CHEMISTS in the US and China have developed a new method to convert waste polyethylene (PE) into liquid vehicle fuels and waxes.
More than 100m t of PE is produced every year, more than any other plastic, and used to make various largely disposable items including plastic bags, films, bottles and packaging. It accounts for up to 60% of the plastic found in municipal solid waste, but it is relatively inert and difficult to degrade. The researchers at the University of California, Irvine (UCI) and the Shanghai Institute of Organic Chemistry (SIOC), led by UCI’s Zhibin Guan, sought a way to address the problem of plastic pollution and create a new source of fuel.
The most common approach to PE recycling involves pyrolysis reactions, which take place at more than 700?C, or the use of caustic chemicals, to break down the strong C-C and C-H bonds in PE. Guan and the team instead used a process called cross-alkane metathesis. This uses relatively mild reaction conditions and widely available, cheap petroleum ethers.
The waste PE is mixed with petroleum ethers (or another short-chain alkane), with the catalysts in a sealed vessel and heated to 175?C. The team used a “pincer”-ligated iridium complex as the initial catalyst, which dehydrogenates the alkanes, forming a mixture of alkenes and an Ir-H2
complex. The second catalyst, based on rhenium, breaks down the chains. The shorter chains are then rehydrogenated by the IrH2
complex, producing saturated alkanes, a mixture of diesel fractions, and waxes, which can be easily separated. All the plastic was completely degraded within one day.
The precise nature of the products can be affected be several factors, including the exact structure of the catalysts, the form of plastic waste (waste bottles produced 64% oils and 36% waxes, while waste food films produced 72% oils and 28% waxes), and the time of the reaction. A longer reaction time results in more of the oils being broken down.
“To the best of our knowledge, the degradation of real-world postconsumer PEs under such mild reaction conditions is unprecendented,” say the researchers. “Featuring high efficiency, mild reaction conditions, and fine control of degradation products, this method shows distinct advantages over traditional pyrolysis processes.”
The team is now working on developing longer-lasting catalysts and developing catalytic systems for other polymers. Science Advances