SK12282000A3 - Method for producing hydroxylammonium salts - Google Patents

Abstract

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

Description

Technical field

The present invention relates to a process for the preparation of hydroxylammonium salts by catalytic reduction of nitric oxide with hydrogen in the presence of an acid and a hydrogenation catalyst.

BACKGROUND OF THE INVENTION

Hydroxylamine is widely used especially in the preparation of caprolactam. It is currently being prepared on an industrial scale, inter alia, by reduction of nitric oxide with hydrogen. Nitric oxide is in turn obtained by the reaction of ammonia and oxygen by the Ostwald process and is used directly in the synthesis of hydroxylamine. Here, nitric oxide is purified in a gas scrubber and converted to hydroxylamine by reaction with hydrogen below 50 ° C using a suitable precious metal catalyst in a sulfuric acid slurry. This results in small amounts of ammonium sulfate and nitrous oxide as by-products. In general, supported platinum catalysts are used whose selectivity to hydroxylamine is maximized by partial poisoning of the catalyst.

DE-A-956 038 discloses a process for the preparation of hydroxylammonium salts in which the reduction of nitric oxide on platinum / graphite catalysts is obtained by reducing the precipitation of platinum on suspended graphite supports, if necessary with the addition of catalytic poisons such as sulfur compounds , selenium, arsenic, or tellurium. These catalysts have the disadvantage that their reactivity and selectivity decrease rapidly with prolonged use.

DE-A-40 22 851 describes platinum / graphite nitric oxide hydrogenation catalysts whose selectivity is related to the apparent density, compressive strength and porosity of the graphite support.

DE-A-40 22 853 describes platinum / graphite catalysts in which the graphite support has a certain particle size distribution in the range from 1 to 600 µm. Using these catalysts, the selectivity of hydroxylamine formation in nitric oxide hydrogenation can be increased.

In the known processes for the preparation of hydroxylamine, the selectivity of nitric oxide formation, the space-time yields and the operating life of the catalysts can be improved.

Nitric oxide prepared by the combustion of ammonia contains other NO _{X 1} compounds such as NO ₂ and N ₂ O, as well as many other impurities in ppm amounts, the most important of which are H 2 S, CO ₂ , CO, CH ₄ and other hydrocarbons. DE-A-15 42 628 has already described a process for selectively removing nitric oxide and / or molecular oxygen by catalytic reduction with reducing gases by means of catalysts containing silver and optionally manganese in the form of their oxide compounds as active ingredients. However, this process is too complicated for industrial use because it requires an additional hydrogenation step and an additional catalyst. In addition, the removal of NO ₂ is insufficient and the concentration of nitrous oxide increases.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved process for preparing hydroxylammonium salts by reducing NO with hydrogen, which is technically simple and gives high selectivities and space-time yields. In addition, the catalyst should have a relatively long service life.

We have found that this goal can be achieved by purifying nitric oxide in a simple manner by treating porous silicon or aluminum-based oxides or activated carbon, since we have found that the purity of the nitric oxide used affects the above parameters.

Thus, the present invention provides a process for preparing hydroxylammonium salts by catalytic reduction of nitric oxide with hydrogen in the presence of an acid and a hydrogenation catalyst, wherein the nitric oxide is purified prior to catalytic reduction by at least one treatment with porous silicon and / or aluminum oxides or activated carbon.

In particular, porous oxides which can be used are silicas such as silica gel and molecular sieves. Molecular sieves are, as is known, natural or synthetic zeolites, i.e., aluminum silicates containing alkali metal or alkaline earth metal cations and having the formula

ManO-AfeOs.xSiOj.yHjO, where M is an alkali metal or an alkaline earth metal, n is a cationic valence ax> 2. The pore diameter of molecular sieves is often given in Ĺ, ie the molecular sieve labeled 3 A has a pore diameter of 3 Ĺ , 3 nm).

According to the present invention, molecular sieves 5A and especially molecular sieves 4A are preferred.

Oxides, especially molecular sieves, are often used in a state containing only a small amount of water or in a substantially anhydrous state.

The activated carbon that can be used is commercial activated carbon such as Degusorb, Contarbon BA, Supersorbon K or Desorex A.

According to the present invention, it has been found that it is particularly advantageous to dry the nitric oxide before cleaning. Conventional materials may be used for this purpose, but it has been found that one of the porous oxides mentioned above, in particular silica gel, is particularly useful for the drying step. Thus, in a particularly preferred embodiment, the nitric oxide is first treated with silica gel for drying (and initial purification) and then molecular sieving for further purification.

Both drying and cleaning can be carried out in one or more stages and by one or more drying or cleaning agents. Generally, the drying and purification is carried out in conventional apparatuses, but it has been found to be advantageous to carry out the drying and purification in suitably sized towers.

Typically, from 1000 to 1500 ml of detergent and about the same amount of desiccant is used to clean about 10 standard 1 / h NO. Cleaning towers are generally regenerated by baking after about 8 to 12 hours of operation. The drying towers are baked after about 300 to 400 operating hours.

The temperature and pressure used in the purification and drying of nitric oxide may vary within wide limits. Generally, the drying and / or cleaning is carried out at 20 to 500 ° C, in particular from 20 to 150 ° C, and at a pressure in the range of from 100 millibars to 100 bar, in particular from 800 millibars to 2 bar, but most preferably at approximately atmospheric pressure. .

Analysis of nitric oxide before and after purification shows that, in particular, the content of nitrogen oxides other than N ₂ O and NO ₂ , which appear in amounts of several percent, and the content of H ₂ S, which appears in amounts of several ppm, is greatly reduced. This can be seen from Table 3 in Example 3.

The hydrogenation following the purification step is carried out in a known manner. Hydrogen and nitric oxide are preferably reacted in a molar ratio in the range from 1.5: 1 to 6: 1. Particularly good results are obtained when the molar ratio of hydrogen to nitric oxide is maintained in the reaction zone from 3.5: 1 to 5: 1.

The hydrogenation catalysts used are those usually used for this purpose. Preferred are the platinum-type graphite catalysts described in DE-A-40 22 853. More specifically, the catalyst is treated in an acid solution, preferably in the acid in which the hydrogenation is carried out, before hydrogenation. This leads to activation of the catalyst.

The hydrogenation is carried out in the presence of an acid, preferably a strong mineral acid, for example nitric acid, sulfuric acid or phosphoric acid, or an aliphatic C 1 -C 5 -monocarboxylic acid, for example formic, acetic, propionic, butanoic or valeric acid, preferably formic or acetic. Acid salts such as ammonium hydrogen sulphate are also suitable. Generally, 4 to 6 N aqueous acid solutions are used and care is taken that the acid concentration does not fall below 0.2 N during the hydrogenation. Additional acid is added if necessary.

The ratio of acid to catalyst greatly depends on the catalyst used. It is generally in the range of from 1 to 150 g of catalyst per liter of mineral acid. In the case of the aforementioned platinum catalyst described in DE-A-40 22 853, the ratio is preferably in the range from 1 to 100 g, in particular from 20 to 80 g of catalyst per liter of mineral acid.

The hydrogenation is generally carried out from 30 to 80 ° C, preferably from 35 to 60 ° C. The pressure during hydrogenation is usually selected in the range from 1 to 30 bar, preferably from 1 to 20 bar (absolute).

The process of the present invention for the preparation of hydroxylammonium salts yields significantly higher hydroxylammonium selectivity and lower selectivity to nitrous oxide. In addition, the space-time yield of catalysts that are exposed to purified nitric oxide in accordance with the process of the present invention increases considerably compared to the use of untreated nitric oxide. In addition, the operating life of the catalysts used is increased. As a result, the catalyst needs to be regenerated less frequently, which improves the process economy.

The following examples illustrate the invention without limiting its scope.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

(a) 40 g of Asbury graphite having a particle size of 2 to 50 µm and 0.5310 g of hexachloroplatinic acid hexahydrate were stirred overnight at 80 ° C in an aqueous solution containing 3.87 ml of concentrated hydrochloric acid and 0.87 ml of concentrated nitric acid . Sodium carbonate was added to the obtained suspension until the pH was 2.75. Then 2.5 g of sodium acetate was added as a buffer. Then 5 mg of elemental sulfur was added and after 2 minutes of waiting, the obtained suspension was mixed with 14.1 g of a 40% 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 it gives a black precipitate in an alkaline solution).

The catalyst thus prepared was separated from the reaction mixture by filtration through a glass frit and washed with distilled water until the pH of the rinse ceased to be acidic. The dried catalyst contained 0.5% by weight of platinum.

b) 3.6 g of the catalyst prepared in a) were suspended in 120 ml of 4.3 N sulfuric acid and 7.75 l / h of a 35% v / v mixture was injected into the suspension at 40 ° C with vigorous stirring (3500 rpm). nitric oxide, which was previously passed through a drying tower filled with 800 ml of silica gel and then through a cleaning tower filled with 1200 ml of 4A molecular sieves (Carl Roth GmbH, Karlsruhe), and 65% by volume of hydrogen. After 4 hours, the catalyst was separated and the liquid phase analyzed. The catalyst which separated was mixed with 120 ml of 4.3 N sulfuric acid and the reaction continued. This procedure was repeated every four hours and will be referred to as a batch. The catalyst was tested until the hydroxylamine synthesis varied from less to 0.5% (activation phase) from dose to dose. This required 20 batches. In 21st batch, unclean NO was used; purified batch NO was reused in 22nd batch as in previous batches. In each case, the liquid phase was analyzed. The selectivities achieved in these batches are shown in Table 1.

Table 1

brew 20 21 22 Cleaning NO Molecular sieves 4A - Molecular sieves 4A NH ₂ OH selectivity [%] 90.93 87,17 90.76 NH ₃ selectivity [%] 8.47 8.40 8.92 N ₂ O selectivity [%] 0.60 4.42 0.32 NO Conversion [%] 95.16 94.75 94.79 Spatial-time yield 0,872 0,833 0,867

It can be seen from Table 1 that the use of unpurified NO leads to a substantial increase in the NO2 selectivity, in particular at the expense of the hydroxylamine selectivity. If purified NO is reused in the next batch, the previous batch will be reached again.

Example 2

Pressing NO through the drying tower

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

Example 3

Pushing NO through drying tower and scrubbing tower

3.6 g of the catalyst prepared in Example 1a) were suspended in 120 ml of 4.3 N sulfuric acid and 7.75 l / h of a 35% v / v mixture was injected into the suspension at 40 ° C with vigorous stirring (3500 rpm). nitric oxide, which was previously passed through a drying tower filled with 800 ml of silica gel and then through a cleaning tower filled with 1200 ml of 4 A molecular sieves (Carl Roth GmbH, Karlsruhe), and 65% by volume of hydrogen. After 4 hours, the catalyst was separated and the liquid phase analyzed. The catalyst which separated was mixed with 120 ml of 4.3 N sulfuric acid and the reaction continued. This procedure was repeated every four hours. The reaction was stopped after 50 batches. The selectivity for nitrous oxide at this point was 0.41%. The results obtained are shown in Table 2.

Table 2

Example 2 Example 3 Cleaning NO Drying tower Drying tower, cleaning tower NH ₂ OH selectivity [%] 89,70 90.05 NH ₃ selectivity [%] 7.87 9.27 N ₂ O selectivity [%] 2.43 0.68 NO Conversion [%] 97.34 95.88 Spatial-time yield 0,880 0,871 Service life [number of batches] 30 > 54

From the table, it can be seen that the catalyst exposed to NO, which had previously been extruded only through a drying tower, has a shorter operating life at comparable hydroxylamine selectivities and space-time yields. It is therefore clear that the molecular sieves remove additional impurities from the NO, leading to accelerated aging of the catalyst. In order to clearly show the effect of the purification, the impurity content in the untreated NO and in the NO purified by different methods (drying tower containing silica gel or molecular sieves 4A) is shown in Table 3.

Table 3

NO n ₂ i%] O ₂ [ppm] WHAT [Ppm] CO ₂ [ppm] ₄ [PPm] c ₂ h ₄ [PPm] N ₂ O [%] ₂ [%] H ₂ S [PPm] steel a bottle 0.82 1.0 <1 <5 <2 1 0.6677 .7942 4.1 through the drying tower 0.57 1.2 <1 <5 <2 1 .6883 0.0090 1.1 via molecular sieves 4A 0.62 1.2 <1 <5 <2 <1 0.2574 0.0044 0.4

Values were obtained by separating the NO to be analyzed in columns of the above type or by the methods described.

Molecular sieves 5A; 50 m; 0.32 mm; (inert gases)

Activated carbon; 2 m; 4 mm; (H)

Poraplot Q; 50 m; 0.53 mm; (carbon dioxide)

Al ₂ O 3 / KCl; 100 m; 0.53 mm; (hydrocarbons) gas chromatography, atomic emission technology (H ₂ S)

Infrared spectroscopy (N ₂ O, NO ₂ )

Claims (8)

PATENT CLAIMS Process for the preparation of hydroxylammonium salts by catalytic reduction of nitric oxide with hydrogen in the presence of an acid and a hydrogenation catalyst, characterized in that the nitric oxide prepared by combustion of ammonia prior to the catalytic reduction is purified by at least one treatment with porous silicon and / or aluminum-based oxides. Method according to claim 1, characterized in that the porous oxide used is molecular sieves or silica gel. Process according to claim 1 or 2, characterized in that the nitric oxide is subjected to a drying step before purification. Method according to claim 2 or 3, characterized in that the nitric oxide is first passed through silica gel and then through molecular sieves, in particular molecular sieves having a pore size of 4A. Process according to any one of the preceding claims, characterized in that the purification is carried out at 20 to 500 ° C, in particular from 20 to 150 ° C. Process according to any one of the preceding claims, characterized in that the cleaning is carried out at a pressure in the range from 100 to 100 bar, in particular from 800 to 2 bar. Process according to any one of the preceding claims, characterized in that the catalytic reduction is carried out at 30 to 80 ° C and a pressure in the range from 1 to 30 bar. Process according to any one of the preceding claims, characterized in that the hydrogenation is carried out using a molar ratio of hydrogen to nitric oxide in the range from 3.5: 1 to 5: 1.