Date of Award

January 2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Alena Kubatova

Second Advisor

Irina Smoliakova

Abstract

Lignin may serve as a potential source of renewable chemicals and as a possible wealth of materials for replacement of petroleum-based fuel and petrochemicals. Lignin is a plant component that constitutes the second most common natural polymer on earth, behind only cellulose, and is the most common natural polymer with an aromatic network. Technical lignins (isolated from chemical processing of raw lignin) are produced as waste in the papermaking and biorefinery industries; an estimation of U.S. waste lignin is about 24 million tons yearly, more than the estimated 10.5 million tons of plastics discarded annually. The exact structures of natural lignin and technical lignins are still not known, thus research continues on characterization of the many forms of technical lignins, which can differ substantially. In this work, we have developed a gel permeation chromatography (GPC) method by HPLC with a variable wavelength UV-Vis detector; this was applied to raw lignin and technical lignins in order to establish a feasible method of determining molecular weights for a polymer which is insoluble in a pure aqueous or a pure organic solvent. Characterization of lignin was continued with a modified Folin-Ciocalteu method for quantification of phenolic hydroxyl groups in lignin model compounds and technical lignins. Additionally, analysis of four factors of the experiment were statistically evaluated using a 24 full factorial (ANOVA) design of experiment, giving information on main influences and interactions of the method. Fractionation of lignins was carried out by preparative size exclusion chromatography. Further analysis of molecular weight distribution in the individual fractions was performed by electrospray ionization high resolution time-of-flight mass spectrometry (ESI HR TOF-MS), thermal carbon analysis (TCA) and thermal desorption-pyrolysis-gas chromatography-mass spectrometry (TD-Py-GC-MS). Additional information about phenolic and aliphatic hydroxyl groups was supplied through phosphitylized standards and lignin samples evaluated via 31PNMR analysis. Oxidative depolymerization of alkali lignin was accomplished through addition of hydrogen peroxide to a water matrix at various percentages (v/v), also with variation of added methanol as a co-solvent. Lignin samples with initial pH values of 3, 7 and 11 were evaluated for wt% of solubilized (depolymerized) material under two sets of filtration, and analyzed for pH change as well. Depolymerization was also done through subcritical water (SW) treatment of alkali lignin. TCA and TD-Py-GC-MS analyses of 300 °C SW samples were performed as described above, while the mass range for MS analysis was 10 – 550 m/z. This range had a lower limit which allowed monitoring of noncondensable gases (H2O, N2, O2, CO2). In addition, a novel method of mass balance was implemented through normalization of TCA and TD-Py-GC-MS data. SW treated samples were compared to untreated lignin profiles to determine the predominant species yielded at each temperature fraction. The process of condensation with concomitant gas formation through the temperature fractions was monitored through elemental analysis as C/H and C/O ratios. A summary of results finds that GPC method development allowed a determination of THF:water ratios which in turn led to complete solubilization in extraction solvents. FC method development resulted in quantitative phenolic OH count per nmol carbon in whole technical lignins and solubilized alkali lignin samples. Fractionation methodology was found to effectively limit MW ranges within individual fractions, although not to the extent expected. Both hi-MW and low-MW compounds outside expected ranges were found in every fraction. Oxidation of lignin by hydrogen peroxide did show depolymerization of samples, but this may have been due primarily to thermal effects. Peroxide reactions resulted in excessive ring-opening which in turn allowed a large amount of condensation and an actual increase in MW and a loss of solubilized material due to filtration of condensed material. Additionally, the lignin in basic and acidic solutions showed a very noticeable buffering effect. Subcritical water treatment of lignin samples resulted in a good mass balance for depolymerized materials in the liquid fraction; the extent of degradation was found to be more extensive than thought when looking at the GPC profiles.

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