Abstract
To investigate the potential for the utilization of cotton stalk, ammonia hydrothermal treatment was applied to fractionate the samples into aqueous ammonia-soluble and ammonia-insoluble portions. The ammonia-soluble portion was purified to yield lignin fractions. The lignin fractions obtained were characterized by wet chemistry (carbohydrate analysis) and spectroscopy methods (FT-IR, 13C and 1H-13C HSQC NMR spectroscopy) as well as gel permeation chromatography (GPC). The results showed that the cotton stalk lignin fractions were almost absent of neutral sugars (0.43%–1.29%) and had relatively low average molecular weights (1255–1746 g/mol). The lignin fractions belonged to typical G-S lignin, which was composed predominately of G-type units (59%) and noticeable amounts of S-type units (40%) together with a small amount of H-type units (~1%). Furthermore, the ammonia-extractable lignin fractions were mainly composed of β-O-4′ inter-unit linkages (75.6%), and small quantities of β-β′ (12.2%), together with lower amounts of β-5′ carbon-carbon linkages (7.4%) and p-hydroxycinnamyl alcohol end groups.
Highlights
Lignocellulosic biomass has enormous potential as a replacement for fossil fuels due to its abundance and chemical composition
This demonstrated the potential of ammonia hydrothermal treatment for high purity lignin separation
The lignin extraction efficiency from cotton stalk dissolved in the ammonia hydrothermal system at the reaction time of 2, 4, 6, 8, and 10 h showed that the yield of five lignin fractions and milled wood lignin (MWL) were 14.6, 15.2, 15.7, 16.2, 16.6, and 8.2% (Klason lignin), respectively
Summary
Lignocellulosic biomass has enormous potential as a replacement for fossil fuels due to its abundance and chemical composition. (b) regulation of individual enzymes in the biosynthetic pathway, and (c) regulation of gene expression affect the lignin content, quality, and distribution [7,8] Through these methods, the designed plants either deposit less lignin or produce lignins that are more amenable to chemical degradation [9,10,11]. As a lignocellulosic feedstock with high potential, the utilization efficiency of this lignocellulosic biomass is highly dependent on its structural properties, such as the relative content, composition, accessibility, and reactivity of the three cell wall components. In order to investigate the potential for the utilization of cotton stalk, separation of lignin components from cotton stalk based on ammonia hydrothermal treatment was investigated. H-13C correlation two-dimensional nuclear magnetic resonance (1H-13C 2D NMR) spectroscopy
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