A new method to convert waste plastic to fuel and raw materials has been developed by an international research team led by the Department of Energy’s Pacific Northwest National Laboratory has the potential to become a game-changer in the chemical recycling space. They reported their discovery in today’s issue of Science.
Typically, recycling plastics requires ‘cracking’ or splitting apart the tough and stable bonds that also make them so persistent in the environment. This cracking step requires high temperatures, making it expensive and energy intensive.
The present study, however, describes how the research team has successfully discovered that this can also be achieved at a much lower temperature, which costs far less energy.
The researchers did so by combining the cracking step with a second reaction step that immediately completes the conversion to a liquid gasoline-like fuel without unwanted by-products. The second reaction step uses alkylation catalysts - the same catalysts currently deployed by the petroleum industry to improve the octane rating of gasoline.
The alkylation reaction immediately follows the cracking step in a single reaction vessel, at the temperature of a warm oven (70 degrees C/158 degrees F).
“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 Oliver Y. Gutiérrez, a study author and 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.”
In their study, the research team, co-led by scientists from the Technical University of Munich, Germany, pointed to separate, recent developments by the petroleum industry to commercialise the second part of the process reported here for crude oil processing.
“The fact that industry has successfully deployed these emerging alkylation catalysts demonstrates their stable, robust nature,” said Johannes Lercher, a senior author of the study, director of PNNL’s Institute for Integrated Catalysis, and professor of chemistry at TUM.
And, while the process is currently at lab stage, because the alkylation catalysts used in the process are already being implemented by industry, opens possibilities for a quick scale-up to an industrial process .
“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,” Lercher noted.
The researchers also point to a limitation on the findings of the study. The process works for low-density polyethylene products such as plastic films and squeezable bottles, and polypropylene products that are not typically collected in kerbside recycling programmes in the United States. High-density polyethylene would require a pretreatment to allow the catalyst access to the bonds it needs to break.
Petroleum-based plastic waste is an untapped resource that can serve as the starting material for useful durable materials and for fuels. More than half of the 360 million tons of plastics produced globally each year are the plastics targeted in this study.
“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,” said Lercher. “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.”
This research study, published Feb. 24, 2023, was supported by the Department of Energy Office of Science.
This short animation illustrates how this newly developed plastic upcycling process works for low-density polyethylene products (LDPE, plastic resin code #4), such as plastic films and squeezable bottles, and polypropylene products (PP, plastic resin code #5) that are not typically collected in curb-side recycling programs in the United States.
(Animation by Sara Levine | Pacific Northwest National Laboratory)