MXPA98002151A - Preparation of a catalyst for hidrogenac - Google Patents

Abstract

The present invention relates to the preparation of a hydrogenation catalyst by reducing platinum in the oxidation state +4 (PL (IV)), with a selective reducing agent in an aqueous medium in the presence of a carbon-containing carrier, to platinum in the oxidation state +2 (PL (II)), then the poisoning of the resulting platinum with a selective reducing agent containing sulfur and then reducing the partially poisoned platinum in this form to metallic platinum (PL (O)) and then the treatment in a manner known per se, wherein (a) (PL (II)) is partially poisoned with a selective reducing agent containing sulfur, the reducing agent is used in an amount corresponding to 15-70 mol% of the amount of a selective reducing agent containing sulfur, which would be necessary to reduce Pt (IV)) to Pt (II), as long as the amount of Pt (IV) corresponds to the amount of Pt (II) used and which is going to poison, and the Pt (II) parcialme The poison is then reduced with an alkaline metal format in Pt (0), or (b) Platinum in an oxidation state greater than +2 is partially poisoned and subsequently reduced or simultaneously by means of a metal format alkaline to Pt (0), and the hydrogenation catalyst, the use of the hydrogenation catalyst for the preparation of hydroxylammonium salts, the processes for the preparation of the hydroxylammonium salts and a process for regenerating the hydrogenation catalysts containing

Description

PREPARATION OF A CATALYST FOR HYDROGENATION The present invention relates to an improved process for the preparation of a catalyst for hydrogenation by reducing platinum in the +4 oxidation state with a selective reducing agent, in an acidic aqueous medium, in the presence of a carbon-containing carrier, to platinum in oxidation state +2, subsequently the poisoning of the resulting platinum with a selective reducing agent containing sulfur and then the reduction of the partially poisoned platinum in this form into metallic platinum and then the treatment in a manner known per se. The present invention further relates to the use of alkali metal formats for the preparation of the hydrogenation catalysts, to the hydrogenation catalysts prepared according to the invention, to a process for the preparation of hydroxylammonium salts and a process for regenerating the Platinum-based hydrogenation catalysts. Noble metals, such as palladium, platinum or ruthenium, which are applied to various carriers, such as silica, alumina, graphite or activated carbon, as described in "Katalytische Hydrierungen im otganische chemischen Laboratorium" F. Zimal osi, Ferdinand Enke Verlag , Stuttgart (1965), are suitable for hydrogenating organic and inorganic compounds. The high dispersion of the noble metal in the catalyst carrier is essential for the activity of these catalysts. The fact (see Structure of Metallic Catalyst, JR Anderson, Academic Press (1975), page 164 et seq.), That, under the conditions of the reaction, the particle size of the applied noble metal increases as a result of agglomeration, the dispersion decreases and the elemental noble metal is separated from the carrier material, is disadvantageous in relation to the design of the process. German Patent 1,088,037 describes a process for the preparation and regeneration of a special catalyst to obtain hydroxylamine by first reducing the platinum in the +4 oxidation state with a selective reducing agent in an acidic aqueous medium in the presence of a carrier containing carbon in platinum in the +2 oxidation state, then the partial poisoning of the resulting platinum with a selective sulfur-containing reducing agent and then the reduction of the poisoned platinum resulting in platinum metal with a format and then the treatment in a manner known per se. The disadvantages of this process are the insufficient selectivity with respect to hydroxylamine, the excessively large amount of ammonia and nitrous oxide that are obtained and the insufficient conversion of nitric oxide and the unsatisfactory space-time yield. DE-C 40 22 853 establishes that the selectivity with respect to hydroxylamine in the hydrogenation of nitric oxide can be increased by using graphite-supported platinum catalysts in which the graphite has a particle size of 1-600 μ.

German Patent 956,038 describes platinum catalysts with graphite support which are obtained by precipitating platinum in suspended graphite carriers, with or without the addition of poisons, such as sulfur compounds, selenium compounds, arsenic compounds or tellurium compounds . These catalysts are suitable for the catalytic hydrogenation of nitric oxide. These catalysts have the disadvantage of reactivity, and selectivity decreases rapidly during prolonged use. DE-C 40 22 851 establishes that the selectivity is related to the bulk density, the compressive strength and the porosity of the graphite carrier in the preparation of hydroxylamine by hydrogenation of nitric oxide in the presence of platinum catalysts supported on graphite. The catalysts used in the processes of the aforementioned German patents have the disadvantage that, due to the agglomeration of the active components, only relatively short lives of the catalyst can be obtained. DE-A 43 11 420 describes the preparation of a hydrogenation catalyst which is obtained by treating a metal salt of platinum with finely divided sulfur in the presence of a dispersant and subsequently reducing the salt of the platinum metal in metallic platinum. Although sodium format is also mentioned as a reducing agent, according to DE-A 43 11 420, formic acid is particularly preferred. Tests with sodium format were not carried out in the corresponding German Patent. There are also no indications that the platinum particle size has a decisive effect on the mechanical stability of the catalyst, the selectivity and the spectrum of the by-product. An object of the present invention is to provide an improved process for the preparation of hydrogenation catalysts, which ensures longer lives for the catalysts used together with at least the same selectivity and a superior space-time yield. In addition, the by-products must be reduced, in particular nitrous oxide and ammonia. We have found that this objective is achieved by an improved process for the preparation of a hydrogenation catalyst by reducing platinum in the oxidation state +4 (Pt (IV)) with a selective reducing agent in an acidic aqueous medium in the presence of a carrier that contains carbon to platinum in the oxidation state +2 (Pt (II)), then the poisoning of the resulting platinum with a selective reducing agent containing sulfur and then reducing the partially poisoned platinum in this form to metallic platinum (Pt (0) ) and then the treatment in a manner known per se, wherein. (a) The (Pt (II)) is partially poisoned with a selective reducing agent containing sulfur, the reducing agent is used in an amount corresponding to 15-70% mol of the amount of a selective reducing agent that would be necessary for reduce Pt (IV)) to Pt (II), as long as the amount of Pt (IV) corresponds to the amount of Pt (II) used and that is going to be poisoned, and the partially poisoned Pt (II) is then reduced with an alkaline metal format in Pt (0), or (b) Platinum in an oxidation state greater than +2 is partially poisoned and subsequently reduced or simultaneously by means of an alkaline metal format at Pt (0). ). We have also found the use of alkali metal formats for the preparation of the hydrogenation catalysts, hydrogenation catalysts prepared according to the invention, a process for the preparation of hydroxylammonium salts and a process for the regeneration of the hydrogenation catalysts to Platinum base. According to the invention, in variant (a) the platinum in an oxidation state +2 is found by a selective reducing agent containing sulfur in an acidic aqueous medium in the presence of a carbon-containing carrier. The partially poisoned platinum in this form is then reduced to metallic platinum with an alkali metal format and then treated in a manner known per se. The amount of selective sulfur-containing reducing agent is chosen so that it is used in an amount corresponding to 15-70, preferably 20-65''mole of the amount of a selective reducing agent that would be necessary to reduce Pt (IV ) to Pt (II), as long as the amount of PT (IV) corresponds to the amount of Pt (II) used and poisoned. In variant (b), the platinum in an oxidation state greater than +2 is poisoned, according to the invention, and subsequently or simultaneously reduced by means of an alkaline metal format to Pt (0). According to the invention, alkali metal formats are used, preferably lithium format, sodium format and potassium format, particularly preferably sodium format. In a particular embodiment, platinum in the +2 oxidation state, which can be obtained by reducing the platinum in an oxidation state greater than +2, particularly preferably Pt (IV), with a selective reducing agent, is used in the variant (a). As a rule, dithionites, in particular sodium dithionite (Blankit®), sulfoxylic acid derivatives, in particular the product obtained by the action of formaldehyde in sodium hyposulfite known as Rongalit®, sulfuric acid and sulfites can be used as the selective reducing agent. In general, the selective reducing agent is used in an equivalent amount for the dissolved platinum having an oxidation state greater than 2, preferably +4. When a selective sulfur-containing reducing agent is used, in variant (a) as a poison that is capable of reducing platinum in the oxidation state, +4 to platinum in the oxidation state +2 (for example, Blankit®), the reduction is carried out, according to the invention, initially only to platinum in the oxidation state +2. Observations to date have shown that the endpoint of this reaction is detectable from a large change in potential that is measured during reduction. The partial poisoning is then carried out, according to the invention, by adding a certain amount above and above that necessary to reduce Pt (>; II), preferably Pt (IV), to Pt (II) and corresponding to 15-70, preferably 20-65% mol of the amount used to reduce Pt (IV) to PT (II). The reduction with the alkali metal format is then carried out according to the invention. If the poison used is a compound that is not capable of reducing platinum in an oxidation state greater than +2, in particular Pt (IV) to Pt (II) (variant (b)), then, according to the invention, platinum is first poisoned and subsequently or simultaneously reduced to platinum (0) (metallic platinum) with an alkali metal format as a reducing agent. These poisons are conventional poisons based on sulfur, selenium, arsenic or tellurium, such as sodium dithionite, alkali metal thiosulfates, hydrogen sulphide, alkali metal sulphide, alkali metal polysulfides, thiourea, telluric acid or arsenic acid or combinations of these. Particularly preferred is the use of elemental sulfur, in particular wettable sulfur, having a particle size of less than 500μ, preferably less than 50μ. The molar ratio of platinum to sulfur, selenium, arsenic or tellurium to be poisoned is usually chosen in the range from 20: 1 to 3: 1, preferably from 10: 1 to 5.1. Partial poisoning is usually carried out by methods known per se, as described, for example, in DE-C40 22 853. The reduction of alkali metal to metal platinum is preferably carried out after it has been carried out. out the partial poisoning. Particularly suitable platinum (IV) compounds are water-soluble compounds, such as hexachloroplatinic acid and its alkali metal and ammonium salts, such as disodium, dipotassium and diammonium hexachlorplatinate. The molar ratio of platinum used to alkali metal format is usually chosen in the range from 1000: 1 to 10: 1, preferably from 100: 1 to 20: 1. The carbon containing carrier that is used, as a rule, is suspended in graphite or activated carbon, in particular electrografite grade, particularly preferably those electrographite grade having a particle size from 0.5 to 600 μ, preferably from 1 to 70 , particularly preferably from 2 to 50 μ. The amount of platinum is in general from 0.2 to 2, preferably from 0.5 to 1% by weight, based on the total weight of the platinum catalyst supported on graphite. According to the invention, the reduction of the platinum is carried out in an aqueous solution, the weight ratio of water to platinum is chosen, usually from 1000: 1 to 100: 1, preferably from 500: 1 to 100: 1. In addition, the reduction is carried out in a slightly acidic range, the pH usually being from 4.5 to less than 7, preferably from 5 to 6. The pH is generally established by adding buffer salts, such as alkali metal acetate, in particular sodium acetate. In a preferred embodiment of variant (a), Blankit® (sodium dithionite) is used as a selective reducing agent. As a rule, the amount of Blankit added is only sufficient for the solution potential, measured by means of a glass electrode, from 420 to 500 mV, preferably from 440 mV to 480 mV. Upon completion of platinum (IV) reduction in platinum (II), whose observations to date have been shown to be noticeable from a large change in Blankit® potential, an amount of Blankit® more or less than necessary to reduce Platinum IV in platinum (II) is added until a certain desired potential is reached. This potential characterizes the poisoning state of the catalyst and is usually from 440 mV to 200 mV, preferably from 270 mV to 340 mV.

The molar ratio of the alkali metal format in platinum is chosen to be in general from 1000: 1 to 10: 1, preferably from 100: 1 to 20: 1. The temperature of the reduction is generally chosen from 50 to 95 ° C, preferably from 60 to 90 ° C. In addition, atmospheric pressure is advantageously employed. The pH after reduction to metal platinum depends essentially on the type of reducing agent chosen and is usually from 5 to 8, particularly preferably from 5 to 6.5. After completion of the reduction, the catalyst is normally treated in the normal manner, for example by filtering it from the reaction mixture and washing it advantageously with water, preferably until the washing water is neutral. The observations to date have shown that the size of the platinum particles prepared according to the invention is generally not greater than 3.5 nm, obtained by determining the linear amplitude at the average height by X-ray diffraction. The observations to date have shown that the catalyst obtained by the novel process is suitable for the hydrogenation of organic and inorganic compounds.

The novel catalysts are preferably used to hydrogenate olefinically or acetylenically unsaturated compounds and to hydrogenate carboxylic acids, aldheides or ketones to the corresponding alcohols or nitriles to the corresponding amines. In addition, novel catalysts are suitable for hydrogenating inorganic substances such as oxygen, but in particular for the preparation of hydroxylammonium salts by hydrogenating nitric oxide in aqueous mineral acids. During the preparation of the hydroxylammonium salts, a molar ratio of hydrogen to nitric oxide is usually maintained from 1.5.1 to 6: 1, preferably from 3.5 to 5: 1. Observations to date have shown that particularly good results are obtained if it is ensured that a molar ratio of hydrogen to nitric oxide of 3.5: 1 to 5: 1 is maintained in the reaction zone. Acids which are advantageously used are strong mineral acids such as nitric acid, sulfuric acid or phosphoric acid, or aliphatic CI-C2 monocarboxylic acids, such as formic acid, acetic acid, propionic acid, butyric acid and valeric acid, preferably formic acid and acetic acid. Acid salts, such as ammonium bisulfate, are also suitable. As a rule, aqueous acids of 4 to 6 normal are used, and the concentration of the acid is usually not allowed below 0.2 normal during the course of the reaction. The hydrogenation of nitric oxide was generally carried out from 30 to 80 ° C, preferably from 35 to 60 ° C. In addition, the pressure during hydrogenation is usually chosen from 1 to 30, preferably from 1 to 20 bar (absolute). The ratio of the mineral acid to the catalyst depends essentially on the platinum metal and the reactor pressure and, in the case of platinum in general, it is from 1 to 100, preferably from 30 to 80 g of platinum / graphite catalyst per liter of mineral acid . In a more preferred embodiment, in particular in the preparation of hydroxylammonium salts, the catalyst is treated with hydrogen (activation) before the hydrogenation in acid solution, advantageously in the mineral acid in which the hydrogenation is carried out. The depleted platinum metal catalysts can be regenerated with the aid of the novel process by contacting the platinum metal of the catalyst in solution, usually by means of an acid or an acid mixture, and if required, separating the insoluble components. The solution of the metal salt of platinum obtained is then neutralized and the metal salt of the platinum is then treated by the novel process described above. The observations to date have shown that the novel catalysts are superior to the catalysts known for the same purpose, in relation to the activity, selectivity and life of the catalyst.

Examples The size of the graphite particles was determined using a MALVERN Mastersizer (see, also verfahrenstechnik 24 (1990), 36 is seq.) Faunhofer diffraction was measured at a wavelength of 633 nm. The particle size distribution was determined in a range from 1 to 600 μ by choosing an auxiliary lens having a focal length f = 300 nm. For the measurement, a pinch of the investigated powders was added to a liter of 0.1% concentration by weight of an aqueous solution Nekanil 910 (BASF AG; Nekanil 910 is a nonylphenol that reacted with from 9 to 10 mol of ethylene oxide; Properties: transparent, viscous liquid, density at 20 ° C: 1.04 g / cmJ; pour point: below minus 10 ° C; pH of a 1% by weight solution from 6.5 to 8.5). Before the measurement, the resulting mixture to be investigated was subjected to an ultrasound treatment for 1 minute. The size of the platinum particles can be determined by means of X-ray diffraction determining the linear amplitude at the average height.

Example 1 a) 40 g of graphite from Becker-Pennrich, with a particle size from 28 to 50 μ and 0.5310 g of hexachloroplatinic acid (IV) hexahydrate was stirred overnight at 80 ° C with 40 ml of the aqueous solution It contained 3.87 ml of concentrated hydrochloric acid and 0.87 ml of concentrated nitric acid. Sodium carbonate was added to the resulting suspension until a pH of 2.75 was reached. 2.5 g of sodium acetate were then added to buffer. Then an aqueous solution of sodium dithionate 4.58% by weight of concentration in an amount sufficient to reduce platinum "1 * in platinum2" (detectable by a large change in potential at 460 V) was added. To poison the catalyst with sulfur, the same solution of sodium dithionite [sic] that was used to reduce platinum'1 * in platinum2 + was added in an amount in an amount corresponding to 60 mole% of the amount that was used to reduce Pt + to Pt2 +. The potential of the solution then obtained, determined by means of a glass electrode, was 355 mV. 14.1 g of an aqueous solution of sodium formate 40% by weight concentration (83 mmol) were then added to the suspension thus obtained, and the stirring was carried out for 4 hours at 80 ° C. After this time, the platinum was no longer detectable with hydrazine hydrate (it produces a black precipitate in alkaline solution in the presence of platinum). The catalyst thus prepared was isolated from the reaction mixture by filtration through a glass frit and washed with distilled water until the pH of the wash water was no longer in the acid range. The dry catalyst contained 0.5% by weight of platinum. (b) 4.8 g of catalyst prepared in accordance with a) were suspended in 120 ml of sulfuric acid 4.3 N, and 7.75 1 / h of a mixture of 35 vol.% nitric oxide and 65 vol.% hydrogen were passed at 40 ° C with vigorous stirring (3500 rpm). After 4 hours, the catalyst was separated and the liquid phase analyzed. After this, 120 ml of sulfuric acid 4.3 N were added to the separated catalyst and the reaction was allowed to continue. This process was repeated every 4 hours. The reaction was terminated after the selectivity with respect to nitrous oxide formed exceeded the established upper limit of 10%. The experimental results are shown in the following table.

Comparative Example 1 The procedure was as in Example 1, except that the poisoning was carried out using 56 mol%, based on the amount that was used, for the reduction of platinum (II), of sodium dithionite, 6.25 ml. of concentrated formic acid was used for precipitation, to produce zero-valent platinum. The results obtained are shown in the following table.

Example 2 a) 40 g of a graphite from Becker-Pennrich, with a particle size from 28 to 50 μ and 0.5310 g of hexachloroplatinic acid (IV) was stirred overnight at 80 ° C with 40 ml of the aqueous solution It contained 3.87 ml of concentrated hydrochloric acid and 0.87 ml of concentrated nitric acid. Sodium carbonate was added to the resulting suspension until a pH of 2.75 was reached. 2.5 g of sodium acetate were then added to buffer. After this, 6.25 mg of elemental sulfur were added and, after a waiting time of 2 minutes, 14.1 g of an aqueous solution of sodium formate 40% by weight (83 mmol) were added to the resulting suspension and taken performed the stirring for 4 hours at 80 ° C. After this time, the platinum was no longer detectable with hydrazine hydrate (it produces a black precipitate in alkaline solution in the presence of platinum). The catalyst thus prepared was isolated from the reaction mixture by filtration through a glass frit and washed with distilled water until the pH of the wash water was no longer in the acid range. The dry catalyst contained 0.5% by weight of platinum. b) The catalyst was tested as described in accordance with Ib).

Comparative Example 2 - similar to DE-A 43 11 420 The procedure was as in Example 2, except that 6.25 mg of elemental sulfur was used for the poisoning and 6.25 ml of concentrated formic acid was used for the precipitation to produce zero-valent platinum . The results obtained are shown in the following table. lí >

Claims (9)

1. CLAIMS A process for the preparation of a hydrogenation catalyst by reducing platinum in the oxidation state +4 (Pt (IV)) with a selective reducing agent in an acidic aqueous medium, in the presence of a carbon-containing carrier, in platinum in the oxidation state +2 (Pt (II)), then the poisoning of the resulting platinum with a selective reducing agent containing sulfur and then reducing the partially poisoned platinum in this form in metallic platinum (Pt (0)) and then the treatment in a form known per se, wherein (a) The (Pt (II)) is partially poisoned with a selective reducing agent containing sulfur, the reducing agent is used in an amount corresponding to 15-70 mol% of the amount of a selective reducing agent that would be necessary to reduce Pt (IV)) to Pt (II), as long as the amount of Pt (IV) corresponds to the amount of Pt (II) used and that is going to be poisoned, and Pt (II) partially poisoned then will r educe with an alkaline metal format in Pt (0), or (b) Platinum in an oxidation state greater than +2 is partially poisoned and subsequently reduced or simultaneously by means of an alkaline metal format to Pt ( 0).
2. The process, according to claim 1, wherein the Pt (II) in variant (a) is obtained by reducing platinum in an oxidation state greater than +2 with a selective reducing agent.
3. The use of a format for the preparation of a hydrogenation catalyst by reducing Pt (IV) with a selective reducing agent in an acidic aqueous medium, in the presence of a carbon containing carrier, in Pt (II), then poisoning the resulting platinum with a selective reducing agent containing sulfur and then reducing the partially poisoned platinum in this form to give metallic platinum (Pt (0)) and then the treatment in a manner known per se, wherein (a) the (Pt (II)) is partially poisoned with a selective reducing agent containing sulfur, the reducing agent is used in an amount corresponding to 15-70% mole of the amount of a selective reducing agent that would be necessary for reduce Pt (IV)) to Pt (II), as long as the amount of Pt (IV) corresponds to the amount of Pt (II) used and that is going to be poisoned, and the partially poisoned Pt (II) is then reduced with an alkaline metal format at Pt (0), or (b) platinum in an oxidation state greater than +2 is partially poisoned and subsequently reduced or simultaneously by means of an alkaline metal format at Pt (0). ).
4. 4. The process according to claim 1 or 2, wherein the selective reducing agent that is used in variant (a) is sodium dithionite.
5. 5. The process, according to claim 1, 2 or 4, wherein the alkaline metal format that is used is sodium format.
6. 6. The process according to claim 1, 2, 4 or 5, wherein the size of the platinum particles is not greater than 3.5 nm. The hydrogenation catalyst obtainable by one of the processes according to any of claims 1, 2, or 4 to 6. 8. The use of a hydrogenation catalyst according to claim 7 or that is prepared by a process according to any of claims 1, 2, or 4 to 6 for the preparation of hydroxylammonium salts. 9. A process for the preparation of hydroxylammonium salts by reducing nitric oxide with hydrogen in the presence of a hydrogenation catalyst, wherein the hydrogenation catalyst used is a hydrogenation catalyst according to claim 7 or which is prepared by a process of according to any of claims 1, 2 or 4 to 6. A process for regenerating a platinum-based hydrogenation catalyst in a manner known per se, wherein Pt (IV) is reduced with a selective reducing agent in an aqueous medium. acid in the presence of a carrier containing carbon to Pt (II), the resulting platinum is then partially poisoned with a selective reducing agent containing sulfur and the partially poisoned platinum in this form is then reduced to metallic platinum (Pt (0)) and the treatment is then carried out in a manner known per se, (a) (Pt (II)) is partially poisoned with a selective reducing agent containing sulfur, the reducing agent is used in an amount corresponding to 15-70% mole of the amount of a selective reducing agent that would be necessary to reduce Pt (IV)) to Pt (II), as long as the amount of Pt (IV) corresponds to the amount of Pt (II) used and that is going to be poisoned, and the Pt (II) partially poisoned then it is reduce with an alkaline metal format to Pt (0), or (b) platinum in an oxidation state greater than +2 is partially poisoned and subsequently reduced or simultaneously by means of an alkaline metal format to Pt ( 0). SUMMARY OF THE INVENTION The present invention relates to the preparation of a hydrogenation catalyst by reducing platinum in the oxidation state +4 (Pt (IV)), with a selective reducing agent in an acidic aqueous medium in the presence of a carbon-containing carrier, to platinum in the oxidation state +2 (Pt (II)), then the poisoning of the resulting platinum with a selective reducing agent containing sulfur and then reducing the platinum partially poisoned in this way to metallic platinum (Pt (0)) and then the treatment in a manner known per se, wherein (a) (Pt (II)) is partially poisoned with a selective reducing agent containing sulfur, the reducing agent is used in an amount corresponding to 15-70 mol% of the amount of a selective sulfur-containing reducing agent, which would be necessary to reduce Pt (IV)) to Pt (II), as long as the amount of Pt (IV) corresponds to the amount of Pt (II) used and which goes to poison, and the Pt (I I) partially poisoned then reduced with an alkaline metal format in Pt (0), or (b) Platinum in an oxidation state greater than +2 is partially poisoned and subsequently reduced or simultaneously by means of a format of alkali metal to Pt (0), and the hydrogenation catalyst, the use of the hydrogenation catalyst for the preparation of hydroxylammonium salts, the processes for the preparation of the hydroxylammonium salts and a process for regenerating the hydrogenation catalysts containing platinum.