Safe and Functional Ionic Liquid Systems for Solubilization and Depolymerization Processes
Open AccessWe have utilized the Principles of Green Chemistry to guide our design of processes that enable a circular economy, two important aspects that are important for implementing sustainable practices. Our efforts have been focused on overcoming obstacles that are associated with ionic liquids (IL) and catalysts used for upgrading biomass in hopes of furthering their implementation in biorefinery processes. We addressed the IL toxicity via computational screening and experimental design. Additionally, we were able to lower processing costs of ILs by utilizing them in catalytic amounts as opposed to solvents, which in turn improved mass transfer of these systems. We took advantage of IL’s excellent solvating ability and combined them with heterogeneous catalysts in hopes of mitigating issues associated with active site accessibility to afford more abundant transformations of biomass constituents. Additionally, we utilized IL analogues, deep eutectic solvents (DES), to bypass concerns of ILs not derived from renewable sources and examined the influence of their properties for biomass transformations. Chapter 1 introduces the background of green chemistry and its application in our research. We outline the principles we implement in our own research and our development of processes that enable a circular economy. We discuss the potential of biomass derived platform chemicals as well as give background for ILs, DES, and heterogeneous catalysts. Chapter 2 reviews our design strategies for developing safer ILs. We focus on understanding the structure-property-activity relationships of IL constituents such as the roles of cations and anions toward the dehydration of fructose to 5-hydroxymethyl furfural (5-HMF), an important platform chemical. We determined the structure-property-activity relationships are distinct from the structure-property-toxicity relationships which allows for design for safety without sacrificing function. Chapter 3 begins our developments of lignin valorization with an IL designed for safety and function. We apply a low-cost, dilute IL system toward organosolv lignin depolymerization and optimize solvent conditions. We aim to understand the IL capabilities for fractionating different lignin linkages, and we demonstrate that this IL system is successful and used in substantially lower amounts than previously reported literature. This chapter lays the groundwork for our development of different lignin valorization techniques in hopes of avoiding the common pitfalls of using technical lignins. Chapter 4 continues to build on the work done in Chapter 3. Using our knowledge of the IL capability for lignin fractionation, we combine it with a heterogeneous palladium-doped catalyst with a two-fold goal of i) extracting lignin from whole lignocellulose and ii) tandem depolymerization to small molecules. We demonstrate that there is a synergistic effect between the IL and catalyst likely due to the IL solvation and fractionation capabilities that exposes more lignin to the active sites of the catalyst. Chapter 5 explores biomass-derived IL analogues, DES, and their capabilities toward delignification. We discuss more detail and background of DES. We also aim to understand the structure-function relationships between the hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) and to determine the effect of solvent composition and pH toward lignin extraction of whole pine biomass. Chapter 6 reviews the work done in the previous chapters as well as highlights other opportunities of ILs and heterogeneous catalysts toward industrially and synthetically relevant processes. In addition, the challenges of implementing these methods in industrially scaled up processes is discussed as well as suggestions for future focus from an industrial perspective.
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