What Is Raney Nickel Definition Preparation Properties and Hydrogenation Uses
When the alloy of aluminium and nickel gives a solid entity, this result is called the Raney Nickel. Raney or Spongy Nickel plays a key role in organic chemistry, where the element is used as a catalyst or reagent during hydrogenation chemical reactions. Upon inspection of its microparticles, this Raney Nickel is said to be a finely grated powder-like substance, where each of its pores forms a 3-dimensional shape. From understanding its properties to the preparation and applications, let us learn everything important about Raney Nickel.
Table of Contents
The Basic Information about Raney Nickel
Spongy Nickel, Skeletal Catalyst, Sponge Metal Catalyst, are all the different names of one compound which is Raney Nickel. The credits for the discovery of this compound goes to Murray Raney in 1926, who was an American mechanical engineer and developed this element for hydrogenation of vegetable oils.
Deriving from the Aluminium-Nickel’s alloy, the Raney Nickel is said to be a solid substance but present as a fine powdery element. This is a compound that is given high importance in the fields of organic chemistry, for hydrogenation reactions in particular.
A vast majority of the available Raney Nickel grades are noted to be a grey substance, strictly under ambient temperature conditions. Let’s now see how one can prepare Raney Nickel using a few chemical elements.
Preparing Raney Nickel Chemically
The primary conditions to prepare Raney Nickel is the alloy of nickel and aluminium. First, heat the aluminium until getting a molten stage and then mix nickel into this mixture. Then, with a method known as ‘quenching’, the molten state of the aluminium-nickel alloy is cooled down for some time.
Meanwhile, the catalyst’s activity can be intensified by adding a third metal, such as zinc or chromium into the mixture obtained. Chromium and zinc here are stated to be the ‘promoters’. Concentrated Sodium Hydroxide is then added to the final alloy, forming hydrogen gas and sodium aluminate. This step can be equated as follows:
2Al + 6H2O + 2NaOH → 2Na[Al(OH)4] + 3H2
After this process, filter out a minimal amount of aluminium from the alloy as NiAl3 and Ni2Al3 using the leaching process. This will result in leaving only the catalyst behind and this Raney Nickel catalyst is cleansed using distilled water in room temperature. This final step is important before storing the result, to remove the sodium aluminate that is leftover from the reaction.
Note that the water used should be oxygen-free (degassed).
Raney Nickel and its Important Properties
Given below are the important physical and chemical properties regarding Raney Nickel and its structure:
Grey coloured-powder under macroscopic conditions but appears solid externally.
Irregularly sized shapes and 3-D figures were observed inside every single grain of the power under an electron microscope and formed using the leaching process.
Both Thermally and Structurally stable.
Raney Nickel is labelled as both ‘harmful’ and ‘flammable’.
Mineral acids like that of the Hydrochloric Acid solute Raney Nickel completely.
6.5 grams per cubic centimetre is the density.
The pH level of this Nickel form is 8.5-12.0.
A few grades of this Nickel form is said to be a Pyrophoric substance.
The catalytic activity of the substance is directly proportional to the amount of Nickel present in it. Which means, the more nickel available, the higher is the catalyst’s activity.
The commercial form of Raney Nickel is available in both active and inactive modes.
Stable grade forms are used in air slurries.
100m2 of Active Raney Nickel surface is noted to be from a 1 gram of the same substance.
Less soluble with other chemical compounds present in a clinical laboratory.
With measurement processes of BET, more than half the structure of the Raney Nickel consists of Nickel only.
The phase of production plays a vital role in deciding the activation level of this Nickel form.
Applicability of the Raney Nickel Catalyst
As stated before, hydrogenation reactions majorly prefer Raney Nickel for its good reactivity and high range of oxygen absorption capacity inside its pores. Let us consider the example of benzene conversion.
Raney Nickel Catalysts is used in producing cyclohexane by the conversion of benzene. Again, this can be oxidized with the same element to create adipic acid.
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This element further catalyzes organic reactions such as desulfurization. To state an example, Raney Nickel Catalyst is used in the Mozingo reduction of thioacetal into the hydrocarbon Ethane.
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Adding to the stated, Raney Nickel is also preferred in the removal of Sulfides and Thiols from the heteroaromatic, aromatic and aliphatic compounds. The result of removing the thiophene and sulfur from the reaction is the formation of an alkane.
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Furthermore, Raney Nickel is also an important catalyst for the reactions including, nitro compounds conversion into amines (vice-versa), getting different kinds of paraffin from olefins, in the formation of sorbitol from dextrose, converting acetylenes to paraffin, and even to obtain alkanes from alkynes and alkenes.
Conclusion
Spongy Nickel, Skeletal Nickel, or Raney Nickel is obtained from the alloy of aluminium and nickel, discovered by Murray Raney, a mechanical engineer in America. This appears solid grey but is a fine powdery material under microscopic conditions. Raney Nickel is available in different grades and can be sold commercially both as an inactive and active reagent. Organic chemistry gives high importance to this compound since hydrogenation, reduction, oxidation, and many other chemical reactions are enhanced and prepared using the Raney Nickel Catalyst.
FAQs on Raney Nickel Catalyst in Organic and Industrial Chemistry
1. What is Raney nickel?
Raney nickel is a highly porous, finely divided form of nickel used as a heterogeneous catalyst, mainly for hydrogenation reactions. It is prepared from a nickel–aluminium alloy by selectively leaching out aluminium with sodium hydroxide solution, leaving behind a sponge-like nickel structure.
- Composition: Mostly nickel with small residual aluminium
- Form: Grey, finely divided solid (usually stored wet)
- Main use: Catalytic hydrogenation of alkenes, alkynes, nitriles, and aromatic compounds
2. How is Raney nickel prepared?
Raney nickel is prepared by treating a nickel–aluminium alloy with concentrated sodium hydroxide to dissolve aluminium. The preparation involves the following steps:
- A Ni–Al alloy is first prepared by melting nickel and aluminium together.
- The alloy is crushed into small particles.
- It is treated with hot NaOH(aq), which dissolves aluminium as sodium aluminate.
2Al(s) + 2NaOH(aq) + 6H2O(l) → 2Na[Al(OH)4](aq) + 3H2(g)
After leaching, a porous nickel skeleton remains, which is washed and stored under water to prevent oxidation.
3. Why is Raney nickel stored under water?
Raney nickel is stored under water because it is pyrophoric and can ignite spontaneously in air. Due to its very high surface area and finely divided structure:
- It reacts rapidly with oxygen in air.
- It may catch fire when dry.
- Moist storage prevents contact with air and reduces oxidation.
4. What is the use of Raney nickel in hydrogenation?
Raney nickel is used as a catalyst for hydrogenation, where it adds hydrogen (H2) across multiple bonds. It accelerates reactions such as:
- Hydrogenation of alkenes to alkanes
- Reduction of alkynes to alkanes
- Conversion of nitriles to amines
CH2=CH2(g) + H2(g) → CH3–CH3(g) (in presence of Raney nickel).
The catalyst provides a surface where hydrogen and the organic molecule adsorb and react efficiently.
5. How does Raney nickel work as a catalyst?
Raney nickel works as a heterogeneous catalyst by adsorbing reactants on its surface and lowering the activation energy of the reaction. The mechanism generally involves:
- Adsorption of H2 on the nickel surface and its dissociation into hydrogen atoms.
- Adsorption of the unsaturated organic compound.
- Surface reaction between adsorbed species.
- Desorption of the hydrogenated product.
6. What is the difference between Raney nickel and nickel powder?
The main difference is that Raney nickel is a highly porous, activated catalyst, while ordinary nickel powder has a much lower surface area and catalytic activity.
- Raney nickel: Prepared from Ni–Al alloy, highly porous, very active, often pyrophoric.
- Nickel powder: Finely divided metallic nickel, less porous, lower catalytic efficiency.
7. Can Raney nickel reduce aromatic compounds?
Yes, Raney nickel can hydrogenate aromatic compounds to cycloalkanes under suitable conditions of temperature and pressure. For example, benzene can be reduced to cyclohexane:
C6H6(l) + 3H2(g) → C6H12(l) (in presence of Raney nickel).
This reaction typically requires elevated temperature and pressure because aromatic rings are stabilized by resonance and are less reactive than alkenes.
8. Is Raney nickel a homogeneous or heterogeneous catalyst?
Raney nickel is a heterogeneous catalyst because it is in a different physical phase from the reactants. In most hydrogenation reactions:
- Raney nickel is a solid (s).
- Hydrogen is a gas (g).
- The organic substrate is often liquid (l) or dissolved.
9. What are the advantages of using Raney nickel?
The main advantages of Raney nickel are its high catalytic activity, relatively low cost, and reusability. Key benefits include:
- Large surface area for efficient adsorption
- Effective hydrogenation of many functional groups
- Less expensive than noble metal catalysts like Pd or Pt
- Can be separated by filtration after reaction
10. What are the safety precautions when handling Raney nickel?
Raney nickel must be handled carefully because it is pyrophoric and may ignite when dry. Important safety measures include:
- Always store and handle it under water or an inert atmosphere.
- Avoid allowing it to dry in air.
- Keep away from open flames and oxidizing agents.
- Dispose of used catalyst according to hazardous waste guidelines.
