Structural analysis of pyrolytic lignins isolated from switchgrass fast pyrolysis oil

Authors: FORTIN, MICHAEL - University Of Sciences - Philadelphia; BEROMI, MEGAN - University Of Sciences - Philadelphia; LAI, A - University Of Sciences - Philadelphia; TARVES, PAUL - University Of Sciences - Philadelphia; Mullen, Charles; Boateng, Akwasi; WEST, NATHAN - University Of Sciences - Philadelphia

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2015
Publication Date: 10/24/2015
Citation: Fortin, M., Beromi, M.M., Lai, A., Tarves, P.C., Mullen, C.A., Boateng, A.A., West, N.M. 2015. Structural analysis of pyrolytic lignins isolated from switchgrass fast pyrolysis oil. Energy and Fuels. 29:8017-8026.

Interpretive Summary: Fast pyrolysis is a process of heating in the absence of oxygen that can produce a liquid product (bio-oil) from non-food biomass such as switchgrass. Bio-oil is a liquid that has the potential to be refined into renewable gasoline and diesel fuels or renewable chemicals. A fraction of the bio-oil, which is specifically derived from lignin (and therefore termed pyrolytic lignin), is non-volatile and highly viscous or even solid. This fraction, because of its chemical and physical properties, makes refining of bio-oil to fuels via traditional catalytic processes difficult. However, it can be separated, enabling the refining of the remainder of the bio-oil and subsequent conversion of the pyrolytic lignin via alternative routes to renewable chemicals. In this paper, several chemical techniques were used to determine the structure and properties of this material to guide efforts for developing appropriate conversion processes. The result of these studies provided an increased understanding of the chemical structure of the pyrolytic lignin derived from switchgrass. This study will be of interest to those working on developing biorefineries based on biomass for which lignin is a co-product.

Technical Abstract: Structural characterization of lignin extracted from the bio-oil produced by fast pyrolysis of switchgrass (Panicum virgatum) is reported. This new information is important to understanding the utility of lignin as a chemical feedstock in a pyrolysis based biorefinery. Pyrolysis induces a variety of structural changes to lignin in addition to reduction in molecular weight. The guaiacol structural units remain largely intact, and some hemicellulose stays covalently linked to the lignin. However, two-dimensional 1H-13C HSQC NMR analysis shows an absence of y-methylene hydrogens from beta-O-4 linkages implying rearrangements in the propyl linking chains have occurred. A significant decrease in the number of methoxy groups on the glucose units in hemicellulose is observed, but the aromatic rings in the lignin retain their methoxy groups. Ferulate and hydroxyl phenol esters are still present in the pyrolyzed lignin, but in lower concentration than in unpyrolyzed switchgrass lignin.