Conversion of Mixed Plastics into Useful Chemicals

As reported in Science on October 13, mixtures of plastics, which are usually difficult to recycle, have been broken down into useful, smaller chemical ingredients in a two-step process.

A team led by Gregg Beckham, a chemical engineer at the US National Renewable Energy Laboratory (NREL) in Golden, Colorado, has come up with a two-step, hybrid process that uses chemistry and then biology to decompose a mix of the most common plastics that make it into recycling plants: high-density polyethylene (HDPE), a soft plastic often found in food packaging; polystyrene, which includes styrofoam; and polyethylene terephthalate (PET), a strong, lightweight plastic used to make drink bottles.

“Only a few works have reported chemical recycling of plastic mixtures before,” says Ning Yan, a chemist at the National University of Singapore and one of the few researchers to have developed a system capable of that. “Combining chemical and biological pathways to convert plastic mixture is even more rare,” he adds.

Two-step process

The first step of the process borrows from a common industrial method to make terephthalic acid, one component of PET. This uses oxygen and chemical catalysts to break down carbon bonds in the mixed plastic waste, which makes the resultant compounds more digestible for the bacterium. The process was inspired by a 2003 study led by Walter Partenheimer, a chemist at chemicals company DuPont in Wilmington, Delaware, who used it to break down single plastics into chemicals such as benzoic acid and acetone.

But Beckham wanted to turn the organic-acid molecules into something more easily commoditized. To do that, the team turned to microbes — specifically, the bacterium Pseudomonas putida, which can be engineered to use different small organic molecules as a source of carbon. “It’s quite an interesting organism,” says Beckham. The team engineered the microorganisms to consume the oxygenated organic molecules that the researchers made from the different plastics using their ‘autoxidation’ reaction: dicarboxylic acids from polyethylene, terephthalic acid from PET and benzoic acid from polystyrene.

The bacteria produced two chemical ingredients that are each used to make high-quality performance-enhanced polymers or biopolymers. “Biology can take multiple carbon sources and funnel them into a single product, in this case a molecule which can be used to make a highly biodegradable polymer,” says Susannah Scott, a chemist at the University of California, Santa Barbara.

“The cool thing about synthetic biology, metabolic engineering and this idea of biological funneling… is that as long as the organism can eat or consume the oxygenated intermediates, then potentially one could make anything,” says Beckham.

The researchers developed their process using a mix of pure polymer pellets, but also tested it on mixed plastics found in everyday products. “We purchased HDPE in the form of milk containers, PET from the vending machine outside my office in single-use beverage bottles. And then polystyrene or Styrofoam cups,” says Beckham.

Temperature limitations

But one issue is the temperature that the autoxidation reaction is run at. At the moment, each plastic reacts best at a different temperature, and the one that the team uses for the mixture corresponds to the most resisting of the reactions. More fundamental chemistry is needed to work out exactly how this reaction works in practice and improve the yields of the reactions, says Stahl.

But he adds that many companies already work with autoxidation processes, to turn xylene into terephthalic acid, a PET precursor molecule. “There’s a lot of in-house knowledge built in, and if one or more of these companies would choose to explore this, I think they could offer a lot of technical know-how,” Stahl says. Beckham says the team is working on an economic analysis and life-cycle assessment of its process.

Another problem will be to put the smaller molecules that the bacteria produce for sale, because demand for those products is much smaller than the quantity of waste plastics, says Yan. “Whether the process will be scaled up depends on economic competitiveness,” he says.

https://www.science.org/doi/10.1126/science.abo4626?cookieSet=1