Structural characterization of alkaline hydrogen peroxide pretreated grasses exhibiting diverse lignin phenotypes

Abstract

BackgroundFor cellulosic biofuels processes, suitable characterization of the lignin remaining within the cell wall and correlation of quantified properties of lignin to cell wall polysaccharide enzymatic deconstruction is underrepresented in the literature. This is particularly true for grasses which represent a number of promising bioenergy feedstocks where quantification of grass lignins is particularly problematic due to the high fraction of p-hydroxycinnamates. The main focus of this work is to use grasses with a diverse range of lignin properties, and applying multiple lignin characterization platforms, attempt to correlate the differences in these lignin properties to the susceptibility to alkaline hydrogen peroxide (AHP) pretreatment and subsequent enzymatic deconstruction.ResultsWe were able to determine that the enzymatic hydrolysis of cellulose to to glucose (i.e. digestibility) of four grasses with relatively diverse lignin phenotypes could be correlated to total lignin content and the content of p-hydroxycinnamates, while S/G ratios did not appear to contribute to the enzymatic digestibility or delignification. The lignins of the brown midrib corn stovers tested were significantly more condensed than a typical commercial corn stover and a significant finding was that pretreatment with alkaline hydrogen peroxide increases the fraction of lignins involved in condensed linkages from 88–95% to ~99% for all the corn stovers tested, which is much more than has been reported in the literature for other pretreatments. This indicates significant scission of β-O-4 bonds by pretreatment and/or induction of lignin condensation reactions. The S/G ratios in grasses determined by analytical pyrolysis are significantly lower than values obtained using either thioacidolysis or 2DHSQC NMR due to presumed interference by ferulates.ConclusionsIt was found that grass cell wall polysaccharide hydrolysis by cellulolytic enzymes for grasses exhibiting a diversity of lignin structures and compositions could be linked to quantifiable changes in the composition of the cell wall and properties of the lignin including apparent content of the p-hydroxycinnamates while the limitations of S/G estimation in grasses is highlighted.

Relationships between Biomass Composition and Liquid Products Formed via Pyrolysis

“…An analysis of more subtle compositional differences, in which compositional factors are varied across different samples, may aid in refining biomass-bio-oil relationships. For example, genetic mutants that vary in only one component relative to near isogenic, unmutated "wild-type" plants can directly address relationships between starting components and products (Li et al, 2012). In addition to genetically determined compositional differences, biomass composition also depends on growth conditions and developmental stage, which relates to harvest time.…”

Chemical properties of bio-oils produced by fast pyrolysis of bamboo

“…First, one proton radical is withdrawn from the γ-hydroxymethyl group by pyrolysis followed by homolytic cleavage at the β-γ bond in the side chain of lignin to release carbon dioxide. Li et al [28] reported the formation of 4VP by the pyrolysis of herbaceous plants. They used analytical pyrolysis to evaluate the lignin structure and did not aim to produce bio-oil.…”

Structural Characterization of Lignin in Wild-Type versus COMT Down-Regulated Switchgrass

“…Although lignocellulosic resources, such as energy crops and agricultural and forest residues, are readily becoming available for bioethanol production, their processing requires a costly pretreatment step to overcome their natural recalcitrance toward biological deconstruction to simple sugars (Sun and Cheng, 2002;Himmel et al, 2007;Pu et al, 2008;Somerville et al, 2010). Lignocellulosic biomass is a complex composite consisting primarily of three biopolymers (i.e., cellulose, hemicelluloses, and lignin) and its recalcitrance has been attributed to several factors such as cellulose accessibility to enzymes, lignin content/structure, lignincarbohydrate complexes, as well as the presence and structure of hemicelluloses (Wyman et al, 2005;Li et al, 2012;Leu and Zhu, 2013;Pu et al, 2013). The goal of pretreatment is to reduce the recalcitrance of biomass by disrupting/modifying the ligninpolysaccharide matrix.…”