Chemists have devised a catalytic method to convert the most prevalent forms of plastic waste, specifically polyolefin bags and bottles, into gases like propylene and isobutylene. These gases serve as fundamental components for manufacturing polypropylene and other plastics. Utilizing low-cost solid catalysts that can be produced on a large scale, this process presents a promising opportunity for establishing a circular economy centered around these disposable plastics.
A novel chemical process can virtually vaporize commonly discarded plastics and transform them into the hydrocarbon building blocks necessary for the creation of new plastics.
The catalytic method, developed at the University of California, Berkeley, is effective for the two primary types of consumer plastic waste: polyethylene, mainly found in single-use plastic bags, and polypropylene, which is present in various hard plastics, such as microwaveable containers and suitcases. It is also capable of efficiently breaking down a blend of these plastic types.
When scaled effectively, this process could facilitate the establishment of a circular economy for many disposable plastics by converting plastic waste back into the monomers needed to synthesize polymers. This would lessen the reliance on fossil fuels for producing new plastics. Developed in the 1980s, clear plastic water bottles made from polyethylene terephthalate (PET) were intended for recycling. However, the quantity of polyester plastics is negligible compared to that of polyethylene and polypropylene, which are collectively known as polyolefins.
“Everyday items like lunch bags, laundry detergent bottles, and milk jugs predominantly consist of polyethylene and polypropylene — these are the polyolefins that surround us,” explained John Hartwig, a UC Berkeley chemistry professor and lead researcher. “What we can now potentially achieve is to take these items and revert them to their original monomers using the chemical reactions we have developed, which can break typically resilient carbon-carbon bonds. This effort brings us closer than ever before to achieving a similar circularity for polyethylene and polypropylene as we currently have with polyesters in water bottles.”
Hartwig, along with graduate student Richard J. “RJ” Conk, and chemical engineer Alexis Bell, a UC Berkeley Graduate School professor, will detail their catalytic process in an upcoming edition of the journal Science.
A circular economy for plastics
Polyethylene and polypropylene account for nearly two-thirds of global post-consumer plastic waste. Approximately 80% of this waste finds its way into landfills, gets incinerated, or is discarded carelessly into the environment, often becoming microplastics in waterways and oceans. The remaining waste is typically recycled into lower-value products like decking materials, flowerpots, and sporks.
To mitigate this waste issue, researchers have sought methods to transform these plastics into higher-value materials, such as the monomers that can be polymerized to create new plastics. This approach aims to develop a circular polymer economy that reduces the need for petroleum-based plastic production, which contributes to greenhouse gas emissions.
Two years ago, Hartwig’s team at UC Berkeley pioneered a technique for converting polyethylene plastic bags into the monomer propylene (or propene) for use in creating polypropylene plastics. This earlier process utilized three distinct custom heavy metal catalysts: one to introduce a carbon-carbon double bond into the polyethylene polymer, while the other two facilitated the splitting of the polymer chain at this double bond, progressively removing carbon atoms and generating propylene (C3H6) molecules until the polymer vanished. However, these catalysts were dissolved in the liquid reaction, making their recovery in an active form challenging.
The new approach replaces the costly, soluble metal catalysts with affordable solid catalysts that are prevalent in the chemical industry’s continuous flow processes, allowing for catalyst reuse. Continuous flow methods are capable of being scaled up to manage larger quantities of material.
Conk began experimenting with these catalysts after consulting with Bell, who is renowned for his expertise in heterogeneous catalysts within the Department of Chemical and Biomolecular Engineering.
By synthesizing a catalyst made of sodium on alumina, Conk discovered it effectively cracked various polyolefin polymer chains, leaving one end with a reactive carbon-carbon double bond. Another catalyst, tungsten oxide on silica, combined the carbon atom from the end of the chain with ethylene gas, continuously introduced into the reaction chamber, creating a propylene molecule. This method, known as olefin metathesis, allows for repeated access to the double bond by the catalyst until the entire polymer chain is converted to propylene.
The same reaction occurs with polypropylene, yielding a mix of propene and a hydrocarbon called isobutylene, used in the chemical industry for producing polymers and high-octane gasoline additives.
Interestingly, the tungsten catalyst outperformed the sodium catalyst in breaking down polypropylene chains.
“Sodium is about as inexpensive as it gets,” Hartwig noted. “And tungsten is an abundantly available metal used extensively in the chemical industry, unlike our earlier ruthenium catalysts that were more sensitive and pricey. The combination of tungsten oxide on silica and sodium on alumina works like mixing two different types of dirt together to deconstruct the entire polymer chain, resulting in higher yields of propene from ethylene, and a blend of propene and isobutylene from polypropylene, outperforming those more intricate and costly catalysts.”
Like a string of pearls
A significant benefit of the new catalysts is that they eliminate the need to remove hydrogen to form breakable carbon-carbon double bonds within the polymer, a characteristic of the researchers’ earlier polyethylene deconstruction process. Double bonds present a vulnerability for polymers, similar to how reactive carbon-oxygen bonds in polyester or PET enhance recyclability. In contrast, polyethylene and polypropylene lack this vulnerability due to their incredibly robust long chains of single carbon bonds.
“You can visualize the polyolefin polymer as a string of pearls,” Hartwig explained. “The locks at the end keep the pearls from falling out. However, if you sever the string in the middle, you can now extract one pearl at a time.”
Using both catalysts, they converted a roughly equal mixture of polyethylene and polypropylene into propylene and isobutylene — both gaseous at room temperature — achieving nearly 90% efficiency. For each type of plastic separately, the yield increased even further.
Conk experimented with plastic additives and various plastic types in the reaction chamber to assess how impurities influenced the catalytic reactions. Minor amounts of contaminants had little effect on the conversion efficiency, but traces of PET and polyvinyl chloride (PVC) significantly impaired the efficiency. However, this issue may be manageable since current recycling practices already sort plastics type.
Hartwig pointed out that although many researchers aspire to redesign plastics for easier reuse, today’s difficult-to-recycle plastics will continue to pose challenges for many years ahead.
“Some might argue that we should eliminate all polyethylene and polypropylene and shift to entirely new circular materials. However, that’s not going to happen for many decades. Polyolefins are inexpensive and possess desirable properties, making them widely used,” Hartwig remarked. “If we can find a way to establish a circular system for them, it would significantly impact the industry, which is precisely what we’ve accomplished. The concept of a commercial plant capable of performing this process is now within reach.”
Other contributors to the research include graduate students Jules Stahler, Jake Shi, Natalie Lefton, and John Brunn from UC Berkeley, along with Ji Yang from Lawrence Berkeley National Laboratory. Shi, Hartwig, and Bell are also associated with Berkeley Lab. This research was funded by the Department of Energy (DE-AC02-05CH11231).
