EP1169129A1 - Method for producing platinum metal catalysts - Google Patents

Abstract

The invention relates to a method for producing catalysts by immersion coating a metallic support with at least one platinum metal. An aqueous medium which comprises at least one platinum metal complex, at least one reduction agent and at least one complexer and which has a pH value of more than 4 is brought into contact with the metallic support in order to deposit the platinum metal, the platinum metal being deposited in the form of discreet, immobilised particles. The invention also relates to the catalysts obtained using this method and to their use for producing hydrogen peroxide and for hydrogenating organic compounds.

Description

Process for the production of platinum metal catalysts

description

The present invention relates to a novel process for the preparation of catalysts by electroless deposition of at least one platinum metal on a metallic support, the catalysts obtainable by this process, the use of the catalysts for the synthesis of hydrogen peroxide from the elements, for the hydrogenation of organic compounds and a process for Production of hydrogen peroxide using these catalysts.

Catalysts which contain platinum metals as catalytically active substances are used in a variety of forms and are of great technical importance, e.g. in the reduction or hydrogenation of organic compounds and in the catalytic purification of exhaust gases from industry and traffic.

For technical applications, use is made of supported platinum metal catalysts, which only use small amounts of the expensive noble metals on mostly catalytically inactive support materials with a large surface area, e.g. Have coal, aluminum oxide, silicon oxide, ceramic or other mineral carriers.

Catalysts in which the support can be used in the form of larger units are particularly easy to handle, e.g. in the form of granules, pearls or in particular fabrics, nets or moldings, such as monoliths. Such supported catalysts are mostly used as fixed bed catalysts and enable the economically advantageous continuous implementation of catalytic processes.

The catalytically active metals are usually applied to such porous supports by impregnating or impregnating the supports with solutions of the salts or organometallic compounds of the catalytically active metal and then immobilizing them by precipitation, hydrolysis, tempering, calcining and / or forming. This usually requires repeated heating of the impregnated catalyst to temperatures between 200 and 1200 ° C. For example, DE-A-2 317 560 describes the production of a catalytic device by impregnating a mineral monolith with a melt of trialkylaluminum at about 120 ° C., treating it with steam at 120 ° C./18 psi and then burning at 400 ° C. The procedure is then repeated with tetraalkylzirconium and the oxidic support obtained in this way soaked with a hexachloroplatinate is annealed at 300 ° C. and formed.

A disadvantage of such porous catalysts is, in addition to the complex production, the low resistance to strongly acidic reaction media. In addition, the use of such porous catalysts as fixed bed catalysts usually leads to a strong, undesirable pressure drop in the reaction vessel.

In order to circumvent the aforementioned disadvantages, attempts have been made for some time to coat metallic supports with catalytically active metals, in particular platinum metals. On the one hand, metallic supports are characterized by increased resistance, on the other hand, metals can be processed into thin sheets, wires, knitted fabrics and nets, which are characterized by a large surface area with favorable flow behavior. However, the application of the catalytically active metal to the metallic support is problematic. For example, EP-A-0 198 435 discloses a process for the preparation of catalysts, the active component, e.g. B. precious metals, are evaporated onto the carrier. Metallic supports can also be steamed using this process, but the process is technically very complex and requires expensive equipment. The process is also unsuitable for molded catalyst parts such as monoliths, wire mesh rings and coils.

Electrochemical plating processes or electroless plating processes, such as those used to refine material surfaces, lead to smooth, uniform coatings. The palladium-coated metals obtained are suitable, for. B. as an inexpensive replacement for gold-coated metal parts in the electronics industry. However, substrates coated in this way are unsuitable as catalysts.

For this reason, attempts have been made to coat metallic supports in soluble form by impregnating or impregnating them with the catalytically active metal. In order to obtain the porous surface necessary for impregnation or impregnation, either the surface of the metallic carrier is oxidized or, as described for example in EP-A-0 075 124, a porous oxide layer is applied to the metallic carrier and this , as previously described for the porous non-metallic supports, coated with the catalytically active metal. The catalysts obtained in this way show good catalytic activity on the one hand, and on the other hand they are not suitable for many reactions in aggressive media, because they can detach, especially at low pH values. solution of the oxide layer and thus irreversible deactivation of the catalyst.

Another process for the production of supported noble metal catalysts on porous oxidic supports is the electroless deposition of noble metal salts from aqueous solutions with reducing agents described in EP-A-0 878 235 in the presence of complexing agents such as ammonium chloride, EDTA or DTPA As a rule, the porous carrier must be activated by soaking with sensitizers before the deposition process. Formaldehyde or aqueous solutions of silver nitrate, titanium salts or tin halides are mentioned as suitable sensitizers. The palladium catalysts produced by this process show good activity as hydrogenation catalysts in the anthraquinone process for the preparation of H ₂ O ₂ in the organic phase, but, like most catalysts based on oxidic supports or coatings, are not suitable for wet-chemical processes in the presence of aggressive chemicals .

DE-A-196 42 770 discloses a process for the production of hydrogen peroxide by continuous reaction of hydrogen and oxygen over palladium catalysts in an aqueous or alcoholic medium. The metal-supported catalysts used in the examples are obtained by electroless deposition of Pd salts in a strongly acidic environment.

In "Catalysis of Organic Reactions (Scarrows and Prunier, ed.),

Marcel Dekker Inc., New York, 1995, pp. 115-124

JR Kosak the production of metal-supported precious metal catalysts and their use for the direct synthesis of H ₂ 0 ₂ from hydrogen and oxygen. The noble metal is applied to the metallic support by electroless plating with palladium chloride or palladium and platinum chloride in the presence of sodium hypophosphite as a reducing agent. The reduction of the noble metal takes place in a strongly acidic solution in the presence of the carrier metal and leads to the formation of finely divided noble metal particles in the solution, which becomes cloudy and turns gray. As the reaction time progresses, the solution decolors to the extent that the precious metal is deposited on the carrier in the form of a black coating.

In "Studies of Surface Science and Catalysis, Vol. 118, pp. 63-72, JP Reymond describes the production of metal-supported palladium catalysts and their use for the hydrogenation of acetophenones. The production of the catalyst is based on the work of Kosak, but also here Palladium chloride in a strongly acidic solution (pH <2.2) with sodium hypophosphite as detergent is deposited. Reymond also observed clouding and darkening of the aqueous reaction medium before the palladium began to be deposited on the metallic support, which Reymond attributed to the formation of very fine palladium particles in the aqueous medium. Reymond describes that the formation of the palladium particles in the solution and the deposition on the metallic carrier are simultaneous processes, the formation of the particles in the aqueous medium proceeding faster than the deposition on the metallic carrier. Reymond concludes from the fact that the catalysts produced in this way have a good catalytic activity in comparison with catalysts produced according to conventional plating regulations by separating noble metals from homogeneous solution. Only the separation of noble metals from a solution inhomogeneous by failed noble metal particles is too catalytic active deposits on the metal supports.

A disadvantage of the catalysts produced by the Kosak and Reymond processes is that the catalytically active coating thus obtained has insufficient adhesion. The insufficient adhesion manifests itself in the detachment of precious metal particles both when cleaning the catalyst after manufacture and when used in the catalytic process. This can lead to the gradual deactivation of the catalyst and contamination of the reaction medium with metal particles. In addition, the activity and selectivity of the catalysts prepared in this way are at least insufficient for economical direct synthesis of H ₂ O ₂ from hydrogen and oxygen, in particular when using hydrogen / oxygen mixtures below the explosion limit. The primary formation of the metal particles in the liquid medium entails the risk that the suspended palladium is not completely deposited on the carrier and the particles are often attached very loosely to the metallic carrier.

The object of the present invention is therefore to provide an improved process for the production of metal-supported platinum metal catalysts which ensures that the expensive platinum metal is deposited as completely as possible and that the noble metal adheres well to the metal support. Furthermore, the catalysts should have a high catalytic activity and selectivity for the direct synthesis of H ₂ 0 ₂ from hydrogen and oxygen. In addition, the catalysts should be characterized by improved service lives. The present object was achieved by providing a process for the production of metal-supported catalysts, in which one has an aqueous reducing solution which contains at least one platinum metal complex, at least one reducing agent and a complexing agent and a pH of greater than 4, with the metallic Bring carrier for the deposition of the platinum metal in contact, the deposition of the platinum metal on the carrier surface in the form of discrete, immobilized - ie firmly anchored - particles from the homogeneous, aqueous medium. In the following, platinum metal complexes are to be understood as meaning both uncharged or charged coordination compounds and salt-like compounds of platinum metals. For the purposes of the present invention, the aqueous reaction medium is "homogeneous" when no clouding or discoloration due to the precipitation of metal particles can be observed. In comparison to the prior art described above, when the coating is carried out according to the invention, no intermediate turbidity due to precipitated metal particles can be observed. Nevertheless, contrary to the assumption of Reymond (see above), the catalysts produced according to the invention have excellent catalyst properties. In addition, essentially quantitative deposition of the platinum metal from the solution can be achieved. Surprisingly, the catalytic coatings produced according to the invention have high abrasion resistance even under heavy mechanical stress, such as, for example, in hydrogen peroxide synthesis due to high exposure to circulating gas and strong liquid circulation. Even after a long period of operation, there is no mechanical detachment.

In the method according to the invention, the platinum metal is preferably deposited without current, i. H. not electrochemically, but by adding a reducing agent to the solution.

Platinum metals in the sense of the invention are preferably the noble metals of subgroup 8 of the periodic table, namely rhodium, iridium, palladium, osmium, iridium and platinum. Ruthenium, rhodium, palladium and platinum are preferred, palladium and platinum are particularly preferred. The catalysts according to the invention can comprise several platinum metals. All combinations of the platinum metals mentioned are conceivable, preference is given to combinations of palladium and platinum, of palladium and rhodium, of palladium and iridium, of palladium, platinum and rhodium and of palladium, platinum and iridium. Palladium and platinum are particularly preferred as a combination. In the combinations with palladium, palladium is preferably the main platinum metal component. The palladium content is then preferably above 40% by weight, preferably above 60% by weight and particularly preferably above 80% by weight, based on the total platinum metal content. The further platinum metals optionally contained as secondary constituents can each account for up to 30% by weight, preferably up to 20% by weight and particularly preferably up to 15% by weight of the total platinum metal content. The platinum metals preferably comprise 80 to 100% by weight of palladium and 0 to 20% by weight of platinum or iridium. In most cases, 1 to 3 of the platinum metals mentioned make up more than 95% by weight of the amount of platinum metal used. If, in addition to a main platinum metal, other platinum metals are also present, these are generally present in amounts of greater than 0.001% by weight, preferably greater than 0.01% by weight, e.g. B. about 0.1% by weight, about 1% by weight or about 5% by weight.

In addition to platinum metals, the catalytically active component can contain further elements as additional components or impurities. Additional components that can influence the activity and / or selectivity of the catalyst are e.g. B. metals such as cobalt, nickel, copper, silver, gold, chromium, molybdenum, tungsten, manganese, rhenium, aluminum, tin, lead, arsenic, antimony and bismuth, and non-metals such as boron, carbon, silicon, nitrogen and phosphorus . The metals and non-metals mentioned can be present in the catalytically active coating both in ionic and in nonionic form. In addition, the catalytically active component may contain other elements (metals and non-metals) as impurities, e.g. B. by the fact that the catalytically active components used contain impurities, or that during the process for the preparation of the catalysts according to the invention, components of the components used in the process according to the invention are incorporated into the platinum metal coatings, such as. B. alkali and alkaline earth metals, phosphorus, boron and halogens.

The additional components can be present at 0.001 to 25% by weight, based on the platinum metal content. Additional components used as promoters or dopings generally make up 0.01 to 20% by weight, preferably 0.1 to 15% by weight and in particular 0.5 to 10% by weight, based on the platinum metal content.

In the process according to the invention, the platinum metals are preferably used as platinum metal complexes. Platinum metal complexes in which the platinum metal is present in the oxidation states +1 to +4 are preferably used. Four-coordinate complexes are preferred. The process according to the invention is preferably suitable for the production of platinum metal catalysts in which palladium is the main platinum metal component.

Palladium (II) complexes are preferably suitable for the production of catalysts which contain palladium, and in particular of catalysts which contain palladium as the main platinum metal component. Palladium (II) complexes in which palladium has the coordination number 4 are particularly suitable.

Such combinations of platinum metal ions and ligand are preferably selected whose complex formation constant is> 1,000 and in particular> 10,000.

Suitable combinations of ligands and counterions for palladium complexes and for platinum metal complexes other than palladium can be selected according to the principle of charge neutrality. Suitable negatively charged ligands are e.g. B. selected from halides and pseudohalides, such as fluoride, chloride, bromide, iodide, CN, OCN and SCN, Cx-C _ö carboxylic acids, such as formic acid, acetic acid and propionic acid and their salts, chelating ligands, such as ethylenediaminetetraacetic acid (EDTA ), Nitrotriesriacetic acid, 1,2-diaminocyclohexanetetraacetic acid and its salts, aminophosphonic acids such as nitrilomethylenephosphonic acid, diketonates such as acetylacetonate, hydroxycarboxylic acids such as glycolic acid, lactic acid, tartaric acid and gluconic acid, and their salts. Suitable as electroneutral ligands are e.g. B. alkyl nitriles such as acetonitrile, amines such as ammonia, primary, secondary and tertiary Ci-C _ö alkyl amines such as ethylamine, n-propylamine, isopropylamine, n-butylamine, tert-butylamine, in Hexyla, dimethylamine, Diethylamine, diisopropylamine, di-n-butylamine, trimethylamine, triethylamine, tripropylamine, N, N-dimethylethylamine, N, N-dimethylisopropylamine and N, N-dimethylbutylamine, di-, tri-, tetra- and poly - Amines, such as ethylenediamine, diethylenetriamine and triethylenetetraamine, non-aromatic and aromatic cyclic amines, such as pyrolidine, piperidine, morpholine, piperazine, pyrrole and their n-Ci-Cg-alkyl derivatives, pyridine and phenanthroline, phosphines, such as phosphine, trimer , secondary and tertiary C ₆ alkyl and ^c ₆ ^-C i ₂ ^ar ylphosphines, in particular triphenylphosphine, and sulfides, such as C ₆ -C ₆ mono- and dialkyl sulfides, C ₆ -C ₂ mono- and di-aryl sulfides and oxygen compounds, di-C ₆ alkanols and phenols and their ethers.

The halides chloride and bromide are particularly preferred as complex ligands; Amines, especially ammonia and triethylamine, cyanide and ethylenediaminetetraacetic acid, and the di-, tri- or tetra-alkali metal (such as sodium) or ammonium salts thereof. 8th

Alkali metals such as lithium, sodium and potassium, alkaline earth metals such as magnesium and calcium, nitrite, nitrate and ammonium are preferably suitable as counterions.

5 Suitable platinum metal complexes are preferably at least 0.01% by weight soluble in water at room temperature (25 ° C.). According to the invention, the platinum metal complex (s) are used in an aqueous medium, in such a concentration that the platinum metal content of the solution is in the range from 0.01 to 0-20.0 g / l, preferably in the range from 0. 1 to 2.0 g / 1 and especially

■ preferably in the range of 0.15 to 1.0 g / 1, such as. B. in the range of 0.15 to 0.25 g / 1, 0.2 to 0.5 g / 1 or 0.35 to 0.8 g / 1.

5 Preferred palladium complexes are H ₂ PdHal ₄ , M ₂ PdHal, M ₂ Pd (CN),

(NH ₄ ) ₂ PdHal ₄ , Pd (NH ₃ ) ₄ Hal ₂ , Pd (NH ₃ ) ₄ (N0 ₃ ) ₂ and Pd (NH ₃ ) ₄ (CN) ₂ , where M is alkali metals, especially sodium and potassium, and shark represents halogen atoms, in particular chlorine, bromine or iodine.

Preferred preferred platinum metal complexes are (NH) IrCl ₆ , HPtCl, (NH ₄ ) ₂ PtCl ₄ , Na ₂ PtCl ₄ and K ₂ PtCl ₄ .

In addition, the aqueous medium contains at least one reducing agent in completely or partially dissolved form. Suitable as 5 reducing agents are all substances or mixtures of substances whose redox potential lies below the redox potential of the platinum metal complex used, i.e. that is, substances with a standard potential in aqueous medium of less than +0.5 volts, but preferably with a standard potential less than 0 volts. The reducing agent or reducing agent mixture is at least at least

1 wt .-%, preferably at least 10 wt .-%, soluble at room temperature (25 ° C) in the aqueous medium. In preferred embodiments of the present invention, the reducing agent or the reducing agent mixture is practically completely soluble in the aqueous medium.

Examples of suitable reducing agents are carboxylic acids, such as formic acid, citric acid, lactic acid, tartaric acid and in particular the salts of the carboxylic acids, preferably the alkali metal, alkaline earth metal, potassium, ammonium and C 1 -C 20 -alkylammonium salts, phosphorous or hypophosphorous acid, the salts of phosphorous or hypophosphorous acid, in particular the alkali metal or alkaline earth metal salts, Cι-Cιo-alkanols, such as methanol, ethanol and isopropanol, sugar, such as aldoses and ketoses in the form of mono-, di- and oligosaccharides, especially glucose, Fructose and lactose, aldehydes such as formaldehyde, hydrogen boron compounds or borohydrides such as boranes, metal boranates and borane complexes, e.g. B. Dibo- 9 ran, sodium borohydride and aminoboranes, in particular trimethylamine borane, hydrazine and alkylhydrazines, such as methylhydrazine, hydrogendithionites and dithionites, in particular sodium and potassium hydrogen dithionite, sodium, potassium and zinc dithionite, hydrogen sulfites and sulfites, in particular sodium - and potassium hydrogen sulfite, sodium, potassium and calcium sulfite, hydroxylamine and urea, and mixtures thereof.

Preferred reducing agents are sodium and potassium hypophosphite, 10 ammonium formate, trimethylaminoborane, sodium borohydride, sodium dithionite and sodium hydrogen dithionite, and mixtures of ammonium formate and sodium hypophosphite.

As a rule, at least one redox equivalent, based on the sum of the platinum metals and additional components (e.g. promoters / doping components), of reducing agent is used. The reducing agent is preferably used in excess. A molar ratio of reducing agent to platinum metal of 10: 1 to 100: 1 and particularly preferably 20: 1 to 20 60: 1, for example about 30: 1, about 40: 1 or about 50: 1, is particularly suitable.

The electroless deposition of the platinum metal is advantageously carried out at a pH of the aqueous medium of greater than 4, preferably greater than 6, such as. B. 7 to 14, especially 8 to

25 12. To do this, it is generally necessary to add at least one base to the aqueous medium containing the platinum metal complex and the reducing agent in order to achieve the desired pH. For the purposes of the present invention, bases are all substances or compounds which are suitable for the pH of the

30 aqueous medium to the desired value. In particular, those bases are used which have complex stabilizing properties, i. H. at least partially have a Lewis base character. The base is preferably selected from metal oxides, metal hydroxides, in particular alkali metal hy-

35 hydroxides, such as sodium hydroxide and potassium hydroxide, metal carbonates, in particular alkali metal and alkaline earth metal carbonates, such as lithium carbonate, sodium carbonate, potassium carbonate, magnesium carbonate and calcium carbonate, nitrogen bases, in particular ammonia, primary, secondary and tertiary amines, such as those previously mentioned

40 described the nitrogen-containing complex ligand. Buffer systems are also suitable, in particular those from the aforementioned bases, the salts of the aforementioned bases and / or suitable acids. Particularly preferred bases are ammonia and sodium hydroxide solution.

45 For the purposes of the invention, aqueous media are substances or mixtures of substances which are liquid under the process conditions and contain at least 10% by weight, preferably at least 30% by weight and in particular at least 50% by weight of water. The part other than water is preferably selected from inorganic or organic substances which are at least partially soluble in water or at least partially miscible with water. For example, the substances other than water are selected from organic solvents, -C ₂₂ alkanols, in particular methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, pentanols and hexanols, C -Cs- Cycloalkyl ethers, such as tetrahydrofuran, pyrans, dioxanes and trioxanes, C 1 -C ₂ -dialkyl ethers, such as dimethyl ether, dibutyl ether and methyl butyl ether, and customary auxiliaries, such as are used in processes for electroless deposition.

The aqueous medium preferably contains less than 40%, in particular less than 30% and particularly preferably less than 20% organic solvent.

In preferred embodiments of the method according to the invention, the aqueous medium is essentially free of organic solvents.

In addition to the platinum metal complex, the reducing agent and the base, the aqueous solution additionally contains at least one complexing agent, preferably with at least one halogen, nitrogen, oxygen and / or phosphorus atom. Complexing agents in the sense of the present invention are ions or compounds which are able to stabilize metal ions in aqueous media. As a rule, such complexing agents are donors or salts of donors. Suitable donors usually have a lone pair of electrons that can interact with the metal ions. Complexing agents which have the above-mentioned hetero atoms as donors are particularly suitable for the process according to the invention. Examples of suitable complexing agents are the metal salts, in particular the alkali metal and alkaline earth metal salts, of the compounds previously mentioned as complex ligands of the platinum metals.

Particularly suitable as complexing agents are hydrogen halide acids, such as hydrogen bromide, hydrogen chloride and hydrogen iodide, the metal salts of the hydrogen halide acids mentioned, in particular the alkali metal and alkaline earth metal salts, and also tin dihalides, zinc dihalides, ammonium salts, such as ammonium chloride, ammonium bromide, ammonium nitride, ammonium ammonium ammonium, , Alkaline earth metal and ammonium salts of carbon acids and hydroxycarboxylic acids, e.g. B. sodium and / or potassium tartrate.

As a rule, platinum metal complex, reducing agent, base and complexing agent can be added to the aqueous medium in any order. Preferably, at least a portion of the base is added to the aqueous medium before the reducing agent is added.

In one embodiment of the process according to the invention, the platinum metal complex, if appropriate the complexing agent and / or the base, is initially introduced into the aqueous medium and then the reducing agent is added.

As a rule, the process according to the invention is carried out at temperatures in the range between 0 and 100 ° C., preferably in the range from 30 to 100 ° C. and in particular in the range from 40 to 85 ° C.

Metallic supports in the sense of the invention are preferably structures which, at least on the (outer) surface which can be reached by the reaction medium, have essentially solid metal (ie in oxidation state 0). Suitable metals for the metallic supports are all metals and alloys which have sufficient stability under the production conditions of the platinum metal catalysts according to the invention and / or under the conditions of use of the catalysts produced therewith. Suitable metals are e.g. As magnesium, aluminum, titanium, vanadium, chromium, molybdenum, tungsten, iron, cobalt, nickel, copper, silver and zinc as well as mixtures and alloys thereof. As further alloy or minor components, elements such as B. the non-metals boron, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur and other transition metals may be included. As a rule, the further alloy or secondary constituents, with the exception of carbon in a proportion of less than 20% by weight, preferably less than 15% by weight and in particular less than 5% by weight per element, based on the total weight of the metallic support. Carbon can be contained in the metallic carriers in amounts of up to 25% by weight. If the metallic supports have further alloy or secondary constituents, these are generally present in amounts of at least 0.01% by weight and preferably about 0.1% by weight, based on the total weight of the metallic support. The metallic carrier preferably consists essentially of steel or iron, copper, aluminum, silver, nickel, chromium, tungsten, titanium and mixtures and / or alloys thereof. Preference is given to high-alloy stainless steel or metals that are formed by Protect fertilizers from a passivation layer against further corrosion, e.g. chrome steels, chrome nickel steels, chrome nickel titanium steels and chrome nickel molybdenum steels, V4A steels and heat-resistant steels with the material numbers 1.4539, 1.4571, 1.4016, 1.4767, 1.4401, 2.4610, 1.4765, 1.4847, 1.4301 and 1.4742, as well as alloys like 1.4742 and Hastelloy.

The metallic supports can be used directly as sheets, perforated sheets, grids, wires or preferably as wire nets, woven or knitted fabrics and in particular in the form of shaped bodies. Shaped bodies in the sense of the invention are preferably spatial structures made of the carriers described above, which, for. B. can be formed by rolling, bending, pressing and the like, for. B. fillers such as Raschig rings, saddle bodies, Pall® rings, wire spirals, wire mesh rings, with or without a web, and monoliths. Monoliths are to be understood here as shaped bodies in the form of ordered packings which are installed in the reactor and which, owing to a large number of flow channels, have a large surface area, based on their volume. Preferred shaped bodies have channels with hydraulic radii (for definition see VDI Heat Atlas, section LE 1) in the range from 0.1 to 10 mm. Fabrics of various types of weave can be produced from wires and fibers of the metals and materials mentioned, such as smooth fabrics, body fabrics, braid fabrics, five-shaft atlas fabrics and other special weaves. These fabrics are preferably combined to form multi-layer fabric associations. Suitable fabric-shaped monolithic catalyst supports are described in EP-A-198 435. Metal foams and metal sponges are also suitable, in particular open-cell or open-pore metal foams or sponges.

Particularly suitable monoliths are made up of several layers of corrugated, kinked and / or smooth fabric, which are arranged in such a way that adjacent layers form more or less closed channels. The hydraulic diameter of the channels is preferably in the range from 1 to 10 mm, in particular from 1.5 to 3 mm (as defined in VDI Heat Atlas, section LE 1). The channels can be straight or curved. Multi-layer fabrics are preferably used, in which smooth and corrugated or kinked fabrics alternate. Monoliths in which the fabrics are partially or completely replaced by sheets, knitted fabrics or expanded metals can also be used. While packings are generally added to the reactor as loose bed, monoliths are preferably installed in the reactor, in particular in such a way that the channels are inclined against the direction of flow of the reaction medium. The fabric layers themselves are preferably installed parallel to the flow direction in the reactor. If several of these units are connected in series, the installation is preferably carried out in such a way that the throughflow channels are inclined alternately in opposite directions to the direction of flow. The structural units are preferably installed in such a way that the fabric layers of two successive structural units form an angle of preferably approximately 90 ° to one another. How winding modules made of corrugated or kinked and possibly also of flat fabric layers are also suitable.

The deposition of the catalytically active component, i.e. H. the platinum metals and optionally promoters and / or the doping components can be carried out before or after the shaping of the metallic supports into shaped bodies. The deposition of the catalytically active components is preferably carried out after the molding into shaped bodies. When shaping the shaped bodies and / or attaching them in the reactor, care must be taken to ensure that the largest possible proportion of the support surface of the catalyst can be achieved in terms of flow in a favorable manner in terms of flow technology, and that the pressure drop in the reactor is as small as possible. This is particularly important for processes with liquid reaction media.

With sufficient mechanical strength, suitable metallic supports preferably have a geometric surface area of greater than 0.5 m / l, in particular greater than 1.5 m ² / l and preferably greater than 2.5 m ² / l.

Before the platinum metal is applied to the metallic support, it is preferably cleaned thoroughly, e.g. B. by treatment with aqueous surfactant solutions and / or galvanosalt solutions, as are common in electroplating, and / or by treatment with organic solvents, such as ethyl acetate, acetone and water, optionally with the aid of ultrasound. Preferably, the metallic carrier can be subjected to a surface treatment prior to the deposition of the platinum metal, which leads to an enlargement of the surface of the metallic carrier and / or to an improvement in the adhesion of the platinum metal to the carrier, such as e.g. B. Etching the surface of the support with acids such as hydrohalic acids, especially hydrochloric acid and hydrobromic acid, nitric acid, sulfuric acid, phosphoric acid and mixtures thereof. The etching of the surface is preferably carried out in such a way that 0.1 to 15% by weight, preferably 1 to 5% by weight, of the carrier is dissolved.

The active component, ie the platinum metal or the platinum metals and the additional components which may be present, as a rule make up 5 × 10 ^-4 to 5% by weight, in particular 10 ^-3 to 1% by weight, particularly preferably 0.1 to 1.0 wt .-%, based on the entire catalyst mass (carrier + catalytically active coating).

If an additional complexing agent is added to the solution, 0.1 to 10,000 equivalents, preferably 1 to 1000 equivalents, particularly preferably 10 to 600 equivalents of the complexing agent, based on the platinum metal component, are generally used.

For example, the metallic carrier is only brought into contact with the aqueous medium when the aqueous medium contains at least the platinum metal complex, the reducing agent, at least part of the base and optionally the additional complexing agent. The aqueous medium preferably already contains all of the components used in the electroless deposition before the aqueous medium is brought into contact with the metallic support. Likewise, the carrier can first be brought into contact with all of the components mentioned above, except for the platinum metal. The platinum metal is then at reaction temperature or a z. B. added up to 30 ° C lower temperature. “Reaction temperature” in the sense of the present invention means the temperature at which the platinum metal particles are deposited on the carrier.

The deposition of precious metals on metallic supports is known from the prior art as electroplating or electroless plating. In these methods, the formation of a thin, non-porous, smooth layer, i. H. a non-porous smooth film, aimed at the carrier. Such smooth layers are used for example for electrical contacts for. B. printed circuits and membranes required. A supported palladium catalyst produced according to a typical plating specification shows only a low activity of hydrogen peroxide formation from hydrogen and oxygen.

The catalysts of the invention differ fundamentally from coated metallic supports produced according to the plating regulations, although the chemicals used for the electroless deposition of the noble metal according to the invention can be completely or partially the same.

Active or selective catalysts for the hydrogenation of organic compounds and for the hydrogen peroxide synthesis are formed by one or more of the following measures according to the invention for the reaction of the components described above with one another: (1) The platinum metal complex, the complexing agent and the reducing agent are brought to the reaction temperature in an essentially aqueous medium in the presence of the cleaned and preferably etched support, ie in less than 120 minutes, preferably in less than 30 minutes. Depending on the reaction mixture and carrier, the reaction temperature can be between room temperature and the boiling point of the aqueous solution. If the mixture at comparatively low temperatures of e.g. B. 40 to 60 ° C tends to form smooth, catalytically inactive coatings of palladium on the support, then a temperature which is at least 10 ° C higher, but preferably a temperature which is at least 20 ° C higher.

(2) Another measure according to the invention is that the substantially aqueous mixture of all components in the absence of the carrier z. B. is tempered for about 5 to 600 minutes before the carrier is brought into contact with the substantially aqueous solution of the reactants. This tempering takes less time, the higher the temperature is selected and the stronger the reducing agent. In the system palladium chloride, ammonium chloride, ammonia and hypophosphite, for example 30 to 90 minutes at 60 to 90 ° C. are advantageous. If the temperature is first tempered in the absence of the carrier, a temperature can also be selected at which a smooth coating would form when the carrier was installed immediately after all the reactants had been mixed.

(3) Another measure which leads to the formation of a supported catalyst according to the invention instead of a support with a smooth coating consists in the fact that the platinum metal salt concentration or platinum metal complex concentration is compared to the conditions under which a smooth layer is formed by the factor increase at least 2, d. H. for example about 0.5 g / 1 palladium in the case of one

Solution, consisting of palladium chloride, ammonia, ammonium chloride and a reducing agent, to about 1 to 20 g / 1.

(4) Another measure which leads to the formation of a catalyst according to the invention consists in the mixture of all

Components shortly before, simultaneously with or shortly after immersing the carrier in the saline solution, are mixed with a platinum metal sol (vaccine sol) produced in another way. Such a sol can be produced using known techniques, in the simplest case by mixing a platinum metal salt solution or platinum metal complex solution with a solution of sodium hypophosphite or another reducing agent in Water. The amount of platinum metal used as the inoculation sol can be 1 to 20% of the total, preferably 5 to 15%.

(5) A further measure according to the invention can consist in destabilizing the mixture of all dissolved components by using one or more of the complexing agents in a low concentration than is necessary for a platinum metal complex solution which is stable at the chosen reaction temperature with respect to the reducing agent is. In the case of the solution of palladium chloride, ammonium chloride, ammonia and hypophosphite, for example, the amount of concentrated ammonia for a solution stable at 65 ° C. can be reduced from 100 to 160 ml / 1 to 75 ml / 1 or less to make the solution for the invention Adjust the assignment of the metallic carrier, d. H. to destabilize.

(6) A further measure can consist in destabilizing the essentially aqueous solution of the platinum metal compound at a given temperature by increasing the concentration of the reducing agent and thus preparing it for the use according to the invention. When using palladium and a temperature of 60 ° C, the desired effect is achieved, for example, with ammonium chloride and ammonia as a complexing agent with> 15 g / 1 sodium hypophosphite as a reducing agent.

According to a preferred embodiment of the invention, catalytically active coatings are obtained by heating the reaction solution or the reaction mixture before the deposition. Temperature control is preferably carried out for 5 to 600 minutes, in particular 10 to 300 minutes and particularly preferably 15 to 180 minutes. The reaction solution or the reaction mixture, i. H. the aqueous medium, which contains platinum metal complex, reducing agent, base and complexing agent, preferably in less than

120 minutes, in particular in less than 60 minutes and preferably in less than 30 minutes, brought to a temperature of 30 ° C., preferably 20 ° C. and preferably 15 ° C. above or below the desired reaction temperature for the deposition and then kept at this temperature. The reaction solution or the reaction mixture is particularly preferably temperature-controlled at a temperature which is up to 30 ° C., preferably up to 20 ° C., lower than the deposition temperature, or approximately at the desired separation temperature. After tempering, the metallic support is brought into contact with the reaction solution or the reaction mixture and the temperature of the reaction mixture is given. if brought to a different temperature suitable for the deposition.

In a further embodiment of the method, the reaction solution or the reaction mixture is mixed with a separately prepared seed sol, if necessary in addition to tempering. Such a vaccine sol can be prepared in a variety of ways (for example according to Kosak, op. Cit. 0.), most simply by mixing a platinum metal salt solution or platinum metal complex solution with a reducing agent in an aqueous medium. The vaccine sol should then be added before, during or after the metallic carrier has been brought into contact with the reaction solution or the reaction mixture. The amount of platinum metal used to produce a vaccine sol preferably makes up 1 to 20% by weight, preferably 5 to 15% by weight, of the total amount of platinum metal used in the invention.

In the process according to the invention, it has proven to be advantageous to ensure sufficient circulation of the reaction solution or the reaction mixture during the deposition of the platinum metal on the support, eg. B. by pumping or stirring.

The reaction time required for the deposition of the platinum metal on the metallic supports is generally between 5 and 500 minutes, preferably 10 and 300 minutes and particularly preferably between 15 and 120 minutes.

In the process according to the invention, preferably more than 70% by weight, preferably more than 80% by weight and particularly preferably more than 90% by weight of the platinum metals used are deposited on the metallic support. As a rule, the platinum metal is bonded so tightly to the metallic support that it is not significantly replaced by contact with liquids and gases when used in catalytic reactions. There is no clouding due to the precipitation of the platinum metal.

Additional components, in particular the elements suitable as promoters or doping components, can optionally be added together with the platinum metal into the aqueous medium, so that the deposition of the platinum metal and the installation of the additional components take place essentially simultaneously. The additional components can also be added to the reaction solution towards the end or after the platinum metal deposition has ended, as a result of which the additional components are preferably installed on the surface of the active component. The additional components can also in a separate second step on the inventive Catalysts are applied, e.g. B. by vapor deposition, preferably as described in EP-A-0 198 435, or by electroless or non-currentless deposition from aqueous and non-aqueous media. The application of additional components to the catalysts according to the invention in a separate second step is particularly advantageous if you want to apply them specifically to the surface of the active component. Furthermore, other deposition conditions that deviate from the conditions according to the invention can be selected for the second step. 0

The catalysts obtained according to the invention can then be formed at temperatures from 0 to 500 ° C., preferably 10 to 350 ° C., and pressures between normal pressure and 200 bar gauge pressure. The formation can, for example, in the presence of water 5 and / or hydrogen, preferably hydrogen, at 10 to 200 ° C, preferably 30 to 150 ° C, and normal pressure or 1 to 150 bar, preferably 10 to 100 bar and particularly preferably 30 to 70 bar. Typically, formation lasts 0.1 to 10 hours, preferably 1 to 5 hours. In a preferred embodiment of the process according to the invention, the catalysts are formed in the presence of the aqueous reaction medium which is described below for the synthesis of water peroxide according to the invention.

In a preferred embodiment of the process according to the invention, the catalysts according to the invention are prepared by at least 0.1 to 30 g / 1, preferably 0.15 to 3 g / 1 and particularly preferably 0.15 to 0.5 g / 1 at least one platinum metal complex, optionally 0.01 to 5 g / 1,

30 preferably 0.01 to 0.5 g / 1 and preferably 0.01 to 0.05 g / 1, at least one further element compound and, based on the platinum metal, at least 20, preferably 50 and particularly preferably at least 100 equivalents of a complexing agent and at least 10 to 100, preferably 20 to 80 and particularly preferred

35 dissolves 40 to 60 equivalents of a reducing agent in an aqueous medium.

Another object of the present invention is a catalyst obtainable by one of the methods described above.

40

The invention also relates to platinum metal catalysts with a metallic support and a catalytically active coating applied thereon, which are characterized in that the catalytically active coating on the support surface

45 immobilized, discrete platinum metal particles with an average particle diameter of less than about 1 μm, preferably less than about 100 nm. The platinum metals preferably have tallparticles have an average diameter of more than about 1 nm and can have, for example, diameters in the range from about 20 to 100 nm. The discrete particles have an approximately spherical shape. In addition, the platinum metal particles essentially form a coating formed as a monolayer on the carrier, while according to the prior art an uneven deposition, often in the form of mechanically unstable agglomerates, is obtained.

In particular, the subject matter is a catalyst in which the metallic support consists essentially of steel, iron, copper, aluminum, silver, nickel, chromium, tungsten, titanium and mixtures and / or alloys thereof. Such catalysts preferably have a platinum metal content in the range from 0.01 to 50 g / kg of support. In the direct synthesis of hydrogen peroxide from the elements, the catalysts obtainable by this process preferably have a selectivity of greater than 70%, in particular greater than 80% and particularly preferably greater than 85%.

The catalysts of the invention are preferably suitable for the hydrogenation of organic and inorganic compounds and in particular for organic compounds such as olefins, for. B. ethylene, propylene, acetylene and butadiene, carbonyl compounds, for. B. aldehydes, ketones, aromatics, such as. As benzene, and particularly preferred for the hydrogenation of oxygen.

The present invention further provides a process for the production of hydrogen peroxide, in which a catalyst, as described above, is brought into contact in an essentially aqueous solution with an oxygen / hydrogen mixture with a mixing ratio in the range from 4: 1 to 30: 1.

The present invention also relates to the use of the catalysts according to the invention for the synthesis of hydrogen peroxide from the elements, both according to the anthraquinone process or an analogous process, and by direct synthesis, ie by direct reaction of oxygen and hydrogen on a platinum metal catalyst in one liquid or gaseous medium, preferably by a method as described above. Suitable methods are e.g. B. described in WO 98/16463. The use of the catalysts according to the invention for the direct synthesis of H ₂ 0 ₂ is particularly preferred.

Suitable reactors for the synthesis of H0 ₂ are described for example in EP-A-068 862, EP-A-201 614 and EP-A-448 884. Tubular reactors in which cylindrically constructed catalyst units are fitted are particularly preferred, since here can form a uniform flow, which allows particularly good reaction management. Also tubular reactors with beds of shaped catalyst parts, such as. B. net rings are suitable.

The reaction is usually carried out with the reactor flooded. Water and / or -CC ₃ alkanols, in particular water and / or methanol, are preferably used as the reaction medium. If water is used as the reaction medium, up to 20% by weight of the alcohol, preferably methanol, can be added to it. If an alcoholic reaction medium is used, it can contain 40% by weight, preferably up to 20% by weight and particularly preferably up to 5% by weight of water. Water is very particularly preferably used as the sole reaction medium. To stabilize the hydrogen peroxide against decomposition, acids, the pKa value of which is preferably lower than that of acetic acid, in particular mineral acids, such as sulfuric acid, phosphoric acid or hydrochloric acid, are added to the reaction medium. The acid concentration is usually at least 10 ^-4 mol / liter, preferably 10 ^-3 to 10 ^-1 mol / liter. Furthermore, traces of bromide or chloride are generally added in concentrations of 1 to 1000 ppm, preferably 5 to 700 ppm and particularly preferably 50 to 600 ppm. But other stabilizers, such as. B. formaldehyde can be used.

The reaction gas, which in addition to hydrogen and oxygen can also contain inert gases such as nitrogen or noble gases, generally has 0 ₂ : H ₂ ratios in the range from 2: 1 to 1000: 1. Molar ratios in the range from 5: 1 to 100: 1, in particular 4: 1 to 60: 1 and particularly preferably in the range from 20: 1 to 50: 1 are preferred. The oxygen used in the reaction gas can also be added to the reaction gas in the form of air.

In a preferred embodiment, the reaction gas is circulated. In this case, the molar ratio in the fresh gas mixture is close to the stoichiometry, preferably in the range from 1.5: 1 to 0.5: 1. The molar ratio 0 ₂ : H ₂ in the cycle gas should be in the range from 5: 1 to 1000: 1, preferably in the range from 20: 1 to 100: 1. The reaction can be carried out at normal pressure and also under excess pressure up to 200 bar. The pressure is preferably 10 to 300 bar, in particular 10 to 80 bar. The reaction temperature can be in the range from 0 to 60 ° C, preferably in the range from 5 to 60 ° C and in particular from 15 to 45 ° C. The partial pressures of the reaction gases in the reaction gas mixture in the reactor and in the cycle gas are preferably selected such that the hydrogen concentration is below the lower explosion limit under reaction conditions located.

Reaction gas and reaction medium can be carried out in cocurrent or in countercurrent to one another, preferably in cocurrent, the liquid phase forming the continuous phase and the reaction gas the discontinuous phase. In the preferred vertical reactor design (upright reactor), the reaction gas and reaction medium are preferably passed through the reactor in cocurrent from bottom to top. Here, hydrogen can be fed to the reactor via one or more intermediate feeds downstream of the oxygen or air feed point. The empty pipe speed of the reaction gas and reaction medium is in the range from 50 to 1000 m / h, preferably in the range from 150 to 300 m / h.

The process described enables hydrogen peroxide solutions with hydrogen contents above 2% by weight, preferably in the range from 3 to 25% by weight, to be prepared. The concentration can be preselected in the desired manner by adjusting the material flows. The selectivity of the hydrogen peroxide formation is z. B. above 65%, preferably 70%. Long-term studies have shown that even after more than 40 days of operation, there is no or only a slight decrease in the catalyst activity and selectivity.

The present invention is illustrated by the following examples without restricting it.

Examples

I. Manufacture of catalyst supports

I.a Manufacture of stainless steel mesh monoliths

A corrugated and a smooth mesh made of stainless steel (material no. 1.4539, mesh size 200 μm, wire diameter 140 μm) were placed on top of one another and thus rolled into a cylindrical monolith with a height of 5 cm and a diameter of 5 cm, that an axial cavity with a diameter of 16 mm was created in the center. The ends of the nets were fixed by welding spots. The mesh level spacing of the smooth mesh was at least 1 mm. The monolith obtained was successively treated in an ultrasonic bath with ethyl acetate, acetone and distilled water and then dried.

Ib pretreatment of the monolith The monolith prepared and purified as described under Ia was treated at 60 ° C. for 180 minutes with concentrated hydrochloric acid (37%) and then rinsed several times with distilled water.

I.c Manufacture of wire mesh rings from stainless steel mesh

Two mesh strips measuring 17 x 3 mm made of stainless steel (material no. 1.4539, mesh size 150 μm, wire diameter 100 μm) were placed on top of each other and formed into wire mesh rings with a diameter of 3 mm and a height of 3 mm. 45 g (approximately 77 ml) of the wire mesh rings were successively treated with ethyl acetate, acetone and distilled water in the ultrasonic bath and then dried.

I.d Pretreatment of the wire mesh rings

The wire mesh rings prepared as described under I.c were at 60 ° C for 60 minutes with concentrated hydrochloric acid

(37%) treated and then rinsed several times with distilled water.

The results of the application test of the catalysts VKl, VK2 and EK1 to EK12 in the direct synthesis of hydrogen peroxide are summarized in Table 1.

II. Production of the non-inventive catalysts VKl and VK2

VKl:

1,500 ml of fully demineralized water were heated to 70 ° C. in a 2,000 ml beaker and an aqueous solution of Na ₂ PdCl (40 g; 1% strength solution, based on the palladium content) was added. Then a monolith prepared as described under Ib was suspended in the beaker. 20 g of a 0.83% strength aqueous solution of NaH ₂ P0 _{2 were then} added with stirring and the reaction mixture was heated to 80 to 88 ° C. The reaction mixture became cloudy and turned dark. The reaction mixture was stirred at this temperature for a further 90 minutes. During this time the solution faded and became clear again. The monolith, which had turned dark gray, was then separated from the reaction mixture and rinsed off with water, a fine black solid becoming detached. In order to determine the amount of palladium metal deposited, the washing liquid was combined with the solid and the reaction mixture, the soluble components are dissolved in aqua regia and then the palladium content of the resulting solution is determined. A deposition of 44% of the palladium offered on the monolith could be calculated from this data.

VK2:

A monolith prepared according to Ib is suspended in a beaker in such a way that it was completely covered by the reaction solution subsequently added. A solution of 9.6 g of NaH ₂ PO _{2 was then added} . 1 H ₂ 0, 21.6 g ammonium chloride and 134 ml (25%) ammonia solution with stirring in 180 ml deionized water and 19.06 g of an aqueous solution of Na ₂ PdCl ₄ (1%, based on the palladium content) , the reaction solution was gradually warmed to 65 ° C. in the course of 2 hours and stirred at this temperature for 60 minutes. The monolith was then removed from the reaction solution and rinsed off with water. No solid was detached from the support. The reaction solution was then combined with the washing solution and the palladium content of the combined solutions was determined. A deposition of> 99% of the palladium offered on the monolith could be calculated from this data.

III. Production of the catalysts EK1 to EK12 according to the invention

EKl:

A solution of 9.6 g was placed in a 1000 ml beaker

NaH ₂ P0 ₂ . 1 H ₂ 0, 21.6 g ammonium chloride and 134 ml aqueous ammonia solution (25%) in 660 ml deionized water. 24.12 g of aqueous Na ₂ PdCl ₄ solution (1%, based on the palladium content) were then added, the solution was heated to 75 ° C. and stirred at this temperature for 60 minutes. The solution remained colorless and clear all the time. A monolith prepared according to Ib was then suspended in the reaction solution and the reaction mixture was stirred at 75 ° C. for a further 65 minutes. The reaction solution remained colorless and clear all the time. The monolith was then removed and rinsed with water. No solid was detached from the support. The reaction solution and washing solution were then combined and the palladium content of the solution was determined. A deposition of 99% of the palladium offered on the monolith could be calculated from this data. EK2:

A solution of 9.6 g NaH ₂ P0 ₂ .1 H ₂ 0, 21.6 g ammonium chloride and 134 ml aqueous ammonia solution (25%) in 560 ml deionized water was prepared in a 1000 ml beaker. Then 41.4 mg of lead (II) nitrate, dissolved in 100 ml of water, and 24.12 g of an aqueous Na ₂ PdCl solution (1%, based on the palladium content) were added, the mixture was heated to 65 ° C and stirred for a further 60 minutes at this temperature. A monolith prepared according to Ib was then suspended in the solution, the reaction mixture was heated to 75 ° C. and then stirred at this temperature for a further 210 minutes. The reaction solution remained colorless and clear all the time. The monolith was then removed from the reaction mixture and rinsed with water. No precipitate was detached from the carrier. The reaction solution and washing solution were then combined and the palladium content of the combined solution was determined. A deposition of more than 99% of the palladium offered on the monolith could be calculated from this data.

EK3:

A solution of 9.6 g of NaH ₂ PO ₂ was placed in a 1,000 ml beaker. 1 H ₂ 0, 21.6 g ammonium chloride and 134 ml aqueous ammonia solution (25%) in 560 ml deionized water. 55 mg (NH) ₂ IrCl ₆ , dissolved in 100 ml of water, and 24.12 g of an aqueous Na ₂ PdCl solution (1%, based on the palladium content) were then added, the mixture was heated to 65 ° C and stirred at this temperature for a further 60 minutes. A monolith prepared according to Ib was then suspended in the solution, the reaction mixture was heated to 75 ° C. and stirred at this temperature for a further 170 minutes. The reaction solution remained colorless and clear all the time. The monolith was then removed from the reaction mixture and rinsed with water. No solid was detached from the support. The reaction solution and washing solution were then combined and the palladium content of the combined solution was determined. A deposition of 96% of the palladium offered on the monolith could be calculated from this data.

EK4:

A solution of 9.6 g of NaH ₂ PO ₂ was placed in a 1,000 ml beaker. 1 H ₂ 0, 10 g of ethylenediaminetetraacetic acid tetrasodium salt and 134 ml of aqueous ammonia solution (25%) in 650 ml of completely deionized water. 24.12 g of an aqueous Na ₂ PdCl ₄ solution (1%, based on the palladium content) were then added, the reaction mixture was heated. tion solution to 75 ° C and stirred for a further 60 minutes at this temperature. A monolith prepared according to Ib was then suspended in the reaction solution and the reaction mixture was stirred at 75 ° C. for a further 40 minutes. The aqueous solution remained colorless and clear all the time. The monolith was then removed from the reaction mixture and rinsed with water. No solid was detached from the support. The reaction solution and washing solution were then combined and the palladium content of the combined solution was determined. A separation of

Calculate 96.4% of the palladium offered on the monolith.

EK5: A solution of 9.6 g of NaH ₂ P0 ₂ was placed in a 1,000 ml round-bottomed flask with a Teflon blade stirrer. 1 H ₂ 0, 21.6 g ammonium chloride and 134 ml aqueous ammonia solution (25%) in 560 ml deionized water. 24.12 g of an aqueous NaPdCl solution (1%, based on the palladium content) were then added with stirring, and the reaction solution was heated up

75 ° C and stirred for a further 60 minutes at this temperature. Then 45 g of the wire mesh rings produced according to I.d were added to the reaction solution and the reaction mixture was stirred at 75 ° C. for a further 180 minutes. The reaction solution remained colorless and clear all the time. Then the wire mesh rings were separated from the reaction solution and rinsed with water. No solid was detached from the rings. The reaction solution and washing solution were then combined and the palladium content of the combined solution was determined. A deposition of more than 99.5% of the palladium offered on the rings could be calculated from this data.

EK6: In a 2000 ml round bottom flask with a Teflon blade stirrer, a solution of 19.2 g of NaH ₂ P0 ₂ was. 1 H ₂ 0, 43.2 g of ammonium chloride and 268 ml of aqueous ammonia solution (25%) in 1 320 ml of completely deionized water. 48.24 g of an aqueous Na ₂ PdCl ₄ solution (1%, based on the palladium content) were added with stirring, the reaction solution was heated to 75 ° C. and the mixture was stirred for a further 40 minutes at this temperature. 45 g of the wire mesh rings produced according to Id were then added to the reaction solution and the mixture was stirred for a further 180 minutes at a temperature in the range from 75 ° C. to 80 ° C. The reaction solution remained colorless and clear all the time. Then the wire mesh rings were separated from the reaction solution and rinsed with water. There was no solid from the rings replaced. The reaction solution and washing solution were then combined and the palladium content of the combined solution was determined. A deposition of more than 99% of the palladium offered on the rings could be calculated from this data.

EK7:

In a 1000 ml round bottom flask with a Teflon blade 1. a solution of 8.64 g of NaH ₂ P0 ₂ was. 1 H ₂ 0, 19.4 g of ammonium chloride, 121 ml of aqueous ammonia solution (25%) and 19.5 g of an aqueous NaPdCl ₄ solution (1%, based on the palladium content) in 594 ml of deionized water. A second solution of 1.224 g NaH ₂ P0 ₂ . 1 H ₂ 0, dissolved in 80 ml of water, and 4.62 g of an aqueous Na ₂ PdCl solution (1%, based on the palladium content). The 2nd solution was allowed to stand until it began to turn brown and was added dropwise to the 1st solution within 5 minutes of the start of browning. The reaction mixture obtained was then heated to 40 to 45 ° C. with stirring. After 20 minutes at this temperature, 45 g of the wire mesh rings produced according to Id were added to the reaction solution, which had become colorless and clear again, and the reaction mixture was heated to 75 ° C. within 30 minutes. The wire mesh rings were then separated from the reaction solution and rinsed with water. A small amount of black precipitate remained in the reaction solution. The reaction solution was then combined with the solid and the washing solution, the solid was brought into solution with aqua regia and the palladium content of the resulting solution was determined. A deposition of more than 89% of the palladium offered on the carriers could be calculated from this data.

EK8:

In a 1000 ml round bottom flask with a Teflon blade stirrer, a solution of 9.6 g of NaH ₂ P0 ₂ was. 1 H ₂ 0, 21.6 g of ammonium chloride and 32.4 ml of aqueous ammonia solution (25%) in 160 ml of completely deionized water. 24.12 g of an aqueous Na ₂ PdCl solution and 0.24 g of an aqueous H ₂ PtCl solution (in each case 1% based on the palladium or platinum content) were added with stirring, and the reaction solution was heated to 75 ° C and stirred for a further 20 minutes at this temperature. Then 45 g of the wire mesh rings produced according to Id were added to the reaction solution and the reaction mixture was stirred for a further 120 minutes at 75 ° C. The reaction solution remained colorless and clear all the time. Then the wire mesh rings were separated from the reaction solution and rinsed with water. It did not become a solid detached from the carriers. The reaction solution and washing solution were then combined and the palladium and platinum content determined. A deposition of 87% of the palladium offered and 58% of the platinum offered on the carriers could be calculated from this data.

EK9:

In a 1 000 ml round bottom flask with a Teflon blade stirrer

Solution of 9.6 g NaH ₂ P0 ₂ . 1 H ₂ 0, 21.6 g ammonium chloride and 134 ml aqueous ammonia solution (25%) in 660 ml deionized water. 24.12 g of an aqueous Na ₂ PdCl solution (1%, based on the palladium content) were added, the solution was heated to 75 ° C. with stirring and the solution was stirred at this temperature for a further 25 minutes. 45 g of the wire mesh rings produced according to Ic were then added to the reaction solution and the reaction mixture was stirred at 75 ° C. for a further 120 minutes. The reaction solution remained colorless and clear all the time. The wire mesh rings were then separated from the reaction solution and rinsed with water. No solid was detached from the carriers. The reaction solution and washing solution were then combined and the palladium content of the combined solution was determined. From this data, a deposition of 84.6% of the palladium offered on the carriers could be calculated.

EK10:

45 g of the wire mesh rings produced according to Id were introduced into a cylindrical 2 1 vessel with a heating jacket and circulation pump, and a solution of 9.6 g of NaHP0 ₂ . 1 H ₂ 0, 21.6 g of ammonium chloride and 32 ml of an aqueous ammonia solution

(25%) in 160 ml of deionized water. The reaction mixture was heated to 56 ° C. Then 12.06 g of an aqueous Na ₂ PdCl solution (1%, based on the palladium content) and 0.12 g of an aqueous H ₂ PtCl solution (1%, based on the platinum content) were added and the mixture was heated Reaction mixture with circulation at 68 ° C in 40 minutes. The aqueous solution remained colorless and clear all the time. Then the wire mesh rings were separated from the reaction solution and rinsed with water. No solids were detached from the carriers. The reaction solution and washing solution were then combined and the palladium content of the combined solution was determined. A deposition of 90% of the palladium offered on the carriers could be calculated from this data. EK11:

In a cylindrical 2 1 vessel with heating jacket and circulating pump, a solution of 99.7 g of NaH ₂ P0 ₂ was. 1 H ₂ 0, 224.4 g ammonium chloride and 337 ml aqueous ammonia solution (25%) in 1 663 ml deionized water. 250.6 g of an aqueous Na ₂ PdCl ₄ solution (1%, based on the palladium content) were added, and the reaction solution was heated to 58 ° C. with stirring in 25 minutes. Then 468 g of the wire mesh rings produced according to Id were added and the reaction mixture was heated to 70 ° C. and circulated for a further 25 minutes. The aqueous solution remained colorless and clear all the time. Then the wire mesh rings were separated from the reaction solution and rinsed with water. No solid was detached from the carriers. The reaction solution and washing solution were then combined and the palladium content of the combined solution was determined. A deposition of 96% of the palladium offered on the carriers could be calculated from this data.

EK12:

468 g of the wire mesh rings produced according to Id were introduced into a cylindrical 2 1 vessel with a heating jacket and circulation pump and then a solution of 99.7 g of NaH ₂ P0 ₂ heated to 45 ° C. 1 H ₂ 0, 224.4 g of ammonium chloride, 337 ml of an aqueous ammonia solution (25%) and 250.6 g of an aqueous NaPdCl solution (1%, based on the palladium content) in 1 663 ml of deionized water. The reaction mixture was then heated to 80 ° C. with circulation within 120 minutes. A violent gas evolution was already observed at a temperature of around 56 ° C, which, however, steadily decreased with the progress of the reaction and was practically complete at the end of the test period. The aqueous solution remained colorless and clear all the time. The wire mesh rings were then separated from the reaction solution and rinsed with water. No solid was detached from the carriers. The reaction solution and washing solution were then combined and the palladium content of the combined solution was determined. A deposition of 99% of the palladium offered on the carriers could be calculated from this data.

All the catalysts obtained (VKl, VK2, EKl to EK12) were treated with hydrogen for 3 hours at 65 ° C. and 50 bar pressure before the application properties were checked. IV. Testing the application properties

The properties of the catalysts obtained were checked in the direct synthesis of hydrogen peroxide from hydrogen and oxygen (Examples B1 to B14) and in the case of EKII also in the hydrogenation of acetophenone (Example B15).

For this purpose, the catalysts VKl and VK2 not according to the invention (comparative examples VB1 and VB2 in Table 1) and the catalysts EKl to EK4 according to the invention (examples B1 to B5 according to the invention in Table 1) were placed in a 300 ml autoclave with stirrer, thermostatting and pressure maintenance 50 bar used as a reaction vessel. In each of these autoclaves, a catalyst monolith was installed centered around the stirrer axis so that it was supplied with liquid and gas evenly by the stirrer. In the autoclave floor there were supply lines for oxygen, hydrogen and the reaction medium. There was a drain in the autoclave lid from which the product / gas mixture was continuously removed. After subtracting the volumes for all internals, an effective reaction volume of 200 ml was available. Water containing 544 ppm hydrogen bromide and 1,200 ppm phosphoric acid was used as the reaction medium. The autoclave was flooded with the reaction medium and sealed. The autoclave was then tempered and the reaction medium, oxygen and hydrogen were passed continuously through the reaction vessel at constant flow rates. The hydrogen content in the discharge gas was determined using a thermal conductivity detector. The H ₂ 0 ₂ content in the liquid discharge was determined by titration. The selectivity of hydrogen peroxide was based on the hydrogen consumed in the reactor.

The autoclave described above was used for examples B6 to B12 according to the invention, but this was provided with a metal basket for holding the catalysts EK5 to EK10 (wire mesh rings), which was attached to the lid of the autoclave. The basket had a cylindrical recess in the center for the shaft of the stirrer, so that the respective catalyst was supplied with liquid and gas evenly. Water was used as the reaction medium, with a hydrogen bromide content of 121 ppm and a phosphoric acid content of 5,000 ppm. The reaction vessel was flooded with the reaction medium and sealed. A constant stream of reaction medium, hydrogen and acid was then passed through. fabric through the reactor. The product / gas mixture was removed continuously from the autoclave lid.

The reaction conditions and results of the reaction are summarized in Table 1.

The reaction temperature T, the total reaction time ti and the time t ₂ after which the hydrogen conversion and the hydrogen peroxide discharge were constant are also given in Table 1.

For example B13 and B14:

The catalyst (B13: EKII; B14: EK12) was fed into a double-jacket metal tube reactor with an inner diameter of 2.2 cm and a length of 2.00 m. The tubular reactor charged with the catalyst was connected to a circulation pump for the gas circuit and to a cooling / heating circuit. The apparatus was then filled with a solution of 121 ppm hydrogen bromide and 5,000 ppm phosphoric acid in water as the reaction medium and sealed. The reaction medium was passed through the apparatus at a constant rate of 500 ml / h. The entire system was adjusted to a pressure of 50 bar by supplying nitrogen using a pressure maintaining valve. The gas circulation (B13: 2 500 Nl / h; B14: 15 600 Nl / h) was set using the gas circuit pump. The nitrogen in the gas circuit was then replaced by a mixture of oxygen and hydrogen, the ratio of the two gases being set to a hydrogen content of 3%. A stream of 44 Nl of circulating gas / h was constantly branched off from the gas circuit and passed into a heat conduction detector for determining the hydrogen content in the exhaust gas. The supply of hydrogen and oxygen was controlled via mass flow meters. The quantities of fresh gas and exhaust gas were continuously recorded during the reaction. The reaction medium emerging from the reactor was separated from the circulating gas in a separator and conveyed out of the plant. The hydrogen peroxide content in the separated reaction medium was continuously monitored by titration with KMn0. Table 1:

) Total reaction time b) Time after which the H ₂ conversion was constant

Example B15

For example B15 according to the invention, the 300 ml autoclave described in Examples B6 to B12, described above, was used, the one for receiving 9.2 g of the catalyst EKII (wire mesh rings) with a metal basket which is attached to the lid of the autoclave was provided. The basket had a cylindrical recess in the center for the shaft of the stirrer so that the catalyst was supplied with liquid and gas evenly. The autoclave was charged with 200 ml of cyclohexane and 8 g of acetophenone, sealed and freed of air by injecting 100 bar of nitrogen twice. The autoclave was then heated to 120 ° C. and 250 bar of hydrogen were injected. While pressing in the consumed hydrogen, stirring was continued at this temperature for 60 minutes. A total of 1.59 liters of hydrogen were consumed. Gas chromatographic analysis of the product obtained showed complete conversion of the acetophenone with formation of 1-phenylethanol (81%) and ethylbenzene (19%).

Claims

claims

1. A process for the preparation of catalysts by electroless deposition of at least one platinum metal on a metallic support, wherein an aqueous medium which comprises at least one platinum metal complex, at least one reducing agent and at least one complexing agent and has a pH of more than 4 with which brings metallic carrier into contact for the deposition of the platinum metal, the deposition of the platinum metal taking place from an essentially homogeneous aqueous solution in the form of discrete, immobilized particles.

2. The method of claim 2, wherein in the solution about 1 to

1,000 equivalents of complexing agents, based on the platinum metal, are contained.

3. The method according to any one of the preceding claims, wherein the reducing agent is used in a 10 to 100-fold molar excess, based on the platinum metal.

4. The method according to any one of the preceding claims, wherein the deposition at a reaction temperature in the range of

40 to 85 ° C is carried out.

5. The method according to any one of the preceding claims, wherein adjusting the pH with at least one base, selected from ammonia, primary, secondary and tertiary amines, alkali and alkaline earth metal hydroxides and alkali and alkaline earth metal carbonates.

6. The method according to any one of the preceding claims, wherein the complex of platinum metal has a complex formation constant of

> 1,000.

7. The method according to any one of the preceding claims, wherein the platinum metal comprises 80 to 100 wt .-% palladium and 0 to 20 wt .-% platinum or iridium.

8. The method according to any one of the preceding claims, wherein at least 0.1 to 30 g / 1 of at least one platinum metal complex, optionally 0.001 to 5 g / 1 of at least one further element compound, and, based on the platinum metal, at least 20 equivalents of a complexing agent and little dissolves at least 10 to 100 equivalents of a reducing agent in an aqueous medium.

9. The method according to any one of the preceding claims, comprising 5 at least one of the following measures:

(1) heating the aqueous medium to reaction temperature in less than about 120 minutes and in the presence of the metallic support;

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(2) tempering the aqueous medium in the absence of the metallic support;

(3) increasing the platinum metal ion concentration in the aqueous medium;

(4) adding a platinum metal seed sol to the aqueous medium;

(5) destabilization of the platinum metal complex by reducing the complexing agent concentration in the aqueous medium; or

(6) destabilization of the platinum metal complex by increasing the reducing agent concentration.

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10. Catalyst obtainable by a process according to one of claims 1 to 9.

11. Platinum metal catalyst, with a metallic carrier and a catalytically active coating applied thereon, characterized in that the catalytically active coating comprises discrete platinum metal particles immobilized on the carrier surface with an average particle diameter of less than approximately 1 μm. 35

12. The catalyst of claim 11, wherein the platinum metal particles have an average diameter in the range of about 20 to 100 nm.

40 13. A catalyst according to claim 11 or 12, wherein the platinum metal particles form a coating formed essentially as a monolayer.

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14. Catalyst according to one of claims 10 to 13, wherein the metallic support consists essentially of steel, iron, copper, aluminum, silver, nickel, chromium, tungsten, titanium and mixtures and / or alloys thereof.

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15. A catalyst according to any one of claims 10 to 14, wherein the platinum metal content is in the range of about 0.01 to 50 g / kg of support.

16. A catalyst according to any one of claims 10 to 15, wherein the selectivity in the direct synthesis of hydrogen peroxide from the elements is> 70%.

17. Use of a catalyst according to any one of claims 10 15 to 16 for the synthesis of hydrogen peroxide from the elements and for the hydrogenation of organic compounds.

18. Process for the production of hydrogen peroxide by direct synthesis, wherein a catalyst according to one of the approaches

20 Proverbs 10 to 16 with an oxygen-hydrogen mixture with a mixing ratio in the range of about 5: 1 to 100: 1 in contact.

19. The method of claim 18, wherein the contacting

25 at a temperature of about 10 to 60 ° C and a pressure in the range of about 10 to 300 bar.

20. The method according to claim 18 or 19, wherein about 200 to 7,500 ppm of at least one acid is added to the aqueous solution.

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