NMR Characterization of Plastic Pyrolysis Oils Obtained over Clay Catalysts

The accumulation of plastic waste has become a significant environmental challenge, stimulating the development of efficient recycling technologies capable of converting polymers into valuable products. In this study, polypropylene wastes were thermocatalytically converted into liquid hydrocarbons using three naturally occurring types of clay as catalysts, namely, kaolin, illite, and bentonite. The obtained liquid products were investigated using one- and two-dimensional NMR spectroscopy, including 1H, 13C, and COSY techniques. Quantitative evaluation of hydrocarbon group composition was performed using established NMR correlations to determine the content of paraffins, olefins and aromatic compounds as well as fuel-related parameters. The results demonstrated that paraffins were the predominant constituents in all pyrolysis oils, accounting for more than 70 vol.%, while olefins and aromatics were present in smaller amounts. Although all catalysts promoted the formation of liquid mixtures of hydrocarbon, noticeable differences in product composition were observed. Oil obtained over illite exhibited increased aromaticity and lower olefin content, whereas kaolin produced a product characterized by the highest isoparaffin index and estimated research octane number. The findings indicate that variations in catalysts influence the characteristics of polypropylene-derived oils and may be used to tailor products’ properties for their utilization as fuel additives or petrochemical feedstocks.