Deep Eutectic Solvents Synthesis, Characterization and Applications in Pretreatment of Lignocellulosic Biomass (2017)

There has been an increased interest in green solvents and biofuels with the growing environmental awareness across the globe. Conventional methods of biofuel production involve the use of large quantities of molecular solvents and ionic liquids (ILs), but they have the drawbacks of high vapor pressure (organic solvents), toxicity, and recyclability in terms of a sustainability point of view. Deep eutectic solvents (DESs) have recently emerged as green alternatives to molecular solvents and ionic liquids (ILs). They are defined as eutectic mixtures formed between quaternary ammonium, phosphonium or sulfonium salts and hydrogen bond donors (HBDs) with the freezing temperature lower than their individual components. Sixteen different novel DESs were synthesized by varying the components (ChCl, AcChCl, U, and G) and their composition, employing various methods such as such as mixing (vortex), heating and mixing (thermal treatment, microwave irradiation, sonication, and shaking incubation), and evaporating the solvent (freeze drying and rotary evaporation). The DESs formation by solvent evaporation took longer time than mixing, and heating and mixing took the least amount of time. The DESs synthesized by different methods varied the amount of water present which affected their freezing point. The freezing temperature of DESs was always found to be much lower than the melting point of their starting materials, which is a characteristic feature of eutectic solvents. The freezing point of DESs depended on the composition and components of QASs and HBDs. The shifts in the representative peaks and broadening of the involved bonds in the FTIR spectra, and the shift in the resonance signal upfield in 1H-NMR spectra confirmed the formation of eutectics and hydrogen bonds in DESs. Comprehensive characterization of the synthesized DESs was carried out. DESs physicochemical and thermal properties including pH, density, refractive index (RI), surface tension, viscosity, octanol-water partition coefficient (Kow), flammability, freezing temperature (Tf), and decomposition temperature (Td) depended on their components and composition. Among the QASs and HBDs studied, the latter had the greater effect on most of the measured properties. A good agreement between the measured various properties of DESs was observed. Most of the measured physicochemical and thermal properties of DESs were comparable to the imidazolium ILs and lower or higher than organic solvents. Miscibility and solvatochromic parameters, including polarity and Kamlet-Taft parameters, of synthesized DESs were determined. The studied DESs were miscible in polar protic solvents and immiscible in nonpolar solvents. Miscibility in polar aprotic solvents depend on the ability to form hydrogen bonds. The DESs studied fall into the category of polar hydrogen bond accepting (HBA) solvents due to their high ET(NR) or ET N and β values. DESs E(NR) and ENT polarities were comparable to short-chain alcohols and imidazolium ILs, but Kamlet-Taft parameters α, β and π* were quite different. DESs ET(NR) and ENT polarities and hydrogen bond donating (HBD) acidity parameter α decreased, and HBA basicity parameter β and dipolarity/polarizability parameter π* remain unchanged with an increase in temperature from 25 to 45 °C. DESs ET(NR) and EN T polarities, HBD acidity parameter α and dipolarity/polarizability parameter π* increased, and HBA basicity parameter β remain unchanged with an increase in water content from zero to 20 wt %. The equation of lines derived from the studies of effect of temperature and added water content to predict the DESs polarity and Kamlet-Taft parameters. Genotoxicity and cytotoxicity of DESs and their individual components were evaluated using the Ames test and LDH, and MTT assays. A synergetic effect was observed because the toxicity of DESs was higher than their individual components. Therefore careful use of the term nontoxic must be considered. The genotoxicity and cytotoxicity of the DESs under study are far less toxic than imidazolium ILs at studied concentration, time and cell lines. The utilization of DESs for the dissolution of biopolymers such as cellulose, hemicellulose, and lignin was evaluated. The DESs selective dissolution of lignin was further explored and an extraction method optimized to selectively extract lignin from lignocellulosic biomass. The DESs extracted up to 80% of the lignin from PCG and SWG biomass was achieved. The extracted lignin was confirmed by FTIR and 1H NMR spectroscopy studies. A new source of DESs-extracted lignin was produced because the extraction was carried out with less harsh solvents at moderate temperature. The DESs were recovered simply by adding water and evaporating, which allowed reuse up to three times without losing significant activity. The selective extraction of lignin and ease of recovery of eutectic solvents demonstrated that DESs pretreatment is a promising green procedure in the biofuel production from lignocellulosic biomass. As DESs research for applications in this regard is still in its infancy, further investigations are still needed to successfully apply DESs as solvents in pretreatment processes at large-scale industrial applications.