Ozonation of a β-0-4 Dimer Lignin Model: By-Product identification and Reaction Pathways

Abstract

In order to better understand the reaction mechanisms that lead to the β-O-4 cleavage and the production of aromatic aldehydes during the ozonolysis of lignin structures, the goal of our work was to determine the degradation pathways of a lignin model compound (β-O-4 dimer: G-G phenolic alkyl-aryl-ether dimer) subjected to molecular ozone in aqueous solution. At low pH (used to favor the molecular ozone reaction mode), the β-O-4 dimer is rapidly dehydrated to produce a styrene derivative. The original dimer and its dehydrated derivative were found to react differently with ozone. The Cβ-H bound of the original dimer is activated by the nearest hydroxyl group which permits an electrophilic addition of molecular ozone, leading to the production of guaiacol and vanillin. Guaiacol is further degraded in para-hydroquinone and para-benzoquinone as end-reaction by-products. Molecular ozone reacts on the dehydrated derivative (styrene) according to a dipolar cyclo-addition which leads to the production of homovanillin and guaiacol. Probably because of the strong reactivity of ozone with activated aromatic ring and possibly due to secondary reaction such as Bayer-Villiger cleavage, vanillin was detected as a trace compound.