Singlet oxygen mediated degradation of lignin—isolation of oxidation products from steam-exploded lignin from pine

Abstract

Lignin obtained from steam explosion of pine was fully characterized. Elemental analysis, GPC, and ultraviolet and ¹H and ¹³C NMR spectra revealed that the obtained lignin contains both guaiacyl and syringyl units. Lignin was dissolved in acetonitrile–ethanol and treated with visible light in the presence of both oxygen and Rose Bengal for different irradiation times. Column chromatography of the residue showed the presence of six compounds: trans-sinapyl alcohol, 4-hydroxy-3,5-dimethoxybenzaldehyde, 4-hydroxy-3,5-dimethoxyphenylacetone, 4-hydroxy-3-methoxybenzaldehyde, cis-sinapyl alcohol, and sinapyl aldehyde. The total amount of fine chemicals increases with the irradiation time. However, it increases rapidly during the first eight hours, but increases slowly after this period. The most important compounds obtained were sinapyl alcohol and 4-hydroxy-3,5-dimethoxybenzaldehyde, but sinapyl alcohol was obtained in the highest amounts after eight hours' irradiation, while the highest amounts of 4-hydroxy-3,5-dimethoxybenzaldehyde were obtained after irradiation for 4 h. After 48 h irradiation comparable amounts of sinapyl aldehyde were obtained. We obtained only compounds derived from the syringyl units in lignin in agreement with the hypothesis that the guaiacyl units are more easily oxidised.