Cooperative catalytic effects between aqueous acidic ionic liquid solutions and polyoxometalate-ionic liquid in the oxidative depolymerization of alkali lignin

The oxidative depolymerization of alkali lignin can be achieved by using Keggin structure polyoxometalate-ionic liquid (POM-IL) as catalyst under aerobic conditions. The effects of catalysts such as [HC(4)im](3)PMo12O40, [HC(4)im](5)PV2Mo10O40, [HC(2)im](5)PV2Mo10O40, and [HC(4)im](4)SiMo12O40 (abbreviated as POM-IL1, POM-IL2, POM-IL3, and POM-IL4, respectively), the acidity of aqueous IL solutions on lignin conversion, and aromatic product distribution were investigated. Under optimized conditions, phenolic products (mainly m (p)-cresol, veratrole, vanillin) had the highest yield and the selectivity was as high as 75.0% using POM-IL1 as the catalyst. POM-IL2 and POM-IL3 were used as the catalysts, the highest conversion of 76.0% of lignin was achieved, and the overall yield of ketone products were higher on lignin samples, in which the main products include acetovanillone, 3-methoxyphenol, and 4-methylcatechol, and the highest selectivity of ketone products was 76.8%. The results show that the lignin fragments could be successfully oxidized into platform compounds in aqueous acidic ILs (AILs) solutions catalyzed by Keggin structure POM-IL. In addition, the role of AILs and POM-ILs in the oxidative depolymerization of alkaline lignin was analyzed, and the possible reaction mechanism between AIL-water/POM-IL/lignin system was speculated.

Automated summary

The oxidative depolymerization of alkali lignin was achieved using Keggin structure polyoxometalate-ionic liquid (POM-IL) as a catalyst under aerobic conditions. The effects of different catalysts and the acidity of aqueous IL solutions on lignin conversion and product distribution were investigated. Phenolic products had the highest yield and selectivity when POM-IL1 was used, while ketone products had the highest conversion and selectivity when POM-IL2 and POM-IL3 were used. The results demonstrate the successful oxidation of lignin fragments into platform compounds using aqueous acidic ILs (AILs) catalyzed by Keggin structure POM-IL.