US – Polyethylenes represent about 60% of plastics in landfills around the world, where they degrade slowly. This is why researchers from the University of California looked into reusing an existing pair of catalysts to break a wide range of polyethylenes, turning them into liquid fuels and other valuable chemicals, according to a published article in the journal Science.

This project could boost recycling efforts to disintegrate million tons of plastic garbage collectors living in cities or polluting the world’s oceans.

As the name suggests, polyethylenes are made of many copies of ethylene, a building block simple hydrocarbon with two carbon atoms surrounded by four hydrogen atoms.

Catalysts connect millions of these ethylenes in long chains, which can be linear or branching and vary in strength and density. In most cases, polyethylenes are inert and resistant to decomposition. In the article, Zhibin Guan, a chemist who led the research, explains that the durability is due to a simple fact: all links between atoms are single bonds, which are highly stable and hard to break.

To change that, Guan and colleagues researchers led by chemist Zheng Huang at the Chinese Academy of Sciences in Shanghai studied use of joined two existing catalysts. These catalysts are typically used to link short hydrocarbons, called alkanes, such as very long and valuable hydrocarbon chains, like those found in diesel fuel.

When the two catalysts are added to a batch of short alkanes, the first catalyst is responsible for stripping hydrogen atoms of adjacent carbon atoms in individual alkane molecules. Then the newly free chemical handles together form a double bond between adjacent carbon atoms. Note that the double bonds create a weak link in the short alkane exploited by the second catalyst and end up dividing them.

The research team wondered if the same process could work backwards to separate very long chains of polyethylene, which can contain up to millions of carbon atoms. To find out, they mixed polyethylene waste such as garbage bags with liquid alkanes, and then added the two catalysts. Again, the first handled remove hydrogen from adjacent carbon atoms in both the long chain and the short polyethylene alkanes to form double bonds; the second phase was to divide the molecules. The result, the researchers report, is that they continue to break down the long chains until they reach the size found in fuels and other valuable hydrocarbons.

One of the problems that research faces is that the catalysts decompose polyethylene slowly; this may occur in the course of a day or more, in addition to being expensive and only serve to separate a few thousand polymer chains, much unless million carried out by most commercial catalysts. Guan and his colleagues are working to overcome these problems, in the hope that can in a single day it can extract millions of tons of plastic waste.

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