Raney Nickel: Preparation, Properties, and Applications

Raney Nickel is a solid entity that is formed from the alloy of nickel and aluminium. It plays a vital role in organic chemistry as it is used as a catalyst and is often employed in hydrogenation reactions. The name Raney Nickel is coined after engineer Murray Raney. The appearance of Raney Nickel is fine powder-like. Each pore of Raney Nickel forms a 3-D shape. It is often also referred to as ‘Skeletal Catalyst’ or ‘Spongy Nickel’. In this article, we will study this reducing agent in detail, and how it plays an important role in organic chemistry. 

Keyterms: Nickel, Aluminium, Organic chemistry, Hydrogenation reactions, Skeletal Catalyst, Spongy Nickel, Reducing agent, Chromium

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What is Raney Nickel? 

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Raney Nickel is a fine-grained solid which is also referred to as the spongy nickel. Raney Nickel is majorly comprised of nickel and is derived from a nickel-aluminum alloy. American engineer Murray Raney developed this solid for certain hydrogenation purposes where it is employed as a catalyst. This light grey powder also acts as a reagent and a catalyst in the organic field of chemistry and hence is a vital part of organic chemistry. The terms “skeletal catalyst” or “sponge metal catalyst” are associated with Raney Nickel. Some Raney nickel is known to ignite spontaneously in air below 54 degrees. It exists in multiple grades, the majority of which are grey solids under fine conditions.

Raney Nickel

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Preparation of Raney Nickel 

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Raney Nickel is prepared in the following way:

• Nickel is first dissolved in molten aluminum and then a cooling technique is followed to prepare Raney Nickel. 
• Depending on the ratio of the alloy, this cooling process is done in several phases, where a small amount of third metal is added. This third metal is called a promoter which may be zinc or chromium and it increases the catalytic activity. 
• When the binary alloy mixture changes to a ternary one, it leads to leaching during activation.  

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

• During the activation process. The powdered alloy is treated with a concentrated solution of sodium hydroxide and the underlying chemical reaction promotes leaching. The result is sodium aluminate. 
• Temperature between 70 degrees Celsius is used to leach the alloy. During this time, the area of Raney Nickel decreases and undergoes several structural rearrangements. 
• This partial removal of the surface is known as selective leaching. 

Preparation of Raney Nickel

Raney Nickel is a strong catalyst that cannot be decomposed easily. This way, it can be stored and reused. Commercial Raney Nickel is available in inactive and active forms. The catalyst needs to be washed in degassed water at a suitable temperature before storage.  

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Properties of Raney Nickel 

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Here are some salient properties of Raney Nickel:

• When Raney Nickel is observed under a microscope, each particle seems like a 3-D mesh with irregularly shaped pores. 
• Raney Nickel has a large BET surface area and is structurally stable because of the leaching and activation process. 
• The surface area is determined by a BET measurement using gas-like hydrogen. 
• Since Ni is an active metal of the catalyst, its surface area is directly proportional to the reactions.  
• Commercially available Raney Nickel has a nickel surface area of approximately 100 m² per gram of catalyst. The solubility of Raney nickel that is used in laboratory solvents, is negligible. 
• It has a high density, about 6.5 grams per cubic centimeter. 
• Certain mineral acids are capable of soluting this Raney Nickel completely. Hydrochloric acid is such an example. 
• For industrial uses, Raney Nickel is found in both active and inactive mode. This catalyst, in its stable form, is often used in the preparation of slurries.
• It is flammable in nature, making it harmful when not used with proper precautions.

Raney Nickel as a Catalyst to form alkane

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Applications of Raney Nickel 

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Raney Nickel is used in various industrial processes. Owing to its stability, ability to absorb hydrogen through its pores, and superior catalytic activities, it is also used in organic synthesis. Let us learn the various industry applications of Raney Nickel. 

• Industrial applications 

Industries use Raney Nickel to reduce benzene to cyclohexane which can further be oxidized to adipic acid. The reduction of the benzene rings is a hard process.  

Reduction process showing how Cyclohexane is reduced to Benzene using Raney Nickel 

• Organic synthesis 

Raney Nickel is used for desulfurization. Thioacetals can be reduced using this catalyst. It is also used in the reduction process of compounds with multiple bonds. Raney Nickel is said to reduce heteroatom-heteroatom bonds too.  

Organic synthesis 

Raney Nickel is also used as a catalyst-

1. To convert Dextrose to sorbitol, 
2. To convert nitro compounds like nitriles to amines, olefins, and acetylenes to kinds of paraffin, and so on.

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Safety measures for handling Raney Nickel 

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Raney Nickel should ideally be kept in an inert atmosphere. Activated Raney Nickel has a big surface area full of hydrogen gas. So much that, even after supplying it as a water slurry, the residue contains enough hydrogen gas to ignite when exposed to air. Exposure to Raney Nickel for a longer period of time may irritate the respiratory or pulmonary tracts. If continued, it may cause pneumonitis or turn carcinogenic.  

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Things to Remember

• Raney Nickel is a fine-grained solid which is also referred to as the spongy nickel. 
• Raney Nickel is majorly comprised of nickel and is derived from a nickel-aluminum alloy.
• When we reduce a double bond (carbon-carbon), Raney Nickel adds hydrogen to it. 
• Raney Nickel is used in organic synthesis because it's highly stable and emits a high catalytic activity under room temperatures. 
• Commercially available Raney Nickel has a nickel surface area of approximately 100 m² per gram of catalyst.
• Chemists checked that Raney Nickel has almost negligible solubility when it comes in contact with some common laboratory solvents. 
• During the activation process of Raney Nickel, the aluminium leaches out the NiAl₃ and Ni₂Al₃ phases present in the alloy.
• To eliminate unwanted traces of sodium aluminate, the catalyst is washed with distilled water at a suitable temperature. 

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