Lignin degradation in oxygen delignification catalysed by [PMo7V5O40](8-) polyanion. Part II. Study on lignin monomeric model compounds

Vanillyl and veratryl alcohols were used as monomeric lignin model compound s in experiments aiming to enhance the understanding of the lignin oxidatio n mechanisms involved in the oxygen delignification in acidic media catalys ed by [PMo7V5O40](8-) (HPA-5) heteropolyanion. Based on the model compound experiments, the first step of lignin oxidation is suggested to be a one-el ectron oxidation by the catalyst. This step is followed by coupling reactio ns and/or by further oxidation by the catalyst. The ratio of coupling/oxida tion reaction products depends on the oxidation conditions and decreases wi th temperature raise. The higher oxidation rate of vanillyl alcohol as comp ared to the veratryl alcohol in the reaction system O-2/HPA-5 was explained by the redox properties of the catalyst. The insufficiently high redox pot ential of HPA-5 did not allow the efficient oxidation of veratryl alcohol. The former was oxidised by VO2+ ions dissociated from the HPA-5 under the a cidic conditions. The reactivity of different lignin structural units in th e reaction system O-2/HPA-5 was estimated by electrochemical methods using various lignin model compounds.