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Raney nickel , also called spongy nickel, is a fine-grained solid composed mostly of nickel derived from a nickel–aluminium alloy. 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.'''See:
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Raney Nickel is a registered trademark of W. R. Grace and Company. Other major producers are Evonik and Johnson Matthey.
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. 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. 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) water is preferred for storage to prevent oxidation of the catalyst, which would accelerate its aging process and result in reduced 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 area 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. Additionally, the solubility of Raney nickel is negligible in most common laboratory solvents, with the exception of such as hydrochloric acid, and its relatively high density (about 6.5 g cm−3) also facilitates its separation from a liquid phase after a reaction is completed.
Other industrial applications of Raney nickel include the conversion of:
, and can be removed from aliphatic, aromatic, or heteroaromatic compounds. Likewise, Raney nickel will remove the sulfur of thiophene to give a saturated alkane.
When reducing a carbon-carbon double bond, Raney nickel will add hydrogen in a Syn addition fashion.
In contrast to the pyrophoric nature of some forms of Raney nickel, nickel silicide-based catalysts represent potentially safer alternatives.
Raney alloys include FeTi and other non Nickel alloys. FeTi has been considered for low pressure Hydrogen Storage. Aldricimica Acta (free from Sigma nee Aldrich) has a complete list of Raney alloys.
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 allergy to nickel, such as skin rashes ("nickel itch").
Nickel is also rated as being a possible human carcinogen by the IARC (Group 2B, EU category 3) and teratogen, while the inhalation of fine aluminium oxide particles is associated with Shaver's disease.
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. This is now a common alloy composition for modern Raney nickel catalysts. Other common alloy compositions include 21:29 Ni/Al and 3:7 Ni/Al. Both the activity and preparation protocols for these catalysts vary.
Following the development of Raney nickel, other alloy systems with aluminium were considered, of which the most notable include copper, ruthenium and cobalt. Further research showed that adding a small amount of a third metal to the binary alloy would promote the activity of the catalyst. Some widely used promoters are zinc, molybdenum and chromium. An alternative way of preparing enantioselective Raney nickel has been devised by surface adsorption of tartaric acid.
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