Chemistry + biology, new processes make waste plastic useful

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Plastic mixtures are often difficult to recycle. A study published Oct. 13 in Science says that after two steps, plastic mixtures can be broken down into smaller and useful chemical components.

Recycling these “stubborn” plastics exacerbates the environmental problems facing the planet. Although existing chemical methods can break down their long polymer chains, these methods are difficult to implement on a large scale.

To this end, Gregg Beckham’s team, a chemical engineer at the National Renewable Energy Laboratory, has developed a new decomposition process. Ning Yan, a chemist at the National University of Singapore and one of the researchers, said: “Only a few studies have previously reported chemical recycling of plastic mixtures, and it is even rarer to combine chemical and biological pathways to break down plastic mixtures. ”

The team first used cobalt or manganese-based catalysts for catalytic oxidation to break down tough polymer chains into oxygenated organic acid molecules. The method was inspired by a 2003 study by DuPont chemist Walter Partenheimer, which used this method to break down individual plastics into chemicals such as benzoic acid and acetone.

Still, Beckham hopes to convert organic acid molecules into something easier to commercialize. To do this, the team turned to microbes for “help.” Among them, the modified Pseudomonas odori can use different small organic molecules as carbon sources.

“It’s a very interesting creature.” Beckham said. The team designed these microbes to consume oxygenated organic acid molecules, which the researchers synthesized from different plastics through auto-oxidation reactions, including dicarboxylic acid in polyethylene, terephthalic acid in PET, and benzoic acid in polystyrene.

This process produces two chemical compositions that are used to make high-quality performance-enhancing polymers and biopolymers, respectively. Susannah Scott, a chemist at the University of California, Santa Barbara, said: “Biological methods can take multiple carbon sources and convert them into a single product. In this case, a molecule could be used to make a highly biodegradable polymer. ”

The researchers developed the process using a plastic mixture of pure polymer particles, which were also tested on plastic mixtures of everyday products.

“We bought milk packaging bottles made of high-density polyethylene from supermarkets, single-use beverage bottles made of PET from vending machines, and Styrofoam cups made of polystyrene or polystyrene,” Beckham said. ”

Shannon Stahl, a co-author at the University of Wisconsin-Madison, said scaling the process is a challenge, and one of the problems is the temperature of the auto-oxidation reaction. “More basic chemistry is needed to determine how this reaction works and thus increase the reaction yield.”

Still, Stahl adds, many companies are already working on auto-oxidation to convert xylene into terephthalic acid, a PET precursor molecule. Beckham said the team is conducting economic analysis and production cycle assessments of its processes.

Another problem is that the market demand for small molecule products produced by the process is much smaller than the amount of plastic waste. “Whether this technology will scale up depends on its market competitiveness.” Yan said. (Source: Wang Fang, China Science News)

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