Researchers have found a way to turn plastic waste into hydrogen using sulfuric acid recovered from old car batteries and energy from sunlight. The process not only breaks down difficult-to-recycle plastics but also produces useful chemicals and clean fuel in a single reactor.
Plastic waste remains a growing environmental challenge, particularly materials such as polyethylene terephthalate (PET), which is widely used in drink bottles and food packaging. Lead-acid batteries present another issue, as recycling operations typically focus on recovering the lead while leaving the sulfuric acid largely underutilized.
A team at the University of Cambridge has now combined these two waste streams in a process that not only recovers useful chemical compounds from plastic but also generates hydrogen using solar energy.
Turning Plastic Waste Back Into Raw Materials
The process begins with PET waste. Researchers ground plastic bottles into fine flakes before mixing them with concentrated sulfuric acid and heating the solution to 140°C (284°F).
As detailed in a release from theThe University of Cambridge, the treatment breaks the plastic down into its original components: ethylene glycol and terephthalic acid, both of which are valuable industrial chemicals. The terephthalic acid naturally precipitates from the mixture, making it easy to recover. Rather than relying on newly produced sulfuric acid, the team used acid extracted from spent lead-acid batteries.
“Sulfuric acid is a component of car batteries, but when they are recycled, they only recover the lead component,” said Kay Kwarteng, the study’s lead author. “We could extract the battery acid and use that instead. It makes a strong argument for sustainability.”
The remaining solution is rich in ethylene glycol, which serves as the feedstock for the next stage of the process.
Using Sunlight to Produce Hydrogen
Converting ethylene glycol into hydrogen posed a challenge because conventional methods typically require alkaline conditions. The solution obtained from the plastic recycling stage, by contrast, is highly acidic.
To bridge that gap, the researchers developed a molybdenum-based catalyst capable of operating in the acidic environment. Described in the journal Joule00031-0?_eturnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS2542435126000310%3Fshowall%3Dtrue), the catalyst becomes active when exposed to visible light.
“Once we expose the catalyst to light, it oxidizes the ethylene glycol which generates electrons,” Kwarteng explained. “These electrons can convert protons,” — present in the acid mixture — “to hydrogen, and they oxidize the ethylene glycol to acetic acid.”
What makes the study stand out is that both stages happen within the same reactor. While plastic depolymerization and hydrogen generation have each been demonstrated before, combining them into one process had not been achieved previously.
More Than a New Way to Produce Hydrogen
The researchers believe the chemistry could be useful for more than fuel production. Erwin Reisner, professor of energy and sustainability at the University of Cambridge, said the same chemistry can be adapted for hydrogenation reactions used throughout the chemical industry.
Instead of releasing hydrogen gas, the process can transfer fuel production directly to other compounds introduced into the reactor. Many hydrogenation reactions currently depend on hydrogen derived from fossil fuels.
The team explored this possibility in a follow-up study published in Angewandte Chemie International Edition, using plastic-derived hydrogen to convert nitrogen-containing compounds into chemical building blocks used in pharmaceutical manufacturing.
“When we use plastics for this hydrogenation, we reduce the carbon footprint by half,” Kwarteng said.
The researchers are now adapting the technology for flow reactors, which continuously convert reactants into products. Speaking to Live Science, Amit Kumar, a catalysis researcher at the University of St Andrews, praised the use of multiple recycled materials while noting that demonstrating the light-driven chemistry at larger scales will be an important next step.
