From Citrus to Cymene: Understanding the Hydrothermal Conversion of Limonene

Limonene, a key intermediate formed during the hydrothermal liquefaction (HTL) of waste tyres, is a well-established, major product from the pyrolysis of tyre rubber. Here, its transformation in supercritical water as both a polyisoprene model compound, and HTL intermediate was studied. At 450°C, limonene is consumed within 30 min, forming an oil consisting of a mixture of aromatic species, of which p-cymene is the major product. Water effects this transformation by facilitating the isomerisation of the initially exocyclic double bond of limonene into the ring, affording α-terpinene that then dehydrogenatively aromatises. In contrast, pyrolysis is known to favour limonene ring opening and radical-mediated rearrangements. The susceptibility to aromatisation of a variety of six-membered ring structures was investigated under the same conditions and found to require at least two double bonds within the starting molecule for aromatisation within 30 min. Based on the comparison of stainless-steel reactors with and without borosilicate glass inserts to those coated internally with silicon or gold, we propose that the combination of the water as solvent and the presence or absence of mildly acidic sites on these reactor surfaces is responsible for the product selectivity differences observed under HTL conditions compared to pyrolysis.