A Comparison of the Structural Changes Occurring in Lignin during Alcell and Kraft Pulping of Hardwoods and Softwoods

Abstract

During this investigation, softwood and hardwood chips were pulped to different extents using conventional kraft and Alcell protocols. The dissolved and residual lignins were then isolated and their functional group distributions examined using quantitative 31P NMR. For both wood species, the solubilized kraft lignins contained the highest abundance of phenolic hydroxyl groups at all degrees of delignification, while the residual kraft lignins contained the lowest. This may be related to the considerably greater solvating abilities of alkaline aqueous media as opposed to those of ethanol toward inducing solubilization of the phenolic moieties. Consequently, at a given degree of delignification, a greater proportion of phenolic units are retained in the Alcell pulps, contributing to their documented higher reactivity with oxygen. As far as residual lignins are concerned, the data showed that condensed phenolic units are formed in greater abundance within softwood than in hardwood pulps for both processes. More specifically, condensed