Production of Vanillin by Oxidation of Pine Kraft Lignins with Oxygen

Abstract

Vanillin was obtained by oxydation of pine kraft lignins from Westvaco and Portucel with oxygen in alkaline solution. In addition to vanillin, acetovanillone is also obtained when the oxidation is carried out under high oxygen partial pressure. Vanillin concentration vs. reaction time curve shows a maximum. Therefore, vanillin undergoes degradation during the oxidation process, the rate of which depends on the oxidation conditions. Relative molecular mass distribution (RMMD) of the lignin does not change during the heating-up and hydrolysis (120-130 o C for 120 min) phases. However, it changes with concomitant formation of vanillin when oxygen is introduced into the reaction mixture in the reactor after hydrolysis phase. RMMD of the resulting degraded lignins becomes almost the same at the end of the oxidation period (reaction times between 280 and 360min). The reaction time needed to reach the maximum vanillin concentration in the reaction mixture decreases with increasing oxygen partial pressure. By contrast, higher total pressure improves vanillin production. On the other hand, the degradation of vanillin is larger and the vanillin yield decreases, even approaching to zero, if the reaction is not interrupted at the proper reaction time. Temperature has a similar effect on the vanillin concentration. Increase in lignin concentration does not result in improvement of the maximum vanillin yield at all concentration levels investigated; the higher the concentration the lower the yield. Sodium hydroxide reduces the vanillin degradation rate. Pine kraft lignin from Westvaco does not produce vanillin by oxidation with copper (II) sulfate pentahydrate. The pine kraft lignin produces vanillin in higher yield on nitrobenzene oxidation. The kraft lignin from Pinus pinaster (Portucel) produces vanillin in slightly lower yield. On direct oxidation with oxygen at alkaline pH, the black liquor from the kraft cook of P. Pinaster produces vanillin in higher yield than the oxidation of corresponding isolated kraft lignin under the same reaction conditions