MXPA00001527A - Method for producing catalysts containing metal nanoparticles on a porous support, especially for gas phase oxidation of ethylene and acetic acid to form vinyl acetate - Google Patents

Abstract

The invention relates to a method for producing a catalyst containing one or several metals from the group of metals comprising the sub-groups Ib and VIIIb of the periodic table on porous support particles, characterised by a first step in which one or several precursors from the group of compounds of metals from sub-groups Ib and VIIIb of the periodic table is or are applied to a porous support, and a second step in which the porous, preferably nanoporous support to which at least one precursor has been applied is treated with at least one reduction agent, to obtain the metal nanoparticles produced in situ in the pores of said support.

Description

METHOD TO PRODUCE CATALYSTS CONTAINING METAL NANOPARTICLES ON A POROUS SUPPORT, ESPECIALLY FOR OXIDATION OF GAS PHASE OF ETHYLENE AND ACETIC ACID TO FORM VINYL ACETATE DESCRIPTION The invention relates to a process for producing a catalyst consisting of one or more metals selected from the group of metals spanning the transition groups Ib and Vlllb of the periodic table of the elements on porous support particles. In the present, the metals are present as nano-sized particles in the finished catalyst. In particular, the invention relates to the production of "coated" catalysts on porous support, preferably nanoporous, by this method. The catalysts, preferably coated catalysts, can be used for many heterogeneously catalyzed reactions such as hydrogenations and oxidations. Among other things, the Pd / Au coated catalysts are extremely well suited to the gas phase oxidation catalysts of ethylene and ascetic acid to give vinyl acetate. In the present, the catalytically active metals are deposited in the form of a cover on or in the outermost layer of the support. They are often produced by penetrating the support with metal salts within a surface region and subsequent precipitation by alkalis to form water soluble Pd / Au compounds. GB-A-1 283 737 describes the production of a noble metal-coated catalyst by prepreg of the support with an alkaline solution and saturation with 25-90% water or alcohol. Subsequent impregnation with Pd salts and reduction of precipitated salts to the metal gives coated catalysts in which the penetration depth of the noble metal is said to be up to 50% of the pellet radius. According to US-A 3,775,342 and 3,822,308, the coated catalysts are produced by impregnation of the support with a solution of Pd / Au salts and with an aqueous base, preferably NaOH, which results in the precipitation of non-soluble palladium hydroxide. Gold hydroxide in a covered surface area of the pellets. The hydroxides that have been fixed in the shell in this way are then reduced to metals. GB-A-1 521 652 obtains coated catalysts of the white egg type, ie only an inner ring of the spherical SiO2 support contains the noble metals while the inner core and a thin outer shell remain virtually free of the noble metal, by a comparable process (prepreg with Pd, Au salts, drying, base precipitation, reduction). US-A 4,048,096 teaches the precipitation of water-insoluble Pd and Au compounds on the support prepreg with Pd / Au salts using sodium silicates instead of NaOH. The thickness of the cover is less than 0.5 mm. US-A 5,567,839 precipitates Pd and Au compounds not soluble in water on the support prepreg with Pd / Au salts using barium hydroxide instead of NaOH. The thickness of the cover is 1 mm. The catalyst can also be doped with barium acetate. EP-AO 519 435 describes the production of a Pd / Au / K or Pd / Cd / K coated catalyst in which a specific support material is washed with an acid prior to impregnation and is treated with a base after the impregnation. US-A 5,314,858 refers to the double fixation of the noble metal in an outer shell by means of two separate beakers using NaOH. WO-A-94/08714 achieves particularly uniform covers by rotary movement of the support impregnated with Pd, Au salts during the fixing step, i.e. immersed in the alkaline fixing solution (NaOH). EP-AO 723 810 uses the pretreatment (impregnation) of the support with metal salt solutions to produce a support that is preferably doped with Al, Zr, Ti and is subsequently used for the base precipitation described above to form a catalyst coated with Pd / Au / K.

US-A-5,347,046 discloses the use of Cu, Ni, Co, Fe, Mn, Pb, Ag as promoters in Pd / Au systems on SiO2 supports pretreated with alkali metal hydroxide and alkali metal silicate. Another method for producing coated catalysts is metal pre -ucleation and subsequent deposition of the designed amount of noble metals. Japanese patent application published 48-10135 / 1973 describes the production of a Pd / Au coated catalyst. Here, a small amount of reduced metal (gold) on the porous support in a pretreatment step. Subsequent impregnation results in the deposition of Pd in a surface area having a thickness of about 15% of the radius of the particle. US-A 4,087,622 teaches the production of coated catalysts by pre-nucleation with Pd / Au (reduced) metal cores in a low concentration, impregnating the porous support of SiO2 or AI2O3 with a Pd / Au salt solution, drying and then reducing the salt of Pd / Au to metal. This pre-nucleation step is followed by deposition of the catalytically necessary amount of noble metal, i.e. the average amount which is then concentrated in a cover near the surface. The use of different variants of "deficiency techniques" also allows coated catalysts to be obtained. These include, among others: > deficiency of precipitants, for example NaOH, in combination with multiple precipitation; > deficiency of impregnation solution (less than the pore volume of the support); > limitation of contact time during the absorption of noble metals; > insufficient concentration of noble metal (per impregnation step) combined with multiple impregnation; and > combinations of the variants mentioned above. EP-A-0 565 952 describes the formation of catalysts of Pd / K / Au, Pd / K Ba and Pd / K / Cd type cover by atomizing a solution of suitable metal salts by means of ultrasound and then applying it to the support particles in such limited amount and within such a time of restriction and beginning the drying in such a way that the catalytically active metal salts can not penetrate the core of the support particles, but only towards an outer part of varying thicknesses, ie the cover. According to EP-AO 634 214 the coated catalysts are obtained by spraying a viscous solution of suitable metal salts in the form of drops or liquid jets onto the support particles, wherein the volume of solution in each dew step is 5-80. % of the pore volume of the support particles and drying is started immediately after the spray.

EP-AO 634 209 obtains coated catalysts by impregnation of the support particles with a viscous solution of suitable metal salts, wherein the volume of solution in each impregnation step is 5-80% of the pore volume of the support particles and drying is started immediately after each impregnation step. According to EP-AO 634 208, coated catalysts are obtained by impregnating the support particles with a viscous solution of salts of the suitable elements and then drying them, wherein the volume of solution in the impregnation is more than 80% of the pore volume of the support particles and the impregnation duration and the drying start time are made so short that the specified metal salts are present in a cover of 5-80% of the pore volume of the support particles after the end of drying US-A 5,576,457 relates to Pd / Cd / K coated catalysts which are doped with Zr and / or Re, wherein the shell can be produced as described in EP 0634208, EP 0634209 or EP 0634214. US-A 5,591 , 688 describes fluidized bed VAM catalysts (VAM = vinyl acetate (monomer)) consisting of Pd-Ba, Au, La, Nb, Ce, Zn, Pb, Ca, Sr, Sb on silica, alumina or zirconia, using free precursors of halide. US-A 5No. 536,693 discloses fluidized bed VAM catalysts consisting of Pd-Au, Cd, Bi, Cu, Mn, Fe, Co, Ce, U which are produced by grinding a fixed-bed catalyst prepreg pre-impregnated with Pd-M and forming Composed with a binder consisting of silica, alumina, zirconia or titania. GB-A-2 006 261 describes Ru-containing catalysts for Fischer-Tropsch synthesis. The production and stabilization of nano-sized noble metal particles in solution is prior art. Other common terms for such solutions are suns or colloids. A summary description can be found in G. Schmid, Cluster and Colloids, From Theory to Applications, VCH Weinheim 1994. Stable sols are produced by reducing metal salt solutions with a reducing agent in the presence of a stabilizer which envelops the particles of nanometric sizes and avoids the additional agglomeration of particles of nanometric sizes. With a suitable choice of reducing agent and stabilizer, it is possible to produce monomodal sols having a narrow particle size distribution. The resulting particle sizes are in the region of <; 200 nm. The sun-impregnation techniques for applying the sols from aqueous solution to supports is also known. Thus, for example, DE-A 195 00 366 describes the production of Pd-coated catalysts for hydrogenations by applying the Pd as a highly diluted sol to a support by impregnation or by spraying on it, with a resulting cover thickness of less than 5%. μm.

This low cover thickness is not critical for many hydrogenation reactions, but it can be a problem in other reactions, for example, the synthesis of VAM, because the very low noble metal content leads to a reduction in activity. In the present, covers in the scale of 5 - 1000 μm, which can accommodate a sufficiently large amount of noble metal, would be desirable. The Pd content of VAM catalysts is in the region of 1% by weight and is therefore very high compared to hydrogenation catalysts (0.05 -0.5% by weight). In "Catalyst Preparation Science IV" (Eds .: Delmon, Grange, Jacobs, Poncelet), Elsevier Science Publishers, New York, 1987, pp. 669-687, Michel and Schwartz describe the preparation of particles of Pd-Au of nanometric size, bimetallic, monodisperse, having 3 different microstructures (alloy, cover Au on a Pd core and vice versa) and its application to a carbon support by adsorption from the colloidal solution. Schmid, West, Malm, Bovin, Grenthe (Chem. Eur. J., 1996, 2, No. 9, 1099) produce Pd / Au catalysts for the hydrogenation of hexane by dip coating a TiO2 support in colloidal Pd solutions. / Au. DE-A 44 43 705 describes the preparation of monometal and bimetal colloids of stabilized surfactant from insoluble precursors which are water-soluble at a high concentration; these are subsequently used for adserosive application to support catalysts from aqueous solution. DE-A 44 43 701 describes coated catalysts which are obtained by coating the supports with the catalytically active metals in aqueous solutions of monometallic or bimetallic colloids of these metals, with the colloids being stabilized by strongly hydrophilic surfactants. The sun-impregnation technology is thus based on a two-step process, that is, the preparation of the sols by means of a reduction step and, if appropriate after further isolation and purification steps, the subsequent fixation to a support. This method consisting of a plurality of steps is relatively complicated per se. It is therefore an object of the present invention to provide a simple process for producing a catalyst consisting of one or more metals selected from the group consisting of metals spanning the transition groups Ib and Vlllb of the periodic table of the elements, with the exception of Ru, on porous support particles. A further object of the invention is to provide a process for producing supported catalysts coated by sol which can be carried out without great expense. Another object of the invention is to provide an improved process for producing coated catalysts on porous, preferably nanoporous supports, in which the thickness of the resulting coating is sufficient for the preparation of vinyl acetate (VAM synthesis). A further object of the invention is to produce a coated, active and selective VAM catalyst, based on Pd / Au in a fast and non-expensive manner in a small number of process steps while at the same time allowing the thickness of the cover to be easily controlled . These objects and also additional objects that are not listed in more detail but that can be derived or inferred from the introductory discussion of the prior art are achieved by a method of the type mentioned at the beginning and having the features of claim 1. Advantageous modifications of the method of the invention are claimed in the dependent claims which are dependent on claim 1. A process for producing a catalyst comprising 1 or more metals selected from the group consisting of metals spanning transition groups Ib and VIII of the Periodic table of the elements, with the exception of Ru, on porous support particles, comprising, in a first step, applying one or more precursor (s) selected from the group of compounds consisting of the transition metal group compounds Ib and Vlllb of the periodic table, with the exception of Ru to a pore support SW; and, In a second step, treating the porous support to which at least one precursor has been applied with at least one reducing agent to give metal particles of nanometric size produced in situ in the pores of the support; It provides, in a particularly advantageous manner, a process that ameliorates the known procedures both with respect to the simplicity of carrying out the procedure and in terms of universal applicability as well as the quality of the resulting process products in a manner that could not be been contemplated before. In carrying out the method of the invention, a number of advantages compared to known methods are discovered: = > In this way, instead of the complicated sun impregnation technique (comprising the steps of: sun preparation, support loading, fixing), a very simple procedure is used comprising fewer steps in which the sun prepares in situ in the pores of the support by reduction. In the present, the preparation of the sol and its fixation to the support are achieved simultaneously in a "single vessel reaction" having fewer steps or steps than in the case of the processes known from the prior art. = > The subsequent reduction step, in particular, which is required in a conventional manner, becomes unnecessary in the method of the invention, because the formation of the cover structure and the reduction to the metals occur simultaneously in one step. = > The technique according to the invention makes it possible to obtain, in a simple way, coated catalysts whose cover thickness can be matched to requirements more easily than in the case of known techniques. = > In particular, larger cover thicknesses are also possible, if desired, than when the conventional sun-impregnation technique is used, in which the diffusion of the sols from the outside to the pores of the support is also obstructed by the effect of mechanical cut. = > The type filler covered with metal salts in the impregnation by known techniques and the rapid removal of the water during drying, for example under reduced pressure, in addition to the reduction method of the invention promote the formation of covers or allow a further reduction in the thickness of the covers, if this is desired. In addition, the process of the invention allows higher noble metal charges on the support, saves procedural steps and the treatment of intense energy with highly diluted solutions is avoided. = > The invention makes it possible to obtain catalyst particles of significantly better uniformity, an essentially narrower monomodal particle size distribution and smaller particle sizes compared to conventional preparation techniques. = > It is very possible advantageously, in the case of a well defined pore structure of the support, to fix the colloid size exactly through the pore size of the support, so that the monomodal distributions of colloids can be more simply produced. = > In known techniques, impurities in sols lead to larger particle sizes and particle agglomeration. In contrast, the apparatus and meticulously clean solvents (distilled water twice) required for the preparation of sols become completely unnecessary in the process used according to the invention, ie the preparation in situ. = > In the case of VAM catalysts, the invention has the advantage of tremendous time savings (and therefore cost savings) in the production process compared to industrial processes involving the precipitation of noble metal hydroxides using NaOH followed by a step of reduction, because according to the invention the cover can be produced in a few minutes while the precipitation of NaOH extends for more than 20 hours. = >; Due to the possible small particle size, the uniformity of the particle size distribution and the large possible cover thickness, the coated catalysts obtained by the process of the invention exhibit high activities and selectivity and have good long-term stability. In the present invention, the pores in a surface area of the support system are used as "microreactors" for the in situ synthesis of stabilized or non-stabilized colloids which, after final drying, are fixed as finely divided nanometer particles at support. The support materials which can be used in the processes of the invention are therefore all porous materials which have a suitable porosity, ie they are microporous, nanoporous or mesoporous, and are essentially inert for objects of designed use and the methods of production. The support materials can have any shape and the shape can be matched to the use. In an advantageous embodiment of the process of the invention, use is made of an inert, porous, preferably nanoporous support, comprising silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, oxide mixtures of the aforementioned compounds, oxides mixed of said compounds and / or aluminosilicates in the form of powders, sheets, strips, membranes, rods, plates, tablets, wagon wheels, monoliths, spheres, chips, rings, solid extruded materials, hollow extruded materials, stars or other bodies formed. It is particularly advantageous to use SiO2, AI2? 3, mixed oxides of SiO2 and AI2Ü3 or mixtures of these oxides in the form of spheres, tablets, rings, stars or other bodies formed as supports. The diameter or length and thickness of the support particles is generally from 3 to 9 mm. The surface area of the support, measured by the BET method, is generally 10-500 m2 / g, preferably 20-250 m2 / g. The pore volume is generally from 0.3 to 1.2 ml / g.

Of particular interest are porous aluminum supports, preferably nanoporous, for example in the form of membranes, tablets, spheres or powders. The nanoporous support materials described herein as preferred are known per se, as are the microporous or mesoporous supports. In this way, for example, nanoporous aluminum oxide support membranes are commercially available: they have nanopores arranged in a regular manner having a pore width in the range of 1 to 500 nm and a depth of up to 500 μm. The pore density is normally in the range of 109 to 1012 pores / cm2. Abstracts describing the structure, production and properties of porous anodic oxide films are given by J.W. Diggle et al., Chem. Rev. 69, 365-405 (1969) and J.P. Gullivan et al., Proceeding of the Royal Society of London, 317 (1970), 51 ff .; Additional information can be found at C.A. Foss et al. J. Phys. Chem. (1994), 98, 2963-2971 and C.K. Preston et al., J. Phys. Chem. (1993), 97, 8495-8503. The non-porous structures can be generated in principle and preferably by anodic oxidation of metal surfaces, preferably aluminum surfaces, in an aqueous solution consisting of a diprotic or triprotic acid. The acids which are suitable for this purpose are, in particular, sulfuric acid, oxalic acid, phosphoric acid and chromic acid. The anodic oxidation of aluminum to produce the membranes to be used according to the invention is usually carried out at a low temperature, for example from 0 to 5 ° C, and preferably using sulfuric acid or oxalic acid as an electrolyte because this allows thick and hard porous films to be obtained. In the production of the films, for example, a highly pure aluminum foil forms the anode in an electrochemical cell. The anodization is carried out with precise control of potential and current. The pore diameter depends on the electrolyte, the temperature and the anodizing voltage, with the diameter increasing with increasing voltage: a guide in the case of sulfuric acid as an electrolyte is 1.2 nm in pore width per volt of applied potential. The use of oxalic acid allows thicker films to be produced than when sulfuric acid is used. After the anodic oxidation, the non-oxidized aluminum on the barrier side may, in a known manner, be dissolved in an acid bath or ground (see, for example US-A 4,687,551), giving nanoporous AI2O3 membranes having a closed surface (barrier side) and an open surface (= pore openings). The grinding of the membranes towards the bottom region of the pores gives first membranes having an open side and a half open side (= very small pore openings); the additional grinding makes it possible to obtain membranes having pore openings of approximately equal width which go from one side to the other. Alternatively, the pores passing through can also be obtained by etching with, for example, KOH in glycol, with the membrane being placed on the etch bath with the barrier side in contact with the bath. In the process of the invention, among others, one or more precursor (s) selected from the group of compounds consisting of the transition metal compounds of groups Ib and Vlllb of the periodic table, with the exception of Ru, is applied to a porous support. This application or loading can be carried out in many ways which are known per se to those skilled in the art, while immobilization of the metal compound (s) in the form of metal or alloys on the support is possible. In this way, for example, deposition from the gas phase by known CVD techniques per se is possible. A preferred method of modification allows the noble metal compound (s) to be applied to the porous support by spraying, immersion, impregnation, spray drying, high coating or coating by fluidized bed, preferably by impregnation. The loading of the support with the noble metal compound (s) can be carried out in one or more sequential steps, with, if desired, drying phases which can be inserted between the individual fixing steps. As active metals that can be concentrated on the support, if desired in a shell, all the reducible metals of the transition groups IB and Vlllb of the periodic table, in particular all the noble metals in those groups, including their mixtures, are adequate.

In a preferred modification of the process of the invention, one or more compound (s) of metals selected from the group consisting of copper, silver, gold, iron, cobalt, nickel, rhodium, osmium, iridium, palladium and platinum is / are applied to the support. Among those, the compounds of Pd, Au, Pt, Ag, Rh, are preferred.

Cu, Ir, Ni and / or Co. Particular preference is given to compounds of Pd, Au, Pt, Ag and / or Rh. In a further advantageous variant of the process of the invention, one or more palladium compound (s) alone or one or more palladium compound (s) together with one or more compound (s) of metals selected from the group consisting of copper, silver , gold, iron, cobalt, nickel, rhodium, osmium, iridium and platinum is / are applied to the support. An extremely advantageous variant comprises applying one or more palladium compound (s) together with one or more gold compound (s) to the support. An essential measure in the method of the invention is to treat the porous support to which at least one precursor has been applied (active metal precursor compound) with at least one reducing agent to give nano-sized metal particles and / or alloy particles produced in situ in the pores of the support. Suitable reducing agents are all compounds which are capable of reducing the metal compounds used, preferably salts, particularly preferably salts of Pd and Au, to the metals. In a particular embodiment of the method, use is made of one or more reducing agents selected from the group consisting of citrates such as potassium citrate, sodium citrate, ammonium citrate; hydrazine, hydroxylamine, sodium hypophosphite, alkali metal borohydrides such as sodium borohydride, potassium borohydride; gaseous reducing agents such as hydrogen, carbon monoxide; formaldehyde, formats, acetates, oxalates, suitable sulfanilates such as sodium hydroxymetalsulphinate, and monohydric or dihydric alcohols such as ethanol, ethylene glycol. Among these, preference is given to citrates (alkali metal / alkaline earth metal / ammonium), formats, acetates, alkali metal borohydrides, oxalates and suitable sulfanilates. An advantageous embodiment of the invention utilizes ammonium citrate, potassium citrate and / or sodium citrate as a reducing agent. Particular preference is given to potassium citrate. The reducing agent is generally used in a stoichiometric amount based on the metal compound (s), but is preferably used with a small excess. The excess can be, for example, from 1.1 to 2, preferably from 1.1 to 1.5, mole equivalents. In the process of the invention, the in situ reduction is preferably carried out at temperatures of room temperature to 150 ° C.

In a particularly advantageous embodiment of the invention, a solution of the metal compound (s) is applied to the porous supports; for example, the supports are impregnated by fixing in or immersion in a solution. This solution can be basically a solution of the metal compound (s) in an acidic and organic solvent. In this way, it is possible to apply an aqueous solution, a solution in an organic solvent or a mixture thereof to the support. As solvents, it is possible to use all the compounds in which the selected salts are soluble and which can be easily removed again by drying after impregnation. Particular preference is given to using water as a solvent. In the present, the nature and purity of water is only of subordinate importance. It is possible to use deionized water, distilled water or double distilled water. Likewise, mixed water can also be used as long as the materials present therein do not have an adverse effect on the process of the invention to produce the catalysts. The nature of organic solvents can vary depending on the nature of the metal compound (s) to be dissolved. For example, unsubstituted carboxylic acids, in particular acetic acid, are especially suitable for salts such as acetates. Water is especially suitable for chlorides.

The additional use of an additional solvent is advantageous when the salts are not sufficiently soluble in acetic acid or in water. As additional solvents, it is possible to use those that are inhertes and are mixable with acetic acid or water. Examples of additives to acetic acid are ketones such as acetone and acetylacetone, also ethers such as tetrahydrofuran or dioxane, acetonitrile, dimethylformamide as well as hydrocarbons such as benzene. Good results can also be obtained by using, as an organic solvent, methanol, ethanol, ethylene glycol, N-methylpyrrolidone, dimethylformamide, dimethylacetamide and / or tetrahydrofuran or a mixture of these substances with water. The noble metal compounds serve as precursors, ie a compound of the metals that can be converted to the metal by reduction. These can be ionic and non-ionic compounds. The salts are by far the most preferred metal compounds to be used as precursors. Suitable salts are all salts of metals that are soluble and do not contain constituents that can poison the catalyst, for example sulfur. Preference is given to acetates and chlorides. If palladium precursor compounds are used, preference is given to soluble palladium compounds, in particular water-soluble salts selected from the group consisting of palladium (II) acetate, palladium (II) chloride, palladium (II) nitrate and tetrachloro- sodium palate (II) [Na2PdCI4]. In the case of chlorides, PdCl2 and Na2PdCl are particularly preferred precursors. Additional soluble metal compounds which can be preferably used, in particular water-soluble salts, are tetrachloroauric acid (III) [HAuCI4], gold acetate (III) [Au (OAc) 3J, potassium aurate [KAuO2], acid hydrate hexachloro-platinic (IV), hexachlorohydric acid hydrate (IV) and / or rhodium chloride hydrate (III). In the case of chlorides, it should generally be ensured that the chloride ions are removed from the catalyst before use. This is achieved by washing the doped support, for example, with water, after the metals have been fixed to the support by reduction to nanometric sized metal particles. The metal compounds are usually used in concentrations of 0.1 to 100 g per liter, preferably 1 to 50 g per liter, based on the solvent. Although suitable catalysts consisting of nano-size particles on a support are obtainable without additional additives, the application of the precursor (s) to the porous, preferably nanoporous support, and / or the reduction of the support to which the precursor (s) has / have been applied in the process of the invention are / is carried out preferably in the presence of a colloid stabilizer or a plurality of colloid stabilizers. Suitable stabilizers are all compounds that are capable of complexing the nano-sized particles obtained by reduction by wrapping them and are therefore capable of preventing further growth and agglomeration of the nano-sized particles. Stabilizers which may be used for the purposes of the invention include, inter alia, betaines, surfactants, polymers such as polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyacrylamide (PAA), polyelectrolytes, citrates, substituted phosphines, substituted sulfanilic acids , chlorides, amino acids or mixtures thereof. It is also possible, inter alia, to use copolymers which are formed of monomers containing betaine groups plus additional monomers such as acrylic acid, acrylic esters, acrylamides, vinyl carboxylates, vinylalkyl ethers, N-vinylpyridine, N-vinylpyrrolidone or N-vinylcarboxamides. In a favorable embodiment of the process, one or more compound (s) selected from the group consisting of betaines, PVP, phosphines, citrates, oxalates, formates, acetates, sulphanilates, PVA and PAA is / are added as a colloid stabilizer. Preference is given to betaines, PVP, PVA, citrates, substituted phosphines, substituted sulfanilic acids and / or chlorides.

Particular preference is given to potassium citrate, ammonium citrate, PVP K 30, dimethyldodecylammoniumpropane sulfonate. In the process of the invention, the stabilizers are normally used in an amount of 5 to 1000% by weight, based on the metal or metals. The addition of the stabilizer can take place in any order. The stabilizer can be added to the metal compound with which the support is impregnated. The support can first be impregnated with the colloid stabilizer. The impregnated support can be brought into contact with the colloid stabilizer. The stabilizer or colloid stabilizers can also be used together with the reducing agent. Subsequent stabilization (after reduction) is also possible under some circumstances. In a very particular variant of the invention, use is made of one or more compounds that act simultaneously as a colloid stabilizer and as a reducing agent. This means that the reducing agent and the stabilizer can also be identical. In this way, for example, potassium citrate acts as a reducing agent and as a stabilizer in the case of Pd / Au. Preference is therefore given, according to the invention, to using ammonium citrate, potassium and / or sodium as reducing agent and colloid stabilizer. Potassium citrate is especially advantageous.

The stabilizer can remain on the nano-sized particles after they have been fixed on the support or can be removed without the presence of the stabilizer interfering with the catalytic function. The complete or partial removal of the stabilizer can, if required, be carried out, for example, hydrolytically using a solvent, thermally or oxidatively, for example by incinerating in air at 300 to 500 ° C, either prior to installation of the catalyst in the reactor or otherwise in situ in the reactor. The application of the metal compound (s) and their reduction to the support can be carried out successively in two steps or in a "one-pot reaction". In a variant, it may be preferred that the first and second steps are carried out in succession. This advantageously allows the porous, preferably nanoporous support, to which at least one metal compound has been applied to be subjected to a drying step before reduction. This allows, for example, that several "covers" of metal compounds be applied to the support by multiple repetition of impregnation and drying. Alternatively, it is also preferred that the first step and the second step be carried out in a single container process without isolation, purification or drying of the porous, preferably nanoporous support, to which the precursor (s) has / have been. applied.

In a preferred embodiment of the invention, a support is preimpregnated first with essentially aqueous salt solutions of reducible active metals, the prepreg of which does not have to lead to a covering, ie the support is under some circumstances, "completely impregnated". However, a cover and therefore a coated catalyst can also be produced. This is achieved, for example, by incomplete specific impregnation of the support and / or by carrying out the reduction in an appropriate manner. Impregnation and if desired after a drying step, subsequent treatment with a reducing agent under such conditions (concentration, temperature, time, etc.) and in the presence or absence of stabilizers results in the active metals being reduced to metals in the oxidation state O, so that they can be concentrated as nano-sized particles in a support body shell to produce a shell-type or white-egg type catalyst. In this way, the application of the precursor (s) and / or the reduction is / are preferably carried out so that the metal compounds are reduced in the pores of the support in a covered surface area to generate the corresponding metals or alloys in the form of nano-sized particles stabilized or not stabilized to give a coated catalyst.

Gaseous reducing agents such as H2 or CO or ethylene can only be used when a covering structure has already been produced on the impregnation with the metal salts. In relation to the mechanism of formation of cover in the production of a catalyst coated with noble metal on porous supports, formed of ceramic, it can be assumed, without restricting the invention to a mechanism, that the rapid reduction takes place at the internal interface between the active metal salt and the reducing agent to give the nano-sized particles, the particles are immobilized in the outer shell due to their size (including the stabilizer shell) and the additional active metal salt diffuses from the inner regions of the formed body towards the surface so that it is similarly reduced inside the cover after reaching the front of the reducing agent which moves slowly inwards and is deposited on the support. A significant advantage of the invention is, inter alia, that particularly stable covers of relatively large thickness can be produced. A cover thickness in the range of 5 μm to 5000 μm is preferably obtained. Coated catalysts having nano-sized particles having an average particle diameter on the scale of 1 to 100 nm in the pores and in the shell are advantageously obtained. This means that the particles of the cover do not agglomerate or agglomerate only a little.

The support can, before, during and / or after the in situ generation of the nano-sized particles, be loaded with additional activators, in particular alkali metal acetates, and, if desired, promoters, for example Zr, Ti compounds. , Cd, Cu, Ba and / or Re. A particularly interesting modification of the process therefore includes the application of one or more activators and / or promoters, after, before or during the application of the precursor (s) and / or the reduction . Some preferred catalyst systems that can be produced according to the invention, preferably coated catalysts, consist, for example, not only of palladium and gold but also of potassium acetate as activator and / or cadmium or barium compounds as promoters. The metal contents of particularly preferred catalysts are as follows: The Pd content of the catalysts Pd / K / Cd and Pd / K / Ba is generally from 0.6 to 3.5% by weight, preferably from 0.8 to 3.0% by weight, in particular from 1.0 to 2.5% by weight. The Pd content of the Pd / Au / K catalyst is generally 0.5 to 2.0% by weight, preferably 0.6 to 1.5% by weight. The content K of the three types of catalysts is generally 0.5 to 4.0% by weight, preferably 1.5 to 3.0% by weight. The Cd content of the Pd / K / Cd catalyst is generally from 0.1 to 2.5% by weight, preferably from 0.4 to 2.0% by weight.

The Ba content of the Pd / K Ba catalyst is generally from 0.1 to 2.0% by weight, preferably from 0.2 to 1.0% by weight. The Au content of the Pd / K / Au catalyst is generally 0.2 to 1.0% by weight, preferably 0.3 to 0.8% by weight. At least one salt of each of the elements to be applied to the support particles (for example Pd / K / Au, Pd / K / Cd, Pd / K / Ba) must be applied. It is possible to apply a plurality of salts of an element, but it is normal to apply exactly one salt of each of the three elements. The necessary amounts of the salts can be applied in one step or by multiple impregnation. The salts can be applied to the support by known methods such as staggered, spray, vapor deposition, immersion or precipitation. In the method of the invention, it is naturally only the noble metal salts, ie Pd and Au, which are reduced to the (noble metal particles of corresponding nanometric size and not the "base" constituents K, Cd, Ba The latter can be applied to the support together with the noble metal salts or otherwise beforehand or afterwards, Normally, according to the method of the invention, a Pd / Au cover is first produced and the support is then impregnated. with potassium acetate solution, with the K being distributed evenly over the cross section of the pellet.

If a plurality of noble metals must be fixed to the support (for example Pd, and Au), alloys or structured nanostructures, ie gold on palladium or palladium on gold, they can also be produced by the method of the invention. The coated catalysts produced by the process of the invention can be used for many heterogeneously catalyzed reactions. These include, inter alia, aminations, hydrogenations, dehydrogenations, dehydrocyclizations, hydroxylations, oxidations, epoxidations, skeletal isomerizations and also combinations of those reaction types for objective conversion of organic molecules. The impregnated and reduced shaped bodies can be used, in particular after activation, as coated catalysts for hydrogenation, oxidation and isomerization reactions, particularly preferably for selective hydrogenation reactions and partial oxidations. Examples of these include: selective hydrogenation of tip, selective hydrogenation of butadiene, selective hydrogenation of acetylene, selective hydrogenation of butinol, selective hydrogenation of octadiene to octene, selective hydrogenation of benzene to cyclohexene, hydrogenation of carbon monoxide, hydrogenation of carbon dioxide , hydrogenation of maleic anhydride, hydrogenation of NOx to NH3 or NH2OH, carboxamides from nitriles, amines from carboxylic acids, aromatic amine, in particular the reaction of benzene with ammonia to give aniline, reductive amination of aldehydes and ketones amines, Wacker synthesis, acetaldehyde from ethylene, oxidation of maleic anhydride butane, oxidation of carbon monoxide, oxidation of alcohols to aldehydes, ketones or carboxylic acids, oxidations of alkenes to alcohols, aldehydes and ketones, oxidations of aldehydes and Ketones to carboxylic acids, hydroxylation n of aromatics, for example oxidation of benzene to phenol or toluene to cresol, oxidation of propylene to acrolein or acrylic acid, ammonioxidation of, for example, toluene to benzonitrile or from propylene to acrylonitrile, epoxides can be converted to aldehydes / ketones and hydrogenation conditions in alcohols, for example, derivatives of ethylene oxide to give the corresponding phenylacetaldehydes or under hydrogenation conditions to phenylethanols. The Pd / Au coated catalysts produced according to the invention can be advantageously used in the synthesis of vinyl acetate to prepare vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases . The vinyl acetate preparation is generally carried out by passing acetic acid, ethylene and oxygen or oxygen containing gases at temperatures of 100 to 220 ° C, preferably 120 to 200 ° C, and pressures of 1 to 25 bar, preferably from 1 to 20 bar, on the finished catalyst, with the unreacted components being able to be circulated. The oxygen concentration is advantageously maintained below 10% by volume (based on the gas mixture without acetic acid). However, under some circumstances, dilution with inert gases such as nitrogen or carbon dioxide is advantageous. Carbon dioxide is particularly suitable for dilution because it is formed in small amounts during the reaction. In the synthesis of vinyl acetate (VAM), it has been found that the supported catalysts used for the synthesis of ethylene, acetic acid and oxygen preferably consist of Pd and an alkali metal, preferably K. As additional additives, they are used successfully. Cd, Au or Ba. The metal salts can be applied to the support by staggering, spraying, vapor deposition, immersion or precipitation. In the case of Pd / Au / K catalysts, it has been discovered that it is advantageous to apply the two noble metals in the form of a cover on the support, ie the noble metals are present only in a zone close to the surface while the regions further inside the support body are virtually free of noble metals. The thickness of these catalytically active coatings is approximately 0.1-2 mm. The process can be carried out more selectively using coated catalysts than when catalysts are used in which the support particles are impregnated just to the core ("fully impregnated"), or the capacity can be increased.

In the latter case, it is possible to maintain the reaction conditions unchanged compared to fully impregnated catalysts and produce more vinyl acetate per reactor volume and unit of time. This makes the accumulation of the obtained crude vinyl acetate easier, because the content of vinyl acetate in the gas leaving the reactor is higher, which also leads to an energy saving in the treatment section. Suitable treatments are described, for example in US 5 066 365, DE 34 22 575, DE 34 08 239, DE 29 45 913, DE 26 10 624, US 3 840 590. If, on the other hand, the plant capacity is the reaction temperature can be decreased and as a result the reaction can be carried out more selectively to the same total yield, thus saving starting materials. In this case, the amount of carbon dioxide that is formed as a byproduct and therefore must be removed as well as the loss of ethylene that goes in associated with this removal are also reduced. Additionally, this procedure leads to a lengthening of the operating life of the catalyst. For this reaction, the invention provides a one-step process for producing supported catalysts coated with sol by generating the sols in situ from the pores of the support by reduction, ie the preparation of the sol and fixation to the support are carried out simultaneously in one step. In this way, the cover thickness can be more easily matched to the requirements, in particular larger cover thicknesses are possible than in the case of the sun-impregnation technique in which the diffusion of the sun from outside to the pores of the Support is also obstructed by the mechanical effect. Additionally, higher noble metal charges on the support are possible, procedural steps are saved and intensive energy treatment with highly diluted solutions is highly avoided. If desired, in the case of well-defined pore structures, in the support, the colloid size can be precisely fixed through the pore size of the support, so that monomodal distributions of colloids can be obtained more easily. simple. The apparatus and meticulously clean solvents (distilled water twice) required for the preparation of sols become completely unnecessary in the preparation in situ. Impurities in the sols lead to larger particle sizes and particle agglomeration. The type filler covered with metal salts in the prepreg by known techniques and the rapid removal of water during drying, for example under reduced pressure, in addition to the reduction method of the invention promote the formation of covers or allow an additional reduction in the thickness of the cover, if this is desired. The catalysts obtained according to the invention have a significantly uniform active metal distribution and a higher noble metal dispersion than the VAM catalysts produced in a conventional manner. The high dispersion is also greatly maintained in long-term operation due to the reduced agglomeration of the noble metal particles, as a result of which the deactivation of the catalysts obtained according to the invention is slower and long operating lives are obtained. The production methods of the invention advantageously lead to an essentially monomodal distribution and very narrow particle size. Additionally, the average noble metal particle diameters are significantly smaller than in the case of conventional catalysts. This results in a highly active metal surface area and thus a high catalytic activity. The following examples serve to explain and illustrate the invention without being restricted to them.

EXAMPLE 1 200 g of supports of SiO2 and (Siliperl AF125, Engelhard) having a BET surface area of 300 m2 / g were sprayed discontinuously at a temperature of 30-32 ° C with a 3.33 g hydrochloric acid solution (18.8 mmoles) of palladium chloride and 1.85 g ( 4.7 mmole) of auric acid in 500 ml of water over a period of 35 minutes in a coating unit. The support spheres were subsequently dried and sprayed with 20 g of tripotassium citrate hydrate dissolved in 200 ml of water over a period of 25 minutes. At a drum rotation speed of 10 rpm the spraying was carried out discontinuously at 1 bar. The inlet temperature (hot air temperature) was 60 ° C and the product temperature was 32-30 ° C. This gave a homogenously impregnated coated catalyst having a cover thickness of 400 μm. The diameter of the nano-sized particles was determined by means of TEM. The average particle diameter is 30 nm.

EXAMPLE 2 g of the same support as in Example 1 were impregnated with a solution of 335 mg of palladium chloride and 186 mg of auric acid by staggering and dried. At 65 ° C, the support was impregnated with 1.52 g of trisodium citrate dihydrate in 19.6 ml of water and, after being allowed to stand for 3 hours at 65 ° C, it was dried. After cutting through a representative number of pellets, the cover thickness was measured by optical microscopy and XPS line sweepers. The thickness of the cover is 1 mm. The diameter of the nano-sized particles was determined by means of TEM. The average particle diameter is 40 nm.

EXAMPLE 3 g of SiO2 supports (Aerosil 200, Degussa) were impregnated with a solution (19.6 ml) of 325 mg (1.89 mmoles) of chloride on palladium and 189 mg (0.473 mmoles) of auric acid by staggering and dried. The supports were moistened with 19 ml of water by staggering and impregnated with 1.68 g of tripotassium citrate in 10 ml of water and dried. After cutting through a representative number of pellets, the cover thickness was measured by means of SEM. The cover thickness is 140 μm. The diameter of the nano-sized particles was determined by means of TEM. The average particle diameter is 60 nm.

EXAMPLE 4 Palladium chloride (335 mg) and auric acid (186 mg) were dissolved in water (19.6 ml) and applied by staging to supports Siliperl AF 125 SiO2 (10.0 g). The supports were dried and impregnated with an aqueous solution of potassium formate (0.5 g) and sodium sulfanilate. (0.2 g) and dried again. The Pd / Au ratio is Pd: Au = 8: 2.

EXAMPLE 5 186 mg of auric acid were applied to Siliperl AF 125 by staggering, the supports were dried at 120 ° C and reduced by means of citrate solution (19.6 ml). After 12 hours, the dark gray spheres were dried and subsequently impregnated with 16 ml of an acetic acid solution of palladium acetate (424 mg, 1.89 mmol) at 60 ° C. The drying was carried out in a vacuum drying oven where the Pd salt was thermally reduced to 120 ° C. The diameter of the nano-sized particles was determined by means of TEM. The average particle diameter is 20 nm.

EXAMPLE 6 325 mg of palladium chloride and 186 mg of auric acid were dissolved in water. 20 g of supports (Siliperl AF 125) were placed in a round bottom container (250 ml) and impregnated with a solution of auric acid and palladium chloride (19.6 ml). The supports were subsequently dried for 4 hours at 120 ° C. They were impregnated with a viscous solution of potassium citrate (10 ml), stirred and dried again. The cover had a thickness of 75 μm and is black.

EXAMPLE 7 Auric acid (189 mg, 0.473 mmol) and palladium chloride (325 mg, 1.89 mmol) were dissolved in water. The SiO2 support (20.0 g, type D11-10, BASF) was impregnated with the solution and subsequently dried for 5 hours at 120 ° C. After cooling, water (A = 16 ml, D = 19 ml) was added to the various supports in order to finally allow a solution of potassium citrate (1.68 g in 10 ml) to diffuse. During the addition of water, the support color changed from beige to white. The support was dried for 5 hours at 120 ° C.

Reactor tests: The catalysts produced in the examples and comparative examples are tested in a tubular microreactor of the fixed bed having a capacity of 36 ml. The gases are dosed through MFCs, the acetic acid is dosed using an LFC (Bronkhorst). The gases and acetic acid are mixed in a gas mixture tube loaded with packing elements. The reactor outlet is left at atmospheric pressure and passed through a glass condenser. The condensed material collected is analyzed offline using GC. The non-condensable gases are determined quantitatively by in-line GC.

Before the measurement, the catalyst is activated in the reactor as follows: The catalyst is heated from 25 ° C to 155 ° C under N2 at atmospheric pressure. At the same time, the gas temperature increases until 150 ° C, and the gas mixing temperature increases to 160 ° C. The conditions are maintained for some time. Subsequently, ethylene is supplied and the pressure is increased to 10 bar. After a while, acetic acid is dosed and the conditions are maintained for some time. After activation, the catalyst is run and measured as follows: Oxygen is added downstream of the gas mixing tube and the oxygen concentration is increased stepwise to 4.8% by volume (first measurement) and then to 5.2% by volume (second measurement). Care should always be taken to ensure that the explosion limits of the ethylene / O2 ignitable mixture are not exceeded. At the same time, the reactor temperature increases to 170 ° C. The reaction is monitored continuously using gas chromatography. When the reaction is proceeding in a fixed state, ie with a constant reactor temperature and constant concentrations of vinyl acetate and CO2 in the product gas stream, sampling is initiated. During a period of 1 hour, a liquid sample and a number of gas samples are taken. The product gas flow is determined using a gas meter. After the test is complete, the oxygen concentration is first decreased stepwise. The compositions of the catalysts used are shown in table 1. The obtained reactor results are shown in the table TABLE 1 Catalyst data TABLE 2 Catalyst performance in the microreactor Advantages and additional embodiments of the invention can be derived from the following claims.

Claims (27)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for producing a catalyst comprising one or more metals selected from the group of metals comprising the transition groups Ib and Vlllb of the periodic table of the elements on porous support particles, which comprises, in a first step, applying at least one solution of at least one metal compound of the group of metals of transition groups Ib and Vlllb of the periodic table, with the exception of ruthenium, as one or more precursor (s) of the group of those metals to a porous support and, in a second step, treating the porous support to which at least one precursor has been applied with at least one reducing agent in such a way that a sol is produced in the pores of the support by reduction of the metal compound and is fixed to the support in this step.

2. The process according to claim 1, wherein an inert, porous, preferably nanoporous support, consisting of silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, oxide mixtures of the aforementioned compounds , mixed oxides of the mentioned compounds and / or aluminum silicates in the form of powders, sheets, strips, membranes, rods, plates, tablets, wagon wheels, monoliths, spheres, chips, rings, solid extruded materials, hollow extruded materials or Star is used.

3. The process according to claim 1 or 2, wherein the metal compound (s) is / are applied to the porous support by staggering, spraying, dipping, impregnation, spray drying, high coating or coating. fluidized bed, preferably by impregnation.

4. The method according to one or more of the preceding claims, wherein one or more compound (s) of metals selected from the group consisting of copper, silver, gold, iron, cobalt, nickel, ruthenium, rhodium, osmium , iridium, palladium and platinum is / are applied to the support.

5. The method according to one or more of the preceding claims, wherein one or more palladium compound (s) alone or one or more palladium compound (s) together with one or more selected metal compound (s). of the group consisting of copper, silver, gold, iron, cobalt, nickel, rhodium, osmium, iridium and platinum is / are applied to the porous support.

6. The method according to one or more of the preceding claims, wherein one or more palladium compound (s) together with one or more gold compound (s) are applied to the porous support.

7. The process according to one or more of the preceding claims, wherein one or more reducing agents selected from the group consisting of citrates such as potassium citrate, sodium citrate, ammonium citrate; hydrazine, hydroxylamine, sodium hypophosphite, alkali metal borohydrides such as sodium borohydride, potassium borohydride; gaseous reducing agents such as hydrogen, carbon monoxide; formaldehyde, formats, acetates, oxalates, sulphanilates such as sodium hydroxymetansulfinate; and monohydric or dihydric alcohols such as ethanol, ethylene glycol; are used.

8. The process according to one or more of the preceding claims, wherein the reducing agent used is potassium citrate, sodium citrate and / or ammonium citrate.

9. The process according to one or more of the preceding claims, wherein a solution of the metal compound (s) is applied to the porous support, preferably nanoporous.

10. The process according to claim 9, wherein an aqueous solution, a solution in an organic solvent or a mixture thereof is applied to the support.

11. The process according to claim 10, wherein the water is used as a solvent.

12. The process according to claim 10, wherein methanol, ethanol, ethylene glycol, N-methylpyrrolidone, dimethylformamide, dimethylacetamide and / or tetrahydrofuran or a mixture of one or more of these substances with water is used as an organic solvent.

13. The process according to one or more of the preceding claims 9 to 12, in which soluble palladium compounds, in particular water-soluble salts, selected from the group of Pd precursors consisting of palladium (II) acetate, palladium chloride ( II), palladium (II) nitrate and sodium tetrachloropaladalate (II) [Na2PdCI] are used.

14. The process according to one or more of the preceding claims 9 to 13, in which soluble metal compounds, in particular water-soluble salts, selected from the group of metal precursors consisting of tetrachloroauric acid (III), gold acetate (lll) [Au (OAc) 3], potassium aurate [KAuO2], hexachloroplatinic acid hydrate (IV), hexachlorohydric acid hydrate (IV) and rhodium chloride hydrate (III) are used.

15. The method according to one or more of the preceding claims, wherein the application of the precursor (s) to the porous support, preferably nanoporous, and / or the reduction of the support to which the precursor (s) is / has Applied is / are carried out in the presence of a colloid stabilizer or a plurality of colloid stabilizers.

16. The method according to claim 15, wherein one or more compound (s) selected from the group consisting of betaines, PVP, citrates, oxalates, formats, acetates, sulfanilates, PVA and PAA is / are added as a stabilizer colloid.

17. - The method according to claim 15 or 16, in which use is made of one or more compounds which act simultaneously as a colloid stabilizer and as a reducing agent.

18. The process according to claim 17, wherein potassium citrate, sodium citrate and / or ammonium citrate is / are used as reducing agent and colloid stabilizer.

19. The method according to one or more of the preceding claims, in which the first and second steps are carried out successively.

20. The process according to claim 19, wherein the porous, preferably nanoporous support to which at least one metal compound has been applied is subjected to a drying step before reduction.

21. The method according to one or more of the preceding claims 1 to 18, wherein the first and second steps are carried out in a single container process without isolation, purification or drying of the porous support, preferably nanoporous, to which the precursor (s) has / have been applied.

22. The method according to one or more of the preceding claims, wherein the application of the precursor (s) and / or the reduction is / are carried out in such a way that the metal compounds are reduced in the pores of the support in a shell-like area near the surface to give the corresponding metals or alloys in the form of stabilized or unstabilized nanometer-sized particles to produce a coated catalyst.

23. The process according to claim 22, wherein a cover thickness in the range of 5 μm to 5000 μm is obtained.

24. The method according to one or more of the preceding claims, in which the catalysts having metal particles and / or alloy particles which have an average particle diameter in the range of 1 to 100 nm in the pores and / or cover are obtained.

25. The method according to one or more of the preceding claims, wherein one or more activators and / or promoters is / are applied after, before or during the application of the precursor (s) and / or the reduction.

26. The method according to one or more of the preceding claims, wherein the reduction in situ is carried out at temperatures of room temperature to 150 ° C.

27. The use of the catalyst obtainable according to one or more of the preceding claims 22-26 for preparing vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases.