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.