Grocery bags, used face masks, and food wrappers contain plenty of potentially useful raw materials. However, in the past, it has proven to be much more economical to keep manufacturing more of these single-use plastics than to recover and recycle them.
A new method to convert low-density plastic waste to fuel and raw materials promises to help close the carbon cycle. Image Credit: Melanie Hess-Robinson | Pacific Northwest National Laboratory.
Now, an international team of researchers guided by the Department of Energy’s Pacific Northwest National Laboratory has cracked the code that stymied earlier attempts to break down these persistent plastics. The team has published their findings in the recent issue of the journal Science.
Low Temperature and Reaction Control
Typically, the process or recycling plastics has required the tough and stable bonds that also make them so persistent in the environment to be ‘cracked’ or split apart. This cracking step necessitates high temperatures, making it costly and energy demanding.
The novelty here is combining the cracking step with a second reaction step that immediately completes the conversion to a liquid gasoline-like fuel without unwanted byproducts.
The second reaction step makes use of alkylation catalysts. These catalysts offer a chemical reaction presently utilized by the petroleum sector to enhance the octane rating of gasoline.
Crucially in the current study, the alkylation reaction immediately follows the cracking step in a single reaction vessel near room temperature (70 °C/158 °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. 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.
Oliver Y. Gutiérrez, Study Author and Chemist, Pacific Northwest National Laboratory
In their study, the research team, co-guided by researchers from the Technical University of Munich (TUM), Germany, highlighted separate and recent developments by the petroleum sector to commercialize the second step of the process stated here for crude oil processing.
The fact that industry has successfully deployed these emerging alkylation catalysts demonstrates their stable, robust nature. 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.
Johannes Lercher, Study Senior Author and Director, Institute for Integrated Catalysis, Pacific Northwest National Laboratory
In their research, the team notes a limitation in their results. The process is effective for low-density polyethylene products (LDPE, plastic resin code #4), such as squeezable bottles and plastic films, and polypropylene products (PP, plastic resin code #5) that are not normally collected in curb-side recycling programs.
High-density polyethylene (HPDE, plastic resin code #2) would necessitate a pretreatment to enable the catalyst access to the bonds it has to disintegrate.
Seeing Waste Plastic as Future Fuel and New Products
Petroleum-based plastic waste is an unexploited resource that can act as the starting material for beneficial durable materials as well as for fuels. Over half of the 360 million tons of plastics generated worldwide annually are the plastics explored in this study.
Yet, looking at a mountain of plastic and seeing its true value requires an innovator’s mindset, a chemist’s ingenuity, and a realist’s understanding of the economics involved. These researchers are attempting to alter the dynamic by applying their knowledge in efficiently breaking chemical bonds.
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.
Johannes Lercher, Study Senior Author and Director, Institute for Integrated Catalysis, Pacific Northwest National Laboratory
Johannes Lercher is also a professor of chemistry at the Technical University of Munich (TUM). This study was published on February 24th, 2023, and is supported by the Department of Energy Office of Science.
What Is Plastic Upcycling?
A 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 curbside recycling programs in the United States. Video Credit: Sara Levine | Pacific Northwest National Laboratory.
Journal Reference:
Zhang, W., et al. (2023) Low-temperature upcycling of polyolefins into liquid alkanes via tandem cracking-alkylation. Science. doi.org/10.1126/science.ade7485.