Scientists from the NTU School of Chemistry, Chemical Engineering and Biotechnology (CCEB) have found an efficient and effective plastic upcycling method that makes use of a commercially available catalyst and light to turn most non-biodegradable plastics into useful products at ambient temperature and pressure.
The problem with plastic waste
Plastic is ubiquitous in our daily lives; nearly everything we use is made from plastic or has plastic components within it. A large proportion of these are single-use plastics, such as packaging materials or disposable items. Consequently, the amount of plastic waste that we produce is staggering, with an estimated 15 million tonnes of plastic entering our oceans each year.
Unfortunately, we only recycle 9 percent of the plastic we produce globally, leaving the rest to be incinerated, buried in landfills, or dumped in the environment, according to a 2019 report by The Guardian.
Scientists have been trying to develop ways to recycle plastics chemically, using processes such as hydrogenolysis, where hydrogen gas is used to break chemical bonds, and pyrolysis, where materials are heated without the use of oxygen.
There has also been recent research into photochemically upcycling plastics. However, these processes often involve the use of high temperatures and expensive noble metals, and they generate unnecessary greenhouse gases.
The uphill task of recycling, and upcycling, plastics is compounded by the disparate types of plastics that are in circulation (Figure 1).
1. Polyethylene Terephthalate (PET), 2. High-Density Polyethylene (HDPE), 3. Polyvinyl Chloride (PVC or Vinyl), 4. Low-Density Polyethylene (LDPE), 5.Polypropylene (PP), 6.Polystyrene (PS), 7.Other
Most of the 91 percent of unrecycled plastic waste consists of non-biodegradable polyolefins that are commonly used in packaging materials due to their longer lifespans, such as polyethylene and polypropylene. Unfortunately, polyolefins are rarely recycled, whereas the main recycling process widely practiced now is the mechanical recycling of polyethylene terephthalate (PET).
A bright breakthrough
In their efforts to discover an improved photocatalytic reaction to deal with plastic waste, the team from NTU CCEB, led by Associate Professor Soo Han Sen, in collaboration with the NTU Nanyang Environment and Water Research Institute (NEWRI), the NTU School of Civil and Environmental Engineering (SCEE), and the Institute of High Performance Computing (IHPC) developed a novel process that uses a commercially sourced base metal photocatalyst and which can be carried out at ambient temperatures and pressure.
This reduces the amount of energy needed while also decreasing the unnecessary production of greenhouse gases. In their study, the team focused on polyolefins and polystyrene since these plastics made up most of the unrecycled plastics.
The team’s process involved the use of a unique tandem carbon-hydrogen (C-H) oxidation and carbon-carbon (C-C) cleavage reaction, facilitated by a single photocatalyst, vanadyl acetylacetonate, or V(O)(acac)2. This compound contains vanadium, a relatively affordable base metal.
In C-H oxidation, an oxygen atom is added to and replaces hydrogen in a carbon-hydrogen bond, forming a new carbon-oxygen (C-O) bond, while during a C-C cleavage, the covalent bond between two carbon atoms is broken, resulting in two separate products.
The resulting recovered products from the upcycling process include carboxylic acids and oxygenated oligomers. The carboxylic acids – formic, acetic, and benzoic – serve as platform chemicals, i.e., base materials to produce a sizable range of end-products with a myriad of applications. Benzoic acid alone has a global market worth of US$1.5 billion in 2022.
The acid can be further reduced to the organic compound methylcyclohexane, which has great potential to be a liquid organic hydrogen carrier, or LOHC. Methylcyclohexane can reversibly store and release hydrogen, allowing for the safe and efficient transportation and storage of hydrogen in liquid form.
Future “mining” of plastic waste
With a lower energy requirement for upcycling plastic waste, the use of a commercially available base metal photocatalyst, and the production of useful recovered products, the team’s efforts have established the fundamental concepts underlying a more sustainable and economically promising future alternative to fossil fuels: one that “mines” plastic waste.
Their work has the potential to help offset greenhouse gas emissions in the production of platform chemicals and aid in improving the circularity of the plastic life cycle.