Effects of Cation Structure and Acidity on Hydrogen Sulfate-Based Protic Ionic Liquid Fractionation Efficacy of Lignocellulosic Biomass

High Resolution Image Download MS PowerPoint Slide Alkylammonium hydrogen sulfate ([HSO 4 ] – ) ionic liquids (ILs) have proven effective fractionation solvents for lignocellulosic biomass. While a number of process parameters have been studied, there has been relatively little investigation into the effect of cation structure for these tunable solvents. Four alkylammonium cations were screened in this study, with three different degrees of substitution. Decreasing degree of substitution was found to enhance solution acidity as measured by the Hammett acidity function, strongly affecting fractionation behavior. Excess acid was added to a tertiary alkylammonium [HSO 4 ] – IL to replicate the behavior of primary and secondary alkylammonium [HSO 4 ] – ILs. The Hammett acidity scale alone was not sufficient to predict and compare the different PIL acidities and their effect on fractionation severity. Methylbutylammmonium hydrogen sulfate ([MBA][HSO 4 ]) was identified as a promising fractionation solvent, generating pulps with high cellulose content (>90%) and saccharification yield (>90%) at a range of different conditions, and at shorter durations than current benchmark ILs. Primary alkylammonium [HSO 4 ] – ILs were found to be ineffective fractionation solvents, due to insufficient lignin removal despite reasonable cellulose pulp contents. Sustainability metrics showed that despite low E-factors, process mass intensity can be significantly high due to high solvent use during washing steps.