Catalysts and Waste

In chemical manufacture, waste is produced when reactions produce side products of little or no value, as well as the desired product.

Catalysts allow reactions to be developed that make the desired product, but few or no co-products. They may also reduce products from competing reactions, making product separation easier.

Phenol

Improved methods for the production of phenol have resulted in much reduced waste. The development of new zeolite catalysts have enabled benzene to be oxidised to phenol using dinitrogen monoxide (N2O), with nitrogen the only other product. This reaction is particularly useful in nylon manufacture link to nylon section at Production method 4 - Alternative Hexanedioic Acid Routes, where N2O is produced by other processes and would otherwise require treatment as waste.


diagram: catalysts used in ethanoic acid manufacture

Ibuprofen

Ibuprofen is one of the four commonly used painkillers. The original method for making it used six steps, with aluminium chloride (AlCl3) as a catalyst, and several other reagents. Although behaving as a catalyst, the AlCl3 cannot be recovered, and must be disposed of.

A newer method uses two true catalysts, hydrogen fluoride and Raney Nickel (an alloy of nickel and aluminium). These can be recovered and re-used many times.

The effect of using better catalysts has been to improve the atom economy of the process, and reduce the number of steps needed from six to three.


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