Phenolation of lignin has been used in the past to study lignin structure.  However, phenolation can also substantially improve the reactivity of lignin for phenol substitution in the synthesis of phenol-formaldehyde resin. A simplified, economically viable process has been developed for the phenolation of lignin using low acid dosages. The exhaustive description of phenolated lignin structures as well as the phenolation mechanism is still deficient. In the present study, mill wood lignin isolated from loblolly pine and hardwood autohydrolysis lignin were phenolated in the presence of sulfuric acid as catalyst. Sophisticated characterization has been undertaken by elemental composition, methoxyl-content analysis and various NMR techniques to elucidate the phenolated lignin structures and phenolation mechanism. The major lignin substructures that reacted include β-O-4’, β-5’, β-β’ and dibenzodioxocin, resulting in the incorporation of phenol at C<sub>α </sub>and C<sub>β </sub>positions. The γ-hydroxyl groups underwent significant elimination reaction by releasing formaldehyde, only small amount of it remained in phenolated lignin. The released formaldehyde caused further phenol incorporation at C5 and C6 positions of the aromatic ring. The highly conjugated intermediates such as vinyl ethers and stilbenes are favored during phenolation process. The results indicated that the structures of phenolated lignins from softwood and hardwood are similar. Phenolated lignins are composed of mainly of triphenylethane units and diphenylmethane units. The condensed structures such as biphenyl, 4-O-5’ survived the phenolation reaction.

Elucidation of the phenolated lignin structures and phenolation mechanism

255th National Meeting (2018)

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