Reactivity of Lignin and Lignin Models Towards UV-Assisted Peroxide

The comparative reactivities of a series of guaiacyl and syringyl lignin model compounds and their methylated analogues towards alkaline peroxide and UV-alkaline peroxide were investigated. The overall reaction was followed by monitoring the reduction of the substrate as a function of time, and in every case, the reaction showed pseudo-first-order kinetics. The reaction rates of most lignin models having identical sidechains with alkaline peroxide and with UV-alkaline peroxide were in the order syringyl > guaiacyl > 3,4,5-trimethoxyphenyl > veratryl. Thus phenols react faster than their methyl ethers, and an extra ortho methoxyl group promotes the reaction. Lignin models possessing electron-donating side-chains had generally higher reaction rates than those with electron-withdrawing sidechains. The reaction rates of the series of benzoic acids were 2–4 times higher at pH 11 than at pH 5. UV-peroxide degradation of a eucalypt kraft lignin was faster than that of a pine kraft lignin, and degradation was 1.4–1.6 times faster at pH 11 than at pH 5. The data are consistent with the formation of higher amounts of reactive radicals under alkaline conditions, and aromatic rings with greater electronegativities promoting reactions with the radicals.