In-depth insight into cyclic methyl-siloxanes in waste tires pyrolysis oil (WTPO), high silicon content gasoline, and high silicon content diesel

Waste tire pyrolysis oil (WTPO) serves as a valuable feedstock for sustainable fuels and high-value-added chemicals. However, the silicon content and siloxane content of WTPO have not been observed in detail, and the similarity of siloxane morphology between WTPO and commercial gasoline and diesel has not been discussed in depth. To address this gap, our research group collected 24 WTPO samples from China, along with 3 commercial gasoline samples and 4 commercial diesel samples exhibiting abnormal silicon content, between 2023 and 2025. The silicon content and siloxane content of the samples were determined by inductively coupled plasma optical emission spectrometer (ICP-OES) and gas chromatography-quadrupole mass spectrometry (GC-MS), respectively. The results indicate that the silicon content ranges of WTPO are <1 ppm to 3674 ppm. Analysis of siloxane morphology revealed that cyclic permethylsiloxanes (Dn, where n denotes the number of Si atoms) are widely present in WTPO. In the detected WTPO samples, the maximum contents of D4, D3, and D5 reach 2251 ppm, 923 ppm, and 475 ppm, respectively. Furthermore, the type and content distribution of cyclic methyl-siloxanes in high silicon content WTPO, high silicon content gasoline, and high silicon content diesel samples demonstrate a strong similarity. This study emphasizes that the potential threats posed by silicon-containing molecules in WTPO to the environment, organisms, and industrial equipment should be evaluated in advance of large-scale commercialization.