Raney nickel should not be confused with Rieke nickel.
Raney nickel, also called spongy nickel, is a fine-grained solid composed mostly of nickel derived from a nickel–aluminium alloy.[1] Several grades are known, of which most are gray solids. Some are pyrophoric, but most are used as air-stable slurries. Raney nickel is used as a reagent and as a catalyst in organic chemistry. It was developed in 1926 by American engineer Murray Raney for the hydrogenation of vegetable oils.[2] [3] Raney is a registered trademark of W. R. Grace and Company. Other major producers are Evonik and Johnson Matthey.
The Ni–Al alloy is prepared by dissolving nickel in molten aluminium followed by cooling ("quenching"). Depending on the Ni:Al ratio, quenching produces a number of different phases.During the quenching procedure, small amounts of a third metal, such as zinc or chromium, are added to enhance the activity of the resulting catalyst. This third metal is called a "promoter".[4] The promoter changes the mixture from a binary alloy to a ternary alloy, which can lead to different quenching and leaching properties during activation.
In the activation process, the alloy, usually as a fine powder, is treated with a concentrated solution of sodium hydroxide. The simplified leaching reaction is given by the following chemical equation:
2 Al + 2 NaOH + 6 H2O → 2 Na[Al(OH)<sub>4</sub>] + 3 H2The formation of sodium aluminate (Na[Al(OH)<sub>4</sub>]) requires that solutions of high concentration of sodium hydroxide be used to avoid the formation of aluminium hydroxide, which otherwise would precipitate as bayerite. Hence sodium hydroxide solutions with concentrations of up to 5 M are used.
The temperature used to leach the alloy has a marked effect on the properties of the catalyst. Commonly, leaching is conducted between 70 and 100 °C. The surface area of Raney nickel (and related catalysts in general) tends to decrease with increasing leaching temperature.[5] This is due to structural rearrangements within the alloy that may be considered analogous to sintering, where alloy ligaments would start adhering to each other at higher temperatures, leading to the loss of the porous structure.
During the activation process, Al is leached out of the NiAl3 and Ni2Al3 phases that are present in the alloy, while most of the Ni remains, in the form of NiAl. The removal of Al from some phases but not others is known as "selective leaching". The NiAl phase has been shown to provide the structural and thermal stability of the catalyst. As a result, the catalyst is quite resistant to decomposition ("breaking down", commonly known as "aging"). This resistance allows Raney nickel to be stored and reused for an extended period; however, fresh preparations are usually preferred for laboratory use.[6] For this reason, commercial Raney nickel is available in both "active" and "inactive" forms.
Before storage, the catalyst can be washed with distilled water at ambient temperature to remove remaining sodium aluminate. Oxygen-free (degassed) water is preferred for storage to prevent oxidation of the catalyst, which would accelerate its aging process and result in reduced catalytic activity.
Macroscopically, Raney nickel is a finely divided, grey powder. Microscopically, each particle of this powder is a three-dimensional mesh, with pores of irregular size and shape, the vast majority of which are created during the leaching process. Raney nickel is notable for being thermally and structurally stable, as well as having a large Brunauer-Emmett-Teller (BET) surface area. These properties are a direct result of the activation process and contribute to a relatively high catalytic activity.
The surface area is typically determined by a BET measurement using a gas that is preferentially adsorbed on metallic surfaces, such as hydrogen. Using this type of measurement, almost all the exposed area in a particle of the catalyst has been shown to have Ni on its surface. Since Ni is the active metal of the catalyst, a large Ni surface area implies a large surface is available for reactions to occur simultaneously, which is reflected in an increased catalyst activity. Commercially available Raney nickel has an average Ni surface area of 100 m2 per gram of catalyst.
A high catalytic activity, coupled with the fact that hydrogen is absorbed within the pores of the catalyst during activation, makes Raney nickel a useful catalyst for many hydrogenation reactions. Its structural and thermal stability (i.e., it does not decompose at high temperatures) allows its use under a wide range of reaction conditions.[7] [8] Additionally, the solubility of Raney nickel is negligible in most common laboratory solvents, with the exception of mineral acids such as hydrochloric acid, and its relatively high density (about 6.5 g cm-3)[9] also facilitates its separation from a liquid phase after a reaction is completed.
Raney nickel is used in a large number of industrial processes and in organic synthesis because of its stability and high catalytic activity at room temperature.[10] [11]
In a commercial application, Raney nickel is used as a catalyst for the hydrogenation of benzene to cyclohexane. Other heterogeneous catalysts, such as those using platinum group elements are used in some cases. Platinum metals tend to be more active, requiring milder temperatures, but they are more expensive than Raney nickel. The cyclohexane thus produced may be used in the synthesis of adipic acid, a raw material used in the industrial production of polyamides such as nylon.[12]
Other industrial applications of Raney nickel include the conversion of:
Raney nickel is used in organic synthesis for desulfurization. For example, thioacetals will be reduced to hydrocarbons in the last step of the Mozingo reduction:[13]
Thiols,[14] and sulfides can be removed from aliphatic, aromatic, or heteroaromatic compounds. Likewise, Raney nickel will remove the sulfur of thiophene to give a saturated alkane.[15]
See also: Reduction of nitro compounds. It is typically used in the reduction of compounds with multiple bonds, such as alkynes, alkenes, nitriles, dienes, aromatics and carbonyl-containing compounds. Additionally, Raney nickel will reduce heteroatom-heteroatom bonds, such as hydrazines,[16] nitro groups, and nitrosamines. It has also found use in the reductive alkylation of amines