Catalytic co-pyrolysis of sewage sludge and softwood using CuO/CoO/CeO2/HZSM-5 hybrid catalysts: Influence of catalyst synthesis techniques on bio-oil product yields and quality

This study investigates the catalytic co-pyrolysis of sewage sludge (SS) and softwood (SW) using CuO/CoO/CeO 2 /HZSM-5 hybrid catalysts synthesized via co-impregnation (CI) and sequential impregnation (SI). X-ray diffraction confirmed the successful incorporation of metal oxides while preserving the parent MFI zeolite structure. Compared with CI, the SI catalyst exhibited superior metal dispersion, higher Cu (18.9 wt%) and Co (11.8 wt%) loadings, and a more uniform surface morphology. BET analysis revealed that SI possessed a larger surface area (205.78 m²/g) and higher pore volume (0.06 cm³/g) than CI (187.84 m²/g, 0.03 cm³/g), indicating better preservation of the zeolitic textural properties. Thermogravimetric analysis further showed greater thermal stability for the SI sample. In pyrolysis experiments, SI achieved the highest feedstock conversion (66 %) and gas yield (42.6 wt%), alongside reduced biochar formation (34 wt%). Conversely, CI produced bio-oil with a higher aromatic content (34.31 %), dominated by phenol (20.61 %). Elemental analysis showed that the non-catalytic SS–SW system yielded bio-oil with the highest carbon content (68.97 wt%) and heating value (32.29 MJ/kg), while the CI catalyst offered an optimal trade-off between heating value (28.61 MJ/kg), oxygen reduction, and hydrocarbon enrichment. These findings demonstrate that the catalyst synthesis method strongly influences phase structure, metal dispersion, and pyrolysis product distribution, providing key insights for the rational design of catalysts for efficient waste-to-biofuel conversion. • Direct comparison of co- and sequential impregnation for hybrid catalysts. • Both catalysts retained the MFI zeolite structure after metal oxide incorporation. • CI catalyst yielded 34.31 % aromatics and highest phenol content (20.61 %) in bio-oil. • SI catalyst maximized gas yield (42.6 wt%) and conversion efficiency (66 %).