Decomposition of lignin model compounds by Lewis acid catalysts in water and ethanol

• Catalytic effects of Lewis acidic salts such as metal acetates and chlorides were investigated.
• Representative di-aromatic fragments were used as simple lignin model compounds.
• Reactions were carried out in two different solvent media (water and ethanol).
• High reaction temperatures (300–400 °C) were required to prevent condensation reactions.
• According to the type of the solvent, different reaction mechanisms were predicted.

The conversion of benzyl phenyl ether, diphenyl ether, diphenyl methane and biphenyl as representative model compounds for α–O4, 5–O4, α 1 (methylene bridges) and 5–5′ lignin linkages was investigated. We compared the use of metal chlorides and acetates. The reactions were studied in sub- and supercritical water and supercritical ethanol between 300 and 400 °C. At low temperature in water, Lewis acids mainly catalyzed condensation of hydrolysis products of the dimeric model compounds. At higher temperature, mono-aromatic products were formed. The yield of monomeric products was higher in ethanol than in water. The preference for ethanol is due to extensive alkylation of the mono-aromatic products, which inhibits their condensation into larger products. The highest yields of deoxygenated mono-aromatics were obtained using Lewis acid catalysts at 400 °C in supercritical ethanol. The preferred Lewis acid catalysts were Fe, Cu, Ni and Al chlorides.

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