(385ae) Sustainable Processing of Lignin for Commercial Applications

As the search for renewable alternatives to petroleum based precursors increases, interest in lignin valorization continues to grow. Lignin is an abundant biopolymer inherent in lignocellulosic biomass and known for its rich molecular functionality and structure (high aromaticity, carbon content, hydroxyl groups, carbonyl groups); and its cost-effective nature (costing about $0.5-1 per kg). As such, lignin could serve as an excellent alternative for petroleum based precursors, cutting down cost while keeping the desirable properties of these applications, making it suitable for diverse applications ranging from carbon fibers and foams to polyurethane foams and phenolic or benzyl compounds. Despite its potential, it is still underutilized due to drawbacks such as limited accessibility in the US, difficulty in purification, wide distribution of molecular weight (MW), low molecular weight (MW), and poor compatibility with easily accessible polymers.

My research work is on developing sustainable methods to produce, process, and characterize lignin for commercial, chemical, and material applications. I aim to unlock the potential of lignin which includes increasing the quantity of lignin in the US, processing the lignin to improve quality, and quantifying the quality through analytical methods while preserving its renewability, cost-effective, and energy-efficient characteristics. The processing and mechanical properties of lignin-based products such as carbon fibers and composites are dependent on the quality of the lignin (molecular weight (MW), glass transition temperature (T_g), and purity). Recent research indicates that elevating MW and T_g while decreasing the impurities of lignin positively impacts the processing and mechanical properties (tensile strength and modulus) of lignin-based carbon fibers.

The aims for my work include

• Developing and optimizing lignin recovery processes to improve the commercial availability of lignin: A major contributing factor to the high cost of lignin production is the evaporation of the black liquor (by-product containing lignin after cellulose is extracted) and filtration step for extracting lignin. I have worked on designing and optimizing various recovery processes that balance cost and quality. This involves reactor design, process flow development, tubing sizing, and system optimization. I then characterize the extracted lignin through NMR, TGA, GPC, and HPLC to measure MW, purity, and thermal properties of the lignin.

• Improving the properties of lignin using solvent fractionation with hot aqueous organic solutions: To improve MW and purity control, I use a physical fractionation method known as Aqueous Lignin Purification with Hot Agents (ALPHA). This technique utilizes inexpensive, green solvents (e.g., acetone, ethanol, and acetic acid) with water at elevated temperatures to form two distinct liquid phases: a lignin-rich phase and a solvent-rich phase. This phase separation facilitates impurity removal and enables continuous processing. I investigate the influence of operating conditions on phase behavior, MW, and purity of the resulting fractions.

• Improving the properties of lignin through chemical modification via esterification with citric acid: To increase MW and tailor polarity, I have explored lignin interlinking via esterification. Traditional esterification uses acid anhydrides or halides—reagents that require toxic catalysts and produce hazardous byproducts. In contrast, I employ citric acid, a naturally occurring and non-toxic alternative, to create ester linkages. This approach not only raises MW but also improves compatibility with common polymers like polyethylene and polystyrene by enabling better polarity control.

• Comprehensive characterization of lignin to quantify commercial and molecular value: All extracted and modified lignins are systematically characterized using NMR, TGA, GPC, FTIR, DSC, and HPLC to assess MW, purity, functionality, and thermal properties—metrics essential for evaluating their suitability in downstream applications.

• Utilization of lignin in the advanced products through collaborative efforts: Finally, I am working collaboratively to incorporate lignin into high-performance products such as carbon fibers, polymer foams, hydrogels, and membranes. These efforts aim to demonstrate lignin’s viability in real-world commercial and industrial applications.