Refinement of limonene epoxides from the light distillate of tire pyrolysis oil via catalytic epoxidation

The light fractions of tire pyrolysis oil (TPO) are rich in valuable chemicals, such as limonene, but the utilization of TPO-derived limonene or limonene derivatives was limited by the difficulties to separate limonene from the rest compounds, especially p-cymene which has a similar molecular structure and physical properties (molecular size, melting point and boiling point) as limonene. In order to utilize limonene and separate p-cymene, this study took advantage of the difference in oxidizability between limonene and p-cymene, and proposed a method of epoxidizing TPO before the separation. After epoxidation, limonene was successfully converted into limonene epoxides, while aromatic compounds in TPO such as p-cymene were inert and non-oxidizable. Limonene epoxides have a higher boiling point than p-cymene, so the limonene epoxides can be easily refined by distillation. In order to accelerate the oxidation of limonene, a peroxophosphotungstate catalyst was used as a sustainable solvent-free process. The fraction of TPO between 160 and 190 °C which has 298 g/L limonene was tested in this study. The epoxidation of the TPO160-190 could reach complete conversion to limonene 1,2-oxides and diepoxide limonene. The resulting oil could be divided into p-cymene enriched fraction and limonene epoxides enriched fraction by simple distillation, and the limonene epoxides enriched fraction contained more than 80% of limonene epoxides. The catalytic mechanism was proposed with the assistance of FT-IR, GC–MS, and 31P NMR analysis. This work presents a novel strategy of epoxidizing limonene for limonene epoxide and p-cymene separation, which could bring a breakthrough for limonene epoxide and p-cymene production from waste-derived oils.