With plastic pollution now one of the world’s most serious environmental problems, researchers worldwide are devising novel strategies to divert plastic products away from landfills and oceans or convert them to more biodegradable materials.
The most common method of chemically recycling plastic is through a process called pyrolysis, which involves heating the material to high temperatures and converting it to fuel or new plastic products. This method has been used for decades and is increasingly being investigated by some of the world’s largest chemical companies as a scalable technology.
Scientists have developed a new method for converting polyethylene, the world’s most widely used plastic, into jet fuel and other liquid hydrocarbon products. The process is more energy-efficient than existing methods and takes about an hour to complete.
In a study published in Chem Catalysis, led by Dr. Hongfei Lin, Dr.Linda Voiland and Chuhua Jia at the Washington State University collaborated with Professor Jim Pfaendtner and Dr. Nadia N.Intan at the University of Washington and Dr. Janani Sampath from the University of Florida, described the process for converting polyethylene — the most frequently produced polymer, primarily used in packaging and plastic containers such as bottles — into ingredients for jet fuel by combining the transition metal ruthenium, which was used as part of a catalyst, with the commonly used solvent hexane.
The process was highly efficient, converting up to 90% of plastic waste into liquid fuel and lubricants. “We’re quite excited that it has the potential to produce a wide variety of products,” Dr. Lin, the study’s corresponding author, told Science Featured Series in an interview. “We discovered a synergy between the solvent and catalyst that results in a very nice combination with numerous control knobs for tuning the reaction conditions and optimizing the entire process,” he explained. “For instance, if you want to maximize jet fuel production yield, we can help. And if the market requires more diesel fuel, we can also produce it, as well as lubricants.”
Pyrolysis, Lin explained, could also convert plastic waste to fuel within an hour with sufficient heat. Still, pyrolysis typically requires temperatures greater than 400 degrees Celsius, compared to the 200 to 220 degrees Celsius required for the new system he and his team developed. “We nearly halved the temperature,” he explained. “A lower temperature implies that we would use less external energy to power this conversion system, implying a higher energy efficiency.”
Obtaining faster kinetics at a significantly lower temperature is a complicated chemical engineering problem. The researchers were taken aback by the new process’s speed and efficiency; Dr. Lin explained: “This is unexpected. We reviewed the literature and discovered that many people used a similar process but required nearly 24 hours to achieve meaningful conversions of similar products — even at higher temperatures.”
Dr. Lin has a history of developing novel methods for producing jet fuel; in a previous paper, he and colleagues demonstrated that jet fuel could be synthesized from terpenoids, a class of organic chemicals found in mint, pine, gumweed, and eucalyptus, among other plants. However, the new approach is promising not only because of its speed and low temperature but also because the underlying principles can be applied to different types of plastics. “The underlying principles can be used to expand research into other types of polymers, not just polyethylene,” Dr. Lin explained. According to the Office of Commercialization at Washington State University, a patent application for the new process of converting plastic waste to jet fuel is pending with the United States Patent and Trademark Office. Apart from obtaining patent approval, there is also the issue of scaling up the technology and overcoming the logistical challenges associated with implementing it in manufacturing and recycling facilities.
As Dr. Lin stated, “How to effectively connect with those waste plastics and transport them to processing facilities is not a task for a chemical engineer,” he added. “The primary impediment to chemical recycling of plastic, particularly in the United States, is a lack of an efficient connection system. In residential areas, trash bins are provided, and recyclables are mixed in with other waste.”
Lin’s research team is currently developing another catalytic process for depolymerizing mixed plastic waste, which could help reduce the enormous cost of recycling. “We want to develop a sequential catalytic process that can handle commingled plastics without physically separating them; we want to break down those polymers at the molecular level,” he explained. “We have some cutting-edge data demonstrating the feasibility of this sequential conversion process.”
Chuhua Jia, Shaoqu Xie, Wanli Zhang, Nadia N. Intan, Janani Sampath, Jim Pfaendtner, Hongfei Lin,
Deconstruction of high-density polyethylene into liquid hydrocarbon fuels and lubricants by hydrogenolysis over Ru catalyst, Chem Catalysis, 2021, https://doi.org/10.1016/j.checat.2021.04.002.