Effect of Oxidation on the Structures and Properties of Lignin

Abstract

The effect of three kinds of oxidants, hydrogen peroxide, copper oxide and ammonium persulfate, on the structures and properties of modified lignin were investigated. It is found out that a low dosage of oxidant can result in the increase in the content of active phenolic hydroxyl and carboxyl. It is most likely to cleave β-O-4 and C-C bond in lignin molecule. Compared to ammonium persulfate and copper oxide, hydrogen peroxide exhibits relatively mild oxidation to lignin. The oxidation of lignin with excessive ammonium persulfate and hydrogen peroxide can cause free radical polymerization of alkali lignin, leading to the increase of molecular weight and the decrease of the sulfonation degree of the sulfonated lignin followed by oxidation and hydroxymethylation (OSAL). Ammonium persulfate exhibits a stronger capacity to initiate free polymerization reaction compared to hydrogen peroxide, so it is suitable to use a low dosage of ammonium persulfate for the oxidation of alkali lignin. Copper oxide could not cause a free radical polymerization, therefore the molecular weight of the sulfonated lignin is lower, and the sulfonation degree is higher. OSAL with a good dispersive effect to cement paste should simultaneously possess a high sulfonation degree and an appropriate molecular weight.