Characteristics of lignin structural conversion in a phase-separative reaction system composed of cresol and sulfuric acid

The chemical structures of lignocresols derived from native lignins by the phase-separative treatment with cresol and sulfuric acid were characterized by spectral analyses and chemical degradations. The changes in the structure of lignin during the conversion process were discussed. Lignocresol had few conjugated systems, being pinkish white, its brightness comparable to milled wood lignin. Spruce Lignocresol included 0.64 mol/C-9 of cresolic nuclei in the molecule (0.9 mol/C-9 in birch lignocresol), 77% of which were linked to lignin C alpha-positions through carbon-carbon linkages, 16% possibly to C gamma-positions, and the remaining 7% etherified to lignin side chains through its phenolic hydroxyl groups. The molecular weight <((M)over bar w)> of lignocresol was ca. 3500 in spruce, lower in birch. Most of the beta- and gamma-positions in the side chains of C-9 units remained intact, except the coniferyl alcohol and aldehyde units. These structural features were constant during the reaction time up to 60 min. It is concluded that the fragmentation of lignin in the phase-separative reaction system was principally due to the cleavage of benzyl aryl ethers, and the skeleton of lignocresol represents those of lignin subblocks formed by the dehydrogenative polymerization of monolignols.