Photochemically Induced Solid‐State Degradation, Condensation, and Rearrangement Reactions in Lignin Model Compounds and Milled Wood Lignin

Abstract

Abstract— Milled wood lignin produced from alkaline hydrogen peroxide‐bleached softwood thermomechanical pulp (TMP) fibers was adsorbed on pure cellulose and irradiated for variable periods of time under oxygen and/or nitrogen. The absolute amounts of β‐O‐4 ethers, phenolic hydroxyl groups, carboxylic acids and various condensed phenolic units were quantified, nondestructively, using 31P NMR spectroscopy. Photoirradiation was found to severely cleave the β‐O‐4 ethers present in lignin with the concomitant formation of new phenolic units. The rate of this cleavage was found to be faster under oxygen than under nitrogen. The catalytic role of oxygen can be rationalized by invoking the formation of peroxy free radicals that may initiate new sites of radical generation (e.g. ketyl radicals) within the lignin causing the subsequent breakdown of the β‐O‐4 linkage. The photoirradiation was found to cause a net increase of the C5‐related condensed phenolic units in lignin. Among these, Cα‐C5 and/ or Cβ‐C5 phenolic moieties were found to predominate. Similar irradiation of a binary mixture of model compounds resembling structures present in softwood lignin resulted in 18 products that were identified and quantified using gas chromatography coupled to mass spectrometry. The photochemical products obtained supported the phenacyl and ketyl mechanistic pathways to pho‐toyellowing, whereas the detection of β‐5 coupling products (common in both the model compound and milled wood lignin studies) may point a new avenue toward the formation of light‐induced products that has not been previously considered in the solid state.