Microwave directly accelerated cleavage of C-C and C-O bonds of lignin by copper oxide-peroxide reaction.

Model erythro, phenolic and non-phenolic lignin β -O-4 dimer compounds were reacted with copper oxide and H 2 O 2 at the electronic field maximum position of a single-mode 2.45-GHz microwave system equipped with a cavity resonator. The reaction products obtained via microwave heating and an oil bath heating in the same reaction vessel and same temperature profile were quantitatively compared. Dimer degradation proceeded through consecutive elementary reactions. The phenolic dimer was dehydroxylated and followed by the spontaneous cleavage of C α -C β and C-O-C bonds to produce guaiacol, vanillin, and vanillic acid. The reaction of the non-phenolic dimer produced veratric acid, veratraldehyde, and guaiacol. Microwave irradiation accelerated cleavage of the side chain and the oxidation of vanillin to vanillic acid. However, no acceleration of veratraldehyde oxidation to veratric acid or aromatic ring cleavage to produce dicarboxylic acids was observed. The selective acceleration of elementary reactions during the degradation of model lignin compounds indicated that microwaves interacted with reaction intermediates that were sensitive to electromagnetic wave.