Lignin Depolymerization and Deoxygenation In Ionic Liquids
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Lignin, an amorphous aromatic biopolymer that comprises up to 30% of plant matter, was dissolved and depolymerized in ionic liquids under acidic conditions. The acidic ionic liquid 1-methylimidazolium chloride (HMIMCl) acts as a combination solvent and catalyst that facilitates degradation of lignin into smaller fragments. This degradation occurs through the catalytic hydrolysis of the β-O-4 ether linkage, the most prominent linkage in the lignin polymer. Studies with guaiacylglycerol-β-guaiacyl ether and veratrylgycerol-β-guaiacyl ether as phenolic and non-phenolic lignin model compounds demonstrated that acid catalyzed degradation of lignin in HMIMCl proceeded efficiently at 150 °C, primarily by eliminating water to produce an enol ether intermediate. This intermediate then undergos hydrolysis of the β-O-4 ether linkage, which is analogous to degradation of the β-O-4 linkage in the lignin polymer. These process conditions were next applied to lignin that was extracted from pine wood biomass through dissolution in the HMIMCl ionic liquid and subsequent precipitation. Analysis with gel permeation chromatography indicates significant decrease in size after treatment in HMIMCl at 150°C for 40 min. After depolymerization, the lignin fragments will be subjected to hydrodeoxygenation (HDO), while in the 1-butyl-3-methyl imidazolium chloride ionic liquid, over supported metal catalysts. Preliminary HDO studies using phenol with ceria- or zirconia-supported copper and nickel catalysts show complete conversion and suggest a viable path to HDO of lignin.