Strong interactions during lignin pyrolysis in wood - A study by in situ probing of the radical chain reactions using model dimers

The β-ether type lignin model dimers [1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-propanol and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1-propanol] have been pyrolyzed in the presence of wood and its constituent polymers (i.e., milled wood lignins, cellulose, and hemicelluloses) in a closed ampoule under N2 at 200-350 °C. A “dimer probe method” based on changes in the reactivity of the dimers in this system has been developed to study lignin radical chain reactions in wood. Two pathways for the cleavage of the β-ether linkages proceeded in the presence of lignins to give the corresponding Cα = Cβ and Cα = O monomers at temperatures as low as 200-250 °C, where the pure dimers were stable. This result suggests that the radical chain reactions of lignins proceed within this temperature range. The effects of the wood polysaccharides on the reactivity were also studied to develop a deeper understand of their influence on the lignin reactions in the cell wall. Interestingly, the effects of the wood polysaccharides were determined to be significant and highly dependent upon the type of polysaccharide and the pyrolysis temperature. For hemicellulose, xylan activated both phenolic and non-phenolic dimers, whereas, glucomannan strongly inhibited the non-phenolic dimer. Notably, cellulose exhibited inhibitory effects at temperatures below 300 °C, although these effects disappeared at temperatures greater than 350 °C, where cellulose decomposes rapidly. The results obtained for two different wood samples [Japanese cedar (Cryptomeria japonica, a softwood) and beech (Fagus crenata, a hardwood)] could be explained in terms of the sum of the influences of the wood constituent polymers. These results therefore, suggest that the radical chain reactions of the lignin in beech occur much more effectively than those in cedar, most likely because of the differences in their hemicellulose compositions. Parameters having an influence on the selectivity of the reaction pathway have also been discussed in terms of the hydrogen abstraction reactivity.