A review of dipentene (dl-limonene) production from waste tire pyrolysis

•Dipentene is formed via intramolecular cyclization of allylic radicals.•A least 2.5 wt% of a steel-free tire can be converted into dipentene.•Vacuum pyrolysis and basic additives can potentially further improve this yield.•The purification of a tyre-derived dipentene remains a significant challenge.

The present review concerns the production of dipentene (dl-limonene) from the pyrolysis of waste tires and is divided into three parts, which discuss (1) the mechanism of dipentene formation from polyisoprene, (2) the reported dipentene yields from experimental studies and (3) the separation and purification of dipentene from a tire-derived oil. Dipentene is formed via an intramolecular cyclization reaction of the allylic radicals formed by random scission of the β bonds with respect to the double bonds in the polyisoprene chains. Dimerization of two isoprene molecules to form dipentene seems unlikely under pyrolytic conditions. Especially at higher temperatures, the formed dipentene transforms further into either isoprene or aromatic compounds, the latter possibly via a diallyl diradical. Both the pyrolysis operating conditions and the tire type and brand have significant influence on the dipentene yield. Among these, the pyrolysis temperature is the most important variable, with temperatures between 400 and 500 °C resulting in maximum dipentene yields. Vacuum pyrolysis and basic additives have been propounded as a means of further improving this yield, but additional results are required to confirm this conclusively. Based on the presently available information in the literature, at least 2.5 wt% of a steel-free tire can be converted to dipentene. Finally, the literature has shown that it is not a trivial exercise to obtain a highly concentrated dipentene fraction of sufficient quality from the tire-derived oil. In particular, the removal of sulfur-containing compounds and the separation of dipentene from 1,2,3-trimethylbenzene, m- and p-cymene and indane are challenges that need to be addressed in the future.