Raney nickel

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Raney nickel 1. Introduction: On the surface, Raney nickel appears as a fine gray powder; however, at the microscopic level, each tiny particle in this powder consists of a three-dimensional porous structure. This porous structure increases its surface area significantly. The large surface area contributes to high catalytic activity, which is why Raney nickel is widely used as a heterogeneous catalyst in organic synthesis and hydrogenation reactions in industrial production. Since “Raney” is a registered trademark of Grace Chemicals Company, strictly speaking, only the products manufactured by that company’s Davidson Chemical Division can be called “Raney nickel”. “Metal framework catalysts” or “sponge-metal catalysts” are used to refer to catalysts with a microporous structure that possess physical and chemical properties similar to Raney nickel. 2. Production: Commercially, the nickel-aluminum alloy required for the production of Raney nickel is obtained by melting catalytically active metals (nickel, iron, or copper) with aluminum in a furnace; the resulting melt is then quenched and cooled, followed by crushing it into uniform fine particles. When designing the alloy composition, two factors need to be considered. First is the composition ratio of nickel to aluminum in the alloy; as this ratio changes, different nickel/aluminum phases are formed during quenching, and these phases have varying leaching properties, which can result in significantly different porous structures in the final product. Typically, nickel and aluminum of equal mass are used for fusion. The second is the proportion of the third metal added. During the quenching process, a small amount of a third metal, such as zinc or chromium, is sometimes added. Their addition alters the composition and phase diagram of the alloy, resulting in different leaching properties and thereby higher catalytic activity; hence they are called \"promoters\". The nickel-aluminum phase diagram shows the NiAl3, Ni2Al3, and NiAl phases that are closely related to the leaching reaction. 3. Activation: The high catalytic activity of Raney nickel stems from the catalytic properties of nickel itself as well as its porous structure. This porous structure is obtained by removing aluminum from nickel-aluminum alloys using a concentrated sodium hydroxide solution; this process is known as leaching. The simplified leaching reaction is as follows: 2Al + 2NaOH + 6H2O → 2Na + 3H2. Since the leaching reaction enhances the catalyst’s activity, and the hydrogen gas produced is stored within the catalyst, this process is also referred to as activation. The surface area of the finished product is usually measured through gas adsorption experiments (such as with hydrogen). Experiments found that nickel was present in almost all of the contact areas. The average nickel contact area of commercial Reney nickel is 100 /g. There are mainly three factors that affect the outcome of the leaching reaction: the composition of the alloy, the concentration of sodium hydroxide used, and the temperature of the leaching reaction. As mentioned earlier, the alloy contains various nickel-aluminum phases; during the leaching process, the aluminum contained in the NiAl3 and Ni2Al3 phases is reacted away first, while the aluminum in the NiAl phase reacts more slowly and can be retained by adjusting the leaching time. This is why it is referred to as \"selective leaching\". In typical activated Raney nickel, nickel accounts for 85% by mass, which means that 2/3 of the atoms are nickel. The aluminum remaining in the NiAl phase can help maintain this porous structure, providing the catalyst with structural stability and thermal stability. The concentration of sodium hydroxide used in the leaching reaction needs to be relatively high, generally reaching 5 moles per liter, in order to rapidly convert aluminum into water-soluble sodium aluminate (Na), thereby preventing the formation of aluminum hydroxide precipitates. Once aluminum hydroxide precipitate is formed, it blocks the existing pores, preventing the remaining sodium hydroxide solution from reaching the alloy, which makes it more difficult for the remaining aluminum to react. This results in a decrease in the surface area of the product’s porous structure, thereby reducing its catalytic activity. The porous structure that gradually forms during the leaching process has a strong tendency to reduce its surface area; structural rearrangements occur as the pore walls combine with each other, resulting in the destruction of the porous structure. An increase in temperature accelerates the movement of atoms, thereby increasing the tendency for structural rearrangement; as a result, both the surface area and catalytic activity of Raney nickel decrease as the leaching reaction temperature rises. On the other hand, if the leaching temperature is too low, the leaching reaction proceeds too slowly. Therefore, the commonly used leaching reaction temperature ranges from 70 to 100 degrees Celsius.