Approach tackles most commonly used synthetic plastic
A new way of recycling millions of tons of plastic garbage into liquid fuel has been devised by researchers from the University of California, Irvine and the Shanghai Institute of Organic Chemistry (SIOC) in China.
The issue of plastic pollution
“Synthetic plastics are a fundamental part of modern life, but our use of them in large volume has created serious environmental problems,” said UCI chemist Zhibin Guan. “Our goal through this research was to address the issue of plastic pollution as well as achieving a beneficial outcome of creating a new source of liquid fuel.”
Guan and Zheng Huang, his collaborator at SIOC, together with their colleagues have figured out how to break down the strong bonds of polyethylene, the most common commercially available form of plastic. Their innovative technique centers on the use of alkanes, specific types of hydrocarbon molecules, to scramble and separate polymer molecules into other useful compounds. The team’s findings were published recently in Science Advances.
Scientists have been seeking to recycle plastic bags, bottles and other trash generated by humans with less toxic or energy intensive methods. Current approaches include using caustic chemicals known as radicals or heating the material to more than 700 degrees Fahrenheit to break down the chemical bonds of the polymers.
In this newly discovered technique, the team degrades plastics in a milder and more efficient manner through a process known as cross-alkane metathesis. The substances needed for the new method are byproducts of oil refining, so they’re readily available.
Guan said the US-China joint team is still working on a few issues to make it more efficient. That includes increasing the catalyst activity and lifetime, decreasing the cost, and developing catalytic processes to turn other plastic trash into treasure.
Alkane metathesis is a process in which alkanes are covalently rearranged to give a new distribution of alkane products . Because PE is essentially composed of ultra-long alkanes, we envisioned that alkane metathesis could be used to cleave PE chains. Here, we report a mild and efficient degradation of PE into liquid fuels and waxes using inexpensive and readily available light alkanes, such as petroleum ether, as the reagents. These alkanes with low carbon number are major constituents of a variety of refinery and petrochemical streams, the Fischer-Tropsch process , and some biomass conversion pathways We show that the cross metathesis between these low-value light alkanes and PE results in very efficient degradation of various grades of PE. In particular, we demonstrate that the wastes of commercial PE plastics can be selectively degraded into transportation fuels and waxes under mild conditions in a controllable manner.
Our strategy for PE degradation is based on a tandem catalytic cross alkane metathesis (CAM) process developed by Goldman et al. and Huang et al., which involves one catalyst for alkane dehydrogenation and another catalyst for olefin metathesis (Fig. 1A). First, the dehydrogenation catalyst Ir removes hydrogen from both PE and a light alkane in a sealed system to form unsaturated species and Ir-H2. Next, the olefin metathesis catalyst scrambles the alkenes, resulting in breakdown of PE chains. Finally, hydrogenation of the newly formed alkenes with Ir-H2 affords saturated alkanes. The metathesis of PE with the light alkane reduces the chain length of PE when an internal double bond of PE scrambles with a double bond of the light alkene. In the presence of a large excess of light alkanes, the initial CAM products should react further with the light alkane to deliver the secondary CAM products with even shorter chain length. After multiple cycles of CAM with light alkanes, PE will be eventually converted to short hydrocarbons suitable for transportation oils. A unique advantage of this process is that the excess light alkane used for the degradation dissolves PE to form a dilute solution with low viscosity, avoiding mass and heat transfer issues encountered in the conventional catalytic pyrolysis processes involving PE melts
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