Depolymerization of lignin for biomass processing in ionic liquids
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There is growing need for technologies to displace traditional petroleum resources. Towards this goal, lignocellulosic biomass is seen as a potential renewable resource for the production of fuels and commodity chemicals. One of the most difficult components of lignocellulose to process is lignin, which is a complex, amorphous aromatic polymer that acts as one of the structural components in plants. Ionic liquids are a class of compounds that are composed completely of anions and cations that, in some cases, can completely dissolve lignocellulosic biomass. The research performed for this dissertation aims to advance the technologies of lignocellulose processing through effective depolymerization of lignin in ionic liquids. Lignin fragments from this depolymerization could be used as a feedstock for further processing into aromatic commodity chemicals or polymers. Additionally, by removing lignin, biomass becomes much more accessible to enzymatic or chemical saccharification as a step towards fermentation into ethanol or other fuels. Both base and acid catalyzed methods were explored, although the base promoted depolymerization of lignin in ionic liquids did not show much promise, as the reaction was never shown to be catalytic. Acidic routes towards lignin depolymerization were more successful. Using the acidic ionic liquid 1-H-3-methylimiazolium chloride, the ether linkages in lignin model compounds could be hydrolyzed with high yields. This technology was also applicable to the whole lignin macromolecule. The mechanisms of this reaction, as well as the effects on lignin were explored with various neutral and acidic ionic liquids, using HPLC, GPC, NMR, FT-IR, and mass spectrometry for analysis of samples. To demonstrate the applications of this technique, pine wood was treated with the acidic ionic liquids to open the structure of the wood to enzymatic saccharification through the removal of lignin and hemicellulose.