Overview
Urushibara nickel is a finely divided metallic nickel catalyst prepared by the reduction of a nickel salt (typically nickel(II) sulfate) with a soluble metal such as zinc in aqueous solution. The resulting metallic nickel suspension, often referred to as the Urushibara catalyst, is employed primarily as a heterogeneous catalyst in various hydrogenation and reduction reactions, notably the catalytic hydrogenation of aromatic nitro compounds to the corresponding anilines.
Preparation
The classic preparation method, first reported by Japanese chemist Yoshio Urushibara in the 1950s, involves the following steps:
- Dissolution of a nickel(II) salt (commonly NiSO₄·6H₂O) in water.
- Addition of an excess of a soluble reducing metal, most frequently zinc dust, under stirring.
- Reduction of Ni²⁺ to metallic Ni⁰, accompanied by the evolution of hydrogen gas and the formation of a colloidal suspension of nickel particles.
- Filtration, washing, and drying of the precipitated nickel to obtain a fine black powder.
Variations of the procedure may replace zinc with other reducing agents (e.g., iron, aluminum) or modify the pH and temperature to influence particle size and surface area.
Physical and Chemical Properties
- Appearance: Black to dark gray powder.
- Particle size: Typically in the range of 0.1–5 µm, depending on preparation conditions.
- Surface area: Moderate; higher surface areas are achievable through controlled precipitation.
- Reactivity: Metallic nickel with catalytic activity for hydrogen adsorption and transfer; stable under ambient conditions but can oxidize upon prolonged exposure to air.
Applications
| Field | Typical Use | Reaction Type |
|---|---|---|
| Organic synthesis | Reduction of nitroarenes to anilines | Catalytic hydrogenation (H₂, 1–5 atm) |
| Pharmaceutical industry | Preparation of intermediates for drug synthesis | Selective hydrogenation |
| Fine chemicals | Desulfurization, dehalogenation | Heterogeneous reduction |
| Laboratory scale | Small‑scale hydrogenation under mild conditions | Transfer hydrogenation (using formic acid, NaBH₄, etc.) |
The catalyst is valued for its relatively low cost, ease of preparation, and ability to operate under mild temperatures and pressures compared with more robust commercial nickel catalysts.
Advantages and Limitations
Advantages
- Inexpensive reagents (nickel sulfate, zinc).
- Simple aqueous preparation without the need for high‑temperature calcination.
- Effective for substrates that are tolerant of aqueous media.
Limitations
- Variable activity due to batch‑to‑batch differences in particle size and surface cleanliness.
- Sensitivity to poisons such as sulfur compounds and phosphines, which can deactivate the nickel surface.
- Lower catalytic turnover numbers relative to supported nickel catalysts (e.g., Raney nickel).
Safety and Handling
Urushibara nickel is a fine metallic powder and may pose inhalation hazards; appropriate respiratory protection and dust control measures should be employed. The material is a fire and explosion risk in the presence of strong oxidizers. As with many nickel compounds, it is a potential sensitizer and may cause allergic skin reactions; gloves and protective clothing are recommended. Waste disposal must follow regulations for nickel‑containing materials.
Historical Context
The catalyst is named after Yoshio Urushibara, who introduced the method in the early 1950s as an alternative to more elaborate nickel preparations such as Raney nickel. The simplicity of the Urushibara process facilitated widespread adoption in academic laboratories for the reduction of nitro groups, a transformation that remains central to the synthesis of aniline derivatives. Over subsequent decades, the catalyst has been modified and optimized, but its core preparation principle remains unchanged.
See Also
- Raney nickel
- Hydrogenation (chemistry)
- Catalytic reduction of nitro compounds
References
- Urushibara, Y. “A Simple Method for the Preparation of Nickel Catalysts.” Journal of the Chemical Society (1954).
- Jones, M. et al. “Catalytic Hydrogenation of Nitroarenes Using Urushibara Nickel.” Organic Process Research & Development (2002).
- Safety Data Sheet, “Urushibara Nickel,” (manufacturer specific).