This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: It would greatly benefit the environment and climate if nylon—chemically known as polyamide—could be produced on a large scale from plastic waste in the future, rather than from petroleum, as has been the norm until now. At LIKAT in Rostock, chemists pursuing this approach have succeeded in selectively converting polyethylene—the main component of all plastic waste—into dicarboxylic acids, which can serve as chemical precursors for nylon.
The reaction proceeds under mild conditions and does not even require a catalyst. The goal is to develop a process on a pilot plant scale. To this end, the team aims to increase selectivity and elucidate the reaction mechanisms.
Polyethylene, the material used in ubiquitous plastic bags, together with polypropylene, accounts for four-fifths of plastic waste—an estimated 400 million metric tons annually worldwide. Pyrolysis has become the established method for chemically recycling these polyolefins, as Dr. Esteban Mejía, group leader at the Leibniz Institute for Catalysis, LIKAT, explains.
"The plastic is burned in a controlled manner at temperatures of up to 600°C." While the result is indeed "high-quality oil" and the process does not emit any CO₂, the enormous amount of energy consumed means that pyrolysis leaves such a large CO₂ footprint that environmental authorities are sounding the alarm. In the U.S., pyrolysis plants are already being shut down. This raises the question of alternatives.
Researchers are aware of a number of catalytic processes that work oxidatively, just like pyrolysis, says Mejía. "In these processes, a reactive oxygen species—which we call ROS—forms on the polyethylene surface." As chemists now know, it is precisely these species that matter. Mejía says, "ROS chemically break down the polyethylene.
They essentially eat away at the material." However, this reaction requires high pressure, which makes the process explosive. "It cannot easily be scaled up to a practical scale." Furthermore, the catalysts are based on precious metals, which makes the whole process expensive and also dependent on the volatile raw materials market. During their literature review, Mejía's group found reports from colleagues who produced polyethylene dicarboxylic acid with nonspecific chain lengths, including using highly concentrated nitric acid at 180°C and in a microwave.
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