These newer catalysts are increasingly
being used. Their advantages in this reaction are:
- The catalyst can be removed, regenerated
and returned for re-use more or less indefinitely
- No hazardous waste or acidic emission
results from the use of zeolite catalysts
- They are relatively cheap
- They work with lower quality feedstock,
yet produce a higher quality product
- They produce a higher proportion of
1 -methylethylbenzene, and little propylbezene, reducing energy
used in purification
more information on zeolite catalysts, see the catalysis site
Production of 1-methylethylbenzene
Cumene is made from benzene
The reaction is catalysed by acid, and can be carried out in either
the vapour or liquid phase.
||Solid, pelletised phosphoric
acid (H3PO4) is used as a catalyst in
the vapour phase reaction. Phosphoric acid is very corrosive,
and disposal of waste from this process can be a problem.
The catalyst cannot easily be regenerated when it reaches
the end of its useful life.
||The Lewis acid aluminium chloride
(AlCl3) is used as the catalyst in the liquid phase.
This is an example of a Friedel-Crafts catalyst (see catalysis
site for more details) (link to cat site). It has the advantage
that the reaction will take place at 100°C and normal
atmospheric pressure, but corrosive waste is produced, and
the catalyst cannot easily be regenerated.
In both cases the 1-methylethylbenzene
(cumene) has to be separated from other products of the reaction,
which will include the other addition product, propylbenzene.
Oxidation of 1-methylethylbenzene (cumene)
The reaction is carried out at temperatures
in the range 90 - 130ºC and pressures of 1-10 atmospheres.
Careful control of acidity levels, temperature and pressure are
vital as at higher temperatures, the hydroperoxide is unstable
and can decompose violently.
To help reduce the risk of this happening, only
25% of the cumene is allowed to react at any one time in order
to keep the concentration of the hydroperoxide within safe limits.
Un-reacted cumene has to be separated out and recycled, adding
to the costs.
This step also produces the major impurities
of this process.
Decomposition of cumene hydroperoxide to phenol
The hydroperoxide is mixed with dilute sulphuric
acid at 60 - 70ºC, to produce both phenol and propanone (acetone)
as products. The decomposition of cumene hydroperoxide to phenol
and acetone is often described as a cleavage reaction.
Two useful products are therefore obtained,
phenol and propanone. Unfortunately this means the production
of phenol is in part dependent on the demand for propanone, which
is rising at a lower rate than phenol, and so in the future propanone
may become a waste product.
The overall process, from benzene to phenol
and propanone, is:
If we assume that both phenol and propanone
are desired products, and that the yield is 100%, this will clearly
give an atom economy of 100% - all the atoms in
the reactants appear in the products.
In practice the yield is more likely to be in
the region of 90%, but even taking this into account, and assuming
all the propanone is waste, still gives an atom economy of 55.7%,
comparing favourably with the old sulphonation process at 36.7%.
The Mitsui company has developed the process
to include recycling the propanone. It is converted to propene,
a reactant in the first step, using two further
steps. Though this may help address overproduction of propanone,
a five step process using hydrogen is not an ideal solution.
What would the atom economy be if all the propanone is considered