Scientists find cheaper method of recycling plastics

Although there are a lot of potentially useful raw materials bound up in used plastic products such as food wrap or grocery bags, until recently it has been much cheaper to continue producing more of these single-use plastics than to recover and recycle them. Now, a team of researchers led by the Department of Energy’s Pacific Northwest National Laboratory (PNNL) has managed to crack the code that stymied prior attempts to break down these persistent plastics.

Usually, recycling plastics requires “cracking” or splitting apart the powerful and stable chemical bonds that also make these products so persistent in the environment. This process requires very high temperatures, and is thus expensive and energy intensive. 

However, the experts succeeded in combining the cracking step with a second reaction step which immediately completes the conversion to a liquid gasoline-like fuel without unwanted byproducts. This second step deploys what are known as alkylation catalysts, which provide a chemical reaction currently deployed by the petroleum industry to improve the octane rating of gasoline. Crucially, this reaction immediately follows the cracking step in a single reaction vessel, near room temperature (70 degrees Celsius/158 degrees Fahrenheit).

“Cracking just to break the bonds results in them forming another one in an uncontrolled way, and that’s a problem in other approaches,” said study co-author Oliver Y. Gutiérrez, a chemist at PNNL. “The secret formula here is that when you break a bond in our system, you immediately make another one in a targeted way that gives you the end product you want. That is also the secret that enables this conversion at low temperature.”

“The fact that [the petroleum] industry has successfully deployed these emerging alkylation catalysts demonstrates their stable, robust nature,” added senior author Johannes Lercher, the director of PNNL’s Institute for Integrated Catalysis. “This study points to a practical new solution to close the carbon cycle for waste plastic that is closer to implementation than many others being proposed.”

The scientists found that this process functions only for low-density polyethylene products (LDPE), such as plastic films and squeezable bottles, and polypropylene products (PP), which are not usually collected in recycling programs in the U.S. By contrast, high-density polyethylene (HPDE) would require a pretreatment in order to allow the catalyst access to the bonds that it needs to crack.

“To solve the problem of persistent waste plastic, we need to reach a critical point where it makes more sense to collect it and return it to use than to treat it as disposable. We’ve shown here that we can make that conversion quickly, at mild conditions, which provides one of the incentives to move forward to that tipping point,” Lercher concluded.

The study is published in the journal Science.

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By Andrei Ionescu, Earth.com Staff Writer

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