Palladium clusters and their use as catalysts

Details Palladium clusters and their use as catalysts Palladium clusters and their use as catalysts

1998-11-23

2001-03-06

Barts, Samuel (Department: 1621)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Metal, metal oxide or metal hydroxide

C502S339000, C502S344000, C585S275000, C252S301500, C556S016000, C556S136000

Reexamination Certificate

active

06197720

ABSTRACT:

The invention relates to colloidal palladium composed of palladium clusters, a rocess for its preparation, its use, colloidal palladium-containing heterogeneous atalysts, a process for their preparation, and their use.
Metal colloids are systems in which metal particles with a diameter of the order of about 1 nm to 1 &mgr; are present. The extremely fine-particle metal itself is referred to as colloidal metal. It may be in undiluted form, dispersed in a continuous phase or adsorbed at a phase boundary. A dispersion thereof in a solvent is referred to as a colloidal solution of the metal.
By metal clusters are meant metal particles which consist of only a few, up to a few thousand, metal atoms and are at the lower end of the abovementioned scale of sizes for colloidal metal.
The preparation of metal colloids has been known for a long time. Normally, metal salts are reduced in solution in the presence of stabilizers to the metal. The stabilizers are substances which are able to coordinate with the metal and thus protect it from agglomeration. It is also known that properties such as size and size distribution of the formed colloidal particles are influenced inter alia by the choice of the reducing agent and of the protective ligand, the ratio of the protective ligand to the metal ion, the solvent and the anion present in the metal salt. However, control of the particle size is not possible because of the complex mechanism of formation; on the contrary, reliance on empirical optimization is necessary.
Metal colloids from noble metals such as palladium are employed on a large scale as catalysts. For these, particularly small particle sizes are desirable because for the same amount of catalyst the catalyst surface area available increases in inverse proportion to the particle diameter. The activity of the catalysts is therefore usually directly related to the size of the catalytically active metal particles.
A number of reactions catalyzed by metal colloids also display the phenomenon of structural sensitivity, i.e. the selectivity of the reaction is related to the size of the catalytically active particles. In order to be able to exploit this effect, the catalyst particles must show a narrow size distribution.
DE-C 44 12 463 describes the preparation of colloidal solutions of palladium by reducing palladium salts with a number of reducing agents such as phosphites, hypophosphites, boranes, ascorbic acid, hydrazine and formaldehyde in the presence of protective colloids, employing as protective colloids polymers such as polyvinylpyrrolidone, poyvinylpyridine, polyvinyl methyl ketone, polyvinyl alcohol, polyvinyl acetate, polyacrylate, alkyl- and hydroxyalkylcelluloses. The publication contains no information on the particle size and its distribution. However, it is known that palladium colloids prepared by this method have a very wide distribution, with particle sizes in the range from a few nm up to about 50 nm.
Chinese Journal of Reactive Polymers 1 (1992), 48-53 discloses the preparation of colloidal palladium by reducing palladium(II) chloride in methanol in the presence of poly-N-vinyl-2-pyrrolidone as protective ligand with methanol/NaOH as reducing agent. Palladium particles with a diameter of from 1 to 3 nm and an average diameter of 2 nm are obtained. The disadvantage of this process is the use of palladium chloride because chloride can act as catalyst poison. In addition, polymeric protective ligands have a strong shielding effect and can be removed only with difficulty.
J. Am. Chem. Soc.
115(1993), 2046-48 describes the preparation of colloidal palladium by reducing palladium(II) acetate with hydrogen in the presence of phenanthroline as protective ligand. 90% of the resulting clusters have a particle size from 3.15 to 3.6 nm. The process requires the use of gaseous hydrogen as reducing agent, which complicates the chemical and safety engineering.
J. Am. Chem. Soc.
116(1994) describes the electrolytic preparation of palladium clusters. This entails the nobel metal in the form of a foil undergoing anodic oxidation and cathodic reduction to palladium(0), employing conductive tetraallkylammonium salts as stabilizer. The resulting palladium colloids have an average particle size which varies in the nanometer range depending on the current density used in the electrolysis, and have a relatively wide distribution of particle sizes. The process makes uses of physiologically unacceptable organic solvents such as acetonitrile/THF.
It is an object of the present invention to provide colloidal palladium with particles of defined size, in particular with the smallest possible size and a narrow distribution of particle sizes, and moreover to avoid the disadvantages described above.
We have found that this object is achieved by providing colloidal palladium composed of palladium clusters with an average particle diameter 0.2 nm≦d≦2 nm, with at least 80% of the palladium clusters having a particle diameter which differs by not more than 0.2 nm from the average particle diameter, it being possible for the palladium clusters to have protective ligands on the surface.
The object is further achieved by a process for preparing colloidal palladium from a colloidal solution of palladium which is obtained by reacting a palladium salt with a reducing agent in a solution containing the palladium salt, the reducing agent and a protective ligand at from 0 to 300° C., employing a branched-chain alcohol having 4 to 30 carbon atoms, where the longest chain has 3 to 18 carbon atoms, as reducing agent, and employing phosphines and/or aromatic nitrogen bases as protective ligands.
The distribution of the particle diameters of the palladium clusters can be represented mathematically by the following relation:
x=d+02 nm
∫[N(x)/N
tot
]dx≧0.8
x=d−0.2 nm
where
x is the particle diameter
N(x) is the number of particles with diameter x
N
tot
is the total number of particles and
d is the average particle diameter.
The novel colloidal palladium has a particularly narrow particle size distribution. Thus, in general, 80% of the novel palladium clusters have a particle diameter differing by not more than 0.2 nm, preferably by not more than 0.1 nm, from the average particle diameter. It is particularly preferred for 90% of the clusters to have a particle diameter which differs by not more than 0.1 nm from the average particle diameter.
The novel colloidal palladium clusters may comprise one or more other metallic components. Other metallic components which are preferred are metals of main groups III and IV, such as gallium, germanium, tin and lead, and transition metals such as Re, Ru, Os, Rh, Ir, Pt, Ag and Au. These may be present in proportions of from 0.1 to 99% by weight.
The novel palladium clusters may have protective ligands on their surface. The protective ligands prevent agglomeration of the palladium particles formed in the preparation of the colloidal palladium by reducing palladium salt in solution. In general therefore, at least some of the palladium clusters have protective ligands adsorbed on the surface immediately after the reduction of the palladium salt to colloidal palladium. The palladium clusters may still have protective ligands on their surface even at later processing stages, for example after isolation of the colloidal palladium from the colloidal solution of palladium formed and, where appropriate, redispersion in a liquid medium, or else after adsorption of the clusters on a carrier. However, it is possible to remove the protective ligands from the surface of the clusters. This generally requires special measures such as heat treatment of the colloidal palladium.
The colloidal palladium is prepared by initially preparing a colloidal solution of palladium. The colloidal palladium can be isolated from this. The colloidal solution of palladium is prepared by reacting a palladium salt in solution with a reducing agent in the presence of a protective ligand. It has been found that colloidal palladium with a very narrow distribution of partic

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