Ionic Liquid Solvent Properties as Predictors of Lignocellulose Pretreatment Efficacy

Abstract

Effective pretreatment of lignocellulosic biomass is vital to its bioconversion to a usable liquid fuel. A growing body of work has focused on using room temperature ionic liquids (RTILs) to pretreat lignocellulose for subsequent fermentation. However, little is known about the physicochemical parameters of RTILs that promote effective pretreatment. In this work we examine the relationship between the Kamlet–Taft α, β, and π* solvent polarity parameters of different RTILs ([Emim][OAc], [Bmim][OAc], and [Bmim][MeSO4]) and effective pretreatment of lignocellulosic biomass. We find the β parameter is an excellent predictor of pretreatment efficacy. Acetate containing RTILs (β > 1.0) remove >32% of lignin from maple wood flour and significantly reduce cellulose crystallinity, resulting in >65% glucose yields after 12 h cellulase hydrolysis. Pretreatment in [Bmim][MeSO4] (β = 0.60) results in the removal of only 19% of the wood flour's lignin with no decrease in crystallinity, and no improvement in sugar yield over untreated wood flour. The addition of water and the dilution of the acetate anion with the methyl sulfate anion decrease the β value and subsequently have a negative impact on lignin extraction, cellulose crystallinity, and sugar yields.