Despite global efforts to transition to a circular plastics economy, more than 75% of the 400 metric tons of plastic produced annually worldwide still end up as waste. Addressing this challenge, researchers at the University of Illinois Urbana-Champaign have presented an ingenious method to recycle a specific form of plastic that proves more difficult to recycle than its counterparts.
In their recent publication in Nature Communications, the researchers detail their groundbreaking process that harnesses renewable energy sources for plastic recycling, aligning with the drive toward a circular plastics economy. Spearheading this innovation were Yuting Zhou, a co-author and postdoctoral associate, and two chemistry professors at Illinois: Jeffrey Moore, an expert in polymers, and Joaquín Rodríguez-López, an authority in electrochemistry.
The project originated from Moore's prior experience with Poly(phthalaldehyde), a type of polyacetal, and Polyoxymethylene (POM), a high-performance acetal resin widely used in industries such as automotive and electronics. POM is a thermoplastic that can be molded when heated and solidifies with robust strength and rigidity upon cooling, often replacing metal in applications like mechanical gears. It's produced by various firms under different names like Delrin by DuPont.
Recycling POM presents challenges due to its highly crystalline properties, which make breakdown complex. While it can be melted and re-molded, the original material's qualities are lost, limiting its utility. Zhou explained, “When the polymer was in use as a product, it's not a pure polymer. It will also have other chemicals like coloring additives and antioxidants. So, if you simply melt it and remold it, the material properties are always lost.”
The Illinois research team's method capitalizes on electricity, drawn from renewable sources, and operates at room temperature. This electro-mediated process disassembles the polymer, breaking it down into monomers – the building blocks of polymers.
While significant advancements have been made in repurposing common synthetic plastics like PE, PET, and PS over the past five years, POM remains relatively unexplored, especially using electricity as a driving force.
In their study, the researchers demonstrate their technique using Delrin. The process commences by dissolving small polymer beads in a liquid to weaken the polymer chain's bonds. Subsequent electrocatalysis breaks down the Delrin polymer chains into monomers.
Central to their success is the organic solvent used to dissolve the plastic. After experimenting with various solvents, Zhou discovered that Hexafluoroisopropanol (HFIP) was effective. It not only dissolves the polymer but also likely facilitates depolymerization during electrocatalysis by acting as a proton donor catalyst.
The HFIP generates acid during electrolysis, which the researchers believe is pivotal in breaking down the polymer into monomers. Zhou stated, “That's what we hypothesize is happening… It's more leaning toward an electro-mediated acid depolymerization process.”
After successfully proving their technique on small beads of pure POM, the team tackled a commercial Delrin product – keck clip sheds, commonly used in chemistry labs. Again, their process succeeded.
Zhou finds this work encouraging, proving the feasibility of using electricity to break down plastic, despite the challenges and limitations. The team plans to explore electrocatalysis further, aiming for the selective upcycling of POM into formic acid and its integration into a flow system.
Ultimately, the researchers aspire to inspire engineers and synthetic chemists to contemplate electricity as a tool for deconstructing synthetic plastics. Their paper, titled “Heterogeneous Electromediated Depolymerization of Highly Crystalline Polyoxymethylene,” is published in Nature Communications, offering a significant stride toward sustainable plastic utilization.
Source: University of Illinois at Urbana-Champaign