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Catalysts

The distinction between chemical and catalytic uses is somewhat blurred. For purposes of clarification, applications of cobalt salts as driers in paints and inks and as curing agents for unsaturated polyester resins are treated as chemical rather than catalytic.

By definition, a catalyst is not consumed in a chemical reaction, but there are always “in process” losses as well as losses in the catalyst recovery, recycling and operations.

Cobalt has major uses in the petrochemical and plastic industries, as both a hetero and homogeneous catalyst. We can deal here with the three main uses which, between them, account for 2,700 tonnes of cobalt (metal).

Firstly, hydro-treating and desulphurisation catalysts for oil and gas. These (CoMOX) catalysts are typically 3-5% cobalt oxide (Co3O4) and 14% MoO3 (molybdenum trioxide), the balance being Al2O3 (alumina).

Hydrodesulphurisation is a process common to all refineries. In it, the feedstock (or crude oil fraction) and hydrogen are passed over a catalyst at elevated temperature and pressure. The aim is to convert the organic sulphur to hydrogen sulphide (H2S). All crude oils contain sulphur in quantities varying from .1 to 2.5% depending on their origin. It must be removed for health and safety reasons amongst others. The CoMOX catalysts are universally used for this are very resistant to poisoning and degradation. They can be constantly regenerated and retain their usefulness for several years. They account for the largest amount of cobalt used in this field.

The second major use of cobalt is in the mixed cobalt acetate/manganese sodium bromide homogeneous catalyst for production of terephthalic acid (TPA) and di-methylterephthalate (DMT). These materials are used to manufacture resin for plastic bottles and also to make the new ultra strong plastics used in recording tapes. It is also the source of the better-known terylene. But anything containing polyester – that’s what it is, courtesy of cobalt, PET, polyethylene terephthalate. The TPA or DMT are made from paraxylene by oxidation with nitric acid or air. The process is catalysed homogeneously using cobalt acetate or bromide with manganese present. TPA has a catalyst with a ratio of 1 Co:3 Mn and DMT, a higher cobalt ratio (10:1). However, recycling is easier from DMT spent reactants than TPA and all in all, TPA production uses the same (if not a larger) amount of cobalt (total 1,700 tonnes of cobalt is used worldwide in this process).

The other large cobalt use in the catalyst field is hydroformylation where the end products are alcohols and aldehydes for plastic and detergent manufacture.

Although the cobalt can be introduced to the systems as metal, oxide, hydroxide or as an inorganic salt, the active catalyst species is a carbonyl in a semi-regenerative cycle system [Hco(Co)4].

The OXO reaction is known as hydroformylation and involves the addition of hydrogen and formyl groups across the double bond of alkenes. The reaction is of the type:


catalyst
H2C = CH2 + Co + H2 -> H3C – CH2CHO

The products being intermediates for a wide variety of chemicals, especially alchohols but also amines and acids.

So these are the 3 main catalysts:

1.  Hydro-treating, desulphurisation – cobalt molybdenum oxide –

     CoMOX
2.  Terephthalic acid + dimethylterephthalate – cobalt acetate +

     manganese

3.  OXO catalysts – freshly reduced cobalt metal, carbonyls or cobalt

     salts (transformed in situ to carbonyl)

Catalysis is a complex subject but cobalt’s use in the above 3 situations and in fact in many others, depends on the following facts:

a) The oxidation-reduction properties of cobalt and its ability to

     demonstrate several valencies I + II + III with easy electron transfer

     between these states.
b) Cobalt’s ability to form complexes by accepting atoms from other

     molecules.
c) Cobalt chemicals in solution and in polymerisation systems can

     decompose to give more than one ion to take part in catalysis, i.e.

     carboxylates – Co(RCoO)2 -> Co2+ + 2RCo2-
d) Solid cobalt compounds have vacancies in their crystal lattices which

     can take part in catalysis.

 

    
 
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