Power Shift is Ellyn Lapointe’s ongoing Gizmodo series exploring advances in green technology, with a focus on renewable energy, grid modernization, and emissions reduction.
Plastic recycling and energy decarbonization represent two of the most pressing sustainability challenges of our time. A new study offers a solution that could address both problems simultaneously: transform plastic garbage in clean hydrogen.
While the concept is not new, the approach, described in an article published in Proceedings of the National Academy of Sciences earlier this month—significantly improves on conventional methods. Called “ATT,” short for alkaline heat treatment, the reaction produces high-purity hydrogen at much lower temperatures without requiring extensive waste sorting. Furthermore, it does not directly generate greenhouse gas emissions.
Two problems, one solution
Because plastic recycling requires expensive sorting and processing, only a small fraction of the world’s discarded plastic is recycled.
“In practice, discarded plastics are often mixed, contaminated with food, adhesives, labels, dyes and other additives, or combined into multi-layer packaging,” co-author Woo Jae Kim, a professor of chemical engineering and materials science at Ewha Womans University in South Korea, told Gizmodo in an email. “Therefore, sorting and cleaning them can be technically difficult and more expensive than producing new plastic from fossil resources.”
Previous research presented that in 2022, the global recycling rate remained stagnant at just 9%, while 40% ended up in landfills and 34% was incinerated. Meanwhile, plastic use is expected to continue growing, rising from 464 megatons in 2020 to 884 megatons in 2050, according to one study. projection.
At the same time, the world urgently needs clean energy sources. Hydrogen is often touted as a promising fuel because it can be burned like oil or gas, but it does not release planet-warming carbon dioxide (CO2). The problem is that there are no easily extractable sources of pure hydrogen available on Earth. If we want to use it, we have to do it.
To solve both problems, chemical engineers are exploring various ways to transform plastic waste into clean hydrogen. Two methods that have gained a lot of attention in recent years are pyrolysis and gasification.
Pyrolysis heats plastics in an oxygen-free environment, breaking them down into oil, carbon and gases (including hydrogen). The process produces relatively low carbon emissions, but only works well with certain types of plastic and therefore requires extensive sorting and refining.
Gasification works differently, partially oxidizing plastics at much higher temperatures to produce a mixture of hydrogen, carbon monoxide and hydrocarbons. Because gasification can handle mixed plastics without extensive sorting, it is generally considered a more cost-effective approach, but the high pressures and extreme temperatures it requires make it energy-intensive, resulting in substantial CO2 emissions.
To eliminate these drawbacks, this new study proposes using alkaline heat treatment to recycle mixed plastic waste. The authors adapted their ATT process from a method that Kim had developed with Ah-Hyung “Alissa” Park, professor of chemical and biomolecular engineering at the University of California, Los Angeles. They designed the original process to convert biomass, such as seaweed, into hydrogen in a carbon-neutral way, but wondered whether a similar approach could also be useful for recycling mixed plastics.
A cleaner conversion
In the lab, Kim, Park and their colleagues used the modified ATT to convert the three most common plastics—polyethylene terephthalate (PET), polyethylene (PE), and polypropylene (PP)—into high-purity hydrogen. ATT breaks down plastic by mixing it with sodium hydroxide (NaOH) and heating it. Thanks to the alkaline conditions provided by NaOH, it does not require as much heat as gasification.
Initially, the process produced significantly more hydrogen from PET than from PE or PP. These two plastics are made entirely of carbon-hydrogen bonds, so they are chemically inert under alkaline conditions. To solve that problem, the researchers briefly exposed PE and PP to mild heat and oxygen before the main reaction. This pretreatment allowed all three plastics to decompose efficiently.
Using this approach, the researchers produced hydrogen yields of 43.7, 51.9, and 30.2 millimoles of hydrogen gas per gram of PET, PE, and PP, respectively, yields comparable to those achieved by pyrolysis and gasification. What’s more, their post-reaction analysis showed that the carbon emissions from the reaction were negligible.
Julie Zimmerman, a professor of chemical and environmental engineering and vice provost for planetary solutions at Yale University, said the study presents an “interesting and potentially important reaction concept,” but it is too early to say it is already a scalable and sustainable route for converting mixed plastic into clean hydrogen.
“The authors demonstrate a credible chemical pathway to produce high-purity hydrogen from PET and pre-oxidized PE and PP, including a controlled mixture of the three plastics,” Zimmerman said. “However, milligram-scale experiments, lengthy oxidation pretreatment, substantial use of alkalis, and high final temperatures establish chemical feasibility rather than technical or economic feasibility.”
Researchers agree that optimizing the process and evaluating its economic viability will require more research. While the reaction produced negligible direct CO2 emissions, they will need to perform a full life cycle analysis to understand its overall carbon footprint, Kim said. The team also needs to develop an efficient way to recycle the sodium hydroxide reagent and test whether the method works with plastic waste containing food residue, moisture, additives and other contaminants.
While there is still much work to be done, the study marks an important step towards a more efficient and potentially cleaner way of converting plastic into hydrogen. As trash and carbon emissions continue to pile up, finding innovative solutions to these problems will become increasingly important.
