Closing the plastic loop with a one-pot recycling process

Closing the plastic loop with a one-pot recycling process
Closing the plastic loop with a one-pot recycling process

Photo credit: Hans Braxmeier from Pixabay

When we first started using plastics about 70 years ago, not much thought was given to the effects of their lifespan and the fact that they can take centuries to decompose. As plastics have diversified and become easier to make, the planet now spans about 8.3 billion tons of the material – almost every piece of plastic ever made – without enough technology or incentives to shrink that growing pile. Plastic is cheaper and easier to make and throw away than it is to recycle.

UC Santa Barbara researchers Susannah Scott and Mahdi Abu-Omar are poised to change this decade-old paradigm. How? With a one-pot, low-temperature catalysis, in which polyethylene – a polymer that is contained in around a third of all plastics manufactured with a global value of around US $ 200 billion per year – is processed into high-quality alkyl aromatic molecules on which it is based many industrial chemicals and consumer goods. Adding value to what would otherwise become garbage could make the recycling of plastic waste more attractive and practical and lead to an environmentally friendly result.

“Here’s one possible solution,” said Scott, who and her colleagues have published their research in Science. Her efforts, she said, are among a growing list of possible actions that can be taken to transform the linear, wasteful economy of plastic into a more sustainable, circular one.

“This is a demonstration of what can be done,” she said.

A second life for plastic waste

There’s no denying that modern existence owes a lot to plastics, from the packaging that keeps food fresh, to the sterile materials used for medical applications, to the cheap, lightweight parts found in many of our affordable ones , durable goods are included.

“There are many positive things about plastics that we need to keep track of,” said Scott, 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 find that there is this really serious end-of-life problem that is an unintended consequence.”

The property that makes plastics so useful also makes them so durable, the researchers explained. It is their chemical inertness – they generally do not react to other components of their environment. Plastic pipes rust or do not get into the water supply, plastic bottles can store corrosive chemicals, plastic coatings can withstand high temperatures.

“You can stick 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 protects your water perfectly,” said Scott.

However, this quality of inertness also means that plastics are broken down naturally very slowly and artificially in a very energy-intensive manner.

“They are made up of carbon-carbon and carbon-hydrogen bonds and are very difficult to recycle chemically,” said Abu-Omar, a professor of chemical engineering who specializes in energy catalysis and holds the UCSB Mellichamp Chair in Green Chemistry. Although a lot of research has gone into learning how plastics can be reduced to their basic components for sustainability reasons, energy costs “have plagued the field for a long time,” the researchers said. Even the benefit of converting these building blocks into high quality molecules is limited when it is cheaper to do the same from extracted petroleum.

“On the other hand, if we could convert the polymers directly into these higher-quality molecules and completely eliminate the high-energy step of returning to these building block molecules, we would have a high-quality, low-energy process,” said Scott.

This innovative way of thinking led to a new catalytic tandem method that not only generates high quality alkyl aromatic molecules directly from plastic waste from polyethylene, but also does so efficiently, inexpensively and with low energy consumption.

“We lowered the temperature of the transformation hundreds of degrees,” said Scott. According to the paper, conventional processes require temperatures between 500 and 1000 ° C to break the polyolefin chains into small pieces and reassemble them into a mixed product of gas, liquid and coke, while the optimal temperature for this catalytic process is in the vicinity of 300 ° C. The relatively mild reaction conditions help break down polymers more selectively into most of the larger molecules within a lubricant range, the researchers said. “And we’ve simplified the number of steps in the process because we’re not doing multiple transformations,” said Scott.

In addition, the process does not require any solvent or added hydrogen, just a platinum-on-aluminum oxide (Pt / Al2O3) catalyst for a tandem reaction that both breaks these hard carbon-carbon bonds and rearranges the molecular “backbone” of the polymer to form characteristic six-sided rings with these structures – high quality alkyl aromatic molecules that are widely used in solvents, paints, lubricants, detergents, pharmaceuticals and many other industrial and consumer products.

“The formation of aromatic molecules from small hydrocarbons is difficult,” added the paper’s lead author, Fan Zhang. “Here, when aromatics are formed from polyolefins, hydrogen is formed as a by-product and is used to cut the polymer chains in order to make the entire process more economical. As a result, we get long chain alkyl aromatics, and that is the fascinating result. ”

This method represents a new direction in the plastics lifecycle where waste polymers can become valuable raw materials instead of ending up in landfills or worse, in waterways and other sensitive habitats.

“This is an example of a second use where we could produce these raw materials more efficiently and in a more environmentally friendly way than from petroleum,” said Abu-Omar. Research still needs to be done on where and how this technology is most effective. However, this strategy could help reduce the accumulation of plastic waste, regain its value and potentially reduce our reliance on the petroleum from which plastics are derived.

“We dig a hole in the ground, we produce, we make, we use, we throw away,” said Abu-Omar. “In a way, this really breaks that mindset. There are interesting scientific discoveries here that lead us to new discoveries, new paradigms and new methods of chemistry. ”

Reference: F. Zhang et al., Upcycling of polyethylene to long-chain alkyl aromatics by tandem hydrogenolysis / aromatization, Science (2020). DOI: 10.1126 / science.abc5441

Press release from UC Santa Barbara

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