Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Carbon monoxide component
Reexamination Certificate
2003-08-21
2004-09-14
Nguyen, Cam N. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Carbon monoxide component
C502S345000, C502S346000
Reexamination Certificate
active
06790423
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to catalysts for the full oxidation of volatile organic compounds (VOC), particularly hydrocarbons, and to a method for the full oxidation of volatile organic compounds (VOC) by using said catalysts.
BACKGROUND OF THE INVENTION
The total combustion of VOC to CO
2
and H
2
O becomes necessary in view of the toxicity and environmental impact of most unburnt VOC. The goal is to minimize the release of VOC into the atmosphere and the forming of CO, which is in turn a toxic component.
The catalysts used most for VOC combustion are:
a) catalysts based on noble metals, which are characterized by a high cost but also by excellent performance in terms of VOC conversion, and operate at temperatures from 200 to 450° C. according to the reactivity of the compound;
b) catalysts based on mixed oxides, typically chromites of copper or of other metals, or barium hexaaluminate, which are characterized by a lower cost but are active in more drastic conditions (temperatures from 400 to 600° C.). This second class of catalysts is also used for catalytic combustors for power generation units. In this case, they operate at temperatures above 900° C.
The types of catalyst used for the combustion of VOC are not free from drawbacks; high cost (for those based on noble metals) and poor activity (for the second class, accordingly requiring operation at higher temperatures, in conditions in which morphologic or structural transformations are facilitated).
SUMMARY OF THE INVENTION
The aim of the present invention is to eliminate the drawbacks of known types of catalyst for the full oxidation of VOC.
In particular, an object of the present invention is to provide catalysts for catalytic combustion which are characterized by high activity, high resistance to temperature, extreme operating conditions, low cost and easy production even as composites and thin films.
Another object of the present invention is to provide catalysts for VOC oxidation with high selectivity for carbon dioxide CO
2
with respect to carbon monoxide CO.
Another object of the present invention is to provide catalysts which lead to full oxidation of the VOC in stoichiometric and non-stoichiometric mixtures of VOC and oxygen (oxidizing or reducing conditions), so that the mixture of gases produced due to VOC oxidation contains no carbon monoxide but contains only carbon dioxide.
Another object of the present invention is to provide a method for the full oxidation of VOC which avoids carbon monoxide removal operations and the known negative consequences of the presence of carbon monoxide in the environment.
Another object is to provide a method for oxidizing VOC to CO
2
which utilizes the full potential of the VOC oxidation reaction, with evident energy-related advantages.
Another object is to provide a method for eliminating carbon monoxide from gas mixtures that contain it together with oxygen.
BRIEF DESCRIPTION OF THE INVENTION
This aim, these objects and others which will become apparent from the detailed description of the invention are achieved by catalysts according to the present invention, which comprise one or more non-stoichiometric crystalline compounds conventionally referenced by formulas which respectively correspond to:
1) A
14
Cu
24
O
41
2) B
4
Cu
5
O
10
(BCuO
2
is also cited in the literature)
and by a method for oxidizing VOC and CO to CO
2
according to the present invention, which uses the same catalysts.
In the first of the above cited formulas, A is Sr or a solid solution of Sr with alkaline-earth metals, alkaline metals, lanthanides; in the second formula, B is Ca or a solid solution of Ca with alkaline-earth metals, alkaline metals, or lanthanides.
DETAILED DESCRIPTION OF THE INVENTION
Examples of catalysts according to the invention have the approximate formula
Sr
14
Cu
24
O
41
or
Ca
4
Cu
5
O
10
Both of these compounds and their derivatives by substitution are widely known in the literature (for Ca
4
Cu
5
O
10
(mentioned as CaCuO
2
): Roth et al, J Am Ceram Soc, Vol. 72, p. 1545 (1989); for Sr
14
Cu
24
O
41
: McCarron et al, Mat Res Bull, Vol. 23, p. 1355 (1988)), and particularly for the compound Sr
14
Cu
24
O
41
there is a vast body of literature associated with its unusual electronic properties. Although it is not possible to formulate exactly the above components, they are unequivocally distinguished by their chemical-physical properties and particularly by the powder diffraction profiles, which correspond to the ones listed in JCPDS international powder diffraction tables, on cards 43-0025 and 46-0054 for the compounds referred to as Sr
14
Cu
24
O
41
and CaCuO
2
, respectively.
The fixing properties of said compounds and derivatives thereof have been described in patent application BO98A000593 herein incorporated by reference. The same patent application describes the use of said compounds to fix gases and gas fixing devices which comprise said compounds.
The inventors have now found that surprisingly said compounds, if prepared so as to have a large specific surface area, preferably larger than 25 m
2
/g (as measured by the BET method), act as catalysts for VOC oxidation. The inventors have found that the reaction for full oxidation of VOC in the presence of the catalysts according to the invention occurs with a high conversion of VOC even at low temperatures.
Moreover, the inventors have found that the catalysts according to the present invention allow VOC oxidation (even when the conversion is partial) with total selectivity toward the forming of CO
2
even in conditions in which there is a significant deficit of oxygen with respect to the stoichiometric mix. The expression “total selectivity” is used to reference the fact that VOC oxidation occurs until only CO
2
and H
2
O are obtained. In other words, in conditions of oxygen deficit, while the quantity that corresponds stoichiometrically to the quantity of oxygen that is present is converted into CO
2
and H
2
O, the other fraction of VOC remains unconverted.
The temperature ranges over which the catalytic oxidation process is active depend crucially on the volatile organic compound to be oxidized. Considering methane as the most stable and least easily oxidizable hydrocarbon, the activation temperatures of the reaction for full oxidation of methane constitute the upper limit of the activation temperatures for any VOC: in particular, the activation temperatures of methane are from 300° C. to 350° C. and from 350° C. to 400° C. for the compounds A
14
Cu
24
O
41
and B
4
Cu
5
O
10
, respectively. The maximum utilization temperatures of the catalysts instead correspond to the decomposition temperatures of the compounds A
14
Cu
24
P
41
and B
4
Cu
5
O
10
, which are proximate to 1000° C. and 750° C., respectively.
The methods for full oxidation of VOC according to the present invention may be performed in combustion chambers or in reheat chambers or flue gas chambers.
The catalytic oxidation reaction of the catalysts according to the present invention occurs on a fixed bed or on a fluid bed.
The catalysts according to the present invention can be in the form of granules.
Advantageously, the catalysts according to the present invention include a substrate material. The substrate can be an inert substrate in the form of a thin film or a composite material. Preferably, the substrate material is constituted by porous substrates which are inert with respect to the above described active materials, such as Al
2
O
3
, TiO
2
, ZrO
2
, CeO
2
, MgO, on which the active material is deposited by impregnation with the aqueous solution of soluble salts (for example nitrates or citrates or acetates or mixtures thereof) of the constituent metals in the correct stoichiometric ratios.
The catalysts according to the invention preferably comprise 5 to 20% by weight of a non-stoichiometric crystalline compound, conventionally designated by a formula which corresponds to A
14
Cu
24
O
41
(I), where A is Sr or a solid solution of Sr with alkaline-earth metals, alkaline metals, lanthanides; or a non-stoichiometric crys
Calestani Gianluca
Dionigi Chiara
Matacotta Francesco Cino
Nozar Petr
Consiglio Nazionale delle Ricerche
Nguyen Cam N.
Wood Phillips Katz Clark & Mortimer
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