A major reason straws, plastic bags and yogurt containers aren’t recycled is that it’s much cheaper to make these products from scratch than to recover them. A new recycling technique reported in the journal Science promises a cost-effective way to convert these ubiquitous plastic waste items into high-quality liquid fuels.
The low-temperature method should be much less energy-intensive and expensive than conventional recycling. “To solve the problem of persistent waste plastic, we need to reach a critical point where it makes more sense to collect it and return it to use than to treat it as disposable,” said Johannes Lercher, director of the Institute for Integrated Catalysis at Pacific Northwest National Laboratory and co-author of the study.
Less than 10 percent of the billions of tons of plastic waste people have generated has ever been recycled, according to the United Nations Environment Programme. Bans on single-use products such as straws, grocery bags, and takeout containers are likely the best way to reduce their use. But better recycling methods could help reduce waste from plastic products for which alternatives are not easily available, such as bottle caps, bread bags, and cling wrap.
These products are made of polyethylene and polypropylene, two of the world’s most commonly used plastics. What makes them useful, namely their strength and resistance to high temperatures, becomes a bane when it comes to recycling the materials. The strong carbon–carbon bonds in these plastics requires very high temperatures to break, making the process energy intensive.
Once the bonds break, the smaller molecules that are created quickly form new bonds, giving unwanted compounds. These byproducts then have to be broken down again, adding time and complexity.
Lercher and colleagues developed a way to convert these plastics into useful fuels at temperatures below 100°C, without creating any side products. They conduct the chemical reaction in the presence of a new type of catalyst called an alkylation catalyst in an aluminum chloride-based solution, which is acidic.
In these reaction conditions, as soon as the carbon bonds in the plastic break apart, new bonds form in a controlled way, giving gasoline-like compounds called alkanes. The alkanes can be used either as a fuel or as raw material for new plastics.
The entire reaction takes place in a single reaction vessel. It takes only three hours and occurs at 70°C. By comparison, existing recycling techniques typically require two stages, take much longer and need temperatures of over 200°C.
According to the researchers, the new method works for low-density polyethylene (type 4 in the recycling symbol) as well as for polypropylene (type 5), both of which aren’t commonly collected and processed in curbside recycling. These plastics make up around half of the 360 million tons of plastics produced globally each year.
The catalyst that the researchers use is already used by the petroleum industry. But it will take more research to know whether the technique can be translated to large scale. If so, such a one-step, low-cost method to convert single-use plastics into chemicals, fuels and high-value materials could go a long way in making the plastic economy circular, the researchers say.
Source: Wei Zhang et al. Low-temperature upcycling of polyolefins into liquid alkanes via tandem cracking-alkylation. Science, 2023.
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