Sunlight used to convert plastics into useful chemical

NTU team creates less toxic, affordable catalyst to process non-biodegradable plastics

Scientists here have developed a way to convert plastic waste into useful chemicals using sunlight.

This is the first reported process that can completely break down a non-biodegradable plastic such as polyethylene using visible light and a catalyst that does not contain heavy metals, said Assistant Professor Soo Han Sen, 39, from Nanyang Technological University (NTU).

His team from the School of Physical and Mathematical Sciences created a catalyst that harnesses light energy, such as sunlight, to break down chemical bonds in plastics within days.

This photochemical reaction is more environmentally friendly than most recycling methods, which involve heating, generating greenhouse gases as a by-product, said Prof Soo.

The National Environment Agency (NEA) reported that only 40,700 tonnes - or 4 per cent - of the 949,300 tonnes of plastic waste thrown out in Singapore last year were recycled.

Most of the plastic waste is incinerated, producing greenhouse gases such as carbon dioxide, while the leftover mass-burn ash is transported to Semakau Landfill, which, it is estimated, will run out of space by 2035.

Developing zero-waste solutions, such as this method to convert plastic waste into useful chemicals, is part of the NTU Smart Campus vision to develop a sustainable future.

The three main plastics which can be converted by this method into useful chemicals are polyethylene, polypropylene and polystyrene. These make up 80 per cent to 90 per cent of the plastics in the world.

The method is one of two known processes that use sunlight to convert plastic waste into useful chemicals, said Prof Soo. The other method, photoreforming, was developed by scientists from the University of Cambridge last year.

  • 4%

    Percentage of the 949,300 tonnes of plastic waste thrown out in Singapore last year that was recycled, according to the National Environment Agency.

Photoreforming combines plastic with water and sunlight to produce hydrogen gas, which can be used for energy generation by hydrogen fuel cells. However, the catalyst it uses contains cadmium, a toxic heavy metal.

The NTU research team uses a vanadium-based catalyst instead.

Vanadium is an affordable metal that is less toxic and about 1,000 times more abundant than cadmium. As a catalyst, it is thus more abundant, affordable and environmentally friendly, the NTU team said.

The team has demonstrated how the catalysed reaction can break down non-biodegradable consumer plastics like polyethylene within six days.

This converts the plastic into formic acid, a naturally occurring preservative and antibacterial agent that can also be used for energy generation by power plants and hydrogen fuel cell vehicles.

The laboratory process involves dissolving the plastic in acetonitrile, an organic solvent, by heating the mixture to 85 deg C.

The catalyst is added, and this mixture is exposed to artificial sunlight at room temperature.

According to Prof Soo, the catalyst has a 100 per cent conversion rate, where all plastic processed is converted into formic acid.

Carbon dioxide gas is also produced as a by-product.

The NTU research team will work with the Agency for Science, Technology and Research's Institute of Chemical and Engineering Sciences (Ices) and Institute of Materials Research and Engineering to develop a continuous flow system for the process.

With such a system, little to no carbon dioxide gas is produced, and hydrogen gas is produced instead, thus making this process carbon neutral, said Prof Soo.

Dr Balamurugan Ramalingam, 45, a scientist at Ices, believes the approach of using light to convert plastic into useful chemicals is remarkable. He said: "Light is a sustainable source of energy and can be harvested from the sun. This can potentially pave the way for converting plastic into useful commodity chemicals."

However, improvements can still be made to the catalyst, said Prof Soo. A lot of energy is now required to recover the catalyst, which is in powder form, after completing the chemical reaction.

He and his research team of four scientists hope to make a solid catalyst that does not dissolve during the process, so that it can be more easily reused.

He expects that, with sufficient resources dedicated to developing the catalyst, it should be ready for commercial use in about five years.

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A version of this article appeared in the print edition of The Sunday Times on December 15, 2019, with the headline Sunlight used to convert plastics into useful chemical. Subscribe