Greener Industry - Phenol - Production Methods Summary

Alternatives

One alternative method that has been in use since 1961 makes phenol by oxidising methylbenzene (toluene). The main advantage of this process is that phenol is the only major product, so there is no dependence on the market for any other by-product. Its disadvantage is that it requires considerably more energy, and it may only become more common if a catalyst can be developed that will enable the reaction under milder conditions.
For more on this process, click here.

Early Production Methods

Phenol was first manufactured commercially by distillation from coal tar, but the energy required and quantities produced meant this method could not be sustained. Synthesis by the sulphonation of benzene was implemented by BASF in 1899 and continued until the 1960s. One of its main disadvantages is the poor atom economy of the reaction, a factor that has become increasingly important as the chemical industry seeks to develop more sustainable processes. For more details of the atom economy of this process, click here.

The sulphonation route produced large quantities of waste, and used aggressive reagents, and so several other methods, mostly using benzene as the starting point, were developed, though these also used reagents like chlorine, sodium hydroxide and hydrogen gas and so are not in use.

Developments

Converting benzene to phenol is an oxidation, and these are often the most challenging reactions for chemists. The aim is to develop a process that will add one oxygen atom in one step - a transformation easily carried out by natural enzymes.

Benzene can be oxidised to phenol through several different routes, and so research is currently trying to establish a catalyst that will result in an economically viable process. Two fruitful directions are:

The development of microreactor technology (a reaction vessel that would fit on a large desktop!) using benzene, oxygen and hydrogen
Oxidation using nitrous oxide (N₂O) and a zeolite catalysis

Using Biomass

Continuing to use benzene as the main feedstock preserves dependence on non-renewable fossil fuel resources, so other directions of research seek to use renewable starting materials to make products previously made from phenol, and includes:

Using phenol-rich liquids from biomass, particularly grasses and softwoods, from which phenol can be extracted or which can be used directly in its place.

Click for information on progress with these developments

Cumene Route

The main method used to manufacture phenol since the 1960s has been through the oxidation of 1-methylethylbenzene, commonly called cumene, which is made from benzene.
diagram: 1-methylethylbenzene (Cumene)
Starting with benzene and propene, the whole process includes three steps, with the only other major product being propanone (acetone). Although there is a market for propanone as a solvent, demand for phenol is rising faster than demand for propanone.

This is one reason why alternative processes are being developed, combined with the fact that it is a three-step process with a potentially explosive intermediate. The possibility of converting excess propanone to propene for re-use is being explored by some companies. Click here for more on the cumene oxidation process.