Plastic Waste Could Become Clean Hydrogen

Plastic waste could become a source of clean hydrogen through a lower-temperature chemical process that handles mixed materials without directly producing significant greenhouse gas emissions.

The approach, called alkaline thermal treatment (ATT), is described in a paper published earlier this month in Proceedings of the National Academy of Sciences. Researchers used it to convert three common plastics—polyethylene terephthalate (PET), polyethylene (PE), and polypropylene (PP)—into high-purity hydrogen.

Why mixed-plastic recycling remains difficult

Only 9% of global plastic waste was recycled in 2022, according to previous research. Another 40% went to landfills and 34% was incinerated. Plastic use is projected to rise from 464 megatons in 2020 to 884 megatons by 2050.

Recommended reading

Fiber-optic sensors keep tracking alive when GPS fails

Sorting is a major barrier. Discarded plastics are often contaminated with food, adhesives, labels, dyes, additives, or multilayer packaging, making them expensive and technically difficult to clean and process.

“In practice, discarded plastics are often mixed, contaminated with food, adhesives, labels, dyes and other additives, or combined in multilayer packaging. Sorting and cleaning them can therefore be technically difficult and more expensive than producing new plastic from fossil resources.”

Existing conversion methods each have drawbacks. Pyrolysis heats plastic without oxygen, producing oil, char, and gases including hydrogen. It generates relatively low carbon emissions but works best with selected plastics, requiring extensive sorting and refining.

Gasification can process mixed plastics, but it partially oxidizes them at extremely high temperatures and pressures. The result is a mixture of hydrogen, carbon monoxide, and hydrocarbons, along with substantial energy use and CO2 emissions.

How alkaline thermal treatment works

ATT mixes plastic with sodium hydroxide (NaOH) and heats it. The alkaline environment allows the reaction to run at much lower temperatures than gasification. The researchers adapted the technique from a process developed by Kim and Ah-Hyung “Alissa” Park to convert biomass such as seaweed into hydrogen in a carbon-neutral way.

PET decomposed readily under alkaline conditions. PE and PP were more resistant because they consist entirely of carbon-hydrogen bonds. The team solved that problem by briefly exposing those plastics to mild heat and oxygen before the main reaction.

The modified process produced hydrogen yields of:

• 43.7 millimoles per gram of PET
• 51.9 millimoles per gram of PE
• 30.2 millimoles per gram of PP

Those results were comparable to yields reported for pyrolysis and gasification. Post-reaction analysis also found that direct carbon emissions were negligible.

The results do not yet establish a commercially viable recycling system. Julie Zimmerman, an endowed professor of chemical and environmental engineering and vice provost for planetary solutions at Yale University, called the work an “interesting and potentially important reaction concept,” while warning that the experiments were performed only at milligram scale.

“The authors demonstrate a credible chemical pathway for producing high-purity hydrogen from PET and pre-oxidized PE and PP, including a controlled mixture of the three plastics. However, the milligram-scale experiments, lengthy oxidation pretreatment, substantial alkali use, and high final temperatures establish chemical feasibility rather than technical or economic viability.”

The researchers say they still need to optimize the process, assess its economics, and conduct a full life-cycle analysis. They also must find an efficient way to recycle the sodium hydroxide and test mixed waste containing food residue, moisture, additives, and other contaminants.