Cellulose in Quaternary Ammonium-based Solvents: Dissolution, Modification and Coagulation

Doctoral thesis, 2023

Processing cellulose through dissolution and regeneration is essential for many applications, yet challenges are posed because it does not readily melt or dissolve in commonly used aqueous and organic solvents. Consequently, extensive efforts have been devoted to developing novel and efficient solvents for cellulose, as well as exploring new functionalization routes. Existing limitations associated with current solvents, such as solution instability, limited dissolution capacity, specific temperature requirements, solvent-related side reactions and a narrow concentration range, are nevertheless significant.

Water-based solvents are of particular interest since they can be applicable in large scales. This thesis mainly focuses on quaternary ammonium hydroxides (QAHs) since they enhance cellulose dissolution compared to extensively studied NaOH(aq) with the aim of gaining deeper insight into the impact of the structural characteristics of the quaternary ammonium-based solvents on cellulose dissolution, modification and coagulation. Initially, cellulose etherification in previously investigated solvents, benzyltrimethylammonium hydroxide, tetramethylammonium hydroxide and NaOH(aq) was investigated in situ using spectroscopy and rheology techniques. The results revealed enhanced reagent solubility, improved cellulose solution stability during the reaction and less pronounced cascade reaction in QAHs, either alone or in combination. With the importance of the solvent structure highlighted for cellulose modification and inspired by the industrially used N-methylmorpholine N-oxide, a series of morpholinium salts with different alkyl chain lengths combined with hydroxide, acetate or chloride counterions were prepared and their ability to dissolve cellulose was investigated. This study aimed to examine the structural properties of the solvent governing dissolution which indeed highlighted the importance of both the cation and anion structures: morpholinium chlorides were incapable of dissolving cellulose whereas morpholinium acetates combined with DMSO and aqueous morpholinium hydroxides functioned as cellulose solvents. While alkyl chains longer than ethyl enabled room-temperature dissolution in aqueous morpholinium hydroxides (likely through improved stabilization of the hydrophobic regions of cellulose), they decreased the dissolution ability of morpholinium acetates in DMSO-based systems. Finally, the behaviour of cellulose solutions in benzyltrimethylammonium hydroxide and two of the newly developed morpholinium hydroxides during coagulation was studied which highlighted the importance of hydrophobic structural motifs for the consistency of the formed gels.