Lignin fate and characterization during ionic liquid biomass pretreatment for renewable chemicals and fuels production

Abstract

The fate of lignin from wheat straw, Miscanthus, and Loblolly pine after pretreatment by a non-toxic and recyclable ionic liquid (IL), [C₂mim][OAc], followed by enzymatic hydrolysis was investigated. The lignin partitioned into six process streams, each of which was quantified and analyzed by a combination of a novel solution-state two-dimensional (2D) nuclear magnetic resonance (NMR) method, and size exclusion chromatography (SEC). Pretreatment of biomass samples by [C₂mim][OAc] at 120 and 160 °C enhances hydrolysis rates and enzymatic glucan digestions compared to those of untreated biomass samples. Lignin partitioning into the different streams can be controlled by altering the ionic liquid pre-treatment conditions, with higher temperatures favoring higher lignin partitioning to the IL stream. 2D NMR bond abundance data and SEC results reveal that lignin is depolymerized during ionic liquid pretreatment, and lignin of different molecular masses can be isolated in the different process streams. SEC suggested that higher molecular mass lignin was precipitated from the ionic liquid, leaving smaller molecular mass lignin in solution for further extraction. Lignin obtained as a residue of enzymatic hydrolysis contained the highest molecular mass molecules, similar in structure to the control lignin. The results suggest that isolated lignins via IL pretreatment from all three feedstocks were both depolymerized and did not contain new condensed structures. This finding leads to the possibility that lignin obtained from this IL pretreatment process may be more amenable to upgrading, thereby enhancing biorefinery economics.