Investigation of Interactions between 1,3 Dialkyl Imidazolium Ionic Liquids and Lignocellulosic Polymers

Lignocellulosic biomass is a potential energy source for fuels, chemicals and materials production in a sustainable manner. A network of covalent and non-covalent bonds between the three main polymers of biomass, i.e., cellulose, hemicellulose and lignin, results in a compact structure that is resistant to chemical and biological attacks and therefore challenging for the efficient utilization of lignocellulosic biomass. Ionic liquids (ILs) have been reported to disrupt the bonds between these polymers and dissolve biomass at temperatures below 100 ˚C. Research through the years has shown that biomass pretreatment with IL brings out the selective dissolution of biomass polymers and reduces cellulose crystallinity with enhanced saccharification rates. However, the specific nature of interactions between the biomass polymers and ILs and the role of IL ion pairs remain largely unknown. In this work, we report on the progress achieved in understanding interactions between ILs and biomass polymers. Herein using four different imidazolium ILs, we developed a unique experimental approach using spectroscopic, scattering and rheological techniques to investigate interactions between individual biomass polymers and ILs. The spectroscopic approach provided evidence for IL cation and anion roles. Scattering studies not only offered conformational details on polymers but also provided a quantitative estimation of the qualitative data acquired using spectroscopy. Rheology studies helped identify dispersive interactions not accounted for by the other two techniques. Finally, our multi-length scale approach helped us identify the two ILs with differential interactions towards cellulose and lignin. These two ILs were selected to perform an in-situ biomass dissolution study using small-angle neutron scattering (SANS), which showed that cellulose microfibril architecture altered in size and shape as a function of time in acetate anion IL as opposed to the formate anion IL. In summary, the molecular level information on IL biomass polymers presented in this work will aid in making a rationale choice of IL ideal for the field of biomass processing.