How chemists are tackling the plastics problem

We tend to lump all plastics into one category, but water bottles, milk jugs, egg cartons, and credit cards are actually made from different materials, as you've probably noticed while trying to figure out what can go in your recycling bin.

Once they've reached a recycling facility, the plastic must be separated, a process that can be slow and costly, and ultimately limits which materials, and how much of them, are recycled.

Now researchers have developed a new process that can transform a mixture of several types of plastics into propane, a simple chemical building block that can be used as fuel or converted into new plastics or other products. The process works because, although their exact chemistry can differ, many plastics share a similar basic recipe: they are made of long chains of mostly carbon and hydrogen.

Coupled with policies and environmental protections, reinventing recycling could play a role in preventing some of the worst damages from plastics.

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. Not only does recycling keep plastics out of landfills and oceans, new ways to produce building blocks for plastics could help cut emissions as well.

What we're really trying to do is think about ways that we can see these waste plastic materials as a valuable feedstock," says Julie Rorrer, a postdoctoral fellow in chemical engineering at MIT and one of the lead authors of the recent research.

A major benefit of the new approach Rorrer and her colleagues developed is that it works on the two most common plastics used today: polyethylene and polypropylene. Into the reactor goes a mixture of the plastics that make bottles and milk jugs, and out comes propane. The approach has high selectivity, with propane making up about 80% of the final product gases.

This is really exciting because it's a step toward this idea of circularity," Rorrer says.

To lower the energy needed to break down plastics, the process uses a catalyst with two parts: cobalt and porous sand-like material called zeolites. Researchers still aren't sure exactly how the combination works, but Rorrer says the selectivity likely comes from the pores in the zeolite, which limit where the long molecular chains in plastics react, while the cobalt helps keep the zeolite from being deactivated.

The process is still far from being ready for industrial use. Right now, the reaction is done in small batches, and it would likely need to be continuous to be economical.

Rorrer says the researchers are also considering what materials they should use. Cobalt is more common and less expensive than some other catalysts they've tried, like ruthenium and platinum, but they are still searching for other options. Better understanding how the catalysts work could allow them to replace cobalt with cheaper, more abundant catalysts, Rorrer says.

The ultimate goal would be a fully mixed-feed plastic recycling system, Rorrer says, and that framework is not completely far-fetched."

Still, achieving that vision will take some tweaks. Polyethylene and polypropylene are simple chains of carbon and hydrogen, while some other plastics contain other elements, like oxygen and chlorine, that could pose a challenge to chemical recycling methods.

For example, if polyvinyl chloride (PVC), widely used in bottles and pipes, winds up in this system, it could deactivate or poison the catalyst while producing toxic gas side products, so researchers still need to figure out other ways to handle that plastic.

Scientists are also pursuing other ways to accomplish mixed-feed plastic recycling. 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, involving chemical oxidation, cuts up long chains, creating smaller molecules that have 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 the plastics generates products that can then be gobbled up by soil bacteria that have been tweaked to feast on them. By altering the metabolism of the bacteria, researchers could eventually make novel plastics, like new forms of nylon.

The research is still a work in progress, says Alli Werner, a biologist at the National Renewable Energy Laboratory and one of the authors of the Science study. In particular, the team is working to better understand the metabolic pathways bacteria are using to make the products so that they can speed up the process and produce larger amounts of 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 biologists like Werner and chemical engineers like Rorrer turn their attention to new plastic recycling methods, they open up opportunities to rethink how we deal with the vast amounts of plastic waste.

This is a challenge that the community is well suited to tackle," Rorrer says. 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."