Elucidation of the Structures of Residual and Dissolved Pine Kraft Lignins Using an HMQC NMR Technique

Abstract

Comparative studies on the structures of residual and dissolved lignins isolated from pine kraft pulp and pulping liquor have been undertaken using the ¹H−¹³C HMQC NMR technique, GPC, and sugar analysis to elucidate the reaction mechanisms in kraft pulping and the lignin reactivity. A modified procedure for the isolation of enzymatic residual lignins has resulted in an appreciable decrease in protein contaminants in the residual lignin preparations (N content < 0.2%). The very high dispersion of HMQC spectra allows identification of different lignin moieties, which signals appear overlapped in 1D ¹³C NMR spectra. Elucidation of the role of condensation reactions indicates that an increase in the degree of lignin condensation during pulping results from accumulation of original condensed lignin moieties rather than from the formation of new alkyl−aryl structures. Among aryl−vinyl type moieties, only stilbene structures are accumulated in lignin in appreciable amounts. Benzyl ether lignin−carbohydrate bonds involving primary hydroxyl groups of carbohydrates have been detected in residual and dissolved lignin preparations. Structures of the α-hydroxyacid type have been postulated to be among the important lignin degradation products in kraft pulping. The effect of the isolation method on the lignin structure and differences between the residual and dissolved lignins are discussed. Keywords: Residual lignin; kraft lignin; kraft pulping; NMR; 2D HMQC technique; lignin−carbohydrate complex; condensation reactions; lignin degradation products