Oxygen Enhances Polyoxometalate-based Catalytic Dissolution and Delignification of Woody Biomass in Ionic Liquids

Abstract

Complete dissolution and over 90% delignification of Southern yellow pine (<0.125 mm) can be achieved in the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) at 110 °C for 6 h by the catalytic action of polyoxometalate in the presence of an appropriate O2 feed. Cellulose-rich materials (CRMs), or pulps, and hemicellulose with a limited lignin content and free lignin were subsequently recovered by adding antisolvents to the IL solution, followed by filtration. Comparison of wood processing in [C2mim][OAc]/POM with or without O2 revealed that the presence of oxygen can greatly facilitate the dissolution, delignification, separation of hemicellulose, and oxidation of lignin. The main products from lignin oxidation were extracted from the IL using benzene and then tetrahydrofuran, and were shown by gas chromatography−mass spectrometry (GC–MS) to be methyl vanillate, acetovanillone, vanillic acid, methyl 3-(3-methoxy-4-hydroxyphenyl) propionate, and methyl 4-hydroxybenzoate. This study suggests that treating wood with a [C2mim][OAc]/POM/O2 system could be a viable strategy to separate wood components with high efficiency and obtain cellulose with high purity for materials or biorefinery applications, particularly those that desire smaller lignin oxidation fragments for further processing.