An efficient, low-energy method can upcycle polyethylene plastic waste into valuable molecules, report researchers.
When we started using plastics about 70 years ago, not much thought—if any—went to the implications of their lifespan and the fact that they can take centuries to decompose. Consequently, as plastics have diversified and become easier to manufacture, the planet is now stuck with some 8.3 billion tons of the stuff—almost every bit of plastic ever produced—without enough technology or incentives to shrink that growing pile. Plastic is cheaper and easier to produce and throw away than it is to recycle.
Researchers Susannah Scott and Mahdi Abu-Omar are poised to shift this decades-old paradigm. How? With a one-pot, low-temperature catalytic method that upcycles polyethylene—a polymer found in about a third of all plastics produced, with a global value of about $200 billion annually—into high-value alkylaromatic molecules that are the basis of many industrial chemicals and consumer products.
Adding value to what would otherwise become trash could make plastic waste recycling a more attractive and practical pursuit with an environmentally beneficial outcome.
“Here’s a potential solution,” says Scott, who reports the work with colleagues in the journal Science. Their effort, she says, is one in a growing list of possible measures that can be taken to turn plastic’s linear, wasteful economy into a more sustainable, circular one.
“This is a demonstration of what can be done,” she says.
Perks and problems
There’s no denying that modern existence owes a lot to plastics, from the packaging that keeps foods fresh, to the sterile materials used in medical applications, to the cheap, lightweight parts that go into many of our affordable, durable goods.
“There are many positive things about plastics that we have to keep in view,” says Scott, a professor of chemistry and of chemical engineering at the University of California, Santa Barbara. “At the same time, we realize that there is this really serious end-of-life issue which is an unintended consequence.”
The property that makes plastics so useful is also what makes them so persistent, the researchers explain. It’s their chemical inertness—they generally don’t react to other components of their environment. Plastic pipes don’t rust or leach into the water supply, plastic bottles can store caustic chemicals, plastic coatings can resist high temperatures.
“You can put one of these pipes in the ground and a hundred years later you can dig it up and it’s exactly the same pipe and it keeps your water completely safe,” Scott says.
But this quality of inertness also makes plastics very slow to break down naturally and very energy intensive to do so artificially.
“They’re made with carbon-carbon, and carbon-hydrogen bonds, and they’re very difficult to chemically recycle,” explains fellow chemistry and chemical engineering professor Abu-Omar, who specializes in energy catalysis. Though much research effort has gone into learning how to reduce plastics to their basic components for sustainability purposes, the energy cost “has plagued the field for a long time,” the researchers say. Even the benefit of converting these building blocks into high-value molecules is limited when it’s cheaper to do the same from extracted petroleum.
“On the other hand, if we could directly convert the polymers to these higher-value molecules and completely cut out the high-energy step of going back to these building block molecules, then we have a high-value process with a low energy footprint,” Scott says.
A cool way to make valuable molecules
That innovative line of thinking produced a new tandem catalytic method that not only creates high-value alkylaromatic molecules directly from waste polyethylene plastic, but also does so efficiently, at low cost, and with a low energy requirement.
“We brought the temperature of the transformation down by hundreds of degrees,” Scott says. Conventional methods, according to the paper, require temperatures between 500 and 1,000°C (932 to 1832°F) to break down the polyolefin chains into small pieces and reassemble them into a mixture product of gas, liquid, and coke, while the optimal temperature for this catalytic process hovers in the neighborhood of 300°C (572°F). The relatively mild reaction condition helps break down polymers in a more selective way to a majority of larger molecules within a lubricant range, the researchers explain. “And, we simplified the number of steps in the process because we’re not doing multiple transformations,” Scott says.
In addition, the process requires no solvent or added hydrogen, just a platinum on alumina (Pt/Al2O3) catalyst for a tandem reaction that both breaks those tough carbon-carbon bonds, and rearranges the polymer’s molecular “skeleton” to form structures with those characteristic six-sided rings—high-value alkylaromatic molecules that find widespread use in solvents, paints, lubricants, detergents, pharmaceuticals, and many other industrial and consumer products.
“Forming aromatic molecules from small hydrocarbons is difficult,” adds the paper’s lead author, postdoctoral researcher Fan Zhang. “Here, during aromatics formation from polyolefins, hydrogen is formed as a byproduct and further used to cut the polymer chains to make the whole process favorable. As a result, we get long-chain alkylaromatics, and that’s the fascinating outcome.”
This method represents a new direction in the lifecycle of plastics, one in which waste polymers could become valuable raw materials instead of winding up in landfills, or worse, in waterways and other sensitive habitats.
“This is an example of having a second use, where we could make these raw materials more efficiently and with better environmental impact than making them from petroleum,” Abu-Omar says. Research must still be conducted to see where and how this technology would be most effective, but it’s one strategy that could help mitigate the accumulation of plastic waste, recoup their value, and perhaps reduce our dependency on the petroleum that plastics come from.
“We dig a hole in the ground, we produce, we make, we use, we throw away,” Abu-Omar says. “So in a way, this is really breaking that way of thinking. There’s interesting science to be done here that will lead us into new discoveries, new paradigms, and new ways of doing chemistry.”
Coauthors are from UC Santa Barbara, the University of Illinois at Urbana-Champaign, and Cornell University.
Source: UC Santa Barbara