Oxidation of Lignin Model Compounds with Cobalt‐sulphosalen Catalyst in the Presence and Absence of Carbohydrate Model Compound

Abstract

A method was developed to study the activity and selectivity of potential oxygen delignification catalysts. In the method, phenolic and non‐phenolic lignin model compounds (2,2′‐biphenol and veratryl alcohol) were oxidised in the presence and absence of carbohydrate model compound (dextran). In this work, useful information was obtained about the catalytic performance of the water‐soluble cobalt‐Schiff base complex, Co‐sulphosalen. Under the reaction conditions typical for industrial oxygen delignification (T = 90°C, pH = 12, p(O2) = 8 bar), Co‐sulphosalen was an active catalyst in the oxidation of 2,2′‐biphenol and veratryl alcohol. Unfortunately, it also catalyzed the depolymerization of dextran. The generation of hydrogen peroxide and the reactions of hydrogen peroxide played a central role in the Co‐sulphosalen catalyzed oxidation of the lignin compounds and especially in the depolymerization of dextran. The selectivity of the delignification reactions was improved by eliminating the effect of the generated hydrogen peroxide by adding phenylglyoxylic acid to the reaction mixture.