Pyrolysis-gas chromatography-mass spectrometry of two trimeric lignin model compounds with alkyl-aryl ether structure

Abstract

Thermal degradation of ω-guaiacoxy-acetoguaiacone-benzylether (A) and ω-syringoxy-acetoguaiacone-benzylether (B) has been investigated for a better understanding of the thermolytic behaviour of lignins. Earlier differential scanning calorimetric studies revealed that degradation starts below 200° C. Direct insertion probe experiments also demonstrated that splitting of linkages between aroxy and acetoguaiacone-benzylether (C) moieties commences at 175° C. As a consequence the presence of the following compounds was observed: guaiacol and C (from A) and syringol and C (from B). Simultaneously to these splitting mechanisms a rearrangement occurs between the ω-carbon of compound C and the phenolic oxygen of the aroxy residues, resulting in the formation of 2-methoxybenzaldehyde and acetovanillone-benzylether (from A) and 2,6-dimethoxybenzaldehyde and acetovanillone-benzylether (from B). Pyrolysis-gas chromatography-mass spectrometry experiments between 240 to 600°C corroborated these results. The higher the pyrolysis temperature the more peaks are displayed in the pyrograms. The pyrograms obtained at 600° C show 40 to 70 major peaks which were evaluated. The relative retention times, M + values and in most cases the assignments to defined compounds are given. Many ‘unknown’ substances could be identified as condensation products derived from benzyl radicals. Flow diagrams illustrate the main splitting mechanisms of compounds A and B.