CN1291962A - Method for producing hydroxylammonium salts - Google Patents

Abstract

The invention relates to a method for producing hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen in the presence of an acid and a hydrogenation catalyst, the nitrogen monoxide being purified before the catalytic reduction at least once by treatment with a porous oxide based on silicon and/or aluminum or with activated carbon.

Description

Process for the preparation of hydroxyammonium salts

The invention relates to a method for preparing hydroxylammonium salt by catalytically reducing nitrogen monoxide with hydrogen in the presence of acid and a hydrogenation reaction catalyst.

Hydroxyammonium salts are widely used, especially for the preparation of caprolactam. Today it is prepared on an industrial scale especially by reducing nitric oxide with hydrogen. Nitric oxide is obtained by sequentially reacting ammonia and oxygen by the Ostwald process, and is directly used in the synthesis of hydroxylammonium. Here, nitric oxide is purified in a scrubber and converted to hydroxylammonium by reaction with hydrogen at temperatures below 50°C using a suspension of a suitably supported noble metal catalyst in sulfuric acid. This produces small amounts of by-products ammonium sulfate and nitrous oxide. In general, partial poisoning maximizes the utility of supported platinum catalysts with selectivity for hydroxylammonium.

German Patent No. DE-A-956038 describes a method for the preparation of hydroxyammonium salts, wherein the reduction of nitric oxide is carried out on a platinum/graphite catalyst, which is formed by reducing and precipitating platinum on a suspended graphite support. obtained, provided appropriate additions of poisons such as sulfur, selenium, arsenic or tellurium compounds. A disadvantage of these catalysts is that the reactivity and selectivity decrease rapidly upon prolonged use.

German Patent No. DE-A-4022851 describes platinum/graphite catalysts for the hydrogenation of nitric oxide, the selectivity of which is related to the apparent density, compressive strength and porosity of the graphite support.

German Patent No. DE-A-4022853 describes platinum/graphite catalysts in which the graphite support has a specific pore size distribution in the medium between 1 and 600 microns. When using these catalysts, the selectivity to hydroxylammonium formation in the hydrogenation of nitric oxide can be improved.

In the known processes for the preparation of hydroxylammonium, the selectivity of nitric oxide formation, the space-time yield and the operating life of the catalyst can be improved.

Nitric oxide produced by ammonia combustion contains other _NOx compounds such as _NO2 and _N2O and many further impurities in the ppm range, the most important being _H2S , _CO2 , CO, _CH4 and other hydrocarbons . German Patent No. DE-A-1542628 has described a process for the selective removal of nitrogen dioxide and/or molecular oxygen with a reducing gas by catalytic reduction using a catalyst comprising silver in the form of oxidizing compounds and possibly manganese as an active ingredient. However, this method is too complicated for industrial use because it requires an additional hydrogenation reaction step and an additional catalyst. In addition, the removal of _NO2 was insufficient and the concentration of nitrous oxide increased.

It is an object of the present invention to provide an improved process for the preparation of hydroxylammonium salts by reduction of NO with hydrogen, which is technically simple to carry out and gives high selectivity and space-time yield. In addition, the catalyst will have a higher operating life.

It has been found that porous oxides or activated carbon based on silicon or aluminum achieve this purpose in a simple manner, since the purity of the nitrogen monoxide used has been found to influence the parameters mentioned above.

The present invention therefore provides a process for the catalytic reduction of nitric oxide with hydrogen in the presence of an acid and a hydrogenation catalyst to prepare hydroxylammonium salts, wherein the nitric oxide is treated with silicon and/or aluminum at least once before the catalytic reduction. The porous oxide of the substrate is either purified by treatment with activated carbon.

Porous oxides that can be used are especially silicon dioxide such as silica gels and molecular sieves. Molecular sieves, as known, are natural or synthetic zeolites, i.e. aluminum silicates, which contain alkali metal or alkaline earth metal cations and have the formula:

M ₂ / _n O·Al ₂ O ₃ · _x SiO ₂ ·yH ₂ O, wherein M is an alkali metal or an alkaline earth metal, n is the valence of a cation, and x≥2. Molecular sieve pore diameter is often represented by _, that is, a molecular sieve marked 3A has a pore opening of 3_ (0.3 nanometers).

According to the present invention, the preferred molecular sieve is 5A, especially molecular sieve 4A.

The oxides, especially molecular sieves, are often used in a state containing some water or in a substantially anhydrous state.

Activated carbons that can be used are commercial activated carbons such as Degusorb, Contarbon BA, Supersorbon K or Desorex A.

According to the invention, it has been found to be particularly advantageous to dry the nitric oxide prior to purification. Conventional materials can be used for this purpose for the drying step, but it has been found that the use of the above-mentioned porous oxides in the drying step is particularly useful, especially silica gel. In a particularly preferred embodiment, nitric oxide is thus first treated with silica gel for drying (and initial purification) and then further purified with molecular sieves.

Both drying and purification can be performed in one or more stages, using one or more drying or purifying agents. In general, drying and purification are carried out in conventional apparatus, but it has been found advantageous to carry out in appropriately sized columns.

For the purification of approx. 10 standard l/h of NO, usually 1000 to 1500 ml of purifying agent and about the same desiccant are used. Purification columns are typically regenerated with bakeout after about 8 to 12 hours of operation. The drying tower is baked out after about 300 to 400 operating hours.

Both the temperature and pressure used in nitric oxide purification and drying can vary widely. Typically, drying and/or purification is carried out at 20 to 500°C, especially 20 to 150°C, at a pressure in the range of 100 mbar to 100 bar, especially 800 mbar to 2 bar, but most conveniently at atmospheric pressure.

Analysis of nitric oxide before and after purification showed, inter alia, that the content of other nitrogen oxides, such as N ₂ O and NO ₂ , which occurred in the percentage range, and the content of H ₂ S , which occurred in the ppm range, were greatly reduced reduce. This can be seen from Table 3 of Example 3.

The hydrogenation after the purification step is carried out in a manner known per se. Preferably hydrogen and nitric oxide are present in a molar ratio ranging from 1.5:1 to 6:1. Especially good results are obtained when the molar ratio of hydrogen to nitric oxide in the reaction zone is maintained at 3.5:1 to 5:1.

The hydrogenation catalysts used are customary for this purpose. It is preferred to use a platinum/graphite catalyst as described in German Patent No. DE-A-4022853. In particular, the catalyst is treated in an acid solution prior to the hydrogenation reaction, preferably in the acid in which the hydrogenation reaction is carried out. This results in catalyst activation.

The hydrogenation reaction is carried out in the presence of an acid, preferably a strong mineral acid such as nitric acid, sulfuric acid or phosphoric acid, or an aliphatic C ₁ -C ₅ -monocarboxylic acid such as formic acid, acetic acid, propionic acid, butyric acid Or valeric acid, formic acid or acetic acid are preferred. Acids such as ammonium bisulfate are also suitable. Typically, 4 to 6N aqueous acid is used, and care is taken to ensure that the acid concentration does not fall below 0.2N during the hydrogenation reaction. Additional acid is added if necessary.

The acid to catalyst ratio depends significantly on the catalyst used. Typically, between 1 and 150 grams of catalyst per liter of mineral acid. In the case of the platinum catalyst described in the above-mentioned German Patent No. DE-A-4022853, the ratio is preferably between 1 and 100 grams of catalyst per liter of mineral acid, especially 20 to 80 grams.

The hydrogenation reaction is usually carried out at 30 to 80°C, preferably at 35 to 60°C. The pressure during the hydrogenation reaction is often chosen within the range of 1 to 30 bar, preferably 1 to 20 bar (absolute).

The process of the present invention for the preparation of hydroxyammonium salts results in significantly higher hydroxyammonium selectivity and lower nitrous oxide selectivity. Furthermore, according to the method of the present invention, the space-time yield of the catalyst exposed to pure nitric oxide is considerably increased compared to that using untreated nitric oxide. Furthermore, the operating life of the catalysts used is also increased. As a result, catalyst regeneration is less frequent, which improves the economics of the process.

The following examples are illustrative of the invention and not limiting:

Example 1

a) At 80°C, in an aqueous solution comprising 3.87 ml of concentrated hydrochloric acid and 0.87 ml of concentrated nitric acid, 40 g of graphite from Asbury with a particle size of 2 to 50 microns and 0 5310 g of hexachloroplatinum(IV) acid hexahydrate were stirred overnight. Sodium carbonate was added to the resulting suspension until the pH was 2.75. Next, 2.5 g of sodium acetate was added as a buffer. Then 5 mg of elemental sulfur was added, and after waiting 2 minutes, the resulting suspension was mixed with 14.1 g of a 40% strength by weight aqueous sodium formate solution (83 mmol) and stirred at 80° C. for 4 hours. After this time, platinum could no longer be detected with hydrazine hydrate (in the presence of platinum a black precipitate was obtained in basic solution).

The catalyst prepared in this way was separated from the reaction mixture by filtration through a glass frit and rinsed with distilled water until the pH of the rinse was not in the acid range. The dry catalyst contained 0.5% platinum by weight.

Suspend 3.6 g of the catalyst prepared in a) at 40°C in 120 ml of 4.3N sulfuric acid with vigorous stirring (3500 rpm), and 7.75 l/h 35% volume ratio of nitric oxide, which is first filled with A drying column of 800 ml of silica gel was then passed over the suspension filled with 1200 ml of molecular sieve 4A (Carl Roth GmbH, Karlsruhe) and a mixture of 65% by volume hydrogen. After 4 hours, the catalyst was separated and the liquid phase was analyzed. Next, the separated catalyst was mixed with 120 ml of 4.3N sulfuric acid and the reaction was continued. This step is repeated every 4 hours and will be designated as a batch. Catalysts were tested until hydroxylammonium synthesis (activated phase) changed less than 0.5% from bath to bath. This required a catalyst operating time of 20 water baths. In the 21st water bath, unpurified NO was used; in the 22nd water bath, the purified NO from the previous water bath was used again. The liquid phase in each case was analyzed. The selectivities achieved in these water baths are shown in Table 1.

Table 1 water bath 20 twenty one twenty two No purification Molecular Sieve 4A - Molecular Sieve 4A Selectivity to NH ₂ OH (%) 90.93 87.17 90.76 Selectivity to become _NH3 (%) 8.47 8.40 8.92 Selectivity to N ₂ O (%) 0.60 4.42 0.32 No conversion (%) 95.16 94.75 94.79 space-time yield 0.872 0.833 0.867

As can be seen in Table 1, the use of unpurified NO resulted in a considerable increase in _NO2 selectivity, mainly at the expense of hydroxylammonium selectivity. If purified NO is used again in a subsequent water bath, the value in the previous water bath changes.

Example 2

Pass NO through drying tower

At 40°C, 3.6 grams of the catalyst prepared in Example 1a) were suspended in 120 milliliters of 4.3N sulfuric acid with vigorous stirring (3500 rpm), and 7.75 l/h of 35% by volume nitric oxide, which was first Pass the suspension through a drying tower filled with 800 ml of silica gel, with a mixture of 65% by volume hydrogen. After 4 hours, the catalyst was separated and the liquid phase was analyzed. Next, the separated catalyst was mixed with 120 ml of 4.3N sulfuric acid and the reaction was continued. Repeat this step every 4 hours. The reaction was stopped when the selectivity to nitrous oxide exceeded a predetermined upper limit of 5%. The experimental results are shown in Table 2.

Example 3

Pass NO through drying tower and purification tower

At 40°C, 3.6 grams of the catalyst prepared in Example 1a) were suspended in 120 milliliters of 4.3N sulfuric acid with vigorous stirring (3500 rpm), and 7.75 l/h of 35% by volume nitric oxide, which was first The suspension was passed through a drying column filled with 800 ml of silica gel and then 1200 ml of molecular sieve 4A (Carl Roth GmbH, Karlsruhe) with 65% by volume hydrogen. After 4 hours, the catalyst was separated and the liquid phase was analyzed. Next, the separated catalyst was mixed with 120 ml of 4.3N sulfuric acid and the reaction was continued. Repeat this step every 4 hours. The reaction was stopped after 50 water baths. At this time, the selectivity to nitrous oxide was 0.41%. The experimental results are shown in Table 2.

Table 2 Example 2 Example 3 No purification drying tower Drying tower + purification tower Selectivity to NH ₂ OH (%) 89.70 90.05 Selectivity to become _NH3 (%) 7.87 9.27 Selectivity to N ₂ O (%) 2.43 0.68 No conversion (%) 97.34 95.88 space-time yield 0.880 0． 871 Operating life (water bath) 30 >54

As can be seen from the table, catalysts exposed to NO previously passed only through the drying tower had shorter operating lifetimes at comparable hydroxylammonium selectivity and space-time yields. Clearly, therefore, molecular sieves free NO from additional impurities resulting in accelerated maturation of the catalyst. In order to clearly represent the purification effect, the impurity content in unpurified NO and NO purified in different ways (drying tower containing silica gel or molecular sieve 4A) is shown in Table 3.

table 3 NO N ₂ [%] O ₂ [ppm] CO [ppm] CO ₂ [ppm] CH ₄ [ppm] C ₂ H ₄ [ppm] N ₂ O [%] NO ₂ [%] H ₂ S[ppm] cylinder 0.82 1.0 <1 <5 <2 1 0.6677 0.7942 4.1 through drying tower 0.57 1.2 <1 <5 <2 1 0.6883 0.0090 1.1 on molecular sieve 4A 0.62 1.2 <1 <5 <2 <1 0.2574 0.0044 0.4

This value is obtained by separating the NO to be analyzed in a column of the type specified below or using the method specified.

Molecular sieve 5A; 50 meters; 0.32 mm; (inert gas)

Activated carbon; 2 m; 4 mm (hydrogen)

Poraplot Q; 50 m; 0.53 mm (CO2)

Al ₂ O ₃ /KCl; 100 m; 0.53 mm (hydrocarbons)

GC Atomic Radiation Technology (H ₂ S)

IR spectrum (N ₂ O, NO ₂ )

Claims (9)

1. A method of catalytic reduction of nitrogen monoxide with hydrogen in the presence of an acid and a hydrogenation reaction catalyst to prepare hydroxylammonium salts, wherein the nitrogen monoxide passes through a silicon and/or aluminum-based porous oxide at least once before the catalytic reduction Or purified by activated carbon treatment. 2. A method according to claim 1, wherein the porous oxide used is a molecular sieve or silica gel. 3. A method according to claim 1 or 2, wherein the nitric oxide is subjected to a drying step prior to purification. 4. A method according to claim 2 or 3, wherein the nitric oxide is first passed over a silica gel and then over a molecular sieve, especially a molecular sieve with a pore size of 4A. 5. Process according to any one of the preceding claims, wherein the purification is carried out at 20 to 500°C, especially 20 to 150°C. 6. Process according to any one of the preceding claims, wherein the purification is carried out at a pressure in the range of 100 mbar to 100 bar, in particular 800 mbar to 2 bar. 7. Process according to any one of the preceding claims, wherein the catalytic reduction is carried out at a temperature of 30 to 80°C and a pressure of between 1 and 30 bar. 8. A method according to any one of the preceding claims, wherein the hydrogenation reaction is carried out using a molar ratio of hydrogen to nitric oxide of between 3.5:1 and 5:1. 9. A method according to any one of the preceding claims, wherein the acid used is sulfuric acid, nitric acid or phosphoric acid.