By Karen Peattie

RESEARCHERS at the University of Glasgow say they have discovered a “greener, cleaner way” to produce a common chemical used by industries including pharmaceuticals.

In a new paper published in the journal Cell Reports Physical Science, Glasgow researchers demonstrate a new method of creating anilines – chemicals commonly used in the manufacture of products including dyes, plastics and insulation, and pharmaceuticals like paracetamol.

The Glasgow team has developed a method of aniline production which works at room temperature and pressure, and which uses protons and electrons created by the electrolysis of water to drive the process of reducing the nitrobenzenes to anilines.

Dr Mark Symes, senior lecturer at the University of Glasgow’s School of Chemistry, who developed the technique with graduate student Athanasios Stergiou, said the “exciting result” builds on a decade of work at Glasgow in developing new forms of mediators. “We’ve been able to demonstrate for the first time a way to produce high yields of common anilines using the mediator we’ve developed at room temperature, without the need for high pressures,” he said.

Industrial chemical plants have for decades created anilines by reacting organic compounds called nitrobenzenes with hydrogen at high temperatures and pressures. The reactions, according to the researchers, often involve catalysts made from precious metals.

They said: “These processes, which often produce tons of anilines at once, are energy-intensive, frequently inefficient, and can create chemical waste by-products which can be harmful to the environment. The hydrogen used in the reactions is often derived from unsustainable sources including fossil fuels.”

Dr Symes added: “Although direct electrochemical reduction of nitrobenzenes has been investigated for more than 40 years, it produces a number of undesirable side products and delivers low yields of anilines.”

Usually, electrolysis involves passing an electrical current through a positively-charged electrode and a second, negatively-charged electrode, both of which are submerged in water. The electricity splits the water molecules into hydrogen gas at one electrode and oxygen gas at the other.

The University of Glasgow’s breakthrough process works by passing an electrical current through water, into which a special type of molecule called a redox mediator has been dissolved. “The mediator molecules collect the protons and electrons produced by electrolysis of water and deliver these to the nitrobenzenes,” the researchers noted.

“The mediator molecules also prevent direct electro-reduction of nitrobenzenes at the electrode surface. In this way, the process of nitrobenzene reduction is made cleaner, cutting down the likelihood of unwanted side-products being created during electrolysis.”

In the paper, the researchers show the effectiveness of their technique by reducing 12 different types of nitrobenzenes to form anilines, ranging from those commonly used in industry to more unusual products. Their results show that the process is more than capable of cleanly producing anilines, in four cases producing yields of over 99 per cent.

“Electrochemistry is inherently scalable, so we believe that it would be possible to stack mediator modules to enable industrial production of anilines,” noted Dr Symes. “If those large-scale reactions were powered using renewable sources of electricity, it could go a long way towards making the $11 billion (£9.2bn) aniline production industry more environmentally-friendly.

“At the other end of the scale, we’ve been able to show that it is possible to cleanly produce smaller quantities of more exotic anilines, like ortho-iodides, which are difficult to create by other methods. That could help make those anilines more economical to produce, and more accessible to researchers and industry in the future.

“Now that we’ve demonstrated the effectiveness of this approach to aniline production, we are beginning to explore the potential of applying it to other reactions. We hope to create similarly transformative benefits elsewhere.”

The team’s paper, titled High Yield and Selective Electrocatalytic Reduction of Nitroarenes to Anilines using Redox Mediators, is published in Cell Reports Physical Science. The research was supported by funding from the Engineering and Physical Sciences Research Council (EPSRC) and the Royal Society.