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
1. What is Raney Nickel and why is it often called a 'skeletal catalyst'?
Raney Nickel is a solid catalyst composed of fine grains of a nickel-aluminium alloy. It is called a skeletal catalyst because it is prepared by selectively removing aluminium from the alloy using a concentrated sodium hydroxide solution. This process, known as leaching, leaves behind a porous, sponge-like nickel structure with an extremely high surface area, which is responsible for its high catalytic activity.
2. What is the fundamental difference between regular nickel powder and Raney Nickel?
The fundamental difference lies in their structure and catalytic activity. Regular nickel powder is a simple metallic element with a relatively low surface area. In contrast, Raney Nickel is a specially prepared, highly porous form of nickel derived from an alloy. This 'skeletal' structure gives it a vast surface area, making it a much more effective and active catalyst for chemical reactions like hydrogenation.
3. How is Raney Nickel prepared from its Ni-Al alloy?
Raney Nickel is prepared through a specific metallurgical and chemical process:
- Alloy Formation: First, an alloy of nickel and aluminium (typically 50% of each) is created by melting them together.
- Quenching: The molten alloy is rapidly cooled or 'quenched' to form a brittle solid.
- Leaching: This solid is then treated with a hot, concentrated solution of sodium hydroxide (NaOH). The NaOH selectively reacts with and dissolves the aluminium, leaving the nickel behind. The reaction is: 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂.
- Washing: The resulting porous nickel is washed with distilled water to remove any residual sodium aluminate (NaAlO₂), yielding the active catalyst.
4. What are the primary industrial and laboratory applications of Raney Nickel?
Raney Nickel is a versatile catalyst primarily used for hydrogenation reactions in organic chemistry. Its key applications include:
- Reduction of Alkenes and Alkynes: Converting carbon-carbon double and triple bonds to single bonds (e.g., producing alkanes).
- Conversion of Functional Groups: Reducing nitro compounds to amines, nitriles to amines, and carbonyls (aldehydes/ketones) to alcohols.
- Desulfurization: Removing sulfur atoms from organic compounds, as seen in the Mozingo reduction.
- Industrial Production: Manufacturing sorbitol from dextrose and adipic acid from benzene (a precursor for nylon production).
5. Why is Raney Nickel considered hazardous and how must it be stored safely?
Raney Nickel is considered hazardous because its activated form is pyrophoric. This means its high surface area and high reactivity cause it to spontaneously ignite upon exposure to air. Due to this fire risk, it must be handled in an inert atmosphere and stored submerged under water or another inert solvent like ethanol to prevent contact with oxygen.
6. How does Raney Nickel's structure enable it to be such an effective catalyst?
Raney Nickel's effectiveness as a catalyst is a direct result of its unique structure. The leaching process creates a sponge-like material with countless microscopic pores. This structure provides an immense surface area, which acts as the active site for reactions. It can adsorb large quantities of hydrogen gas, holding the H₂ molecules in an activated state, ready to react with other organic molecules that come into contact with the catalyst's surface.
7. Can the catalytic activity of Raney Nickel be modified for specific reactions?
Yes, the activity and selectivity of Raney Nickel can be modified. This is often achieved by adding a small amount of a third metal, known as a 'promoter', to the initial nickel-aluminium alloy. For example, adding metals like chromium or zinc can enhance the catalyst's performance for specific types of hydrogenation reactions, making it more selective or more robust under certain industrial conditions.
