Microwave-enhanced alkali treatment of Pinus yunnanensis: Physiochemical characterization of the dissolved lignins |
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Authors: | Bai-Liang XueMing-Fei Li Feng Xu Run-Cang Sun Gwynn Jones |
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Institution: | a College of Forestry, The North-Western University of Agricultural and Forest Sciences and Technology, Yangling 712100, China b Institute of Biomass Chemistry and Technology, Beijing Forestry University, Beijing 100083, China c State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China d The BioComposites Centre, University of Wales, Bangor LL57 2UW, United Kingdom |
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Abstract: | Pinus yunnanensis was subjected to water-bath and microwave treatments in 1% NaOH aqueous solutions at 100 °C with various ratios of bath heating time to microwave heating time (0/120, 20/100, 40/80, 60/60, 80/40, 100/20 and 120/0 min). The lignins dissolved in the alkali liquors were separated and purified, and their physicochemical features were comparatively characterized by sugar analysis, GPC, FT-IR, 13C and HSQC NMR, as well as thermogravimetric analysis (TGA). The results showed that the lignin fractions extracted with microwave heating (20-120 min) had high molecular weights and polydispersities (Mw 3150-5710 g/mol, Mn 2130-3020 g/mol, Mw/Mn 1.48-2.00) as compared to those prepared without microwave heating (Mw 3080 g/mol, Mn 2080 g/mol, Mw/Mn 1.48). The most striking characteristic of all lignin fractions was the almost absence of associated sugars (0.16-3.25%). The TGA results indicated that the thermal stability of the lignin fraction increased with the increment of the molecular weight. FT-IR and NMR spectra suggested that the lignin fractions showed similar structures which were mainly composed of guaiacyl (G) and minor amounts of p-hydroxyphenyl (H) units. Moreover, HSQC NMR spectrum of a typical lignin fraction (prepared with microwave heating for 120 min) revealed that it contained dominant amounts of β-O-4′ linkages (64.6%) and phenylcoumaran (β-5′) substructures (25.8%) together with small amounts of resinol (β-β′) substructures (6.7%) and coniferyl alcohol end groups (2.9%). |
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Keywords: | Microwave heating Lignin Pinus yunnanensis FT-IR NMR |
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