Preparation and Reactions of Diels-Alder Adducts of Lignin-Derived Quinones

Abstract

The goal of chemical pulping is to liberate the carbohydrate fibers from lignocellulosic materials, such as wood, through a delignification process. Pulping catalysts, such as anthraquinone, promote delignification rates such that lower cooking times and less severe conditions can be employed. As a result of their action, catalysts lead to improved selectivity, i.e., preferen- tial degradation of lignin versus degradation of cellulose and other carbo- hydrates. A disadvantage to the use of pulping catalysts is that they are relatively expensive and are lost during the pulping process. However, quinone structures can be generated from lignin through the use of several oxidative techniques. Modification of these quinones through Diels-Alder reactions with various dienes can enhance their stability and give them the characteristics needed to act as delignification catalysts. Although significant catalytic activity is exhibited by naphthoquinone and phenanthrenequinone structures, these compounds do not achieve the high degree of activity demonstrated by anthraquinone. Pulping with an isomeric mixture of 2,6- and 2,7-dimethylanthraquinone, however, has shown that these compounds are superior to anthraquinone in terms of both reduction in kappa number and increase in yield. The most promising synthetic pathway to the production of useful deligni- fication catalysts, such as dimethylanthraquinone, appears to be a two-step approach in which lignin is first degraded into monomeric units, such as vanillin and syringaldehyde, and these smaller compounds are subsequently oxi- dized to methoxy- or 2,6-dimethoxy-p-benzoquinone. -2- Nearly quantitative yields of p-benzoquinones may be obtained from com- pounds containing benzylic hydroxyl groups through the use of potassium nitro- sodisulfonate. Hydrogen peroxide is effective in generating p-benzoquinones and related hydroquinones from compounds containing a-carbonyl groups (up to 96% combined yield). Peracetic acid is an effective oxidant on compounds with either a-hydroxyl groups of a-carbonyl groups; up to a 93% combined yield of quinones and related compounds, which can be easily converted into quinones, may be obtained. Potassium nitrosodisulfonate, peracetic acid, and electrochemical anodic oxidation have been used successfully to generate p-benzoquinones from lignin, but yields are often low. Highest yields (up to 16%) have been obtained with potassium nitrosodisulfonate. Thus, lignin or simple phenolic compounds obtained from lignin may be oxi- dized to methoxy- or 2,6-dimethoxy-p-benzoquinones. These quinones may then be converted into anthraquinone compounds through Diels-Alder reactions using dienes such as isoprene. In the future this may result in the production of low-cost pulping accelerators which could lead to very cost-effective pulping processes.