Nylon has long been produced from petroleum-based raw materials. However, this is quite harmful to the environment because of the use of non-renewable fossil resources, as well as the need for large amounts of energy and the emission of climate-damaging nitrous oxide during the production process. A research group from the Helmholtz Centre for Environmental Research (UFZ) and the University of Leipzig has now developed a process to produce adipic acid (one of the two components of nylon) from phenol by electrochemical synthesis and using microorganisms. The team also claims that phenol can be replaced by waste materials from the wood industry. These wastes can be used to produce bio-based nylon. The research work was published in the journal Green Chemistry.
In T-shirts, stockings, shirts and ropes, or as a component of parachutes and car tires, polyamide is widely used as a synthetic fiber. In the late 1930s, this synthetic polyamide was named "nylon". Nylon 6 and nylon 6,6 are two polyamides that account for about 95% of the global nylon market. Until now, they have been produced from fossil raw materials. However, this petrochemical process is harmful to the environment because it emits about 10% of climate-damaging nitrous oxide globally and requires large amounts of energy. Our goal is to make the entire nylon production chain environmentally friendly," says Dr. Falk Harnisch, head of the e-biotechnology working group at the Helmholtz Centre for Environmental Research (UFZ). Our goal is possible if we use bio-based waste as a raw material and make the synthesis process sustainable."
Researchers at the University of Leipzig, led by Drs. Falk Harnisch and Rohan Karande, describe how this was achieved in an article published in Green Chemistry. For example, nylon is composed of about 50% adipic acid, which until now has been extracted industrially from petroleum. In the first step, phenol is converted to cyclohexanol and then to adipic acid. This energy-intensive process requires high temperatures, high pressures and large amounts of organic solvents. It also releases large amounts of nitrous oxide and carbon dioxide. Researchers have now developed a process by which they can convert phenol to cyclohexanol using an electrochemical process, explains Dr. Falk Harnisch: "The chemical transformation behind it is the same as the established process. However, the electrochemical synthesis uses electrical energy instead of hydrogen, it takes place in an aqueous solution, and only requires ambient pressure and temperature."
Dr. Mijel Chávez Morejón, a chemist at the Helmholtz Centre for Environmental Research (UFZ) and first author of the study, said, "In order for this reaction to proceed as quickly and efficiently as possible, a suitable catalyst is needed. This will maximize the number of electrons required for the reaction and increase the efficiency of the conversion of phenol to cyclohexanol. In laboratory experiments, the carbon-based rhodium catalyst showed the best productivity (almost 70% of electrons and slightly more than 70% of cyclohexanol). The relatively short reaction time, efficient yields, effective use of energy and synergy with biological systems make this process attractive for co-production of adipic acid." In an earlier study, two other Helmholtz Centre for Environmental Research (UFZ) working groups led by Dr. Katja Bühler and Dr. Bruno Bühler discovered how Pseudomonas syringae converts cyclohexanol to adipic acid in a second step. According to Dr. Rohan Karande, "Until now, it has not been possible to convert phenol to cyclohexanol by microorganisms. We have filled this gap with electrochemical reactions."
Researchers at the University of Leipzig have filled another gap in the production of environmentally friendly nylon by developing an alternative to phenol produced from fossil raw materials. To do this, they used monomers such as eugenol, catechol and guaiacol, all of which are produced as degradation products of lignin, a waste product of the wood industry. "For these model mixtures, we have been able to show that they can be processed and synthesized all the way to adipic acid," says Falk Harnisch.
However, there is still a long way to go before lignin-based nylon reaches the market. For example, so far, scientists have achieved a yield of 57% over the entire 22-hour process (i.e., from monomer of lignin residue to adipic acid via microbial and electrochemical reaction steps). According to Micjel Chávez Morejón, "It's a very good yield." These results are still only based on lab tests at the ml level. In the next two years, prerequisites will be created for scaling up the process. This technology transfer requires not only a better understanding of the overall process, but also the use of real lignin blends instead of model blends, as well as improvements in the electrochemical reactor.Falk Harnisch and Rohan Karande agree: "The process for lignin-based nylon exemplifies the great potential of electrochemical-microbial processes, as it is possible to intelligently combination approach to establish an optimal process chain."
