Correlating lignin structure to aromatic products in the catalytic fast pyrolysis of lignin model compounds containing β–O–4 linkages

Lignin model compounds, phenethoxybenzene, 1-methoxy-2-phenethoxybenzene, and 1-phenethoxy-4-propylbenzene were reacted over various zeolites in the vapor phase at 600 °C to elucidate the role of the zeolite type during the catalytic fast pyrolysis (CFP) of molecules containing β–O–4 linkages. The goal was to determine how different zeolites including hierarchical zeolites upgrade lignin fragments, ultimately for converting the lignin portion of lignocellulosic biomass to chemicals or fuels. Using three different catalysts, HZSM-5, H-beta, and HY, the model lignin compounds were upgraded by catalytic fast pyrolysis at 600 °C. The total acidity or Bronsted acidity of the catalysts were held similar when comparing the three different zeolite types to understand the role of each zeolite on the product distribution. The optimal catalyst for the production of benzene from all three model compounds was H-beta, but the least amount of coke was produced with HZSM-5. Compared with the analogous microporous HZSM-5 catalyst, the hierarchical materials resulted in increased liquid carbon molar yields. Although H-beta produced the highest amount of benzene with 1-methoxy-2-phenethoxybenzene, HZSM-5 was able to produce more liquid products from this model lignin fragment. To analyze the catalysts with intact model compounds, vaporization studies were conducted to prevent the thermal cleavage of the Cβ–O bond in the β–O–4 linkages. These vaporization studies resulted in lower liquid yields and higher coke yields. Thus, it is important for CFP to generate fragments of the β–O–4 linkages before catalyst interaction, and catalyst selection for the conversion of lignin should greatly depend on feed compositions.