The air we breathe is thickening with pollutants. Among the most recalcitrant pollutants, polypropylene – employed for purposes akin to food packaging and automotive parts – and polyethylene, ubiquitous in plastic bags, bottles, toys, and even mulch, will likely persist for an extended period before degrading in landfills.
However, the process can be troublesome, potentially producing large quantities of the potent greenhouse gas methane. Polyethylene and polypropylene, two polyolefins, are produced through the polymerization of their respective monomers, ethylene and propylene, which are predominantly sourced from fossil fuels. Despite their reputation for being notoriously difficult to break, the bonds of polyolefins are particularly challenging to disrupt.
Researchers at the University of California, Berkeley, have developed a method for recycling polymers utilizing catalysts that cleave their molecular bonds, effectively transforming them into gaseous propylene and isobutylene at ambient temperatures. These recyclable gases can be converted into fresh plastic products.
As a result of polypropylene and polyethylene are notoriously difficult and expensive to separate from one another in a mixed waste stream, it is crucial that a recycling process be developed for both polyolefins.
Breaking It Down
The recycling process employed by the group utilizes an isomerizing ethenolysis method, relying on a catalyst to break down olefin polymer chains into their constituent monomers. Due to their molecular structure, polyethylene and polypropylene exhibit exceptional resistance to chemical reactions, attributed to the long sequences of single carbon-carbon bonds within these polyolefin molecules. While many polymers feature at least one carbon-carbon double bond, this molecular structure is often significantly easier to break.
Although the same research team had previously attempted isomerizing ethenolysis, their earlier efforts were hindered by the use of expensive metals whose purity was compromised over time, limiting their effectiveness in converting all plastic to fuel. The successful combination of sodium on alumina with tungsten oxide on silica, despite the necessity for higher temperatures, ultimately yielded a more cost-effective and efficient process.
When exposed to sodium on alumina, each plastic undergoes a breakdown process that cleaves every polymer chain into shorter fragments, forming fragile carbon-carbon double bonds at both terminals. Interruptions by chains persisted repeatedly throughout the timeline. The resulting compounds then underwent a subsequent process, commonly known as olefin metathesis. As they were exposed to a steady stream of ethylene gas flowing into a reaction chamber, accompanied by treatment with tungsten oxide on silica, the carbon-carbon bonds underwent cleavage, leading to their breakage.
When exposed to high temperatures, the polymer chain reaction breaks down all carbon-carbon bonds in polyethylene and polypropylene, releasing carbon atoms that are subsequently incorporated into ethylene molecule structures. “The presence of ethylene is crucial in this process, serving as a coreactant,” Dr. R.J. stated. A leading researcher on the study, Conk spoke to Ars Technica about the findings. The damaged hyperlinks subsequently interact with ethylene, thereby eliminating them from the molecular structure. Without ethylene, no response can occur.
The complex process involves the total transformation of polyethylene into propylene, which is then converted into a mixture of propylene and isobutylene alongside polypropylene.
This technique exhibits pronounced selectivity, yielding a substantial quantity of targeted products – namely, propylene and isobutylene – with polyethylene and polypropylene as their respective precursors. Propylene, a crucial raw material for the chemical industry, and isobutylene, a versatile monomer employed in various polymers, including synthetic rubber and as a fuel additive, are both in high demand.
Mixing It Up
Researchers sought to investigate the outcome when polypropylene and polyethylene were combined for isomerizing ethenolysis treatment, given that plastics often blend together at recycling facilities. The reaction was successful in converting the feedstock into a mixture of propylene and isobutylene, with only a slight excess of propylene over isobutylene.
Occasionally, mixtures may also comprise contaminants in the form of excess plastics. The group also needed to determine if the response remained effective in the presence of contaminants. Researchers manipulated discarded plastic items, along with a centrifuge and a bread bag infused with varying polymer residues, including polypropylene and polyethylene, to explore their properties. While the production of propylene and isobutylene was significantly reduced,
Another potential approach could involve exploring the use of distinct plastics, such as PET and PVC, alongside polypropylene and polyethylene, to determine if this variation yields any noticeable effects. However, these adjustments significantly diminished the output. To ensure successful recycling of polypropylene and polyethylene products, any residual contaminants must be meticulously removed beforehand.
While this recycling method holds promise in potentially preventing massive amounts of waste, its scalability would need to be significantly increased to have a meaningful impact. The analysis team’s decision to scale up the experiment yielded identical results, hinting at a positive outcome in the long term. Despite these efforts, significant infrastructure development is still necessary before meaningful reductions in plastic waste can be achieved.
The researchers expressed their hope that the research would yield practical approaches for generating novel polymers, according to the report. “As a result, the need to manufacture these crucial commodity chemicals sourced from fossil carbon and accompanied by substantial greenhouse gas emissions can be drastically reduced.”
