DE2009374C3 - 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 4- 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.

If the metal oxide mixture in the catalyst is to have the required chemical activity, it must 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 (III) is converted and from the mixed product by precipitation with alkali and calcining of the Product of the essentially iron-III-oxide containing Catalyst is obtained.

This usual precipitation of the hydroxides entrains significant amounts of undesirable contaminating anions and it is not possible to wash and /.u remove them to a reasonable extent, especially because of the extremely strong absorption potential of iron hydroxide towards various anions such as chloride, sulfate, phosphate and arsenal . Therefore, iron oxide catalysts obtained by the precipitation of Fe (OH) ₂ or Fe (OH) 3 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 reduce both the activity and the service life of the catalysts. 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 or HCl, respectively to the resulting gas and can thereby poison any other catalyst used in subsequent processes.

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

Lead oxide can be used as an activator for the iron oxide catalyst with very good success. If, however, sulphate impurities are present at the same time, as they normally occur in the production of Fe ₂ O 3, the activating effect of PbO, which is clear with a fresh catalyst, disappears after a certain time, so that the addition of PbO no longer offers any significant advantage. However, you can achieve a lasting effect with PbO, if

jo 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.

To produce iron oxide or hydroxide precipitates with a negligibly low sulfur content, where possible a reagent should be used that can be recovered and thereby a enables 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 the fact that solutions of soluble ice (III) salts are obtained an ammonium or alkali salt of oxalic acid is added, the resulting coarse crystalline precipitate washes to remove soluble impurities and then treated with an alkaline solution,

V) the resulting flaky iron oxide or hydroxide is filtered off, washed and dried and the water-soluble oxalates are recovered from the filtrate.

According to the invention it is possible to use high temperature catalysts to produce the iron oxide obtained by precipitation 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

bo one assumes mixed solutions that, in addition to the soluble iron salts contain other soluble salts. At the first step after the invention the water-soluble iron salts are converted into slightly soluble, coarsely crystalline iron salts, which are easily

b5 are 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-II-sulfate with sodium oxalate the iron-II-oxalate failed. The above solution containing sodium sulfate is poured off and the coarse crystalline precipitate of iron oxalate washed very thoroughly, to remove traces of sulfate. In the hydrolysis of iron oxalate with sodium hydroxide Then a suitable precipitate of iron oxide or hydroxide and the sodium oxalate is formed recovered.

It turns out that the properties of iron oxalate are particularly suitable for the process according to the invention are. 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 sulfate and chloride ions from the precipitate and hydrolyzes easily without breaking the acid decomposed in the process, which increases the economic efficiency of the process.

After precipitation of the iron-ll-oxalate it is not necessary to isolate the sparingly soluble precipitate of the 2 ^r> 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 which is equivalent to the amount of iron oxalate. The mixture is then kept at a temperature between 50 and 100 ⁰ C until hydrolysis occurs and a flaky and ultra-pure Fe (OH) 2 precipitate is formed, which is filtered off from the alkaline Na, K or NH ₄ - Oxalate solution can be separated. 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 O ₃ and PbO. The water-soluble oxalates released during hydrolysis are collected and reused for precipitation.

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

50

example 1

100 TeUcFeSO ₄ · 7 H ₂ O 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 41,000 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 (OHJj was listed in water b5 slurried. To this was added 2 parts of chromic acid was added and the mixture heated V ₂ hours at 100 ⁰ C. The precipitate was filtered off and dried.

The finished catalyst was ^{burned at 400 °} C. for 2 h. 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, except that after the addition of the chromic acid and after heating to 100 ^° C. for 1/2 hours, 6 parts of lead nitrate were added. Yield 33.0 parts (theoretical yield 34.2 parts).

Example 4

The procedure was as in Example 3, but only 2 u parts of lead nitrate were 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 ₁

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

Example 3: 84.3% Fe ₂ O ₃ , 4.0% Cr ₂ O ₃ , 11.7% PbO

Example 4: 89.5% Fe ₂ O ₃ , 4.3% 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 I, 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 direct precipitation of Fe (OH) ₂ from an ice, sulfate or iron 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 I. 2, 3 and 4 denotes. Table 1 shows the chlorine and sulfur content in all catalysts.

Tabel

C content S content (%) (o / o) example 1 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 0.234

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% CO ₂ . The steam: dry gas ratio was 1: 1. The reaction temperature was 400 ° C. and the pressure 1 atmosphere. The measured conversion was used to calculate the rate constant according to the equation

r = kfPco-Pcocil
calculated in which Pcoa ^rfer CO partial pressure is at equilibrium. These rate constants are given in Table 2.

Table 2 Measured Rate constant after at first after ι 300 days 50 days 4.5 example 7.6 6.7 4.0 1 7.3 6.5 5.3 2 13.0 8.9 5.7 3 14.1 9.2 4th Comparison 3.3 example 6.0 5.1 3.0 1 8.7 4.7 3.2 2 9.0 5.2 3.1 3 10.7 5.0 4th

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 reduced, the catalyst can be activated even further by adding PbO, while Sulfur-containing catalysts are only activated briefly by PbO.

Claims (1)

Claim: Process for the production of high-temperature catalysts based on ice oxide by treatment of soluble iron (il) salts with an alkaline solution, filtering off, washing and Drying, characterized in that solutions of soluble iron (II) salts are added Add ammonium or alkali salt of oxalic acid to the coarsely crystalline precipitate Washes removal of soluble impurities and then treated with an alkaline solution, the resulting flaky iron oxide or hydroxide is filtered off, washed, dried and dried out recovered the water-soluble oxalates from the filtrate.