3D-printed monolithic ZSM-5@nano-ZSM-5: Hierarchical core-shell structured catalysts for enhanced cracking of polyethylene-derived pyrolysis oils

The catalytic upgrading of polyethylene-derived pyrolysis oils using ZSM-5 zeolites enables the cost-effective production of valuable chemicals from plastic waste, fostering a circular economy through resource recycling. To achieve high-performance catalyst design, balancing active site accessibility and selectivity is crucial. Here, we report a monolithic ZSM-5 @nano-ZSM-5 core-shell catalyst (mZ@nZ) fabricated via vat photopolymerization, in situ crystal engineering, and a one-step desilication-recrystallization treatment. The micromorphologies and microstructures of the core-shell catalysts are investigated using electron microscopies and physical adsorption methods. The mZ@nZ catalyst features a self-supporting face-centered cubic macrostructure with a layer of compositionally heterogeneous ZSM-5 zeolites, which comprises a catalytically benign nano-ZSM-5 shell (Si-rich) and a catalytically active ZSM-5 core (Al-rich). Abundant mesopores are created in mZ@nZ through desilication and zeolite agglomeration processes. This design offers three significant advantages: (1) enhanced active site accessibility via in situ zeolite growth, (2) surface passivation using a Si-rich shell to minimize side reactions, and (3) improved diffusion facilitated by the hierarchical structure, reducing light olefin secondary reactions and coke formation. Compared to pristine monolithic ZSM-5 and a reference pelletized catalyst, the mZ@nZ catalyst demonstrates superior performance in terms of higher light olefin yields and selectivities, with enhanced resistance to coking in the model compound and real LDPE pyrolysis oil cracking reactions. This research showcases a rational design approach at both macroscopic and microscopic scales, offering valuable insights for advancing polyolefin waste upgrading and novel zeolite catalyst design.