Reactions of Lignin with Peroxymonophosphoric Acid: The Degradation of Lignin Model Compounds

Summary Peroxymonophosphoric acid (Pp) has been shown to be an excellent delignification chemical for kraft pulps. However, the reaction mechanism of Pp with lignin has not been elucidated. Therefore, the reactions of Pp with non-phenolic lignin model compounds have been investigated at 70 and 90°C and compared to those of peroxymonosulfuric acid (Px). Pp and Px appear to follow similar reaction pathways, however, the reaction kinetics are significantly different. Both Pp and Px react with 1,2-Dimethoxy-4-methyl-benzene as electrophiles, with aromatic hydroxylation being the main initial reaction. For 1-(3,4-Dimethoxy-phenyl)-ethanone, both Px and Pp react via the Baeyer-Villiger reaction, in which the initial step is nucleophilic attack by the peracid.The reaction with 1-(3,4-Dimethoxy-phenyl)-ethanol occurs primarily via carbocation formation followed by subsequent reactions with the peracid. Changes in pH dramatically affect the reaction rates of Pp with all the model compounds investigated, whereas with the exception of the Baeyer-Villiger reaction, changes in pH did not affect the Px reactions. Below pH 2, Pp is a better electrophile than Px, but above pH 2, Px is better. In addition, while Px remains reactive, Pp is not reactive at pH above 7 for the model compounds investigated.