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Waste plastic can be processed into hydrogen gas and carbon nanotubes.
A pilot project has demonstrated a technique for processing plastic into carbon nanotubes and hydrogen gas. The approach offers the possibility of turning waste plastic into a source of clean fuel and high value carbon if it can be scaled up for industrial use.
The new method, developed by an international team that included researchers at Saudi Arabia’s King Abdulaziz City for Science and Technology, pulverizes plastic in a one-step process using microwaves and a catalyst made of iron oxide and aluminium oxide. The key to the method is that the plastic isn’t directly heated since it is transparent to the microwaves.
The microwaves heat the catalyst, which then heats the plastic around it. This means that the catalytic process happens at the interface between the two materials, with the remaining plastic staying cool until it comes into contact with the catalyst. As a result, there are fewer side reactions, making this process more efficient than existing approaches
In proof-of-concept experiments, the team showed that waste plastic from supermarket products can be processed in 30-90 seconds to produce hydrogen gas and carbon nanotubes, recovering 97% of the hydrogen in the plastic.
Chemical engineer Chunfei Wu of Queen’s University Belfast, who wasn't involved in the study, describes this as an interesting advance. “This approach offers the advantage of fast processing and higher carbon yield,” he says, but notes that the fraction of the carbon that forms nanotubes could be small. “That means the total carbon nanotube yield could be low.”
Wu also notes there will be challenges in scaling up the process. “The study used clean plastics, but in practice the plastic will be dirty. If the catalyst is mixed directly with dirty plastic, its performance will be very different and the process could be challenging. There will also be challenges related to scaling up of microwave processing.”
One of the paper’s lead authors, Tiancun Xiao of the University of Oxford, says large-scale industrial microwave processes are already used in the mining, food, and petrochemical industries, so this may not present a great hurdle. He says the team is working to scale the method up for industrial use. “This new type of catalysis has great potential as a route out of the plastic Armageddon we face and a path towards a hydrogen-based economy, particularly for developing countries,” he says.
Jie, X. et al. Microwave-initiated catalytic deconstruction of plastic waste into hydrogen and high-value carbons. Nat. Catal. http://dx.doi.org/10.1038/s41929-020-00518-5 (2020).