Biomass processing using ionic liquids: Effects of 3-methylimidazolium cations and carboxylate anions

As the global transportation and industrial sectors continue to grow, fuels, chemicals, and products derived from lignocellulosic biomass have become a key alternative to petroleum-based products. Lignocellulosic biomass is composed of lignin, cellulose, and hemicellulose linked together in a rigid structure. This spatial arrangement contributes to its resistance to degradation and requires pretreatment and/or separation before being processed to produce valuable chemicals and fuels. Biomass pretreatment has mainly been optimized to convert carbohydrates into monosugars. However, better sustainability is attained when the entire feedstock is utilized to produce fuel and value-added chemicals and products. To achieve this goal, an integrated biorefinery will require a highly selective and economically viable fractionation process. Although traditionally used for pretreatment, recent studies have found ionic liquids to be ideal solvents for biomass dissolution, “activation”, and fractionation to produce various end products for biorefinery and industrial applications.Previous works have demonstrated that the IL 1-ethyl-3-methylimidazolium acetate ([EMIM]Acetate) is ideal for the above processes to produce sugars as well as lignin-based products. However, our study shows that three other ILs with 3-methylimidazolium cations and carboxylate anions (1-ethyl-3-methylimidazolium formate ([EMIM]Formate), 1-allyl-3-methylimidazolium formate ([AMIM]Formate), and 1-allyl-3-methylimidazolium acetate ([AMIM]Acetate)) are effective for biomass dissolution, with [AMIM]Formate having a 40% increase in biomass solubility compared to [EMIM]Acetate. Both [AMIM]Formate and [EMIM]Acetate are further evaluated for their activation and fractionation capability by studying crystallinity changes and enzymatic conversion rates of cellulose and hemicellulose into soluble sugars. Our findings show that although [AMIM]Formate is better at biomass dissolution, [EMIM]Acetate is better for biomass activation and fractionation. Following activation using [AMIM]Formate, biomass retains its most of its crystallinity and acetyl groups, whereas activation using [EMIM]Acetate significantly reduces crystallinity and acetyl groups, leading to higher enzymatic conversion of cellulose and hemicellulose. Future studies should investigate the potential for in situ saccharification in ILs using commercial cellulases and hemicellulases, as our preliminary data show that enzymes remain active in these two ILs. Ultimately, this research will provide technological breakthroughs needed to develop a robust means of biomass fractionation and subsequent conversion into high value organics and biofuels.