Date of Award
7-26-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Lignocellulosic biomass is produced photosynthetically from atmospheric CO2 and is one of the few sources of renewable, reduced carbon that is generated on a scale comparable to demand for chemical commodities. For a sense of scale, the U.S. Energy Security and Independence Act targets the production of approximately 16 billion gallons per year of second-generation lignocellulosic biofuels by 2025. That production would generate approximately 62 million tons per year of dry lignin byproducts. The focus of this dissertation is on the valorization of these lignin components of biomass. Currently, lignin is either discarded or burned for local heat or electricity production and otherwise provides little industrial value. However, techno-economic analysis shows that utilization of lignin is critical to the economic viability of carbohydrate-derived commodity bioproducts. Research has been conducted for decades to identify thermal, thermochemical, and catalytic routes to transform lignin into products that have value beyond its heat content, but commercial implementation has been restricted due to limitations such as the structural diversity of lignin and the refractory behavior of lignin subunits upon deconstruction. Thus, there is a need to explore new lignin processing concepts. While lignin is a highly recalcitrant polymer that evolved over millions of years to resist depolymerization, it does have molecular moieties that represent valuable phenolic and aromatic structures trapped within this polymeric structure that may be amenable to targeted cleavage. Breaking lignin interunit linkages unlocks these phenolic and aromatic moieties that can then be upgraded into other value-added materials and chemicals, but the universe of nonselective side reactions needs to be controlled. The main research goal of this dissertation is to overcome the tendency of lignin to depolymerize into diverse product streams that are not suitable for downstream upgrading. This is done by understanding how best to engineer processes that increase selectivity of β-O-4 bond cleavage at lower temperatures and how the resulting products are best processed further. To that end, this dissertation first curates a framework for how lignin depolymerizes. It then develops an understanding of the kinds of products that are formed from catalytic lignin depolymerization and the mechanisms that naturally occur to produce them. Afterwards, a biorefinery process leveraging these principles is demonstrated. This dissertation contains four major chapters: 1) creating and validating polymer kinetic models for lignin disassembly, recombination, and extraction from biomass, 2) applying these polymer kinetic models in the context of in-situ lignin breakdown reactions to understand dominant chemical pathways, 3) leveraging positive matrix factorization to decompose complex mass spectrometry data and reveal insights about how catalytic lignin depolymerization proceeds into specific detectable products, and 4) demonstrating a hybrid conversion approach that combines the advantageous kinetics of thermocatalytic depolymerization with the selectivity of biological upgrading.
Language
English (en)
Chair
Marcus Foston