Metal carbonyls catalytic properties

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

On the other hand, hi- or multi-metallic supported systems have been attracting considerable interest in research into heterogeneous catalysis as a possible way to modulate the catalytic properties of the individual monometalUc counterparts [12, 13]. These catalysts usually show new catalytic properties that are ascribed to geometric and/or electronic effects between the metalUc components. Of special interest is the preparation of supported bimetallic catalysts using metal carbonyls as precursors, since the milder conditions used, when compared with conventional methods, can render catalysts with homogeneous bimetallic entities of a size and composition not usually achieved when conventional salts are employed as precursors. The use of these catalysts as models can lead to elucidation of the relationships between the structure and catalytic behavior of bimetalUc catalysts. 

The appropriate decarbonylation of surface carbonyl species can lead to the retention of the metal frame. When the original metal frame is preserved under reaction conditions, and it is properly characterized, a direct structure-catalytic properties relationship can be established, and the catalytic behavior of supported metaUic clusters can be differentiated from that of the supported metallic particles. 

Notably, the use of heteronuclear surface carbonyl species can lead to the preparation of well-defined supported bimetallic entities that can be used as model catalysts to study the promoter effect of a second metal. The close intimacy achieved between the two metals in the surface carbonyl species is related to the structural characteristics and catalytic properties of the final catalyst In the preparation of supported, tailored, multi-component catalysts, the use of metal carbonyl surface species still deserves to be studied to further explore the exciting field of nano-sized entities in catalysis. 

In the present review we shall describe recent developments in the catalysis of reactions by dicobalt octacarbonyl. Although many of the reactions to be described do not necessarily involve dicobalt octacarbonyl directly in the catalytic cycle, but some derivative, there are several reasons for choosing this compound as a starting point. The most important reason being that dicobalt octacarbonyl is a reasonably stable, commercially available, fairly well characterized compound which easily gives active catalytic intermediates. Although by no means unique in their catalytic properties, the cobalt carbonyls do provide a particularly active and versatile example of metal carbonyl catalysis. Their catalytic reactions are also by far the most investigated and best understood. 

In spite of some declining industrial interest, the last 5 years have seen an unusual academic interest in the catalytic properties of the metal carbonyls. This has been part of a wider surge of interest in the organometallic chemistry of the transition metals and its application to homogeneous catalysis. Reactions such as Ziegler polymerization, the Oxo reaction, and the Wacker process are but a few of the many reactions of unsaturated molecules catalyzed in the coordination sphere of transition metal complexes (20). These coordination catalyses have much in common, and the study of one is often pertinent to the study of the others. 

In the effort to modify the catalytic properties of rhodium, Wilson and co-workers prepared Rh/Fe/Si02 and other two-metal-silica combinations (74) methanol yields over a Rh/Fe/Si02 catalyst shown in Table XIV were higher than those over the Rh/MgO, Rh/ZnO, and Rh/LaB6 catalysts prepared from cluster carbonyls, but the selectivity of the latter toward methanol was better than that of bimetallic rhodium-iron catalysts. 

Rare earth oxides are useful for partial oxidation of natural gas to ethane and ethylene. Samarium oxide doped with alkali metal halides is the most effective catalyst for producing predominantly ethylene. In syngas chemistry, addition of rare earths has proven to be useful to catalyst activity and selectivity. Formerly thorium oxide was used in the Fisher-Tropsch process. Recently ruthenium supported on rare earth oxides was found selective for lower olefin production. Also praseodymium-iron/alumina catalysts produce hydrocarbons in the middle distillate range. Further unusual catalytic properties have been found for lanthanide intermetallics like CeCo2, CeNi2, ThNis- Rare earth compounds (Ce, La) are effective promoters in alcohol synthesis, steam reforming of hydrocarbons, alcohol carbonylation and selective oxidation of olefins. 

Methanol is currently the largest volume carbonylation product and is made by passing syngas (CO -f H2 Section 4.1.2) over a solid Cu-Zn oxide catalyst. Most of the other carbonylation reactions are catalyzed by the later c -block transition metals, often under homogeneous conditions in solution. This is despite a public perception that the use of heavy metals (such as the complexes of the 4d and 5d transition metals) is generally undesirable. However their extremely effective catalytic properties now make their use mandatory in many... 

The preference for rhodium was known from the investigations conducted by Monsanto. Diversifying from these patents, the available low-cost catalyst metals were studied which have catalytic properties comparable with those of rhodium. In the presence of alkali metal salts and Group VIB metal carbonyls, Ni-catalyzed carbonylation operates under mild conditions of nearly 190 °C and 40 bar [57]. Nevertheless, only limited success was found with other catalyst... 

Metal carbonyls proved to be superior to earlier catalytically active systems, where in particular strong acids were used [4], because the conditions, namely pressure and temperature, that had to be applied led to skeletal isomerization of the substrates and resulted predominantly in the formation of branched isomers of carboxylic acids. Metal carbonyls were of great advantage over the older catalysts in this respect. Despite the fact that it was possible to optimize the catalyst metal, the ligands, and the promoters for nearly every carbonylation reaction, enabling the reactions to take place under milder conditions than had been previously used, these processes could only be realized industrially after the development of appropriate reactor materials because of the corrosive properties of the reaction media and products. 

Catalytic properties of supported clusters identified as primarily Ira or Ir6 were reported by Xu et al [15], who investigated a structure-insensitive reaction, toluene hydrogenation. The support was NaY zeolite or, for comparison, MgO. EXAFS spectroscopy (Table 3) showed that the first-shell Ir-Ir coordination numbers characterizing both the fresh and used MgO-supported catalysts made by decarbonylation of supported [Ir4(CO)i2] or [HIr4(CO)ii] are indistinguishable fi om 3, the value for a tetrahedron, as in [Ir4(CO)i2] and [HIr4(CO)n]. The decarbonylated clusters retained or nearly retained this metal frame. EXAFS data show that the decarbonylated Ire clusters had metal fi ames indistinguishable from the octahedra of the precursor hexairidium carbonyls, indicated by the coordination number of approximately 4. 

Acidic zeolites are most commonly used, but metal-containing zeolites also have catalytic properties. For example, ion exchange of Rh into faujasites produces methanol carbonylation catalysts the Rh complexes work in much the same way as the analogous soluble catalyst (see 14.2.3.1) . ... 

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