Effect of side-chain structure on hydrothermolysis of lignin model compounds

Abstract

Hydrothermal-controlled selective cleavage of CβO and CαCβ bonds was well realized via modification the side-chain structure of lignin. To elucidate the reaction pathway and the origin of selectivity, the sites of the benzylic alcohol (Cα–OH) and benzylic ketone (CαO) on the cleavage mechanisms of βO4 lignin dimers were investigated. Results clearly demonstrated the hydrothermolysis process could be divided into three stages: (1) intramolecular elimination reactions, (2) the cleavage of CαCβ or CβO bonds and (3) secondary reactions of intermediates. Reaction pathways were highly susceptible to the side-chain structure. The CαCβ and CβO bonds of 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-ethanone tended to directly cleave and form aromatic monomers. However, many different species dimers were produced via the intramolecular elimination reactions when the hydroxyl group was introduced at the Cα or Cγ position, which participated in a variety of reaction mechanisms, resulting in more complex products distribution. In addition, the results also indicated that dehydration of Cα-OH group significantly facilitated the cleavage of CβO bonds, but oxidation of the Cα-OH into Cα=O group weakened the CαCβ bond. Based on the compositions and structures of products, several cleavage pathways of dimers were proposed, explaining well the origin of selectivity.