We are increasingly conscious of the need to recycle waste products, but it is never quite so easy as rinsing and sorting your waste into the appropriate bins, especially when it comes to plastic.
Despite our best intentions, only around 16% of plastic is recycled into new products — and, worse, plastics tend to be recycled into low quality materials because transformation into high-value chemicals requires substantial amounts of energy, meaning the choices are either downcycling or prohibitively difficult. The majority of single-use plastics end up in landfills or abandoned in the environment.
This is a particular problem when it comes to polyolefins such as polyethylene (PE) and polypropylene (PP), which use cheap and readily available raw materials. Approximately 380 million tonnes of plastics are generated annually around the world and it is estimated that, by 2050, that figure will be 1.1 billion tonnes. Currently, 57% of this total are polyolefins.
Why are polyolefins an issue? The strong sp3 carbon–carbon bonds (essentially long, straight chains of carbon and hydrogen atoms) that make them useful as a material also make them particularly difficult to degrade and reuse without intensive, high energy procedures or strong chemicals. More than most plastics, downcycling or landfill disposal tend to be the main end-of-life options for polyolefins.
Polyethylene is used to make plastic bags and packaging.
Now, however, a team of scientists from MIT, led by Yuriy Román-Leshkov, believe they may have made a significant step towards solving this problem.
Previous research has demonstrated that noble metals, such as zirconium, platinum, and ruthenium can help split apart short, simple hydrocarbon chains as well as more complicated, but plant-based lignin molecules, in processes with much lower temperatures and energy.
So the team looked at using the same approach for the long hydrocarbon chains in polyolefins, aiming to disintegrate the plastics into usable chemicals and natural gas. It worked.
First, they used ruthenium-carbon nanoparticles to convert more than 90% of the hydrocarbons into shorter compounds at 200 Celsius (previously, temperatures of 430–760 Celsius were required).
Next, they tested their new method on commercially available, more complex polyolefins without pre-treatment (an energy intensive requirement). Not only were the samples completely broken down into gaseous and liquid products, the end product could be selected by tuning the reaction, yielding either natural gas or a combination of natural gas and liquid alkanes (both highly desirable) as preferred.
Polypropylene is used in bottle caps, houseware, and other packaging and consumer products.
The researchers believe that an industrial scale use of their method could eventually help reduce the volume of post-consumer waste in landfills by recycling plastics to desirable, highly valuable alkanes — but, of course, it's not that simple. The team says that more research into the effects of moisture and contaminants in the process is required, as well as product removal strategies to decrease the formation of light alkanes which will be critical for the industrialisation of this reaction.
However, they believe the path they're on could lead to affordable upcycling technology that would better integrate polyolefins into the global economy and incentivise the removal of waste plastics from landfill and the environment.
More about the study can be read here: