Ruthenium catalysts unpromoted

IV. Unpromoted and Carboxylic Acid-Promoted Ruthenium Catalysts... 

Although the ruthenium species observed during CO reduction in the absence of promoters is Ru(CO)s, its concentration can be reduced to unobservable levels by promoters which cause the formation of ionic ruthenium complexes. Because this system differs from unpromoted ruthenium catalysts in as many respects—rates, selectivities, catalytic species observed, and mechanism—it is addressed separately in this section. 

Unpromoted ruthenium catalysts have been shown to become unstable toward metallization with increasing temperature and/or decreasing CO pressure. Ionic promoters, especially halides, are found to provide a large stabilizing effect toward precipitation of metal, and significantly higher... 

The rates to methanol and ethylene glycol have qualitatively similar dependences on pressure (Fig. 18) and promoter concentration (Fig. 20), but somewhat different dependences on catalyst concentration (Fig. 19). The latter study shows that the ethylene glycol-producing reaction is more highly dependent than the methanol-forming reaction on processes intermolecular in catalyst components. There appears to be no evidence that all of the methanol and ethylene glycol are formed from a common precursor. Indeed, since methanol is known to be produced by unpromoted ruthenium catalysts, this product could be formed by several independent pathways in the promoted system. 

A mechanism possibly involving intermolecular hydride transfer in this promoted ruthenium system is thus very different from the reaction pathways presented for the cobalt and unpromoted ruthenium catalysts, where the evidence supports an intramolecular hydrogen atom transfer in the formyl-producing step. Nevertheless, reactions following this step could be similar in all of these systems, since the observed products are essentially the same. Thus, a chain growth process through aldehyde intermediates, as outlined earlier, may apply to this ruthenium system also. 

Ruthenium catalysts have also been found to be active in synthesis gas conversion, although simple unpromoted systems give only methanol (and methyl... 

Following the development of sponge-metal nickel catalysts by alkali leaching of Ni-Al alloys by Raney, other alloy systems were considered. These include iron [4], cobalt [5], copper [6], platinum [7], ruthenium [8], and palladium [9]. Small amounts of a third metal such as chromium [10], molybdenum [11], or zinc [12] have been added to the binary alloy to promote catalyst activity. The two most common skeletal metal catalysts currently in use are nickel and copper in unpromoted or promoted forms. Skeletal copper is less active and more selective than skeletal nickel in hydrogenation reactions. It also finds use in the selective hydrolysis of nitriles [13]. This chapter is therefore mainly concerned with the preparation, properties and applications of promoted and unpromoted skeletal nickel and skeletal copper catalysts which are produced by the selective leaching of aluminum from binary or ternary alloys. 

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