Efficient new catalyst converts mixed plastic waste into propane


Plastic waste is one of the most pressing environmental problems of our time, and sorting different types makes recycling tricky. Now, engineers at MIT have developed an effective new catalyst that breaks down mixed plastics into propane, which can then be burned as fuel or used to make new plastic.

The ubiquity of plastic in our modern world means that huge amounts of the stuff ends up in the environment, and there are worryingly few places that seem to be untouched by it. Plastic is now found from the north to the south poles, from the seafloor to the top of Mt Everest, and is working its way up the food chain to the point it can now be found inside our own bodies.

Plastics have very strong carbon bonds, which makes them resilient and reliable during use but a real pain to recycle. Worse still, different types of plastic require different recycling methods, making it difficult to sort and recycle at scale. But the MIT team has now proposed a new technique that can deal with multiple plastics mixed together, converting them into a single product, propane, that itself has many uses.

The key is a catalyst that consists of a porous crystal called a zeolite, which is stuffed with cobalt nanoparticles. While other catalysts break carbon bonds in unpredictable places, producing varied end-products, the new catalyst break the bonds in a specific and repeatable location.

That location means it essentially shears off one propane molecule, leaving the rest of the hydrocarbon chain behind, ready to undergo the process over and over. This works on multiple types of plastic, including the most commonly used ones like polyethylene (PET) and polypropylene (PP).

In tests on real-world samples of mixed plastics, the team found that the catalyst and the process converted around 80% of the plastic into propane, without producing methane as a by-product. The resulting propane can be used directly as a relatively low-impact fuel, or as a feedstock to make new plastics in a partially-closed loop system. Importantly, the ingredients for the catalyst – zeolite, cobalt and hydrogen – are relatively cheap and easy to come by.

As intriguing as the study is, the researchers say that future work will need to focus on how the technique might be scaled for use in real-world plastic recycling streams, as well as how it might be affected by contaminants like glues and labels.

The research was published in the journal JACS Au.

New Atlas, 10 October 2022
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