Thermal behaviour of cellulose materials

Studying the high-temperature treatments of biomass-derived materials is essential to understand their thermal decomposition and characterize useful carbonized products in a frame of environmental sustainability. The goal of this thesis is the mechanistic understanding of thermal degradation (150-800 °C) in oxidizing and inert atmospheres of two types of cellulose materials, also identifying applications for the product. Cellulose filaments (CFs) are microfibril bundles and heterogeneous fibrillar mass extracted through mechanical refinement. Cellulose nanocrystals (CNCs) are produced through dissolution of amorphous regions and may contain sulfate (S-CNC) or carboxylate (C-CNC) surface groups, depending on the preparation conditions. Acid groups of as-prepared CNCs can be neutralized with alkali counterions. CNC suspensions can be freeze-dried or air-dried forming birefringent aerogels and iridescent chiral nematic films, respectively. The kinetics and thermochemistry of thermal degradation of cellulose materials, as well as their morphological and chiroptical modifications, were studied by several techniques, including thermogravimetric analysis, solid-state NMR spectroscopy, and scanning electron microscopy. From these and other techniques, it was deduced that CFs have a simple degradation mechanism, and the highest stability among the materials studied (325 °C), despite abundant amorphous regions and inhomogeneous fibrous mass. When fully gasified, CFs emit a large fraction of alcohol-based gases, including biofuels. CNC-H aerogels decompose in complex ways below 200 °C, with abundant char and sulfur evaporation at high temperatures. Sodium counterions in S-CNC-Na aerogels improve the stability up to 300 °C, where partial surface rehydration and formation of sodium hydroxide occur, while carbonization yields highly condensed structures. In their air-dried form, the thermal stability of S-CNC films can be improved with larger alkali counterions and the cholesteric structure is maintained even after prolonged thermal treatment, advantageous for potential applications as temperature sensors. In contrast to S-CNC, C-CNC aerogels are more thermally stable in acid form. Here, the presence of sodium often accelerates the degradation by decomposition into sodium carbonate. Higher carboxylate content and specific surface area were found to shift C-CNC degradation towards lower temperatures, as well as catalyzing decarboxylation in acid form. The results of this thesis will inform the development of novel cellulose materials with high thermal stability.Science, Faculty ofChemistry, Department ofGraduat