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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