Date of Award
Doctor of Philosophy (PhD)
Increasing world population has led to great demands for fuels, chemicals, and materials, and has raised concerns over the depletion of non-renewable resources and the environmental impacts of their processing and utilization. However, the biorefinery concept suggests lignocellulosic biomass can be used as an alternative resource for producing a range of fuels, chemicals and materials to fulfill these demands in a sustainable way. Hence, fermentation technologies are widely developed to efficiently utilize lignocellulosic carbohydrates. However, the non-carbohydrate fraction of biomass, lignin, is still considered as waste and is under-utilized as low-grade fuel, mainly for local heat and electricity production. Since lignin comprises 30% of the total carbon in lignocellulosic biomass, the under-utilization of lignin violates a major goal of the biorefinery concept: to efficiently convert the renewable carbon and energy stored in biomass into a range of products with higher value. Meanwhile, the aromatic structure of lignin suggests that selective deconstruction of lignin has great potential in generating fuels and platform chemicals. However, the conversion of lignin into economical transportation fuels and value-added chemicals is currently limited by the insufficient development of conversion technologies. Thus, this dissertation presents four research studies focused on understanding the lignin phenomena required for its selective deconstruction into a narrow distribution of desired value-added products. (Study 1) The structural complexity of lignin makes typical reaction network and kinetic analysis difficult. Thus, using lignin model polymers, this study focuses on understanding the reaction network and kinetics involved in the cleavage of aryl ether linkages (β-O-4) in a polymer via copper porous metal oxides (CuPMO) catalyst in methanol (MeOH). (Study 2) CuPMO catalyzes not only cleavage of aryl ether linkages but also the undesired reduction of aromatic rings. This study focuses on understanding reaction networks that prevent reduction of aromatic rings upon the addition of dimethyl carbonate (DMC) to the lignin depolymerization reaction with CuPMO in MeOH. (Study 3) Instead of using lignin model polymers, another approach to resolve reaction networks and kinetics for lignin is to develop novel methods of analyzing lignin product distributions. This study applies positive matrix factorization (PMF) analysis to gas chromatography-mass spectrometry (GC-MS) data obtained on low molecular weight products in an effort to simplify the analysis of lignin depolymerization. (Study 4) This study exploits in-situ magic angle spinning (MAS) solid-state nuclear magnetic resonance (ssNMR), a novel technique to monitor the depolymerization network and kinetics of lignin. Studies in this dissertation demonstrate that CuPMO catalyst can effectively convert lignin into value-added phenolic products. Moreover, with the addition of DMC, the aromaticity of lignin-derived products can be enhanced by stabilizing phenolic intermediates against further hydrogenation. In addition, this dissertation also illustrates the application of novel techniques to characterize lignin during depolymerization and lignin-derived products after depolymerization. First, PMF analysis is shown to provide valuable structural information on a large number of lignin depolymerization products generated under different conditions. Then, in-situ MAS ssNMR is used to obtain a better understanding of the depolymerization reaction network and kinetics of lignin. Lastly, a future perspective is presented, detailing the probable next stage of study in the conversion of lignin into desired chemicals.
Marcus B. Foston
John D. Fortner, John T. Gleaves, Cynthia Lo, Kevin D. Moeller,