DE2009374B2 - Process for the production of high-temperature catalysts based on iron oxide - Google Patents

Description

The invention relates to a method for the production of high-temperature catalysts, particularly are used for the so-called high temperature water gas reaction.

This response can be given by the following equation:

CO + H ₂ O-CO ₂ + H ₂

Chromium-activated iron oxide, i.e. iron oxide, is often used for this reaction. H. a mixture of iron and chromium oxides, used in powder form.

When the metal oxide mixture in the catalyst the should have the necessary chemical activity, soot it be made in a special porous shape, which has a large specific surface. These Shape cannot be obtained by mechanically crushing the solid material. A suitable one A method to obtain the required surface is that of a mixture of water-soluble salts of these metals by basic reagents, such as alkali or alkaline earth hydroxides or aqueous ammonia, which precipitates hydroxides. For example, in GB-PS 9 40 380 is the production of a iron oxide-containing catalyst described in the iron (II) sulfate by heating partially to iron (HI) is converted and from the mixed product by precipitation with alkali and calcining of the Product of the essentially iron-IIl-oxide-containing catalyst is obtained.

In this usual precipitation of the hydroxides, substantial amounts of undesirable contaminating anions are entrained and it is not possible to wash and remove them to an adequate extent, especially because of the extremely strong absorption potential of iron hydroxide towards various anions such as chloride and sulfate. Phosphate and arsenal. Therefore, iron oxide catalysts obtained by the precipitation of Fe (OH) ₂ or Fe (OH) j contain 1500-5000 ppm of such undesirable anions, especially sulfur compounds, even after extremely thorough washing. Since iron sulfate and chloride are cheap, these water-soluble salts are preferably used as starting materials for the production of Fe-Cr oxide conversion catalysts.

However, these chlorides and sulfates are catalyst poisons and set both activity and The life of the catalytic converters. In addition, when using the catalyst, chloride and sulfate are converted into volatile compounds such as HCl and H ₂ S. Except that the catalyst itself is poisoned, such a catalyst containing sulfate and / or chloride gives H ₂ S and HCl, respectively to the resulting gas and can thereby poison any other catalyst used in subsequent processes.

This is e.g. B. the case with the so-called low-temperature catalysts, which are increasingly being used in Second converters are used, which are connected to the water gas generators, in which such High temperature catalysts are used. Around To avoid such pollution it is necessary to remove and dispose of the gas escaping from a sulfur- or chlorine-containing catalyst ... until that the resulting water gas is sufficiently pure, which obviously affects the effectiveness of the process

? n affects

Lead oxide can be used with very good success as an activator for the iron oxide catalyst can be used. However, if sulphate impurities are present at the same time are how they are normally used in the manufacture of Fe ₂ Oj occur, the activating effect of PbO disappears, which is clear after a certain time with a fresh catalyst, so that the addition of PbO no longer brings any clear advantage. However, a lasting effect can be achieved with PbO if an iron oxide catalyst is activated which is very pure and contains no more than 20 to 50 ppm sulfur.

It is the object of the present invention to develop improved high-temperature catalysts which have a significantly lower sulfur content, d. H.

is iron oxide or hydroxide precipitates with a produce negligibly low sulfur content, using a reagent if possible, that can be recovered, thereby enabling a continuous process.

This object is achieved by a method for the production of high-temperature catalysts Iron oxide base by treating soluble iron (II) salts with an alkaline solution, filtering off, washing and drying, which is characterized by What is net is that an ammonium or alkali metal salt of oxalic acid is added to solutions of soluble iron (II) salts adds the resulting coarsely crystalline precipitate to remove soluble impurities, washes and then treated with an alkaline solution, the resulting flaky iron oxide or hydroxide filtered off, washes and dries as well as from the filtrate recovers water-soluble oxalates.

According to the invention it is possible to produce high-temperature catalysts by precipitation obtained iron oxide with a sulfur content of less than 50 ppm, advantageously less than 20 ppm, and can be activated with PbO.

Mixed oxide catalysts can of course be prepared according to the invention by one starts from mixed solutions which, in addition to the soluble iron salts, also contain other soluble salts contain. In the first step according to the invention, the water-soluble iron salts become slightly soluble coarsely crystalline iron salts that easily be completely freed from impurities can, as the coarsely crystalline precipitate, in contrast to the iron oxides or hydroxides, cannot tends to absorb and hold onto anions.

When using sodium oxalate and ferrous sulfate, you can use the inventive method an iron oxide with a high specific Obtained surface that has a sulfur content of only 10 ppm. First, a solution of Iron-U-sulfate with sodium oxalate the iron-II-oxalate precipitated The supernatant sodium sulfate-containing solution is poured off and the coarsely crystalline Iron oxalate precipitate washed very thoroughly to remove traces of sulfate. In the Hydrolysis of the iron oxalate with sodium hydroxide then results in a suitable precipitate of iron oxide or hydroxide and the sodium oxalate is recovered.

It turns out that the properties of iron oxalate are particularly suitable for the process according to the invention. It is sparingly soluble, easily crystallizes in regular crystals, the surface of which does not tend to be absorbed, making it easier to remove of sulphate and chloride ions from the precipitate and readily hydrolyzes without decomposing the acid, thereby reducing the economics of the Procedure is increased.

After the iron-H-oxalate has precipitated, it is not necessary to isolate the slightly soluble precipitate from the liquid phase.

After the sulfate and / or chloride has been completely removed from the iron oxalate suspension, the hydrolysis step can be carried out as follows: An amount of NaOH, KOH or NH4OH is added to the pure suspension that is equivalent to the amount of iron oxalate Maintained temperature between 50 and 100 ° C until hydrolysis occurs and a flaky, id ultra-pure Fe (OH) precipitate is formed, which can be separated from the alkaline Na, K or NH ₄ oxalate solution by filtering off. After washing out, the resulting Fe (OH) 2 suspension is combined with water-soluble compounds, especially nitrates, which give the oxides desired for activating the iron oxide, such as Cr ₂ O3 and PbO. The water-soluble oxalates released during hydrolysis are collected and reused for precipitation.

The invention is explained in more detail by the following examples, in which under parts always Parts by weight are to be understood.

example 1

100 parts of FeSO ₄ · 7 HjO were dissolved in 264 parts of pure cold water. 48 parts of sodium oxalate (Na ₂ C ₂ O ₄ ) were then added with stirring. There was no need to heat. After one hour a precipitate of iron oxalate FeC ₂ O ₄ · 2 H ₂ O had formed. The sulfate concentration was decreased by repeated decanting and washing on a filter until a qualitative sulfate analysis was negative. The purified iron oxalate was slurried in 300 parts of pure water and 29 parts of NaOH or 41 parts of KOH were added. The mixture was heated to 95 ° C. for 1 h and the Fe (OH) ₂ formed was then filtered off and washed. The filtrate was collected to recover the oxalate. After washing, the pure Fe (OH) _{2 was} slurried in water. To this end, 2 parts of chromic acid were added and the mixture was heated to 100 ^° C. for _{1/2 hour.} The precipitate was filtered off and dried.

The finished catalyst was ^{calcined at 400 °} C. for 2 hours. Yield 28.4 parts (theoretical yield 30.2 parts).

Example 2

The procedure was as in Example 1, with the exception that 71 parts of FeCl ₂ · 4 H ₂ O were used as starting material. The yield was 29.0 parts (theoretical yield 30.2 parts).

Example 3

The procedure was as in Example 1 but heating -stündigem 6 parts of lead nitrate was added to 100 ⁰ C after the addition of the chromic acid, and after> / _2. Yield 33.0 parts (theoretical yield 34.2 parts).

Example 4

The procedure was as in Example 3, but only 2 1/2 Parts of lead nitrate added. Yield 31.2 parts (theoretical yield 32.2 parts).

The chemical compositions were as follows found for each example.

Example 1: 94.2% Fe ₂ O ₃ , 4.8% Cr ₂ O ₃

Example 2: 94.2% Fe ₂ O ₃ , 4.8% Cr ₂ O ₃

Example 3: 843% Fe ₂ O ₃ , 4.0% Cr ₂ O ₃ , 11.7% PbO

Example 4: 89.5% Fe ₂ O ₃ , 43% Cr ₂ O ₃ , 6.2% PbO

For comparison, a number of catalysts were prepared which had identical compositions to those prepared according to Examples 1, 2, 3 and 4, but in the preparation of which the step of FeC ₂ O ₄ · 2 H ₂ O precipitation was omitted. That is, these comparative examples were prepared by directly precipitating Fe (OH) ₂ from an iron sulfate or ferric chloride solution. These precipitates were washed out in the same manner as described above until the washing water no longer contained sulfate. Activation with PbO and Cr ₂ O ₃ was carried out in the same way.

These examples are referred to as Comparative Examples 1, 2, 3 and 4. Table 1 shows the chlorine and Sulfur content in all catalysts.

Table 1

Cl content S content example t 0.0037 0.002 2 0.0043 0.002 3 0.0051 0.002 4th 0.0042 0.002 Comparison example 1 0.0073 0.213 2 0.0916 0.004 3 0.0081 0.167 4th 0.0070 0534

The chemical activity of the samples was calculated by determining the conversion of a dry gas with the composition 60% H ₂ , 20% N ₂ , 10% CO and 10% 2O ₂ . The steam: dry gas ratio was 1: 1. The reaction temperature was 400 ⁰ C jnd the pressure is 1 atmosphere. The measured conversion was used to calculate the rate constant according to the equation
r = k (Pco-Pca _G )

calculated, in which Pcoai is the CO partial pressure at equilibrium. These rate constants are given in Table 2.

Table 2

Measured Rate constant after at first after ι 300 days 50 days example 4.1 1 7.6 6.7 4.0 2 73 6.5 53 3 13.0 8.9 5.7 4th 14.1 9.2 Comparison example 3.3 1 6.0 5.1 3.0 2 8.7 4.7 3a 3 9.0 5.2 3.1 4th 10.7 5.0

From Table 1 it can be seen that the process of the invention is very effective in reducing the sulfur content and is far superior to the known precipitation process.

Table 2 shows that when the sulfur content is decreased, the catalyst is increased by the addition of PbO can be activated even further, while sulfur-containing catalysts can only be activated for a short time by PbO to be activated.

Claims (1)

Claim: Process for the production of high-temperature Ks.talysatoren based on iron oxide by treating soluble iron (II) salts with a alkaline solution, filtering off, washing and drying, characterized in that an ammonium or alkali salt of oxalic acid is added to solutions of soluble iron (II) salts, the resulting coarsely crystalline precipitate washes and removed to remove soluble impurities then treated with an alkaline solution, the resulting flaky iron oxide or -hydroxide is filtered off, washed and dried and the water-soluble oxalates are recovered from the filtrate.