Chinese chemists have demonstrated how solid-state NMR spectroscopy can be used to determine the composition of a plastic mixture and use the information obtained to convert the plastic into simple organic compounds. Specifically, the scientists extracted benzoic acid, the amino acid alanine, lactic acid, bisphenol A, terephthalic acid, and other products from a mixture of eight different polymers. The results of the study were published in the journal Nature.
Every year, the chemical industry produces approximately 400 million tons of polymers used in everyday life and industry. After use, most plastic is incinerated or sent to landfills. For example, in 2022, only ten percent of all plastic produced was recycled. One of the challenges in recycling is that household products often contain mixtures of different polymers, each of which must be separated and recycled. Therefore, chemists are searching for effective methods for separating mixed plastics.
One such method was presented by chemists led by Ma Ding from Peking University. They used two-dimensional solid-state NMR spectroscopy to determine the composition of a polymer sample. This method allows for the analysis of solid samples for the presence of functional groups unique to each polymer.
The scientists tested their approach on a mixture of polystyrene (PS), polylactate (PLA), polyurethane (PU), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyethylene (PE), and polypropylene (PP), which they prepared themselves from pure polymers. They first confirmed the mixture's composition using solid-state NMR and then began processing.
Processing the mixture involved several steps. First, using the solvents tetrahydrofuran and hexane, the chemists separated the PS and converted it to benzoic acid. Next, using pure tetrahydrofuran, they extracted PU, PC, and PVC from the mixture. Using glycolysis, the scientists obtained ethylene carbonate and bisphenol A from the PC, and from the remaining PC and PVC mixture, they obtained a mixture of liquid dichloroalkanes and an aromatic amine salt.
Next, the chemists separated PLA from the solid residue containing PLA, PET, PE, and PP using an ammonolysis reaction and converted it into alanine. The scientists then hydrolyzed PET in the presence of alkali, producing terephthalic and lactic acids. The researchers then cracked the mixture of PE and PP, resulting in a mixture of alkanes.
Thus, the chemists demonstrated that, using solid-state NMR, several solvents, and established processing methods, a mixture of eight polymers can be converted into simple organic compounds. They also demonstrated that their composition determination and processing method can handle plastics of unknown composition, collected from various laboratories and households.
Plastic production leads to environmental pollution with microplastics. For example, we previously reported on how microplastics were found on the summit of Everest and in human blood. Microplastics are suspected of negatively affecting the human digestive system.
