Raney-Nickel

Raney nickel is a solid catalyst composed of fine grains of a nickel- aluminum alloy, and is used in many industrial processes. Raney nickel was developed in 1926 by the American Engineering Murray Raney as an alternative catalyst for the hydrogenation of vegetable oils. In modern processes of Raney nickel is used as a heterogeneous catalyst in many syntheses of complex organic molecules, such as natural compounds in hydrogenation reactions.

Raney nickel is formed by leaching a nickel aluminum block by means of concentrated sodium hydroxide solution. This " activation " solves a major part of the aluminum from the alloy, leaving a porous structure that can affect chemical reactions greatly due to their large surface. A typical catalyst is comprised of about 85 percent of the mass of nickel, which corresponds to about two nickel atoms per aluminum atom.

This form of nickel is pyrophoric, that is, they begin to burn with oxygen on contact.

Production

The alloy

Commercially, the alloy is produced by melting the active metal - in this case nickel, to produce other Raney -type catalysts, iron and copper may be used - and aluminum prepared in a crucible, followed by quenching. The resulting metal is ground to a fine powder and then sieved to the desired particle sizes.

The initial composition of the alloy is crucial, as the quenching produces different Ni / Al phases that behave differently in alkalis. In this way, the resulting porosity of the final product have large fluctuations. In most cases, the starting alloy on equal weights of nickel and aluminum, which is the same ratio used Murray Raney in discovery of Raney nickel.

During quenching, small amounts of a third metal, such as zinc or chromium, can be used. This additional metal increases, the catalytic activity and is therefore referred to as the "promoter ." Through the promoter, however, the alloy and its phase diagram is changed to a ternary alloy, which results in addition to any other method of deterrence also changed behavior during activation by itself.

The activation process

The porous structure of the catalyst produced by activation of the aluminum by means of sodium hydroxide solution. The simplified reaction liquor is given by the following equation:

The hereby resulting sodium aluminate ( Na [ Al (OH ) 4] ), forces the use of concentrated sodium hydroxide solutions in order to prevent a precipitation of aluminum hydroxide in the form of bayerite. Here, the alkalis have concentrations of up to 5 mol / L. The bayerite can clog the pores of the structure formed and adversely affect the efficiency and activity of the catalyst due to the reduced surface thereby.

The temperature of the liquor also has a significant influence on the surface properties of the catalyst, with usual temperatures from 70 to 100 ° C range. With Raney nickel and Raney -type catalysts, reduced temperature rising during the activation process, the resulting surface. This is due to structural transformations within the alloy, which is similar to run the sintering.

Before storage, the catalyst is washed with distilled water at room temperature to remove remaining traces of sodium. Deoxygenated water is preferably used for storage, in order to prevent oxidation of the catalyst, which accelerates the aging process and reduce the catalytic activity.

Properties

Macroscopically Raney nickel from such a finely divided gray powder. Microscopically, however, similar to each particle a three-dimensional grid whose openings - pores of irregular size and shape - are created mainly by the leaching. Raney nickel is thermally and structurally stable and has a high BET surface area. These properties result mainly from the activation process and contribute to the relatively high catalytic efficacy.

During activation of the aluminum is dissolved out of the NiAl3 and Ni2Al3 phases of the alloy remaining aluminum is usually in the form NiAl. This removal of the aluminum from the specific phase is referred to as " selective leaching ." The NiAl phase assisted structural and thermal stability of the catalyst which is resistant to degradation thereby. This resistance allows Raney nickel, to be stored and used for a long time; However, most fresh preparations are used for laboratory use. For this reason, commercial Raney nickel is offered in both "active " and " inactive " form.

The surface is often determined by BET measurements, for which a mainly adsorbed by metallic surfaces gas - about hydrogen - is used. On such measurements, it was demonstrated that almost the entire exposed surface is composed of the particles of nickel. Since nickel is the active metal of the catalyst, so a large area for chemical reactions is offered, which is reflected in the catalytic activity. Commercially available Raney nickel has an average nickel surface area of 100 m² per gram.

Due to the high efficiency and the adsorption of hydrogen within the pores is Raney Nickel, a suitable catalyst, in many hydrogenation reactions. Its stability - for example the fact that it does not decompose at a high temperature - also allows a wide range of reaction conditions. With the exception of mineral acids - for example hydrochloric acid - is Raney nickel hardly soluble in most laboratory solvents and facilitates its high density, which is between 6 and 7 g / cc, the separation of the liquid phase after a reaction.

The particle size is normally 20 to 50 microns, and in special cases up to 10 microns or 90 microns.

Applications

Raney nickel is used for its stability and activity at room temperature in a variety of industrial processes and laboratory synthesis. It is typically used in the reduction of components having multiple bonds - such as alkynes, alkenes, nitriles, polyenes, aromatic compounds and materials of the carbonyl group. In addition, Raney nickel, reduced heteroatom -heteroatom bonds, such as organic nitro compounds and nitrosamines. The alkylation of amines, and the amination of alcohols, represents a further field of application

An example of the use of Raney nickel in the industry is the following reaction, where benzene is reduced to cyclohexane. Reduction of the aromatic structure of the benzene ring is difficult to achieve with other chemical methods, but can be achieved by means of Raney nickel. Other heterogeneous catalysts, such as those that use the elements of the iron-platinum group, can also be used, but are usually more expensive than Raney nickel. After this reaction, cyclohexane can be used as for the synthesis of adipic acid, a raw material for the production of polyamides such as nylon.

In the reduction of carbon-carbon double bonds, Raney nickel added (addition) hydrogen. In addition to the use as catalyst of Raney nickel can be used as a reagent for the desulfurization of organic compounds. For example thioacetals be reduced to hydrocarbons:

Nickel sulphide precipitates, while Ethan can escape as a gas. Similar transformations are the Clemmensen reduction and the Wolff-Kishner reaction.