Origin of the mediator losses in electrochemical delignification processes: primary and secondary reactions of violuric acid and N,N′-dimethylvioluric acid radicals with lignin model compounds

Abstract

The use of mediator-based processes for the delignification of wood pulps is of great interest as they promise to be environmentally as well as economically interesting alternatives to the currently dominating chlorine-based processes. Mediator-based processes suffer from the fact that substantial amounts of the mediator molecules are irreversibly lost during the reactions. We have now analyzed in detail the primary and in addition the secondary reactions of the oxime type mediators violuric acid and dimethylvioluric acid with lignin model compounds and softwood pulp in order to find the reasons for the repeatedly reported loss of mediator molecules during enzymatic and electrochemical delignification processes. Using a set of lignin model compounds, representing the various structural subunits in the lignin network, we were able to demonstrate that the loss of mediator is not due to the primary reaction of the mediator radicals with the lignin but occurs in a secondary reaction where the activated lignin subunits react with the mediators in the solution. The primary reaction of the mediator radicals is hydrogen abstraction from phenols and activated aromatic rings which leads to the formation of phenoxyl and phenyl radicals. The secondary reaction is the formation of colored semi-stable covalent adduct molecules from an activated lignin subunit with a mediator molecule in the solution leading to polycyclic N-hydroxy compounds. Only in this secondary reaction mediator molecules are removed from the solution due to covalent attachment to the lignin network. No reaction with the cellulose content in the pulp is observed. The findings reported in this paper point the way to an improved mediator design and appropriately modified processes, where the secondary mediator reactions are suppressed, and removes a major road block towards a technical application of the mediator-based delignification procedures.