Researchers at the University of Johannesburg (UJ) Department of Chemical Sciences on June 28 announced that, in processes in which compressed hydrogen is not needed, the use of a stable three-phase emulsion to transform a toxic waste product into valuable feedstock makes hydrogenation into a safe, low-energy process.

“Hydrogenation is necessary to manufacture many of today’s medicines. It is a backbone process for the pharmaceutical and chemical industries. However, hydrogen is expensive and safety measures to prevent explosions in factories and laboratories are costly.

"If compressed hydrogen is not needed, significant savings are possible. It also means that many chemical processes can be much safer and easier to work with.”

The researchers convert nitrobenzene into aniline using a catalysed hydrogenation process in a Pickering emulsion.

Nitrobenzene is produced in huge amounts globally as waste from chemical manufacturing. Aniline, meanwhile, is a significant industrial commodity and is a feedstock for many chemical products, including many medicines.

The emulsion process has the potential to be a much safer industrial hydrogenation process than those currently in use, says UJ Department of Chemical Sciences researcher Professor Reinout Meijboom.

The process the researchers designed uses toluene to dissolve the nitrobenzene. This forms the first, organic or toluene phase of the process. For the second aqueous phase, they dissolve sodium borohydride in water.

The catalyst is the third phase in the process. It consists of modified silica microspheres and palladium. They also use a bimetallic catalyst, where palladium is combined with cobalt or nickel.

If the three phases are added together, but not mixed into an emulsion, the combination can be stored for days or weeks, says Meijboom.

“A small amount of hydrogenation takes place, but the process only really gets going once a proper emulsion is formed.”

“The catalyst also acts as a stabilising emulsifier. When the three phases are mixed into an emulsion, the catalyst starts the hydrogenation process. The formation of the emulsion takes a few seconds. The reaction takes about two hours at laboratory scale,” says Meijboom.

The hydrogen needed for hydrogenation is supplied by the dissolved sodium borohydride. Hydrogenation happens efficiently at room temperature, which saves energy. There is no need for stored or piped hydrogen. This removes most of the explosion risk from the process, he adds.

After the hydrogenation completes, the resulting emulsion is stable enough for storage for a few days, before separating out the aniline.

“By using emulsion chemistry, we have one system where catalyst, emulsifier, aqueous and organic phases all mix up into an extremely stable system,” says Meijboom.

“The use of solid particles as catalysts and emulsifier, or stabiliser, poses less threat to the environment than conventional surfactant. The composition is less toxic,” says UJ Department of Chemical Sciences researcher and study co-author Dele Peter Fapojuwo.