A vial containing a solution of polyethylene, a common plastic, and a novel catalyst that can break down the plastic. Photo credit: Lester Kok.
Researchers at Nanyang Technological University, Singapore (NTU Singapore) have invented an environmentally friendly way to transform non-biodegradable plastic waste into useful chemicals by harnessing sunlight.
A team led by Nanyang Assistant Professor Han Sen Soo from the School of Physical and Mathematical Sciences has developed a catalyst (a substance that enables and speeds up chemical reactions) that does not contain toxic heavy metals and can break down most common plastics. The broken-down plastic is turned into formic acid, a chemical used in fuel cells. The advance was reported in the journal Advanced Science in October 2019.
Plastics are used virtually everywhere in the modern world, including in consumer electronics, clothing, and engineering materials. Non-biodegradable plastic waste is often indiscriminately discarded, and ends up polluting marine ecosystems. In Singapore, most plastic waste is incinerated, which produces greenhouse gases such as carbon dioxide.
Scientists have attempted to find ways to convert waste plastics into useful chemicals, but none have proven widely successful thus far. In one approach, known as photoreforming, plastic is combined with water and sunlight to produce hydrogen gas, but this requires catalysts containing cadmium, a toxic heavy metal. Other methods require the plastic waste to be treated with caustic alkaline solutions that are dangerous to handle.
The catalyst developed by the NTU team is a type of photocatalyst, meaning that it uses light energy to drive a chemical reaction. Photocatalysts allow for eco-friendly chemical reactions powered by sunlight, whereas most reactions performed in industry require heat, usually provided by burning fossil fuels. Another advantage of the new photocatalyst is that, unlike common catalysts, it does not contain expensive or toxic metals such as platinum or ruthenium, only the inexpensive base metal vanadium.
Three of the researchers performing the photocatalysis experiment. From left: Dr. K. F. Chin, Asst. Prof. H. S. Soo, and Dr. M. Đokić.
Photo credit: Lester Kok.
Many plastics are non-biodegradable because they contain extraordinarily inert chemical bonds called carbon-carbon bonds, which cannot be broken by ordinary chemical treatments. The new vanadium-based photocatalyst is 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.
The NTU team reported that the photocatalyst breaks down carbon-carbon bonds in over 30 different types of chemicals tested. Most promisingly, it can quickly break down polyethylene, the most commonly-used plastic in the world and a major contributor to plastic pollution in the ocean.
In a typical laboratory test, the plastic sample is first dissolved by heating to 85 degrees Celsius in a solvent. The photocatalyst, in powder form, is then added to the solution, and the sample is exposed to artificial sunlight for a few days. Following this procedure, the team found that photocatalyst completely breaks down the polyethylene over six days. By contrast, polyethylene in the natural environment takes an estimated 500—1000 years to decompose when exposed to sunlight.
The broken-down polyethylene is turned into formic acid, an industrially useful chemical that can be used in fuel cells or converted into other chemicals.
“The overarching goal of this research is to develop sustainable and cost-effective methods to harness sunlight to manufacture fuels and other chemical products,” said Prof. Soo. “This new chemical treatment is the first that can completely break down a non-biodegradable plastic using visible light and a catalyst that does not contain heavy metals.”
The research was supported in part by NTU’s Artificial Photosynthesis Laboratory, and was funded by an A*STAR Advanced Manufacturing and Engineering (AME) Individual Research Grant, which aims to convert fundamental scientific ideas into useful applications.
Currently, the research team is pursuing improvements to the process that will allow the breakdown of plastics to produce other useful chemical fuels, such as hydrogen gas.
S. Gazi, M. Đokić, K. F. Chin, P. R. Ng, and H. S. Soo, Visible Light Driven Cascade Carbon-Carbon Bond Scission for Organic Transformations and Plastics Recycling, Advanced Science (2019)