Production and Advanced Spectroscopic Characterization of Solvothermal Liquefaction Products of Polystyrene, Styrene–Butadiene Rubber, and Scrap Tire Waste

Solvothermal liquefaction (STL) is a promising thermochemical technique for recycling organic waste materials. In this study, three polymer feedstocks, high molecular weight polystyrene (PS), styrene-butadiene rubber (SBR), and scrap tire waste (STW), were subjected to STL under subcritical (270 °C) and supercritical (320 °C) conditions, using toluene as the solvent. The resulting liquid products were characterized by atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry (APPI FT-ICR MS), carbon-13 nuclear magnetic resonance (¹³C NMR), and Fourier transform infrared (FT-IR) spectroscopy. All feedstocks yielded significant amounts of liquid products. PS was fully liquefied at both temperatures, whereas SBR and STW left 16-22 wt % solid residues. Spectroscopic analyses showed that PS primarily decomposed into styrene monomers and small oligomers. In contrast, SBR and STW produced complex mixtures of aliphatic and polycyclic aromatic hydrocarbons (PAHs) and their derivatives. The SBR-derived liquids were rich in large, alkylated PAHs, while STW mainly generated polyisoprene oligomers with sulfur and oxygen functionalities. Supercritical conditions resulted in higher liquid yields than the subcritical ones but also increased the aromaticity of the products. Based on the results, STL is a promising thermochemical technique for waste polymer valorization but requires fine-tuning and further product downstream fractionation and processing.