Simulated autohydrolysis of aspen milled wood lignin in presence of aromatic additives. Changes in molecular weight distribution

The reactions between aspen milled wood lignin (MWL) and 2‐naphthol, resorcinol, and p‐hydroxybenzoic acid were studied under simulated autohydrolysis conditions. The material after the reaction was separated into a dioxane‐insoluble fraction (DI), a dioxane‐soluble but ether‐insoluble fraction (DSEI), and an ether‐soluble fraction (ES). In the absence of additive, the lignin first depolymerized and remained solvent soluble, then repolymerized to become insoluble. With 2‐naphthol as additive, the amount of DI lignin decreased proportionally to the amount of additive present. When more than 0.1 mole of 2‐naphthol/C‐9 unit was present, no DI lignin was formed, that is, all the lignin was soluble. Gel permeation chromatography studies indicated that the additive was acting effectively as a blocking agent, preventing lignin repolymerization. Small amounts of resorcinol acted as a repolymerizing agent since two or more fragments of lignin could readily condense with a molecule of the very active additive. As a result of this bridging effect, the amount of DI lignin increased. As the amount of resorcinol was increased, it behaved more like a blocking agent and a reduction in the amount of DI lignin was observed. The presence of a large excess of resorcinol resulted in the formation of soluble lignin only. The molecular weight distributions supported the proposed dual role of resorcinol as a bridging or blocking agent. The use of an excess of p‐hydroxybenzoic acid prevented the formation of DI material and resulted in low molecular weight soluble lignin.