Chemical and radiation-chemical radical reactions in lignocellulose materials

Chemical and radiation-chemical radical reactions in lignocellulose materials were explored by 3-cm and 2-mm ESR spectroscopy. Background (intrinsic) singlet signals at g=2.003 from wood pulp and lignin and those arising during reaction of lignocellulose materials with acids and chlorine were attributed to radicals with conjugated C--C bonds. The 2-mm ESR signal with 3D anisotropy of g-factor from o-semiquinone radical ions formed in reaction of lignin with NaOH was recorded for the first time. The singlet signals derived from cellulose γ-irradiated at 77 K and marked out during post-thermal reactions were assigned to radicals with conjugated bonds. In wetted cellulose, a triplet signal with αβH≅2.7 mT and imposed quadruplet structure (0.5-0.7 mT) from three γ-protons was detected at 300 K and attributed to S4-radicals. The triplet signals derived from S2- and S3-radicals in pyranose cycles of cellulose exhibited higher values of αβH (3.0-3.2 mT) and lower thermal stability (up to 250 K). In radiolyzed cotton pulp, detected were ESR signals derived from formyl radicals formed upon rupture of the S5--S6 bond in pyranose cycles. Heating up irradiated samples under O2 was accompanied by formation of peroxide radicals. Photoinduced recombination of trapped electrons with S1-radicals was found to proceed as a chain reaction with a kinetic length of about 25 units. Photolysis (λ≥360 nm) of radiolyzed cellulose enhanced the disclosure of pyranose cycles and, as a result, the evolution of CO2 by a factor of 2-2.5.