A study on the catalytic pyrolysis characteristics of waste oil based on different molecular sieves

The pore structure and acidic sites of the catalyst significantly influence the catalytic pyrolysis process of waste oil. This study utilized waste clay oil generated from the industrial decolorization and refining process of soybean oil as feedstock. Thermogravimetric analysis and pyrolysis/gas chromatography-mass spectrometry were employed to investigate the effects of different molecular sieves on the thermodynamic properties, kinetic parameters, and pyrolysis product distribution of the waste oil. The distribution of pyrolysis products and kinetic parameters were primarily regulated by pore structure and acidic sites. ZSM-5 with medium pore sizes and moderate acidity limits some competitive reactions and adverse reactions. Under its catalysis, the product distribution of monocyclic aromatic hydrocarbons (MAHs) was most concentrated, exhibiting the highest peak area percentage and the greatest reduction in apparent activation energy. A detailed product analysis revealed that molecular sieve pore inhibited the formation of 1-methylnaphthalene. The pore space restriction of molecular sieves promoted the formation of multi-substituted alkylbenzenes and inhibits the formation of mono-substituted linear alkylbenzenes. Furthermore, for ZSM-5 with varying SiO₂/Al₂O₃ ratios, the catalyst acidity gradually decreases with increasing SiO₂/Al₂O₃ ratio, leading to a reduction in the peak area percentage of aromatics. The appropriate Brønsted acid sites (BAS)/Lewis acid sites (LAS) ratio promotes the formation of MAHs through synergistic effects, while delaying catalyst deactivation. In this study, ZSM-5/120 with a BAS/LAS ratio of 1.36 was identified as the optimal catalyst for MAHs production.