Can cobalt-based catalysts revolutionize plastic recycling?

11 Oct 2022 --- A cobalt-based catalyst could be the answer to circularizing the plastic packaging waste economy, according to new research from the Massachusetts Institute of Technology (MIT), US. The university’s research team has discovered the material could break down mixed plastics into propane, which can then be burned as fuel or used to make new plastic.

The solution offers an answer to the two central challenges in today’s recycling systems: a vast disparity in plastic chemical compositions, many of which are multi-layered and mixed (making them harder to reprocess), and varying sortation systems for separating these materials at the end-of-life stage.

While many new methods of sortation are being developed, the huge variety of collection, material types and mechanical or chemical recycling systems make the entire recycling industry highly insufficient. Calls to harmonize all these aspects throughout the US have not yet been answered, and recycling rates remain low – estimates remain at around 9% of total plastics ever produced.

However, the MIT team has proposed a new chemical process using a catalyst that can deal with multiple plastics mixed, which converts the waste into a single product – propane. Propane, a liquified petroleum gas, can then be used as fuel for stoves, heaters and vehicles or as a feedstock for producing new plastic products.

The chemical process turning plastics to propane. Image credit: MITRomán-Leshkov, a lead researcher on the project, explains that because the long-chain molecules in plastics are held together by carbon bonds, which are “very stable and difficult to break apart,” existing techniques for breaking these bonds tend to produce a random mix of different molecules.

This then requires complex refining methods to separate out into usable specific compounds. “The problem is,” he says, “there’s no way to control where in the carbon chain you break the molecule.”

Cobalt to the rescue?
The study team used a new catalyst made of a microporous material called zeolite, which contains cobalt nanoparticles. Despite zeolites being studded with tiny pores less than a nanometer in width, the team assumed there would be little interaction between the zeolite and the polymers.

However, the opposite was shown: the polymer chains enter the pores, and synergistic action between cobalt and the acid sites in the zeolite break down the chain at the same point.

This means the cleavage corresponded to chopping off exactly one propane molecule without generating unwanted methane, leaving the rest of the longer hydrocarbons ready to undergo the process again. This method could work on a variety of plastics, such as PET and PP, say the researchers. Cobalt mining in DRC is a contentious issue, with much of it produced through forced labor.

The team then tested this system on an example of mixed recycled plastic, producing what they say are “promising results.” The process converted around 80% of the plastic into propane without producing methane as a by-product. The materials needed for the process, zeolites and cobalt, are both cheap and widely available.

However, cobalt’s low cost comes with a price – around 70% of the world’s supply is sourced from the Republic of Congo, where an estimated 25,000 children work in unregulated and often deadly conditions.

Avoiding methane emissions
The fact that 80% of the propane is cut off without producing methane is an important element in developing an effective chemical recycling process. Recently, Zero Waste Europe reported that greenhouse gas emissions from pyrolysis are roughly nine times higher than those of traditional mechanical recycling processes, questioning the efficacy of advanced recycling methods.

In future work, the researchers say they will need to focus on how the cobalt-based catalyst technique might be scaled for use in real-world plastic recycling streams and how it might be affected by contaminants like inks, glues, and labels attached to plastic containers.

The MIT team is also continuing to study the system's economics and analyze how it can fit into today’s systems for handling plastic and mixed waste streams. “We don’t have all the answers yet,” Román-Leshkov says, “but preliminary analysis looks promising.”

By Louis Gore-Langton

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