Carbonyls in Cellulose: An Investigation into Formation Mechanisms, Analytical Methods, and their Consequential Properties for Fiber Engineering Applications

This thesis focused on the introduction of carbonyls into the cellulose structure with the wider aim to expand the scope of applications of cellulosic fibers. Thereby, two separate reaction classes were critically re-investigated. First, the periodate oxidation of cellulose combined with modification of the resulting aldehyde functionalities. Second, the carbonization mechanism of cellulose with a focus on the thermal dehydration reactions below 300 °C. Periodate oxidation was investigated for its potential to introduce more flexible segments in the rigid cellulose structure. The low temperature dehydration represents a key yield determining step during the preparation of cellulose-based carbon fibers. To obtain further insights into these well researched topics novel analytical techniques were applied. A recently developed solution state NMR method relying on an ionic liquid electrolyte for the dissolution of crystalline cellulosic materials proved to be useful and was further expanded to a full analytical protocol. The studies on periodate oxidation were significantly hampered by different side reactions. Thus, the research focus shifted on their quantification and mitigation. An updated workup scheme for dialdehyde celluloses was elaborated and an indirect size exclusion chromatography protocol allowed to follow the partial depolymerization reactions in the early stages of periodate oxidation. For further modification the applicability of reductive amination and borohydride reduction was screened. Also there, issues with unwanted degradation or incomplete conversion were encountered and could be highlighted. Overall, the studies on periodate oxidation resulted in an improved understanding of the underlying side reactions. However, they ultimately prevented a meaningful application of this modification strategy for fiber engineering purposes. In the fundamental studies on the thermal dehydration reactions of cellulose different reaction intermediates could be isolated and identified. Application of solution state NMR showed that the first thermal transformations result in depolymerization to levoglucosan end capped structures. In contrast to prevailing mechanistic proposals in the cellulose-based carbon fiber community, the initial dehydration reactions do not include elimination reactions in the pyranose structures. Instead, a polyfuran was isolated as first carbonization intermediate and could be thoroughly characterized. This suggested a localized dehydration mechanism occurring on the reducing end groups, as recently postulated in the literature focusing on other fields of cellulose pyrolysis. This observation also suggests that the chemistry of cellulose carbonization is similar to the carbonization reactions of other sugars. Moreover, the occurrence of a polyfuran intermediate has considerable implications for the potential reactions occurring at higher temperatures, which so far evaded proper analytical characterization.