The world produces more than two million metric tons of nylon every year. That nylon, used to make clothes, ropes, fishing nets and parachutes among other things, is made today from petroleum-based raw materials using immense amounts of energy.
But researchers now report a way to create the key building block of nylon from plant-based sources. The method, reported in the journal Green Chemistry, combines a low-energy chemical process and bacterial transformation of chemicals, and could slash the energy use and greenhouse gas emissions of making nylon.
Nylon is a 50-50 mix of two chemical components. One of them is adipic acid, which is made from petroleum-derived phenol in a two-step chemical process. The first step is to convert phenol to cyclohexanol in an energy-intensive process that requires high temperatures and harsh chemicals. The second step converts cyclohexanol into adipic acid in a process that releases a lot of nitrous oxide, also known as laughing gas. Nitrous oxide is 300 times more potent a greenhouse gas as carbon dioxide, and also depletes the ozone layer in the atmosphere.
The research team from Helmholtz-Centre for Environmental Research and Leipzig University in Germany wanted to make the entire nylon production chain environmentally friendly. “This is possible if we access bio-based waste as feedstock and make the synthesis process sustainable,” said Falk Harnisch, professor of electrobiotechnology.
They start by substituting fossil-based phenol with phenols that come from a biological source. Namely, chemicals such as catechol, and guaiacol that are derived from lignin, the fibrous component that gives plants their strength and is a waste product of the pulp and paper industry.
To convert those bio-based phenols to cyclohexanol, the researchers use electricity and a special carbon-based catalyst instead of heat and hydrogen as is conventionally done. This brings down energy use, and the electricity could come from renewable sources. The process converts over 70 percent of the chemicals into cyclohexanol. Then the team uses a bacterium, Pseudomonas taiwanensis, to convert cyclohexanol into adipic acid in a second step.
The process takes 22 hours, giving a yield of 57 percent overall going from the lignin-derived phenols all the way to adipic acid.
Earlier this year, apparel company Lululemon started selling shirts made of plant-based nylon. Biotechnology company Geno makes that nylon by using bacteria to ferment plant sugars such as glucose or sucrose. “This plant-based feedstock might create competition for food with humans, feed and land use conflicts,” says Harnisch.
Using lignin, which is a waste product, is more sustainable. In 2020, researchers at the University of Edinburgh reported using bacteria to directly convert lignin-derive guaiacol into adipic acid. The Leipzig team’s process is faster and has higher yield.
Yet, it’s still an early-stage demonstration and there is a long way to go before this bio-based nylon could be made at large enough scale for the market. Right now, the researchers are working with millileters of solution. Through partnerships with others in academia and industry, the team hopes to build improved reactor prototypes that can be scaled up for producing biobased nylon, Harnisch says.
Source: Micjel Chávez Morejón et al. Integrated electrosynthesis and biosynthesis for the production of adipic acid from lignin-derived phenols. Green Chemistry, 2023.
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