Dual-catalyst catalytic pyrolysis of poplar sawdust: A systematic study on first-layered catalysts

Dual-catalyst catalytic fast pyrolysis (CFP) technology is effective in converting biomass waste into hydrocarbon-rich bio-oils. However, systematic studies on dual-catalyst biomass CFP are limited, especially with respect to the best-performed dual catalyst design. In this context, dual-catalyst CFP of poplar sawdust was conducted with 18 in-situ first-layered catalysts, including 6 inorganic salts (NaCl, KCl, MgCl₂, ZnCl₂, K₃PO₄, and K₂HPO₄), 7 metal oxides (SiO₂, Al₂O₃, CaO, MgO, ZnO, Fe₂O₃, and CuO), and 5 porous materials (SBA-15, MCM-41, Y zeolite, Beta zeolite, and SAPO-34), while the ZSM-5 zeolite was fixed as an ex-situ second-layered catalyst. The effects of the 18 first-layered catalysts on bio-oil production were investigated and compared. Applying the porous materials as the first-layered catalysts increased the bio-oil yields compared to the inorganic salts and metal oxides. In particular, mesoporous SBA-15 combined with ZSM-5 led to the highest bio-oil yield of 19.4 wt% and a satisfactory hydrocarbon selectivity of 30.5 area%. The synergistic effects of the dual-layered catalysts were also studied. The first-layered catalysts cracked the primary biomass volatiles into smaller oxygenates, while the second-layered ZSM-5 deoxidized these molecules to hydrocarbons. In addition, the porous materials exhibited better synergy with ZSM-5 by simultaneously deoxygenating all the oxygenate types. The dual catalysts configured by Beta + ZSM-5 converted the most oxygenates into hydrocarbons, and the hydrocarbon selectivity (41.4 area%) was twice that in the blank experiment (Thermal/ZSM-5). Therefore, combining the porous materials and ZSM-5 delivers a promising catalyst for dual-catalyst biomass CFP.