We’re grappling globally with a persistent plastic predicament. Our heavy reliance on non-biodegradable single-use plastics, coupled with inefficient recycling processes, means that a lot of plastic still ends up languishing in landfills or choking up sensitive environments.
Today, conventional plastic recycling involves mechanically breaking down specific types of plastics into small pellets that can be turned into new plastic products. A promising alternative approach called pyrolysis uses high temperatures and powerful catalysts to chemically transform waste plastics, upcycling them into a wider range of valuable materials such as fuels and chemical feedstocks.
However, plastic pyrolysis faces its own technical hurdles: its crucial catalysts need to be regularly replaced due to issues such as coking, where built-up carbon residues block active sites, and poisoning, where other chemicals deactivate their catalytic properties.
To tackle this dilemma, a joint team of researchers from A*STAR’s Institute of Materials Research and Engineering (IMRE) and the Institute of Sustainability for Chemicals, Energy and Environment (ISCE2) have been on the hunt for next-generation catalysts for pyrolysis.
“Considering the disadvantages of traditional options such as zeolites and metal oxides, we sought an inexpensive inorganic alternative that is readily available, continuously produced and exhibits significant catalytic efficacy,” said Jason Lim, Group Leader of IMRE’s Sustainable Supramolecular Materials Lab.
Co-led by Lim and Enyi Ye, Group Leader of IMRE’s Nano+ Lab, the team earmarked incineration fly ash (IFA), a byproduct from municipal waste incinerators that typically ends up in landfill. While IFA is rich in catalytic compounds like calcium hydroxychloride (CaClOH), its potential as a catalyst for pyrolysis had been largely ignored till now.
Using IFA, the researchers explored pyrolysis of three widely-used plastics—polyethylene (PE), polypropylene and polystyrene—in their pure forms, as well as samples of everyday plastic waste. In a series of control experiments, they compared IFA’s effectiveness versus other materials to understand CaClOH’s potential role in plastic recycling.
The researchers reported that IFA significantly improved the pyrolytic conversion of virgin high-density PE (HDPE) into liquid hydrocarbons from 46.7 to 92.8 percent. They also discovered that HDPE plastics, commonly used in bottles and grocery bags, yielded the highest amount of liquid product while minimising the formation of char, a solid byproduct.
“CaClOH’s amphoteric nature lets it act as both an acid and a base, giving it a unique reactivity compared to other calcium-containing basic materials like calcium oxide,” said Lim. “It promotes the formation of positively-charged intermediates during pyrolysis, reducing the odds of chemical crosslinking that would normally result in waxes, and increasing the production of low-weight liquid hydrocarbons, which can be useful as fuels, solvents and chemical precursors.”
The team envisions repurposing abundant, low-cost plastic waste into valuable hydrocarbon feedstocks through IFA pyrolysis to minimise landfill volume and foster a sustainable circular economy. To pave the way, the group is currently exploring other classes of catalysts for plastic upcycling.
“We would also like to stabilise post-pyrolysis IFA so it can be safely used in other areas such as construction materials,” added Ye. “We aim to transform IFA from a waste product into a resource with diverse applications.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE) and the Institute of Sustainability for Chemicals, Energy and Environment (ISCE2).
