The Condensation Reactions of Lignin Model Compounds in Alkaline Pulping Liquors

Abstract

Abstract The significance of condensation reactions at the 5-position of the aromatic ring of lignin to the overall pulping process has been evaluated using lignin model compounds. Reactions of 5-hydroxymethylcreosol, as a model for 5-substituted lignins formed when formaldehyde is released during pulping, have shown that such groups will react via ortho-quinonemethides to give alkali-stable carbon-carbon bonded condensation products. The amount of condensation depends on the type of pulping liquor used, with less formed in the kraft and soda-anthraquinone (AQ) processes than in the soda process. In these liquors the hydrosulphide or anthrahydroquinone (AHQ) ions present can compete effectively with phenoxide ion for the reactive o-quinonemethide intermediate. Reduced AQ reacts with the o-quinonemethide to form oxanthrone- and anthrone-adducts which are still detectable at 170°C. Such reactions, particularly the reduction of AQ to its anthrone oxidation state, may be one of the reasons why AQ is consumed during pulping. Degradation products of glucose have also been found to decrease the amount of lignin-lignin type condensation products.