Raney nickel

Raney nickel is an amorphous solid composed of finely divided grains of a nickel/aluminium alloy. In organic synthesis Raney nickel is commonly used as a heterogeneous catalyst for hydrogenation reactions.

Since Raney® is a registered trademark of W. R. Grace and Company, only those products by the Grace Davison division are properly called Raney nickel. Alternatively, the more generic term skeletal catalyst is used to refer to catalysts that have physical and chemical properties similar to those of Raney nickel.

Properties

Preparation

Alloy Preparation

Alloys are prepared commercially by melting the active metal (nickel in this case) and aluminium in a crucible and quenching the resultant melt, which is then crushed into a fine powder. This powder may be screened for a specific particle size range depending on the application the catalyst may be required for.

Alloy composition is very important because the quenching process produces a number of different Ni/Al phases that have different leaching properties. This results in markedly different porosities and crystallite sizes in the end product. The most common starting alloy used in industry contains an equal amount per weight of nickel and aluminium, coincidentally the same ratio Murray Raney used in his discovery of Raney nickel.

Activation

The porous structure of the catalyst arises from the selective removal of aluminium from alloy particles using aqueous sodium hydroxide. The leaching reaction is given by:

The formation of sodium aluminate (NaAlO₂) requires that high concentrations of excess sodium hydroxide are used in order to avoid the formation of aluminium hydroxide which precipitates as bayerite causing blocking of the pores and loss of surface area, thus reducing catalyst activity.

The temperature used to leach the alloy has a marked effect on the surface properties of the catalyst. The surface areas of Raney® nickel (and skeletal catalysts in general) decrease with increasing temperature of leaching due to structural rearrangements that may be considered analogous to sintering.

After leaching the catalyst can be washed with distilled water at ambient temperature, and can be stored under deareated distilled water to prevent oxidation.

History

Murray Raney graduated as a Mechanical Engineer from the University of Kentucky in 1905. 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 oxide. Believing that better catalysts could be produced, he formed his own research company in 1921. In 1924 a 50% nickel-silicon 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.

Subsequently Raney produced a 50% nickel-aluminium 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. It may be of interest to note that Raney's choice of an alloy with 50% nickel-silicon content was fortuitous and without any real scientific basis. However, this is the preferred alloy composition for production of Raney® nickel catalysts currently in use.

Following the development of Raney® nickel, other alloy systems were considered. Some of the most notable include copper, ruthenium and cobalt. Sometimes a small amount of a third metal may be added to the binary alloy to enhance the activity of the catalyst, and as such they are called promoters. Some common promoters are zinc, molybdenum and chromium.

References

1. US1628190 – Murray Raney's patent on his nickel-aluminium catalyst.

External Links

• Grace Davison – Product page for Raney® catalysts. Includes a history section.
• US1563587 – US patent of Raney's original nickel-silicon catalyst.