The Difference of Reactivity between Syringyl Lignin and Guaiacyl Lignin in Alkaline Systems

The difference of reactivity between syringyl lignin and guaiacyl lignin in alkaline systems were investigated by using syringyl and guaiacyl types of β-aryl ether lignin model compounds, softwood and hardwood dioxane lignins, and softwood and hardwood meals. In the lignin model study, β-aryl ether of syringyl lignin model was cleaved much faster than that of guaiacyl lignin model by soda and soda/anthraquinone treatment. The formation of coniferyl alcohol and sinapyl alcohol from guaiacyl lignin model and syringyl lignin model, respectively, was almost proportional to the cleavage of β-aryl ether under these conditions. Based on the model studies, coniferyl alcohol and sinapyl alcohol from isolated lignin and protolignin by soda and soda/anthraquinone treatments were determined in order to evaluate the extent of β-aryl ether cleavage in the terminal units of lignin. The results in both the dioxane lignins study and the wood meals study also indicate that β-aryl ether of syringyl lignin is cleaved much more easily than that of guaiacyl lignin and that there is no difference of reactivity of guaiacyl lignin in hardwood lignin and softwood lignin. This high reactivity of syringyl lignin may contribute the faster delignification rate of hardwood than softwood.