Synergistic and antagonistic effects of ZnO/Zn2+ species and Brønsted/Lewis acid sites variations on the catalytic conversion of polyethylene to aromatics

The catalytic pyrolysis of polyethylene for aromatic production is a promising method for waste plastic disposal and resource recovery. This study investigated the nature of metal species and the distribution of acidic sites in low-density polyethylene (LDPE) scission-aromatization reactions using metal-loaded zeolite catalysts. Under the influence of the 1.0Zn/MOR40 catalyst, LDPE achieved a maximum aromatic selectivity of 45.71 % at 500 °C, based on the total product composition. The results indicated that catalysts with a lower Brønsted acid site to Lewis acid site ratio (BAS/LAS) and a higher concentration of [ZnOH] + species favored the reaction, promoting the generation of olefin intermediates and aromatics while suppressing polycyclic aromatic hydrocarbons (PAHs) formation. However, there was an optimal range for the BAS/LAS ratio and [ZnOH] + species concentration. Reaction results involving squalene and triacontane demonstrated that excessive dehydrogenation of reactants and insufficient BAS could hinder the progression of aromatization reactions, resulting in an increased proportion of PAHs. This study provides a comprehensive understanding of the balance between the synergistic and antagonistic effects of metal-acid sites on the aromatization process.