Note: Descriptions are shown in the official language in which they were submitted.
2142889
The invention concerns a catalyst of reduction of the NOX and
more particularly of NO and NO2 as well as a process of reduction
of these NOX.
One knows numerous processes of transformation of the NOX in
nitrogen.
One of these processes consists in using, as a reducing
agent, carbon monoxide or hydrogen and as a catalyst, a catalyst
based on precious metals supported on alumina or silica.
But in an oxidizing atmosphere, the oxidation of the hydrogen
and carbon monoxide by gaseous oxygen precedes the reduction of
NO by hydrogen and this reaction of reduction will therefore only
take place when all the oxygen has been consumed.
This means that in the presence of a high excess of oxygen,
it is necessary to use processes of selective reduction. The most
common process consists in using ammonia as a reducing agent of
the NOX in the presence of a catalyst constituted of V2Os and WO3
deposited on TiO2. This catalyst and this process enable the
reduction of the NOX into nitrogen even in the presence of
sulphur poisons.
But the catalytic materials have a limited lifetime and the
installation requests an important investment. Further the
storage of ammonia as well as its use are delicate.
IWAMOTO et al., propose in Catal. Today, 10, (1991), 57, to
replace ammonia by hydrocarbons. Thus selective reducing
hydrocarbons would be the hydrocarbons having more than two
carbons and particularly propane and propene. The catalysts used
214~889
are zeolites with a high Si/Al ratio exchanged or not with
transition metals.
S. SUBRAMANIAN et al. in Ind Eng. Chem. Res., 31, (1992),
2460, have studied the selective reduction of the NOX by methane
in the presence of a catalyst constituted of palladium supported
on alumina and conclude that the simultaneous elimination of
methane and of NO cannot take place with this type of catalyst.
However, in the European patent application
N~ EP 0,499,286 A2, Y. KAWAI discloses catalysts constituted of
Co-Ag and of Co-Pd on zeolite which would enable to eliminate
simultaneously the carbon monoxide, the NO and the methane even
in the presence of 5% of oxygen and at 500~C. In the same
conditions a catalyst constituted of palladium supported on a
zeolite of the MFI type would not convert NO.
LI and ARMOR in the US patent N~ 5,149,512 use a catalyst
constituted of a zeolite, particularly of the MFI type, exchanged
with Co for the selective reduction of the NOX.
But cobalt is not usable in industrial processes and in
catalytic exhausts of automobile vehicles because it can
transform itself in carbonyl cobalt complexes highly toxic and
polluting for the environment.
Another disadvantage of the catalysts of the prior art
disclosed in particular by LI, BATTAVIO and ARMOR in "Effect of
Water Vapor on the Selective Reduction of NO by methane over
Cobalt-Exchanged ZSM-5" Journal of catalysis 142,561-571, (1993),
is their loss of activity in the presence of water.
3 21~2889
To palliate the above disadvantages and in contrast with this
state of the technique, the present invention proposes a catalyst
of reduction of the NOX by methane or any mixture containing in
majority methane such as for example natural gas, in an oxidizing
atmosphere, constituted of a zeolite of the MFI type exchanged
w th palladium the weight of which is comprised between 0.3% and
2~ of the total weight of catalyst.
According to a characteristic of the catalyst of the
invention, the weight content of palladium is of about 0.5%
According to another characteristic of the invention, the
catalyst is obtained from palladium II tetramine hydroxide as a
precursor of Pd.
According to yet another characteristic of the invention, the
catalyst is activated under oxygen, before utilization by a
process comprising the steps of : (a) elevation in temperature
from room temperature up to 300~C with a rate of elevation in
temperature of 0.5~C/mn, (b) plateau of 300~C during an hour, (c)
elevation in temperature from 300~C up to 500~C with a rate of
elevation in temperature of 0.5~C/mn, (d) maintaining at 500~C
during an hour, and (e) lowering in temperature from 500~C down
to 300~C.
According to still another characteristic of the catalyst of
the invention, after the precited step (e) a step (f) of sweeping
with an inert gas such as helium is realized.
According to a further characteristic of the catalyst of the
invention, the used zeolite of the MFI type has a Si/Al ratio
higher than 15.
4 214~89
The catalyst of the invention enables the reduction of the
NOX in the presence of up to 10% by volume of H2O in the form of
vapor and/or of up to 10% by volume cf ~2 and/or of up to 0.8% by
volume of CO.
According to a particularity of the invention, the selective
reduction of the NOX is effected at a temperature comprised
between about 350~C and 400~C.
The invention has also for object to propose a process of
reduction of the NOX, in a oxidizing atmosphere, by methane or
any mixture containing in majority methane such as for example
natural gas, which comprises a step of reacting a reaction medium
comprising among others methane, oxygen and NOX with a catalyst
constituted of a zeolite of the MFI type exchanged with between
0.3% and 2% by weight of palladium, with respect to the total
weight of catalyst used which enables to obtain the selective
reduction of the NOX into N2.
According to a particularity of the process of the invention,
the zeolite of the MFI type is exchanged with about 0.5% by
weight of palladium with respect to the total weight of catalyst
used.
According to another particularity of the process of the
invention, the precursor of Pd is the palladium II tetramine
hydroxide.
According to yet another particularity of the process of the
invention, the catalyst is activated, before utilization, under
oxygen, by a process comprising the steps of (a) elevation in
temperature from room temperature up to 300~C with a rate of
5 21 l2889
elevation in temperature of 0.5~C/mn, (b) plateau of 300~C during
1 hour, (c) elevation in temperature from 300~C up to 500~C with
a rate of elevation in temperature of 0.5~C/mn, (d) maintaining
at 500~C during one hour, and (e) lowering in temperature from
500~C to 300~C with oxygen.
Still according to a particularity of the process of the
invention, after the above step (e) a step (f) of sweeping with
an inert gas such as helium is realized.
According to a characteristic of the process of the
invention, the said reaction medium can contain up to 0.8% by
volume of CO which is then simultaneously oxidized.
According to another characteristic of the process of the
invention, the said reaction medium can contain up to 10% by
volume of H2O in vapor form.
According to yet another characteristic of the process of the
invention, the said reaction medium can contain up to 10% by
volume of ~2
According to a last characteristic of the process of the
invention, the said step of reacting is effected at a temperature
comprised between about 350~ and about 500~C.
Other details, characteristics and advantages of the
invention will appear better in the course of the detailed
description that will follow and that is done in reference to the
appended figures wherein :
Figure 1 illustrates the influence of the content of Pd
exchanged in a zeolite of the MFI type, on the percentage of CH4
and of NO converted to N2 or (N2O + N2).
6 21428~9
Figure 2 illustrates the influence of the content of Pd
exchanged in a zeolite of the protonated zeolite type with large
pores, on the percentage of CH4 and of NO converted to N2 or (N2O
+ N2) -
The zeolite of the MFI type, prepared according to a process
known by itself presents a Si/Al ratio very variable that can
attain extreme values (1000). This carrier has then served for
the preparation of catalysts based on ions of metals of
transition by cationic exchange between the ions of zeolite (H+,
Na+, Ca2+,...) and those of the metals of transition. This
exchange enables one to obtain metallic ions dispersed and
stabilized within the zeolitic network.
The zeolites exchanged with precious metals according to the
invention were prepared by cationic exchange using adequate
precursors. The palladium (II) tetramine hydroxide is a
particularly preferred precursor.
The following protocol was followed :
5 g of zeolite are put in a beaker, then the precursor is
added after having been diluted in 50 cm3 of distilled water.
Agitation is maintained during an hour at a temperature of 25~C
to obtain the equilibrium of exchange. The solid is then filtered
and washed with an abundant quantity of distilled water (about 1
litre). It is then dried in the oven at 120~C overnight.
This preparation process enables one to obtain a catalyst
based on palladium exchanged in the MFI zeolite which is
interesting in that it has Pd2+ ions, in low quantities indeed,
7 21~28~ 9
but well separated the ones from the others which limits doubly
the possibilities of clustering.
The catalytic tests were effected in a reactor with a flow-
through bed constituted of a U-tube without sinter wherein one
places a little pad in wool of quartz on which is deposited
200 mg of catalyst.
The catalyst is then in situ activated under oxygen by
proceeding to a slow temperature elevation (0.5~C/mn) from 25~C
up to 300~C followed by a plateau of 300~C during 1 hour before
being brought to a temperature of 500~C with a rate of elevation
between 300~C and 500~C of 0.5~C/mn. The temperature is then
maintained at 500~C during 1 hour before proceeding at a lowering
to 300~C under oxygen. One then sweeps the catalyst during
30 minutes with helium before injecting the reaction mixture
comprising gases such as methane, NOX, carbon monoxide, oxygen
and water in the form of vapor.
The exhaust gases are analysed with two chromatographs or
infrared analysers depending on the nature of the gas to analyse.
The first chromatograph is a chromatograph with a catharometric
detection (T.C.D.), INTERSMAT IGC 121 ML. The column of
separation is a CTR column of a length of 2 metres, a diameter of
1/4 of an inch of stainless steel containing a molecular sieve 5A
and PORAPAK Q. The vector gas is helium. The oven, the injector
and the detector are at a temperature of 40~C. The sensitivity of
the detector is 250 mA.
The second chromatograph is a chromatograph with a flame
ionization (F.I.D.) INTERSMAT IGC 120 FB. The column has a length
8 214288g
of 2 metres and a diameter of 1/8 of an inch and contains PORAPAK
Q-
The vector gas is helium. The oven temperature is 130~C. Thetemperature of the injection is 175~C and that of the detector is
170~C.
The Si, Al, Pd contents have been analysed by atomic
absorption.
The invention is based~on the surprising discovery and that,
contrarily to the teachings of the prior art that a catalyst
constituted of a zeolite of the MFI type exchanged with between
0.3% and 2% by weight of palladium with respect to the total
weight of catalyst presents numerous proprieties that will be
described in relation with the examples that will follow.
In these examples, it ls the reduction of NO rather than that
cf NO2 that has been studied because it is admitted by the man
skilled in the art that NO is representative of the NOX.
Spacial velocities of 35.000 h-1 were used for the
realization of the tests effected in the Examples 1 to 35 and 39
to 43.
Examples 1 to 5.
Different catalysts constituted of MFI zeolite with a Si/Al
ratio = 15 exchanged with different contents in Pd were
synthesized as previously described and tested at 500~C with a
reaction mixture comprising 2% by volume of oxygen, 0.1% by
volume of methane and 0.2% by volume of NO.
9 2192889
The convers_ons and selectivities cor espording to the
nlt-2 ac iv_-y cf the cata yst are ~egrolped in the following
T2b e 1 :
Example Content Conversion Conversion Selec~ivitv Se!ec~ivity
N~ of Pd in of CH4 in % of NO in % in N2O in % in ~-2 in %
%
0.3 42 28 0 l 00
2 0.49 78 40 0 l 00
3 0.74 69 28 7 93
4 0.98 73 32 0 lO0
1.94 96 27 8 92
Table 1
These results have also ~een represen ed in the form of
curves 1 anc 2 in the appended figure 1.
On the graph of figure 1, one has represented the evolution
of the conversions of methane (curve 1) ard of NO ~curve 2)
versus the content of Pd (weight %) of tr.e catalyst.
It is then noticed that the conversions of NO and of CH4 are
simultaneously high for palladium contents comprised between
about 0.4% and about 0.6% by total weight of catalyst, the
maximum of activity presenting itself around a palladium content
of 0.5% by weight with respect to the total weight of the
catalyst.
Further, the selectivity in N2 is excellent with this
catalyst. ~hus the formation of N20 , un~esirable, is nearly nil.
- 21~2889
Examples 2 and 6 to 10.
Catalysts constituted of a zeolite of the MFI type with a
S-/Al ratio of 15 exchanged with about 0.49~ by weight of
palladium for the examples 2 and 6 to 8 and with about 0.97% by
weight of palladium for the examples 9 and 10 were tested at
500~C with a reaction mixture containing between 0.2% and 10% by
volume of ~2~ 0.1% by volume of CH4 and 0.2~ by volume of NO.
The conversions and selectivities corresponding to the
initial activity of the catalyst are regrouped in the following
Table 2.
Example Concentration Conversion Conversion Selectivity Selectivity
N~ of ~2 in % of CH4 in of NO in % in N2O in % in N2 in %
%
6 0.2 62 37 0 100
7 0.4 72 40 0 100
8 0.8 78 40 0 1 00
- 2 2 78 40 0 100
9 0.2 39 23 5 95
98 26 25 75
Table 2
It can be seen from these examples that the excellent
capability of converting NO of the catalys s of the invention
11 2l~2889
hardly depends on the content of oxygen when that one varies in
the range of 0.2% to 10% by volume.
Examples 2 and 11.
The reagents and conditions of reactions are similar to those
of the examples 2 and 6 to 8, but the oxygen concentration is set
to 2% in volume and the conversions and selectivities correspond
to the initial activity of the catalyst then at the activity of
said catalyst after 15 h of sweeping (and therefore of reaction).
The results are regrouped in the following Table 3.
Example Time inConversion Conversion Selectivity Selectivity
N~ hoursof CH4 in % of NO in % in N20 in % in N~ in %
2 0 78 40 0 100
11 15 78 40 0 100
Table 3
These results mean that the catalyst of the invention is
stable with time.
Examples 11 to 13.
The reagents and conditions of reaction are similar to those
of the examples 2 and 11. The stability of the catalyst after 15
hours of reaction having been established in the previous
12 21 ~ 2889
examples, 0.8% by volume of CO was added to the reaction mixture
after 15 hours.
The results of these tests are regrouped ln the following
Table 4.
Example Time Concen'¢a~ion Conversion Conversion Selectivity Selecavity
N~ in in CO of CH4 in of NO in % in N20 in N2
hours in% % in % in%
1 1 15 0 78 40 0 100
12 16 0.8 75 40 0 100
13 20 0.8 78 40 0 100
Table 4
As shown by the results regrouped in this Table, the
introduction in the reaction mixture of 0.8% of CO modifies
neither the conversion of methane nor that of the NO. The carbon
monoxide is totally converted to CO2 at temperatures very
inferior to those needed for the reduction of NO by methane. This
property is important because the gases issued from the
combustion of methane may contain CO. The latter may come either
from an incomplete oxidation of methane or from WGS (water-gas-
shift) :
C~2 + H2 = CO + H20,
or also from the reforming of methane :
CH4+H20= CO+3H2
_ 13 21~288 9
In every cases CO appears as a supplementary reducer present
in the effluents. CO may then react with NO to provide CO2 and
N2. This reaction competes with the reduction of NO by CH4.
Thus one could have thought that the presence of CO in the
reaction mixture would have caused a decrease in the methane
conversion. In fact, it is not so, the catalyst of the lnvention
enabling not to modify the conversions of CH4 even in the
presence of 0.8~ of CO. Furthermore the selectivity in N2 is not
modified.
Examples 14 to 17.
Tests were effected with catalysts constituted of a zeolite
of the MFI type with a Si/Al ratio of l9 exchanged with about
0.97% by weight of Pd and with a reaction mixture comprising l.6%
by volume of oxygen, 0.1% by volume of CH4 and 0.2% by volume of
NO and in which water was introduced in the form of vapor, in
order to test the activity and the selectivity of this catalyst
in the presence of water.
Indeed the gases issued from the combustion of methane
contain vapor of water and the latter is known to deteriorate the
performances of this type of catalysts.
The results are regrouped in Table 5.
21~2889
14
Example Time in Concentration Conversion Conversion Selectivity Selectivity
N~hours afterin H20 of CH4 in of NO in % in N20 in N2
the beginning in % % in % in %
of the
sweeping
14 0 0 89 38 5 95
0 5.3 91 32 10 90
16 1 7.5 92 31 19 81
17 2 10 89 27 6 94
Table 5
These results show that up to a content of water of about 10%
in volume, the activity of the catalyst remains excellent. The
conversion of methane is constant and that of NO moves from 38 to
27%.
Examples 18 to 21.
The catalyst was then tested with natural gas and that in the
presence of oxygen, in order to test the applicability of this
catalyst to the treatment of the exhaust gases of motors of
vehicles functioning with natural gas or equipped with a
bicarburation system (gasoline/natural gas).
The reagents and conditions of reactions are similar to those
of the examples 2 and 6 to 8 but the content of palladium is set
to 0.97%.
21 ~2889 ~
The volumetric composition of the natural gas ls the
following :
CH4 = 91.37%
N2 = 0-70%
C2H6 = 6.70%
C3Hg = 1.10%
isoC4H1o = 0.04%
nC4H1o = 0.04%
neoCsH12 = 0.01%
isoCsH12 = 0.02%
nC5H12 = 0-02%
The conversions and the selectivities corresponding to the
initial activity of the catalyst are regrouped in the following
Table 6.
Example Concentration Conversion Conversion of Conversion Selectivity Selectivity
N~ in 02~f CH4 in hydrocarbons of NO in %in N20 in N2
in % % other than in % in %
CH4in%
18 0.2 91 100 28 6 94
19 0.4 94 100 30 7 93
0.8 95 100 31 6 94
21 2 89 100 32 10 90
Table 6
16 21 ~2889
These results show that the activity of the catalyst of the
invention is just as good, even when using natural gas.
Examples 22 to 24
Finally,-assays were realized with catalysts identical to
those of the examples 14 to 17 in the presence of a mixture
representative of exhaust gases coming from a combustion of
gasoline (stoechiometric mixture) and at different temperatures
of reaction.
Indeed in the context of an application to a vehicle equipped
with a system with a bicarburation gasoline/natural gas, the
catalytic exhaust must be able to treat exhaust gases issued from
the combustion of these two fuels.
The reaction mixture then comprises 5025 ppm of CO, 685 ppm
of NO, 1060 ppm of C3H6 and 6940 ppm of ~2
The results of these tests are regrouped in the following
Table 7.
Example Temperature Conversion Conversion of Conversion Selectivity Selectivity
N~ in ~C of CO C3H6 ~f NO in N20 in N2
in % in % in % in ~/O in ~/O
22 360 56 70 44 0 1 00
23 409 100 98 74 15 85
24 440 100 100 100 0 100
Table 7
21~2889
Therefore, the catalyst shows an excellent activity for
exhaust gases issued from the combustion of natural gas and
gasoline at temperatures comprised between about 350~C and about
450~C
Moreover, for comparison purposes, different catalysts have
been tested :
- catalysts constituted of a zeolite of the mordenite type
with large pores exchanged with different contents in Pd,
- catalysts constltuted of zeolite of the mordenite type with
large pores and MFI type exchanged or impregnated with cokalt at
varying concentrations.
Examples 25 to 29.
In these examples, the protonated zeolite with large pores is
obtained by firing a commercial ammoniated mordenite of a
composition corresponding to a Si/Al ratio of 5.5.
This NH4M zeolite is fired to eliminate NH3 and to obtain the
protonated form of the mordenite.
This zeolite is then exchanged with palladium to different
contents and tested at a temperature of 500~C with a reaction
mixture comprising 2% by volume of oxygen, 0.1% by volume of CH4
and 0.2% by volume of NO.
The conversions and selectivities corresponding to the
initial activity of the catalyst are regrouped in the followlng
Table 8.
18 2112889
Example Content of Conversion Conversion of Selectivity Selectivi~
N~ Pd in % of CH4 in NO in % in N2O in in N2
% % in%
0.52 67 18 0 100
26 0.7 98 27 0 100
27 0.87 100 24 0 100
28 0.95 91 16 4 96
29 1.26 100 19 0 100
Table 8
Besides, these results have been reported in figure 2 which
illustrates the influence of the content of Pd in a zeolite of
the mordenite type with large pores on the activity of the
catalyst. In this figure, one has plotted on the absciss the
percentage by weight with respect to the total weight of catalyst
of Pd exchanged in a zeolite of the mordenite type with large
pores and on the ordinate the corresponding percentages of
converted CH4 (curve 3) and NO (curve 4).
On the whole range of contents of palladium studied, one can
note that the activity of this catalyst in converting NO is lower
than that of the catalysts of the invention.
Examples 30 to 35.
Assays were also effected on catalysts constituted of zeolite
of the MFI type exchanged or impregnated with cobalt.
21~2889
19
The catalyst constituted of MFI zeolite exchanged with cobalt
was prepared by exchanging the protons of the MFI zeolite with
cobalt ions.
Other catalysts were prepared by impregnation of cobalt on a
carrier of zeolite of the MFI type and on a carrier of zeolite of
the protonated mordenite type with large pores according to the
following operation mode :
the precursor was cobalt nitrate dissolved in water that had
been added to a known weight of zeolite. One left it under
agitation during 1 hour then the water was removed by evaporation
at 60~C in an evaporator under reduced pressure. After drying
overnight at 120~C, the solid was fired under oxygen with a slow
elevation in temperature of 0.5~C per minute followed by two
successive plateaux of 300~C during 1 hour and cf 500~C during 1
hour.
These catalysts based on cobalt were tested at a temperature
of 500~C with a reaction mixture containing 2% by volume of
oxygen, 0.1% by volume of methane and 0.2% by volume of NO.
The conversions and selectivities corresponding to an
activity of the catalyst after variable times of sweeping (and
consequently of reaction) are regrouped in the following Table ~.
2142889
Example Catalyst Time in Conversion Conversion Selectivit Selectivit
N~ hours after of CH4 of NO y in N2O y in N2
the in % in ~/0 in % in %
beginning of
the
sweeping
0.4%Co/MFI (e) 0 26 5 0 100
31 0.4%Co/l~:FI (e) 2 26 4 0 100
32 4.65%Co/HMLP* 0 86 33 0 100
(i)
33 4.65%Co/HMLP* 21 88 38 0 100
(i)
34 4.3%Co/MFI (i) 0 80 7 0 100
4.3%Co/MFI (i) 1 81 9 0 100
Table 9
* : mordenite type with large pores
(e) : prepared by exchange
(i) : prepared by impregnation
These results show that only a catalyst constituted of a
zeolite of the mordenite type with large pores impregnated with
4.65% by weight of cobalt with respect to the total weight of
catalyst presents an activity comparable with that of the
catalyst of the invention. However this type of catalyst can lead
to the production of toxic complexes of cobalt carbonyl.
21 21~2889
Examples 36 to 38.
The following tests were realized with a reaction mixture
comprising 1000 ppm of CH4, 2000 ppm of NO and 2~ Of ~2 on a
catalyst of the invention, at a temperature of 500~C, in order to
confirm the influence of the spacial velocity on the conversion
of CH4 and NO.
Indeed, the higher the spacial velocity, the lower the time
of contact of the reaction medium with the catalyst, which leads
normally to a decrease in the percentages of converted CH4 and
NO.
The conversions of H4 and NO obtained at different spacial
velocities are regrouped in the following Table 10.
ExampleSpacial velocit,v Conversion of Conversion of
N~ in h-l CH4 in % NO in %
36 35000 90 22
37 17500 98 29
38 8750 100 31
Table 10
The Examples 36 to 38 show that a decrease in the spacial
velocity of a factor 4 increases the conversion of NO of about
40% and the conversion of CH4 of about 10%.
Examples 39 to 43.
22 21 42889
The influence on the conversion of NO, of the quantity of
reducer (methane) used has been studied.
A catalyst constituted of a zeolite of the MFI type with a
Si/Al ratio = 18, exchanged with 0.97% of Pd was tested in the
presence of 2~ of oxygen, at a temperature of 500~C and a spacial
velocity of 35000 h-1. The concentration in NO was maintained
constant at 2000 ppm.
The results are regrouped in the followlng Table 11.
ExampleConcentration inCH4 Concentration Conversionof
N~ in ppm in NO in ppm NO in %
39 1000 2000 33
2000 2000 43
41 3000 2000 45
42 4000 2000 45
43 5000 2000 43
Table 11
The results regrouped in Table 11 show an increase in the
percentage of NO converted when the concentration in methane
increases from 1000 ppm to 2000 ppm. After 2000 ppm and up to
5000 ppm of CH4, the percentage of converted NO remains constant.
The catalyst according to the invention presents consequently
the advantage of being able to be used in an oxidizing
atmosphere, more particularly in an atmosphere containing up to
10% of oxygen, with methane or natural gas or gasoline, in the
presence of up to 10% of water, and that in difficult conditions,
23 21~2 889
that is with a low concentration in Pd, high spacial velocities
and low concentrations in reducer. Further it does not lead to
the production of toxic compounds, such as CO which is totally
converted into C02, which would allow in particular a use as a
catalyst in the catalytic exhausts of vehicles functioning with
natural gas.
Of course, the invention is in no way llmited to the
embodiments described and illustrated which were given by way of
examples only.
Thus, the catalyst according to the invention could be used
in other operation conditions, for example, with different
concentrations in NO, CH4, whether in fixed or fluidized bed.
That is to say that the invention comprises all the technical
equivalents of the means described as well as their combinations
if they are effected according to its spirit.
