DEGRADATION MECHANISM OF LIGNIN BY STEAM EXPLOSION .7. STEAM TREATMENT OF SYRINGYL TYPE LIGNIN MODEL COMPOUNDS

To explain the degradation mechanism of lignin by a steam explosion, s yringylglycerol beta-sinapylalcohol ether was used in the present stud y. This model compound was subjected to a steam treatment, 230 degrees C, 2.9 MPa, for 4 min. Steam treatments of sinapyl alcohol-DHP (Dehyd rogenation polymer) (S-DHP), sinapyl alcohol-DHP with mannan or syring yl-lignin carbohydrate complex (S-LCC), and white birch (Betula piatyp hylla, Sukatchev var. japonica, Hara) wood also were conducted. The ma jor degradation product obtained from steamed syringylglycerol-beta-si napylalcohol ether was syringaresinol. Other products found were syrin galdehyde, sinapaldehyde, and dihydrosinapyl alcohol. On the other han d, beta-oxysinapyl alcohol and its isomers as the major degradation pr oducts of acidolysis, were not detected. These results suggest that wi th the conditions used, the beta-ether linkage of syringylglycerol-bet a-sinapylalcohol ether was cleaved homolytically and produced two sinp yl alcohol radicals. Homocoupling of these radicals mainly occurred to give syringaresinol. It is concluded that the main degradation mechan ism of lignin by a steam explosion is not an acidolysis but a homolyti c cutting reaction followed by the homocoupling of the beta-radical to produce syringaresinol.