Leipzig research team develops process for biobased nylon

In T-shirts, stockings, shirts, ropes or even as a component of parachutes and car tyres, polyamides are used everywhere as synthetic fibres for which the name nylon was created at the end of the 1930s. Nylon-6 and nylon-6,6 are two polyamides that make up around 95 per cent of the global nylon market and are produced from fossil raw materials. However, this petrochemical process is harmful to the environment - on the one hand because it emits around ten per cent of nitrous oxide (laughing gas), which is harmful to the climate, and on the other because it requires a lot of energy. „Our goal is to make the entire nylon production chain greener. This is possible if we use bio-based waste as starting materials and make the synthesis process sustainable," says Prof. Dr Falk Harnisch, Head of the Electrobiotechnology Working Group at the Helmholtz Centre for Environmental Research (UFZ).

The Leipzig researchers led by Falk Harnisch and Dr Rohan Karande (Leipzig University/ Research and Transfer Centre for Bioactive Matter b-ACTmatter) have described how this can be achieved in an article for the scientific journal Green Chemistry. Nylon, for example, consists of around 50 per cent adipic acid, which has so far been obtained industrially from crude oil. In a first step, phenol is converted to cyclohexanol, which is then converted to adipic acid. This energy-intensive process requires high temperatures, high gas pressure and organic solvents, and also releases a lot of nitrous oxide and carbon dioxide. The researchers have now developed a process in which they can convert phenol into cyclohexanol using an electrochemical process. „The underlying chemical conversion is the same as in the established processes: However, the electrochemical synthesis replaces the hydrogen gas with electrical energy, takes place in an aqueous solution and only requires ambient pressure and room temperature," explains electro-biotechnologist Falk Harnisch. A suitable catalyser is needed to ensure that this reaction runs as quickly and efficiently as possible. This should maximise the yield of electrons required for the reaction and the efficiency of how much cyclohexanol is ultimately produced from phenol. In laboratory experiments, the best yields were achieved with a carbon-based rhodium catalyst with almost 70 per cent electrons and just over 70 per cent cyclohexanol. The relatively short reaction time, the efficient yield and the effective use of energy as well as synergies with the biological system make this process attractive for the combined production of adipic acid," says Dr Micjel Chávez Morejón, UFZ chemist and first author of the study. How the bacterium Pseudomonas taiwanensis converts cyclohexanol into adipic acid in a second step had already been discovered in earlier research work by two other UFZ working groups led by Prof Dr Katja Bühler and Prof Dr Bruno Bühler. „Until now, it had not been possible to make the reaction of phenol to cyclohexanol take place microbially. We have closed this gap with the electrochemical reaction," summarises Dr Rohan Karande, who is now continuing this work at the University of Leipzig in cooperation with the UFZ.

And the Leipzig researchers were able to close yet another gap in green nylon production by developing an environmentally friendly alternative to phenol, which is produced from fossil raw materials. To do this, they used monomers such as syringol, catechol and guaiacol, all of which are a by-product of lignin - a waste product of the timber industry. We have been able to show for these model substances that together we can go all the way to adipic acid," says Falk Harnisch. And Rohan Karande adds: "Around 4.5 million tonnes of adipic acid are produced worldwide. If we develop wood residues for this, it would have a decisive impact on the global market.

However, there is still a long way to go before lignin-based nylon is ready for the market. The scientists have so far achieved a yield of 57 per cent for the 22-stage overall process, i.e. from the monomers from lignin residues using electrochemical and microbial reaction steps to adipic acid. „That is a very good yield“, says Micjel Chávez Morejón. The results are still based on laboratory tests on a millilitre scale. For this reason, the prerequisites for bringing the process to the litre scale are to be created over the next two years. This technology transfer requires not only a better understanding of the entire process, but also the use of real lignin mixtures instead of model mixtures and the improvement of electrochemical reactors. Falk Harnisch and Rohan Karande agree: „The process for the lignin-containing nylon is an example of the great potential of electrochemical-microbial processes, as an optimal process chain can be built up through the intelligent combination of different components.

The process for the development of biobased nylon is funded by the UFZ programme for innovation „transfun“, which supports the implementation of ideas in applications at the UFZ. The project funds provided totalling 250,000 euros are supplemented by the University of Leipzig's own contributions.

Press release of the "idw - Informationsdienst Wissenschaft" from 04.07.2023