The method of oxidation of ammonia

 

(57) Abstract:

A method is proposed for the oxidation of ammonia, wherein the ammonia and air react in the presence of an oxidation catalyst containing the oxides of cerium and lanthanum, and (b) cobalt and cobalt, cerium and lanthanum are present in such proportions that the atomic ratio of cerium plus lanthanum to cobalt is in the range from 0.8 to 1.2, the said oxides are present as phase mixed oxide, and less than 30 at.% cobalt is present in the form of free cobalt oxides. The technical result is an increase in the activity and selectivity of the catalyst used in the method of the oxidation of ammonia. 3 table.

The present invention relates to the oxidation of ammonia. For the production of nitric acid and hydrogen cyanide is widely used oxidation of ammonia. To obtain nitric acid is subjected to air oxidation of the ammonia before the formation of nitrogen oxides, and hydrogen cyanide mixture of ammonia and methane (typically natural gas) is subjected to oxidation by air. In both methods to achieve the oxidation of ammonia over a catalyst direct gas mixture at elevated temperature. In this case, undesirable such is that in addition to the good of his activity has good selectivity.

For many years, for catalysts used are usually platinum, and sometimes mixed with other precious metals, in the form of nets or fabrics made of metal wire. Such catalysts have good activity and selectivity, but they also have disadvantages: the catalysts are not only very expensive, but their metals exhibit considerable volatility at the temperatures encountered during their applications, so they are lost in the flow of gases. Although enough is known as to apply the tool in the form of traps, set down the stream to trap volatile metal for subsequent retrieval, however, the service life of the catalyst due to the constant volatility of the metal decreases, and must often be replaced. In addition, significant capital costs for the extraction of metals from traps installed downstream, as well as making new meshes or fabrics for catalysts.

Thus, such catalysts of noble metals, it is desirable to replace the other.

It is known that cobalt oxide is active for the oxidation of ammonia. With the aim of improving the activity and the voter is selected metals.

For example, as catalysts for the oxidation of ammonia propose to apply a composition based on oxides of lanthanum/cerium oxide (3)/Cobalt oxide having the General formula LaI-xCExCOOC(where: x=0-1) and received by the special co-deposition method, as described, for example, in the patent CN-A-86108985. It is specified that, as shown by tests conducted on a small scale, such materials have good activity and selectivity, however, there is an assumption that their activity and/or selectivity decrease with operating temperatures at the upper end of the temperature range commonly used for the oxidation of ammonia (interval 800-1000oC).

It was found by us that this type of catalyst is important that the weight of the cobalt was present as phase mixed oxide, for example, as Perovskites patterns R3(RE = rare earth metal) or in the form in which oxygen is not stoichiometric and he is not present as free cobalt oxides, such as oxides of cobalt (II,III) Co3O4or co cobalt Soo. We believe that if a significant amount of cobalt present as free oxides in vremeni to catalyze oxidation during adverse reactions, for example, nitrogen or nitrous oxide, whereas if the mass of cobalt locked in phase mixed oxide, for example? as Perovskite structure, the ability to oxidize more limited the required oxidation.

The manufacture of the catalyst simply by co-deposition of oxide components or compounds, which are easily decomposed in it) or by evaporation of a solution of a mixture of thermally degradable salts, for example nitrates, the desired metals with subsequent annealing at moderate temperatures, for example 600-900oC, optional blocks a lot of cobalt in the mixed phase oxides, for example Perovskite structure, even if the components are present in the required proportions. To obtain the required structure required heat treatment of the product. According to the above-mentioned application CN-A-86108985 catalysts were subjected to calcination at a temperature of 900oC for 5 hours before use, resulting catalyst containing more than 30 at.% cobalt in the form of free oxides. We believe that this heat treatment is not appropriate and to reduce the number of present free of cobalt oxide requires heat treatment at a higher temperaturesare 1150oC, can lead to the decomposition of mixed oxide phases, selection of free cobalt oxides. Alternatively, or additionally, the removal of free cobalt oxides of the composition can be applied stage, for example, the composition can be washed with ammonia solution or other solution containing a complexing agent for cobalt. An example of such a complexing agent is ethylenediaminetetraacetic acid.

Thus, the present invention provides for obtaining a oxidation catalyst containing the oxides of (a) at least one element a selected from rare earth metals and yttrium, and (b) cobalt and cobalt and an element a present in such proportions that the atomic ratio of element content And the cobalt is in the range of 0.8-1.2; at least some of the oxides of cobalt and element a is present as a phase mixed oxide, with at least 30%, preferably less than 25% of cobalt (atoms), present in the form of free cobalt oxides.

So, this catalyst contains at least one phase of mixed oxides containing cobalt and at least one element of A. the Catalyst may also coderev A. The atomic ratio of the element a and the cobalt is between 0.8 and 1.2, in particular from 1.0 to 1.2. As free cobalt oxides are preferably less than 25% (by atoms) of cobalt, and in particular preferably as monoxide cobalt, COO, was attended by less than 15% (by atoms) of cobalt. The contents of the various phases can be determined by x-ray (XRF) or thermogravimetric analysis (TGA), which apply in the latter case, the loss in mass associated with the characteristic thermal decomposition of Co3ABOUT4that occurs at a temperature of about 930oC in air. Preferably less than 10%, in particular less than 5% by weight, the composition is free cobalt oxide (II,III), and less than 2% by weight of free carbon cobalt.

As part or all of the element And apply preferably at least one element selected from yttrium, cerium, lanthanum, neodymium and prasetiya. Element And may contain a mixture of at least one element Vv with variable valency selected from cerium and praseodymium, and at least one element Vn with nepremenno valence selected from yttrium and rare earth elements with nepremenno valence, for example Lunt is the element Vn with nepremenno valence was in the range of 0-1, especially 0-0,3. Preferably most of the cobalt is present as Perovskite phase of Asoo3but when the element And contains two or more elements, such as Vv and Vn, it is not necessary that attended mixed Perovskite phase, for example VvxVn1-xCOO3where x is between 0 and 1. Therefore, it may be Perovskite phase, for example VnCoO3or Vv3mixed with other phases, such as Vv2O, Vn2ABOUT3(VvxVn1-x)2O3or VvxVn1-xO2.

As indicated, the catalyst may be in the form in which the oxygen content is not stoichiometric. This is the result of variable valence of cobalt, as well as any variable valence rare-earth element that is present as part or all of the element A.

This catalyst member can be formed by heating a composition containing oxides of cobalt and element And, preferably air, to a temperature in the range of 900-1200oC for receiving the material which is present as free oxides only a small fraction of cobalt.

The composition can be prepared by precipitation, such as the way of what I or hydroxide to precipitate the corresponding metal (core) carbonate, of hydroxides or oxides and calcining to convert the precipitated compounds to oxides. The use of compounds of alkali metals as a basis for the exercise of deposition is less preferable, because they inevitably cause some contamination of the product with sodium, which can act as a catalyst poison. The deposition can be performed alternatively, but less preferably, by adding base to the solution of mixed salts. Any composition can be obtained by forming a solution of a thermally degradable salts, for example nitrates or salts of organic acids, for example oxalates or citrates, metals in appropriate proportions and evaporate the solution to dryness and calcining to perform decomposition to the corresponding oxides. Less preferably the composition can be prepared by mixing pre-formed metal oxides in appropriate proportions.

In another alternative, you can use part or all of the material element And as a carrier to which is applied a coating of cobalt and any remaining element of A. Thus fine oxide of the element a, for example oxide cetana, with subsequent decomposition of salts of cobalt and any item A. Or such material on the carrier can be obtained by deposition of cobalt and perhaps a certain element as well as thermally degradable compounds in finely ground, for example deposited, the oxide of an element or compound that can decompose it.

Whatever the used method for the preparation of compositions of the oxides, the present composition must be subjected to annealing, for example, in air at a sufficiently high temperature and for a sufficiently long period of time to obtain sufficient material with a mixed structure of oxides, for example Perovskite structure, to combine most, if not essentially all, free of oxides of cobalt in one or more phases of the mixed oxides. As stated, the temperature of calcination is preferably in the interval 900-1200oC. the Duration of the required heating will depend on the applied temperature and the applied method of preparation of the composition. In that case, if the heating temperature is below 1100oC, the preferred heating at least 6 hours. On the other hand, preferred prodoljitel the cobalt, free monoxide cobalt. However, the catalysts obtained by evaporation of a solution containing a mixture of organic salts, such as citrates of the respective metals to dryness and calcining, may require heat treatment for more than short periods of time and/or at temperatures 200-300oC below the temperature required for the compositions obtained, for example, deposition. On the other hand, if the catalyst is obtained by calcination of a mixture of pre-formed oxides, may require longer periods of time and/or higher temperatures to obtain the material in which it is present as free oxides only a small fraction of cobalt.

According to the application CN-A-86108985 catalysts was tested in a small scale with catalysts in the form of a layer of coarse powder.

For practical reasons it is undesirable to apply a layer of powder catalyst in the installation of natural size for the oxidation of ammonia, since it is desirable that the catalyst was in this form that it could directly replace the commonly used grid of the noble metal.

In the patent the Czech Republic, CS 2 is viola, promoted with a small amount of cerium oxide (3), chromium oxide (3) and/or aluminum oxide. However, such catalysts, which contain significantly more cobalt than required for Perovskite structure, will inevitably contain a significant amount of free cobalt oxides.

To obtain the corresponding surface area of the catalyst when using wire media, you must provide the carrier a ceramic coating, a thin layer, and then precipitate on this thin layer of active material. As such thin layers are usually applied composition of aluminum oxide or lanthanum oxide. However, with the conventional high-temperature steel media, there is a risk that the material of the thin layer or impurities remaining in it, such as alkali, resulting from the use of alkaline aluminate solutions for the formation of a thin layer can gradually diffuse during the application of the active material, disrupt the desired structure and interfere with the catalytic process.

However, it was found that when using the primary carriers made of heat-resistant ferrite alloy containing aluminum, you can get herestraat the problem of migration of alkaline impurities in the active catalysts.

Catalytic processes using a layer of disordered Packed elements of the catalyst carrier, having many through channels and are supported catalyst and in which the elements of the carrier can be made of such alloys proposed in the United Kingdom patent-And-2077136. In this reference describes the oxidation of ammonia as an example of a catalytic process, for which you can apply such elements. It also provides a link to the United Kingdom patent-And-1568861, which describes how the application of a suitable thin layer that do not use alkaline solutions.

The corresponding iron alloys/aluminium are those described in the patent of great Britain, 2077136, and in particular those which have the following composition in mass %:

Chrome - 10-25

Aluminum - 3-6

Yttrium and/or cerium - 0-1

Cobalt - 0-5

Carbon - 0-0,5

Iron and usual impurities) - Missing to balance the number of

The presence of yttrium and/or cerium preferable because they have a stabilizing effect on the aluminum oxide formed after firing alloy or the final catalyst. To reduce the migration of components of the alloy or a thin layer in the active catalyst is -1% yttrium, cerium and/or 1-3% cobalt, 0-0,5% carbon, the rest being iron and usual impurities.

According to the present invention, the catalyst is produced preferably by the formation of metallic fabric, mesh or lining of the wire alloy of iron/aluminum, applying a thin layer of aluminum oxide, cerium oxide (3), zirconium dioxide or oxide of lanthanum, for example as described in UK patent-And-1568861, and then by applying a dispersion containing the active composition of the oxides, or a solution of compounds able to decompose the active oxides. A thin layer is applied to the alloy preferably after the implementation of the surface oxidation of the alloy by annealing the alloy in air at a temperature of, for example, 1000oC. Thin layer is applied preferably in the form of Zola, and when they use a thin layer of aluminum oxide, it is preferably also contains yttrium oxide and/or cerium oxide (3). Covered with metal fabric, mesh or the lining then subjected to calcination in air at high temperature to reduce the amount of free cobalt oxides. At the same time, this annealing is to provide a sintering between adjacent wires coated for me the other.

It is established that during the firing of the final composition at a high temperature for the formation of the desired structure of mixed oxides with a minimum content of free cobalt oxides are aluminum oxide and lanthanum oxide, if they are used as a thin layer, in the form diffusion layers, fleeting related components, but with time they become relatively stable, thus during the application happens in the future, little migration. Instead of applying the metal primary carrier, you can use the lining, or mesh, or fabric made of ceramics, such as alpha-alumina, fibers or filaments, for example, by weaving; this ceramic primary carrier may have a secondary carrier in the form of a thin layer according to the above.

Alternatively, instead of applying a metal grid or lining can be applied monolithic carrier in the form of cellular or foamed ceramic material, such as aluminum oxide or zirconium dioxide, or monolithic construction, fabricated from an alloy of iron/aluminum, for example, as proposed in the United Kingdom patent-And-2077136, but not necessary to apply you can apply monolithic design with their channels, oriented at an angle relative to the direction of gas flow. As described, these solid carriers may also have a thin layer of the secondary carrier.

Thus, according to the present invention proposed a catalyst for the oxidation containing the primary carrier in the form of a mesh, fabric, lining or monolith formed of a heat resisting alloy of iron/aluminum, or in the form of a mesh, fabric, lining, monolith or foam from a ceramic material, a secondary carrier in the form of free alkali thin layer of aluminum oxide or lanthanum oxide on the primary carrier; and in the secondary carrier active coating of the oxides of (a) at least one element a selected from rare earth metals and yttrium, and (C) cobalt, and the content of cobalt and element And is of such proportions that the atomic ratio of element a to cobalt is in the range of 0.8-1.2 at least some amount of the oxides of cobalt and element a is present in the form of a mixed phase oxides, less than 30%, preferably less than 25%, cobalt (atoms) present in the form of free cobalt oxides.

When using a ceramic honeycomb or prop the m material for the media.

The catalysts in accordance with the present invention, especially those in the form of fabrics, grids or plates, can be used as a direct replacement for conventional catalysts of noble metals essentially without modification of the ammonia oxidation process, except, of course, that you can exclude devices with the usual traps of the noble metal. During the oxidation of ammonia to nitric oxide to nitric acid, the oxidation process can be performed at temperatures of 800-1000oC, in particular 850-950oC, a pressure of 1-15 bar abs. with the concentration of ammonia in the air 5-15%, often about 10% by volume.

In addition to their use for the oxidation reactions of ammonia catalysts can also be used for other okislenii.

The present invention is illustrated in the following examples.

Example 1

By mixing the solution of the nitrates of lanthanum, cerium and cobalt in such proportions that one atom of cerium was 3 atom of lanthanum and 4 atoms of cobalt, prepared catalyst. The solution was subjected to evaporation to dryness, and the obtained powder was probalily in air at a temperature of 1100oC for 8 hours to obtain the structure of the mixed oxides. Thermogram the A.

In the same way, but excluding cerium nitrate and applying these proportions, one cobalt atom was one atom of lanthanum, prepared the second catalyst. Thermogravimetric analysis showed that 13.3% of the cobalt atoms are present in the form of free cobalt oxide.

The catalysts were tested by placing approximately 0.1 g of the obtained powdery catalyst in the tube microreactor and sending through a tube microreactor mixture of helium containing 5% by volume of ammonia and 10% by volume of oxygen, with the linear speed of 5000 m/h. This corresponds to the flow rate 1,8106h-1. The temperature was then raised from the 100oC to 1000oC speed 30oC/min and were analyzed by escaping gas at different temperatures.

For comparison purposes was tested under the same conditions the lining (0,13 g) of 5 layers of metallic fabric and platinum/rhodium (which has been found to provide optimum selectivity for the oxidation of ammonia to nitrogen oxides).

The selectivity, defined as [NOJ/([NO] + 2[N2]), where [NO] and [N2] respectively represent the volume ratio of nitric oxide and nitrogen in the outgoing gas at different temperatures, shown in the table. 1.

One part was progulivali in air at a temperature of 900oC for 6 hours. Other parts were progulivali in air at temperatures of 1000oC, 1200oC 1300oC and 1400oC, respectively. Chemical analysis of the sample calcined at a temperature of 900oC, showed that the atomic relations of the metals were La:CE:4,6: 1,06: 5, that is, he had the atomic ratio of rare earth metal to cobalt is about 1/13, which is consistent with the observation, h is the mass of the catalyst.

X-ray analysis (R) parts calcined at different temperatures was carried out using silica as an internal standard to determine the relative content of the present oxides of cobalt (II, III) and co cobalt. These data were calculated relative atomic content of cobalt present as free cobalt oxides. The results are presented in table 2.

Studies by electron microscopy show that none of the samples did not contain Perovskite mixed phase lanthanum/cerium/cobalt, although it is possible that Perovskite phase lanthanum/cobalt, L3present in the samples calcined at a temperature of 900oC and above, contained a small amount (less than about 2%) of cerium. However, electron microscopy showed that many particles have Perovskite phase lanthanum/cobalt, attached to or deposited on the oxide particles of cerium and/or cerium oxide, doped with lanthanum.

From these data we can see that the monoxide cobalt detected in the samples calcined at high temperatures, can result in decomposition Perovskite phase lanthanum/cobalt and/or oscsid cobalt at a temperature of about 930oC.

The calcined samples were tested for their selectivity for the oxidation of ammonia by the method described in Example 1, except that after increasing the temperature to 1000oC temperature maintained at this level for 10 minutes, and then it decreased at a rate of approximately 30oC / minute. During raising and lowering of the temperature was carried out analyses of the facing strip. The results are presented in table 3.

From these data it is seen that at high operating temperatures (above about 900oC) a decrease in the selectivity for those samples that have a high content of cobalt in the form of free cobalt oxides. More poor selectivity of the samples during the cooling cycle in contrast to the heating cycle may result from the decomposition of free oxides of cobalt (II,III) to less than electoral monoxide cobalt at high operating temperatures; this difference between the data cycle of heating and cooling is less obvious at lower operating temperatures may result in formation during high-temperature part of the transaction test free monoxide cobalt, undergoes a reversible transition back into otara way described in Example 2, with its final calcination at a temperature of 900oC for 6 hours. X-ray diffraction analysis showed that the atomic relations of the metals were La: CE: 8,54:2,08:10. Thermogravimetric analysis showed that 23.8% of cobalt atoms were present in the form of free oxides of cobalt.

Of the catalyst formed small cylindrical pellets, and a sample of the pellets was tested on the selectivity of the method described in Example 2. At the test temperature of 900oC the selectivity was 92%.

The rest of the catalyst pellet was then supported on a wire mesh as the catalyst in the reactor for the oxidation of ammonia in an industrial installation for the production of nitric acid, which worked in the future under typical operating conditions installation for the production of nitric acid (11-12% of ammonia in the air; operating pressure =1.1 bar; inlet temperature = 200oC; outlet temperature = 910-925oC) within 6 months. Then took it for analysis a sample of the catalyst and subjected him to test the selectivity of the method of Example 2. Thermogravimetric analysis showed that only 5.7% of the cobalt atoms are present in the form of free oxide is given, what level of free oxides of cobalt present in the catalyst, decreased significantly during the first 6 months of operation at elevated temperatures, and this was accompanied by an increase in selectivity. Performance of the catalyst after 6 months of operation are the same as those of the fresh catalyst of platinum/rhodium grids.

The operation process of the oxidation of ammonia was then continued for an extension of 6 months under the same conditions as before, and then subjected to the analysis of another sample, which showed that 5.5% of the cobalt atoms were present as free cobalt oxides. This indicates that the content of free oxides of cobalt was stable with only a very slight further decline occurring during the second 6-month period.

The method of oxidation of ammonia, wherein the ammonia and air react in the presence of an oxidation catalyst containing the oxides of cerium and lanthanum, and (b) cobalt and cobalt, cerium and lanthanum are present in such proportions that the atomic ratio of cerium plus lanthanum to cobalt is in the range from 0.8-1.2, characterized in that the said oxides are present as FA

 

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FIELD: hydrocarbon conversion catalysts.

SUBSTANCE: catalyst for generation of synthesis gas via catalytic conversion of hydrocarbons is a complex composite composed of ceramic matrix and, dispersed throughout the matrix, coarse particles of a material and their aggregates in amounts from 0.5 to 70% by weight. Catalyst comprises system of parallel and/or crossing channels. Dispersed material is selected from rare-earth and transition metal oxides, and mixtures thereof, metals and alloys thereof, period 4 metal carbides, and mixtures thereof, which differ from the matrix in what concerns both composition and structure. Preparation procedure comprises providing homogenous mass containing caking-able ceramic matrix material and material to be dispersed, appropriately shaping the mass, and heat treatment. Material to be dispersed are powders containing metallic aluminum. Homogenous mass is used for impregnation of fibrous and/or woven materials forming on caking system of parallel and/or perpendicularly crossing channels. Before heat treatment, shaped mass is preliminarily treated under hydrothermal conditions.

EFFECT: increased resistance of catalyst to thermal impacts with sufficiently high specific surface and activity retained.

4 cl, 1 tbl, 8 ex

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