Each year, more than 390 million metric tons of plastics are produced globally, yet close to half are single-use varieties that can be economically challenging to recycle.
Now, recent research out of the Pacific Northwest National Laboratory suggests a new method for cost-effectively upcycling these plastics, transforming them into fuel and other useful petroleum products.
The study, which PNNL co-led with a team from the Technical University of Munich, Germany, found two separate but compatible processes that are already being utilized by the petroleum industry that could be applied in tandem to the process, given the right conditions.
The new technique allows for the selective upcycling of polyolefins, a class of polymers that include polyethylene (PE) and polypropylene (PP). PE plastics include plastic grocery bags, cling films and squeezable bottles. PP plastics are commonly used for bottle caps, food storage containers and single-serve coffee capsules.
Ordinarily, the molecular bonds that form these single-use plastics require considerable energy in the form of heat to “crack” the highly stable bonds that make these plastics so persistent in the environment. Cracking breaks these carbon-carbon bonds, reducing the molecules into simpler compounds.
However, in existing recycling approaches, the resulting molecules immediately form new bonds in uncontrolled ways. Further processing is then needed to break down the intermediary molecule. Such a two-stage process requires considerable time, energy and infrastructure.
Now, based on a study that was published in the journal Science last week, researchers have demonstrated a new, far more efficient process. It uses what’s known as an alkylation catalyst that promotes a desired controlled bond as the cracking process takes place. The reaction occurs in an ionic solvent that provides a highly acidic environment which promotes the rapid conversion of these plastics.
By combining what was previously a two-step process into a single one that is much more energy-efficient, the researchers were able to transform the waste plastics directly into petroleum compounds, including highly branched liquid alkanes, gaseous isobutane and large alkanes. The resulting gasoline-like compounds can then be used either as a fuel or as the source for new plastics, all without unwanted byproducts.
“Cracking just to break the bonds results in them forming another bond 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.”
The process also occurs rapidly, taking only three hours to achieve full conversion of the plastics.
“This reaction time of three hours is remarkable,” Gutiérrez said. “Because for transformations other people have reported, the required temperatures are above 200 degrees Celsius, and they need much longer to have a similar amount of polymer fully completed.”
Also, because the process operates at much lower temperatures than prior methods, it can reduce energy costs considerably. Many existing two-stage processes require moderate to high reaction temperatures that are typically between 200 to 250 degrees Celsius. This new PNNL/TUM process, which the researchers refer to as tandem cracking-alkylation, can be performed at around 70 degrees Celsius (or 158 degrees Fahrenheit), well below the boiling point of water.
The new process works for low-density polyethylene products, as well as for polypropylene products, both of which aren’t typically collected and processed in curbside recycling. This amounts to around half of all plastics produced annually.
Other plastics, such as high-density polyethylene (HPDE), could also be processed using this technique but would require a pretreatment stage to give the catalyst the access needed to break the bonds. However, there are already other methods available for recycling these plastics that are comparatively more efficient.
While turning the plastic waste into fuel has an appeal, there are vocal critics of the approach. Some worry that it creates the sense that plastics are sustainable, despite nearly always being made from fossil fuels. There are also concerns about the carbon footprint of the fuel — which would likely be lower with PNNL’s methods compared to higher heat strategies. And ProPublica recently reported on federal regulators struggling to manage the health and environmental impacts of plastic-derived fuel production.
It should be noted these are still early days for this process and further work needs to be done to better understand what is occurring at the molecular level so it can be optimized for use at industrial scales. If this approach is scalable, it could be a real game-changer in terms of recycling/upcycling of a tremendous amount of plastic waste. In addition, the low temperature of the process has the added benefit of being less dangerous and would need less protective infrastructure.
“The investment you would have to make would probably be lower because all of a sudden you don’t need thick walls and tanks for safety,” noted Gutiérrez. “It’s safer to work with an ionic solvent that is at 70 degree Celsius versus one that is pressurized 100 atmospheres at much higher temperature.”
The process is also appealing because it could be performed in existing refinery settings, using processes that are already being used at industrial scale. In fact, the chemical reaction these catalysts provide is currently utilized by the petroleum industry to improve the octane rating of gasoline.
“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. “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.”
In the end, work like this could make a significant contribution to a future circular economy, one that is much more economically and environmentally sustainable and far less wasteful.
The research study paper, “Low-temperature upcycling of polyolefins into liquid alkanes via tandem cracking-alkylation” was published in the journal Science on Feb. 24, 2023, and was supported by the Department of Energy Office of Science.