This is the general title for a whole family of catalysts, each potentially customised for a particular reaction. Improved spongy nickel catalysts are one of the areas of study of the IMPRESS Project. You may sometimes hear the material referred to as ‘Raney Nickel’ although this is a registered trade name. Spongy nickel starts its life as blocks of pure nickel and aluminium. Equal masses of the two metals (together with any additional ‘promoters’) are melted together and cast into bars. The material is then either crushed and ground to a fine powder or is melted and turned into minute droplets by a process called ‘gas atomisation’ (rather like a high temperature aerosol spray). The rate of cooling is important as this affects the ‘phases’ of the nickel aluminides present in the catalytic material (e.g. Ni2Al3 and NiAl3). Different cooling rates can lead to varying proportions of the different phases as well as the sizes of the phase grains.
What can you say about the shapes and distribution of particle sizes generated by the two methods? Can you explain any of your observations? Could the method of production have any affect on the effectiveness of the catalyst?
The particle size in both pictures may seem small but the process is not yet finished. The catalytic material is next ‘leached’ with concentrated sodium hydroxide. This removes most of the aluminium leaving an open skeleton largely of metallic nickel with areas remaining of nickel aluminides and oxides of aluminium and the promoter impurities.
The result of the leaching is to massively increase the surface area of the catalytic particles. A typical leached powder has an effective surface area of 100 m2 g-1 and an apparent density of around 6.5 g cm-3.
After leaching the powders have to be stored and transported under deoxygenated water. If they dry out the metallic nickel will react with atmospheric oxygen. This happens with to the surface atoms of any sample of bulk nickel but in the catalyst a large fraction of the atoms are at the surface and so the reaction is extremely exothermic, to the extent that that catalyst can spontaneously burst in to flames and so the powder is described as being ‘pyrophoric’. If large quantities of oxygen are dissolved in the water then the metallic nickel can be slowly oxidised and this leads to ‘aging’ of the catalyst, it gradually becomes less active. This is an important point to realise for industrial chemistry since it shows that even though a catalyst is not used up in a chemical reaction it may, none the less have a limited usable life.
Spongy nickel catalysts have many applications. In particular they react well with hydrogen (a great deal of hydrogen is already adsorbed on to the surface of the catalyst as a result of the leaching process). As a result spongy nickel catalysts are used extensively in the chemical industry in hydrogenation reactions. Typical applications are:
The catalyst breaks open the glucose ring structure and attaches hydrogen atoms to either end. The greatest use of sorbitol is in the food industry where it is used as an artificial sweetener. It is also used a humectant (moisture retainer) in foodstuffs and cosmetics.
The catalyst breaks attaches hydrogen atoms to either end. Most cyclohexane is used as a feedstock in the production of plastics such as polyurethane and polyamides (including the various types of nylon).
Oils and fats are two members of the same family of chemicals, the lipids or more specifically the triglycerides. They consist of a single molecule of glycerol (C3H5(OH)3) attached to three fatty acids (e.g. stearic (octadecanoic) acid CH3(CH2)16 COOH and oleic (cis-9-octadecenoic) acid CH3(CH2)7 CH=CH(CH2)7 COOH). The fatty acids attached to a given glycerol molecule can be of any type, they need not all be the same.
If the fatty acids include one double bond (e.g. oleic acid, the major constituent of olive oil) they ‘monounsaturated’, if there are two or more then they are ‘polyunsaturated’. The only real difference between oils and fats is the number of double bonds; this affects their melting point. If they have a large number of double bonds they have a low melting point and so are liquid at room temperature and are called oils. The fewer the number of double bonds the higher the melting point. If they are solid at room temperature they are called fats. Ultimately if there are no double bonds then it is described as being saturated, since it can accept no more hydrogen atoms into the carbon chain structure.
Generally fats are more useful in foodstuffs, partly because they are easier to work with (try ‘rubbing in’ butter to flour to make a pastry and then try repeating the process with olive oil!) but mainly because they are less likely to go rancid and so spoil (double bonds are sites where the fatty acids can be easily oxidised). However most of the naturally occurring, commercially available triglycerides, such as vegetable oils, are heavily polyunsaturated and thus are oils. The food industry takes polyunsaturated oils and by reacting them with hydrogen over a nickel catalyst, hydrogenates them. This is the process used to convert liquid vegetable oils such as sunflower or olive oil in to margarines and low fat spreads.
First, hydrogen gas (H2) is adsorbed on to the surface of the catalyst. The molecular hydrogen rapidly splits in to adsorbed atomic hydrogen which is free to migrate across the surface. This is exactly the same as in the Haber Process except that here the catalyst is spongy nickel.
Next the unsaturated fatty acid is adsorbed on to the catalyst surface. This breaks the double carbon bond, each carbon making a temporary bond with the catalytic surface. This step can be reversed with the fatty acid being released from the catalyst surface with no net chemical change taking place.
An adsorbed hydrogen atom then binds to one of the carbons, thereby breaking the temporary bond with the catalyst surface. At this point the chain may well rotate to adopt a lower energy state (this could not have happened to the free molecule since double bonds are not free to rotate). This step may also be reversed, with a hydrogen atom binding back on to the catalyst surface.
Finally the last carbon atom binds to an adsorbed hydrogen atom. The altered fatty acid is no longer bound to the catalyst surface and so drifts away.
The result is that the hydrogen reacts with the carbon double bonds and the fatty acids become more saturated. The extent to which a polyunsaturated oil approaches becoming saturated will depend on the temperature of the reactor and the time that the oil is held in it.
It is possible for the unsaturated fatty acid to be desorbed from the surface without any net addition of hydrogen. None the less there may be a chemical change from a cis-fatty acid to a trans-fatty acid. What is this change and how can it come about?