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Raney nickel

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   Dry activated Raney nickel.
   Enlarge
   Dry activated Raney nickel.

   Raney nickel is a solid catalyst composed of fine grains of a
   nickel-aluminium alloy, used in many industrial processes. It was
   developed in 1926 by American engineer Murray Raney as an alternative
   catalyst for the hydrogenation of vegetable oils in industrial
   processes. More recently it is used as a heterogeneous catalyst in a
   variety of organic syntheses, most commonly for hydrogenation
   reactions.

   Raney nickel is produced when a block of nickel-aluminium alloy is
   treated with concentrated sodium hydroxide. This treatment, called
   "activation", dissolves most of the aluminium out of the alloy. The
   porous structure left behind has a large surface area, which gives high
   catalytic activity. A typical catalyst is around 85-percent nickel by
   mass, corresponding to about two atoms of nickel for every atom of
   aluminium. The aluminium which remains helps to preserve the pore
   structure of the overall catalyst.

   Since Raney is a registered trademark of W. R. Grace and Company, only
   those products by its Grace Davison division are properly called "Raney
   nickel". Alternatively, the more generic terms " skeletal catalyst" or
   "sponge-metal catalyst" may be used to refer to catalysts that have
   physical and chemical properties similar to those of Raney nickel.

Preparation

Alloy preparation

   Alloys are prepared commercially by melting the active metal (nickel in
   this case, but iron and copper "Raney-type" catalysts can be prepared
   as well) and aluminium in a crucible and quenching the resultant melt,
   which is then crushed into a fine powder. This powder may be screened
   for a specific particle size range depending on the application the
   catalyst may be required for.

   The initial alloy composition is important because the quenching
   process produces a number of different Ni/Al phases that have different
   leaching properties. This may result in markedly different porosities
   in the end product. The most common starting alloy used in industry
   contains an equal amount per weight of nickel and aluminium,
   incidentally, the same ratio Murray Raney used in his discovery of
   Raney nickel.

   During the quenching procedure, small amounts of a third metal, such as
   zinc or chromium, may be added. This is done to enhance catalytic
   activity, and as such this third metal is called a " promoter". Note
   that the addition of a promoter changes the alloy and its resulting
   phase diagram to that of a ternary alloy, leading to different
   quenching and leaching properties during activation.
   Raney nickel is pyrophoric and must be handled with care. This shipping
   container is filled with vermiculite to protect the sealed bottle
   inside.
   Enlarge
   Raney nickel is pyrophoric and must be handled with care. This shipping
   container is filled with vermiculite to protect the sealed bottle
   inside.

Activation

   The porous structure of the catalyst arises from the selective removal
   of aluminium from alloy particles using aqueous sodium hydroxide. The
   simplified leaching reaction is given by the following chemical
   equation:

          2Al + 2NaOH + 6H[2]O → 2Na[Al(OH)[4]] + 3H[2]

   The formation of sodium aluminate (Na[Al(OH)[4]]) requires that
   solutions of high concentration of sodium hydroxide are used in order
   to avoid the formation of aluminium hydroxide, which precipitates as
   bayerite. Hence sodium hydroxide solutions with concentrations of up to
   5 molar are used. Bayerite may cause blocking of the pores formed
   during leaching, and with the subsequent loss of surface area, it can
   reduce the efficiency and activity of the catalyst.

   The temperature used to leach the alloy has a marked effect on the
   surface properties of the catalyst. Commonly used temperatures range
   from 70 to 100 ° C. The surface area of Raney nickel (and skeletal
   catalysts in general) tends to decrease with increasing leaching
   temperature. This is due to structural rearrangements within the alloy
   that may be considered analogous to sintering, where alloy ligaments
   would start adhering to each other at higher temperatures leading to
   the loss of the porous structure.

   Before storage, the catalyst can be washed with distilled water at
   ambient temperature in order to remove any remaining traces of sodium
   aluminate. Oxygen-free water is preferred for storage in order to
   prevent oxidation of the catalyst, which would accelerate its aging
   process and result in reduced catalytic activity.
   Phase diagram of the Ni-Al system, showing relevant phases.
   Enlarge
   Phase diagram of the Ni-Al system, showing relevant phases.

Properties

   Macroscopically Raney nickel looks like a finely divided gray powder.
   Microscopically, each particle of this powder looks like a
   three-dimensional mesh, with pores of irregular size and shape of which
   the vast majority are created during the leaching process. Raney nickel
   is notable for being thermally and structurally stable as well has
   having a large BET surface area. These properties are a direct result
   of the activation process and contribute to a relatively high catalytic
   activity.

   During the activation process, aluminium is leached out the NiAl[3] and
   Ni[2]Al[3] phases that are present in the alloy, while most of the
   aluminium that remains does so in the form of NiAl. The removal of
   aluminium from some phases but not others is known as " selective
   leaching". It has been shown that the NiAl phase provides the
   structural and thermal stability to the catalyst. As a result the
   catalyst is quite resistant to decomposition ("breaking down", commonly
   known as "aging"). This resistance allows Raney nickel to be stored and
   reused for an extended period; however, fresh preparations are usually
   preferred for laboratory use. For this reason commercial Raney nickel
   is available in both "active" and "inactive" forms.

   The surface area is typically determined via a BET measurement using a
   gas that will be preferentially adsorbed on metallic surfaces, such as
   hydrogen. Using this type of measurement, it has been shown that almost
   all the exposed area in a particle of the catalyst has nickel on its
   surface. Since nickel is the active metal of the catalyst, a large
   nickel surface area implies that there is a large surface available for
   reactions to occur simultaneously, which is reflected in an increased
   catalyst activity. Commercially available Raney nickel has an average
   nickel surface area of 100 m² per gram of catalyst.

   A high catalytic activity, coupled with the fact that hydrogen is
   absorbed within the pores of the catalyst during activation, makes
   Raney nickel a useful catalyst for many hydrogenation reactions. Its
   structural and thermal stability (i.e., the fact that it does not
   decompose at high temperatures) allows its use under a wide range of
   reaction conditions. Additionally, the solubility of Raney nickel is
   negligible in most common laboratory solvents, with the exception of
   mineral acids such as hydrochloric acid, and its relatively high
   density (between 6 or 7 g/cm³) also facilitates its separation off a
   liquid phase after a reaction is completed.

Applications

   Raney nickel is used in a large number of industrial processes and in
   organic synthesis because of its stability and high catalytic activity
   at room temperature. It is typically used in the reduction of compounds
   that have multiple bonds, such as alkynes, alkenes, nitriles, dienes,
   aromatics and carbonyls. Additionally, Raney nickel will reduce
   heteroatom-heteroatom bonds such as nitro groups, and nitrosamines.
   (For further information see Reduction of nitro compounds.) It has also
   found use in the reductive alkylation of amines and the amination of
   alcohols.

   A practical example of the use of Raney nickel in industry is shown in
   the following reaction, where benzene is reduced to cyclohexane.
   Reduction of the aromatic structure of the benzene ring is very hard to
   achieve through other chemical means, but can be effected by using
   Raney nickel. Other heterogeneous catalysts, such as those using
   platinum group elements, may be used instead to similar effect, but
   these tend to be more expensive to produce than Raney nickel. After
   this reaction cyclohexane may be used in the synthesis of adipic acid,
   a raw material used in the industrial production of polyamides such as
   nylon.
   Benzene is routinely reduced to cyclohexane using Raney nickel for the
   production of nylon.

   When reducing a carbon-carbon double bond, Raney nickel will add
   hydrogen in a syn fashion.

   In addition to being a catalyst, Raney nickel will also act as a
   reagent to desulfurize organic compounds. For example, thioacetals will
   be reduced to hydrocarbons:
   Example of desulfurization of thioacetals using Raney nickel.

   Nickel sulfide will precipitate as millerite, while ethane can be
   easily separated through distillation. Similar transformations are the
   Clemmensen reduction and the Wolff-Kishner reduction.

Safety

   Raney nickel is a flammable material.
   Nickel metal is classified as "Harmful".

   Due to its large surface area and high volume of contained hydrogen
   gas, dry, activated Raney nickel is a pyrophoric material that should
   be handled under an inert atmosphere. Raney nickel is typically
   supplied as a 50-percent slurry in water. Care should be taken never to
   expose Raney nickel to air. Even after reaction, Raney nickel contains
   significant amounts of hydrogen gas, and will ignite when exposed to
   air.

   Raney nickel will produce hazardous fumes when burning, and therefore
   the use of a gas mask is recommended when extinguishing fires caused by
   it. Additionally, acute exposure to Raney nickel may cause irritation
   of the respiratory tract, nasal cavities and pulmonary fibrosis if
   inhaled. Ingestion may lead to convulsions and intestinal disorders. It
   can also cause eye and skin irritation. Chronic exposure may lead to
   pneumonitis and other signs of sensitization to nickel like skin rashes
   ("nickel itch").

                     Image:nfpa h1.png Image:nfpa f3.png Image:nfpa r1.png

   Nickel is also rated as being a possible human carcinogen by the ( IARC
   Group 2B, EU category 3) and teratogen, while the inhalation of fine
   aluminium oxide particles is associated with Shaver's disease. Care
   should be taken when handling these raw materials during laboratory
   preparation of Raney nickel. Moreover, activation of Raney nickel
   produces large amounts of hydrogen gas as a by-product, which is also
   highly flammable.

Development

   Murray Raney graduated as a Mechanical Engineer from the University of
   Kentucky in 1909. In 1915 he joined the Lookout Oil and Refining
   Company in Tennessee and was responsible for the installation of
   electrolytic cells for the production of hydrogen which was used in the
   hydrogenation of vegetable oils. During that time the industry used a
   nickel catalyst prepared from nickel(II) oxide. Believing that better
   catalysts could be produced, around 1921 he started to perform
   independent research while still working for Lookout Oil. In 1924 a 1:1
   ratio Ni/Si alloy was produced, which after treatment with sodium
   hydroxide, was found to be five times more active than the best
   catalyst used in the hydrogenation of cottonseed oil. A patent for this
   discovery was issued in December 1925.

   Subsequently, Raney produced a 1:1 Ni/Al alloy following a procedure
   similar to the one used for the nickel-silicon catalyst. He found that
   the resulting catalyst was even more active and filed a patent
   application in 1926. It may be of interest to note that Raney's choice
   of nickel-aluminium ratio was fortuitous and without any real
   scientific basis. However, this is the preferred alloy composition for
   production of Raney nickel catalysts currently in use.

   Following the development of Raney nickel, other alloy systems with
   aluminium were considered, of which the most notable include copper,
   ruthenium and cobalt. Further research showed that adding a small
   amount of a third metal to the binary alloy would promote the activity
   of the catalyst. Some widely used promoters are zinc, molybdenum and
   chromium. Recently, a way of preparing enantioselective Raney nickel
   has been devised by surface adsorbtion of tartaric acid.
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