Rostock chemists dissolve polyethylene into precursors for nylon

Chemists at the Leibniz Institute for Catalysis (LIKAT) in Rostock have found a way to convert polyethylene, the main component of plastic waste, into dicarboxylic acids, which can act as precursors for nylon. The reaction runs under mild conditions and needs no catalyst.

Polyethylene is the stuff of ordinary plastic bags. With polypropylene, it makes up around four in every five pieces of plastic waste, which the source puts at an estimated 400 million metric tons a year worldwide. Nylon, known chemically as polyamide, has until now been made chiefly from petroleum.

The established way of recycling these plastics chemically is pyrolysis, which breaks them down at temperatures of up to 600°C. It yields a high-quality oil, but the energy it demands leaves a hefty carbon footprint. Environmental authorities have raised the alarm, and some plants in the United States are being closed.

Other oxidation methods exist, but they need high pressure, which makes them explosive, and they rely on costly precious-metal catalysts. The team, led by Dr Esteban Mejía, instead started from earlier work that used harsh, concentrated nitric acid, and set about simplifying it. Their version places pieces of plastic bag in highly diluted nitric acid with hydrogen peroxide, heats the mixture to no more than 120°, and the polyethylene dissolves.

Control is the tricky part. “It’s important to maintain a specific heat input regime,” says Mejía. Let the intermediate products linger too long and the result is a muddled mixture.

There is also work to do on precision. Nylon needs dicarboxylic acid whose chains hold exactly six carbon atoms, and the team has so far narrowed its product to between four and 10. They hope to improve on that with Dr Udo Kragl at the University of Rostock, and have applied for project funding. They also plan to study the reaction mechanism, and a colleague at the University of Guayaquil in Ecuador is looking at how light affects the reactive oxygen species involved.

The long-term aim is a process at pilot plant scale. A paper is being prepared for the Journal of the American Chemical Society, and an invitation from Singapore has arrived to present the findings at the Southeast Asia Catalysis Conference in December.

Why it matters

Roughly 8 million metric tons of polyamide are produced worldwide, mostly from petroleum. If waste plastic could one day supply part of that raw material, it would cut reliance on fossil resources and give discarded bags a useful second life. The work is still at the research stage and the product is not yet precise enough for nylon, but a gentler route that avoids a catalyst is a cracking place to begin.