Mechanistic studies of milled and Kraft lignin oxidation by radical species
Accomplishing selective lignin degradation in a controlled manner by breaking C–O and C–C bonds is of great scientific interest although technically challenging. The structural modification of two different types of lignin (milled and Kraft) subjected to four sources of radicals (superoxide, sulfate, hydroperoxide, and hydroxyl radicals) was studied for achieving such a purpose. The treatments with superoxide and hydroperoxide radicals increased the number of alkyl group substituents and H lignin subunits while sulfate and hydroxyl radicals preferentially reacted via lignin side-chain oxidation. The efficiency of lignin cleavage, based on reduced molecular weight (Mw) as a result of oxidation, was found to be generally in the order of superoxide, sulfate, hydroperoxide, and hydroxyl radicals initiators. Characterizing the radical degradation products revealed that C–C cleavage of phenyl coumarin and resinol substructures was the predominant mechanism in superoxide radical oxidation, whereas β-O-4 cleavage and side-chain oxidation were the main pathways in the sulfate radical treatment. Moreover, superoxide and hydroperoxide radical treatments shifted lignin thermolysis peaks to lower temperature. However, in sulfate and hydroxyl radicals methods, due to condensation reactions, some stable lignin-like polymers were generated which required higher degradation temperatures. Collectively, the results demonstrated the different ways of lignin structure alteration by different radical oxidation sources and highlighted the reaction features of each radical tested.