Catalytic Pyrolysis of Waste Polyethylene Using Combined CaO and Ga/ZSM-5 Catalysts for High Value-Added Aromatics Production

Amidst the global focus on recycling and treatment of waste plastics for resource sustainability, it is a huge challenge to convert waste plastics into aromatics, especially high value-added monocyclic aromatic hydrocarbons (MAHs). In this work, catalytic pyrolysis experiments of linear low-density polyethylene (LLDPE) indicated that the selectivity to aromatics increased from 70.35% when using pure HZSM-5 to 94.02% with 7 wt % Ga-modified HZMS-5. Further, the cracking catalyst CaO was utilized to increase the proportion of MAHs in aromatics. Three different catalytic modes, CaO sequential with Ga/ZSM-5 mode, Ga/ZSM-5 sequential with CaO mode, and the mixed mode, were conducted for the combined catalytic pyrolysis of polyethylene. Under such three modes, the selectivities to benzene, toluene, ethylbenzene, and xylene (BTEX) were 45.18%, 52.29%, and 56.21%, respectively. MAHs were inhibited under CaO sequential with the Ga/ZSM-5 mode and facilitated under the other two modes. By contrast, the mixed mode showed the best catalytic synergistic effect involving Ga/ZSM-5 and CaO, where LLDPE pyrolytic gas was randomly and repeatedly exposed to Ga/ZSM-5 and CaO, creating more chances to form shuttle catalysis involving aromatization and cracking. The present study is expected to provide some reference value for recycling and utilizing liquid products via catalytic pyrolysis of waste plastics.