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Abstract:
In the last decades, the interest in the use of cellulose as a raw material for the production of platform chemicals and biofuels has increased in the scientific and industrial communities. This is a consequence of the depletion of fossil resources as well as the assimilation of new concepts such as Green Chemistry and Sustainability.1 One of the biggest challenges for the use of cellulose as a raw material is to overcome its recalcitrance posed by a dense network of hydrogen-bonds between different macromolecular chains of the biopolymer.
Only special solvent systems are capable of completely disassembling the cellulosic network, making its functional groups accessible to chemicals and enzymes. The non-derivatizing solvents present a common characteristic: the presence of ionic species. Among them, one can highlight aqueous NaOH(/urea),2-5 tertiary amine N-oxides (e.g. NMMO),2,3,6,7 amide/LiCl solutions (e.g. DMAc/LiCl),2,3,8,9 inorganic molten salts hydrates,2,3,10 ionic liquids,2,11-16 and ionic liquid solutions in molecular solvents (MS).15,17,18 The last two systems are the most suitable solvents for cellulose.
In 2002, Rogers et al. introduced alkylimidazolium-based ILs as efficient solvents for cellulose.11 For example, cellulose solutions with concentrations up to 25 wt-% can be obtained in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) or 1-ethyl-3-methylimidazolium acetate ([EMIM][AcO]), under low severity dissolution conditions. Later on, Rinaldi18 reported that high cellulose loads (up to 14 wt-%) can be instantaneously dissolved in solutions of a dipolar aprotic molecular solvents (e.g., DMSO) containing a minor mole fraction of [BMIM]Cl or [EMIM][AcO], named Organic Electrolyte Solutions (OES).18
The discovery of OES not only offers a better solvent system for cellulose processing than the parental neat ILs—as described in the state of the art—but opens up a window of opportunities in the chemistry of cellulose. The IL-concentration can be broadly varied in the IL/MS solvent mixture while still keeping cellulose in solution. Therefore, by using OES it is possible to assess whether the ILs plays a role in the chemical transformation of cellulose, beyond, of course, dissolving the biopolymer. In fact, this concept was the ‘seed’ of this thesis.
In Chapter 2, the state of the art concerning biomass utilization, cellulose recalcitrance and IL-based systems as solvent media for cellulose processing is presented. In addition, some advantages of IL/MS solutions over the respective neat ILs are pointed out. Since further chapters treat the acid catalyzed hydrolysis of 1,4-β-glucans (cellulose and cellobiose), the current literature on the chemical nature of 1,4-β-glucans is revised in order to discuss the current results.
The first chapter concerning the results and discussion of this work, Chapter 3, shows a systematic study to assess whether alkylimidazolium-based ILs play a role in the acid-catalyzed hydrolysis of cellulose and cellobiose in [BMIM]Cl/DMSO binary mixtures. This study confirms that the hydrolysis rate of 1,4-β-glucans increases with IL-concentration. Notably, the driving force of this effect is now elucidated through. Inspired by these results, we decided to assess the effect of inorganic salts on the acid-catalyzed hydrolysis of cellobiose, a water soluble model compound of cellulose, as presented in Chapter 4. Once again, the presence of electrolytes appears to enhance the reactivity of glycosidic bonds towards acid-catalyzed hydrolysis. These results significantly broadened the current understanding of the impact of inorganic salts on the hydrolysis of cellulose in aqueous slurries.
In the last two chapters, experimental thermodynamic aspects of cellulose solutions into IL/MS binary solutions are introduced. In Chapter 5, the interactions between fifteen selected ionic liquids (ILs) and cellobiose (CB) are examined by high-precision solution microcalorimetry. A notably correlation was found between the nature of the results of ∆mixH(CB) and the solubility behavior of cellulose. This correlation suggests that ∆mixH(CB) offers a good estimation of the enthalpy of dissolution of cellulose even in solvents in which cellulose is insoluble. Therefore, the current findings open up new horizons for a comprehensive understanding of the thermodynamic factors accounting for the spontaneity of cellulose dissolution in ILs or IL/MS solutions. In Chapter 6, the phase behavior of ternary solutions of cellulose into [EMIM][AcO]/MS mixtures is depicted. Such solutions present a critical temperature below which, typically, two well defined phases are formed. The previous understanding of the energetics of solute-solvent interactions studied in Chapter 5 guided understanding the impact of solution composition on the temperature of phase separation and the final composition of formed phases.
