Date of Award

Summer 8-16-2024

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

Sampath Gunukula

Second Committee Member

M. Clayton Wheeler

Third Committee Member

G. Peter van Walsum

Abstract

The carbon-rich biopolymer lignin has huge potential applications in producing bio-based products. However, the variability in lignin molecular weight, heterogeneity in lignin chemical structure, and limited functionality of lignin obtained from separation techniques prevent it from being widely used. In response to this challenge, a simple and eco-friendly approach was implemented to fractionate kraft lignin using deep eutectic solvents to produce homogenous lignin fractions with unique properties.

The following seven deep eutectic solvents were successfully made: choline chloride/ethylene glycol, choline chloride/levulinic acid, choline chloride/acetic acid, choline chloride/pyrocatechol, tetrabutylammonium chloride/ethylene glycol, tetrabutylammonium chloride/hexanol, and octanoic acid/hexanoic acid. Fourier Transform Infrared Spectroscopy (FTIR), and freezing point measurements were used to characterize Deep Eutectic Solvents (DESs), and the activity coefficient of each component in the mixture was calculated using the measured melting points. All seven DES mixtures showed significant temperature depression with negative deviations in the activity coefficient of each component at the eutectic point. These deviations were used to derive general heuristics for screening potential hydrogen bond donors and hydrogen bond acceptors for DES formation and lignin fractionation.

Among the seven DESs developed, ChCl/Lev and TBACl-EG were used to fractionate kraft lignin. Kraft lignin is sequentially dissolved in ChCl/Lev, and TBACl-EG and stirred for 5 and 15minute respectively at 30°C. Different lignin fractions were obtained by gradual precipitation with water or acetone and centrifugation followed by vacuum filtration. Comprehensive analyses were conducted using thermal analysis (Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), functional group analysis (Nuclear Magnetic Resonance), and Elemental Analysis. The lignin fractions produced showed improved flexibility as compared to the parent kraft lignin with a Tg value reduction from 131°C to about 60°C revealed by DSC analysis. Improved thermal properties in lignin fractions was also observed between temperature ranges of 25°C to 200°C compared to the parent kraft lignin due to the rise in the specific heat capacities in the fractions. Elemental analysis revealed a reduction in carbon content with increased hydrogen content among the lignin fractions after fractionation. The overall thermal stability of lignin fractions over a temperature range of 100-800 was observed in the TGA and DTGA curves of the lignin fractions.

Thermogravimetric analysis (TGA) of the fractions obtained using ChCl-Lev DES revealed mass losses within the temperature range of 100-800°C as follows: the first fraction (F1), which dissolved after 15 minutes of extraction, exhibited a mass loss of 71%; the second fraction (F2), obtained after 30 minutes of extraction, showed a mass loss of 55.9%; the third fraction (F3), collected after 45 minutes of extraction, displayed a mass loss of 59.49%; and the undissolved fraction had a mass loss of 74.5%.

Similarly, for TBACl-EG DES, the weight loss percentages were observed as follows: the undissolved fraction experienced the highest weight loss at approximately 84%, followed by F1 at 77%, F3 at 70%, and F2 at 65%. Comparatively, kraft lignin exhibited a weight loss of approximately 52% at 800°C, retaining more weight due to its higher molecular weight compared to the fractions. Notably, among the fractions, F2 demonstrated greater thermal stability. Finally, proton NMR analysis on the lignin fractions revealed a variation in phenolic hydroxyl concentration and molecular weight, translating into a decrease in signal intensities amongst the lignin fractions.

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