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

Authors: SATHITSUKSANOH, NOPPADON - Joint Bioenergy Institute (JBEI); Holtman, Kevin; YELLE, DANIEL - Forest Products Laboratory; MORGAN, TREVOR - Hawaii Natural Energy Institute; STAVILA, VITALIE - Sandia National Laboratory; PELTON, JEFFREY - Lawrence Berkeley National Laboratory; BLANCH, HARVEY - University Of California; SIMMONS, BLAKE - Sandia National Laboratory; GEORGE, ANTHE - Sandia National Laboratory

Submitted to: Green Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/18/2013
Publication Date: 11/18/2013
Citation: Sathitsuksanoh, N., Holtman, K.M., Yelle, D.J., Morgan, T., Stavila, V., Pelton, J., Blanch, H., Simmons, B.A., George, A. 2013. Lignin fate and characterization during ionic liquid biomass pretreatment for renewable chemicals and fuels production. Green Chemistry. 3(3):1236-1247. doi: 10.1039/c3gc42295j.

Interpretive Summary: Lignin, which is one of the main components isolated from agricultural residues during biofuel production, is generally burned for its energy because it is too “damaged” by pretreatment protocols to be used for higher-value products. Treatment of residue feedstocks has been introduced as a way to pretreat biomass so that a higher grade of lignin is isolated, thus adding value to the entire biorefinery operation. In this study, the fate of lignin from wheat straw, Miscanthus, and Loblolly pine was characterized after pretreatment by a non-toxic and recyclable ionic liquid (IL), [C2mim][OAc], followed by enzymatic hydrolysis. The lignin was characterized by a novel solution-state two-dimensional (2D) nuclear magnetic resonance (NMR) method, and size exclusion chromatography (SEC). It was shown that pretreatment of biomass samples by this ionic liquid at 120 and 160 °C enhances hydrolysis rates and aids enzymatic glucan digestion compared to untreated biomass samples. The results suggest that using these ionic liquids to isolate lignin from these three sources of biomass could be very beneficial in biorefinery operation since the lignin became depolymerized without creating other condensed structures that could complicate further use of the lignin. This finding leads to the possibility that lignin obtained from this IL pretreatment process may be more amenable to upgrading, thereby enhancing biorefinery economics.

Technical 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.