(167c) Solvent Effects on the Dissolution and Precipitation of Cellulosic Biomass from Ionic Liquids

Chemical processing of biomass remains a challenge as the rigid inter- and intra- molecular hydrogen bonding network of cellulose renders it insoluble in nearly all aqueous and organic solvents.  Alternatively, select ionic liquids (ILs) are capable of dissolving significant quantities.  For instance, a promising IL, 1-ethyl-3-methylimidazolium diethyl phosphate [EMIm][DEP] is capable of dissolving more than 13 wt.% cellulose at only 60 ^oC.  While pure ILs have been primarily targeted for biomass pretreatment, understanding the role of mixed solvent systems is important to improve momentum transport (i.e. lower mixture viscosity) and reduce the quantity of expensive IL required; two common criticisms of ionic liquid based processes.  The vast majority of molecular solvents investigated dramatically decrease cellulose solubility in ILs and thus most are considered “antisolvents”.  However, we have recently discovered that select quantities of polar aprotic solvents, when mixed with ILs, (despite having negligible solubility on their own) are capable of enhancing the thermodynamic solubility limit of cellulose in IL mixtures, even beyond the measured cellulose solubility limits of pure ILs.  Furthermore, these “cosolvents” enhance transport properties of IL/biomass mixtures and reduce sensitivity to moisture (H₂O) which is known to severely impact dissolution capacities of cellulose in ILs.  This presentation will illustrate solid-liquid phase equilibrium results for the dissolution of cellulose in various IL/cosolvent, IL/antisolvent, and IL/mixed solvent systems with the ionic liquid [EMIm][DEP].  Spectroscopic techniques including analysis by Kamlet Taft methodology will elucidate molecular interactions between ionic liquid and solvent species and provide an understanding of the solvation sphere around the IL in relation to cellulose dissolution within the various IL-solvent mixtures.  Finally, rheological studies will demonstrate solvent effects on momentum transport.  Our preliminary findings indicate that mixed IL/cosolvent systems could be even better solvents for cellulose dissolution than pure ILs from thermodynamic, transport, and economic perspectives.  These results will be presented and discussed.