Lignin is a heterogeneous aromatic polymer found in woody biomasses, accounting for a significant percentage of all renewable carbon. The primary means by which lignin can be utilized for the synthesis of higher value products is through thermochemical conversion schemes that can include pyrolysis, liquefaction and gasification. One of the most important first steps in thermochemical conversion of lignins is in understanding their structure on a molecular level. Recently, it has been shown that this structure can be computationally modeled using four characteristics: 1) the monomer composition (p-hydroxyphenyl, guaiacyl, and syringyl content); 2) the bond-type composition; 3) the molecular weight characteristics; and 4) the degree of branching in the polymer. While the first three characteristics have been well-studied in numerous previous publications, to date there is comparatively less in-depth study describing how to measure or calculate the degree of branching. In this poster we will present some results, along with their underlying theoretical basis, on the branching coefficient for a range of contrasting lignins. The branching coefficient is defined as the probability that one given aromatic monomer unit in a lignin polymer is bonded to three or more other units.<br/>Using earlier published work, it is possible to show that based on the molecular weight characteristics of lignin (number-averaged and weight-averaged), the branching coefficient can be calculated and that it must be within the range of 0.00 to 0.33. We calculate the branching coefficients using experimental molecular weight data for two kraft lignins from pine and eucalyptus woods (0.18 and 0.03, respectively), one soda lignin from wheat straw (0.01) and one organosolv lignin from wheat straw (0.00). Continuing work will include the analysis of molecular weight for milled-wood lignins from corn stover, Douglas fir and hybrid poplar woods. Additionally, we will explore the nature of the differences in branching between hardwoods, softwoods and grass-type biomasses, and investigate possible correlations between branching determined by molecular weight with important lignin characteristics that can be measured through other analytical methods, including carbon-13 NMR and FTIR spectroscopy.

Analyzing the degree of branching in lignins: Theoretical and experimental results

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.