NTU Singapore chemists harness sun to convert plastic waste into electricity, commercialization to follow

17 Dec 2019 --- Chemists at Nanyang Technological University, Singapore (NTU Singapore) have discovered a method that turns plastic waste into valuable chemicals by using sunlight. In lab experiments, the research team mixed plastics with their catalyst in a solvent, which allows the solution to harness light energy and convert the dissolved plastics into formic acid – a chemical used in fuel cells to produce electricity. PackagingInsights speaks to Assistant Professor Soo Han Sen, the NTU Singapore research team leader, about the significance of the technology in minimizing plastic waste and generating renewable energy and the project’s next steps.

Reporting their work in Advanced Science, Soo’s team made their catalyst from the affordable, biocompatible metal vanadium, commonly used in steel alloys for vehicles and aluminum alloys for aircraft.

When the vanadium-based catalyst was dissolved in a solution containing a non-biodegradable consumer plastic like polyethylene and exposed to artificial sunlight, it broke down the carbon-carbon bonds within the plastic in six days.

This process turned the polyethylene into formic acid, a naturally occurring preservative and antibacterial agent, which can also be used for energy generation by power plants and in hydrogen fuel cell vehicles.

“We aimed to develop sustainable and cost-effective methods to harness sunlight to manufacture fuels and other chemical products. This new chemical treatment 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,” Soo explains.“Our research is hopefully the beginning of an entirely different and more sustainable approach to tackling plastic pollution,” Soo says. 

“Our research is hopefully the beginning of an entirely different and more sustainable approach to tackling plastic pollution while minimizing greenhouse gas emissions. There are already methods to tackle the global plastic pollution such as incineration and pyrolytic reforming, but these methods invariably need large amounts of heat derived from fossil fuels. In other words, they mainly transform the plastics pollution problem into a different (and potentially even bigger) problem by contributing to global warming. We want to overcome these problems by using renewable energy to conduct the reaction and also collect the formic acid products to reuse the carbon,” he tells PackagingInsights.

In Singapore, most plastic waste is incinerated, producing greenhouse gases such as carbon dioxide, and the leftover mass – burn ash – is transported to the Semakau landfill, which is estimated to run out of space by 2035.

Energy from the sun
The vanadium-based catalyst, which is supported by organic groups and typically abbreviated as LV(O), uses light energy to drive a chemical reaction and is known as a photocatalyst. Photocatalysts enable chemical reactions to be powered by sunlight, unlike most reactions performed in industry that require heat, usually generated through the burning of fossil fuels.

Other advantages of the new photocatalyst are that it is low cost, abundant, and environmentally friendly, unlike common catalysts made from expensive or toxic metals such as platinum, palladium or ruthenium.

While scientists have tried other approaches for turning waste plastics into useful chemicals, many approaches involve undesirable reagents or too many steps to scale up. One example is an approach called photoreforming, where plastic is combined with water and sunlight to produce hydrogen gas, but this requires the use of catalysts containing cadmium, a toxic heavy metal. Other methods require plastics to be treated with harsh chemical solutions that are dangerous to handle.

Most plastics are non-biodegradable because they contain extraordinarily inert chemical bonds called carbon-carbon bonds, which are not readily broken down without the application of high temperatures.The research team has demonstrated a proof of concept and aims to commercialize the technology in five years.

The new vanadium-based photocatalyst developed by the NTU Singapore research team was specially designed to break these bonds and does so by latching onto a nearby chemical group known as an alcohol group and using energy absorbed from sunlight to unravel the molecule like a zipper.

As the experiments were conducted at laboratory scale, the plastic samples were first dissolved by heating to 85 degrees Celsius in a solvent, before the catalyst, which is in powder form, was dissolved. The solution was then exposed to artificial sunlight for a few days. Using this approach the team showed that their photocatalyst was able to break down the carbon-carbon bonds in over 30 different compounds and the results demonstrated the concept of an environmentally-friendly, low-cost photocatalyst.

Commercialization on the horizon
The research team is now pursuing improvements to the process that may allow the breakdown of plastics to produce other useful chemical fuels, such as hydrogen gas.

“We plan to create a fully renewable process by turning our process into a photoelectrochemical system that can produce hydrogen gas simultaneously. This way, the hydrogen gas can help to provide a fuel source for the reaction,” Soo explains.

“The biggest challenges that we face now are that our photocatalyst is dissolved together with the plastics and it operates best in an organic solvent. For the scale of the plastics problem, we need to overcome both these problems.”

The next steps for the project, as outlined by Soo, are:

1. Expand the scope of plastics that can be recycled.
2. Convert the system into a photoelectrochemical system so that renewable electricity can be used. This will be more versatile for places where electricity can be generated from sources such as hydroelectric power or geothermal energy.
3. Improve the catalytic activity and reusability.

“I would say that we are on the technological readiness scale of 1-3. We have demonstrated a proof of concept that we can use sunlight to convert the plastics but I am actively seeking funding and talent to tackle the challenges I mentioned above. If I am optimistic, I would say that we need at least five more years to commercialize this technology. We are happy to work with engineers to overcome the obstacles and we welcome collaborations and ideas,” Soo concludes.

By Joshua Poole