The catalytic process vaporizes plastic bags and bottles, producing gases to make new recycled plastics

A new chemical process can essentially vaporize the plastics that dominate today’s waste stream and turn them into hydrocarbon building blocks for new plastics.

The catalytic process, developed at the University of California, Berkeley, works equally well with the two main types of post-consumer plastic waste: polyethylene, a component of most single-use plastic bags, and polypropylene, a component of hard plastics from microwaveable dishes to luggage. It also effectively degrades a mixture of these types of plastics.

This process, if scaled up, could help create a circular economy for many disposable plastics, with plastic waste being converted back into monomers used to make polymers, reducing the use of fossil fuels to make new plastics. Clear plastic water bottles made from polyethylene tetraphthalate (PET), a polyester, were designed in the 1980s to be recycled this way. But the volume of polyester plastics is tiny compared to that of polyethylene and polypropylene plastics, known as polyolefins.

“We have a huge amount of polyethylene and polypropylene in everyday objects, from lunch bags to laundry detergent bottles to milk jugs. A lot of what’s around us is made of these polyolefins,” said John Hartwig, a UC Berkeley chemistry professor who led the research.

“In principle, we can now take these objects and return them to the starting monomer through chemical reactions we have designed that break the typically stable carbon-carbon bonds. In doing so, we are closer than anyone else to giving polyethylene and polypropylene the same kind of circularity that we find in the polyesters in water bottles.”

Hartwig, graduate student Richard J. “RJ” Conk, chemical engineer Alexis Bell, who is a professor in the graduate school at the University of California, Berkeley, and their colleagues will publish details of the catalytic process in the journal Science.

A circular economy for plastics

Polyethylene and polypropylene plastics make up about two-thirds of the world’s post-consumer plastic waste. About 80% ends up in landfills, is incinerated or simply dumped on the streets, often ending up as microplastics in waterways and the ocean. The rest is recycled into low-value plastic, made into decking materials, flower pots and cutlery.

To reduce this waste, researchers have looked for ways to turn plastics into something more valuable, such as monomers that are polymerized to produce new plastics. This would create a circular polymer economy for plastics, reducing the need to make new plastics from petroleum, which generates greenhouse gases.

Two years ago, Hartwig and his team at the University of California, Berkeley, developed a process to break down polyethylene plastic bags into the monomer propylene, also called propene, which could then be reused to make polypropylene plastics.

This chemical process uses three different custom-made heavy metal catalysts: one to add a carbon-carbon double bond to the polyethylene polymer and the other two to break the chain at this double bond and repeatedly cut a carbon atom and, together with ethylene, make propylene (C₃H₆) until the polymer disappeared. But the catalysts were dissolved in the liquid reaction and their lifetime was short, making it difficult to recover them in an active form.

In the new process, expensive soluble metal catalysts were replaced by cheaper solid catalysts, commonly used in the chemical industry for continuous flow processes that reuse the catalyst. Continuous flow processes can be scaled up to process large volumes of materials.

Conk first experimented with these catalysts after consulting with Bell, an expert in heterogeneous catalysts, in the Department of Chemical and Biomolecular Engineering.

By synthesizing a sodium-on-alumina catalyst, Conk found that it efficiently broke or cracked various types of polyolefin polymer chains, leaving one of the two pieces with a reactive carbon-carbon double bond at the end. A second catalyst, tungsten oxide on silica, added the carbon atom at the end of the chain to ethylene gas, which is constantly circulating in the reaction chamber to form a propylene molecule. This latter process, called olefin metathesis, leaves behind a double bond that the catalyst can access again and again until the entire chain is converted to propylene.

The same reaction occurs with polypropylene to form a combination of propene and a hydrocarbon called isobutylene. Isobutylene is used in the chemical industry to make polymers for products ranging from soccer balls to cosmetics and to make high-octane gasoline additives.

Surprisingly, the tungsten catalyst proved even more effective than the sodium catalyst in breaking polypropylene chains.

“There’s nothing cheaper than sodium,” Hartwig said. “And tungsten is an abundant metal on Earth, used extensively in the chemical industry, unlike our ruthenium metal catalysts which were more sensitive and more expensive. This combination of tungsten oxide on silica and sodium on alumina is like taking two different types of dirt and having them disassemble the entire polymer chain together to get even higher yields of propene from ethylene and a combination of propene and isobutylene from polypropylene than we could with these more complex and expensive catalysts.”

Like a pearl necklace

One of the main advantages of the new catalysts is that they avoid having to remove hydrogen to form a brittle carbon-carbon double bond in the polymer, which was a feature of the researchers’ previous method for deconstructing polyethylene. Such double bonds are a polymer’s Achilles heel, in the same way that the reactive carbon-oxygen bonds in polyester or PET make plastic easier to recycle. Polyethylene and polypropylene don’t have this Achilles heel: Their long chains of single carbon bonds are very strong.

“Think of the polyolefin polymer as a string of pearls,” Hartwig says. “The clasps at the end keep the pearls from falling off. But if you cut the string in the middle, you can remove one pearl at a time.”

The two catalysts together converted an almost equal mixture of polyethylene and polypropylene into propylene and isobutylene (both gaseous at room temperature) with an efficiency of nearly 90%. For polyethylene or polypropylene alone, the efficiency was even higher.

Conk added plastic additives and different types of plastics to the reaction chamber to see how the catalytic reactions were affected by contaminants. Small amounts of these impurities did little to affect the conversion efficiency, but small amounts of PET and polyvinyl chloride (PVC) significantly reduced efficiency. This is not a problem, however, because recycling methods already separate plastics by type.

Hartwig noted that while many researchers hope to redesign plastics from the ground up so they can be easily reused, today’s hard-to-recycle plastics will be a problem for decades.

“You could say we should get rid of all polyethylene and polypropylene and use only new circular materials. But the world won’t do that for decades and decades. Polyolefins are cheap and have good properties, so everyone uses them,” Hartwig said. “People say if we could figure out a way to make them circular, that would be a big step forward, and that’s what we’ve done. You can start to imagine a commercial plant that would do that.”

Other co-authors of the study are graduate students Jules Stahler, Jake Shi, Natalie Lefton and John Brunn of UC Berkeley and Ji Yang of Lawrence Berkeley National Laboratory. Shi, Hartwig and Bell are also affiliated with Berkeley Lab.