Researchers in the United States have demonstrated a one-pot method of converting polyethylenes into more valuable chemicals at low temperatures. Early stage research could help address Earth’s growing mountain of plastic waste and offers an alternative route to chemicals currently produced through energy-intensive processes that consume fossil fuels.
Plastics are a versatile class of materials with a myriad of applications and are an omnipresent and indispensable part of modern life today. In 2015, global production of petroleum-based plastics was 380 million tonnes and production is expected to double again in 20 years. However, their stability and chemical inertness also mean they pose a significant problem for waste because they can persist for hundreds of years when disposed of in landfills or into the environment. A 2017 study found that this was the destination of 79% of all plastic waste ever produced, with only 9% recycled. Current industrial recycling methods generally involve simply washing polymers before dissolving and remodeling them. However, these recycled plastics have lower properties and lower value than virgin plastics. And while chemical recycling methods can, in principle, depolymerize some plastics into the monomer, this process is highly endothermic, posing a challenge to its economic viability.
Now Susannah Scott of the University of California, Santa Barbara and her colleagues have demonstrated the conversion of polyethylenes, which account for 36% of all plastic waste, into long-chain alkylatomatic chemicals, which are more valuable than the original plastic. Alkylaromatics are currently mainly produced by reforming the naphtha fraction of crude oil at 500-600 ° C to produce a blend called BTX (benzene-toluene-xylenes), which is then alkylated at a later stage using a strong acid catalyst, generating more environmental energy. problems of waste separation and disposal. The team’s new method involves directly converting polyethylenes into a blend of linear alkyl aromatics, using a platinum catalyst at a temperature of just 280 ° C.
The group discovered their process by accident while experiencing a different reaction. They were initially perplexed by their results because the low temperatures used should not have been sufficient for the aromatization, which is highly endothermic. However, they found that the hydrogen released by aromatization reacted with the polyethylene chains, promoting their decomposition into shorter chains. “[Complete] depolymerization would return the polyethylene back to ethylene and that reaction would be highly endothermic,” explains Scott. “But we won’t go back to ethylene… you can think of taking the big molecules and splitting them with hydrogen to get alkanes, and this is an exothermic reaction. Therefore, coupling the reaction that produces hydrogen with the reaction that consumes hydrogen makes us almost thermo-neutral. ”
Researchers demonstrated the process on a sample of a low-density polyethylene plastic bag and a high-density polyethylene bottle cap, producing a range of linear alkyl aromatics that could have uses ranging from surfactants and lubricants to refrigerants and to insulating oils. . They are working to develop a more scalable continuous process that can tolerate the mixed feed streams commonly found in actual polymer waste. “We have been working with blends and the process continues to work until the contaminants deactivate the catalyst,” says Scott.