Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Patent
1994-03-08
1997-06-24
Caldarola, Glenn A.
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
502327, 502152, 502300, G01J 2104
Patent
active
056417233
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for preparing highly active doped supported catalysts of metal of the groups 6 to 11 of the Periodic Table and the use of catalysts prepared by said process.
BACKGROUND OF THE INVENTION
The activity of a heterogeneous metal catalyst is dependent on the particle size, the uniform distribution on the support of the particles and the formation of incomplete crystal structures. Classical preparation methods for heterogeneous metal catalysts are soaking (impregnation) and precipitation.
DESCRIPTION OF THE PRIOR ART
According to the impregnation method, the metal salt is adsorbed, the excessive solution is removed, and the catalyst precursor is dried. The subsequent calcination at from 300.degree. C. to 500.degree. C. in air causes the conversion of the adsorbed metal salt into oxidic products to be effected. The resulting metal oxides will have to be activated by reduction. In this process, a large part of the active component diffuses into the interior of the support particle and, hence, becomes inaccessable to the substrate, especially in liquid phase reactions.
This drawback can be eliminated by precipitating the metal component onto the support. In the ideal case, the active components forms a thin layer on the support particle (shell catalyst). One measure for the fineness of the distribution on the support particle is the ratio of surface metal atoms to the total of metal atoms in % (dispersity) {J. T. Richardson, Principles of Catalyst Development, Plenum Press, New York (1989) 162}. A disadvantage of the above precipitation method is constituted by the very non-uniform local distribution of the active component on the support as well as by a very broad particle size distribution (FIG. 1).
Art atomic metal distribution is also possible on supports. However, such "monoatomically" covered support catalysts have proven to be absolutely inactive in hydrogenations. They fail to exhibit any tendency towards chemisorption and hydrogen dissociation {W. F. Graydon et al., J. Catal. 69 (1981) 180-192}. It is only with an increase in the cluster size that the metal character of the particles is increased {G. Schmid, Aspects of Homogeneous Catalysis, Vol. 7 (1991) pp. 1-36}. The smallest particle size which allows an adsorptive cleave of hydrogen to be achieved represents a threshold value which is affected by the arrangement of the metal atoms {H. Gentsch, Essener Universitatsberichte 1 (1989) 30-34}. For platinum and nickel, these minimum agglomerates comprise 12 metal atoms.
It is the object of the present invention to obtain a uniform covering of the support particle with metal particles having a narrow size distribution by means of a deposition on support surfaces under mild conditions, especially by avoiding any thermal stress, of highly disperse metals, readily decomposable organometal clusters comprising a defined number of metals per cluster or of isolated metal and/or alloy colloids of a described particle size.
Many attempts have been made for this purpose to employ organometal reagents which are readily capable of being reactively bonded to surfaces {Catalyst Design, pp. 71 et seq.; M. Ichikawa, J. Chem. Soc. Chem. Commun. 1976, 26; Y. Yermakov, Catal. Rev. Sci. Engl. 13 (1976) 77}. However, in practice it has shown to be difficult to remove the organic ligands in a defined manner under mild conditions. It is true, the decomposition of mononuclear or organometal complexes of transition metals on the surface produces a highly disperse metal; however, the agglomerates formed therefrom on the surface are too small so that they cannot display any optimal catalytic action. For example, the palladium complex [C.sub.3 H.sub.5 PdC.sub.5 H.sub.5 ] which has been reactively anchored to activated carbon and then thermally decomposed (particle size below 40 nm) provides only a weak activity in the hydrogenation of vinylacetylene. Larger active metal agglomerates are provided only after sintering the primary particles at temperatures between 200.degree. C. and 600.degr
REFERENCES:
patent: 4102819 (1978-07-01), Petrow et al.
patent: 4513098 (1985-04-01), Tsao
patent: 4900712 (1990-02-01), Bar-Ilan
Bonnemann Helmut
Brijoux Werner
Brinkmann Rainer
Dinjus Eckhard
Fretzen Rainer
Caldarola Glenn A.
Studiengesellschaft Kohle MBH
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