Date of Award
12-2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical and Biomolecular Engineering
Committee Chair/Advisor
Mark Thies
Committee Member
Eric Davis
Committee Member
Mark Roberts
Committee Member
Terry Walker
Abstract
In the emerging bioeconomy, agricultural residues from our nationwide crop harvests are positioned to be a cornerstone of renewable and sustainable fuels, chemicals, and materials. However, to be economically viable, the basic constituents of this plant matter must be separated and funneled to the appropriate application to maximize value and overall usability.
This work focuses on the component that has, to this point, been largely left behind: lignin. Although it is the most abundant aromatic biopolymer on the planet, making up 15-40% of grasses, hardwoods, and softwoods, the processing involved to isolate usable lignins is too complex and expensive to warrant implementation for the high-volume applications lignin is well-suited to address. This is particularly true for agricultural residues, as many pretreatment strategies relegate the lignin to an impurities-laden wash phase that cannot be processed with current technologies.
Curiously, this work follows the reverse order of what one might expect: instead of building from raw biomass pretreatment, lignin-recovery techniques, and ultimately the finishing steps required to produce an application-ready lignin precursor, we began from the precursor and worked our way back to the raw biomass. This unorthodox direction had significant implications on how the project unfolded, and perhaps lent a different perspective that enabled the discoveries made along the way.
With respect to the purification of ag-based lignins, the elevated level of impurities posed a challenge, as the high-value materials that we wished to generate (e.g., carbon fibers) required very low levels of metals contamination. Fortunately, we quickly discovered that our Aqueous Lignin Purification using Hot Agents (ALPHA) process was just as effective at removing impurities from ag-based lignins as from hardwood or softwood lignins; however, tuning the fundamental properties of the lignin polymers (molecular weight, hydroxyl content, etc.) was either ineffective or less-effective than in previous cases.
Following these results, the focus of the overall project ultimately turned to the preceding lignin recovery from biomass step, which generates lignin for the ALPHA process. Notably, the recovery step has historically been the limiting factor for large-scale lignin production from agricultural residues because of the difficulty of the filtration step that separates the lignin precipitate from the lignin-free wash phase after acidification. Fortuitously, we discovered a recovery method that circumvents the filtration roadblock, allowing for a filter-cake-like lignin phase to be isolated by other means. This process, known as lignin Recovery Using Separation by Heat (RUSH), opened the door to vertically integrating biomass-to-precursor processing in-house here at Clemson.
Controlling the entire design space of the raw-biomass-to-precursor process was critical to solving the issue of tuning the molecular properties of lignin. Lignin chemistry is largely defined by the pretreatment step, where many irreversible changes take place, including the cleavage of β-O-4 linkages. Later steps, such as ALPHA, are limited by the properties of the feed lignin, which are dependent on pretreatment. Altering pretreatment conditions to obtain the desired lignin properties, and investigating whether our RUSH and ALPHA processes would retain those desired properties, was key to completing vertical integration. This also allowed for investigation of different pretreatment systems (such as dilute ammonium hydroxide pulping), different feedstocks (hardwood and softwood), and other methods of purification, as covered in this dissertation.
Recommended Citation
Lynn, Bronson, "Isolation and Upgrading of Lignin from Agricultural Sources Using Phase Equilibria" (2024). All Dissertations. 3783.
https://open.clemson.edu/all_dissertations/3783
Author ORCID Identifier
0000-0001-5023-5244
