Stabilizer chemistry of the lyocell process: II—Chemistry, kinetics and products of H2O2-bleaching of ellagic acid and its oxidation products

Ellagic acid (EA, 1 ) is the main transformation product of the stabilizer propyl gallate in the lyocell process and, with its oxidation product, the bis( ortho -quinone) derivative ellagiquinone (EQ, 3 ), can contribute significantly to the discoloration of spinning dopes and regenerated fibers. While the preceding part of this study addressed the solubility, (re)-adsorption, and general oxidation chemistry of these compounds, the present work systematically investigates their kinetics, mechanism, and degradation products upon alkaline hydrogen peroxide bleaching under conditions relevant to industrial fiber finishing ( i.e . , P-stage bleaching at pH 10). Kinetic studies by UV/Vis spectrophotometry revealed that the bleaching of EA follows pseudo -first-order kinetics with respect to EA/ellagiquinone, exhibiting a moderate activation energy of 73.72 kJ/mol and a significantly negative activation entropy (− 101.38 J·mol −1 K −1 ), indicative of a highly ordered transition state in the rate-determining step. Bleaching kinetics of EA and ellagiquinone were found to be identical, demonstrating that the initial oxidation of EA to ellagiquinone is fast and that the rate-determining step occurs in the subsequent degradation of ellagiquinone to the final products. Product analysis by GC/MS and NMR spectroscopy identified the reaction intermediates as carbonate, oxalic acid, mesoxalic acid (= ketomalonic acid), croconic acid, and rhodizonic acid, with their relative proportions varying as a function of reaction time. The observed degradation pathways involve oxidative ring opening of ellagiquinone, followed by parallel chain-shortening and ring-closure processes, ultimately yielding oxalate and carbonate as stable end products. These findings provide a comprehensive mechanistic understanding of the bleaching chemistry of EA-derived chromophores, offering a rational basis for optimizing industrial fiber bleaching protocols in lyocell fiber production.