Raney nickel

Raney nickel

Dry activated Raney nickel
Identifiers
CAS Number	7440-02-0 Y
UNII	7OV03QG267 Y
Properties
Appearance	Light-gray powder
Hazards
GHS labelling:
Pictograms
Hazard statements	H250, H317, H351, H372, H412
Precautionary statements	P210, P273, P280, P302
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Raney nickel

In the activation process, the alloy, usually as a fine powder, is treated with a concentrated solution of sodium hydroxide.^[3] The simplified leaching reaction is given by the following chemical equation:

2 Al + 2 NaOH + 6 H₂O → 2 Na[Al(OH)₄] + 3 H₂

The formation of

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.[7] 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.^{[citation needed]}

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").^[7] This resistance allows Raney nickel to be stored and reused for an extended period; however, fresh preparations are usually preferred for laboratory use.^[8] 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 (

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.^{[citation needed]}

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.^[6] 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.^[6]

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.^[9][10] 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)^[1] also facilitates its separation from a liquid phase after a reaction is completed.

Applications

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.^[6][11][12]

Industrial applications

In a commercial application, Raney nickel is used as a catalyst for the

Other industrial applications of Raney nickel include the conversion of:

• Dextrose to sorbitol
  ;
• amines, for example, 2,4-dinitrotoluene
  to 2,4-toluenediamine;
• Nitriles to amines, for example, stearonitrile to stearylamine and adiponitrile to hexamethylenediamine
  ;
• Olefins to paraffins, for example, sulfolene to sulfolane
  ;
• 1,4-butanediol
  .

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:^[14][15]

Reduction of functional groups

It is typically used in the

In contrast to the pyrophoric nature of some forms of Raney nickel, nickel silicide-based catalysts represent potentially safer alternatives.^[25]

Safety

Due to its large surface area and high volume of contained hydrogen gas, dry, activated Raney nickel is a

Additionally, acute exposure to Raney nickel may cause irritation of the respiratory tract and nasal cavities, and causes pulmonary fibrosis if inhaled. Ingestion may lead to convulsions and intestinal disorders. It can also cause eye and skin irritation. Chronic exposure may lead to pneumonitis and other signs of sensitization to nickel, such as skin rashes ("nickel itch").^[27]

NFPA 704 fire diamond
1 3 1

Nickel is also rated as being a possible human

Development

Murray Raney graduated as a mechanical engineer from the University of Kentucky in 1909. In 1915 he joined the Lookout Oil and Refining Company in Tennessee and was responsible for the installation of electrolytic cells for the production of hydrogen which was used in the hydrogenation of vegetable oils. During that time the industry used a nickel catalyst prepared from nickel(II) oxide. Believing that better catalysts could be produced, around 1921 he started to perform independent research while still working for Lookout Oil. In 1924 a 1:1 ratio Ni/Si alloy was produced, which after treatment with sodium hydroxide, was found to be five times more active than the best catalyst used in the hydrogenation of cottonseed oil. A patent for this discovery was issued in December 1925.^[28]

Subsequently, Raney produced a 1:1 Ni/Al alloy following a procedure similar to the one used for the nickel-silicon catalyst. He found that the resulting catalyst was even more active and filed a patent application in 1926.^[29] This is now a common alloy composition for modern Raney nickel catalysts.^[2] Other common alloy compositions include 21:29 Ni/Al and 3:7 Ni/Al. Both the activity and preparation protocols for these catalysts vary.^[2][30]

Following the development of Raney nickel, other alloy systems with aluminium were considered, of which the most notable include copper,

• Nickel aluminide
• Urushibara nickel
• Rieke nickel
• Nickel boride catalyst
• Raney cobalt, a similar cobalt/aluminum alloy catalyst which is sometimes more selective for certain hydrogenation products (e.g. primary amines via nitrile reduction).^[2]

References