Plastics upcycled into liquids that can store hydrogen energy

Scientists at Nanyang Technological University, Singapore (NTU Singapore) have created a process that can upcycle most plastics into chemical ingredients useful for energy storage, using light-emitting diodes (LEDs) and a commercially available catalyst — and all at room temperature. Described in the journal Chem, the process is very energy-efficient and could be easily powered by renewable energy in the future, unlike other heat-driven recycling processes like pyrolysis.

The NTU innovation overcomes the current challenges in recycling plastics such as polypropylene (PP), polyethylene (PE) and polystyrene (PS), which are typically incinerated or discarded in landfills. The biggest challenge of recycling these plastics — which together account for over 75% of global plastic waste — is their inert carbon–carbon bonds, which are very stable and thus require a significant amount of energy to break. This bond is also the reason why these plastics are resistant to many chemicals and have relatively high melting points.

Currently, the only commercial way to recycle such plastics is through pyrolysis, which has high energy costs and generates large amounts of greenhouse emissions, making it cost-prohibitive given the lower value product of the resulting pyrolysis oil. By contrast, the NTU method uses LEDs to activate and break down the inert carbon–carbon bonds in plastics with the help of a commercially available vanadium catalyst.

First, the plastics are dissolved or dispersed in the organic solvent known as dichloromethane, which is used to disperse the polymer chains so that they will be more accessible to the photocatalyst. The solution is then mixed with the catalyst and flowed through a series of transparent tubes where the LED light is shone on it.

The light provides the initial energy to break the carbon–carbon bonds in a two-step process with the help of the vanadium catalyst. The carbon–hydrogen bonds in the plastics are oxidised — making the bonds less stable and more reactive — after which the carbon–carbon bonds are broken down.

After separation from the solution, the resulting end products are chemical ingredients such as formic acid and benzoic acid, which can be used to make other chemicals employed in fuel cells and liquid organic hydrogen carriers (LOHCs). LOHCs are now being explored by the energy sector as they play critical roles in clean energy development, given their ability to store and transport hydrogen gas more safely.

“Our breakthrough not only provides a potential answer to the growing plastic waste problem, but it also reuses the carbon trapped in these plastics instead of releasing it into the atmosphere as greenhouse gases through incineration,” said Associate Professor Soo Han Sen, who invented the new method. He added that the method can use sunlight or LEDs powered with electricity from renewable sources such as solar, wind or geothermal, allowing for clean and energy-efficient management of plastics in a circular economy.

The NTU team has filed a patent for their photocatalytic process, which has been designed with industrial scalability in mind, through the university’s innovation and enterprise company, NTUitive. The team is now seeking partners to further commercialise the technology, which may contribute towards helping Singapore achieve its 2050 Net Zero Emissions target.

Image caption: Associate Professor Soo Han Sen looking at a bottle of solution with dissolved plastic and vanadium catalyst. Image credit: NTU Singapore.