MXPA00008033A - 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

PREPARATION OF HYDROXYLAMONIUM SALTS The present invention relates to a process for preparing hydroxylamine salts by catalytic reduction of nitrogen monoxide with hydrogen in the presence of an acid and a hydrogenation catalyst. Hydroxylamine is widely used for, in particular, the preparation of caprolactam. It is currently prepared on an industrial scale by, among others, the reduction of nitrogen monoxide with hydrogen. The nitrogen monoxide in turn is obtained by reaction of ammonia and oxygen by the Ost ald process and is used directly in the synthesis of hydroxylamine. In this case, the nitrogen monoxide is purified in a scrubber and converted to hydroxylamine by reaction with hydrogen below 50 ° C using a noble metal catalyst with convenient support in suspension of sulfuric acid. It produces small amounts of ammonium sulfate and dinitrogen monoxide as byproducts. In general, supported platinum catalysts are used whose selectivity for hydroxylamine is maximized by partial poisoning. DE-A-956 038 describes a process for the preparation of hydroxylamine salts in which the reduction of nitrogen monoxide is carried out on catalysts of platinum / graphite that was obtained? by reductive precipitation of platinum on supported graphite supports, if appropriate, with the addition of catalyst poisons such as sulfur, selenium, arsenic or tellurium compounds. These catalysts have the disadvantage that the reactivity and selectivity decreases rapidly with prolonged use. DE-A-40 22 851 discloses platinum / graphite catalysts for the hydrogenation of nitrogen monoxide whose selectivity is related to the bulk density, compressive strength and porosity of the graphite support. DE-A-40 22 853 discloses platinum / graphite catalysts in which the graphite support has a certain particle size distribution in the range from 1 to 600 microns. When these catalysts are used, the selectivity of hydroxylamine formation in the hydrogenation of nitrogen monoxide may increase. In the known methods for the preparation of hydroxylamine, the selectivity of the formation of nitrogen monoxide, the space-time yields and the operating lives of the catalysts can be improved. The nitrogen monoxide prepared by combustion of ammonia contains other NOx compounds such as N02 and C20 as well as numerous other impurities in the ppm range of which the most important are H S, C0, CO, CH4 and other hydrocarbons. DE-A-15 42 628 has already described a process for selectively removing nitrogen dioxide and / or molecular oxygen by catalytic reduction with reducing gases using catalysts containing silver and possibly manganese in the form of oxidic compounds as active components. However, this process is too complicated for industrial use because it requires an additional step of hydrogenation and an additional catalyst. In addition, the separation of N02 is insufficient and the concentration of dinitrogen monoxide increases. An object of the present invention is to provide an improved process for preparing hydroxylamine salts by reducing NO with hydrogen which is technically simple to perform and provides high selectivities and space-time yields. In addition, the catalyst must have a relatively long operating life. We have found that this goal is achieved by purifying nitrogen monoxide in a simple way by treatment with porous oxides based on silicon or aluminum or activated carbon, since we have discovered that the purity of nitrogen monoxide used influences the parameters before mentioned. The present invention therefore provides a process for preparing hydroxylammonium salts by reduction catalyst of nitrogen monoxide with hydrogen in the presence of an acid and a hydrogenation catalyst, wherein the nitrogen monoxide is purified before the catalytic reduction by subjecting it to at least one treatment with porous oxides based on silicon and / or aluminum or with activated carbon. Porous oxides that can be used, in particular, silicon dioxides such as silica gel and molecular sieves. Molecular sieves are, as is known, natural or synthetic zeolites, that is, aluminum silicates containing alkali metal or alkaline earth metal cations and having the formula: M2 / nO • Al2o3 •? Si? 2 • yH2o, where M is an alkali metal or an alkaline earth metal, n is the valence of the cation and x > 2. The pore diameter of molecular sieves is usually given in A, that is, a molecular sieve designated as 3 Á has a pore hole of 3 A (0.3 nm). In accordance with the present invention preference is given to molecular sieves 5A and in particular to molecular sieves 4A. Oxides, in particular molecular sieves, are often used in a state containing little water or practically anh JlPo. The activated carbons that can be used are commercial activated carbons such as Degusorb, Contarbon BA, Supersorbon K or Desorex A. In accordance with the present invention, it has been found particularly advantageous to dry the nitrogen monoxide before purification. Customary materials can be used for this purpose, but it has been found particularly useful to employ one of the aforementioned porous oxides, in particular silica gel, for the drying step. In a particularly preferred embodiment, the nitrogen monoxide is, therefore, first treated with silica gel for purposes of drying (and initial purification) and then with a molecular sieve for further purification. The drying and purification can be carried out in one or more steps and use one or more desiccants or purification agents. In general, the drying and purification are carried out in customary apparatuses, but it has been found advantageous to perform the drying and purification in suitably sized towers. For the purification of approximately 10 1 standard / h of NO, normally from 1000 to 1500 ml of purification agent and approximately the same amount of desiccant are used. The purification towers are usually Jrij ^ afe &Bt = s ^ asrffca ^ s ^ taa. * -; »^ & -I, * regenerate from approximately 8 to 12 hours of drying they are baked after approximately 300 to 400 hours of operation. Both the temperature and the pressure used in the purification and drying of nitrogen monoxide can vary within a wide range. In general, the drying and / or purification are carried out from 20 to 500 ° C, in particular from 20 to 150 ° C, and at a pressure in the range from 100 mbar at 100 bar, in particular, from 800 mbar to 2 bar, but it is more convenient at approximately atmospheric pressure. The analysis of nitrogen monoxide before and after purification shows that, in particular, the content of other nitrogen oxides such as N20 and N02, which occurs in the percentage range, and the H2S content, which occurs in the range of ppm, decrease greatly. This can be seen in Table 3 in Example 3. The hydrogenation following the purification step is carried out in a manner known per se. Preferably, hydrogen and nitrogen monoxide are reacted in a molar ratio in the range from 1.5: 1 to 6: 1. Particularly good results are obtained when a molar ratio of hydrogen to nitrogen monoxide from 3.5: 1 to 5: 1 is maintained in the reaction zone. The catalysts for hydrogenation used are those that are normally used for this purpose. Preference is given to the use of platinum catalysts on graphite as described in DE-A-40 22 853. In particular, the catalyst is treated in an acid solution before the hydrogenation, conveniently in the acid in which the hydrogenation is carried out. This gives rise to catalyst activation. The hydrogenation is carried out in the presence of an acid, preferably a strong mineral acid such as nitric acid, sulfuric acid or phosphoric acid, or a C 1 -C 5 monocarboxylic acid, aliphatic such as formic, acetic, propionic, butyric acid or valeric, preferably formic or acetic acid. Acid salts such as ammonium bisulfate are also convenient. In general, aqueous acids of 4 to 6 N are used and care must be taken to ensure that the concentration of the acid does not fall below 0.2 N during the course of the hydrogenation. If necessary, more acid is added. The ratio of the acid to the catalyst depends significantly on the catalyst used. The general, this is in the range of 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 preferred ratio is in a range from 1 to 100 g, in particular from 20 to 80 g, of the catalyst per liter eaf. of mineral acid. 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 preparing hydroxylammonium salts provides selectively higher selectivity of hydroxylammonium and selectivity of lower nitrogen monoxide. In addition, the space-time yield of the catalysts that are exposed to the purified nitrogen monoxide according to the process of the present invention, is increased considerably compared to the use of untreated nitrogen monoxide. In addition, the operating life of the catalysts used also increases. As a result, the catalyst has to be regenerated less frequently, which improves the economy of the process. The following examples illustrate the invention without restricting it.

Example 1 a) 40 g of Asbury graphite having a particle size from 2 to 50 μ and 0.5310 of hexachloroplatinic acid (IV) hexahydrate were stirred a-fc-a-MÜSSÜIS i • $ & overnight at 80 ° C in an aqueous solution containing 3.87 ml of concentrated hydrochloric acid and 0.87 ml of concentrated nitric acid. The sodium carbonate was added to the suspension obtained until the pH was 2.75. Subsequently, 2.5 g of sodium acetate were added as a buffer. 5 mg of elemental sulfur were then added and, after waiting for two minutes, the resulting suspension was mixed with 14.1 g of a 40% by weight aqueous solution of sodium formate concentration (83 mmol) and stirred at 80 ° C. for four hours. After this time, the platinum could no longer be detected by means of hydrazine hydrate (gives a black precipitate in alkaline solution in the presence of platinum). The catalyst prepared in this way was separated from the reaction mixture by filtration through a glass frit and washed with distilled water until the pH of the washings was no longer in the acid range. The dried catalyst contained 0.5% by weight of platinum. b) 3.6 g of catalyst prepared under a) were suspended in 120 ml of sulfuric acid 4.3 N, at 40 ° C with vigorous stirring (3500 rpm), 7.75 l / h of a mixture of nitrogen monoxide at 35% by volume had previously been passed through a tower of íj ^ íj? "-JA? ^^ Á! S! ¡¡¡¡í1iJ ^ S ^ S? ¿¿¿¿¿dried packed with 800 ml of silica gel and later through a purification tower packed with 1200 ml of molecular sieves 4A (Cari Roth GMBH, Karlsruhe) and 65% by volume of hydrogen were passed to the suspension.After four hours, the catalyst was separated and the liquid phase was analyzed.The catalyst that had been subsequently separated was mixed with 120 ml of acid 4.3 N sulfuric acid and the reaction was continued.

The procedure was repeated every four hours and will be designated as a batch. The catalyst was tested until the synthesis of hydroxylamine changed by less than 0.5% from batch to batch (activation phase). This required a catalyst process time of 20 batches. At lot number 21, then NO not purified was used; in lot 22, again NO purified was used as in the previous lots. The liquid phase was analyzed in each case. The selectivities achieved in these batches are shown in Table 1. 20 Table 1 In Table 1 it can be seen that the use of unpurified NO leads to a significant increase in the selectivity of N02, mainly at the expense of hydroxylamine selectivity. If NO purified is again used in the next batch, the values of the previous batch are obtained again.

Example 2 NO passage through a drying tower 3.6 g of catalyst prepared in Example la) were suspended in 120 ml of sulfuric acid 4.3 N and, at 40 ° C with vigorous stirring (3500 rpm) 7.75 1 / h of a mixture of 35% by volume of nitrogen monoxide that had previously been passed through a drying tower packed with 800 ml of silica gel and 65% by volume of hydrogen was passed to the suspension. After four hours the catalyst was separated and the í% S * ~ ££ jßsÉ & *. ~. *..- liquid phase. The catalyst that had been separated was subsequently mixed with 120 ml of sulfuric acid 4.3 N and the reaction was continued. This procedure was repeated every four hours. The reaction was interrupted when the selectivity for dinitrogen monoxide exceeded the predetermined upper limit of 5%. The experimental results are shown in Table 2.

Example 3 NO passage through a drying tower and a purification tower 3.6 g of catalyst prepared in Example la) were suspended in 120 ml of sulfuric acid 4.3 N and, at 40 ° C with vigorous stirring (3500 rpm) , 7.75 1 / h of a mixture of 35% by volume of nitrogen monoxide that had previously been passed through a drying tower packed with 800 ml of silica gel and then through a purification tower packed with 1200 ml of 4A molecular sieves (Cari Roth GMBH, Karlsruhe) and 65% by volume of hydrogen were passed to the suspension. After four hours the catalyst was separated and the liquid phase was analyzed. The catalyst that had been separated was subsequently mixed with 120 ml of sulfuric acid 4.3 N and the reaction was continued. This procedure was repeated f ^^^^^ t ^ > yes ^^^ imßmeeiiííms s ^? ssí < ^ < ^ "_ A" a? A _-_ wi ^ _ ^ B-a_M__B _ ^ _ ^ _ ^ _ ^ _ ^ _ ^ _-_ M _ ^ _ BB- ^ fiBs¿ ^^ M ..? «. ~ Bl? J ^ nfi? I ^? RavTM_-ffi-W ^ i ^^ r ii '^? Tiaflafafe, ^ *. . every four hours. The reaction was interrupted after 50 batches. The selectivity for dinitrogen monoxide was 0.41% at this point. The results obtained are shown in Table 2.

Table 2 It can be seen from the Table that the catalyst exposed to NO that had previously been passed only through a drying tower has a shorter operating life comparable with hydroxylamine selectivities and space-time yields. Therefore, it is clear that molecular sieves release NO from additional impurities that lead to accelerated aging of the catalyst. To clearly show the effect of purification, the content of the impurities in the unpurified NO and in the NO purified in different forms (drying tower containing silica gel or molecular sieves) & -tea »fc &. | 4A) is shown in Table 3.

Table 3 The values were obtained by separating the NO to be analyzed in the columns of the type indicated below or using the indicated methods. Molecular sieve 5A; 50 m; 0.32 mm; (inert gases) Activated carbon; 2 m; 4 mm; (hydrogen) Poraplot Q; 50 m; 0.53 mm; (carbon dioxide) AI2O3 / KCI; 100 m; 0.53 mm; (hydrocarbons) Atomic emission technique by GC (H2S) IR spectroscopy (N20, N02) aa ^ a »'. ^ Bte-a - ^^ e-'

Claims (1)

1. CLAIMS A process for preparing hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen in the presence of an acid and a hydrogenation catalyst, wherein the nitrogen monoxide prepared by combustion of ammonia is made available and is thus purified before the catalytic reduction by subjecting it to at least one treatment with porous oxides based on silicon and / or aluminum. The process as claimed in claim 1, wherein the porous oxide used is a molecular sieve or a silica gel. The process as claimed in claim 1 or 2, wherein the nitrogen monoxide is subjected to a drying step before purification. The process as claimed in claim 2 or 3, wherein the nitrogen monoxide first passes over silica gel and subsequently onto a molecular sieve, in particular a molecular sieve having the pore size 4A. The process as claimed in any of the preceding claims, wherein the purification is carried out from 20 to 500 ° C, in particular from 20 to 150 ° C. ', ^? ^^ x,: The process as claimed in any of the preceding claims, wherein the purification is carried out at a pressure in the range from 100 mbar to 100 bar, in particular from 800 bar to 2 bar. The process as claimed in any of the preceding claims, wherein the catalytic reduction is carried out from 30 to 80 ° C and at a pressure in the range from 1 to 30 bar. The process as claimed in any of the 10 previous claims, wherein the hydrogenation is carried out using a molar ratio of hydrogen to nitrogen monoxide in the range from 3.5: 1 to 5: 1. fifteen twenty 25