Skeletal metals Preparation

Because the complexes listed in Tables 1-8 are all prepared with metal halides and carborane anions, only one example for each type of metallocarborane (number of B atoms) is given in the following representative equations for clarity, the skeletal framework in the equations have numbered positions and H atoms are omitted ... 

Raney predicted that many other metal catalysts could be prepared with this technique, but he did not investigate them [8], Copper and cobalt catalysts were soon reported by others [4,5], These catalysts were not nearly as active as Raney s nickel catalyst and therefore have not been as popular industrially however they offer some advantages such as improved selectivity for some reactions. Skeletal iron, ruthenium and others have also been prepared [9-13], Wainwright [14,15] provides two brief overviews of skeletal catalysts, in particular skeletal copper, for heterogeneous reactions. Table 5.1 presents a list of different skeletal metal catalysts and some of the reactions that are catalyzed by them. 

Basic information concerning the mechanism of skeletal rearrangement was provided by labeling experiments and kinetic studies. The use of specifically prepared catalysts, such as metal films and alloys, and structure sensitivity studies supplied additional data. The information resulted in establishing two basic processes the bond shift and the cyclic mechanisms.151-154... 

Polymetallic anions, prepared by dissolution of alloys of the alkali and post-transition metals in amine solvents (often with a complexand for the alkali metal cation), have been characterized in crystalline and solution phases. Clusters TlSng3, Ge92 (with 20 skeletal bonding electrons), Sn93- (21 skeletal e) and Bi95+ (22 skeletal e) possess a tricapped trigonal prismatic structure, symmetry D3A, with variations of dimensional detail which correlate with the electron population.291 292 This structure is a ctoso-deltahedron, and with 20 (2h + 2) skeletal electrons can be construed to be three-dimensionally aromatic.292 The 22e clusters M94 (M = Ge, Sn, Pb) occur as the C4v monocapped square antiprism, a nido polyhedron. 

When one component of a bimetallic alloy is leached out, a finely divided metal powder of high surface area results. One of the oldest of these so-called skeletal metal catalysts is Raney nickel10,11. Nickel boride is a more recently developed hydrogenation catalyst prepared by the reduction of nickel salts with sodium borohydride12-14. Bimetallic catalysts are often used to achieve selective saturation of a double bond in bifunctional unsaturated systems, e.g. in dienes. Amorphous metal alloys, a newly developed class of metal catalysts15,16, have also been applied in the hydrogenation of alkenes and dienes. 

The linkage of transition metals to skeletal phosphorus in a phosphazene has presented an unresolved challenge for many years. We have recently succeeded in the preparation of iron and ruthenium phosphazenes by the reaction shown below (8). 

Hafnium is the third element in the same group with Ti and Zr, the transition metals of the Periodic Table it is predicted that Hf02 is enhanced in acidity by sulfate addition up to superacidity. The catalyst, which was obtained by exposing Hf(OH)4, prepared by the hydrolysis of HfCI4, to I M H2S04 and then calcining, was active for the skeletal isomerization of... 

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. 

Alloys are prepared commercially and in the laboratory by melting the active metal and aluminum in a crucible and quenching the resultant melt which is then crushed and screened to the particle size range required for a particular application. The alloy composition is very important as different phases leach quite differently leading to markedly different porosities and crystallite sizes of the active metal. Mondolfo [14] provides an excellent compilation of the binary and ternary phase diagrams for aluminum alloys including those used for the preparation of skeletal metal catalysts. Alloys of a number of compositions are available commercially for activation in the laboratory or plant. They include alloys of aluminum with nickel, copper, cobalt, chromium-nickel, molybdenum-nickel, cobalt-nickel, and iron-nickel. 

The principal advantage of skeletal catalysts is that they can be stored in the form of the active metal and therefore require no prereduction prior to use as do conventional catalysts which are in the form of the oxide of the active metal supported on a carrier. These catalysts can also be prepared on demand by a simple caustic leaching procedure. They have very high activity since the BET surface area (typically up to 100 m2g-1 for skeletal nickel and 30 m2 g l for skeletal copper) is essentially the metal surface area. Skeletal catalysts are low in initial cost per unit mass of metal and therefore provide the lowest ultimate cost per unit mass of active catalyst. The high metal content provides good resistance to catalytic poisoning. 

The addition of a second component in metal catalysts is widely used in order to enhance activity and/or selectivity. In the case of skeletal nickel catalysts it is a simple procedure to add small amounts of a second metal during the alloy preparation stage. Although other metals have been used in laboratory studies, the most common metals used to promote skeletal nickel catalysts employed industrially are Co, Cr, Cu, Fe, and Mo. 

Skeletal catalysts consist of a metal skeleton remaining after the less noble components of an alloy has been removed by leaching with base or acid. The skeleton metals belong almost exclusively to Groups IB and VIIB of the periodic table (Fe, Co, Ni, Cu, and Ag), whereas Al, Zn, Si, and Mg are the most commonly used alloy components. The alloys are prepared by fusion of the components in the proper proportion, Raney pioneered the development of skeleton catalysts. A widely used Ni and Co catalyst, which is highly active... 

Due to proposals that adjacent d and p orbitals can overlap to form delocalized band-type orbitals either in the ground state or the excited state 1-4), there has been much interest in the synthesis and properties of structures whose skeletal frameworks consist of covalently bonded metal atoms. However, early attempts at the rational synthesis of oligomers and polymers containing a mix of d- and p-block metals were frustrated by the general lack of synthetic methodology that can be used to prepare metal-metal bonds under relatively mild conditions, and by the... 

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