Abstract
BackgroundThe requirement for efficient and green conversion technologies has prompted hydrothermal processing as a promising treatment option for sustainable biorefinery industry. The treatment has been applied to process plenty of lignocellulose materials, yielding abundant high value-degraded products, especially the products in the liquid fraction. Therefore, it is essential to systematically evaluate the degraded products in aqueous fraction by comprehensive analysis and structural characterization during the treatment.ResultsRapeseed straw was hydrothermally treated at temperature ranging from 145 to 205 °C for various retention time (15, 30, 60 and 120 min), and the degraded polysaccharides and lignin products in aqueous phase were systematically evaluated by comprehensive analysis and structural characterization. Results showed that with an increase of severity, the polymers were gradually depolymerized resulting in a decrease of the molecular weight from 8430 (log R0 3.26) to 2130 g/mol (log R0 5.08), an increase of oligosaccharides from 19.44 (log R0 2.88) to 99.94 g/kg (log R0 4.32) and an increase of monosaccharides from 0.91 (log R0 2.88) to 30.43 g/kg (log R0 4.37). With the increase of monosaccharide degradation components (8.26 to 125.68 g/kg), the saccharides gradually decreased after its maximum value. The maximum yield of oligosaccharides (99.94 g/kg) accompanying a relatively low level of monosaccharides (17.77 g/kg) was obtained at a high temperature (190 °C) for a short reaction time (15 min). The degraded polysaccharides had a linear backbone of (1 → 4)-linked β-d-xylopyranosyl xylan decorated with branches based on 2D NMR spectra analysis. Lignin was strongly condensed with a decrease of S/G ratio as the severity increased. The yields of the degraded constitutions have a incomplete linear correlation with the treatment severity.ConclusionsThe liquid fractions obtained from hydrothermal treatment were subjected to comprehensive analysis and structural characterization. Results indicated that hydrothermal treatment had a significant influence on the composition and structure of the polysaccharides and lignin in the aqueous phase. The treatment could be adopted to obtain XOS-rich fraction with limited formation of by-products. In addition, the result was expected to further reveal the mechanisms of hydrothermal treatment on rapeseed straw and to facilitate the value-added applications of agricultural residues in the biorefinery industry.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0552-8) contains supplementary material, which is available to authorized users.
Highlights
The requirement for efficient and green conversion technologies has prompted hydrothermal process‐ ing as a promising treatment option for sustainable biorefinery industry
Monosaccharide and oligosaccharide analysis of the liquid fractions The dissolution of hemicelluloses was triggered by hydronium ions generated in situ by autoionization of water and further enhanced by organic acids generated from the degradation of hemicelluloses [37, 38]
(4.67 g/kg raw material (RM)) and monosaccharides (9.25 g/kg RM) were obtained at severities of 3.38, 3.26, 3.70 and 4.32, respectively. These results suggested that the extent of the hemicellulose degradation related to the content of the original amount in rapeseed straw
Summary
The requirement for efficient and green conversion technologies has prompted hydrothermal process‐ ing as a promising treatment option for sustainable biorefinery industry. The treatment has been applied to process plenty of lignocellulose materials, yielding abundant high value-degraded products, especially the products in the liquid fraction. Hydrothermal pretreatment is an environmentally friendly, low cost and effective treatment in the commercial production before downstream biorefinery industry, which is an green innocuous efficient method that utilizes water as the sole solvent under the conditions of high temperature and high pressure in a closed system. The degraded sugars present in the liquid fraction mainly consist of high value-added oligomers, which are potentially used for chemicals, food and pharmaceutical production [13, 15]. Low-cost materials of construction, no requirement for chemical addition and high value-added products make the treatment to have great economic advantages [17, 18]. It is necessary to understand the dissolution mechanism and the products in the liquid phase from the hydrothermal treatment, which would contribute to the biorefinery scenario
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