New Catalyst Turns Mixed Plastic Waste Into Fuel, New Plastics

The huge amount of plastic waste that ends up in the form of microplastics in the oceans, soil and drinking water, but also in human and animal bodies, is one of the main polluters of modern times. Scientists are always looking for new solutions to get rid of plastic waste as efficiently as possible.

One solution could be the use of cobalt-based catalysts. They could break down mixed plastics into propane, which could then be burned as fuel or used to make new plastic.

A study by scientists from the Massachusetts Institute of Technology (MIT) points out that the main problem why the enormous amounts of plastic waste today cannot be easily disposed of is their diversity. Plastics come in so many different varieties, and chemical processes to break them down into a form that can be reused in some way tend to be very specific to each type of plastic.

Waste separation, which we now consider one of the most important methods of plastic recycling, is also inefficient. And much of the plastic material collected through recycling programs ends up in landfills anyway.

The MIT team proposed a new chemical process that uses a catalyst that can process multiple plastics mixed together and convert them into a single product, propane. Propane can then be used as a fuel for stoves, heaters, and vehicles, or as a feedstock in the manufacture of a variety of products.

The new catalyst consists of a microporous material called zeolite that contains cobalt nanoparticles. Although zeolites are riddled with tiny pores less than a nanometer wide, it was logical to assume that there would be little interaction between the zeolite and the polymers. Surprisingly, however, the opposite was found: not only do the polymer chains penetrate into the pores, but the synergistic work between cobalt and the acid sites in the zeolite can break the chain at the same point.

It turned out that this cleavage site was equivalent to cutting off exactly one molecule of propane without producing unwanted methane, leaving the rest of the longer hydrocarbons ready to go through the process over and over again. This works on a variety of plastics such as polyethylene (PET) and polypropylene (PP).

The explorers checked their system on a real example of mixed recycled plastic and gave promising results. The process converted about 80% of the plastic into propane without producing methane as a by-product. The materials needed for the process, zeolites and cobalt, are both fairly cheap and widely available.

In future work, researchers need to focus on how the technique might be scaled for use in real-world plastic recycling streams and how it might be affected by contaminants such as inks, adhesives and labels attached to plastic containers.

Magazine reference:

  1. Guido Zichittella, Amani M. Ebrahim, Jie Zhu, Anna E. Brenner, Griffin Drake, Gregg T. Beckham, Simon R. Bare, Julie E. Rorrer, and Yuriy Román-Leshkov. Hydrogenolysis of polyethylene and polypropylene to propane over cobalt-based catalysts. JACS Au 2022. DOI: 10.1021/jacsau.2c00402

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