Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1998-10-30
2001-04-24
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S213200, C502S065000, C502S067000, C502S075000
Reexamination Certificate
active
06221324
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the destruction of nitrogen oxides in gases such as combustion gases or the effluents from nitric acid synthesis plants, using the process of selective catalytic reduction (SCR) using ammonia.
BACKGROUND OF THE INVENTION
It is known that nitrogen oxides (NO and NO
2
, hereafter NOx) can be reduced to nitrogen N
2
by selective reduction using ammonia (H. Bosch and F. Janssen, Catal. Today, 1988, 369) and thus that these compounds, which are known to contribute to the formation of photochemical fog and acid rain, can be removed from gases discharged into the atmosphere. The reduction of NOx would essentially follow the reactions obeying the following overall equations:
NO
2
+4/3NH
3
→7/6N
2
+2H
2
O
which take place catalytically. Among the catalysts employed, industry has in particular adopted cubic faujasites (FAU) exchanged with copper (European Patent EP 0,483,201 and U.S. Pat. No. 5,536,483) which have excellent activity in the 250-400° C. temperature window and which are particularly well suited to the treatment of tail gases from most nitric acid plants. Until recently, the presence of nitrogen protoxide (N
2
O) in these discharges was barely a matter of concern, this being a gas deemed to be harmless because it is not involved in the formation of acid rain, until account is taken of its not insignificant contribution to the greenhouse effect. Its removal has thus become a concern of public services and industrial companies. It turns out that the gases treated by SCR using ammonia on most catalysts of the prior art, especially on examples of faujasite Y exchanged with copper, may give rise, in certain temperature windows, to the formation of N
2
O.
It is now known, in particular by the use of the profiles of NOx SCR by NH
3
, to distinguish two waves of NO reduction, respectively around 230° C. and above 325° C., and it may be noted in this temperature range that a parasitic reaction of a reduction of NO to nitrogen protoxide N
2
O takes place. It has been possible to correlate this N
2
O formation with a temperature-programmed reduction (TPR) profile, using hydrogen, of the “copper” species in the catalyst, according to a method which was explained in “Characterized Catalysts via Temperature-Programmed Reduction”, Chemtech, 1977, 316-302, by J. W. Jenkins, B. D. McNicol and S. D. Robertson, which authors have developed the TPR technique with analysis by a catharometric cell. The corresponding experimental process is developed below in terms of examples.
DESCRIPTION OF THE INVENTION AND BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
shows a faujasite structure.
FIGS. 2-10
graphically depict results of experiments described in the specification.
It has just been discovered, and it is this which is the basis of the present invention, that certain doubly exchanged copper zeolites do not generate N
2
O over the range of SCR reduction operating temperatures.
The zeolites are microporous crystalline aluminosilicates of general formula:
M
f
n
(AlO
2
)
f
(SiO
2
)
g
hH
2
O
in which
M is the compensating cation for the negative charge generated by replacing SiO
4
tetrahedra by AlO
4
tetrahedra,
n is the oxidation state of M,
f is the number of AlO
2
moles per unit cell,
g is the number of SiO
2
moles per unit cell,
h is the number of H
2
O moles per unit cell.
The SCR catalysts of the prior art are examples of cubic faujasite Y (i.e. those having an Si/Al molar ratio >2.5), the copper of which is the active ion among the compensating ions imposed by the final Si/Al ratio of the zeolite. They will be represented here by
Cu(x)M(100-x)Y
in which
Y represents a cubic faujasite (FAU) lattice,
x is the theoretical degree of copper-ion exchange, expressed as a percentage of the total exchange capacity of the said faujasite,
M being H
+
, Na
+
, K
+
, NH
4
+
or any other cation having the degree of saturation necessary to ensure electrical neutrality of the structure.
The proportion and location of copper ions within the faujasite structure are certainly not independent of its SCR activity and the parasitic production of N
2
O.
FIG. 1
accounts for this structure in which may be distinguished an arrangement of SiO
4
/AlO
4
tetrahedra in truncated cuboctahedra called sodalite cages or &bgr; cages, these communicating via hexagonal prisms and defining larger cavities, the &agr; cages or supercages. Per crystal unit cell of cubic faujasite (FAU), there are 16 hexagonal prisms, 8 sodalite cages and 8 supercages. The diameters of these cavities and their openings are as follows:
Sodalite
Hexagonal
Supercage
cage
prism
Cavity diameter (nm)
1.3
0.66
Opening diameter (nm)
0.74-0.9
0.22-0.26
0.24
In respect of the SCR profile of a copper-exchanged faujasite Y of formula Cu(76)—NaY (the symbol —NaY indicating that, in this particular case, the exchange was carried out on a sodium faujasite Y) for NO conversion and N
2
O production (FIG.
2
), two NO conversion waves, near 230° C. and above 325° C. and two N
2
O formation waves at 240° C. and above 310° C. may be identified. At the same time, the hydrogen TPR diagram of this Cu(76)—NaY shows two peaks P
1
and P
2
resolved by Gaussian deconvolution, at 222° C. and at 327° C. respectively for the low-temperature range (<800° C.) and one peak at 952° C. for the high-temperature range (>800° C.), the ratio of the areas A
1
/A
2
corresponding to the peaks P
1
and P
2
respectively being less than 1.5 in this specific case (FIG.
3
). The interpretation of this that is given is that copper is to a large degree localized in the sodalite cages and that it contributes to the generation of N
2
O.
The SCR and TPR profiles of a faujasite Y ordinarily doubly exchanged with copper and with calcium, and satisfying the formula Cu(44)Ca(28)—NaY (
FIGS. 4 and 5
) are not substantially different from those of a Cu(76)—NaY faujasite and display a behaviour which is interpreted in the same way.
If the SCR and TPR profiles of a copper and calcium doubly-exchanged faujasite are now examined, but using the particular operating method consisting in firstly carrying out the partial calcium exchange, in calcining the result of this first exchange and then in continuing with a copper second exchange, a modification of the TPR diagram (
FIG. 5
) is observed, this diagram no longer including a single low-temperature copper reduction wave around 247° C. and the SCR profile (
FIG. 6
) no longer displays N
2
O production. The interpretation of this that is given is that in this copper faujasite, which always brings about the catalytic destruction of No but no longer generates N
2
O, the copper is mostly located in the large cages.
The invention consists in developing this observation, in applying it to the SCR of nitrogen oxides NOx using ammonia, without generating N
2
O, and in generalizing it to other zeolites as catalytic means of this process. There is no reason to limit the invention to examples of faujasite Y, and faujasites having an Si/Al ratio of from 1 to 20 form part of the invention. It has been verified that the property extended not only to faujasites having sodalite cages in their structure but also to zeolites in which the structural arrangement of the SiO
4
and AlO
4
tetrahedra provides both small cavities accessible through windows having 6 tetrahedra and large cavities accessible through windows formed by at least 8 tetrahedra, and the copper of which occupies only the large cavities.
In order to confer practical reality on the latter proposal, what is demanded of the TPR is to provide its measurable characteristic. These solids having small cavities essentially free of copper and large cavities in which the copper is mainly housed are those which have two waves of hydrogen consumption in temperature-programmed reduction, the first wave at a temperature below 800° C., mainly attributed to the reduction of Cu
2+
ions into Cu
+
ions of the copper localized in the large cavities, the second above 800° C. which is attributed to the reduction of
Coq Bernard
Delahay Gerard
Fajulas Francois
Kieger Stéphane
Neveu Bernard
Grand-Paroiesse S.A.
Griffin Steven P.
Medina Maribel
Smith , Gambrell & Russell, LLP
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