How chemists are tackling the plastics problem
We tend to lump all plastics in one category. But water bottles, milk cartons, and credit cards are made from different materials. This is something you probably noticed when trying to decide what can go in your recycle bin.
Once they reach a recycling facility, the plastic needs to be separated. This can be slow and expensive and limits how many materials and how much are recycled.
Now, researchers have created a new process which can convert a mixture of different types of plastics into propylene, a chemical building block that can be used to make new plastics or other products. Although each plastic is unique in their chemistry, the process works because they share a common basic recipe. They are made up of long chains of carbon and hydrogen.
Combining policies with environmental protections , reinventing recycling could help to prevent some of the most severe plastic-related injuries.
Over 400 million metric tons of plastic are produced each year worldwide. Of that, less than 10% is recycled, about 30% remains in use for some time, and the rest either finds its way to landfills or the environment, or is incinerated. Plastics are also a significant driver of climate change: their production accounted for 3.4% of global greenhouse gas emissions in 2019. Recycling helps keep plastics out the oceans and landfills. However, new ways to make plastic building blocks could also help reduce emissions.
“What we really want to do is think of ways that we can view these waste plastic materials in a valuable feedstock,” says Julie Rorrer ,, a postdoctoral fellow at MIT in chemical engineering and one of the leading authors of the new research.
The major advantage of the new approach Rorrer developed with her colleagues is that it works with the two most commonly used plastics today: polyethylene (or polypropylene). The reactor then releases propane. It is made up of a mixture from the plastics used to make bottles and milk jugs. The approach has high selectivity, with propane making up about 80% of the final product gases.
” This is really exciting, because it’s an important step towards the idea of circularity,” Rorrer states.
HTML3_ To lower the energy required to break down plastics, the catalyst uses two parts: cobalt as well as porous sand-like material called Zeolites. Rorrer believes that the selectivity of the mixture is due to the pores in the Zeolite. These pores limit the reaction of long molecular chains in plastics, while the cobalt keeps the zeolite active. The process is not yet ready for industrial use. The reaction is currently being done in small batches and would need to continue to be economically viable.
Rorrer said that researchers are also looking at what materials to use. They are still looking for other options, though cobalt is more popular and less expensive than other catalysts like platinum and ruthenium. Rorrer believes that a better understanding of the mechanism of catalysts could help them replace cobalt by cheaper, more plentiful catalysts.
The ultimate goal would be a fully integrated plastic recycling system. Rorrer said that .”
However, it will take some tweaks to achieve that vision. Polyethylene and polypropylene, which are simple chains of carbon, hydrogen and oxygen, could pose a problem for chemical recycling methods.
For example, if polyvinylchloride (PVC) is used in pipes and bottles, it could poison or deactivate the catalyst while also producing toxic gas side products. Researchers still need to find other ways to deal with that plastic.
Scientists are also looking for other methods to recycle mixed-feed plastic. In a study published in Science in October, researchers used a chemical process alongside genetically engineered bacteria to break down a mixture of three common plastics.
The first step, which involves chemical oxidation cuts down long chains and creates smaller molecules with oxygen tacked on. The approach is effective because oxidation is “quite promiscuous,” working on a range of materials, explains Shannon Stahl, a lead author of the research and a chemist at the University of Wisconsin. Oxidizing plastics produces products that can be eaten by soil bacteria that has been adapted to eat them. Researchers could create novel plastics by altering the metabolism of bacteria.
The research is still in progress, according to Alli Weber ,, a biologist at National Renewable Energy Laboratory who was one of the authors for the Science study. The team is focusing on understanding the metabolic pathways bacteria use to make the products. This will allow them to speed up the process and produce more useful materials.
This approach could likely be used on a larger scale, as both oxidation and genetically engineered bacteria are already widespread: the petrochemical industry relies on oxidation to make millions of tons of material every year, and microorganisms are used in industries like drug development and food processing.
As chemical engineers like Rorrer and biologists like Werner turn their attention towards new plastic recycling methods, it opens up opportunities to rethink the way we deal with the huge amounts of plastic waste.
” This is a challenge that our community is well-suited to address,” Rorrer said. And she’s noticed a significant influx of new researchers starting to work on plastics: “It seems like everyone and their sister is getting into plastic upcycling.”
I’m a journalist who specializes in investigative reporting and writing. I have written for the New York Times and other publications.