Date of Award

8-2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Biomolecular Engineering

Committee Chair/Advisor

Mark Thies

Committee Member

David Bruce

Committee Member

Eric Davis

Committee Member

Rhett Smith

Abstract

The demand for sustainable polymers that can serve as precursors for carbon-based products, high-value chemicals, and advanced materials is steadily increasing. Lignin, a primary component of biomass and the most abundant aromatic biopolymer, is gaining attention as a promising option. Its molecular structure, abundance, cost-effective nature, and renewability make it an appealing alternative for applications in the automotive, aerospace, healthcare, and energy sectors.

However, despite its potential, lignin utilization remains limited by drawbacks, including low commercial value due to limited sustainable isolation technologies, poor compatibility with polymers, low molecular weight (MW), low glass transition temperature (Tg), difficulty in purification, and broad molecular weight distribution.

The primary objective of this work is to address these challenges and enhance lignin utilization by improving its commercial value and quality (defined by molecular properties and purity) through processing and characterization using analytical methods—all while preserving its renewable, cost-effective, and energy-efficient nature. The processing and mechanical properties of lignin-based materials strongly depend on the lignin quality (MW, Tg, and purity). For instance, increasing MW and Tg while reducing lignin impurities has been shown to significantly improve the performance of lignin-based materials, particularly carbon materials.

In this work, two processes were optimized to recover lignin from by-products of paper mills derived from both woody biomass and agricultural residues. These approaches enable upgrading lignin from a low-value by-product into a potential co-product alongside cellulose. For woody biomass, the batch Sequential Lignin Recovery and Purification (SLRP) process was optimized for different feedstocks, achieving over 85% lignin recovery from black liquor. For agricultural residue, the batch Lignin Recovery Using Separation by Heat (RUSH) process was optimized, and a continuous proof-of-concept system was developed, yielding approximately 70% lignin recovery. The SLRP process produced lignin with MW values ranging from 8 to 35 kDa and 95% purity, while the RUSH process yielded lignin with MW of 25 kDa and 85% purity. Both processes achieved over 80% lignin recovery in batch operation, comparable to or exceeding current standards for lignin recovery.

Furthermore, lignin recovered from corn stover (an agricultural residue) and softwood kraft sources were subjected to a solvent fractionation process, Aqueous Lignin Purification using Hot Agent (ALPHA), to control molecular weight and purify the lignin. This fractionation resulted in a 2-9-fold increase in MW and a corresponding 2-9 fold reduction in impurities.

To further increase lignin MW and Tg, technical lignin was chemically modified via esterification of hydroxyl groups using citric acid, an eco-friendly, bio-based multifunctional compound. This approach, which remains widely unexplored, improves lignin properties, processing, and durability. The modification resulted in a sixfold increase in MW and a 45 ⁰C increase in Tg, along with reduced polarity.

Overall, this work advances both the fundamental understanding and practical utilization of lignin, supporting its development as a sustainable feedstock for high-performance materials.

Author ORCID Identifier

https://orcid.org/0009-0002-4683-9234

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Available for download on Tuesday, August 31, 2027

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