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

One kraft lignin and two lignosulfonates were oxidized in aqueous acidic solutions containing a polyoxometalate (POM). The degradations were carried out in H2O or MeOH/H2O mixtures in the presence of oxygen. The treatment with aqueous H3PMo12O40 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 intermediate

Acidic oxidation of kraft lignin into aromatic monomers catalyzed by transition metal salts

As one of the three main components in woody biomass, lignin is an abundant but underused renewable raw material and carbon source. Owing to its aromatic structure and large availability as a by-product of pulping, its conversion into chemicals is highly attractive. In the present work, the oxidation of a softwood kraft lignin in acidic media was investigated in the presence of a homogeneous catalyst. The objective was to find a cheap but efficient catalyst for the depolymerization of kraft lignin into aromatic monomers. Different transition metal salts were screened and compared to phosphomolybdic acid, which was investigated in previous studies, and to experiments in sulfuric acid without additional catalyst. Vanillin and methyl vanillate were the main monomeric products detected by gas chromatography/mass spectrometry but their formation was only slightly increased by using transition metal salts (up to 6.28 wt% yield). However, the presence of iron or copper chloride resulted in fast formation kinetics and significant amounts of other monomeric products. In addition, an efficient fragmentation of the lignin molecule from a weight-average molecular weight of 3500g mol-1 down to 500g mol-1 was observed by size-exclusion chromatography. The enhanced incorporation of oxygen into the reaction products in the presence of those catalysts was proven by Fourier transform infrared spectroscopy and the influence of the catalyst concentration was studie

Acidic oxidation of kraft lignin into aromatic monomers catalyzed by transition metal salts

ISSN:0018-3830ISSN:1437-434