Many of the materials commonly thrown in household recycling bins, including flexible films, multilayer materials and a lot of coloured plastics, cannot be recycled using conventional methods. And only about 9 per cent of plastic in the US is ever reused, often in low-value products.
But a team at the University of Wisconsin–Madison believes that its new technique can turn low-value waste plastic into high-value products while reducing greenhouse gas emissions by roughly 60 per cent.
The process relies on pyrolysis, a method whereby plastics are heated to high temperatures in an oxygen-free environment to create pyrolysis oil. This oil contains large amounts of olefins – a class of simple hydrocarbons that are central building blocks in today's chemicals and polymers, including various types of polyesters, surfactants, alcohols and carboxylic acids.
In current energy-intensive processes such as steam cracking, chemical manufacturers produce olefins by subjecting petroleum to extremely high heat and pressure. In the new process, olefins are recovered from pyrolysis oil and used in a much less energy-intensive chemical process called homogenous hydroformylation catalysis. This process converts olefins into aldehydes, which can then be further reduced into important industrial alcohols.
"These products can be used to make a wide range of materials that are higher value," said George Huber, who led the study.
These higher-value materials include ingredients used to make soaps and cleaners, as well as other useful polymers.
In recent years, at least 10 large chemical companies have built or announced plans for facilities to produce pyrolysis oils from waste plastics. Many of them run the pyrolysis oil through steam crackers to produce low-value compounds. The new technique could provide a more sustainable and lucrative way to use those oils.
"Currently, these companies don't have a really good approach to upgrade the pyrolysis oil," said postdoctoral researcher Houqian Li. "In this case, we can get high-value alcohols worth $1,200 to $6,000 (£940–£4700) per ton from waste plastics, which are only worth about $100 per ton. In addition, this process uses existing technology and techniques. It's relatively easy to scale up."
The next step for the team is to tune the process and better understand which recycled plastics and catalyst combinations produce which final chemical products.
"There are so many different products and so many routes we can pursue with this platform technology," Huber added. "There's a huge market for the products we're making. I think it really could change the plastic recycling industry."
Last year, E&T looked at whether companies were using chemical recycling processes to "greenwash" the worldwide plastic crisis, rather than more stringent attempts at reducing production of the material.
