DEGRADATION MECHANISM OF LIGNIN BY STEAM EXPLOSION .6. STEAM TREATMENT OF MILLED WOOD LIGNIN

The degradation mechanism of lignin by steam treatment was studied usi ng milled wood lignin (MWL) from Pinus merkusii Jungh. and De Vriess. The MWL was subjected to a steam treatment (2.05 MPa-212 degrees C for 8 min) with linter cellulose. For further comparison with the results from pine MWL, wood chips of pine also were subjected to the same tre atment. From the steamed pine MWL, coniferyl alcohol was found to be a major product followed by coniferaldehyde and vanillin, suggesting th at the oxidation and reduction of the coniferyl alcohol radical, forme d by the homolytic cleavage of the beta-aryl ether bond, had occurred. Furthermore, one electron reduction of the radical was a faster proce ss than oxidation to lead to a greater yield of coniferyl alcohol than coniferaldehyde. These compounds scarcely were detected in the degrad ation products of the acidolyzed MWL. On the other hand, beta-oxyconif eryl alcohol (keto and enol forms) and its isomers (2-propanone and 1- propanone) were found as the major degradation products of the pine MW L by acidolysis. The results obtained from the steamed pine MWL was si milar to that from the steamed pine wood chips. Then, it was concluded that the main degradation mechanism of lignin by the steam treatment was not an acidolysis but a homolytic cleavage with the formation of t he coniferyl alcohol radical.