Radiation-thermal decomposition of lignin: Products and the mechanism of their formation (Review)

Resumo

The mechanism of the radiation-thermal conversion of lignin including the formation of aromatic radical cations and their fragmentation resulting in the appearance of phenoxyl radicals is considered. The multipath formation of phenoxyl radicals occurs with the participation of the reactions of molecules with electrons and small radicals (^•Н and ^•СН₃) and electronic excitation relaxation processes. Phenoxyl radicals are characterized by smaller thermal stability in comparison with that of parent macromolecules. The further thermally stimulated decomposition of these radicals results in the release of monohydric and dihydric phenols from a polymeric chain. The most effective liberation of phenols takes place on the surface of lignin particles, whereas the formation of wood charcoal with the participation of unsaturated products dominates in the bulk. The formation of dihydric phenols is intensified in the presence of alkanes in the irradiated sample; this fact is indicative of an important role of ^•Н and ^•СН₃ radicals in the formation of monomeric phenol products.