Cellulose in quaternary ammonium hydroxide solutions

Licentiate thesis, 2022

An ever-increasing demand to shift away from fossil-based feedstock towards renewable resources has led to more use of bio-based materials. Cellulose being the most abundant biopolymer on Earth has received a considerable attention with a wide range of applications (in textile fibers, films, membranes, etc.), many of which requiring processing through dissolution and/or chemical functionalisation. Cellulose cannot melt when heated or dissolve in the most common aqueous and organic solvents. Thus, a lot of efforts have been made to develop new and effective solvents aiming to address some common limitations associated with the existing solvents including solution instability, low dissolution capacity, specific temperature requirement, side reactions of the solvent itself and narrow concentration range required for the dissolution. Water solution of quaternary ammonium hydroxides (QAHs), classified as relatively green and economical chemicals, show promising potential in cellulose dissolution. In this thesis, to gain deeper insight on QAHs(aq) as derivatisation and dissolution media for cellulose, cellulose etherification in benzyltrimethylammonium hydroxide (Triton B) and tetramethylammonium hydroxide (TMAH), along with NaOH(aq) as well as different mixtures of these bases was investigated. In this study the emphasis was set on in-situ monitoring of the etherification reaction combined with the spectroscopic analysis of the products. The results highlighted the impact of hydroxide base composition on cellulose etherification where QAHs either alone or in combination enhanced the reagent solubility and cellulose solution stability during the reaction compared to NaOH which resulted in a lower solution stability promoting likely side reactions of the introduced substituents. Considering the importance of the dissolution system on functionalisation, a new solvent for cellulose was developed in the next study inspired by the well-known industrially used solvent N-methylmorpholine N-oxide (NMMO). In this study, N,N--dimethyl morpholinium and hydroxide ions were coupled to synthesize N,N-dimethylomorpholinium hydroxide(aq), and the dissolution properties of cellulose in the new solvent were investigated showing mostly molecularly dissolved cellulose chains with good chemical stability upon refrigeration.