New technique turns common plastic waste into fuel

Polyethylene, the plastic and its derivatives used in making the majority of the world's disposable beverage containers, is produced at a staggering volume of over 100 million metric tons each year, most of which ends up in landfills. Now scientists from the University of California, Irvine (UCI), and the Shanghai Institute of Organic Chemistry (SIOC) in China have found a way to turn this waste into a usable liquid fuel.

"Synthetic plastics are a fundamental part of modern life, but our use of them in large volume has created serious environmental problems," says 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."

High-density polyethylene (HDPE), low-density PE (LDPE) and linear low-density PE (LLDPE), are the types of polyethylene used in beverage and food containers because their chemical composition makes them inert and, therefore, not likely to degrade or interact with any of the chemicals normally found in things we're going to eat. Unfortunately, this also means that because they don't react to most chemical compounds, they are not readily or easily broken down by substances generally applied to other recyclable materials.

This fact has often led to various alternative methods of recycling, or upcycling plastic bottles into everything from plastic pellets to make snack bags, plastic string to be woven into other products, and entire villages made of plastic bottle houses.

Unfortunately, the standard industrial method to recycle such plastic waste usually involves less "friendly" forms of disposal. In fact, standard methods probably cause more harm than good as they employ caustic chemicals or require heating the materials to more than 700° F (370° C) to fracture the polymer's chemical bonds, and produce harmful by-products such as hydrocarbon gas, oil, wax, and char in uncontrolled amounts in the process.

To help create a better industrial-scale method of recycling and chemical recovery, researchers from a joint US-China collaboration project at UIC and SIOC have come up with a new method to break down polyethylene that produces fewer toxic by-products and more useful compounds.

Using by-products of petrochemical production known as alkanes (saturated hydrocarbons where the hydrogen and carbon atoms are arranged in a branched formation in which all of the carbon to carbon connections are single bonds), the scientists were able to separate and reconstitute the polymer molecules into other useful compounds.

The process is what is know as cross-alkane metathesis, which is essentially a double-dissolution chemical reaction where two parts of two substances form two new substances. In this case, the technique selectively degrades a range of PE plastics into fuels and waxes under milder conditions and in a more controlled manner than in current industrial processes of plastic breakdown and recovery. In fact, according to the researchers, the new method can completely degrade PE compounds into usable oils and waxes in only one day at just 347° F (175° C).

Future work will see the researchers addressing efficiencies, including increasing catalyst activity and lifetime, reducing running costs, and evolving the processes to help turn other types of plastic waste into usable products.

The results of this research were recently published in the journal Science Advances.

Source: UC Irvine