Date of Award

January 2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Alena Kubatova

Second Advisor

Evguenii I. Kozliak

Abstract

Characterization of lignin and its degradation products, more specifically determination of their molecular weight (MW) distribution, is essential for assessment and applications of these potentially renewable phenolics. In this dissertation a suite of lignomics approaches allowing for comprehensive and accurate characterization of lignin focusing on the low and high MW species was developed. These methods may be used either in combination or independently. The methods were validated and applied to characterizing lignin and the products of its transformation.

Several size exclusion chromatography (SEC) systems were previously deemed to be suitable for MW-based separations and thus are frequently used for lignin analysis, however the nature of secondary non-SEC interactions remains unclear. In this dissertation, several representative gel filtration and gel permeation systems were assessed. This work confirmed that undesired secondary non-SEC interactions may be detrimental and need to be carefully evaluated. From the evaluated SEC columns, only the stationary phase with highly cross-linked porous polystyrene/divinylbenzene provided the most effective separation by MW for both low and high MW model compounds. It was shown that polystyrene and poly(methyl methacrylate) standards may be reliably used for the SEC column calibration if an appropriate stationary phase was utilized. Notably, the column with a higher pore and lower particle size provided a better resolution towards polymeric standards, even though the particle size effect was downplayed in the earlier SEC studies of lignin. It was demonstrated that for several evaluated SEC systems, the separation was strongly affected by functionalities of the analytes and correlated with the compounds’ pKa rather than MW. The separation on the stationary phases featuring polar hydroxyl groups was shown to lead to specific column-analyte secondary interactions, perhaps based on their hydrogen bonding with lignin. A novel approach for lignin mean MW calculation based on mass spectrometry data was implemented. The determined number-average MW corroborated the SEC results.

Furthermore, an electrospray ionization high-resolution time-of-flight mass spectrometry lignomics was developed as a method to expand the lignomics toolkit while targeting the simultaneous detection of low and high molecular weight lignin species. The effect of a broad range of electrolytes and various ionization conditions on ion formation and ionization effectiveness was studied using a suite of mono-, di- and triarene lignin model compounds as well as intact lignin. Contrary to the expectations based on literature, the positive ionization mode was found to be more effective for methoxy-substituted arenes and polyphenols, i.e., species of a broadly varied MW structurally similar to the native lignin. For the first time, an effective formation of multiply charged species of lignin with the subsequent mass spectrum deconvolution was reported. The obtained MW values were in good agreement with those determined by SEC and LDI.

To minimize heterogeneity of lignin, which hinders its characterization by the spectral and thermal methods, narrow MW lignin fractions were obtained by preparative SEC considering the most suitable chromatographic conditions. Characterization of these fractions was performed with a suite of methods while using traditional chemistry approaches as well as nanoparticle characterization. Commercially available alkali lignin was shown to contain the impurities that were structurally different from lignin. The results of thermal carbon analysis suggested that these impurities may have a carbohydrate-like nature.

Furthermore, the developed chromatography and mass spectrometry methods may be widely applied in a challenging field of both natural and synthetic polymer characterization. In this dissertation, the application of the newly developed approaches to characterizing the lignin-derived products and synthetic polymers was shown.

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