Understanding changes in lignin of Panicum virgatum and Eucalyptus globulus as a function of ionic liquid pretreatment

Ionic liquids (ILs) have shown great potential for the reduction of lignin in biomass after pretreatment. Although dilute acid and base pretreatments have been shown to result in pretreated biomass with substantially different lignin composition, there is scarce information on the composition of lignin of IL pretreated biomass. In this work, temperature dependent compositional changes in lignin after IL pretreatment were studied to develop a mechanistic understanding of the process. Panicum virgatum and Eucalyptus globulus were pretreated with 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]). Measurement of syringyl and guaiacyl ratio using pyrolysis–GC/MS and Kamlet–Taft properties of [C2mim][OAc] at 120 °C and 160 °C strongly suggest two different modes of IL pretreatment. Preferential breakdown of S-lignin in both eucalyptus and switchgrass at high pretreatment temperature (160 °C) and breakdown of G-lignin for eucalyptus and no preferential break down of either S- or G-lignin in switchgrass was observed at lower pretreatment temperatures (120 °C).

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► Tracked syringyl and guaiacyl ratios during IL pretreatment of switchgrass and eucalyptus.
► Kamlet–Taft properties of [C2mim][OAc] at 120 °C and 160 °C suggest two mechanisms.
► Preferential breakdown of S-lignin in both eucalyptus and switchgrass was observed at 160 °C.
► Breakdown of G-lignin for eucalyptus was observed at 120 °C.
► Insight on IL pretreatment mechanism demonstrates selective removal of S or G lignin is feasible.