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 NaBH 4 impregnation) were also explored. Increasing the temperature of the catalytic reaction from 450 to 650 °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 NaBH 4 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.