqueous solution is known per se to those
skilled in the
art. Impregnation is preferably carried out by spraying the impregnation
solution of the
appropriate transition metal or a precursor compound thereof onto the zeolite.
The amount of aqueous impregnation solution or the amount of transition metal
or
precursor compound of the at least one transition metal present in this
impregnation
solution is set so that an appropriate amount of transition metal is present
on the zeoli-
te after application to the zeolite and drying and calcination. Methods of
determining
the appropriate amounts are known to those skilled in the art.
In one embodiment of the process, the water present on the zeolite after
application of
the at least one metal according to step (A) of the process is removed, for
example by
drying. Methods of drying a solid are known per se to those skilled in the
art, for exa-
mple filtration, centrifugation and/or heating. In a preferred embodiment,
drying of the
zeolite after process step (A) is effected by heat treatment at a temperature
in the ran-
ge from, for example, 10 to 150 C and a pressure of, for example, atmospheric
pres-
sure or a reduced pressure of, for example, less than 800 mbar. The transition
metal-
comprising zeolite which has preferably been dried in this way is preferably
transferred
directly to step (B).
Step (B):
Step (B) comprises calcination of the zeolite from step (A) to convert, if
applicable, the
precursor compound into the at least one transition metal and to obtain a
zeolite com-
prising the at least one transition metal.
Calcination of a solid is known per se to those skilled in the art. The
zeolite which has
been doped with metal cations in step (A) is preferably calcined at a
calcination tempe-
rature of from 300 to 700 C, preferably from 400 to 600 C, particularly
preferably from
450 to 580 C. Calcination can generally be carried out in any suitable
atmosphere.
Preference is given to using an inert atmosphere, for example a nitrogen
atmosphere.
Calcination is carried out until an appropriately doped zeolite catalyst is
obtained. For
example, calcination is carried out for from 1 to 10 hours, preferably from 3
to 6 hours.
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9
In step (B) of the process, any water still present from the impregnation step
(A) and/or
any water of crystallization present and/or any organic solvent present is/are
firstly re-
moved. In addition, the precursor compound of the at least one transition
metal which
is preferably used is converted into the corresponding transition metal and/or
transition
metal oxide and/or the at least one transition metal is at least partly
incorporated into
the lattice structure of the zeolite.
Step (C):
Step (C) comprises application of the sulfur and/or phosphorus atoms or a
precursor
compound thereof to the doped zeolite from step (B).
Preference is given to applying at least one precursor compound of the sulfur
and/or
phosphorus atoms in step (C) of the process. Examples of appropriate precursor
corn-
pounds are selected from the group consisting of sulfurous acid H2S03,
sulfuric acid
H2SO4, phosphinic acid H3P02, phosphonic acid H3P03, phosphoric acid H3PO4 and
mixtures thereof. Preference is given to sulfuric acid and/or phosphoric acid.
In a preferred embodiment, the doped zeolite obtained in step (B) is
impregnated with
an aqueous solution of the appropriate precursor compound. As indicated for
step (A),
an aqueous solution comprising water can be used. It is also possible to use
an aque-
ous solution comprising, in addition to water, a polar, water-soluble solvent,
for examp-
le alcohols such as methanol, ethanol, propanols or mixtures thereof, in step
(C). An
aqueous solution comprising water as solvent is preferably used in step (C).
Very parti-
Impregnation can be carried out by methods known per se to those skilled in
the art, for
example by bringing the zeolite from step (B) into contact with the
abovementioned
After impregnation, the impregnated zeolite can be dried by all methods known
to those
skilled in the art. Appropriate methods have been mentioned for step (A) and
apply
analogously to step (C).
In the process, it is preferred that no washing of the zeolite catalyst takes
place during
or after step (C) since otherwise sulfur and/or phosphorus atoms would be
removed
again, which is undesirable for the purposes of the invention.
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Step (D) of the process comprises (D) calcination of the zeolite from step (C)
in order to
obtain the catalyst used according to the invention.
Calcination of a solid is known per se to those skilled in the art. The
zeolite which has
5 been doped with transition metal cations and sulfur and/or phosphorus
atoms in step
(D) is preferably calcined at a calcination temperature of from 300 to 700 C,
preferably
from 400 to 600 C, particularly preferably from 450 to 580 C. Calcination can
generally
be carried out in any suitable atmosphere. Preference is given to using an
inert atmo-
sphere, for example a nitrogen atmosphere.
Calcination is carried out until an appropriately doped zeolite catalyst is
obtained. For
example, calcination is carried out for from 1 to 10 hours, preferably from 3
to 6 hours.
In step (D) of the process, any water still present from the impregnation step
(C) and/or
any organic solvent present is/are firstly removed. In addition, the precursor
compound
of the sulfur and/or phosphorus atoms which is preferably used is converted
into the
sulfur and/or phosphorus atoms or oxides thereof and/or the sulfur and/or
phosphorus
atoms are at least partly incorporated into the lattice structure of the
zeolite and/or form
a compound with the at least one transition metal from step (A).
The steps (A) and (C) can optionally also be combined. This can be effected,
for exa-
mple, by the above-described transition metal solution and the above-described
soluti-
on comprising sulfur and/or phosphorus atoms being applied in succession or
simulta-
neously without intermediate calcination and intermediate drying. As an
alternative,
steps (A) and (C) can also be carried out directly in succession without the
intermediate
step (B).
An optional dealumination or demetallation of the zeolite catalyst to be used
according
to the invention can be carried out at any point in the production process
mentioned by
way of example, in particular before step (A) and/or before step (C) and/or
after step
(D). The dealumination or demetallation of a zeolite is known in principle to
those skil-
led in the art.
For example, dealumination or demetallation can be effected by treatment with
H20
vapor. The degree of dealumination or demetallation can be set via the
duration of the
steam treatment. The treatment of the zeolite with steam can, for example, be
carried
out at temperatures of from 300 to 800 C for a period of from 0.5 to 48 hours.
The zeo-
lite can be exposed to pure steam or a mixture of nitrogen and/or air and
steam having
a proportion of water vapor of from 1 to 100% by weight at total pressures up
to
100 bar. A carrier gas can optionally be added to the steam or the water vapor
mixture.
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11
Suitable carrier gases are inert under the treatment conditions; examples are
N2, Ar,
He, H2 or a mixture thereof.
The zeolites can, optionally in addition to the steam treatment, also be
dealumina-
ted/demetallated by means of a mineral acid treatment. The acid treatment can
both
remove extralattice metal from the pores and lead to a further dennetallation
of the lat-
tice. This step can, for example, be carried out in a batch reactor at
temperatures of
from 0 to 120 C at an acid/zeolite ratio of from 1 to 100 cm3/g and at acid
concentrati-
ons of from 0.001 M to the maximum concentration of the acid. Examples of
acids
which can be used for this step are HCI, HF, H2SO4, HNO3 and H3PO4. After the
acid
treatment, the zeolite is separated from the reaction mixture by conventional
methods,
e.g. by filtration or centrifugation.
After production of the zeolite catalyst to be used according to the invention
is comple-
te, this catalyst can be converted into a suitable form. This is generally
carried out by
processes known to those skilled in the art, for example pressing,
pelletization, sieving,
crushing, extrusion. Industrially, the zeolite catalyst used according to the
invention is
preferably used in the form of shaped bodies, e.g. extrudates or honeycomb
bodies, for
example having a particle diameter of from 1 to 10 mm, preferably from 1.5 to
5 mm.
As an alternative, the zeolite can be used as starting material in a suitable
form in the
production of the catalyst used according to the invention.
The use according to the invention can generally be employed in all
applications in
which the content of nitrogen oxides in a gas is to be reduced. In a preferred
embodi-
ment, the invention is used in nitric acid production, in adipic acid
production, for power
station offgases, for gas turbines or for automobile catalysts in the low-
temperature
range. Process gases and offgases comprising nitrogen oxide are obtained in
these
processes and the nitrogen oxides can be removed inexpensively by means of the
pro-
cess described here.
The present invention therefore preferably relates to the use according to the
invention
in nitric acid production, in adipic acid production, for power station
offgases, for gas
turbines or for automobile catalysts in the low-temperature range,
particularly prefe-
rably in nitric acid production.
The present invention also provides a process for reducing the content of
nitrogen oxi-
des in a gas by bringing the gas into contact with a zeolite catalyst as
defnied above.
In a preferred embodiment, gases to be treated according to the invention
comprise
nitrogen oxides selected from the group consisting of dinitrogen monoxide N20,
nitro-
gen oxides NON, where x is 1 or 2, and mixtures thereof. In a preferred
embodiment,
CA 02814409 2013-04-11
12
the gas to be treated contains little (NO/N20 < 0.5) and in particular no
nitrogen oxides
NON. Therefore, a stage for removal of NO is inserted upstream in a preferred
em-
bodiment of the process of the invention. Processes for removal of NO are
known to
those skilled in the art.
Particular preference is given to the nitrogen oxide N20 being catalytically
degraded by
means of the process of the invention so that there is overall a reduction in
the content
of this gas in the gas to be treated.
The gas to be treated according to the invention has a content of dinitrogen
monoxide
N20 of, for example, from 10 ppm by volume to 20% by volume, preferably from
200 ppm by volume to 10% by volume, particularly preferably from 500 to 2000
ppm by
volume.
There is no restriction with regard to the further components present in the
gas to be
treated. Routine and therefore preferred further components comprised in the
gas to be
treated according to the invention are selected from the group consisting of
water,
oxygen, NO, NO2, NH3 and N2 and mixtures thereof.
In general, the temperature at which the gas to be treated is brought into
contact with
the zeolite catalyst in the reaction zone is less than 500 C, preferably less
than 400 C,
very particularly preferably from 250 to 400 C.
The present invention therefore preferably provides the process of the
invention carried
out at a temperature of less than 400 C, very particularly preferably from 250
to 400 C.
In a further embodiment, various zeolite catalysts to be used according to the
invention
or one or more zeolite catalysts to be used according to the invention in
combination
with further catalysts known to those skilled in the art can be used. When a
plurality of
different zeolite catalysts and optionally other catalysts are used, these can
be mixed
with one another or be arranged in succession in the reactor. The latter
arrangement is
particularly advantageous when the zeolite catalyst arranged at the inlet end
catalyzes
particularly NO decomposition, optionally in the presence of nitrogen-
comprising redu-
cing agents, and/or the zeolite catalyst arranged at the outlet end catalyzes
particularly
the decomposition of N20.
Particular preference is given to using a uniform above-described zeolite
catalyst in the
process of the invention.
CA 02814409 2013-04-11
13
The reaction zone can, for the purposes of the present invention, in principle
be confi-
gured in any desired way. It can be present, for example, in a tube reactor or
radial
basket reactor.
The gas laden with nitrogen oxides is usually passed over the catalyst at a
space ve-
locity of from 200 to 200 000h-1, preferably from 5000 to 50 000 h-1,
particularly prefer-
ably from 10 000 to 30 000 h-1, based on the catalyst volume. For the present
purpos-
es, the term space velocity refers to the ratio of the volume of gas mixture
under STP
per hour to the volume of catalyst. The space velocity can thus be adjusted
via the flow
velocity of the gas and/or via the amount of catalyst.
The process of the invention is preferably carried out at a GHSV (gas hourly
space
velocity) of from 2000 to 200 000 standard Igas/Icath (standard I: standard
liters ¨ gas
volume at STP), particularly preferably from 5000 to 50 000 standard
Igas/Icath, very par-
ticularly preferably from 10 000 to 30 000 standard !gas/Icath.
The present invention therefore provides, in particular, the process of the
invention in
which the GHSV (gas hourly space velocity) is from 2000 to 200 000 standard
Igas/Lcath
(standard I: standard liters ¨ gas volume at STP), particularly preferably
from 5000 to
50 000 standard Igas/Icath, very particularly preferably from 10 000 to 30 000
standard
Igas/Icath.
The process of the invention is generally carried out at a pressure in the
range from 1
to 50 bar (a), preferably from 2 to 15 bar (a).
The process of the invention can, in one embodiment, be carried out in the
presence of
at least one reducing agent. According to the invention, all reducing agents
which are
able, under the conditions of the process, to reduce the dinitrogen monoxide
N20 which
is preferably to be degraded are suitable.
The present invention therefore preferably provides the process of the
invention in
which at least one reducing agent is additionally used.
Preferred reducing agents are selected from the group consisting of nitrogen
corn-
pounds, for example NH3, hydrocarbons, for example methane CH4 or propane
C3H8,
CO, SO2, H2 and mixtures thereof. Particularly preferred reducing agents are
selected
from the group consisting of NH3, methane CH4, propane C31-18, H2 and mixtures
there-
of.
CA 02814409 2013-04-11
14
The present invention therefore preferably provides the process of the
invention in
which the reducing agent is selected from the group consisting of nitrogen
compounds,
hydrocarbons, CO, SO2, H2 and mixtures thereof.
Apart from NH3, further suitable nitrogen compounds are, for example, azanes,
hydrox-
yl derivatives of azanes, and also amines, oximes, carbamates, urea or urea
deriva-
tives.
An example of an azane is hydrazine.
An example of hydroxyl derivatives of azanes is hydroxylamine.
Examples of amines are primary aliphatic amines such as methylamine.
An example of a carbamate is ammonium carbamate.
Examples of urea derivatives are N,N'-substituted ureas such as N,N'-
dimethylurea.
Ureas and urea derivatives are preferably used in the form of aqueous
solutions.
The way in which the preferably gaseous reducing agent is introduced into the
gas
stream to be treated can be chosen freely for the purposes of the invention;
the reduc-
ing agent is preferably introduced upstream (in the flow direction) of the
reaction zone.
It can also be introduced, for example, into the inlet line upstream of the
vessel before
the catalyst bed or directly before the bed. The reducing agents can be
introduced in
the form of gases or in the form of a liquid or aqueous solution which
vaporizes in the
gas stream to be treated. The introduction of any reducing agent added into
the gas to
be treated is preferably carried out by means of a suitable device such as an
appropri-
ate pressure valve or appropriately configured nozzles.
The amount of any reducing agent added is generally determined so that, based
on the
nitrogen oxide to be degraded, an approximately equimolar amount of reducing
agent
is present in the reactor.
The oxygen content of the reaction gas is preferably less than 10% by volume,
in par-
ticular less than 5% by volume.
The water content of the reaction gas is preferably less than 10% by volume,
in particu-
lar less than 1% by volume.
In general, preference is given to a relatively low water concentration since
higher wa-
ter contents would make higher operating temperatures necessary. This could,
de-
CA 02814409 2013-04-11
pending on the zeolite type used and the time of operation, exceed the
hydrothermal
stability limits of the catalyst and therefore has to be matched to the
individual case
chosen.
5 The content of nitrogen oxides in the gas stream to be treated can be
significantly re-
duced by the process of the invention. For example, from 10 ppm by volume to
20% by
volume, preferably from 200 ppm by volume to 10% by volume, particularly
preferably
from 500 to 2000 ppm by volume, of the nitrogen oxides, in particular
dinitrogen mon-
oxide N20, present at the beginning are degraded by the process of the
invention using
10 the specific above-described zeolite catalyst.
According to the invention, the nitrogen oxides present are catalytically
degraded by,
preferably, being converted into nitrogen N2 and oxygen 02, in the presence of
a reduc-
ing agent additionally into the oxidation product of this reducing agent, e.g.
in the case
15 of H2 into H20.
The process of the invention can be used, in particular, in nitric acid
production, in adip-
ic acid production, for power station offgases, for gas turbines or for
automobile cata-
lysts in the low-temperature range. In these processes, process gases and
offgases
comprising nitrogen oxide are obtained and can be inexpensively freed of
nitrogen ox-
ides by means of the process indicated here.
Examples
1. Catalyst preparation
Commercially available zeolites in the H form as powder are used as starting
materials
for the catalyst preparation. BEAlo is the sales product PB/H from Zeochem and
MF117
corresponds to PZ 2/25H from the same company. FAU40 alias CBV 780, FERio
alias
CP 914C, BEA140 alias CBV 28014 and MF115 alias CBV 3020E can be purchased
from
Zeochem. BEA1.40 is treated at 450 C in a hydrogen atmosphere for 4 hours
before
transition metal and phosphorus and/or sulfur atoms are introduced. This
process im-
proves the crystallinity and acidity of the zeolite.
All catalysts are firstly impregnated with iron nitrate solution according to
the water up-
take of the zeolite. The amount of solution is thus selected so that the
solution is com-
pletely absorbed by the catalyst and is uniformly distributed in the latter.
The amount of
iron nitrate is selected so that, after calcination at 550 C for 4 hours under
a nitrogen
atmosphere, the indicated amount of iron is comprised in the product. The
phosphorus
and sulfur contents specified are subsequently obtained by impregnation
(according to
the water uptake) with appropriately diluted phosphoric or sulfuric acid and
renewed
CA 02814409 2013-04-11
16
calcination under the conditions indicated above. The powders obtained in this
way are
subsequently compacted without washing or similar process steps and crushed. A
frac-
tion having particle sizes from 0.4 to 0.7 mm obtained by sieving is used in
the subse-
quent testing.
2. Testing
The catalyst obtained in this way is installed and tested in a tube reactor.
The amount
of catalyst corresponds in each case to 0.5 ml. The experiments are carried
out at
1.5 bar (a) and a GHSV (gas hourly space velocity) of 8000 standard
lgas/leath. Gas en-
tering the reactor and gas leaving the reactor are analyzed to determine the
nitrous
oxide content by GC analysis (flame ionization detector) in order to be able
to calculate
the depletion or the conversion.
The mixture of 1000 ppm by volume of N20, 3% by volume of 02, 0.3% by volume
of
H20 and the balance to 100% by volume of N2 will hereinafter be referred to as
base
gas. In this mixture, part of the nitrogen is optionally replaced by further
components,
as follows: 1000 ppm by volume of NO (equilibrium composition of NO and NO2),
2000 ppm by volume of H2, 2000 ppm by volume of NH3 and/or 500 or 2000 ppm by
volume of C3H8. These optional additions are in each case indicated in the
table for the
experiment, with the factors 0.5, 1 and 2 before the addition referring to the
amount
introduced in 1000 ppm by volume increments.
The results of the individual experiments are shown in tables 1 and 2. The
conversion
of N20 in the base gas at 300 and 400 C is reported. In the description of the
catalysts
used, the subscripts indicate the amount of transition metal or S and/or P
present in
percent by weight; the amount of zeolite is not indicated since the sum of
zeolite, tran-
sition metal, S and/or P is in each case 100% by weight. For example, the
catalyst
Fe25PO4-BEA140 consists of 2.5% by weight of Fe, 0.4% by weight of P and
balance to
100% by weight, i.e. 97.1% by weight, of zeolite BEA140. "2 means "not
determined".
The catalysts denoted by "C" in tables 1 and 2 are comparative examples.
-
Table 1: Conversion of N20 at 300 C in the base gas
+NOx +NOx +2+12
No. Catalyst pure +NOx +2+12 +2.C2H6 +0.5.C2H6 +2=NH3
+2+12 +2.C2H6
no 02
_
C1 Fe2.5-BEA10 - 0% - 12% - 100% 84% 53% 67%
2 Fe2.5P0.4-BEA10 - 12%
27% 34% 100% 100% 60% 100%
C3 Fe2.5-BEA140 - - - - - - -
96%
4 Fe2.5P0.4-BEA140 - - - - - - -
- 100%
C5 Cu2.5-FAU40 - - - - - - - -
10%
n
6 Cu2.5P2.2-FAU40 --
- - -
17%
_
0
C7 Fe2.5-FER10 0% 5% 7% 10% 7% 9%
- 14% 89% "
CO
H
8 Fe2.5S2.1-FER10 2% - 7% 11% 18% 12% -
17% 100%
0
9 Fe2.5P0.4-FER10 0% 6% 7% 12% 24% 14% -
32% 100%
I.,
-4
0
Fe2.5P0.7-FER10 5% 5% 8% 12% 19% - 19% -
30% 100% -4 H
UJ
I
11 Fe2.5P1.4-FER10 0% 4% 1% 6% 14% 31% _
16% 78% 0
i
12 Fe2.5P2.2-FER10 - - - - - - -
- 65% H
H
13 Fe2.5P2.9-FER10 0% 3% 0% 5% 8% 18% _
12% 27%
14 Fe2.5P3.6-FER10 - - - - - -
- 17%
C15 Fe2.5-MF117 ' 0% 0% 0% 12% 0% 91% - 14%
n. d.
16 Fe2.5P0.4-MF117 1% 2% 0% 16% 8% 90%_
28% n. d.
17 ' Fe2.5130.7-MF117 2% 4% 13% 15% 8% 81%- 27%
n. d.
18 ' Fe2.5P1.1-MF117 0% 3% 0% 15% 19% 88%- 26%
n. d.
19 Fe2.5P1.4-MF117 0% 4% 0% 14% 7% 92% - 26%
n. d.
_
õ
Table 2: Conversion of N20 at 400 C in the base gas
+NOõ +NO), +2112
No. Catalyst pure +NOõ +21-12 +2.C2H6 +2.NH3
+21-12 +2.C2H6 no 02
C20 Fe2.5-BEAt10 - 78% - - 35% 100% - 100%
21 Fe2.5PO4-BEA10 - 82% - - 48% 100% 69% 100%
C22 Cu2.5-BEA10 - - - - 12% - - -
23 CU2.5P0.2-BEA10 - - - - 14% - - -
24 Cu2.5P0.4-BEA10 - - _ - 16% - - -
0
25 Cu2.5P0.8-BEA10 - - _ - 12% - - -
0
I.,
26 CU2.5P1.2-BEA10 - - - - 13% - -
- co
H
FP
27 CU2.5P1.6-BEA10 - - - - 12% - - -
0
C28 Fe2.5-FAU40 - - _ - - - - 77%
0
29 Fe2.5P2.2-FAU40 - - - -
- - - 100% .... H
I
C30 Fe2.5-FER10 2% 34% 49% 62% 23% 85% 22% 100% 0
i
H
31 Fe2.51D0.4-FER10 2% 73% 78% 89% 39% 100% 34% 100% H
32 Fe2.51D0.7-FER10 9% 74% ' 79% 80% 53% 100% 55%
100%
33 Fe2.5P1.4-FER10 2% 49% 62% 50% 42% 95% 24% 100%
34 Fe2.5P2.2-FER10 - 44% 59% - - - - 100%
35 Fe2.5P2.9-FER10 1% 28% 37% 40% 38% 81% 20% 76% .
36 Fe2.5P3.6-FER10 - 23% 38% - _ _ - 73%
C37 Fe2.5-MF115 - - - - - - - 69%
38 Fe2.5P2.2-MFI15 - - - - - - 74%
C39 Fe2.5-MF117 1% 25% 32% 68% 20% 100% 50% -
40 Fe2.5P0.4-MF117 1% 34% 44% 64% 29% 100% 54% -
41 Fe2.5P0.7-MF 117 1% 34% 29% 63% 26% 100% 37% -
42 Fe2.5P11-MF1 17 2% 29% 35% 74% 28% 100% 47% -
43 Fe2.5P1.4-MFI 17 2% 33% 40% - 69% 19% 100% 49% -
C44 Cu2.5-MF117 - - 0%- 9%
45 Cu2.5P0.2-MF117 - - 8% - 14% -
46 CL12.5P0.4-MF117 - 8% - 16% -
47 Cu2.5P0.8-MF117 - - 8% - 20% -
48 Cu2.5P1.2-MF117 - _ 9% - 18% -
49 Cu2.5P1.4-MF117 - - 9%
0
50 Cu2.5P2.2-MF117 - - 9%
CO
51 Cu2.5P4.1-MF117 - - 8%
0
q3.
0
0
