Spatial proximity between metal oxides and zeolite: a critical factor governing bio-oil production in biomass pyrolysis

Metal oxide–zeolite bifunctional catalysts have demonstrated remarkable effectiveness in biomass catalytic fast pyrolysis (CFP) for the producing hydrocarbon-rich bio-oil. However, the mechanistic role of spatial proximity between metal and acid sites in governing pyrolysis reaction pathways remains insufficiently understood. To address this research gap, this study systematically investigates how the metal–acid proximity governs bio-oil production in biomass CFP. Initial screening of various metal oxides (In₂O₃, ZrO₂, CaO, and ZnO) integrated with ZSM-5 zeolite results in the identification of In₂O₃ as the most effective metal species. Consequently, a series of In₂O₃-ZSM-5 catalyst architectures are designed to probe the effect of metal–acid proximity: dual-bed In₂O₃//ZSM-5 (with millimeter intimacy), physically mixed In₂O₃ + ZSM-5 (with micrometer intimacy), and In₂O₃/ZSM-5 composites (with nanometer intimacy). Results demonstrate that the nanoscale In₂O₃/ZSM-5 configuration performs best in delivering the highest hydrocarbon selectivity of 43.9 area%, which represents 46% improvement relative to the parent ZSM-5. Mechanistic studies employing 4-ethylphenol as a model compound for pyrolysis, coupled with density functional theory calculations, further elucidate the proximity-dependent reaction pathways. The structure–activity relationships established herein provide fundamental insights to guide the rational design of bifunctional catalysts for biomass thermochemical conversion.