Synthesis of novel ionic liquids for cellulose processing

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Title: Synthesis of novel ionic liquids for cellulose processing
Author: Gazagnaire, Eva
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2019
Language: eng
Thesis level: master's thesis
Degree program: Kemian ja molekyylitieteiden maisteriohjelma
Master's Programme in Chemistry and Molecular Sciences
Magisterprogrammet i kemi och molekylära vetenskaper
Specialisation: ei opintosuuntaa
no specialization
ingen studieinriktning
Discipline: none
Abstract: Nowadays one main objective in chemistry is to find environmentally benign alternatives to non-biodegradable materials, like common plastics. Further, as fossil resources are decreasing, novel approaches to utilize renewable materials (like biomass), are becoming increasingly important for the mankind. There is a long-standing interest for utilization of cellulose; it is the most abundant polymer on earth and can be found in many organisms such as plant, algae, and bacteria but also in some animal species. Cellulose is the most abundant biogenic polymer used in the world; its largest source is from wood, which contains up to 50% of cellulose but also other compounds such as lignin (up to 30%), hemicellulose (up to 30%), inorganic salts and proteins. The main challenge is to dissolve cellulose, because of is strong intra- and intermolecular hydrogen bonding, it is not soluble in common molecular organic solvents and does not melt. According to these problems of cellulose dissolution, a novel class of solvents has been designed and called: Ionic liquids (ILs). ILs are made of an organic cation and an organic or inorganic anion, their major difference from classic salts is their lower melting point (under 100°C) They are able to do covalent and ionic bonds as normal organic solvent do, but their special character comes from the fact that ILs are able to do also strong H-bonding and columbic interactions. The mechanism of the dissolution itself has been studied using molecular dynamics and it has been shown that the anion and cellulose build a strong hydrogen-bonding network between them; the cation has a different stabilizing effect on dissolution as it is dominated by Van der Walls and electrostatic interactions. The structure of the IL plays a big role. A non-hindered cation will be more effective and, an anion with high basicity will be the most effective. Many of the ILs studied to dissolve cellulose contain phosphonium cations or halogen anions, such combination leads to toxicity and corrosive action against reaction vessels. To diminish the negative effects, other combinations have to be designed. Chemist started to use superbases as a cation and weak acid such as acetic acid as an anion to form superbase-based ILs. ILs-based on guanidine are known to be chemically and thermally stable, this comes from the high proton delocalisation between the three nitrogen atoms. Even if they are able to dissolve cellulose, their characteristics related to their structure such as high melting point and high viscosity are a problem for lab experiments using a classic magnetic stirring. Also they are limited to 10 wt% cellulose dissolution. A lot of superbase ILs such as immidazolium-based ILs were investigated for cellulose dissolution but they require temperature higher than 90°C enhancing cellulose degradation. So bicyclic guanidine were investigated as a potential class of ILs. Because of their rigid bicyclic structure, they are less affected by steric effect than their acyclic analogue. This explains why chemists started to be interested in bicyclic guanidine species in ILs such as TBD and its methylated version mTBD.

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