High catalytic activity of Raininickel


The high catalytic activity of Raeney nickel comes from the catalytic properties of nickel itself and its porous structure, and the porous structure is derived from the removal of aluminum in nickel-aluminum alloys with concentrated sodium hydroxide solution, this process is called leaching, and the subsequent leaching reaction is as follows:
 
2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2
 
Since the leaching reaction brings the activity of the catalyst, and the hydrogen produced at the same time is stored in the catalyst, it is also called activation. The surface area of the finished product is usually measured by adsorption experiments with gases such as hydrogen. Experiments have found that nickel is present in almost all contact areas. The average nickel contact area of commercial Raininickel is 100 m²/g. [8] 。 There are three main factors that affect the outcome of the leaching reaction, they are the composition of the alloy, the concentration of sodium hydroxide used and the temperature of the leaching reaction.
 
As mentioned earlier, the alloy contains a variety of nickel-aluminum phases, in the leaching process, the aluminum contained in the NiAl3 and Ni2Al3 phases is first reacted off, while the aluminum contained in the NiAl phase reacts more slowly and can be retained by adjusting the leaching time, which is why it is called "selective leaching". Typical activated Raeney nickel contains 85% of the mass of nickel, which means that 2/3 of the atoms are nickel. The aluminum in the remaining NiAl phase can help maintain this porous structure, providing structural stability and thermal stability for the catalyst.
 
The concentration of sodium hydroxide used in the leaching reaction is relatively high, generally up to 5 moles/liter, so as to quickly convert aluminum into water-soluble sodium aluminate (Na[Al(OH)4]) and avoid aluminum hydroxide precipitation. Once a precipitate of aluminum hydroxide is generated, the precipitation will block the holes that have been formed, preventing the remaining sodium hydroxide solution from entering the path of the alloy, making it difficult for the remaining aluminum to react off. This results in a smaller surface area of the porous structure of the product and a decrease in catalytic activity.
 
The porous structure gradually formed during the leaching process has a strong tendency to reduce its surface area, and structural rearrangement occurs, and the pore walls combine with each other, so that the porous structure is destroyed. The increase of temperature will accelerate the atomic movement, increasing the trend of structural rearrangement, so the surface area and catalytic activity of Raney nickel decrease with the increase of the leaching reaction temperature, and if the leaching temperature is very low, it will make the leaching reaction speed too slow, so the commonly used leaching reaction temperature is between 70 to 100 degrees Celsius.

 

 

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Reni nickel catalyst science knowledge(1)

Physical and chemical properties: Reni nickel catalyst before activation is silver-gray amorphous powder (nickel-aluminum alloy powder), with a moderate degree of flammability, partial activation in the presence of water and the production of hydrogen easy agglomeration, long-term exposure to air is easy to weather. Nickel-aluminum alloy powder is activated into gray-black particles, accompanied by active hydrogen, extremely unstable, oxidative combustion in the air, must be immersed in water or ethanol for preservation. It was first used by American engineer Murray Rainey as a catalyst in the hydrogenation of vegetable oils. The preparation process is to treat nickel-aluminum alloy with concentrated sodium hydroxide solution, in this process, most of the aluminum will react with sodium hydroxide and dissolve, leaving a lot of micropores of different sizes. In this way, the surface of Raininickel is a fine gray powder, but from a microscopic point of view, each tiny particle in the powder is a three-dimensional porous structure, this porous structure greatly increases its surface area, and the large surface area brings high catalytic activity, which makes Raininickel widely used as a heterogeneous catalyst in organic synthesis and industrial hydrogenation reactions. Since "Rainey" is a registered trademark of Grace Chemicals, strictly speaking, only products manufactured by the company's Davidson Chemical Division can be called "Lanny Nickel". The term "metal backbone catalyst" or "sponge-metal catalyst" is used to refer to catalysts with a microporous structure and physical and chemical properties similar to Raney nickel.