Reactivity of high and low molar mass lignin in the laccase catalysed oxidation

摘要

Residual lignins from pine kraft pulp (KraftRL), oxygen delignified pine kraft pulp (KraftOORL), and fractionated KraftRL were treated with laccase. The reactivity of lignin substrates in laccase reaction (oxygen uptake) and alterations in molar mass distribution and in content and type of phenolic hydroxyl groups were analysed. KraftRL was fractionated according to the molar mass. Each fraction had an equal level of phenolic hydroxyl groups. The lower the molar mass, the higher was the reactivity and the degree of polymerization. The results indicate that the molar mass is critical for polymerization and not the content of free phenolic hydroxyl groups. Generally, the content of free phenolic hydroxyl groups was only slightly decreased in these polymerization and depolymerization reactions although they are the sites of primary oxidation. Also the structure of lignin was shown to affect the reactivity. The reactivities of KraftRL and KraftOORL were similar when evaluated by oxygen uptake. Although both were polymerized, the polymerization of KraftOORL was not as strong as that of KraftRL. Obviously the competing depolymerization reactions were more pronounced in KraftOORL than in KraftRL.