Scientists invent a bright way to upcycle plastics into liquids that can store hydrogen energy

Sep 15, 2023 (Nanowerk News) Scientists from 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, all at room temperature. The new process is very energy-efficient and can be easily powered by renewable
energy in the future, unlike other heat-driven recycling processes like pyrolysis.
This 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. Globally, only nine per cent of plastics are recycled,
and plastic pollution is growing at an alarming rate. The biggest challenge of recycling these plastics 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.
Developed by Associate Professor Soo Han Sen, an expert in photocatalysis from
NTU’s School of Chemistry, Chemical Engineering, and Biotechnology, the new
method uses LEDs to activate and break down the inert carbon-carbon bonds in
plastics with the help of a commercially available vanadium catalyst. Published this week in the journal Chem (“Upcycling of non-biodegradable plastics by base metal photocatalysis”), the NTU method can upcycle a range of plastics, including PP, PE and PS. These plastics, together, account for over 75 per
cent of global plastic waste.

In developing a green solution to the plastic waste problem, the team wanted to ensure
that minimal extra carbon emissions are generated through the recycling of plastics,
which are long chains of molecules containing carbon atoms. Inventor Assoc Prof Soo said: “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.”

How the plastics are broken down

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. Unlike current and other emerging technologies to recycle plastics such as pyrolysis,
which uses a high-temperature process to melt and degrade the plastics into low-quality
fuels, or carbon nanotubes and hydrogen, the new LED-driven method requires
much less energy. Prof Soo adds that their method is unique in that it can use sunlight or LEDs powered
with electricity from renewable sources such as solar, wind or geothermal, to
completely process and upcycle such a wide range of plastics. This can allow for clean
and energy-efficient management of plastics in a circular economy and increase the
recycling rate of plastics. The process may also help Singapore to reduce the amount of plastic waste from
being incinerated or landfilled, helping the country to meet its Zero-Waste Masterplan, where it aims to increase the overall recycling rate to 70 per cent by 2030 and reduce
waste going to the Semakau landfill, estimated to run out of space by 2035.
Singapore generates around 1 million tonnes of plastic waste annually and only six
per cent of Singapore’s plastic waste is recycled. This study is part of a bigger project, entitled SPRUCE: Sustainable Plastics
RepUrposing for a Circular Economy, which also involves Professor Xin (Simba)
Chang, Associate Dean (Research) from the Nanyang Business School and
Associate Professor Md Saidul Islam from the School of Social Sciences. The interdisciplinary team estimates that if Singapore can upcycle 80 per cent of its
plastics, it could lead to at least a 2.1 million tonnes reduction in carbon dioxide
emissions – about four per cent of the nation’s total greenhouse gas emissions.
In addition, when plastics are upcycled into chemical feedstock, it reduces the need
by the chemical industry to combust fossil fuels to produce chemical feedstock, further
cutting down greenhouse gas emissions. Based on the estimations by Prof Chang and other team members, the economic
benefit of reducing carbon dioxide emissions is estimated to be S$41.40m per year
while the estimated cost savings from avoiding landfill use is about S$41.35 million
per year in Singapore. Plastic reuse and recycling are projected to generate a profitpool
growth of as much as US$60 billion for the chemical industry globally. Prof Chang, an expert in corporate finance, added, “Given that Singapore’s chemical
industry accounts for about one-third of the manufacturing output in 2015, the
integration of plastic upcycling technology into the industry has the potential to yield
considerable positive economic and environmental impact.” Sociology expert Assoc Prof Islam said: “This innovative approach — by transforming
plastic waste into valuable resources like formic acid — not only reduces the burden
of plastic pollution but also addresses the growing demand for sustainable chemicals.
This contributes to a cleaner environment, enhances public health, and creates new
employment opportunities, especially in research, development, and production
sectors, thereby fostering economic growth with a shift towards circular economies.”