Novel photothermal pyrolysis on waste tire to generate high-yield limonene

Waste tire is a typical solid municipal waste with large production capacity. Developing new low carbon-emission method to pyrolyze waste tire and produce high-value limonene is promising. Herein, a novel photothermal pyrolysis system was successfully built, and waste tire was firstly pyrolyzed by photothermal at different temperatures (400 °C, 500 °C, 600 °C, and 700 °C). The photothermal pyrolysis can assist in achieving high yield of limonene, of which the yield can high up to 8.98 wt% at 600 °C and the relative peak areas of Gas Chromatography-Mass Spectrometer (GC–MS) results were all about 30 A.%. cis-1,4-polyisoprene in waste tire partly decomposed into some free radicals and they further reacted to generate limonene by intramolecular cyclization, dimerization, and isomerization. Parts of limonene aromatized to 1-methyl-4-(1-methylethenyl)-benzene and converted to triolefins that formed diene rings, the diene rings further aromatized to xylene and toluene, which were partly combined to generate 1,6,7-trimethyl-naphtalene. The photothermal pyrolysis mechanisms of waste tire were revealed: At higher pyrolysis temperature, waste tire partly decomposed and others polymerized to form amorphous carbons. The small aromatic rings and alkyl-aryl CC bonds further polymerized to be large aromatic rings system. The pores in char further formed and limonene generation was strengthened, because the limonene formation/decomposition was mainly limited in the pores. Subsequently, some pores were blocked at 700 °C, the generation of limonene was thus inhibited.