Insights into the Lignin Dissolution Mechanism of Water Content Tailored-choline Chloride (ChCl) Based Green Solvents for Biomass Pretreatment

Recent works in green solvents research trend towards ternary deep eutectic solvents (DESs) as a breakthrough which enhanced the performance of biomass dissolution and tunability of DESs for varying applications. In this context, water is extremely facile in such purpose as it is both a hydrogen bond acceptor and a donor with limited studies reported on modulating the properties of DES. ChCl/Malic acid 1:1 (CCMA) and ChCl/Lactic acid 1:18 (CCLA) DESs with varying water content were synthesized and characterized. Fourier transform infrared spectroscopy (FTIR) spectra with broadened OH stretching, diminished choline chloride characteristic peak associated with its OH group as well as bathochromic shift of the C=O stretch associated with carboxylic acids demonstrated the formation of hydrogen bond. The addition of water also caused the density of CCMA and CCLA reduced by 2.8 % and 3.6 %, respectively with 50 wt% of water content due to the lowered free volume of DESs. CCLA showed the highest lignin solubility at 6.32 wt% and further addition of water resulted in weakened structure with degradation in dissolution performance. Both CCMA and CCLA water content tailored green solvents which hindered self-degradation from esterification reaction demonstrated great potential to be applied in biomass pretreatment.