Analysis of products from the oxidation of technical lignins by oxygen and H3PMo12O40 in water and aqueous methanol by size-exclusion chromatography

Abstract

One kraft lignin and two lignosulfonates were oxidized in aqueous acidic solutions containing a polyoxometalate (POM). The degradations were carried out in H 2 O or MeOH/ H 2 O mixtures in the presence of oxygen. The treatment with aqueous H 3 PMo 12 O 40 led to the dissolution of the studied lignins in the acidic medium (pH 1-2) and to the formation of up to 6.5 wt% vanillin and 6.2 wt% methyl vanillate based on the weight of dry lignin. The lignin oxidation products were analyzed by size-exclusion chromatography (SEC). For this purpose, a SEC method was developed, which allows the analysis of kraft lignin, lignosulfonates, and reaction products thereof without the need to remove the homogeneous catalyst. This method allows the direct observation of depolymerization and repolymerization reactions and hence the provision of a tool for studying the underlying chemistry. It has been demonstrated that the depolymerization of kraft lignin in water is accompanied by counterproductive condensation reactions. These repolymerization reactions were effectively prevented by addition of methanol, which couples competitively with lignin intermediates.

Lignin repolymerisation in spruce autohydrolysis pretreatment increases cellulase deactivation

“…Wayman and Lora 21 first reported that certain aromatic Fig. 1 Scheme for reactions of lignin in acidic media adapted from Voitl et al 15 (a) cleavage of β-O-4 bond and formation of carbocation; 16 (b) elimination of β-proton giving rise to an enol ether; 16 (c) acid hydrolysis of enol ether; 16 (d) repolymerisation reaction. 17 compounds can act as scavengers in the autohydrolysis of aspen and increase the yield of organic solvent-extractable lignin, allowing to obtain a highly delignified pulp.…”

Multi-catalysis reactions: new prospects and challenges of biotechnology to valorize lignin

“…Due to economic and environmental reasons, production of vanillin from lignosulfonates is a declining process (Sixta 2006). Therefore, Rudolf von Rohr and co-workers investigated an alternative process using kraft lignin as feedstock (Voitl et al 2009;Voitl and Rudolf von Rohr 2010;Werhan et al 2011). Kraft lignin is a by-product of the kraft process, which is presently the most prominent pulping process (Auhorn and Niemela 2007).…”

Lignocellulosics as a Renewable Feedstock for Chemical Industry: Chemicals from Lignin

“…The presence of methanol was found to be necessary to minimise condensation reactions by coupling with reactive intermediates, giving a fifteen-fold enhancement in yield to give approximately 6% vanillin and 6% methyl vanillate [309].…”