Abstract
Conversion of technical lignin into performance biopolymers such as polyurethane offers environmental and economic advantages when combined with production of biofuels from biomass sugars, presenting significant interest towards studying the role of pretreatment on lignin structure and functionality. Co-solvent enhanced lignocellulosic fractionation (CELF) pretreatment, employing acidic aqueous tetrahydrofuran (THF) mixtures, was developed to effectively break down the lignin-carbohydrate matrix and promote extraction of lignin from lignocellulosic biomass with desirable purity and yield. In this study, we report on the effects of CELF pretreatment reaction severity on the molecular structure of CELF-extracted lignin and its impact towards the mechanical properties of resulting lignin-polyurethanes. Reaction temperature was found to play the most significant role, compared to reaction time and acidity, in manipulating structural features such as molecular weight, functionality and intra-polymer structure. At the severe reaction conditions at 180 °C, the order of reactivity for primary lignin interlinkages characterized by semiquantitative HSQC NMR analysis were found to be -ether > phenylcoumaran (5’) > resinol (’) facilitating a high degree of depolymerization yielding a high frequency of free phenolics and reduced aliphatic hydroxyl groups. All side-chain interlinkages were depleted converting guaiacyl subunits into condensed forms, while retaining more uncondensed syringyl subunits. Under the mild 150 °C temperature reaction, CELF lignin had higher molecular weight and retained more -ether interlinkages. The results from CELF lignin-based polyurethane synthesis indicated that the tensile properties depended on the miscibility of CELF lignin with other components and low molecular weight cuts improved the dispersion of lignin in the polyurethane network. Pre-mixing of CELF with poly(ethylene glycol) (PEG) reduced the brittleness and improved the ductility of the CL-PEG polyurethanes.
Highlights
Lignin found in lignocellulosic biomass is a class of heterogeneous biopolymers typically derived from three types of phenylpropanoid subunits: guaiacyl (G), syringyl (S), and p−hydroxylphenyl (H) (Higuchi, 2003)
We report the effects of Co-solvent enhanced lignocellulosic fractionation (CELF) pretreatment reaction severity on the molecular structure of CELF-extracted lignin and its impact toward the mechanical properties of the resulting lignin-based polyurethanes
Mild CELF pretreatment at low temperature was conducted to reduce the changes on lignin chemical structure and preserve high molecular weight, high β–O–4, and aliphatic hydroxyl contents
Summary
Lignin found in lignocellulosic biomass is a class of heterogeneous biopolymers typically derived from three types of phenylpropanoid subunits: guaiacyl (G), syringyl (S), and p−hydroxylphenyl (H) (Higuchi, 2003). The recalcitrance of plant cell wall is designed by nature to be resistant to biological and chemical degradation. In order to reduce the costs associated with processing lignocellulosic biomass to biofuels and biochemicals, pretreatment is often employed to modify the plant cell wall to improve accessibility of cellulolytic enzymes to the crystalline cellulose domains from which fermentable glucose can be released (Mostofian et al, 2016). In order to improve upon conventional aqueous biomass pretreatment methods, the addition of miscible co-solvents greatly improves the dissolution of lignin that is critical in maximizing utilization of all major biomass fractions by subsequent catalytic and biological conversion methods
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