A comparison of the reactivity and efficiency of ozone, chlorine dioxide, dimethyldioxirane and hydrogen peroxide with residual kraft lignin

Residual lignin isolated from a conventional softwood (picea mariana) kraft pulp was reacted in homogeneous solutions with varying charges of ozone, chlorine dioxide, dimethyldioxirane (DMD) and alkaline hydrogen peroxide, followed by quantitative P-31 NMR analyses of their functional groups. This effort supplied plots of relative reactivity of these reagents toward specific functional groups present in lignin, and it revealed salient features that control their reactivity and efficiency. The guaiacyl phenolic units, present on the lignin, were found to be the major sites of attack for all the surveyed oxidative treatments. At a given charge, ozone and chlorine dioxide were found to be most efficient in causing the formation of carboxylic acids on residual kraft lignin. Alternatively, low charges of ozone and DMD may act as activating stages prior to another bleaching stage i.e chlorine dioxide and/or alkaline hydrogen peroxide, since they were found to introduce free phenolic hydroxyl groups in residual lignin. Most of the condensed phenolic units present in residual kraft lignin react with the bleaching reagents surveyed. However, it is likely that certain condensed structures are resistant to attack. At high charges of alkaline hydrogen peroxide the extent of condensed phenolic unit elimination was similar to those observed for chlorine dioxide and ozone.