Raney metals

A significant volume of literature relates to our work. Concerning choice of support, Montassier et al. have examined silica-supported catalysts with Pt, Co, Rh Ru and Ir catalysts.However, these systems are not stable to hydrothermal conditions. Carbon offers a stable support option. However, the prior art with respect to carbon-supported catalysts has generally focused on Ru and Pt as metals.Additionally, unsupported catalysts have also been reported effective including Raney metals (metal sponges).Although the bulk of the literature is based on mono-metallic systems, Maris et al. recently reported on bimetallic carbon-supported catalysts with Pt/Ru and Au/Ru. In contrast, our work focuses primarily on the development of a class of rhenium-based carbon supported catalysts that have demonstrated performance equal to or better than much of the prior art. A proposed reaction mechartism is shown in Figure 34.2 °l... 

Electrocatalytic Hydrogenation of Organic Compounds at Raney Metal Electrodes ... 

In this paper, results on the ECH of representative substrates are presented i) in order to illustrate the advantages and the limitations of the ECH at Raney metal electrodes as a method of hydrogenation of organic compounds and ii) in order to discuss some synthetic and mechanistic aspects. The Raney metal electrodes consisted of Raney metal particles embedded in a nickel matrix (1) or dispersed in a lanthanum polyphosphate matrix (2), or of pressed Raney metal alloy of which the outmost layer only has been leached (3). 

Synthetic Aspects of Electrohydrogenation of Nitro Compounds at Raney Metal (RM) Electrodes in Aqueous Alcohols... 

There are three categories of important experimental facts about the electrohydrogenation of nitro groups at large surface-area electrodes (such as Raney metal electrodes) in an aqueous alcoholic medium and these facts have mechanistic consequences. 

Unprotonated hydroxylamines are not reducible by electron transfer (10) as already mentioned and the electrohydrogenation of nitro compounds at Raney metal electrodes in neutral and basic aqueous alcoholic media gives the corresponding amines as shown by the numerous examples illustrated above. Therefore, in these media, hydroxylamines must be reduced to the amine by an ECH mechanism (eq. [1] followed by eqs. [18] and [19]). 

Nitro groups attached to a primary and secondary alkyl group in a highly basic (pH > 13) medium exist as the nitronate (enolate) anions. These anions must be very difficult to reduce by electron transfer and are surely much more difficult to reduce than water. Since the electrohydrogenation of such nitro compounds to the corresponding amines is veiy efficient at Raney metal cathodes in 0.1 to 0.15 M KOH (or NaOH) aqueous alcohol (pH > 13) (12), as... 

When nitrocumene (39) was electrohydrogenated in the same basic medium as above but at Raney metal electrodes, aminocumene (44) was obtained as expected from an ECH mechanism pathway (eq. [1] followed by eqs. [12] to [19]) but bicumyl (43), resulting from electronation of nitrocumene (39) (Scheme 2), was formed also (Scheme 15) (22). 

It is noteworthy that the relative proportion of amine 44 and bicumyl (43) which reflects the ratio of the rate of electronation to the rate of reaction with M(H) (the competition between electronation and reaction with M(H)), varies with the Raney metal (compare entries 1 and 3 of Table 1, and entries 2 and 4) and with the electrode potential (compare entries 1 and 2). The more negative is the potential, the faster is the rate of electronation and the higher should be the proportion of bicumyl (43) as observed (entries 1 and 2). The less active the Raney metal as hydrogenation catalyst, the slower is the rate of reaction with M(H) (the lower is the amount of M(H) at the surface of the electrode) and the lower is the amount of aminocumene (44). RCu is the least active catalyst and the proportion of aminocumene (44) is indeed the lowest at the RCu cathode (entry 4). 

Table 1 Electrohydrogenation of nitrocumene (39) at Raney metal (RM) electrodes inEt0H-H20 40 60 (w/w) (22).

Electrohydrogenation at Raney metal electrodes is a mild method of hydrogenation, the advantages and disadvantages of which have been pointed out in the Introduction and have been illustrated in the paper with selected... 

Raney Metal Type of Reaction Raw Material Product Ref. 

Raney nickel is made by dissolving an (Al,Ni)-alloy containing 50 % Ni in 20 % NaOH in water. The nickel sponge is black, the area is 80-100 m7g. The nickel contains some A1 and AI2O3. The most used Raney metals are Ni, Co and Fe. 

As is typical for Raney metals made from metals of relatively low melting point, 7 , (Ag) = 1000°C, Raney silver is a much coarser material than Raney nickel, Tm = I650°C. It exhibits a specific surface of no more than 1 m2/g or less. 

Raney metals, in particular Raney Ni, can be pyroforic in air because of the large amount of H dissolved [135]. Some additives are used to alleviate this problem [136], although other solutions have also been proposed. For instance, Raney-Ni can be incorporated into a Ni matrix so that the heat of H2 - 02 recombination is dissipated through the metal [17,137, 138] (Fig. 5). 

Variants of preparation have been proposed [135, 248] including sintering [391] or co-electrodeposition of the precursors [138, 407], and aluminization of the surface of Ni at high temperature whose nature has a definite effect on the resulting electrocatalytic activity [408]. The main features of Raney Ni have been evaluated, including the pore size distribution and the real surface area [93, 135]. It has been found that the composition of the precursor alloys and their particle size have important influence on the adsorption properties of the resulting Raney metal, hence on its electrocatalytic properties [409]. 

Studies with adatoms and microcrystals have clearly shown that the electronic structure of these systems are profoundly different from those of the corresponding bulk material. Electrocatalytic activation can be expected only as the structure of the material is substantially changed (cf. Raney metals). The case of DSA has shown how widely the properties of a material can be varied as it is obtained in a poorly crystalline, dispersed, non-stochiometric form. 

Despite the large amount of data collected in this field, options seem to be restricted to coatings containing Ni or Co, and Mo, with some additives apparently necessary to impart the wanted properties. It follows that the problem of cathode activation is only in part a question of synergism, while the microscopic structure of the catalyst appears to be of greater importance. Thus, Raney Ni still stands out in the group of possible catalysts [110, 531]. The role of Al (or Zn) does not seem to be simply that of sacrificial components. Residual Al present in the structure plays probably a role which can be similar to that of Cd in Ni-Mo. For this reason mixed Raney metals show great promise [415]. Raney Ni-Co (10%) exhibit a very extended... 

Fusion/ alloy leaching (mixed oxides, Raney metals)... 

Although with the selective removal proecdurc the active component and the support are usually verv intimately mixed, it is difficult to control the porous structure and/or the mechanical strength ol the result ing eatalyst bodies Nonetheless the procedure is difficult to beat for the production ol highly loaded supports The most well known example of selective removal, the preparation of Raney metals, where alu minum is selectively removed, leaves behind almost exclusively the desired active metal... 

Other Systems. Evidence for dealloying has been reported in austenitic stainless steel and iron-nickel alloys in acidified chloride-containing solutions, reduction of titanium dioxide in molten calcium chloride, and copper-zinc-aluminum alloy pellets in NaOH solutions to produce Raney metal particles. (Corcoran)5... 

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