Catalysts mixed-metal

A variety of mixed metal catalysts, either as fused oxides (42 7 8) or coprecipitated on supports (25 0) or as physical mixtures of separate catalysts (5P), have been tested in aniline reductions. In the hydrogenation of ethyl p-aminobenzoate, a coprccipitated 3% Pd, 2% Rh-on-C proved superior to 5% Rh-on-C, inasmuch as hydrogenolysis to ethyl cyclohexanecarboxylate was less (61) (Table 1). 

These data show hydrogenolysis to increase with temperature, a general observation supported by many experiments. Here the influence of temperature is less with the mixed-metal catalysts. 

The attainment of regio- and enantioselectivity are continuing challenges, which form the basis of investigations of new catalytic systems. For example, novel mixed-metal catalysts such as (CO)2Co(u-CO)(u-HC=CBu)Rh(CO)2 promote the stereospecific synthesis of Z-only BuC (CHO) = CHSiMe2Ph from 1-hexyne and HSiMe2Ph.1364... 

J. Ojima, Homogeneous Mixed-Metal Catalyst Systems New and Effective Routes to N-Acyl-a-amino-Acids via Carboxylations, J. Mol. Catalysis 37, 25 (1986). 

The use of mixed-metal catalysts can also dramatically affect the products of autoxidations. An example mentioned earlier is the selective oxidation of acetaldehyde to acetic anhydride in the presence of a mixture of cobalt and copper acetates. Another example is the co-oxidation of alkanes and olefins in the presence of both an autoxidation and an epoxidation catalyst (see Section III.B) ... 

GOODMAN Surface Properties of Mixed-Metal Catalysts... 

This so called ensemble or geometric effect, however, is not the only reason for the distinct properties of mixed metal catalysts. The diluent metal can... 

Mixed oxides can also be reduced to the mixed metal catalysts by hydrogen reduction. Such oxides have been prepared by sodium nitrate fusion of mixed metal salts, primarily those of the noble metals, following the procedure used for the prepeiration of Adams catalyst, platinum oxide. 31-136 Non-noble metal alloy powders are prepared by heating mixed nitrates, carbonates or hydroxides in air to convert them to the mixed oxides which are then reduced. The hydroxides can be prepared by coprecipitation from a mixed metal salt solution. ... 

Since the catalytic process takes place on the surface of the mixed metal catalysts, a knowledge of the surface composition of these materials is essential to... 

It can be seen, then, that while the surface composition of a mixed metal catalyst is of critical importance to the outcome of a given reaction, there is little that may be said concerning the optimum surface concentration for a particular reaction. Even if such a prediction could be made it would be difficult to design a catalyst having the prescribed surface composition under the reaction conditions used. Much more needs to be done to optimize the use of such mixed metal catalysts, particularly in synthetically useful reactions. 

Supported mixed metal catalysts can be prepared by almost any of the procedures described above for the production of monometallic species. The discussion concerning the surface composition of these multimetallic species presented in Chapter 12 applies to supported catalysts as well but may be modified by the presence of very small crystallites in the supported catalysts. 2> 3 The factors presented above concerning the location of the metal in the support particle are also applicable here as well and can be used to prepare bimetallic catalysts having specific distribution profiles. 

Fig. 13.21. Reaction of tetraalkyl tin with a supported metal to give a tin containing supported mixed metal catalyst.

Supported mixed metal catalysts are also prepared by other means such as the deposition of bimetallic colloids onto a support O and the decomposition of supported bimetallic cluster compounds.208 The photocatalytic codeposition of metals onto titania was also attempted with mixed results.209 with a mixture of chloroplatinic acid and rhodium chloride, very little rhodium was deposited on the titania. With aqueous solutions of silver nitrate and rhodium chloride, more rhodium was deposited but deposition was not complete. In aqueous ammonia, though, deposition of both silver and rhodium was complete but the titania surface was covered with small rhodium crystallites and larger silver particles containing some rhodium. With a mixture of chloroplatinic acid and palladium nitrate both metals were deposited but, while most of the resulting crystallites were bimetallic, the composition varied from particle to particle.209... 

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