MXPA99005205A - Ammonia oxidation catalyst - Google Patents

Abstract

A catalyst for oxidation reactions, particularly the oxidation of ammonia comprises oxides of (a) at least one element A selected from rare earths and yttrium, and (b) cobalt, said cobalt and element A being in such proportions that the element A to cobalt atomic ratio is in the range 0.8 to 1.2, at least some of said cobalt and element A oxides being present as a mixed oxide phase with less than 25%of the cobalt (by atoms) being present as free cobalt oxides. The catalyst may be supported on a secondary support in the form of an alkali-free alumina or lanthana wash coat on a primary support inthe form of a mesh, gauze, pad, or monolith formed from a high temperature iron/aluminium alloy or a mesh, gauze, pad, monolith, or foam of a ceramic material.

Description

PROCESS FOR AMMONIAC OXIDATION DESCRIPTION OF THE INVENTION This invention relates to the oxidation of ammonia. Oxidation of ammonia is widely used in the manufacture of nitric acid and hydrogen cyanide. In the manufacture of nitric acid, ammonia is oxidized with air to nitric oxide, while in the manufacture of hydrogen cyanide, a mixture of ammonia and methane (often as natural gas) is oxidized with air. In both processes, the gas mixture is passed at an elevated temperature over a catalyst to carry out the oxidation. Lateral reactions, such as the formation of nitrogen or nitrous oxide, are undesirable. Consequently, it is required that, in addition to good activity, the catalyst has good selectivity. For many years, the catalysts used have been platinum, sometimes with alloys with other precious metals, in the form of meshes or gauzes formed of metallic wire. Such catalysts have good activity and selectivity but suffer from the disadvantage that only the catalyst is very expensive, but also at the working temperatures, the metal shows an appreciable volatility and in this way metal is gradually lost in the flow of water. gas. Although it is well known that downstream means can be provided to trap the volatilized metal so that it can be subsequently recovered, due to the continuous volatilization, the duration of the catalyst is short and replacement is often necessary. In addition, the recovery of metals from the downstream trap and the remanufacturing of meshes or catalyst gauzes involves a considerable allocation of working capital. Therefore, it is desirable to provide a substitute for such precious metal catalysts. It is well known that cobalt oxide shows activity for ammonia oxidation. In order to improve activity and selectivity, numerous proposals have been made to incorporate various promoters such as rare earths into a cobalt oxide catalyst. For example, it has been proposed in CN-A-86108985 to use lanthanum / ceria / cobalt oxide compositions of the general formula (wherein x is from 0 to 1) manufactured by a specified coprecipitation route as oxide catalysts for oxidation of ammonia. It has been reported that such materials have good activity and selectivity when tested on a small scale, although there is some suggestion that the activity and / or selectivity decreases at operating temperatures in the upper end of the temperature range normally used for ammonia oxidation. (800-1000 ° C).

We have found it important that, in this type of catalyst, the volume of the cobalt be present as a mixed oxide phase, for example, such as the Perovskite structure RECo03 (RE = rare earth), or a form thereof in the which oxygen is non-stoichiometric, and is not present as free cobalt oxides, for example cobalt-cobalt oxide Co304 or cobalt monoxide CoO. We believe that if an appreciable proportion of cobalt is present as the free oxides, in use at elevated temperatures, for example above about 850 ° C, the free cobalt oxides * are likely to catalyze lateral reaction oxidations, eg, to nitrogen or nitrous oxide, while the volume of the cobalt is "immobilized" in a mixed oxide phase, such as the perovskite structure, and the oxidation capacity is more limited to the oxidation that is desired. Producing a catalyst simply by coprecipitation of the constitutive oxides (or compounds that readily decompose to them) or by evaporation of a solution of a mixture of thermally decomposable salts, for example nitrates, of the desired metals, followed by calcination at moderate temperatures, for example 600-900 ° C, does not necessarily immobilize the volume of cobalt in a mixed oxide phase such as the Perovskite structure even if the components are present in requisite proportions. The heat treatment of the product is necessary to obtain the desired structure. In the document mentioned above CN-A-86108985, the catalyst is calcined at 900 ° C for 5 hours before its use; we consider that such heat treatment is inadequate, and a treatment at higher temperatures and / or for longer times is required to minimize the amount of free cobalt oxide present. However, heating at such a high temperature, above about 1150 ° C, can result in the decomposition of mixed oxide phases, releasing free cobalt oxides. Alternatively or additionally, steps can be taken to remove the free cobalt oxides from the composition: for example, the composition can be washed with an ammonia solution or another solution containing a complexing agent for cobalt. Ethylenediaminetetraacetic acid is an example of such a complexing agent. Accordingly, the present invention provides an oxidation catalyst comprising oxides of: (a) at least one element A which is selected from yttrium rare earths, and (b) cobalt, cobalt and element A are in such proportions so that the atomic ratio of the element A to the cobalt is in the range of 0.8 to 1.2, at least part of the cobalt oxides and of the element A are present as an oxide phase mixed with less than 30%, preferably less of 25% of cobalt (by atoms) present as free cobalt oxides.

Therefore, the catalyst contains at least one mixed oxide phase containing cobalt and at least one element A. The catalyst may also contain oxides of free element A and / or one or more mixed oxide phases containing 2 or more elements A. The atomic ratio of element A to cobalt is from 0.8 to 1.2, particularly from 1.0 to 1.2. Preferably less than 25% (in atoms) of the cobalt is present as free cobalt oxides, and in particular, it is preferred that less than 15% (in atoms) of the cobalt is present as the cobalt monoxide, CoO. The proportion of the various phases can be determined by X-ray diffraction (XRF) or by thermogravimetric analysis (TGA) making use, in the latter case, of the weight loss associated with the characteristic thermal decomposition of Co304 which occurs at approximately 930 ° C in the air. Preferably less than 10%, particularly less than 5% by weight of the composition is free cobalt-cobalt oxide and less than 2% by weight is free cobalt monoxide. Preferably, at least one element selected from yttrium, cerium, lanthanum, neodymium and praseodymium is used as part or all of element A. The element A may comprise a mixture of at least one variable valence element Vv which is selected from cerium and praseodymium and at least one non-variable valence element Vn selected from yttrium and the rare earth elements of non-variable valence such as lanthanum or neodymium. In particular, it is preferred that the atomic ratios of the variable valence element Vv to the non-variable valence element Vn be in the range of 0 to 1, particularly 0 to 0.3. It is preferred that most of the cobalt be present as ACo03 in the Perovskite phase, but when the element A comprises two or more elements, for example Vv and Vn, it is not necessary to have a mixed Perovskite phase, for example VvxVn1. xCo03 where x is between 0 and 1. Therefore, there may be a phase of Perovskita, for example, VnCo03 or VvCo03, mixed with other phases such as Vv203, Vn203, (VvxVn1-x) 203 or ^ n ^^. As indicated above, the catalyst may be in a form in which the amount of oxygen is non-stoichiometric. This arises from the variable valence of cobalt and also of any variable valence of rare earth present as a part, or as the whole of the element A. The catalyst can be formed by heating a composition containing the cobalt oxides and of the element A, preferably in air, to a temperature in the interval 900-1200 ° C in order to produce a material in which only a small proportion of the cobalt is present as free oxides. The compositions can be made by precipitation, for example by adding a solution of soluble salts of the relevant metals to a solution of a base, for example carbonate or ammonium hydroxide, to precipitate the relevant metals such as (basic) carbonates, hydroxides or oxides followed by calcination to convert the precipitated compounds to the oxides. The use of alkali metal compounds as the basis for carrying out the precipitation is less preferred as it inevitably causes some contamination of the product with sodium which can act as a catalyst poisoner. Alternatively, although less preferred, precipitation can be carried out by adding the base to the solution of the mixed salts. Alternatively, the composition can be made by forming a solution of thermally decomposable salts, for example, nitrates or salts of organic acids, for example oxalates or citrates, of the metals in the appropriate proportions and evaporating the solution to dryness followed by calcination to carry out the decomposition to the appropriate oxides. More preferably, the composition can be worked up by mixing preformed oxides of the metals in the appropriate proportions. In another alternative, part or all of the material of element A can be used as a support on which cobalt is coated and any remaining element A. Therefore, an oxide of finely divided element A, for example ceria, can be impregnated with a solution containing a cobalt salt, and possibly also a salt of element A, for example lanthanum salt, followed by decomposition of the cobalt and any salt of element A. Alternatively, such supported material can be manufactured by precipitation by precipitating cobalt and, optionally, part of element A, as compounds that can be decomposed by heat on a finely divided element A, for example precipitate, or a compound that can be decomposed for it. Regardless of the route used to manufacture the oxide composition, the composition must be calcined, for example, in the air, at a sufficiently high temperature and for a sufficiently long time to form sufficient material with a mixed oxide structure, for example the structure of Perovskite to combine most, if not essentially all, of the free cobalt oxides in one or more phases of mixed oxide. As indicated above, the calcination temperature preferably is in the range of 900-1200 ° C. The duration of the necessary heating will depend on the temperature used and the way used to make the composition. If the heating temperature is below 1100 ° C, heating is preferred for at least 6 hours. On the other hand, the duration of the heating at a temperature greater than 1150 ° C is preferably less than 6 hours in order to minimize the decomposition of the phases containing cobalt oxide into free cobalt monoxide. However, catalysts prepared by evaporating a solution containing a mixture of organic salts, for example citrates, from the relevant metals to dryness, followed by calcination, may require heat treatment for shorter times and / or at temperatures of 200-300. ° C below the temperatures necessary for compositions manufactured, for example, by precipitation. On the other hand, if the catalysts are manufactured by calcining a mixture of preformed oxides, longer times and / or higher temperatures may be required to produce a material of which only a small proportion of the cobalt is present as free oxides. In the document CN-A-86108985 mentioned above, the catalysts are tested on a small scale with the catalyst in the form of a bed of a coarse powder. For practical reasons, it is not desirable to use a bed of a pulverized catalyst in a full-sized ammonia oxidation plant: desirably, the catalyst must be in a form such as to allow direct replacement by conventionally used precious metal meshes or gauzes . It has been proposed in the Czech patent CS 266106 to use a catalyst in the form of a stainless steel mesh that supports a coating of a cobalt oxide mixture promoted with small amounts of ceria, chromia and / or alumina. However, such catalysts, which contain much more cobalt than is necessary for the Perovskite structure, will inevitably contain substantial proportions of free cobalt oxides. In order to generate a suitable surface area of catalyst when a wire support is used, it is necessary to provide the support with a ceramic coating, called a washing coating, and then the active material is deposited on this washing coating. Usually, alumina or lanthanum compositions are used as such wash coatings. However, with conventional high-temperature steel supports, there is a risk that the material of the wash coating, or the impurities remaining therein, for example, alkaline substances, result from the use of alkaline aluminate solutions to form the wash coating, which in use may diffuse gradually into the active material by altering the desired structure and interfering with the catalytic performance. However, we have found that by using primary supports manufactured at high temperature of a ferritic alloy containing aluminum, it is possible to obtain a good adhesion of the wash coating to the primary support without the use of alkaline washing solution solutions and this In this way, the problem of migration of alkaline impurities to active catalysts can be avoided. In GB-A-2077136 catalytic processes have been proposed using a bed of randomly packed catalyst support units having a plurality of through passages and in which the catalyst is supported and in which the support units can be supported. manufacture from such alloys. This reference mentions the oxidation of ammonia as an example of a catalytic process for which such units can be used. This reference also mentions GB-A-1568861 for methods of applying a suitable wash coating which does not involve the use of alkaline wash coating solutions. Suitable iron / aluminum alloys are those described in GB-A-2077136 mentioned above, and in particular are those of the following weight composition: chromium 10-25% aluminum 3-6% yttrium and / or cerium 0 -1% cobalt 0-5% carbon 0-0.5% iron (and usual impurities) rest The presence of yttrium and / or cerium is preferred as they exert a stabilizing effect on the alumina formed by calcination of the alloy or the final catalyst. The presence of cobalt may also be desirable to minimize the migration of components from the alloy or the wash coating to the interior of the active catalyst: the preferred alloys contain 15-25% chromium, 4-6% aluminum, 0.3-1% of yttrium, cerium and / or 1-3% cobalt, 0-0.5% carbon, and the rest of iron and usual impurities. In the present invention it is preferred that the catalyst be made to form a gauze, mesh or wire pad of an iron / aluminum alloy, to apply a wash coating of alumina, ceria, zirconia or lanthana, for example as described in GB-A-1568861, and then applying a dispersion containing the composition of active oxides or a solution of compounds can be decomposed to the active oxides. The wash coating is preferably applied to the alloy after carrying out the surface oxidation of the alloy by calcining the alloy in air at, for example, 1000 ° C. The wash coating is preferably applied as a sol and, when an alumina wash coating is used, preferably also contains yttrium and / or ceria. The coated gauze, mesh or pad is then subjected to calcination in high temperature air to reduce the amount of free cobalt oxides. At the same time, this calcination will effect some sintering between the adjacent coated wires to join the gauze, mesh or pad into a sturdy structure at points where the adjacent strands of wire contact each other. During this calcination of the final composition at high temperature to form the desired mixed oxide structure with minimal free cobalt oxides, it is found that alumina, and lanthanum if used as the wash coating, are present as layers diffusers that extend into adjacent components, but that are subsequently relatively stable so that during their use there is little additional migration. Instead of using a primary support, a pad, mesh or gauze formed of ceramic material can be used, for example, alpha alumina, fibers or filaments, for example by corrugation. Such a ceramic primary support may have a secondary wash coating support as in the above. Alternatively, instead of using a gauze, mesh or pad, a monolithic support in the form of a honeycomb or foam of a ceramic material such as alumina or zirconia, or a monolithic structure formed of an iron / aluminum alloy, can be used, for example as proposed in GB-A-2077136, but not necessarily used as a random packed bed of units as proposed in GB-A-2077136. Therefore, monolithic structures can be used with their conduits oriented at pre-set angles to the gas flow direction. Such monolithic supports may again have a secondary wash coating support as described above. According to the present invention, there is further provided an oxidation catalyst comprising a primary support in the form of a mesh, gauze, pad or monolith formed from a high temperature iron / aluminum alloy or mesh, gauze, pad , monolith or foam of a ceramic material, a secondary support in the form of an alumina wash coating or lanthanum free of alkaline material on the primary support; and supported on the secondary support, an active coating of oxides of: (a) at least one element A which is selected from rare earths and yttrium, and (b) cobalt, the cobalt and the element A are in such proportions so that the atomic ratio of element A to cobalt is in the range of 0.8 to 1.2, at least part of the cobalt oxides and element A are present as an oxide phase mixed with less than 30, preferably less than 25% of cobalt (by atoms) present as free cobalt oxide. When a honeycomb or foam ceramic material is used, it can be formed from the catalyst composition and thus the need for a separate support material is avoided. The catalysts of the invention, particularly those in the form of gauzes, meshes or pads, can be used as a direct substitute for the conventional precious metal catalyst essentially without modification to the ammonia oxidation process, except of course that the dispositions can be eliminated. of conventional precious metal trap. In the oxidation of ammonia to nitric oxide for the manufacture of nitric acid, the oxidation process can operate at temperatures of 800-1000 ° C, particularly 850-950 ° C, pressures of 1 to 15 bar absolute, with ammonia in concentrations at air of 5-15%, often about 10% by volume. In addition to the use for ammonia oxidation reactions, the catalyst can also be used for other oxidations. The invention is illustrated by the following examples.Example 1 A catalyst is made by mixing a solution of nitrates of lanthanum, cerium and cobalt in proportions such that 3 lanthanum atoms and 4 cobalt atoms are provided per cerium atom. The solution is evaporated to dryness and the resulting powder is calcined in the air at 1100 ° C for 8 hours to provide a mixed oxide structure. The TGA indicates that 5.8% of the cobalt atoms are present as free cobalt oxide. A second catalyst is prepared by the same route but omitting the cerium nitrate and using such proportions that there is one lanthanum atom per cobalt atom. The TGA indicates that 13.3% of the cobalt atoms are present as free cobalt oxide.

The catalysts were tested by placing approximately 0.1 g of the resulting pulverized catalyst in a microreactor tube and by passing a helium mixture containing 5% by volume of ammonia and 10% by volume of oxygen through the microreactor tube at a linear velocity of 5000 m / h. This corresponded to a space velocity of 1.8 x 106h "1, after which the temperature was increased from 100 ° C to 1000 ° C at a rate of 30 ° C / min and the exhaust gas was analyzed at various temperatures. In comparison, a pad (0.13 g) of 5 layers of a platinum / rhodium gauze (which has been found to provide optimum selectivity for the oxidation of ammonia to nitric oxide) is tested under the same conditions. the selectivity defined as [NO] / ([NO] + 2 [N2]), where [NO] and [N2] respectively represent the volume proportions of nitric oxide and nitrogen in the exit gas, at various temperatures.

Example 2 A mixture of lanthanum, cerium and cobalt compounds is precipitated by the gradual addition of a solution containing nitrates of lanthanum, cerium and cobalt in the atomic proportions La: Ce: Co 4: 1: 5 to a precipitating solution consisting of a mixture of ammonium carbonate and oxalic acid. Through the precipitation the mixture is continuously stirred, the pH between 6 and 7 is maintained at the temperature between 48 and 57 ° C. The suspension is then allowed to sit and a flocculated precipitate is formed. The supernatant liquor is a dark pink color, which indicates that not all the cobalt has precipitated. The precipitate is separated by filtration, dried in air at 120 ° C for 6 hours and then calcined in the air at 600 ° C for an additional 6 hours. The calcined material is divided into several portions, each approximately 10 g.

One portion is calcined in air at 900 ° C for 6 hours. The other portions are calcined in the air at 1000 ° C, 1100 ° C, 1200 ° C, 1300 ° C and 1400 ° C, respectively. The chemical analysis of the sample calcined at 900 ° C shows that the atomic proportions of the metals are La: Ce: Co: 4.6: 1.06: 5, that is, it has an atomic ratio of rare earth to cobalt of approximately 1.13, which is consistent with the observation that not all cobalt has precipitated. Analysis by XRF shows that cobalt represents 22.6% by weight of the catalyst. The XRD analysis of the calcined portions at the different temperatures was performed, using silica as an internal standard, to determine the proportions of cobalt-cobalt oxide and cobalt monoxides that were present. From these data, the atomic ratio of the cobalt present as free cobalt oxide was calculated. The results are shown in the following table.

Electron microscope studies indicate that none of the samples contained a mixed phase of lanthanum / cerium / cobalt perovskite, although it is possible that the lanthanum / cobalt / perovskite present in the samples calcined at 900 ° C and higher contained a small proportion (less about 2%) of cerium. However, it was observed that many of the particles, under the electron microscope, have a lanthanum / cobalt / Perovskite phase attached to, or coated on particles of cerium oxide and / or cerium oxide added with lanthanum. From these data it would appear that the cobalt monoxide observed in the samples calcined at high temperatures may result from the decomposition of the lanthanum / cobalt perovskite and / or cobalt-cobalt oxide: other studies have shown that the cobalt-oxide cobalt decomposes reversibly forming cobalt monoxide at approximately 930 ° C. The calcined samples were tested to determine their selectivity for ammonia oxidation by the method described in Example 1 above, except that after increasing the temperature to 1000 ° C, the temperature was maintained at this level for 10 minutes and then decreased to approximately 30 ° C per minute. Exit gas analyzes were performed during the increase as well as during the decrease in temperature. The results are shown in the following table.

From these data it is observed that there is a decrease in the selectivity at high operating temperatures (approximately above 900 ° C) for those samples having higher proportions of cobalt as free cobalt oxides. The poorer selectivity of the samples in the cooling cycle as opposed to the heating cycle is likely to result from the decomposition of free cobalt-cobalt oxide to a less selective cobalt monoxide at the high operating temperatures: this difference between the cycle data heating and cooling is less apparent at lower operating temperatures, possibly as a result of free cobalt monoxide, which is formed during the higher temperature part of the test procedure, which undergoes the reversible transition back to cobalt-oxide Cobalt as the temperature decreases.

Example 3 Approximately 20 kg of catalyst are prepared by the method described in Example 2 with the final calcination at 900 ° C for 6 hours. The analysis by XRF shows that the atomic proportions of the metals is La: Ce: Co 8.54: 2.08: 10. The TGA analysis shows that 23.8% of the cobalt atoms are present as free cobalt oxides. The catalyst is formed into small cylindrical granules and a sample of the granules is subjected to selectivity tests by the procedure of Example 2. The selectivity at the test temperature of 900 ° C was 92%. The rest of the catalyst granules were then supported on a wire mesh as the catalyst in an ammonia oxidation reactor of a commercial nitric acid plant which then operates under typical nitric acid plant operation conditions (11-7). 12% ammonia in air, 1.1 bar operating pressure, 200 ° C inlet temperature, and 910-925 ° C outlet temperature) for 6 months.

Subsequently, a mixture of catalyst was taken for analysis and tests of selectivity were carried out by the procedure of Example 2. TGA analysis shows that only 5.7% of the cobalt atoms were present as free cobalt oxides, and that the selectivity to temperature test of 900 ° C is 96%. These data show that the level of free cobalt oxide present in the catalyst decreases significantly during the first 6 months of operation at high temperature accompanied by an increase in selectivity. The operation of the catalyst after 6 months of operation is similar to that of a recent platinum / rhodium gauze catalyst. The operation of the ammonia oxidation process subsequently continues for an additional 6 months under the same conditions as the previous ones and subsequently an additional sample is analyzed, which shows that 5.5% of the cobalt atoms are present as free cobalt oxides. This indicates that the free cobalt oxide content has stabilized with only a very small additional decrease that occurs during the second 6 month period of operation.

Claims (9)

1. A process for the oxidation of ammonia, wherein the ammonia and the air are reacted in the presence of an oxidation catalyst comprising oxides of (a) at least one element A which is selected from rare earths and yttrium, and (b) ) cobalt, cobalt and element A are in proportions such that the atomic ratio of element A to cobalt is in the range of 0.8 to 1.2, the process is characterized in that at least part of the cobalt and element A oxides they are present as an oxide phase mixed with less than 30% cobalt, (per atoms), present as free cobalt oxides.

2. The process according to claim 1, characterized in that less than 25% of the cobalt is present, by atoms, as free cobalt oxides.

3. The process according to claim 1 or claim 2, characterized in that less than 15%, by atoms, of the cobalt is present as cobalt monoxide.

4. The process according to one of claims 1 to 3, characterized in that less than 5% by weight of the composition is cobalt-cobalt oxide and less than 2% by weight is free cobalt monoxide.

5. The process according to any of claims 1 to 4, characterized in that part or all of the element A is at least one element that is selected from yttrium, cerium, lanthanum, neodymium and praseodyium.

6. The process according to claim 5, characterized in that the element A comprises a mixture of at least one element Vv of variable valence that is selected from cerium and praseodinium and at least one element Vn of variable valence that is selected from yttrium , and a rare earth element of variable valence.

7. The process according to claim 5, characterized in that the atomic proportions of the variable valence element Vv with respect to the non-variable valence element Vn is in the range of 0 to 0.3.

8. The process according to any of claims 1 to 7, characterized in that the oxidation catalyst is obtained by heating a composition containing cobalt oxides and at least one of the elements A is selected from yttrium rare earths, at a temperature in the range of 900-1200 ° C.

9. The process according to any of claims 8, characterized in that the oxidation catalyst comprises a primary support in the form of a mesh, gauze, pad or monolith formed from a high temperature iron / aluminum alloy or mesh, gauze, pad, monolith or foam of a ceramic material, a secondary support in the form of a wash coating of alumina or lanthanum free of alkaline material on a primary support; supported on the secondary support, an active coating of oxides of: (a) at least one element A which is selected from rare earths and yttrium, and (b) cobalt, the cobalt and the element A are in proportions such that the proportion atomic of the element A with respect to cobalt is in the range of 0.8 to 1.2, at least part of the cobalt oxides and of the element A are present as an oxide phase mixed with less than 30% cobalt (per atoms) present as free cobalt oxides.