Pulping catalysts from lignin .8. Nitrogen dioxide oxidation of lignins to benzoquinones

Several lignin samples have been oxidized with nitrogen dioxide (NO2) in methanol to give low yields of benzoquinones. The yields of 2,6-dimethoxy-p-benzoquinone (DMBQ) from hardwood lignins were highest when the lignin was first extracted to provide a lower molecular weight lignin fraction. The yields of DMBQ were also improved by degrading the lignin to smaller fragments prior to NO2 oxidation. One of the best pretreatments in this regards was heating with copper oxide in aqueous NaOH. Thioacidolysis analysis indicated that p-aryl ether linkages were effectively cleaved by the CuO treatment and that the product mixture contained increased levels of vanillin and syringaldehyde. The best yields of DMBQ from a fractionated lignin and a pretreated lignin were about 10 and 15%, respectively, which corresponds to about 40-60% of the available syringyl units in the lignin. The yields were highest from organosolv Lignins; such lignins are probably less condensed than kraft lignins. The yields were slightly improved when N-hydroxy-succinimide was present.