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Low-Energy Hydrogenation Without H2 Gas

Hydrogenation is an important reaction in many sectors of the chemical process industries (CPI), but H2 gas is not only expensive, its use requires considerable safety measures to prevent explosions.


By Gerald Ondrey

Significant cost savings could be achieved if hydrogenations did not require compressed H2 gas. Researchers at the Dept. of Chemical Sciences, University of Johannesburg have taken a step in this direction by developing a safe, low-energy process for transforming nitrobenzene, a toxic waste product, into aniline, an important feedstock for many chemicals and medicines — without the need for compressed H2.

The researchers use a so-called Pickering emulsion — an emulsion that is stabilized by solid particles — to convert nitrobenzene into aniline. As described in a recent issue of Colloids and Surfaces, the hydrogenation takes place at the aqueous-organic interface of the Pickering emulsion, where the solid catalyst particles also serve to stabilize the emulsion.

The catalyst consists of modified silica microspheres and palladium, as well as a bimetallic catalyst (PdM, where M = Co, Ni). The other two phases of the emulsion are toluene, which dissolves the nitrobenzene, and an aqueous solution of sodium borohydride. The stable droplets of the water-toluene emulsion act as microreactors for the reaction, with the NaBH 4 supplying the hydrogen needed for the reduction.

If the three phases are added together, but not mixed, the combination can be stored for days or weeks, says professor Reinout Meijboom. When the three phases are mixed into an emulsion, which takes a few seconds, the catalyst kick-starts the reaction. At the laboratory scale, the reaction takes about 2 h at room temperature.

The emulsion process has the potential to be a much safer industrial hydrogenation process than those currently in use, says Meijboom.

Originally published at Chemical engineering online

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