A green oxidant and water instead of organic acid were used for converting toluene into benzoic acid

Using platinum nanocatalyst, a two-member team at the Indian Institute of Technology (IIT) Madras has successfully converted petroleum waste-product toluene into benzoic acid. Benzoic acid is used as a food preservative (E210) and medicine for fungal/bacterial infection. Toluene is converted into benzoic acid through selective and controlled oxidation in the presence of a catalyst — binaphthyl-stabilised platinum nanoparticles (Pt-BNP).

Green oxidant

Generally, organic reactions are carried out using organic solvents, which makes it expensive and also generates toxic waste. So in a departure from current practice, the team led by G. Sekar from the institute’s Department of Chemistry has used water as solvent to make it environment-friendly. Also, a green oxidant (70% aqueous tert-butyl hydroperoxide or TBHP) is used for converting toluene into benzoic acid.

“When toluene is oxidised, it gives four products. But when we use the catalyst that we developed, only benzoic acid is produced. No alcohol, aldehyde or ester is produced,” says Prof. Sekar. The yield of benzoic acid varied from 68-96% depending on whether the toluene used is electron-deficient or electron-rich. The results of the study were published in the journal Applied Catalysis B: Environmental.

Central to the work is the novel catalysts that the team developed. Generally, platinum nanoparticles are not stable in nature as they tend to agglomerate and become macroparticles. The catalytic activity is reduced once it becomes macroparticles. The binaphthyl that is bound to platinum nanoparticles acts as a stabiliser and prevents nanoparticle agglomeration.

“Binaphthyl bound to platinum nanoparticles makes the catalyst easy to handle and stable. It is the stability of the catalyst to remain as nanoparticles that allows us to recover it and reuse the catalyst up to five times,” says Prof. Sekar. There was no change in the size of the catalyst even after being reused five times.

Toluene when oxidised gets converted into benzoic acid. Molecular oxygen when used alone does not oxidise toluene and so no benzoic acid is generated. So the researchers used TBHP as an oxidiser. “The catalyst reacts with TBHP to initiate the oxidisation reaction where toluene gets converted into benzoic acid through a series of reaction steps,” says Rajib Saha, a PhD student at IIT Madras and co-author of the paper.

Economical combination

When used alone, a large quantity (four parts of TBHP to 1 part of toluene) of TBHP would be required for the conversion, which will not be economically favourable. In order to reduce the amount of TBHP used, the researchers also used molecular oxygen.

“In the presence of molecular oxygen, only two parts of TBHP are needed for the conversion. So molecular oxygen behaves as a co-oxidiser,” says Prof. Sekar. “Molecular oxygen is cheap, so using it along with TBHP helps in reducing the cost.” The use of TBHP along with molecular oxygen also increased the yield of benzoic acid.