Nuclear Magnetic Resonance Studies. 4. Analysis of Residual Lignin after Kraft Pulping

Abstract

Residual lignin from a series of softwood kraft pulps was isolated and structurally characterized by nuclear magnetic resonance. Results of these analyses showed that residual lignin structure differs depending upon the lignin content of the pulp and the pulping technology employed. The lower lignin content pulps had reduced contents of aliphatic hydroxyl groups and β-O-4 structures and higher contents of phenolic hydroxyl groups, carboxylic acid groups, and condensed structures compared to higher κ number pulps. Analysis of residual lignins isolated from conventional kraft pulps (CK) and extended modified kraft pulps (EK) indicated that the CK pulps had lower contents of β-O-4 structures and higher contents of condensed structures. These differences potentially explain the improved delignification selectivity of EK kraft pulping because residual lignin with a higher content of β-O-4 structures and a lower content of condensed structures would be more reactive under pulping conditions.