Chemists from China have shown how solid-state NMR spectroscopy can be used to determine the composition of a plastic mixture and use the information obtained to process the plastic into simple organic substances. In particular, scientists obtained 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 about 400 million tons of polymers that people use in their homes and industries. After use, most plastic is burned or sent to landfills. For example, in 2022, people recycled only ten percent of all plastic produced. One of the problems in the recycling process is that household products often contain mixtures of different polymers, each of which must be separated and recycled. Therefore, chemists are looking for effective methods to separate mixtures of plastics.
One such method was presented by chemists led by Ma Ding of Peking University. They used two-dimensional solid-state NMR spectroscopy to determine the composition of a polymer sample. This method allows solid samples to be analyzed 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 made themselves from pure polymers. They first confirmed the composition of the mixture using solid-state NMR, and then began processing it.
The processing of the mixture involved several steps. First, using the solvents tetrahydrofuran and hexane, the chemists separated the PS and converted it into 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 a mixture of liquid dichloroalkanes and an aromatic amine salt from the remaining mixture of PC and PVC.
Next, from the solid residue containing PLA, PET, PE and PP, the chemists separated PLA using an ammonolysis reaction and converted it into alanine. Then the scientists hydrolyzed PET in the presence of alkali and obtained terephthalic and lactic acids from it. And the researchers cracked the mixture of PE and PP - as a result, a mixture of alkanes was formed.
The chemists showed that using solid-state NMR, several solvents, and known processing methods, they could turn a mixture of eight polymers into simple organic substances. They also showed that their compositional determination and processing method could handle plastics of unknown composition collected from various laboratories and households.
The production of plastic leads to environmental pollution with its microparticles. For example, we previously reported on how microplastic was found on the summit of Everest and in human blood. Presumably, microplastic has a negative effect on the human digestive system.
