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 PMo 12 O 40 , [HC 4 im] 5 PV 2 Mo 10 O 40 , [HC 2 im] 5 PV 2 Mo 10 O 40 , and [HC 4 im] 4 SiMo 12 O 40 (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. • Oxidative depolymerization of AL in aqueous ILs/POM-ILs was investigated. • AL conversion of 76.0%. was achieved under O 2 pressure of 2.2 MPa at 150 °C. • A cooperative catalytic effect between acidic IL and POM-IL can be found. • Cation and anion types of POM-ILs and acidic ILs affected product distributions.