Effects of Fe-, Zr-, and Co-Modified Zeolites and Pretreatments on Catalytic Upgrading of Biomass Fast Pyrolysis Vapors

To evaluate the influence of metals on the catalytic activity of HZSM-5 zeolite, the catalytic fast pyrolysis of sawdust was investigated using a vertical two-stage reactor with Fe- (different loading amounts), Zr-, and Co-modified zeolite catalysts. Simultaneously, the effects of biomass pretreatments (HCl, NaOH, and NaBH4 impregnation) were also explored. Increasing the temperature of the catalytic reaction from 450 to 650 degrees C led to simplification of the composition of the bio-oil, and there was a marked increase in the content and yield of aromatic hydrocarbons. With respect to modification of the catalyst, 4Co/HZSM-5 produced the highest gas and coke yields, whereas 4Fe/HZSM-5 and 4Zr/HZSM-5 exhibited similar effects on the product yields and demonstrated better features in the catalytic process than HZSM-5, resulting in a content of over 45% aromatic hydrocarbons in the generated bio-oil. The 4Zr/HZSM-5 catalyst promoted the formation of more benzene and its derivatives, whereas 4Fe/HZSM-5 produced a higher yield of naphthalene and its derivatives. With increased Fe loading, catalytic deoxygenation and hydrocarbon formation became more effective. A reaction pathway leading to aromatics is proposed to describe the catalytic pyrolysis process. HCl and NaBH4 pretreatments largely promoted the formation of hydrocarbons, whereas NaOH pretreatment was favorable for the formation of phenolics. The combined utilization of biomass pretreatments and 4Fe/HZSM-5 catalyst was found to be an effective method for upgrading bio-oil.