Lewis-acid catalyzed depolymerization of Protobind lignin in supercritical water and ethanol

• Lewis acids (metal acetates, chlorides and triflates) used for lignin conversion.
• Supercritical ethanol and supercritical water compared.
• Char formation dominates in supercritical water.
• Supercritical ethanol inhibits char formation, high yield of monomeric products.
• Ethanol alkylates reaction products.

The use of metal acetates, metal chlorides and metal triflates as Lewis acid catalysts for the depolymerization of soda lignin under supercritical conditions was investigated. The reactions were carried out at 400 °C in water and ethanol. Lignin conversion in supercritical water led to formation of insoluble char and resulted in low yields of monomeric products. When the reaction was performed in supercritical ethanol, char formation was inhibited and higher yields of low molecular-weight organic products were obtained. The ethanol solvent was also converted in two ways. Firstly, the lignin depolymerization products were alkylated by ethanol. Secondly, ethanol was converted into a range of higher hydrocarbons including paraffins and olefins. Possible mechanisms of the lignin and ethanol conversion reactions are discussed.

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