Sponge nickel catalysts are used in a wide range of hydrogenation reactions in the pharmaceutical industry. Dr Nicholas J. E. Adkins, technology manager at CERAM, describes a new process that provides sponge nickel-based catalysts with superior properties
Raney nickel (generically termed sponge nickel) is a high surface area solid catalyst composed of fine grains of nickel. Developed in 1926 by Murray Raney1 as an alternative catalyst for the hydrogenation of vegetable oils, it is now commonly used as a heterogeneous catalyst in many organic synthesis processes - mainly in hydrogenation reactions.
Such reactions are widely used in the pharmaceutical industry, for example in the hydrogenation of nitriles to amines and stereoselective reductive alkylation.
Traditionally, the catalyst is prepared by alloying nickel and aluminium (usually in a 50:50 wt% ratio), crushing the ingot into a powder and then leaching out the aluminium with alkali to leave finely divided porous nickel.2 This remaining structure has a large surface area, which gives it a high catalytic activity. Other metals that can be processed in a similar way are copper, cobalt, iron, silver and platinum.
Over the past few years, experts at the research and technology company CERAM have developed a spray deposition process that produces an active nickel catalyst supported on a metal foil that can be coiled or formed into a variety of different shapes suitable for use in efficient loop reaction vessels.3
Supported catalysts made using spray deposition provide many benefits. The main one is that with a substrate support, the removal of the cata-lytic material is much easier with no time being wasted separating out the catalyst from the reacted material a problem that occurs with the catalyst in slurry form. A designed benefit of this is the ability of the reactants to move over the catalyst easily in a continuous way when used in a continuous reaction process such as a loop reactor. There is no loose powder to be filtered.4
Spray deposition is a development of gas atomisation processing. In gas atomisation, molten metal is broken up into small droplets and rapidly solidified during freefall before the drops come into contact with each other or with a solid surface. The principal method is to disintegrate a thin stream of molten metal by subjecting it to the impact of high-energy jets of gas (air, nitrogen or argon).5 Particles are then collected at the bottom of the atomisation chamber.
In spray deposition mode the method comprises melting together a Raney metal and aluminium to form an alloy mixture, which is then poured through a nozzle before directing a gas jet of either argon or nitrogen onto the mixture to form a spray of droplets immediately downstream of the nozzle (see Figure 1).
These droplets are then directed onto a metallic substrate, such as iron, mild steel, stainless steel, titanium, nickel or copper. Under normal conditions, the substrate will be in foil form without perforations; it can, however, be a gauze-like material. The process is controlled so an exothermic reaction takes place between the substrate and the sprayed material on initial contact to form an intermetallic bond.
A range of metals can be used for this process. The Raney metal and aluminium are preferably melted together above the liquidus temperature of the alloy mixture. The alloy mixture may be a nickel-aluminium alloy, which ideally contains 40-70 wt% nickel. Alternative alloy mixtures are listed in Table 1.
There are several advantages to producing sponge catalysts using a gas atomisation process. The casting and crushing stage is not required so there are fewer manufacturing steps and because gas atomisation is a high cooling rate process, novel alloy compositions can be processed, leading to a significant improvement in activity.
This increased activity may allow their application in alkaline fuel cells with a significant cost saving as platinum group catalysts are substituted.
The production of sponge nickel catalysts processed by gas atomisation is being further investigated by CERAM in the EU-funded project IMPRESS (Intermetallic Materials Processing in Relation to Earth and Space Solidification).6
The high cooling rates of the gas atomisation process have shown advantages in other projects. For instance, in the production of lanthanum nickel alloy powder for nickel metal hydride batteries, the rapid solidification rate suppresses the formation of high rare earth content phases, increasing cycle life.7
In conclusion, CERAM has developed a novel method for preparation of sponge nickel catalysts using a gas atomisation process. This method has benefits for the pharma industry, in the production of a nickel compound with increased activity compared with traditionally manufactured intermetallics.