Studies on the reactions of phenolic and non-phenolic lignin model compounds with chlorine dioxide

The reaction between chlorine dioxide and four different lignin model compounds (phenolic and non-phenolic, with or without an alpha-hydroxyl group) are studied. The key difference with previous studies is the effective elimination of hypochlorous acid by sulfamic acid during chlorine dioxide treatment. The elimination of hypochlorous acid, an important intermediate formed during chlorine dioxide bleaching, makes it possible to study the consumption of the lignin model compounds solely as a result of the action of chlorine dioxide. Based on almost closed mass balances for the substrate and atomic chlorine it is found that the reaction between ClO2 and lignin model compounds is generally characterized by three independent parallel reactions: 1) demethylation, 2) formation of 2-methoxy-p-quinone and 3) formation of muconic acid monomethyl ester and/or its derivatives. Although, the importance of each reaction depends on the functional groups of the substrate, the demethylation reaction is the most important. It is shown that non-phenolic lignin model compounds do react with ClO2 when ClO2 is supplied in large excess. However at conditions representative of industrial ClO2 application levels, only the phenolic substrates react with chlorine dioxide when both phenolic and nonphenolic lignin model compounds are present. Contrary to the previous model compound studies done without effective elimination of hypochlorous acid, no chloroaromatic material is detected in the present investigation. This supports the view that the reaction intermediate hypochlorous acid (or chlorine) is solely responsible for the formation of chloro organic material during chlorine dioxide bleaching. Reaction mechanisms of the degradation of phenolic and non-phenolic lignin model compounds by ClO2 are also discussed.