Vaporizing Plastics Recycles Them Into Nothing But Gas, Researchers Find

Polypropylene and polyethylene plastics 'can be recycled,' reports Ars Technica. But as 'polyolefin' polymers, 'the process can be difficult and often produces large quantities of the greenhouse gas methane.

'Now, researchers at the University of California, Berkeley have come up with a method of recycling these polymers that uses catalysts that easily break their bonds, converting them into propylene and isobutylene, which are gasses at room temperature. Those gasses can then be recycled into new plastics...'

[T]he previous catalysts were expensive metals that did not remain pure long enough to convert all of the plastic into gas. Using sodium on alumina followed by tungsten oxide on silica proved much more economical and effective, even though the high temperatures required for the reaction added a bit to the cost. In both plastics, exposure to sodium on alumina broke each polymer chain into shorter polymer chains and created breakable carbon-carbon double bonds at the ends. The chains continued to break over and over.

Both then underwent a second process known as olefin metathesis. They were exposed to a stream of ethylene gas flowing into a reaction chamber while being introduced to tungsten oxide on silica, which resulted in the breakage of the carbon-carbon bonds. The reaction breaks all the carbon-carbon bonds in polyethylene and polypropylene, with the carbon atoms released during the breaking of these bonds ending up attached to molecules of ethylene... The entire chain is catalyzed until polyethylene is fully converted to propylene, and polypropylene is converted to a mixture of propylene and isobutylene. This method has high selectivity — meaning it produces a large amount of the desired product.

That means propylene derived from polyethylene, and both propylene and isobutylene derived from polypropylene. Both of these chemicals are in high demand, since propylene is an important raw material for the chemical industry, while isobutylene is a frequently used monomer in many different polymers, including synthetic rubber and a gasoline additive.


'Because plastics are often mixed at recycling centers, the researchers wanted to see what would happen if polypropylene and polyethylene underwent isomerizing ethenolysis together,' the article adds. 'The reaction was successful, converting the mixture into propylene and isobutylene, with slightly more propylene than isobutylene.' The reaction worked, even if there were contaminants from other plastics. And 'When the research team increased the scale of the experiment, it produced the same yield, which looks promising for the future....'
The researchers hope this some day could reduce the demand for chemicals derived from fossil fuels.

Thanks to Slashdot reader echo123 for sharing the article.

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