Document Type

Dissertation - Open Access

Award Date

2020

Degree Name

Doctor of Philosophy (PhD)

Department / School

Chemistry and Biochemistry

First Advisor

Douglas Raynie

Abstract

Extensive use of fossil fuel for different purposes has resulted in significant depletion of it at an alarming rate. Therefore, a potential and sustainable alternative source are badly needed at this moment. Nowadays, lignocellulose biomass is appealing to much interest by the researchers because of its potential nature as a renewable carbon source. It is the most abundant renewable source which is composed of cellulose, hemicellulose, and lignin. Among them, lignin is considered the 2nd most abundantly natural polymer after cellulose which comprises 10-30 % of biomass depending on the source and the environment. Lignin is a three-dimensional cross-linked organic polymer, composed of three different phenylpropanoid units such as coniferyl, sinapyl, and pcoumaryl alcohols. Being a renewable and polyaromatic by nature, lignin is being considered as a potential source of a wide range of chemicals and renewable energy. But it is very important to understand the comprehensive structure of lignin before convert into value-added products. Therefore, our main aim is to characterize the naturally derived lignin by thermal, spectrometric, oxidative, and chromatographic techniques to understand the comprehensive structure and macromolecular features of lignin. To accomplish the goal, the objectives of this work are: 1. Optimize the processing conditions for the extraction of lignin from the non-wood source. 2. To investigate the thermal stability and functional groups of lignin and its residue by thermal and spectrometric methods. 3. To study the molecular weight distribution, and monomeric structure of lignin and its residue by chromatography. 4. Oxidative degradation of lignin into monomers and to optimize the reaction conditions for conversion. In chapter II, thermal behaviors of lignin and its residue after catalytic hydrodeoxygenation reaction were characterized by TGA and DSC. TGA results indicated that both lignin and its residue showed variable weight loss in different temperature ranges, different percentages of residual carbon, and different DTGmax of the sample. Results showed that the weight loss of residue was lower than the lignin throughout different temperature ranges. But a higher percentage of residual carbon (45.85 %) was observed for residue than lignin (25.89 %). On the other hand, the maximum rate of weight loss (DTGmax) for residue was observed at a lower temperature at 420 °C than lignin at 480 °C. DSC results showed a lower melting point for residue(150.48 °C) than lignin (174.40 °C). Moreover, the lignin-residue decomposed at 370 °C, whereas no visible change was observed in lignin around this temperature. So, DSC and TGA analysis revealed that lignin residue was thermally less stable compared to lignin. Additionally, lower melting point with higher residual carbon for residue showed that both thermally stable and unstable compounds were produced during the hydrodeoxygenation reaction. In chapter III, chemical structure, functional groups, and molecular weight of lignin and residue were investigated by FTIR, NMR, and GPC analysis. FTIR study, showed both lignin and residue contain the same functional group, and no further new band was noticed, which suggests residue still contains unreacted lignin or smaller breakdown products with similar chemical properties of lignin. NMR analysis also showed the same chemical functional group present in both lignin and residue. But the quantitative study of NMR showed a different amount of the functional groups. It was shown that lignin is higher in aromatic proton where the residue is higher in the aliphatic proton. Results also showed, aliphatic/aromatic ratio of the residue is 3 times greater than the lignin. The higher aliphatic/aromatic ratio of residue demonstrated that the significant number of aromatic moieties of the lignin have gone with the reaction mixture leaving the aliphatic moiety in the residue. On the other hand, GPC analysis found three distinct peaks for lignin and two for residue. The greater molecular weight distribution and polydispersity were observed which indicated the formation of C-C bonds during the catalytic reaction. The study showed that this kind of bond formation is related to the guaiacol units which are connected to each other at elevated temperature. In chapter IV, lignin and its residue were depolymerized by cupric oxide oxidation and monomeric products were identified and quantified by GC-MS analysis. A chromatographic study showed that lignin produced four monomers and residue produced two monomers, respectively where all of them are characterized as a G moiety. Among the phenolic monomers, vanillin was the major product for both samples. Additionally, the results showed residue contains less amount of vanillin and acetovanillone than lignin and no peaks for guaiacol and homovanillic acid. Oxidative depolymerization of lignin was also carried out at different temperatures and times to optimize the reaction conditions for better yield. The chromatographic study showed four monomers produced in each condition. Among them, vanillin was found as a major product and production of vanillin increased with the increase of both temperatures and times. The production of guaiacol and acetovanillone increased when the temperature reached 150 °C and time at 2 hours then slightly decreased with the increase of both temperature and time. But the production of homovanillic acid significantly decreased when the temperature reached 175 °C but slightly decreased when time reached 2.5 hours. Therefore, our oxidative study found 150 °C and 2.5 hours as an optimization conditions for the better production of phenolic monomers from lignin. In chapter V, lignin was extracted from wheat straw using an accelerated solvent extraction technique. To optimize the extraction conditions, extraction was carried at three different temperatures such as 140 °C, 170 °C, and 200 °C and two different acidic conditions 0.05% and 0.1% for 60 minutes. Then the extracted lignin was characterized by TGA, FTIR, and subcritical water. The study showed that extraction of lignin increased from 13.89 % to 28.69 % when the temperatures increase from 140 °C to 200 °C. But acid concentration showed very little impact on extraction of lignin. It was shown, extraction of lignin increased slightly with the increases of acid concentration at a specific temperature. On the other hand, characterization of lignin with TGA, FT-IR, and liquefaction showed the similar result with the commercial lignin, indicated lignin was successfully extracted from wheat straw. Liquefaction of lignin followed by GC-MS analysis showed 9 phenolic monomeric products with 86% total relative amount.

Library of Congress Subject Headings

Lignocellulose.
Lignin.
Biomass.

Format

application/pdf

Number of Pages

123

Publisher

South Dakota State University

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Chemistry Commons

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Rights Statement

In Copyright