CO2-Assisted Catalytic Pyrolysis of Polyolefins to Aromatics over Mesoporous HZSM-5 and Ga/ZSM-5 Catalysts

Catalytic pyrolysis is considered as a potential route to convert polyolefins to high-value aromatics toward a circular economy. However, the inherent diffusional constraints of zeolites and the formation of excess hydrogen during catalytic pyrolysis limit the selectivity of aromatics. In this study, we propose to construct mesoporous Ga-modified HZSM-5 to eliminate diffusion restriction and improve shape selection catalysis of aromatics. At the same time, we adopt CO2 as a mild oxidizer to remove excessive hydrogen to expedite the aromatization of intermediate alkanes and olefins. Developing the synergistic effect between CO2 and acid sites of Ga/ZSM-5-meso makes the BTEX content exceed 60%. Compared with the N-2 atmosphere, CO2 significantly enhances the yield of aromatics for all of the catalysts (HZSM-5, HZSM-5-meso, Ga/ZSM-5, and Ga/ZSM-5-meso). Significantly, Ga-Lewis acid sites and mesoporous structures promote dehydrogenation to produce H-2, which can be subsequently consumed through reverse water gas shift (RWGS) and methanation reactions, accelerating the equilibrium toward aromatics production. In this case, most of olefins in gas and nonaromatic hydrocarbons (non-AHs) in oil are completely converted into aromatics. Corresponding oil yield and aromatics selectivity in oil reach as high as 63.3 and 100% for CO2-Ga-ZSM-5-meso, respectively, compared with 40.7 and 96.2% for N-2-HZSM-5. Moreover, CO2 has the effect of dealkylation, leading to a notable increase in the relative contents of benzene and toluene. In gas composition, the content of H-2 decreases obviously while the content of CH4 increases, and CO is produced. Finally, the synergetic catalysis mechanism of the CO2-assisted catalytic pyrolysis of polyolefins over Ga/ZSM-5-meso is proposed.