Lewis acid-catalyzed depolymerization of soda lignin in supercritical ethanol/water mixtures

Abstract

The depolymerization of lignin model compounds and soda lignin by super Lewis acidic metal triflates has been investigated in a mixture of ethanol and water at 400°C. The strong Lewis acids convert representative model compounds for the structure-forming linkages in lignin, namely α–O–4, 5–O–4 (C–O–C ether bridge), and α–1 (methylene bridge). Only the 5–5′ C–C linkage in biphenyl was unaffected under the given reaction conditions. Full conversion of soda lignin was achieved without char formation. Lignin was converted into a wide range of aliphatic and aromatic hydrocarbons. Ethanol was involved in the alkylation of the lignin depolymerization products. These alkylation reactions increased the product yield by inhibiting repolymerization of the products. The resulting organic phase consisted of aliphatic hydrocarbons (paraffins and olefins), aromatic hydrocarbons (extensively alkylated non-oxygenated mono-aromatics, mainly alkylbenzenes as well as mono-aromatic oxygenates, mainly phenolics), condensation products (mainly naphthalenes) and saturated oxygenates (ketones and carboxylic acids). Although complete product analysis was not possible, the data suggest that the dominant fraction of lignin was converted into monomeric units with a small fraction with molecular weights up to 650g/mol.