Towards more efficient plastics recycling

A team of Northwestern University presents a cheap nickel catalyst that may appreciably simplify everyday plastics recycling. The catalyst breaks down polyolefins to oils and waxes. We use these plastics, like polyethylene and polypropylene, mainly for packaging. We can subsequently ‘upcycle’ the oils and waxes to more valuable applications, like lubricants, fuels and candles. And there is a special effect: this procedure keeps on working and becomes even more active if the waste has been polluted with PVC.

Polyolefins

Polyolefins are everywhere: we produce squeeze bottles and foils from it, just like milk bottles, bin liners and throw-away cutlery. They are cheap, strong and versatile; but precisely because of those properties, recycling poses a problem. The carbon-carbon bonds that form the essence of them are hard to break down. Consequently, as polyolefins make up almost two thirds of world-wide plastic use – some 220 million tons annually – just a fraction of it (about 1 to 10 percent) will be really recycled. This requires sorting of waste streams, and this is usually done by hand. For even small impurities can render useless complete batches.

The new catalyst for plastics recycling treats exactly that bottleneck. It promotes hydrogenolysis: a chemical process that uses hydrogen and a catalyst to cut long polymer chains into shorter hydrocarbons. Earlier processes often relied on scarce and expensive precious metals like platinum or palladium; but here, a nickel catalyst performs all the work. This catalyst contains one closely defined active site. This works as a kind of surgical knife: just enough to break up the correct bonds, without demolishing the entire polymer.

Precision

That precision carries with it two important benefits. Firstly, it enables to break down branched and straight polyolefins together. Therefore, here chemistry assists in ‘separating’ the waste; mechanical pre-sorting is less required. Secondly, the process operates under milder conditions than comparable nickel-based systems: about 100^oC lower, with half the hydrogen pressure, and using ten times less catalyst. And yet, activity for plastics recycling is ten times higher! In sum: using less energy and materials, and with a higher yield of useful products.

Often, recycling is being blocked by PVC contamination. By its appearance, PVC looks a lot like other plastics; but upon heating it will produce hydrogen chloride (among others) that can ‘poison’ catalysts. But not here! The nickel catalyst not only stays stable in the presence of PVC, but even performs better if the original mixture contains up to one quarter of PVC. That’s good news: for often, materials containing traces of PVC will be dubbed ‘unrecyclable’ from the start. Moreover, the catalyst can be regenerated simply by a cheap alkylaluminium treatment; this renders reuse doable.

Limited options

At present, there is just a limited number of options for polyolefin plastics recycling. Often, mechanical recycling is being practiced, but this requires intensive separation and often results in low-value products. Heating is possible as well, up to 400-700^oC. But this requires a lot of energy and results in a broad spectrum consisting of a rather less selective product mixture. The nickel catalyst makes possible a third avenue: chemical ‘dismantling’ at lower temperatures; this process uses a cheaper metal and has some tolerance for a realistic contamination. The treated plastics are being changed into oils and waxes that can be separated and recycled rather easily. This opens up an avenue towards a more efficient plastics recycling, with less waste being thrown away; and with less microplastics dumped in the environment.

But for the time being, this process only exists on the level of the laboratory. It still needs to be scaled up before actual use; moreover, it will have to be integrated into processes. But if we do not have to sort polyolefins in great detail, and still be able to ‘cut’ them into smaller molecules, the price of recycling will change, and this will render reuse more applicable. That is good news for recyclers, for the wallet and for the environment.

Author

Diederik van der Hoeven

Source

Bio Based Press, 2025-09-24.

Supplier

Northwestern University

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