Abstract
A renewable raw material, rice straw is pretreated for biorefinery usage. Solution-state two-dimensional (2D) 1H-13 C hetero-nuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectroscopy, was used to analyze 13 cultivars of rice straw before and after dilute acid pretreatment, to characterize general changes in the lignin and polysaccharide components. Intensities of most (15 of 16) peaks related to lignin aromatic regions, such as p-coumarate, guaiacyl, syringyl, p-hydroxyphenyl, and cinnamyl alcohol, and methoxyl, increased or remained unchanged after pretreatment. In contrast, intensities of most (11 of 13) peaks related to lignin aliphatic linkages or ferulate decreased. Decreased heterogeneity in the intensities of three peaks related to cellulose components in acid-insoluble residues resulted in similar glucose yield (0.45–0.59 g/g-dry biomass). Starch-derived components showed positive correlations (r = 0.71 to 0.96) with glucose, 5-hydroxymethylfurfural (5-HMF), and formate concentrations in the liquid hydrolysates, and negative correlations (r = –0.95 to –0.97) with xylose concentration and acid-insoluble residue yield. These results showed the fate of lignin and polysaccharide components by pretreatment, suggesting that lignin aromatic regions and cellulose components were retained in the acid insoluble residues and starch-derived components were transformed into glucose, 5-HMF, and formate in the liquid hydrolysate.
Highlights
In order for ethanol to be classified as an environmentally friendly fuel, it must be produced from abundant and renewable resources, such as lignocellulosic materials [1]
Most signals related to aromatic regions, such as p-coumarate, guaiacyl, syringyl, and p-hydroxyphenyl (ROI 1–13 except regions of interest (ROI) 6), and that of the cinnamyl alcohol end group (ROI 14), increased or were unchanged after dilute acid pretreatment (Fig 2)
The results of the present study showed that the starch content in rice straw serve as good indicators of the xylose, formate, and 5-HMF concentrations in the liquid hydrolysate and of the acid-insoluble residue yield generated by dilute acid pretreatment
Summary
In order for ethanol to be classified as an environmentally friendly fuel, it must be produced from abundant and renewable resources, such as lignocellulosic materials [1]. Due to its complex chemical structure, bioethanol production from rice straw requires pretreatment to facilitate subsequent enzymatic hydrolysis [4]. Dilute acid pretreatment has been studied widely due to its low cost, convenience, and effectiveness for a broad range of lignocellulosic biomass sources [5,6,7,8]. Dilute acid pretreatment produces an acid-insoluble residue and liquid hydrolysate. The acid-insoluble residue is composed primarily of cellulose and lignin [9]. This cellulose is a major source of glucose following enzymatic hydrolysis. Understanding of how dilute acid pretreatment alters the lignin and polysaccharide components and acid-insoluble residue in various kinds of rice straws remains limited [13]
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