Abstract
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 [HC4im]3PMo12O40, [HC4im]5PV2Mo10O40, [HC2im]5PV2Mo10O40, and [HC4im]4SiMo12O40 (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.
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