A team of researchers from the Ewha Women’s University (Seoul, South Korea; www.ewha.ac.kr) and the University of California at Los Angeles (UCLA; www.ucla.edu) have taken a step toward addressing one of main challenges in plastics recycling — economically processing mixed plastics streams without the need for presorting. They developed a method, known as alkaline thermal treatment (ATT), that can directly convert mixed polymers, including polyethylene terephthalate (PET), polyethylene (PE) and polypropylene (PP), into hydrogen at temperatures significantly lower than conventional gasification and at atmospheric pressure.
Source: UCLA
Originally investigated for biomass conversion, ATT uses sodium hydroxide (NaOH) as an alkaline catalyst to facilitate polymer degradation under relatively mild conditions. “Unlike conventional gasification, NaOH-assisted ATT enables plastic decomposition at significantly lower temperatures, while producing high-purity hydrogen and minimizing carbon emissions,” the researchers write in a recent issue of Proceedings of the National Academy of Sciences (PNAS).
One focus of the research was on the crucial role that oxygen functional groups play in improving the reactivity of the waste plastics for ATT. Oxygen groups are already present in the polyester PET, but not the polyolefins PE and PP, so the researchers developed a thermal oxidation pretreatment method to render the method effective on mixed waste streams. The pre-oxidation step introduces oxygen functional groups that facilitate polymer decomposition and hydrogen production in ATT.
“A key advancement in this work is the oxidation pretreatment of PP and PE, which enhances their reactivity in ATT and allows efficient hydrogen generation, even from typically resistant polyolefins,” the researchers say.
While hydrogen is produced from the ATT, the carbon that was originally part of the polymer waste ends up sequestered mostly as sodium carbonate, with some in tars and waxes, meaning very little carbon dioxide is produced in the recycling process.
The researchers say their study establishes ATT as “a promising and sustainable solution for plastic-waste management and clean energy production, providing an economically viable low-carbon pathway for hydrogen generation.”