Report of the Basic Energy Sciences Roundtable on Chemical Upcycling of Polymers
Plastics are ubiquitous in modern life. They are primarily made from synthetic carbon-based polymers— organic macromolecules made up of many repeating subunits called monomers—and are designed to be durable and resistant to degradation. Global plastics production has reached a rate of more than 400 million metric tons (MTs) per year, with more than 8 billion MTs produced in the past 50 years. Average production has increased by 36% in the past decade and is projected to grow to 700 million MTs in 2030—about 80 kg of plastics produced for every human on earth. Globally, 20% of discarded plastics are recycled (<10% in the United States), primarily using mechanical processes, and about 25% are incinerated for energy recovery. More than half are deposited into landfills or released into the environment. Thus, discarded plastics pose a long-term environmental challenge: For example, at the current rate of plastics production and disposal, the mass of plastics in the ocean is predicted to exceed the mass of fish by 2050. Current approaches to reducing unwanted plastics are insufficient to address the growing accumulation of discarded plastics. Although incineration eliminates unwanted plastics and recovers some of the energy used to make them, it uses up the potential resource and creates unwanted byproducts. Mechanical recycling—which involves shredding, heating, and remolding of the plastics—is more efficient than making them from petroleum products, using less than half as much energy to generate new plastics. However, mechanical recycling generally degrades, or downcycles, the polymers. Chemical recycling involves deconstruction of polymers by chemical processes to monomers for conversion back into polymers or to molecular intermediates that can be used as fuel or feedstock by the chemical industry. However, present methods (e.g., pyrolysis—high-temperature decomposition in the absence of oxygen), are energy-intensive and require further processing to make products. Chemical upcycling of polymers—the process of selectively converting discarded plastics into chemicals, fuels, or materials with higher value—holds the promise of changing the paradigm for discarded plastic from waste to valued resource. A significant opportunity exists for fundamental research to provide the foundational knowledge required to move toward a circular lifecycle for plastics, in which the chemical constituents of plastics are reformed into polymers or repurposed to give them another life. To identify the fundamental challenges and research opportunities that could accelerate the transformation of discarded plastics to higher-value fuels, chemicals, and materials, the US Department of Energy, Office of Science, Office of Basic Energy Sciences sponsored a Roundtable on Chemical Upcycling of Polymers, which was held near Washington, DC, on April 30–May 1, 2019. This roundtable identified four Priority Research Opportunities to address the complex chemical transformations and physical processes underlying the upcycling of discarded plastics.