Oxidation of 1-Methylethylbenzene

Zeolite Catalysts

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

For more information on zeolite catalysts, see the catalysis site

Production of 1-methylethylbenzene (cumene)

Cumene is made from benzene and propene.
diagram: 1- methylethylbenzene from benzene and propane
The reaction is catalysed by acid, and can be carried out in either the vapour or liquid phase.

Vapour 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.
Liquid phase 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.
diagram: 1- methylethylbenzene to cumene hydroperoxide

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

diagram: cumene hydroperoxide to phenol and propanone

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.

Atom Economy

The overall process, from benzene to phenol and propanone, is:

equation: benzene to phenol and propanone

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%.

Recycling Propanone

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.

Question

What would the atom economy be if all the propanone is considered waste?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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