WO1999008785A2

WO1999008785A2 - Nano-porous aluminium oxide membranes containing noble metal clusters or noble metal nanoparticles

Info

Publication number: WO1999008785A2
Application number: PCT/EP1998/004863
Authority: WO
Inventors: Alfred Hagemeyer; Harald Werner; Uwe Dingerdissen; Klaus KÜHLEIN; Günter Schmid
Original assignee: Celanese Chemicals Europe Gmbh
Priority date: 1997-08-13
Filing date: 1998-08-05
Publication date: 1999-02-25
Also published as: WO1999008785A3; DE19734973A1

Abstract

The invention relates to nano-porous aluminium oxide membranes containing noble metal clusters or noble metal nanoparticles, which membranes were obtained by anodic oxidation and have a close pore radius distribution. Production takes place by coating the membranes with a colloidal solution of ligand-stabiliized noble metal nanoparticles or clusters, by drying and then burning them in the presence of an oxygen-containing gas. The aluminium oxide membranes have an average pore radius of 1-500nm with a standard deviation of the pore radius distribution of 0-20 %, preferably 0-10 %. The aluminium oxide membranes containing noble metal clusters or nanoparticles are used to particular advantage as catalysts during synthesis in the presence of vinyl acetate.

Description

Nano-porous aluminum oxide membranes containing precious metal clusters

description

The invention relates to precious metal supported catalysts, namely nano-porous aluminum oxide membranes containing precious metal clusters, processes for their production and the use thereof, in particular as a supported catalyst for vinyl acetate synthesis.

Precious metal catalysts have long been known per se and are widely used for numerous technical heterogeneously catalyzed oxidation and hydrogenation reactions. Conventionally, the supported catalysts are produced by applying metal salt solutions (for example chlorides, nitrates, acetates) to the (for example oxide-ceramic) support and then reducing them with chemical reducing agents to the metals in oxidation state 0. The noble metal atoms agglomerate to ia non-uniform precious metal particles with a broad particle size distribution. The diameters of the nanoparticles often range from 1 to 100 nm. The non-uniform size distribution of the catalytically active component has an unfavorable effect on the activity and selectivity of the catalysts. The long-term behavior ("service life") of the catalysts is often unsatisfactory, since the nanoparticles are mobile on the support and grow together over time through sintering to form larger aggregates with little activity, so that the catalysts deactivate relatively quickly.

An improvement in the production of precious metal supported catalysts was represented by the brine impregnation technology, in which (by chemical reduction of metal salt solutions) pre-formed nanoparticles (oxidation level 0) are drawn onto the support from colloidal solution. To avoid agglomeration of the nanoparticles in solution and their precipitation, the nanoparticles are stabilized with ligands. Each nanoparticle is surrounded by a protective ligand that prevents direct contact between the metal nuclei and thereby keeps the nanoparticles in solution. A major advantage of the sol coating technology is that the precious metal components are essentially already in a reduced state after the sol has been applied to the carrier. This eliminates a reduction in the active metals at high temperatures, which generally causes the noble metals to sinter together and thereby reduces the catalytic surface.

The prior art teaches methods for producing heterogeneous catalysts which are characterized in that nanoparticles of one or more catalytically active metals are first produced in a separate process step via a sol process and these particles are then immobilized on a support. The general advantage of the sol process is the achievable dispersity of the particles and the narrow distribution of the particle diameters.

General representations of this method can be found among others in (a) B.C. Gates, L.Guzci, H.Knözinger, Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986; (b) J.S. Bradley in Clusters and Colloids, VCH, Weinheim 1994, pp. 459-544.

In nanoparticle-based syntheses of heterogeneous catalysts, so-called stabilizers or ligands or are used to stabilize the metal particles

Protective colloids, especially if you need processable brine with a metal concentration of 0.1% or higher. Stabilizers or protective colloids envelop the metal particles. The ligands can be both neutral and have an electrical charge. This prevents the particles from clumping together.

Examples of low molecular weight stabilizers include oxygen-, phosphorus-, sulfur- or nitrogen-containing ligands as well as cationic, anionic, betaine or non-ionic surfactants. Polyacrylic acid, polyvinyl alcohol or poly (N-vinylpyrrolidone), inter alia, have been used as polymeric protective colloids. Process for the production of hydrosols and organosols of, for example, palladium and Gold has been described several times, as have heterogeneous catalysts produced from it: YLLam and M. Boudart, Journal of Catalysis 1977, 50, 530-540 describe a synthesis of Pd / Au particles of 2-4.5 nanometers, in which one starts from ( Au (en) ₂ ) ³⁺ and (Pd (NH ₃ ) ₄ )) ²⁺ salts are first linked by ion exchange with the acidic groups of one

Manufactures silica carrier and then reduces the fixed metal salts.

J.B. Michel and J.T. Schwarz in Catalyst Preparation Science IV, Elsevier Science Publishers, New York 1987, 669-687 describe the sequential reduction or coreduction of palladium and gold salts using sodium citrate as a reducing agent and at the same time as a stabilizer. Palladium-on-gold or gold-on-palladium particles or Pd / Au alloy particles were obtained. The colloids obtained were immobilized on carbon carriers.

G. Schmid et al. Chem. Eur. J. 1996, 2, 1099-1103 describe bowl-shaped, bimetallic Pd / Au colloids in the size range of 20-56 nanometers, which were produced using the seed growth method. Sulfonated triphenylphosphane and sodium sulfanilate ligands were used as stabilizers. The particles obtained were immobilized on titanium dioxide supports.

EPA 0 672 765 claims the electrochemical production of i.a. Pd / Au sols using cationic, anionic, nonionic or betaine stabilizers. In this process, the metal salts are reduced on the cathode of an undivided electrolysis cell. The betaine-stabilized brines described there require an approximately 5-fold molar excess of stabilizer, based on the metal salt and the use of organic solvents.

DE 4443 701 claims shell catalysts which are said to be suitable as heterogeneous catalysts. The particles are deposited in an outer shell of the carrier grain that is up to 200 nanometers thick. A process for their production by means of a cation-stabilized hydrosol is also claimed. DE-A-195 00 366 describes the production of Pd shell catalysts for Hydrogenation by applying the Pd as a highly diluted sol by impregnation or spraying on a support, resulting in a shell thickness of less than 5000 nanometers. PVP is used as a stabilizer.

If metal particles and alloys of relatively uniform composition and narrow size distribution can also be applied to supports using the sol impregnation technique, the sol-impregnated catalysts often have a lower activity than the conventional catalysts. Sometimes this can be compensated for by a greatly increased metal load.

The cause of the loss of activity lies in the too tightly adhering ligand shell, by which the nanoparticles are still surrounded after the carrier fixation and which block the active centers on the metal surface. Attempts to remove the disruptive ligand shell by extraction with solvents were unsuccessful, since large parts of the nanoparticles were washed off the support again and the ligand shell could only be removed incompletely. Only a part of the ligand molecule can be removed by hydrolysis of the ligands, the head group of the ligand adhering to the metal surface remains on the nanoparticle. If the ligand shell is oxidized with oxygen, air or ozone, the nanoparticles sinter at the necessary high combustion temperatures to form larger aggregates with a non-uniform size distribution, so that the original advantage of the sol impregnation technology is nullified.

Although a whole series of noble metal catalysts on a wide variety of supports are already known, there is still a need for improved noble metal supported catalysts which can be used in a variety of ways and in particular can be used for the synthesis of vinyl acetate.

The object of the invention is to provide supported noble metal catalysts which are particularly suitable for oxidation and hydrogenation reactions, which have ligand-free nanoparticles with a uniform composition and narrow particle size distribution and which are characterized by high activities and Characterize selectivity and which are particularly stable against the effects of higher temperatures and are characterized by long-term stability.

This object is achieved by noble metal clusters or nanoporous aluminum oxide membranes containing noble metal nanoparticles with a narrow pore radius distribution. The average pore radius of the nanoporous aluminum oxide membranes is preferably 1-500 nm and preferably has a standard deviation of the pore radius distribution of 0-20, preferably 0-10%. The noble metal clusters or noble metal nanoparticles advantageously consist of palladium, platinum or binary metal alloys such as palladium-gold alloys.

The invention further relates to a process for the production of noble metal clusters or nanoporous aluminum oxide membranes containing noble metal nanoparticles, which is characterized in that nanoporous alumina membranes obtained with anodic oxidation and having a narrow pore radius distribution are coated with a colloidal solution of ligands, stabilized noble metal nanoparticles or clusters, dried and then in Calcined in the presence of an oxygen-containing gas. The caicination advantageously takes place in the presence of air. In a further advantageous process according to the invention, the caicination is carried out in the presence of pure oxygen. Another advantageous embodiment involves calcining in the presence of ozone. The caicination is preferably carried out at temperatures of 50-1200, in particular at 100-1000, particularly advantageously at temperatures of 200-1000X. It is advantageous to use aluminum oxide membranes with an average pore radius of 1-500nm and a standard deviation of

Pore radius distribution from 0-20%, preferably from 0-10% to use. It is advantageous to carry out the coating with monodisperse nanoparticles or clusters. Palladium, platinum, binary noble metal alloys such as palladium-gold alloys are particularly suitable as noble metals in the context of the invention.

Another object of the invention is the use of palladium-gold noble metal clusters or gold-palladium nanoparticles containing nanoporous Aluminum oxide membranes as a catalyst for the synthesis of vinyl acetate

Nanoporous Alumina (Alumιna) membranes with very narrow ^r pore distribution can be produced by anodic oxidation of Aluminiurnoberfiachen in acidic particular diprotic acids such as sulfuric acid, oxalic acid or tπprotischen acids such as phosphoric acid at temperatures around 0 ° C Details of the Membranhersteiluπg found in JW Diggle et al Chem REv 69 (1969) 365, MM Lohrengel, Mater Science and Engineering 6 (1993) 241, JP O'Sullivan et al, Proceediπgs of the Royal Society of London 317 (1970) 511, H Masuda et al, Science 268 (1995 ) 1466, C Martin et al, Science 266 (1994) 1961, H Masuda et al, Thin Solid Films (1993) 1, H Moskovits et al, IEEE Transitioπ oπ Electron Devices 43 (1996) 1646 In general, a sheet of highly pure Aluminum the anode in an electrochemical cell. The anodizing takes place under precise potential and current control. The pore diameter in the membrane depends directly on the applied voltage The result is a pore size of 1.4 nm per volt. The pore density can vary between 10 ⁹ to 10 ¹² / cm ^2. An atomic force microscopy image of a planar section through a membrane formed at 40 V is shown in FIG. 1

It is clear from Fig. 1 that the uniform pores, like almost parallel channels, extend deep into the membranes

The variation of the pore size ranges from 1 to 500 nm. The thickness of the membrane depends on the anodizing time and can be up to about 500 mm or more, for example 600 - 1000 mm. Such nanoporous alumina films show excellent thermal properties The resulting polymorphic starting material produces over 300 ° C g and over 800 ° C aluminum oxide. The pore diameter widens somewhat, while the size of the membrane shrinks somewhat, so that the overall porosity increases

The surfaces of the aluminum oxide membranes can be chemically modified because they are covered with OH groups. This devatization can be used

CORRECTED SHEET (RULE 91) ISA / EP for example to make the surface hydrophobic.

Aluminum membranes can be used in various forms: a) with a closed and an open side b) with an open and semi-open side, i.e. smaller pore openings and c) bilateral pore opening.

Which modifications you choose depends on the intended reuse. The closed or semi-open side facing the aluminum is called the barrier side, the other, open, solution side.

Metal clusters can be inserted into the pores in various ways.

a) Vacuum induction Metal clusters and colloids can be brought into the pores from solution by applying a vacuum to one side of the membrane. For this purpose, membrane type b) is used so that the particles get stuck in the narrowed 1-2nm pores and do not leave the pore with the solution.

b) immersion process

When a membrane is immersed in a cluster solution, it is drawn into the pores by capillary forces. Depending on the ligand shell of the cluster, organic or aqueous solutions can be used as solvents. Membranes with narrow pores (<10nm) will absorb more clusters than those with larger pores. It is advantageous to evacuate the pores beforehand.

c) Electrophoresis The prerequisite is that the clusters used move in an electric field, which was observed in all cases examined. One side of the membrane is contacted with a metal. It can original aluminum. If the membrane has previously been detached from the aluminum, one side can subsequently be steamed or sputtered with metal. The method also works in the presence of the

Barrier layer. Fig. 2 schematically shows the arrangement for the electrophoretic filling of pores.

With the help of methods 2a-c it is possible to bring soluble clusters into membrane-like, nanoporous aluminum oxide.

The following palladium and platinum clusters were used to fill the aluminum membranes:

1.8nm Pt cluster; Ligand shell made of phen ^* and oxygen according to G. Schmid, B. Morun, J.-O. Malm, Angew. Chem. 101 (1989) 772; Appl. Chem. Int. Ed. Engl. 28 (1989) 778. 2.2nm, 3.0nm and 3.6nm Pd clusters; Ligand shell made of 1, 10-phenanthroline and

Oxygen, or from (-) - chinchonidine and oxygen according to G.Schmid, M.Harms, J.-O.Malm, J.-O.Bovin, J.v. Ruitenbeck, H.W. Zandbergen, Wen T.Fu, J. Am. Chem. Soc. 115 (1993) 2046; G. Schmid, S. Emde, V. Maihack, W. Meyer-Zaika, St. Peschel, J. Mol Catal. A 107 (1996) 95; G. Schmid, V. Maihack, F. Lantermann, St. Peschel, J. Chem. Soc. Dalton Trans. (1996) 589.

After filling it is dried and then in the presence of oxygen e.g. calcined in an oven. However, the special possibilities of these clusters filled with clusters are that they can be heated to temperatures of up to 1000 ° C. The clusters, which have lost their ligand shell due to the oxidation, combine to form larger particles, the diameter of which is determined by the pore size. In this way it is possible to produce catalytically active metal particles of a well-defined size in the carrier material. Since the pore size e.g. can be varied between 1 and 500nm, there is a wide range of precisely adjustable particle sizes.

Fig. 3 shows 18-20nm created from originally 1.5nm clusters Particles.

These noble metal supported catalysts with a uniform particle size of the noble metal nanoparticles can optionally be loaded with further promoters and dopants and can be used for oxidation and hydrogenation reactions. By

Clusters of different metals can be filled in during thermolysis e.g. also produce bimetallic particles.

Pd-Au bimetal supported catalysts produced by the method according to the invention are particularly suitable for the synthesis of vinyl acetate by gas phase oxidation of ethylene and acetic acid.

The catalytic conversion of ethylene, oxygen and acetic acid to vinyl acetate, the starting monomer of an economically important group of polymers, is carried out on an industrial scale in tube bundle reactors in the gas phase. The heterogeneous catalysts used for this purpose contain as a catalytically active component palladium and possibly gold-containing particles which are immobilized on an inert carrier material.

In general, heterogeneous catalysts consist of an inert, porous support material such as moldings, bulk material or powder and the catalytically active components which are located on the surface and in the pores of the support material. When producing such catalysts, it is important to form as many active centers as possible, i.e. to apply the catalytically active components in fine distribution to the support and to anchor them as firmly as possible on the outer and inner surface of the support at the locations which are accessible to the reactants.

The space-time yield and the lifetime of Pd / Au catalysts exert a strong influence on the economy of the vinyl acetate process. However, the currently best catalysts meet the requirements. of sales, selectivity and long-term behavior are insufficient. For example, in most cases sales are only around 10%. In addition, the Cost of the catalyst itself, which is determined by the use of large quantities of the expensive precious metals palladium and gold.

In the previously known processes for the preparation of catalysts for the vinyl acetate process, the reactive centers are produced by applying a solution of compounds of the catalytically active components to the support, for example by impregnation with a solution of salts of the metals in question. The compounds on the support are then converted into the catalytically active components by one or more chemical steps, for example by precipitation and reduction.

An important reason for the high need for precious metals is based on the impregnation process which has been predominantly used up to now, and certain disadvantageous phenomena can occur, as will be explained below.

It can often be observed that during the chemical conversion of the palladium and gold compounds on the carrier, the particle diameters that can be achieved are above 10-20 nanometers. There is usually a relatively broad distribution of particle diameters of around 5 to 100 nanometers. The formation of larger metal particles of a few 100 to 200 nanometers is particularly undesirable. This results in a reduction in the catalytic activity infoige the reduction in the specific metal surface. This can be illustrated by the following example: a cluster, simplified as a cube, which consists of atoms of a metal with an assumed diameter of 0.25 nanometers, contains approximately 87% with an edge length of 1 nanometer, with one

Edge length of 2.5 nanometers 49% and with an edge length of 10 nanometers 0.14% surface atoms.

Another problem arises from the difficulty of depositing the palladium and the gold in a homogeneous distribution on the support. In practice, the carrier often contains gold-rich domains next to districts with a balanced Pd / Au ratio. An uneven distribution is one of the possible causes during the loading process and a different behavior during the fixing process.

It is also known that supported noble metal catalysts under the usual operating conditions, i.e. loss of activity at temperatures of around 150 to 170 ° C for longer periods of operation. This loss is due among other things that the metal particles migrate on the support surface and can unite with other particles, i.e. recrystallize to form larger particles, resulting in a reduction in specific surface area. The smaller the metal particles, the more pronounced this effect. Since corresponding observations have also been made with vinyl acetate catalysts, it is desirable to reduce the rate of migration of the palladium-gold particles by embedding them in a microenvironment which interacts more intensely with the support.

In order to compensate for the loss of activity caused by the reasons outlined above, in practice it has been forced to increase the exposure of the wearer to precious metals accordingly. However, there are limits to the increase in the precious metal content, since the metal particles agglomerate all the more easily with higher loads.

The task was therefore to develop new Pd / Au catalysts with improved properties. their particle size, particle distribution, composition and microstructure and improved service life (long-term behavior).

The use of conventional brine watering technology cannot solve the problems mentioned satisfactorily. The low molecular weight or polymeric compounds previously used as stabilizers or protective colloids for the preformed nanoparticles have various disadvantages. The ligand stabilizers impair, for example, due to the strong interactions of their donor groups with the active metal centers catalytic interactions. For the same reason, it is difficult to separate them from the metal core after they have been applied to a carrier. Polymeric protective colloids can impair the catalytic activity of the metal particles, so that their removal after fixation is desirable. In many cases this does not succeed or only incompletely.

The disadvantages mentioned are overcome by the present invention. According to the method of the invention, bimetallic supported Pd / Au catalysts with improved properties can be obtained. their particle size, particle distribution, composition and microstructure and improved service life (long-term behavior).

Before, during and / or after the fixing of the brine and / or the caicination, the carrier can also be combined with further activators, in particular alkali acetates, preferably K acetate and optionally promoters, for example Zr, Ti, Cd, Ba, Cu connections.

The metal contents of the finished catalysts have the following values:

The Pd content of the Pd / Au catalysts is generally 0.5 to 2.0% by weight, preferably 0.6 to 1.5% by weight. The Au content of the Pd / Au catalysts is generally 0.2 to 1.0% by weight, preferably 0.3 to 0.8% by weight.

In addition to Pd and Au, a preferred catalyst system also contains K acetate as an activator. The K content is generally 0.5 to 4.0% by weight, preferably 1.5 to 3.0% by weight.

The vinyl acetate is generally prepared by passing gases containing acetic acid, ethylene and oxygen or oxygen at from 100 to 220 ° C., preferably from 120 to 200 ° C., and from 1 to 25 bar, preferably from 1 to 20 bar, over the finished catalyst, whereby unreacted components can be circulated. The oxygen concentration is expediently kept below 10% by volume (based on the acetic acid-free Gas mixture). However, dilution with inert substances is also possible _. Gases such as nitrogen or carbon dioxide are beneficial. Carbon dioxide is particularly suitable for dilution since it is formed in small quantities during the reaction.

Claims

claims

1. Noble metal clusters or nano-porous aluminum oxide membranes containing noble metal nanoparticles with a narrow pore radius distribution.

2. Noble metal clusters or noble metal nanoparticles containing nano-porous aluminum oxide membranes according to claim 1, characterized in that the mean pore radius 1-500nm and a standard deviation of the pore radius distribution of 0-20%, preferably 0-10%.

3. Noble metal clusters or noble metal nanoparticles containing nano-porous aluminum oxide membranes according to claim 1 or 2, characterized in that the clusters or nanoparticles contain platinum, palladium or palladium and gold as the noble metal.

4. A process for the production of noble metal clusters or nano-porous alumina membranes containing noble metal nanoparticles, characterized in that nano-porous alumina membranes obtained with anodic oxidation and having a narrow pore radius distribution with a colloidal solution of ligand-stabilized noble metal nanoparticles or

Cluster coated, dried and then calcined in the presence of an oxygen-containing gas.

5. The method according to claim 4, characterized in that one carries out the caicination in the presence of air.

6. The method according to claim 4, characterized in that one carries out the caicination in the presence of pure oxygen.

7. The method according to claim 4, characterized in that one carries out the caicination in the presence of ozone.

8. The method according to at least one of claims 4 to 7, characterized characterized in that the caicination is carried out at temperatures of 50-1200X, preferably at 100-1000 ° C.

9. The method according to claim 8, characterized in that one carries out the caicination at 200 to 1000 ° C.

10. The method according to at least one of claims 4 to 9, characterized in that one uses aluminum oxide membranes with an average pore radius of 1-500nm and a standard deviation of the pore radius distribution of 0-20%, preferably of 0-10%.

11. The method according to at least one of claims 4 to 10, characterized in that platinum, palladium or gold and palladium are used as the noble metal.

12. The method according to at least one of claims 4 to 11, characterized in that monodisperse, colloidal solutions are used.

13. Use of the nano-porous aluminum oxide membranes containing noble metal clusters or noble metal nanoparticles according to one of claims 1 to 4 or produced by a process according to one of claims 4 to 12 as a catalyst in the synthesis of vinyl acetate.