November 29, 2020

They develop an efficient method to recycle plastic waste



Researchers have developed an efficient and energy-efficient method to recycle plastic waste made of polyethylene, a polymer found in approximately one-third of all plastics produced, with a global value of around $ 200 billion a year (about 168,000 million euros), in valuable molecules that can be reused for later use, as published in the journal ‘Science’.

Susannah Scott and Mahdi Abu-Omar of the University of California, Santa Barbara, have developed a one-pot, low-temperature catalytic method that recycles polyethylene into high-value alkylaromatic molecules that are the basis for many industrial and chemical chemicals. consumption.

Adding value to what would otherwise become garbage could make recycling plastic waste a more attractive and practical activity with a beneficial result for the environment.

“Here is a potential solution”, Scott advances who explains that this work is one in a growing list of possible measures that can be taken to turn the linear and wasteful economy of plastic into a more sustainable and circular one. “It is a demonstration of what can be done,” he adds.

“There are many positive things about plastics that we need to consider,” acknowledges Scott, a professor of chemistry and chemical engineering at UC Santa Barbara, who holds the UCSB Mellichamp Chair in Sustainable Catalytic Processing. At the same time, we realize that that there is a really serious end-of-life problem that is an unintended consequence. ”

The property that makes plastics so useful is also what makes them so persistent, the researchers explain. It is their chemical inertness: they generally do not react to other components in their environment. Plastic pipes won’t rust or leak into the water supply, plastic bottles can store caustic chemicals, and plastic liners can withstand 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 the exact same pipe and it keeps the water completely safe,” Scott recalls. But this quality of inertia is what makes plastics decompose very slowly naturally and require a lot of energy to do so artificially.

“They are made with carbon-carbon and carbon-hydrogen bonds, and they are very difficult to recycle chemically,” explains professor of chemical engineering Abu-Omar, who specializes in energy catalysis and holds the UCSB Mellichamp chair in Green Chemistry.

Although a lot of research effort has gone into learning how to reduce plastics to their basic components for sustainability purposes, the cost of energy “has plagued the field for a long time,” say researchers.

“On the other hand, if we could directly convert polymers into these higher-value molecules and completely eliminate the high-energy step of going back to these building block molecules, then we will have a high-value process with a low energy footprint.” Scott points out.

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 does so efficiently, inexpensively, and with a low energy requirement.

“We lowered the temperature of the transformation by hundreds of degrees,” says Scott. Conventional methods, according to the document, require temperatures between 500 and 1000 ° C to break the polyolefin chains into small pieces and reassemble them in a product mixture of gas, liquid and coke, while the optimum temperature for this catalytic process is around in the neighborhood of 300 ° C.

The relatively mild reaction condition helps to break down polymers more selectively into a majority of larger molecules within a lubricant range, the researchers explained. “And we simplify the number of steps in the process because we are not doing multiple transformations,” he adds.

Also, the process requires no solvent or added hydrogen, just a platinum on alumina (Pt / Al2O3) catalyst for a tandem reaction that breaks those tough carbon-carbon bonds and rearranges the polymer’s molecular “backbone” 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.

“It is difficult to form aromatic molecules from small hydrocarbons,” adds the lead author of the article, Fan Zhang. “Here, during the formation of aromatics from polyolefins, hydrogen is formed as a by-product and is used to cut the hydrocarbons. polymer chains to make the whole process favorable. As a result, we get long chain alkyl aromatics, and that’s the fascinating result.

This method represents a new direction in the life cycle of plastics, one in which waste polymers could become valuable raw materials rather than end up in landfills, or worse, in waterways and other sensitive habitats.

“This is an example of having a second use, in which we could manufacture these raw materials more efficiently and with a better environmental impact than making it from oil,” adds Abu-Omar.

However, research remains to be done to see where and how this technology would be most effective, but it is a strategy that could help mitigate the build-up of plastic waste, regain its value, and perhaps reduce our dependence on the oil that plastics come from.

.



Source link