Enhanced aromatics production via catalytic pyrolysis of poly(3-hydroxybutyrate) using ZSM-5 catalysts

The valorization of biodegradable plastics into high-value chemicals offers a sustainable pathway for plastic waste management. In this study, catalytic pyrolysis of poly(3-hydroxybutyrate) (PHB), a representative bioplastic, was systematically investigated over ZSM-5 zeolites with varying degrees of mesoporosity, aiming to maximize the BTEX yield. Catalyst characterization confirmed that mesopore introduction increased external surface area and accessibility of acid sites, while relatively reducing strong Brønsted acidity. Catalytic performance was evaluated, as well as the effects of different pyrolysis atmospheres (N₂, CO₂, CH₄) and catalyst-to-feed (C/F) ratios (1/10, 1/6, 1/3). Compared with non-catalytic pyrolysis, ZSM-5 significantly reduced oxygenates in the oil and promoted the formation of aromatic hydrocarbons. Among the mesoporous catalysts, MEHZ-1 delivered the highest selectivity, producing 56.44 % BTEX (benzene, toluene, ethylbenzene, and xylenes) in N₂ atmosphere. Under CH₄ co-feeding, MEHZ-1 achieved a further increase to 68.20 % BTEX, accompanied by a dramatic decrease in oxygenates from 33.10 % to 23.18 % compared to N₂ atmosphere. Gas and oil analysis revealed that the mesoporous MEHZ-1 facilitated deoxygenation, mainly decarboxylation, and propene oligomerization that contributes to generating aromatics. Increasing the C/F ratio also enriched BTEX at the expense of oxygenates. Overall, the synergy of mesoporosity and optimized Brønsted and Lewis acidity in MEHZ-1, combined with CH₄, most effectively promoted aromatic hydrocarbon production. These findings highlight the potential of tailored mesoporous zeolites for efficient bioplastic upgrading into valuable aromatics, supporting sustainable chemical recycling within a circular economy framework.