Converting a mixture of plastic into a propane gas

People lump all plastics into one category, but you may have noticed while trying to figure out what to put in the recycling bin that water bottles, milk jugs, egg cartons, and credit cards are actually made of different materials.

Once these materials reach a recycling facility, the plastic must be separated, a process that can be slow and expensive, but which limits the types of recycled materials and the amount.

Converting plastic into propane gas

Researchers have developed a new process that can convert a mixture of several types of plastic into propane, a simple chemical substrate that can be used as fuel or turned into new plastics or other products. Although many plastics have different chemical properties, they share a similar basic composition: they are made of long chains of carbon and hydrogen, which is why the process works.

Along with environmental policies and protections, recycling can play an important role in staving off some of the worst damage caused by plastic.

More than 400 million metric tons of plastic are produced each year around the world. Less than 10% is recycled, and about 30% remains used for some time. The rest ends up either in landfills or in the oceans or is... Burn it. Plastics are also a significant cause of climate change, with their production accounting for 3.4% of global greenhouse gas emissions in 2019. Not only does recycling keep plastic out of landfills and oceans, but new methods for producing the basic substrates of plastic can also help reduce emissions.

“What we're really trying to do is think about ways in which we can see these used plastics as valuable feedstocks,” says MIT postdoctoral fellow in chemical engineering Julie Rohrer, one of the lead authors of the latest research.

Targeting the most common plastic

One of the main benefits of the new approach developed by Rorer and her colleagues is that it works on two of the most common types of plastics used today: polyethylene and polypropylene. A mixture of plastic bottles and milk jugs enters the reactor, and propane comes out. This approach has high selectivity, as propane makes up about 80% of the final product gases. “This is really exciting because it is a step towards the idea of a circular economy,” Rorer says.

The process uses a two-part catalyst: cobalt metal and a porous, sand-like material called zeolite, to reduce the energy needed to break down plastic. Researchers are still not entirely sure how this combination works. Still, Rorer says the selectivity advantage likely comes from the pores in the zeolite, which limit the interaction of the long molecular chains in the plastic. At the same time, the cobalt helps prevent the zeolite from being deactivated.

However, this process is still not ready for industrial use. Currently, reaction processes are performed in small quantities, and the reaction must be continuous in order to be economically fruitful.

“Researchers are also thinking about what materials to use,” Rorer says. Cobalt is more common and less expensive than some of the other catalysts they've tried, such as ruthenium and platinum, but they are still looking at other options. “By comprehending the mechanisms underlying catalysts, they may be able to substitute less costly and more plentiful catalysts with cobalt.,” Rorer adds, stating that a fully mixed plastic recycling system is not completely out of the question and would be the ultimate goal.

A vision whose realization requires adjustments to be economically viable

However, achieving this vision requires some adjustments. Simple chains of carbon and hydrogen make up polyethylene and polypropylene, but many other plastics contain additional components like oxygen and chlorine that can make chemical recycling techniques difficult.

For example, if polyvinyl chloride (PVC), widely used in bottles and tubes, is used in this system, it could inactivate or poison the catalyst by emitting toxic gas byproducts, so researchers still need to discover other ways to deal with it. With these plastic materials.

Scientists are also seeking to find other ways to complete mixed plastic recycling operations. In a study published in the journal Science in October, researchers used a chemical process and genetically engineered bacteria to break down a mixture of three common types of plastic.

The first step involves chemical oxidation, where long chains are cut, forming smaller oxygen-containing molecules. This approach is effective because the oxidation is completely mixed, and works on a range of materials, explains the paper's lead author and UW chemist Shannon Stahl.

The oxidation of plastics produces products that can then be broken down by soil bacteria that have been modified to feed on them. By changing the metabolism of bacteria, researchers can make new plastics, such as new forms of nylon.

Allie Werner, a biologist at the National Renewable Energy Laboratory and one of the authors of the study published in the journal Science, explains that the research is still ongoing. The team is working to understand the metabolic pathways that bacteria use to make products so that they can speed up the process and produce larger quantities of beneficial substances.

Considering how commonplace oxidation and genetically modified bacteria are, this strategy might be applied on a bigger scale. Millions of tons of materials are produced annually by the petrochemical sector through oxidation, and microorganisms are employed in food processing and medicine discovery, among other industries.

When biologists like Werner and chemical engineers like Rohrer turn their attention to discovering new ways to recycle plastics, they give us the opportunity to reconsider how we deal with vast amounts of plastic waste.

“Society must address this challenge in appropriate ways,” Rorer says. Rorer has noticed a huge influx of new researchers starting to work on plastic recycling. “It seems like everyone is getting into plastic recycling,” she says.