Catalysts for Plastic Recycling
Chemical process can recycle PET bottles at lower temperatures.
A plastic bottle tossed in the recycling bin may end up being shredded and reused to make a sweater or a carpet, but it won’t be turned into another water bottle. At least not so far. Catalysts being developed by researchers at IBM and Stanford could make it cost-effective to break down polyethylene terephthalate, or PET, plastics into their constituent chemicals for reuse as bottles. The company is working to test its PET-recycling catalyst at a large scale to eventually develop it for industrial use.
Most plastic drink bottles are made from PET, as is anything with the “1” recycling stamp. Typically, the plastic is washed, mechanically ground, and mixed with “virgin” PET to make a polymer that’s not suited for packaging but can be used to make secondary products, including clothes and carpeting.
Mechanical, rather than chemical, recycling is used for PET because it’s too expensive to break the polymer down into its chemical parts, says Dennis Sabourin, executive director of the National Association for PET Container Resources, a trade organization. There are two existing methods for accomplishing the chemical reaction, says Sabourin, but they are “very energy-intensive and have been abandoned because of the cost.” Even with the use of existing catalysts to help the recycling reactions along, these processes must be done at high temperatures and under great pressure, and take a long time. If the new catalysts have “even a modicum of success, it would be big news,” Sabourin says.
The IBM and Stanford researchers, who described their work today in the journal Macromolecules, have developed several new catalysts, one of which can be used to chemically recycle PET in a short time at 75 ºC. PET is made from two feedstocks, one of them an organic acid, the other ethylene glycol, which is relatively inexpensive. The catalyst works in an ethylene glycol solution. When cut up water bottles are placed in the solution, the catalyst causes the organic acid in the plastic to react with the ethylene glycol in solution to make PET that is of the same quality used to make the bottle initially.
The new catalysts are a result of a decade-long research project at IBM to develop better ways to make the polymers used as an insulating layer in computer chips. These layers are traditionally prepared using catalysts that contain metal. Metallic catalysts are highly active, but they’re difficult to remove once the reaction is done, leaving small impurities that can nonetheless interfere with a chip’s performance. These metal impurities can also leach out, becoming an environmental pollutant when the chip is trashed at the end of its lifetime. “To remove that catalyst is cost-prohibitive, so we started looking for a new way to make polymers,” says James Hedrick, lead scientist at IBM’s Almaden Research Center in San Jose, CA.
The IBM researchers turned to organic catalysts, whose design is similar to biological enzymes.
“Organic catalysts have traditionally been heavily criticized because they have low activity and large amounts of them are required” to help a reaction along, says Hedrick. “We found families of catalysts that are as active as metallic catalysts,” some of which are good for working with insulating materials for chips, and some of which can work with PET. The IBM researchers collaborated with a computational chemistry group within the company to model the activity of the catalysts, and have demonstrated them experimentally in the lab.
The PET-recycling catalyst, a type of molecule called a carbene, was inspired by vitamin B1, says Stanford chemistry professor Robert Waymouth. The Stanford and IBM researchers guessed that a similar organic small molecule might be good at catalyzing reactions that string esters together to make long polymers.
The IBM researchers will now collaborate with researchers at the King Abdulaziz City for Science and Technology in Saudi Arabia to test the chemical recycling of PET on a larger scale. “We need to see if what we discovered on the lab bench can work in big reactors,” says Waymouth. In initial tests, they will focus on breaking down the polymer into its constituents. However, the company has also had good results using its organic catalysts to depolymerize PET to make specialized materials such as feedstocks for high-strength plastics that are more valuable but are expensive to make using other pathways. “You start with trash, and build it back up into higher value materials,” says Robert Allen, senior manager of advanced materials chemistry at IBM Almaden.